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When do bolts need washers

When do bolts need washers?

In mechanical equipment, we often see that some bolted connections use washers, some bolted connections do not use washers, some bolted connections use flat washers, some use spring washers, so do you want to use washers in the design?
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Bolt connection with flat washer
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Stud connection — spring washer locking
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Function of washer

Flat washer

When the bolt needs to be removed frequently, in order to protect the surface of the connected part from being scratched, it is generally necessary to place a flat washer at the position of the nut or bolt head, so as to prevent the surface of the connected part from being scratched.
In some joints with accurate friction coefficient, such as high-strength prestressed bolts, flat washer is required at the nut end, because in this case, the bolt, nut and washer are provided by the same manufacturer. The bolt manufacturer needs to deal with the nut and washer to ensure the consistency of friction coefficient between nut and washer.
When the bolt strength is high and the compressive stress of the connected part is low, in order to reduce the compressive stress of the pressure bearing surface of the connected part, it is generally necessary to increase the flat washer.
In the bolt connection, the pressure transmission of the bearing surface of the bolt head or nut is gradually expanding in a trumpet shape. The larger the bearing surface is, the smaller the compressive stress is. Therefore, adding a flat washer can reduce the compressive stress of the contact surface of the connected parts and ensure the effective bolt connection.
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Transfer section of bolt preload in bolt connection
In some cases, in order to ensure that the bolt holes between the two connectors can correspond one by one, we need to increase the diameter of the hole to compensate the position deviation of the hole. When the bolt hole exceeds the standard requirements, we need to add a flat washer.
Sometimes for the convenience of installation and adjustment, the bolt hole will be made into a long hole, so that the installation position of the workpiece can be adjusted within a certain range. When using the long hole, it is necessary to use thickened flat washer, or even special washer or multiple washers.

Spring washer

The main purpose of spring washer is to relax, which is generally used in the occasions where the pre tightening force of bolt is not high and the dynamic load is small.
When the clamping length of the bolt is short, the stress relaxation of the bolt is obvious, and the spring washer can make up for the stress relaxation.
Some experiments show that the spring washer of high-strength bolt connection has almost no anti loosing effect under dynamic load, and also causes the rapid attenuation of bolt clamping force. Therefore, spring washer is rarely used in high-strength bolt connection in foreign countries.
We have made it clear under what circumstances a washer is required for bolted connection, and under what circumstances a washer is not required?

  • 1. Shear stress bolt connection generally does not need washer. First of all, bolt does not need too much preload, and it does not bear axial force in work. In this case, bolt hole is generally small, and bearing surface of nut or bolt head is generally large, so it does not need to reduce compressive stress, and the connected parts can bear it.
  • 2. When using the flange nut or bolt, the gasket is not needed, and the pressure bearing surface of the flange nut is large enough.

Source: China Washer Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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