There are two main principles used for locking devices – friction and positive locking. It should be noted that locking systems using friction should be selected with care because under high vibration friction between adjacent surfaces can be significantly reduced. With positive locking device e.g. use of tab washer very high reliability results provided the locking device is used correctly.

Lock Nut /Jam Nut

The term is used for thin nuts used to lock a thicker nut. Usually a thin nut is fixed above a normal nut. It is recommended by many persons that the thin nut should be adjacent to the joint surface and tightened against the thick nut as shown above. As per them, if thin (lock) nut is placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain. The lock nut principle can also be used to fix a nut in any position on the male screw thread and therefore create a shoulder.

Nylock Nut

Usually nuts are free spinning, but Nylock nuts have a plastic or fiber collar set into the nut which is an interference fit on the male thread that causes resistance to nut turning. This resistance is called "prevailing torque". Prevailing torque is the torque required to turn the nut. The prevailing torque does not go toward tightening the bolt. On assembly the male thread forces its way through the collar and the resultant friction restricts the tendency to unscrew.

The rule of thumb is to add the prevailing torque to the torque value when applying torque to a Nylock nut. This is because the prevailing torque doesn't contribute to bolt tightening. You can use torque wrench to measure Nylock nut torque and then add this value to the bolt's required torque. Usually you wouldn't add prevailing torque to the torque value published by the equipment manufacturer. The equipment manufacturer has already done this for you.

The prevailing torque locknut retains its locking ability even when the preload or tightening torque has been lost. The nylock nuts are more friendly to the threads, locks out moisture and prevents corrosion.

For more information on nylock nuts please refer IS 7002 (Identical to ISO 7040), Prevailing Torque Type Hexagon Nuts (With Non-Metallic Insert), Style 1 – Property Class 5, 8 and 10. The DIN standard number is 982.

All-metal Lock Nuts

As nylock nuts are not suitable in areas exceeding 120 degrees C, for high temperature application all-metal lock nuts are used. Various types of all-metal lock nuts are as under.

Distorted Thread Nut (CLEVELOC Nut)

In this type of nut, the collar of the nut has been slightly crushed at the top to make it oval. When the round bolt reaches the oval portion of the nut it springs the nut back round. This spring action grips the bolt and adds friction that prevents loosening. Such type of nut is also known as Cleveloc nut. Cleveloc is a registered trade name of Forest Fasteners.

Spring Beam Nut (FLEXLOC Nut)

This type of nut has thin slots cut down through the top few threads with the resulting fingers bent slightly inward. At installation, the bolt springs the fingers out and the fingers grip the bolt with a prevailing torque. This type of lock nut is also known as FLEXLOC nut. FLEXLOC is a trademark of SPS Technologies. For more information please visit website of SPS Technologies – http://www.spstech.com

Slotted Nut

In slotted nut, a slot is machined into the nut and the nut is deformed to compress the slot as shown above. When the nut is tightened onto the male thread it forces the nut back to its original geometry. The thread system is locked by the built in friction.

Slotted / Castle Nuts

These nuts have slots in the top face. The nuts are fully tightened and a hole is drilled through the male thread to align with one of the slots. Split cotter pin is then inserted through the nut and the male thread and bent to hold it in position. This is a very effective and positive locking device but is expensive to install.

For more information on slotted / castle nuts please refer IS 2232, Specification for Slotted and Castle Nuts. This standard prescribes the requirements of precision and black grades of slotted and castle nuts in the following diameter ranges.
Precision grade slotted nuts: 4 to 39 mm
Black grade slotted nuts: 12 to 33 mm and
Precision and black grades castle nuts: 12 to 100 mm.

Durlok® self-locking system

Durlok® nuts and bolts have serrations on nut face and below bolt head. These serrations prevent fasteners from loosening. Durlok® is a trademark of Unbrako. For more information on Durlok®, please visit website of Unbrako – http://www.unbrako.in.

Heli-Coil Screw Lock Inserts

There are two designs of Heli-Coil inserts. STANDARD type provides a smooth free-running thread. SCREW-LOCK type provides self-locking torque on the male member by a series of straight segments / chords on one or more of the insert coils. For more information on Heli-Coil inserts, please visit website – http://www.helicoil.in or http://www.emhart.com.

Spring Washers

They are double or single coils of rectangular or square section made from spring steel. The washers prevent rotation of the nut or bolt by the two ends digging into the surface of the two adjacent faces. Since they are not highly reliability, they are generally used for non-critical applications. For more information on single coil spring washer please refer IS 3063, Fasteners – Single coil rectangular section spring lock washers – Specification. This standard covers requirements for single coil rectangular section spring lock washers suitable for use with bolt/nut assemblies involving fasteners of property class 5.8 or less in the size range 2 to 100 mm.

Star Washers

These washers are generally made from spring steel with serrations either on the inside or the outside diameter. Since they are not highly reliability, they are generally used for non-critical applications. They are also known as Shakeproof or Multi-tooth lock washers. For more information on star washer please refer IS 5371, Specification for – Multi-tooth Lock Washers. It covers the requirements for multi-tooth lock washers for use with screws in the diameter range 1.6 to 30 mm.

Tab washers

Tab washers are thin metal washers designed with tabs which project from the outside diameter. The washer is placed below the head of the bolt or the nut and following tightening one or two tabs are bent upwards against the flats of the bolt/nut head. The remaining tab is bent down into a suitable hole in the surface being fastened or over a local edge, if one is available. For more information on tab washers please refer IS 8068, Specification for Tab Washers. It covers the requirement for tab washers of two types for locking bolts and nuts in the bolt diameter range 3 to 52 mm.

As this methods fix the nut or bolt head to the adjacent surface, use two tab washers (one for bolt and other for nut) to lock a set of fastener if the other component is not secured by some other method.

Thread Locking Adhesives

A modern trend is to use engineered adhesives. These are simply applied to the threaded component prior to assembly. The type of adhesive selected will depend on the need for a assembly – permanent or an assembly which requires dismantling. The most well known manufacturer of these systems is Henkel Loctite. For more information on this, please refer an article on it – Anaerobic and Cyanoacrylate Adhesives (in Products Category).

CAUTION:
Many locking devices are single use items and there is always the risk that during an activity requiring screw removal and replacement the maintenance department may not have the same replacement locking device in stock. There is tendency to re-use the existing item. This will result is the screws being re-assembled such that they are not firmly locked.

It is a good practice to ensure that new locking devices are available before critical maintenance work is taken up.

Need for use of a washer

At high preloads the force under the head of a bolt or nut can exceed the compressive yield strength of the clamped material. If this occurs excessive embedding and deformation can result in bolt preload loss. To overcome this hardened washers under the bolt head are used to distribute the force over a wider area into the clamped material. Similar situation can be there if steel fasteners are used for softer clamped material like aluminium at normal preloading. To overcome this plain steel washers are used. If preload is not adequate, fasteners will work loose in service leading to their failure. Due to this, structural bolts are designed to be used with hardened washers.

In practice washers are used for one more application – use of fasteners on over size holes (for example, on enlarged holes in base plate to accommodate fastener to overcome interference due to misalignment of holes in equipment and holes of foundation plate). In such cases, care shall be taken to select a thick washer (preferably thick plate with a hole of required size having large outside diameter to adequately cover enlarged hole) which will not deform during tightening.

