We often see in mechanical equipment that some bolted connections use washers, and some bolted connections do not use washers, some bolted connections use flat washers, some use wave spring washers, and some use wave spring washers plus flat washers. Should we use washers in our design? Today, I will share some knowledge about bolted connection design.
What exactly does a washer do?
Flat washer
In the case where the bolt needs to be disassembled frequently, in order to protect the surface of the connected parts from being scratched, it is generally necessary to place a flat washer at the nut or bolt head position. In this way, the surface of the connected parts can be prevented from being scratched, especially for surfaces that are precision machined.
In some joints where the friction coefficient is required to be relatively accurate, there is no separate test requirement for the friction coefficient. For example, in the bolted connection of wind power, the connection pair of bolts, nuts and washers is generally purchased.
In this case, the bolts, nuts and washers are all provided by the same manufacturer, and the bolt manufacturer needs to deal with the nuts and washers accordingly to ensure the consistency of the friction coefficient between the nuts and washers. At the same time, the supplier is required to test the torque coefficient to meet the technical requirements.
When the bolt strength is relatively high and the allowable compressive stress of the connected parts is relatively low. For example, the requirements for lightweighting of automobiles are getting higher and higher. In the body and even chassis structural parts, a large number of lightweight connection technologies and structures are used. In order to reduce the compressive stress on the bearing surface of the connected parts in the bolted joint and prevent the occurrence of crushing and permanent plastic deformation, it is generally necessary to add a flat washer.
In addition, in the bolt connection, the pressure transmission of the bolt head or nut bearing surface is gradually expanded in a trumpet shape. The larger the bearing surface, the smaller the compressive stress. Therefore, adding a flat washer can reduce the compressive stress on the contact surface of the connected parts and ensure the effectiveness of the bolted connection.
For the connection that needs to be sealed, the stress distribution diameter of the stress cone on the joint surface is increased. In this way, adding gaskets can ensure the sealing effect.
In some occasions, such as automobiles mostly use stamping parts and welding parts, the position of the holes between the two parts cannot be well matched accurately. In order to ensure the one-to-one correspondence between the bolt holes between the two connectors, we need to increase the diameter of the holes to compensate for the positional deviation of the holes. When the bolt hole exceeds the standard requirements, the compressive stress exceeds the strength of the material, in this case, it is necessary to add a flat washer.
Sometimes for the convenience of installation and adjustment, such as the connection between the control arm bracket and the control table, one of the bolt holes in the two bolt connections will be made into a long hole, so that the installation position of the workpiece can be adjusted within a certain range. Thicken flat washers and, more specifically, even use special washers or washers.
Wave spring washer
The main purpose of the wave spring washer is to relax, and it is generally used in occasions where the bolt pre-tightening force is not high and the dynamic load is small. At present, a large number of tests have confirmed that the wave spring washer has little anti-loosening effect. Therefore, the wave spring washer is not recommended for tightening occasions with relatively high pre-tightening force and where the connection is more important.
However, for the electrical connection joint, the wave spring washer still has a certain effect on the final complete loosening, especially when it is loosened to a certain extent, a certain preloading force can still exist when the preloading force is small. At this time, the effect without the wave spring washer can still play a certain role.
When the clamping length of the bolt is relatively short, the stress relaxation of the bolt is obvious, and the wave spring washer can compensate for the stress relaxation. Of course, at this time, it can be considered to use a disc spring instead of a spring washer, and the disc spring can distribute the axial pressure more evenly.
Experiments have shown that under dynamic load conditions, the wave spring washer for high-strength bolted connections has almost no anti-loosening effect, and it also causes rapid attenuation of the bolt clamping force. Therefore, the wave spring washer is rarely used in high-strength bolt connections abroad.
Under what circumstances do we need to use washers for bolted connections, we have already made it clearer, then under what circumstances can we not use washers?
The connection of shear stress bolts (reaming hole bolts) generally does not require washers. First of all, the shear stress bolts do not need too much pre-tightening force, and they do not bear axial force during work. In this case, the bolt holes are generally relatively small. The bearing surface of the nut or bolt head is generally relatively large, and the connected parts can bear it without reducing the compressive stress.
When flange-faced nuts or bolts are used, due to the large contact area of flange-faced bolts or nuts, washers are usually no longer needed to increase the contact area and reduce compressive stress.
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