Often washers are used with fasteners. At times they are used as a standard practice with out knowing why they shall be used. In this article information is given about why a washer is used with fasteners. Information is also given on dimensions for plain round washers and taper washers.
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
| Bolt Size, MM | Inside Diameter, MM | Outside Diameter, MM | Thickness, MM |
|---|---|---|---|
| M1.6 | 1.8 | 4 | 0.4 |
| M1.8 | 2.1 | 5 | 0.4 |
| M2 | 2.4 | 5 | 0.4 |
| M2.2 | 2.6 | 6 | 0.5 |
| M2.5 | 2.9 | 6.5 | 0.5 |
| M3 | 3.4 | 7 | 0.5 |
| M3.5 | 4.0 | 8 | 0.5 |
| M4 | 4.5 | 9 | 0.8 |
| M4.5 | 5.0 | 10 | 1 |
| M5 | 5.5 | 10 | 1 |
| M6 | 6.6 | 12.5 | 1.6 |
| M7 | 7.6 | 14 | 1.6 |
| M8 | 9 | 17 | 1.6 |
| M10 | 11 | 21 | 2 |
| M12 | 14 | 24 | 2.5 |
| M14 | 16 | 28 | 2.5 |
| M16 | 18 | 30 | 3.15 |
| M18 | 20 | 34 | 3.15 |
| M20 | 22 | 37 | 3.15 |
| M22 | 24 | 39 | 3.15 |
| M24 | 26 | 44 | 4 |
| M27 | 30 | 50 | 4 |
| M30 | 33 | 56 | 4 |
| M33 | 36 | 60 | 5 |
| M36 | 39 | 66 | 5 |
| M39 | 42 | 72 | 6 |
| M42 | 45 | 78 | 6 |
| M45 | 48 | 85 | 8 |
| M48 | 52 | 92 | 8 |
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
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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
| Bolt Size | Inside Diameter | Outside Diameter | Thickness |
|---|---|---|---|
| 3/8 | 7/16 | 1 | 0.083 |
| 1/2 | 9/16 | 1-3/8 | 0.109 |
| 5/8 | 11/16 | 1-3/4 | 0.134 |
| 3/4 | 13/16 | 2 | 0.148 |
| 7/8 | 15/16 | 2-1/4 | 0.165 |
| 1 | 1-1/16 | 2-1/2 | 0.165 |
| 1-1/8 | 1-1/4 | 2-3/4 | 0.165 |
| 1-1/4 | 1-3/8 | 3 | 0.165 |
| 1-3/8 | 1-1/2 | 3-1/4 | 0.180 |
| 1-1/2 | 1-5/8 | 3-1/2 | 0.180 |
| 1-5/8 | 1-3/4 | 3-3/4 | 0.180 |
| 1-3/4 | 1-7/8 | 4 | 0.180 |
| 1-7/8 | 2 | 4-1/4 | 0.180 |
| 2 | 2-1/8 | 4-1/2 | 0.180 |
| 2-1/4 | 2-3/8 | 4-3/4 | 0.220 |
| 2-1/2 | 2-5/8 | 5 | 0.238 |
| 2-3/4 | 2-7/8 | 5-1/4 | 0.259 |
| 3 | 3-1/8 | 5-1/2 | 0.284 |
Note: Dimensions are as per ASME B18.22.1
Dimensions for Round Plate Flat Washers
| Bolt Size | Inside Diameter | Outside Diameter | Thickness |
|---|---|---|---|
| 3/8 | 7/16 | 2 | 3/16 |
| 1/2 | 9/16 | 2-1/4 | 3/16 |
| 5/8 | 11/16 | 2-1/2 | 1/4 |
| 3/4 | 13/16 | 3 | 1/4 |
| 7/8 | 15/16 | 3.5 | 5/16 |
| 1 | 1-1/16 | 4 | 3/8 |
| 1-1/4 | 1-3/8 | 5 | 3/8 |
| 1-1/2 | 1-5/8 | 5 | 3/8 |
Dimensions for Round Hardened Steel Washers as per ASTM F436
| Bolt Size | Inside Diameter | Outside Diameter | Thickness | |
|---|---|---|---|---|
| Minimum | Maximum | |||
| 1/2 | 17/32 | 1-1/16 | 0.097 | 0.177 |
| 5/8 | 11/16 | 1-5/16 | 0.122 | 0.177 |
| 3/4 | 13/16 | 1-15/32 | 0.122 | 0.177 |
| 7/8 | 15/16 | 1-3/4 | 0.136 | 0.177 |
| 1 | 1-1/16 | 2 | 0.136 | 0.177 |
| 1-1/8 | 1-3/16 | 2-1/4 | 0.136 | 0.177 |
| 1-1/4 | 1-3/8 | 2-1/2 | 0.136 | 0.177 |
| 1-3/8 | 1-1/2 | 2-3/4 | 0.136 | 0.177 |
| 1-1/2 | 1-5/8 | 3 | 0.136 | 0.177 |
| 1-5/8 | 1-3/4 | 3-1/4 | 0.178 | 0.28 |
| 1-3/4 | 1-7/8 | 3-3/8 | 0.178 | 0.28 |
| 1-7/8 | 2 | 3-1/2 | 0.178 | 0.28 |
| 2 | 2-1/8 | 3-3/4 | 0.178 | 0.28 |
| 2-1/4 | 2-3/8 | 4 | 0.24 | 0.34 |
| 2-1/2 | 2-5/8 | 4-1/2 | 0.24 | 0.34 |
| 2-3/4 | 2-7/8 | 5 | 0.24 | 0.34 |
| 3 | 3-1/8 | 5-1/2 | 0.24 | 0.34 |
| 3-1/4 | 3-3/8 | 6 | 0.24 | 0.34 |
| 3-1/2 | 3-5/8 | 6-1/2 | 0.24 | 0.34 |
| 3-3/4 | 3-7/8 | 7 | 0.24 | 0.34 |
| 4 | 4-1/8 | 7-1/2 | 0.24 | 0.34 |
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.