One problem that can occur with washers is that they can move when being tightened so that the washer can rotate with the nut or bolt head rather than remaining fixed. This can affect the torque tension relationship. In view of this ensure that washer does not rotate while tightening as rotating washer will require higher torque to overcome friction (due to larger friction area) leading to less preloading when tightened to recommended torque value.

A more modern alternative of washer is to use a flange headed nuts and bolts. Take care to replace a flanged fastener with a flanged fastener or normal fastener (of same strength) with a good quality plane machined washer.

In view of above, the best practice is not to use a washer if it is not called for.

Washer Dimensions

Different standards are followed in different countries. In India IS 2016, specification for plain washers is followed. This standard lays down the requirements for plain washers of the following types:

• Machined washers, for precision and semi-precision grade of general purpose bolts and screws, in the diameter range 1.7 to 155 mm.
• Punched washers, type A, for black grade general purpose bolts and screws, in the diameter range 1.8 to 52 mm and
• Punched washers, type B, for slotted head screws in the diameter range 1.8 to 22 mm.

Dimensions of type A washers as per IS 2016

IS 6649, Specification for Hardened and Tempered Washers for High Strength Structural Bolts and Nuts covers the requirements for through hardened and tempered steel washers intended for assembly with large series hexagon, high strength structural bolts and nuts in the size range M16 to M36.

ISO 7089 and ISO 7090 are for plain washers and chamfered plain washers respectively. Chamfered washers are given chamfering on washer outside diameter. They are normal series washers for product grade A. Washer dimensions as per ISO 7089, ISO 7090 and DIN 125 are as under.

Plain Washer Dimensions as per ISO 7089 and 7090 / DIN 125 standards

Note: All dimensions are in MM. ID – Inside Diameter, OD – Outside Diameter.

Flat inch (imperial) washers are made as per ASTM F844 and F436. Standard flat washers made as per ASTM F844 specification are unhardened and intended for general use. Round plate washers are simply a low carbon steel, oversized cut washer designed to provide a larger bearing surface than F844 standard flat washer. Plate washers are commonly used in wood construction to prevent bolt heads or nuts from pulling into the wood. Round Hardened Steel Washers are through hardened or carburized to add strength. They are made as per ASTM F436 specification. They are physically smaller than a standard F844 flat washers. Washer dimensions for various types are as under.

Dimensions for Standard Flat Washer as per ASTM F844

Note: Dimensions are as per ASME B18.22.1

Dimensions for Round Plate Flat Washers

Dimensions for Round Hardened Steel Washers as per ASTM F436

Note: Dimensions are as per ASTM F436

Taper Washers

Flange thickness of a rolled beam or channel is not constant. Due to this, when base frames are made from rolled steel beams and channels, a taper washer is required. Taper washer makes surfaces between bolt head and nut parallel as shown below.

IS 5372, Taper washers for channels (ISMC) covers the requirements for taper washers for use with Indian Standard Medium Weight Channels (ISMC) with bolts in the diameter range of 10 to 39 mm. IS 5374, Taper washers for I-beams (ISMB) covers the requirements for taper washers for use with Indian Standard Medium Weight Beams (ISMB) with bolts in the diameter range 10 to 39 mm.

Caution:
When taper washers are used, check for correctness of their installation as many times they are installed wrongly by inexperienced technicians.

Metric Studs

The end of a stud which is secured in a tapped hole is called metal end (also known as tap end) of the stud. Generally this end has a thread tolerance which results in thread interference. If normal thread is required at this end, it shall be specified in purchase order. The other end is called as nut end.

Metric double end studs are NOT called out by their overall length. To calculate overall length (total length) of a metric double ended stud, add the metal end length to the nominal length.

Metal end length of a stud is a multiple of nominal diameter of a stud (d). The value of multiplier is different for different standard / types. The value of multiplier for various DIN standards is as under.

Thus as per DIN 939, metal end length of M12 stud will be 12 x 1.25 = 15 mm.

General guidelines for calculating nut end length of a stud which is based on nominal length (L) are as under.

Stud lengths as per DIN 835

Various lengths of a stud (for M5 to M16) as per DIN 835 are as under.

Notes:
As multiplier = 2.0 for DIN 835, e = 2d
For nominal length less than 125 mm, Nut End Length (b) = 2d + 6 mm

Nominal Length = l
Overall Length = l + e

Information on IS 1862

IS: 1862 covers the requirements for studs in the diameter range from 3 to 39 mm.

Three types of studs are specified in the standard as under.

Type A:- Recommended for use in Steel.
Type B:- Recommended for use in Cast Iron.
Type C:- Recommended for use in Aluminium Alloys.

The dimensions for the metal end of the stud shall conform to those specified in IS 2186 – ISO Metric external screw threads for interference fit applications. Approximate length of metal end of a stud having nominal diameter d shall be as under.

Type A = 1.0d
Type B = 1.5d and
Type C = 2.0d

Nut end length may be calculated using general guidelines given above in this article.

Studs shall be designated by the name, type, size, nominal length, number of this standard, grade and symbol for mechanical property class. Thus a stud of Type B, 6 mm nominal diameter, 30 mm nominal length, product grade A and made of steel having property class 8.8 shall be designated as under.

Stud B M6 x 30 IS-1862 – A – 8.8

Threaded Rods

Fully threaded rods are also available for general purpose. They are available in variety of materials and with variety of surface treatment (plating). As per DIN 975, they are available in standard length of 1.0 meter. Rods of 2.0 and 3.0 meter lengths are also supplied.

Studs as per Unified (inch) standards

Unified studs are threaded in accordance with ANSI B 1.1.
Information about studs made as per unified (inch) standard is as under.

Continuous Thread Studs

Continuous-thread studs are threaded from end to end and are often used for flange bolting with two nuts. There are two types as under.

Type 1

They are used for general purpose. The length of this type is measured from end to end. Threads are UNRC-2A.

Type 2

They are used for high temperature-pressure piping. These studs are made to the dimensional standard requirements of ANSI B16.5 and have a length measurement requirement different from all other studs, i.e., the length is measured from first thread to first thread, exclusive points. Points are flat and chamfered. Threads are UNRC-2A for all sizes 1 in. and under and 8UNR-2A for all sizes over 1 in. They are made from material as per ASTM A-193, Grade B7 specification. Nuts for these studs are made from material as per ASTM A-194, Grade 2H specification. Nuts are threaded as per UNC-2B.

Partially threaded studs

There are two types of stud as under.

Tap-end Studs

Tap-end studs have a short thread on one end, called the tap end which is threaded to a Class NC5 or Class UNRC-3A fit. This end is for screwing into a tapped hole. The other or nut end is threaded with a Class UNRC-2A fit. Length of the stud is measured overall. The tap end has a chamfered point, but the nut end may have either a chamfered or round point, at the manufacturer's option.

Double-end Studs

Double-end studs have equal-length threads on each end to accommodate a nut and are threaded to a Class 2A fit. Length of stud is measured overall. Both ends have chamfered points, but round points may be furnished on either or both ends at the manufacturer's option. Double-end studs are used for flange bolting or other applications where torching from both ends is necessary or desirable.

