Why are washers used in bolted connections

A washer is usually used in bolted joints. Is it necessary to use a washer, when is it not necessary, and what is the use of a washer? Is it essential to use a washer in bolted joints or the design of bolted joints?

1. Bolted joints - no washers

In what cases do we not need to use washers?

Washers are generally not required for shear-strength bolt (reamed bolt) connections. Shear-strength bolts do not need much pre-strength. They are not subjected to axial force during operation, so the bolt holes are generally relatively small, and the pressure-bearing surfaces of the nuts or bolt heads are typically relatively large, so there is no need to reduce the compressive strength. The connected parts can withstand the force. When flange-faced nuts or bolts are used, the contact area of the flange-faced bolts or nuts is large, so washers are usually unnecessary to increase the contact area and reduce the pressure strength.

2. Bolted Joints - Use of Flat Washers

Flat washers in fasteners are mainly used to reduce pressure, increase friction, seal, dampen vibration, and hold fast.

When are flat washers needed?

In cases where bolts have to be disassembled frequently, it is often necessary to place flat washers on the nut or bolt head to protect the surface of the connected parts from scratches, especially for precision machined surfaces. In some connections where the coefficient of friction requires greater precision, there is no separate test requirement for the coefficient of friction, such as in the case of wind-power bolted connections, where a combination of washers, bolts, nuts, and washers are generally used.

In this case, the bolt, nut, and washer are all supplied by the same Juxin Fasteners. In the case of bolt production, Juxin Fasteners must treat the nut and washer accordingly to ensure that the coefficient of friction between the nut and the washer remains consistent. Suppliers must also test the torque coefficient to meet the technical requirements. When the strength of bolts is relatively high and the permissible strength of connected parts is relatively low. For example, as the demand for lightweight automobiles is increasing, and lightweight joining technologies and structures have been adopted for a large number of vehicle bodies and even chassis structural components, it is generally necessary to increase the number of flat washers to reduce the pressure strength of the connected parts in bolted joints and to prevent compression collapse and permanent plastic deformation. In addition, in a bolted joint, the pressure force transmitted to the pressure surface of the bolt head or nut is gradually expanded in the shape of a trumpet, and the larger the pressure surface, the smaller the pressure strength, so the addition of a flat washer reduces the pressure strength of the contact surfaces of the connected parts and ensures the effectiveness of the bolted joint. For connections that need to be sealed, the diameter of the distribution of the strength of the cone of strength on the bonding surface is increased. In this way, the addition of a gasket ensures the sealing effect.

In some cases, such as in automobiles where stamped and welded parts are used, the position of the holes between the two parts cannot be precisely matched. To ensure that the bolt holes between the two connecting parts correspond, we need to increase the diameter of the holes to compensate for the positional deviation of the holes. If the bolt holes exceed the standard requirements, the compressive force exceeds the strength of the material, which is why we need to increase the number of flat washers.

In some cases, for ease of mounting and adjustment, such as when the control arm support is connected to the control table, one of the bolt holes in the two bolted connections is made into a long hole so that the mounting position of the workpiece can be adjusted within a specific range. When a long hole is used, a thickened flat washer, a special washer, or multiple washers must be used.

3. Stud Connection - Spring Washers to Prevent Loosening

Spring washers increase the friction between the nut and the bolt, which means they provide additional protection against the fastening bolt's loosening due to the vibration of the equipment in operation.

When should spring washers be used?

Spring washers, whose primary purpose is relaxation, are generally used in structures where shock loads are not significant, it is not essential to prevent the loosening of the structure, or where the loosening of a single nut would not cause a significant accident. Extensive testing has confirmed that spring washers are ineffective in preventing loosening, and their use is not recommended for connections of greater importance in tightening situations with higher preload levels. The locking effect of spring washers depends on the compression of the spring washer, the strength of the spring washer, and the edge of the spring washer's aperture catching on the face of the connection. If the face of the connection is more critical and scratches are not permitted, it is necessary to add a flat washer to prevent the spring washer from scratching the face of the connection, but the locking effect will be reduced. If the diameter of the through-hole of the bolt is large, the contact surface area of the spring washer becomes small, and a flat washer is added to increase the contact surface area; for example, in the case of long holes, a flat washer is generally added. Experiments have shown that under dynamic loading, spring washers in high-strength bolted joints have little or no anti-loosening effect and cause a rapid deterioration of the bolt's clamping strength, so spring washers are rarely used in high-strength bolted joints. Do you know the difference between using flat and spring washers in bolted connections? The use of washers is still relatively significant in preventing bolt loosening.

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