What are washers?
Table of Contents
Washers are small round machine components that are indispensable in the machine building industry. They are used in combination with bolts and nuts and help to secure them. Usually made out of metal, they can have many shape variations. Everybody has seen them and many have used them, but what do they actually do? Why are they used?
In order to better understand the uses for a washer we need to consider how the bolt connection works. Partly mentioned in the post about bolt strength the bolt connection is held secured with a combination of friction forces and elastic deformation. Friction forces appear between the thread of the bolt and the thread of the nut and between the bolt head and the material. Elastic deformation occurs in the bolt, when the bolt is tightened the bolt stem elongates elastically.
In order to cause elastic deformation in the bolt adequate torque needs to be applied when tightening. This elastic deformation of the bolt is what is keeping the bolt connection secured even under changing vibrational loads.
Problems that can appear when securing a bolt
For a proper and rigid connection the mating surfaces should be completely flat. Often times that is not the case. Sometimes this can even cause problems. If there are small variations the bolt had can mate on the peeks of these variations. As this is a small surface, this connection is not rigid and after the deformation of the peeks the elastic preload force in the bolt can significantly reduce thus reducing the security of the connection.
Another problem can appear with the material that the bolt is being secured to. If it is a brittle material then the force concentration under the bolt head can cause miniature cracks to appear. Under exploitation these cracks will spread an eventually the material will break under the blot.
Why use washers?
Washers can prevent a lot of problems that appear during the assembly and during the exploitation. Special washers also used to prevent loosening bolts under vibration.
Washers are used to:
Spread out the force of the bolt/nut onto a greater area;
Provide a greater area of contact when tightening a bolt – in order to prevent the bolt head tightening on a few high spots;
Reduce the friction when tightening a bolt/nut – reduce the stick-slip effect;
Help protect the surface under a bolt/nut (regular flat washer).
You should always use washers when you can. Use plain flat washer when:
- Fastening a blot through a slot – for force distribution;
- Mating a bolt to some softer material then steel – for force distribution and surface protection;
- Mating a blot to a painted surface – surface protection.
There are also a variety of washers that have a role in preventing the untightening of a bolt/nut.
Washers for securing a connection during vibration
Washers can secure the bolt in two ways. One is to add friction between the bolt and the mating surface and the other is to add a preload to the bolt connection.
Washers that grip into material
Adding friction is achieved with textured washers that grip onto a mating surface and the bolt head. Some examples of these washers are star washer, some split washers, …
These washers are sure to mess up the surface of the material under the bolt. They should not be used in combination with a regular flat washer as that defeats the purpose.
Washers that preload the bolt
As mentioned a secured fastener connection is based on elastic deformation of the bolt stem in the connection. In order to provide preload onto a bolt a number of washers are designed . This preload is preferable because the material that is fastened can sag or the connection can loosen up a bit during vibration. Some examples of washers that provide preload are belleville washer, wave washer, disc spring washer and others. Special place goes to wedge type washers like nordlock they are one of the most expensive options, but should be the safest.
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