Tightening Methods

In gasketed joints fasteners shall be tightened to create a seal. Thus for this application, one can calculate required pre-load based on internal pressure and number of fasteners. For other applications, required pre-load depends on application and typical values vary from 50% to 80% of the yield strength of the fastener material. For critical applications designer specifies preloading values.

All fastener materials are slightly elastic and must be stretched a small amount to develop clamping force. Within elastic limit, stress is proportional to strain and modulus of elasticity, Young’s modulus (stress / strain = constant) is a property of material. Young’s modulus (tensile or compressive loads) for steel is 30,000,000 psi. Thus for steel fasteners, a stretch of 0.001 inch per inch length of a fastener will develops 30,000 psi clamping force. From this information, one can easily calculate fastener to be stretched for desired pre-load. This method of preloading is very accurate but it requires that the ends of the bolts be properly prepared and also that all measurements be very carefully made. In addition, direct measurements are only possible where both ends of the fastener are available for measurement after installation. Since in most cases it is not possible to measure fastener elongation easily, other indirect methods are used for pre-loading.

Six methods are used to control tightness of a threaded fastener. In order of increasing accuracy they are:

The decision as to which tightening method to use depends primarily on the criticality of the joint. As cost increases with higher accuracy method, generally the method selected will almost always lie between the two extremes. Some applications will allow the high inaccuracy of the "feel" method, while the high cost, highly accurate strain gages are used almost entirely in the laboratory. Information on various methods is as under.

Feel

In this method, fasteners are tightened as per feel of a person based on his work experience. As there is no measurement, quality of work depends on person’s expertise.

Torque wrench

Torque wrench is a manual wrench which incorporates a gauge or other method to indicate the amount of torque transferred to the fastener. In this method, fasteners are tightened by calibrated torque wrench set to a desired torque value.

When torque is applied to a fastener it is used to overcome friction to turn the fastener and stretch the fastener to develop the clamping force. The latter is considered the useful part of the torque. Generally 85% of the torque is used to overcome friction and only 15% is available to produce bolt load. Thus change in the coefficient of friction for different conditions can have a very significant effect on fastener loading. Thus though this is the least expensive and the most popular method for preloading fasteners, it is the least accurate.

Fastener manufacturers usually recommend seating torques for each size and fastener material based on test carried out by them. In absence of information about torque values, the torque tightening equation may be used to decide torque value. Within the elastic range, before permanent stretch is induced, the relationship between torque and tension is essentially linear. Studies have found that there are many variables that have an effect on this relationship. To overcome this, a torque tightening equation (formula) has been developed based on empirical test results as under.

T = KDP Where,
T = torque, Nm (in-pounds).
D = fastener nominal diameter, m (inch)
P = preload, N (pounds)
K = “nut factor,” “tightening factor,” or “k-value”

The nut factor is often assumed to be 0.2 for steel nuts and bolts tightened without lubrication.
More information on friction and nut factor are given in an article reproduced from Loctite, “Torque/Tension Relationship – The Forgotten Factor” at the end of this article.

Turn-of-the-nut

The process of pulling parts of a joint together is called snugging. During snugging most of the input turn is absorbed in the joint with little tension being given to the bolt. The torque required to pull plates together so that direct contact occurs is called snug torque. The snug torque is usually determined experimentally on the actual joint. The snug torque ensures that metal to metal contact occurs at all the interfaces within the joint. At this point the nut or bolt is turned to a predetermined number of degrees in turn-of-the-nut method of tightening. This method also utilizes change in bolt length. In theory, one bolt revolution (360° rotation) should increase the bolt length by the thread pitch. This method is used for slip critical (friction grip) connection of structural members using structural bolts. It eliminates the friction factor. However its accuracy is affected by the care of the workman in measuring the angle the nut or bolt is turned. For more information on this method, please refer article on Structural Bolts.

For using this method with gasket, value of nut or bolt to be rotated after snugging must be developed by tests for each joint because of the "rubberiness" of the joint. The snugging also produces a large variation in preload due to rubberiness of gasket material.

PLI washers

Preload indicating (PLI) washers utilize compression of plastically deformable material of washer under load. The use of load indicating washers is widespread in structural engineering. One type of such washers have small raised pips on their surface which plastically deform as the bolt is tensioned. The correct preload is achieved when a predetermined gap is present between the washer and the under head of the bolt. This is measured using feeler gauges. The smaller the gap, greater is the tension in the bolt. Generally they are not used in mechanical engineering. They are also known as direct tension indicators. For more information on direct tension indicators, please refer – other useful information section in article about Gaskets.

Bolt elongation

Since stress/strain is a constant relationship for any given material, the relationship is used to preload bolt by staining (stretching) it for desired stress (loading). Various methods are used to stretch bolt or measure stretch as the bolt is being loaded. Two such methods used for large size bolts are heat tightening and use of hydraulic tensioner.

Heat Tightening

Heat tightening utilizes the thermal expansion characteristics of the bolt. The bolt is heated to expand. After it has expanded, the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil or any other suitable method. This is not a widely used method and is generally used only on very large size bolts.

Hydraulic Tensioner

In this method a hydraulic tool is used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched, the nut is run down the thread to snug up with the joint. The hydraulically applied load is then removed resulting in tension being induced into the fastener.

Some times special bolts are used which have provision to measure stretch as they are tightened. One such fastener is the Rotabolt which measures bolt stretch (extension) by the use of a central gauge pin which passes down a centrally drilled hole in the bolt.

Bolt elongation can be measured by ultrasonic measurement also.

Strain gages

By using strain gages, the strain produced can be directly detected. Strain gages may be applied directly to the outside surface of the bolt or by having a hole drilled in the center of the bolt and the strain gage installed internally. The output from these gages need instrumentation to convert the gage electrical measurement method. It is an expensive method and not always practical.

Sequence of tightening

The sequence in which bolts or studs are tightened has a substantial effect on the distribution of preload in a joint. Since in most cases, all bolts of a joint are not tightened simultaneously, tightening of a bolt effects preload in other previously tightened bolts in the group. Such effects are called elastic interaction or bolt cross talk. To minimize this, bolts are tightened in a cross bolt tightening pattern. If the joint is critical it is recommended considering a multiple pass tightening sequence. In such a sequence, each bolt is tightened more than once so as to reduce the preload reduction caused by the tightening of the other bolts in the joint. Tightening of bolts as per tightening sequence uniformly preloads all the bolts of a joint. Always run the nuts or bolts down by hand. This gives an indication that the threads are satisfactory (if the nuts will not run down by hand, then there is probably some thread defect – check again and, if necessary, replace defective parts). Cross bolt tightening sequence for various patterns of joints is shown below.

Tightening of stainless steel fasteners

Stainless steel fasteners can unpredictably sustain galling (cold welding) during preloading. Stainless steel self-generates an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the thread surfaces, protective oxides are broken, possibly wiped off, and interface metal’s high points and shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme case this leads to seizing – the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out. The problem can be eliminated by lubricating the threads. In very unfavorable assembling conditions MoS2 or similar lubricants are recommended. It is also recommended to use a very low revolution screw-driving machine or they should be tightened by hand to minimize heat generation during tightening. High temperature increases tendency for galling. Be careful however, if you are using the stainless steel fasteners in food related applications as some lubricants may be unacceptable.

Recommended tightening torque values for preloading fasteners

In case, torque tightening values are not specified, steel fasteners may be preloaded by tightening them to following torque values.

Tightening Torques in Kilograms Meters / Pound Feet

For ready reference tables showing tightening torque and forces for stainless steel fasteners are reproduced below from website of Bufab Stainless AB, Sweden – http://www.bufab-stainless.se

Tigthening torque and forces for A2, A4 and Bumax fasteners

1) The Mv recommendations refer to burr-free surfaces lubricated with a good quality lubricant.
2) The preload applied is calculated as 65% of Rp 0.2 but in practice the value can be expected to vary between 50 – 80 %
3) The Mv-recommendations are calculated assuming a coefficient of friction of 0.16 which requires a good-quality lubricant.

Hankel Loctite had invented anaerobic technology for thread treatment and gasketing in 1950. They are world leader for anaerobic adhesives sealants and cyanoacrylate adhesives. An article on Torque / Tension relationship from their web site is reproduced below.

Torque/Tension Relationship – The Forgotten Factor

If you have used a torque wrench to ensure correct bolt tension, then you have just become a victim of friction – the forgotten factor. Only by controlling this factor can reliability be restored to the threaded fastener. The correct function of a nut and bolt is to clamp individual parts together with sufficient force, so as to prevent relative movement between the parts. When clamped parts move due to the influence of shock, stress, vibration or thermal forces, structural failures can occur rapidly. The key to reliability then is to prevent relative movement by ensuring that sufficient clamp load is generated in the fasteners. This is best achieved by understanding and controlling the friction forces which absorb so much of the applied torque. Modern design trends using smaller, fewer, high strength/low yield bolts torqued into the yield region, place more and more emphasis on good design practices and understanding of the forces involved. Improper lubrication practices can also lead to many types of failure in bolted assemblies. We tighten a screw or bolt by applying a torque to the head or nut until a balance is obtained between the torque applied and the sum of the bolt tension and friction forces. The distribution of these forces is shown in Table 1.

We see, therefore, that in normal course thread fasteners only 15% of the applied torque actually produces clamp load, the rest is absorbed by friction on the tread flanks and under-head bearing surface of the nut and bolt head.

Modern anaerobic thread-lockers such as Loctite 222, 243, 262 play a major role in improving the reliability of bolted assemblies, not simply by preventing premature loss of clamp load but more importantly, by controlling the friction characteristics of the metal surfaces of the fasteners. Even apparently identical fasteners from the same batch of steel, and having undergone the same heat treatment, can exhibit considerable difference in clamp loads, even when torqued to exactly the same levels. The explanation lies in variations in the “K” factor for the fastener. A simplified model for the relationship between the torque applied, the fastener diameter, the force achieved or required, and the “K” factor is:

T = KDF, where
T = Torque – Nm (inch-pounds)
D = Nominal diameter of fastener – m (inch)
F = Clamp Load – N (pounds)
K = “K” factor. An empirical constant which takes into account friction and the variable diameter under the head and threads where friction is acting (it is not the coefficient of friction, although it is related to it).

Values of “K” can be determined experimentally, see Table 2. The range of values for any lot of fasteners tested was plus or minus 14%, however different fasteners lots increased the variation to plus or minus 20%. The variation in friction (and therefore “K”) is wide since it is the result of extremely high pressures acting on surfaces which vary in roughness, oxide levels, plating finish and thickness, and lubrication types and levels.

Note:
These values were obtained using 16 TPI, 3/8 UNC nuts and bolts, where the bolt was captive and the nut was turned. Both the threads and the nut face were lubricated. An unlubricated thrust face, either nut or bolt head, can almost double the “K” value. The dry solvent cleaned bolt would never achieve the clamp load for which it was designed, irrespective of the amount of torque applied, while the bolt lubricated with anti-seize compound (which is not an uncommon practice with the mining industry and heavy engineering) is stretched well into its elastic limit and is a disaster waiting to happen. The application of the modern anaerobic thread-locking compound Loctite 243 substantially reduces the torque tension scatter envelope over identical “”as received” lightly oiled fasteners.

The “K” factor variation ranging from 0.11 to 0.17 found in seemingly identical fasteners here results in a substantial clamp load variation. At exactly the same torque level of 24 foot pounds, variations between 4500 pounds and 6700 pounds are experienced. This is not exactly a recipe for reliable engineering assembly. The same fasteners treated with Loctite 243 would exhibit a variation between 4700 pounds and 5400 pounds, which is close to the design clamp load for such a fastener when tightened to 75% of its proof load.

Clearly, Loctite anaerobic thread-lockers perform a task which is more important than maintaining bolt tension, they provide a reliable means of controlling friction forces so that, once again your torque wrench allows you to achieve the correct tension.

For more information on Loctite and their products, please visit their website – http://www.loctite.com.au

Care to be taken while using hexagon keys

Always ensure to use snug fit keys having full wall contact to get desired tightening torque. A worn out key will slip in the socket and may damage it making it useless to tighten later to required torque even with a good key. A good quality key has high strength and ductility. With a good quality key, far more tightening torque than is needed can be applied without damaging the screw or the key. A good quality key shall twist and shear off clean with out damaging socket when excessively tightened (destruction test). Please remember that cost of a key is negligible as compared to problems one may face later due to bad workmanship due to use of a bad key.

As a thumb rule, a fastener will get adequately tightened when the short arm is inserted in the socket and the long arm is deflected or bent through an angle of 25-30 degrees by the application of force near the end of the long arm.

Product and dimensions standards

Product and dimensions standards used for hexagon key in various countries are as under.

ANSI: B 18.3, B 18.3.2M
IS: 3082
ISO: 2936
DIN: 911
BS: 2470

For ready reference, information on metric hexagon keys made by Unbrako as per ANSI standard from their engineering guide is reproduced below.

Metric Hexagon Keys – Dimensions and Mechanical Properties

Point Selection According to Application

Point of a set screw is that part of the set screw which rotates against the shaft being secured. Point selection is normally determined by the nature of the application. Information on standard point types is as under.

Plain cup

The cup point is the most used style set screw. It is used for fast, permanent and semi-permanent location of parts on shafts with hardness differential of 10-15 Rockwell C points and where cutting in of cup edge on the shaft is acceptable. It is used for soft materials where high tightening torques are impractical.

Knurled cup

They are used for quick and permanent location of gears, collars, pulleys or knobs on shafts. They resist most severe vibration.

Flat

They are use where parts must be frequently re-set, as it causes little or no damage to part it bears against.

Cone

They are used for permanent location of parts. Deep penetration on tightening gives highest axial and holding power. In material over Rockwell C15 point is spotted to half its length to develop shear strength across point. It is also used for pivots and fine adjustment.

Dog

They are used for permanent location of one part to another. They have flat end with the threads stopping short of the end. The end is spotted in hole drilled in shaft or against flat (milled). They often replace dowel pins. Works well against hardened members or hollow tubing.

Oval

They have rounded end. They are used for frequent adjustment without deformation of part it bears against. They are also used for seating against an angular surface (applications where point meets shaft on an angle), circular U-grooves or axial V-grooves (splined). They are covered in ANSI standard.

Selection of Size

Socket set screws offer three types of holding power: torsional (resistance to rotation); axial (resistance to lateral movement); and vibrational. Size selection is an important factor in holding power. As a rule of thumb the screw diameter should be roughly 1/2 that of the shaft. Additional design considerations are as under.

Holding power is almost directly proportional to seating torque in a cup, flat, and oval point screws. Holding power can be increased by increasing seating torque. Greater holding power reduces the number of screws required and the assembled cost of the application. By its penetration, the set screw point can add as a much as 15% to total holding power. Cone points, with deepest penetration, give the greatest increase and oval points, with minimum penetration, the least. Making 1 the index for cup point, holding power values for others can be approximately assumed to be 1.07 for cone point, 0.92 for flat or dog points, and 0.90 for oval point.

Relative hardness between set screw and shaft is also a factor. A 10-point differential between the screw’s normal Rockwell C 50 and shaft should be maintained for full holding power. As much as 15% loss in holding power can result from a lower differential.

Vibration resistance can be achieved by correct size and proper tightening. The UNBRAKO knurl cup set screw offers additional mechanical locking resistance when required.

Product and dimensions standards

Product and dimensions standards used for socket set screws in various countries are as under.

ANSI: B 18.3, B 18.3.6M
IS: 1367 (part 5 and part 14, section 3), 6094
ISO: 4026 to 4029
DIN: 913 to 916
BS: 2470 and BS-EN ISO 4026 to 4029

Information on IS standard is given below.

IS 1367, part 5 specifies the mechanical properties and test methods made of carbon and alloy steel. The property classes are designated by the symbols shown in the table below.

The standard also give information on steel specification and mechanical properties and methods of testing. Limiting values for decarburization test and values of testing torque for various nominal sizes of screws for property class 45H are also given in the standard. The screw shall withstand the test torque given in the standard without splitting or cracking.

The threads shall be ISO metric, 6g as per IS 4218 (ISO 261) and IS 14962 (ISO 965).

Dimensions shall be as per IS 6094.

Most of the information in above article is given from Unbrako’s engineering guide. For ready reference, information on metric socket set screws made by Unbrako as per ANSI standard from their engineering guide is reproduced below.

Metric Socket Set Screws

NOTES:

1. Material / Specifications: ASTM F912M / ANSI B 18.3.6M, ISO 4026 to 4029, DIN 913 to 916
2. Hardness: Rockwell C45-53
3. Angle: The cup angle is 135 maximum for screw lengths equal to or smaller than screw diameter. For longer lengths, the cup angle will be 124 maximum
4. Threads: ANSI B 1.13M, ISO 261, ISO 262 (coarse series only)
5. Thread Class: 4g 6g
6. Grade: 45H

Dimensions and Recommended Seating Torque for Plain Cup Point and Knurled Cup Point Set Screws

Dimensions for Flat Point, Cone Point and Dog Point Set Screws

1936-Series and 1960-Series

This term is generally used in America. The original configuration of socket head cap screws didn't maintain consistent relationships among the nominal shank diameter, head diameter, and socket size throughout the available size range. This limited the performance potential of some sizes. In the 1950s, one socket screw manufacturer in America performed extensive studies to optimize performance based on geometry, fastener material strength, and applications. These studies resulted consistent dimensional relationships throughout the size range. Eventually, these relationships were accepted as industry standards and the year of acceptance – 1960 – was adopted to identify the optimized designs. The term 1936-Series was selected to identify the older style for replacement requirement.

Advantages of socket head cap screws

• As compared to ordinary fasteners, less socket screws of the same size can achieve the same clamping force in a joint.
• As fewer screws are required for a given job, fewer holes are required to be drilled and tapped.
• There is weight reduction as fewer screws are used.
• There will be weight reduction on account of smaller size of the component parts since the cylindrical heads of socket screws need less space than hex heads and require no additional wrench space.

Product and dimensions standards

Product and dimensions standards used for socket head cap screws in various countries are as under.

ANSI: ASTM A574, B 18.3, B 18.3.1M
IS: 2269
ISO: 4762
DIN: 912
BS: 2470, BS-EN ISO 4762

Information on ANSI, IS and ISO standard is given below.

ANSI Standards

As per ANSI, an alloy steel socket head cap screw means a part made in accordance with ASTM A574. The standard requires that dimensions of product shall conform to the requirement of ANSI B 18.3. The finished screws shall have hardness and tensile strength as under.  

The standards for metric, alloy steel socket head cap screws, ASTM A574M also specifies one strength level. This strength level is equivalent to ISO 898 Property Class 12.9 and is similar to the strength level for inch socket heads. The dimensions shall be as per ANSI B 18.3.1M.

IS Standards

IS 2269 covers the requirements for hexagon socket head cap screws in the size range M1.6 to M36. They are made as per product grade A (IS 1367, part 2). Their threads shall be metric coarse pitch, 6g as per IS 4218. The nominal dimensions and preferred length-size combination are given in the specification. Their mechanical properties shall be as under.

As per property class 12.9 (IS 1367, part 3) for steel bolts and
As per A2-70 for d ≤ 20 mm and A2-80 for d > 20 mm for stainless steel bolts (IS 1367, part 14).

As per the standard, screw shall be designated by nomenclature, thread size, length, number of the standard and material. Thus a hexagon socket head cap screw of size M12 and length 70 mm and made from steel shall be designated as: Hexagon Socket Head Cap Screw M10 x 70 IS 2269 – steel.

Caution:

Although the ANSI and IS standard specifies only one strength level, metric socket screws are manufactured around the world to various standards. These standards allow strength levels and materials different from the ANSI and IS standard. For example Property Class 6.8 has a nominal tensile strength (600 MPa) only one half of Property Class 12.9 (1,200 MPa). Therefore, there are metric socket screws in distribution that look similar but have different strengths.

Thus the user of metric socket screws must be aware of the strength level of the fasteners he or she is buying. Purchasing by the simple description metric socket head cap screw can result in one of many strength levels being sold. This can result in undesirable product performance.

ISO Standard

Information on ISO standard 4762 is as under. 

Information on dimensions is as under.

Advantages of Unbrako socket head cap screws

Unbrako has developed and maintained certain manufacturing advantages that continue to set them apart from others. Below is a summary of these advantages.

All socket cap screws have forged heads and hex sockets.

The ASTM A574 (Inch) Specifications and the ASTM A574M (Metric) Specifications mandate cold or hot-formed heads through 1 1/2” or M20 respectively. In all other sizes (sizes above 1 ½” or 20 mm) the specification permits hex sockets to be forged or machined. All unbrako socket cap screw heads and their hex sockets are forged.

All socket cap screws have rolled threads

The ASTM A574 (Inch) Specifications and the ASTM A574M (Metric) Specifications require rolled threads through 5/8” diameter and for screw lengths through 4”, or respectively with metrics requiring rolled threads through M24 diameter, and product lengths up to150 mm inclusive. Larger products may be rolled, cut, or ground at the option of the manufacturer. Ground threads are not produced because of excessive costs. But all Unbrako socket cap screws are roll threaded.

Higher minimum tensile strengths

The ASTM A574 (Inch) Specifications and the ASTM A574M (Metric) Specifications specify minimum tensile strength of 180,000 psi through 1/2”, and 170,000 psi for 5/8” sizes larger, and the appropriate metric equivalents in Mega Pascals The corresponding requirements for core hardness are HRC 39 to 45 for 1/2” or smaller, and HRC 37 to 45 for 5/8” sizes and larger. Unbrako tensile specifications are 10,000 psi higher than standards (1300 or 1250 MPa for metric fasteners) – while maintaining the core hardness range.

Radiused root thread runout

At the beginning of 1966, the thread specification for aircraft fasteners, MIL-S-8879, was released. In Revision A of this spec, it stated that the runout threads must have root radii as large or larger than the normal root radius of the threads. Large radius of runout root provides a smooth form that distributes stress and increases fatigue life of thread run-out. The purpose of this requirement was to strengthen this specific area of the fastener, because this was where most fastener failures occurred. The initial use of this specification was for aircraft fasteners, and while it certainly proved its worthiness in these applications, it has never been added to any commercial fastener standard. However, Unbrako (SPS) saw the merits of this technological advancement, embraced this concept, and developed a Radiused Root Runout as a standard feature on all of their socket cap screws.

“WR” Thread Form

This is an SPS proprietary thread form that fits within the standard UNR thread profile. “WR” stands for WIDE ROOT. This unique thread form has two major differences, and they are both achieved with special thread roll dies made by their Hi-Life Tool division. First, the root of the thread is made as wide as the Unified Screw Thread Series permits. Secondly, they restrict the tolerance of the root radius to the upper, largest 30% of the specification. With this combination, “WR” provides the strongest UNR thread form, and with the greatest opportunity to maximize fatigue resistance.

Etching for thread laps

When threads are rolled onto their products, random samples are being etched in heated acid, and then examined under a stereo microscope to detect thread laps. A thread lap is a flaw. It’s a folding of material when the threads are being rolled. If you don’t check for laps in this method, they’re not likely to be detected. The result will be built-in product defects that then become prime areas to begin fatigue cracking, and then subsequent fastener failures. Because of these efforts, they can certify to ASTM F788, Supplemental Requirement, S1. for Assemblies Subject to Severe Dynamic Stresses on all Unbrako socket cap screws.

Engineered fasteners are often expected to withstand high-fatigue loads or cycles over a prolonged period of time. Above factors and closely controlled manufacturing processes account for the greater fatigue resistance of UNBRAKO socket screws.

Above explains why one shall buy fasteners from a reputed manufacturer. When you buy a product from a reputed manufacturer, you get best quality even without asking it as they are committed to give you best quality product.

Information on ANSI metric socket head cap screws made by Unbrako

For ready reference, information on metric socket head cap screws made by Unbrako as per ANSI standard is reproduced below from Unbrako’s Engineering Guide.

Metric Socket Head Cap Screws as per ANSI Standard

NOTES:

1. Material / Applicable or Similar Specification: ASTM A574M / DIN ENISO4762-alloy steel
2. Property Class: 12.9-ISO 898/1
3. Hardness: Rc 38-43
4. Tensile Stress: 1300 MPa thru M16 size and 1250 MPa over M16 size.
5. Yield Stress: 1170 MPa thru M16 size and1125 MPa over M16 size.
6. Threads: ANSI B1.13M, ISO 261, ISO 262 (coarse series only)
7. Thread Class: 4g 6g

Dimensions, mechanical properties and application data are as under.

Body and Grip Length dimensions are as under (As per ASME / ANSI B 18.3.1M-1986).

LG is the maximum grip length and is the distance from the bearing surface to the first complete thread.
LB is the minimum body length and is the length of the unthreaded cylindrical portion of the shank.

Note: For information on length of M20 and M24, please refer web site of Unbrako.

Countersunk head screws with hexagon socket

Countersunk head screws with hexagon socket are widely used for fastening of plates, strips, and other thin sections in modern equipment and machinery requiring strong and reliable joint. They are also known as flat head socket cap screws. Dimensions standards used for countersunk head screws with hexagon socket in various countries are as under.

ANSI: B 18.3, B 18.3.5M
IS: 6761
ISO: 10642
DIN: 7991
BS: 2470, BS-EN ISO 10642  

For ready reference information on metric countersunk head screws with hexagon socket made by Unbrako as per ANSI standard is reproduced below from Unbrako’s Engineering Guide.

NOTES:

1. Material / Applicable or Similar Specification: ASTM F835M / DIN ENISO10642
2. Dimensions: B18.3.5M
3. Property Class: 12.9
4. Hardness: Rc 38-43 (alloy steel)
5. Tensile Stress: 1040MPa
6. Shear Stress: 630 MPa
7. Yield Stress: 945 MPa
8. Sizes: For sizes up to and including M20, head angle shall be 92°/90°. For larger sizes head angle shall be 62°/60°.
9. Threads: ANSI B1.13M, ISO 262 (coarse series only)
10. Thread Class: 4g 6g

Dimensions and application data are as under.

For more information on socket head cap screws and related products, please refer to  Engineering Guide by Unbrako on their web site: http://www.unbrako.com.au

Application

Structural Bolts are designed to be used with nuts and washers for the connection of structural members. The head of a heavy hex structural bolt is specified to be the same size as a heavy hex nut of the same nominal diameter, thus allowing a single size wrench or socket to be used on the bolt head and the nut. Structural bolts also have a shorter thread length so that the threads can be eliminated from the shear planes of the connection. There are two ways in which these bolts are used: Slip Critical Connections and Snug Tightened. The AISC (American Institute of Steel Construction) has published the “Specification for Structural Joints Using ASTM A325 or A490 Bolts” through RCSC (Research Council on Structural Connections), which describes tightening methods for joints using structural bolts. In many cases (e.g., bearing connections), bolts can be used in the snug tight condition. Snug tight is defined as the tightness that exists when all plies of a joint are in firm contact. It can normally be attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench. If full pretensioning is required, such as in slip critical (HSFG) connections, they are tightened to a load equal to 70% of the minimum tensile strength. Slip critical connections, rely on the friction between the steel plies being clamped together and the high clamp load of the structural bolt / nut to prevent any movement (or slip) of the joint.

Product and dimensions standards

Product and dimensions standards for structural bolts as per ANSI and IS are as under.

ANSI: ASTM A325, A325M, A490, A563, B 18.2.6 and B 18.2.3.7M
IS: 3757 and 1367, part 3.

ANSI Standards

ASTM A325, A325M, A490 and A563

ASTM A325 Bolts are typically supplied as plain or galvanized. They are available in two types, type 1 and type 3 (weathering steel). These bolts are heat-treated to a minimum tensile strength of 120 ksi up to 1” diameter, and 105 ksi over 1”. Dimensions are as specified for heavy hex structural bolts in ANSI/ASME B18.2.6 and threads are UNC (Unified Coarse) as per ANSI/ASME B1.1.

ASTM A325M Bolts are typically supplied as plain or galvanized. ASTM A325M Bolt is equivalent to the properties of an ASTM F568 Class 8.8 Bolt. These properties are essentially identical to Class 8.8 in ISO 898/1. A325M Bolts are produced to the dimensions for heavy hex structural bolts as specified in ANSI B18.2.3.7M. The threads are rolled as specified in ANSI B1.1.3M, to a metric coarse thread with Grade 6g tolerance.

ASTM A490 Bolts are supplied as plain (black) finish. These bolts are heat-treated to a tensile strength range of greater than 150 ksi. Dimensions are as specified for heavy hex structural bolts in ANSI/ASME B18.2.6 and threads are UNC (Unified Coarse) as per ANSI/ASME B1.1.

Type 3 fasteners are used primarily in outdoor structures where the steel is subject to corrosion. As the surface of the steel corrodes or rusts, instead of forming coarse, flaky rust, the type 3 material forms a fine-textured oxide coating that tightly adheres to the base metal. This oxide coating seals the surface of the base metal from further corrosion. The surface appears to be a uniform fine grained, deep brownish-red rust color. The protection of this oxide coating works well in moderate environments such as normal outdoor weather.

ASTM A325 and A490 type 1 bolts shall be used with nuts as per ASTM A 563 – type C, C3, D, DH, ASTM A194 – type 2, 2H and washers as per ASTM F436. ASTM A325 and A490 type 3 bolts shall be used with nuts as per ASTM A563 – type C3, DH3, and washers as per ASTM F436.

For slip critical (HSFG) connection application, these bolts shall be tightened to minimum value loads as under (Minimum values are equal to a load corresponding to 70% of the minimum tensile strength).

# Values of minimum tension load in 1000’s of Pounds (kips) as per RCSC Table 8.1

B18.2.6 – 2006 Fasteners for use in Structural Applications

This Standard covers the general and dimensional data for five products in the inch series recognized as American National Standard. These five structural products include: (a) Heavy Hex Structural Bolts: ASTM A 325 or ASTM A 490; (b) Heavy Hex Nuts: ASTM A 563; (c) Hardened Steel Washers; Circular, Circular Clipped or Beveled: ASTM F 436; (d) Compressible Washer-Type Direct Tension Indicators: ASTM F 959; (e) Twist-Off-Type Tension Control Structural Bolts: Heavy Hex and Round: ASTM F 1852.

B18.2.3.7M – 1979 Metric Heavy Hex Structural Bolts

This Standard covers the general and dimensional data for metric heavy hex structural bolts recognized as American National Standard.

IS Standards

IS 3757: Specification for High Strength Structural Bolts

IS 3757 covers the requirements for large series hexagon, high strength structural steel bolts in property classes 8.8 and 10.9 (as per IS 1367, part 3) and in the size range M16 to M36 with short thread lengths suitable for use in both friction-type and bearing-type structural steel joints.

These bolts shall be used with the appropriate nuts (as per IS 6623, having property classes 8 and 10 as per IS 1367) designed to provide an assembly with a high level of assurance against failure by thread stripping on over tightening and washers (as per IS 6649). IS 6649 covers the requirements for through hardened and tempered steel washers (Hardness: HRc 38-45). Threads shall be ISO metric as per IS 4218. Bolts and nuts threads shall have tolerance class of 6g and 6H respectively.

IS 4000 code of practice for high strength bolts in steel structures

This standard applies to high strength bolts used in both friction type and bearing type shear joints and for tension joints. This standard is complementary to IS 800, `Code of practice for general construction on steel'. Provisions not covered in this standard shall be conforming to IS 800.

This standard covers the requirements for the design, fabrication, assembly and inspection in all types of steel structures, of structural joints using high strength bolts conforming to IS 3757 – 1985 `High strength structural bolts' tensioned to the minimum bolts tension specified in this code.

Tensioning methods for slip critical connections

The AISC Research Council on Structural Connections, RCSC gives four appropriate methods to fully tension bolts for slip critical connections. They are:

1. “Turn-of-Nut” method (where you turn the nut through a certain number of degrees to elongate the bolt).
2. Use of alternative bolts (such as Twist-Off-Type Tension Control Structural Bolts).
3. Load Indicating Washers (or DTI’s), and
4. Calibrated torque wrench method.

The “Turn-of-Nut” installation method is widely used to pretension bolts. In this method, after pre-installation verification testing, bolts are tightened in three steps.

1. Snug-tighten the bolts in the joint.
2. Match-mark the nut and protruding end of the bolt and
3. Rotate the nut by the proper amount listed in the table below.

Nut Rotation from Snug-Tight Condition for Turn-of-Nut Pretensioning^a,b
(RCSC Table 8.2 is reproduced below)

^a – Nut rotation is relative to bolt regardless of the element (nut or bolt) being turned. For required nut rotations of 1/2 turn and less, the tolerance is plus or minus 30 degrees; for required nut rotations of 2/3 turn and more, the tolerance is plus or minus 45 degrees.

^b – Applicable only to joints in which all material within the grip is steel.

^c – When the bolt length exceeds 12db, the required nut rotation shall be determined by actual testing in a suitable tension calibrator that simulates the conditions of solidly fitting steel.

^d - Beveled washer not used.

Matchmakings and required turns are shown below.

Pre-installation verification testing

A tension calibrator is a hydraulic device that indicates the pretension that is developed in a bolt that is installed in it. Such a device is an economical and valuable tool and it must be readily available whenever high-strength bolts are to be pretensioned. Skidmore-Wilhelm is one of the tension calibrator manufacturers- http://www.skidmore-wilhelm.com.

Pre-installation verification testing is carried out as under.

1. Take 3 bolts of each diameter, length, grade and production lot; 3 washers of each diameter and production lot; and 3 nuts of each diameter, grade and production lot as they will be assembled.
2. Assemble the first set of bolt-washer-nut combination into a tension calibrator.
3. Snug the assembly using the same technique to be used in the structure.
4. Match mark the nut, bolt and faceplate of tension calibrator.
5. Apply the required rotation as listed in the above table for the assembly being tested.
6. Verify that the tension on the tension calibrator’s dial gage is at least 5% more than the minimum bolt pretension as listed in the RCSC Table 8.1 given above (Minimum + 5% value).
7. Record the tension achieved in a log book.
8. Remove the assembly and repeat steps 2 through 7 until all three assemblies have been tested.

Hydraulic tension calibrators undergo a slight deformation during bolt pretensioning. Hence, when bolts are pretensioned the nut rotation corresponding to a given pretension reading may be somewhat larger than it would be if the same bolt were pretensioned in a solid steel assembly. Stated differently, the reading of a hydraulic tension calibrator tends to underestimate the pretension that a given rotation of the turned element would induce in a bolt in a pretensioned joint (This is the reason to read Minimum + 5% value in step 6 of Pre-installation verification testing). If the actual pretension developed in any of the fastener assemblies is less than 1.05 times that specified for installation and inspection as per RCSC Table 8.1, the cause(s) shall be determined and resolved before the fastener assemblies are used in the work.

Twist-Off-Type Tensioning of Structural Bolts

Tightening of Tru-Tension® fasteners is given below to explain tightening of bolts by this method.

Tru-Tension® Fasteners are designed to be installed with various types of lightweight portable electric wrenches specifically intended for use with this style of structural fastener. They can be utilized for any applications where A325 – Type I or Type III (weathering steel) and A490 bolts are specified. The installation tool has an inner socket which engages the spline tip of the bolt, while the outer socket engages the nut. The outer socket rotates the nut relative to the bolt spline, and when the tension is sufficient in the fastener, the spline tip simply twists-off, leaving the tightened bolt correctly installed in the connection.

Tightening of fasteners by load indicating washers (or DTI’s), and calibrated torque wrench methods will be covered in an article on tightening methods.

Joint preparation and precaution for slip critical connections

Take care of the following during installation.

Ensure that joint surfaces in contact are clean. The surfaces shall be free of paint or other applied finish, oil, dirt, loose rust, loose scale, burs and other defects which would prevent solid seating of the parts or would interfere with the development of friction between them. Tight mill scale is not detrimental.

No gasket or other flexible material shall be placed between the plies (ply is a single thickness of steel forming part of a structural joint).

Visually inspect the bolts, nuts and hardened washers for damage / cracks etc. and for the correct identification markings.

Align the holes in parts to be joined in such a way that they permit bolts to be placed freely in position. Driving of bolts is not permitted.

After snug tightening all bolts, pretensioning is carried out. Due to pretensioning a bolt, adjoining bolts may loose snug tightening condition. Snug tighten them again before pretensioning them.

Follow proper bolt tightening sequence.

For more information on specification for structural joints using ASTM A325 or A490 bolts by Research Council on Structural Connections (RCSC) please visit their website: http://www.boltcouncil.org

As information is given only as an illustration, it is not complete and you are requested to refer the standard for full information.

IS 1364, Part 2 (identical to ISO 4017) -  Screws/Bolts

IS 1364, part 2 is about hexagon head screws of product grades A and B. From maintenance point of view the most important information is as under.

Product Specification

The standard gives specification and reference standards for the product as under.

Please note following things in above table.

• Thread tolerance is defined for the product by the standard.
• Class 4.6 and 4.8 steel are not used for making fasteners unless agreed.
• Product grade is based on size of product.

Product Dimensions

The standard gives following information on dimensions.

1. Dimensions for the product (e.g. nominal width across flats and its minimum size) and
2. Table showing range of length of fasteners made for a given size (nominal diameter).

Note:
Product information (dimensions) for ISO 4017 is the same as that covered by ISO 4014 with the exception of threading up to the head and nominal length up to and including 200 mm as popular lengths.

In view of above, to illustrate dimensions given by the standard, dimensions given by ISO 4014 (Hexagon head bolts – Product grades A and B) for fasteners up to size M36 is given here. ISO 4014 gives information on thread length (b) in addition to information given in ISO 4017.

ISO 4014-1979

This international standard gives specifications for partly threaded hexagon head bolts with metric dimensions and thread diameters from 3 mm up to and including 36 mm with product grade A for sizes M 3 to M 24 and with product grade B for sizes M 30 and M 36.

For information on range of length of fasteners, please refer the standard.

Method of Designation

The standard has given example to designate a product as under.

Using information given in the standard, a hexagon head screw with thread size M12, nominal length equal to 80 mm and property class 8.8 can be designated / specified as under for its procurement.

Hexagon head screw ISO 4017 – M12 x 80 – 8.8 (as per ISO standard) and
Hexagon head screw IS 1364 (part 2) – ISO 4017 – M12 x 80 – 8.8 (as per IS standard).

Please note that when you designate a product using a standard, it takes care of many things. For example, in above case it takes care of general requirement as per ISO 8992, type of thread (as per ISO 261, ISO 965-2), thread tolerance (6g in this case), mechanical properties (as per ISO standard 898-1), product grade as per ISO 4759-1, surface finish, limits for surface discontinuities (as per ISO 6157-1 and ISO 6157-3) and acceptability as per ISO 3269. It also takes care of various dimensions of screw.

Note:
In ISO standard comma (,) is used as a decimal marker while in Indian Standard the current practice is to use point (.) as the decimal marker.

IS 1364, Part 3 (identical to ISO 4032) – Nuts

IS 1364, Part 3 : Hexagon Nuts, Style 1 (Size Range M 1.6 to M 64) – is identical with ISO 4032.

IS standard 1363 Part 3, 1364 Parts 3 to 6 and 13722 to 13724 are on coarse and fine threaded nuts (equivalent ISO standard numbers are 4032 to 4036 and 8673 to 8675). As same spanner is to be used for tightening either bolt or nut, width across flats for nuts is same as that for bolt / screw head. However their thickness is different as per nut styles. For general applications style 1 nut are used. Thickness of nut for various styles is as under.

Style 1 – 0.8 d (ISO 4032)
Style 2 – 1.0 d (ISO 4033)and
Thin Nuts – 0.5 d (ISO 4035), where d is nominal diameter of fastener.

Method of Designation

The standard gives example to designate a product. As per ISO 4032, hexagon nut, style 1, with thread M12 and property class 8 is designated as follows:

Hexagon nut ISO 4032 – M12 – 8

Important specifications with reference standards, nut dimensions for preferred threads for style 1 nut as per ISO 4032 and dimensions of nuts as per unified (inch) standard are as under. For full information on style 1 / unified nuts and other types of nuts, please refer relevant standards.

Important specifications with reference standards for style 1 nut as per ISO 4032

Nut dimensions for preferred threads as per ISO 4032 (Metric Nut Style 1)

All Dimensions are in millimeters

For Ready Reference – Dimensions of hex nuts, hex jam nuts and heavy hex nuts as per Unified (inch) Standard.

Hex Nuts and Hex Jam Nuts

Heavy Hex Nuts

Tolerances

The standard specifies tolerance for bolts, screws, studs and nuts. It specifies tolerance level and tolerances of form and position. Tolerance levels for many features of fasteners are given in the standard. For example, tolerance level for thread (one of the features) is given below.

Feature	Tolerance for Product Grades
	A	B	C
Internal thread (nuts)	6H	6H	7H
External threads (bolts, screws and nut end of studs)	6g	6g	8g

Tolerances of form and position are used to specify tolerance for the following.

• Concentricity, symmetry and run-out
• Perpendicularity
• Parallelism and
• Straightness.

Detail on tolerances is not covered in the article as it is not useful for maintenance.

Note:
Tolerance of form and position are important for use of fasteners in critical applications. To ensure that you get quality product, it is recommended that you buy fasteners manufactured by reputed company.

Product Grades

The product grades covered in this standard refer to the quality of the product and to the size of the tolerances where Grade A is the most precise and Grade C, the least precise. The above designation of product grades as A, B and C is a change from designation of product grades employed in 1967 version. The following table gives the equivalent product grades in the old and revised versions.

It can see that tolerance 6g for external threads for product grades A and B makes them more precise than product grade C having tolerance 8g. As per IS 1363 (Part 1) – 1984, no finish is required for hexagon head bolts of product grade C except for the thread. A small die seam across the bearing surface is permissible.

In general for industrial use, product grades A and B are specified as wide / high tolerances are given for product grade C.