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What is a washer

What is a washer?

The washer refers to the part between the connected part and the nut. Generally flat metal ring is used to protect the surface of the connected part from nut scratch and disperse the pressure of nut on the connected part.


Classification of washers

The washer is divided into: flat washer-c, large washer-a and C, extra large washer-c, small washer-a, flat washer-a, flat washer chamfered type-A, high-strength washer for steel structure, spherical washer, conical washer, square bevel washer for I-beam, square bevel washer for channel steel, standard spring washer, light spring washer, heavy spring washer, internal tooth lock washer and internal serration lock washer Washer, external tooth lock washer, external serrated lock washer, single lug locking washer, double ear locking washer, outer tongue retaining washer, and locking washer for round nut.
Flat washer is generally used in the connector, one is soft, the other is hard and brittle, its main role is to increase the contact area, disperse pressure, prevent the soft texture from being crushed. The basic function of the spring of the spring washer is to give the nut a force after tightening the nut, and increase the friction force between the nut and the bolt! The material is 65Mn (spring steel), the hardness of heat treatment is hrc44 ~ 51hrc, and the surface is oxidized.
Washer (spring) is a kind of elastic cushion or locking washer, which can prevent bolt from loosening. The working principle of the lock washer is very simple. It consists of two pieces of washer. The outer side has a radial convex surface, while the inner side is a helical tooth surface. When assembling, the inner helical tooth surfaces are relative, and the outer radial convex surface is occluded with the contact surfaces at both ends. When the connecting parts are vibrated and the bolts tend to be loose, only the internal helical tooth surfaces of the two washers are allowed to move relatively, which will produce lifting tension, so as to achieve 100% locking.
Spring washer is widely used in the bearing and non bearing structures of general mechanical products. It is characterized by low cost, convenient installation and suitable for parts with frequent assembly and disassembly. Automatic selection of washer is included, but the locking capacity of spring washer is very low! Especially in Europe and the United States, the use rate of high reliability products is very low, especially the important load-bearing structure connection has been abandoned. Our country has some applications in military industry, but it has been improved to stainless steel. Steel spring washer has been banned in CASC for a long time! It is also said to be very unsafe, there are two reasons: 1 is the “expansion circle”, 2 is hydrogen embrittlement.
Spring washer is often called spring washer in screw industry. Its material has stainless steel and carbon steel, carbon steel is also iron. Commonly used spring washer specifications and sizes are m3, M4, M5, M6, M8, M10. M12, M14, M16. These specifications are more commonly used.
In mechanical equipment, we often see that some bolted connections use washers, some do not use washers, some bolt connections use plain washers, some use spring washers, and some use spring washers plus plain washers.
So, do we need to use washer in design? Today, let’s do a detailed review.
Bolted connection – without washer, as shown in Figure 1

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Figure 1

Bolt connection – use flat washer as shown in Fig. 2

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Fig. 2

Bolt connection – spring washer locking, as shown in Fig. 3

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Fig. 3

What exactly does the washer do?

Flat washer

In the situation where the bolt needs to be disassembled 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, especially for the surface with precision machining.
In some joints where the friction coefficient is required to be more accurate, there is no separate test requirement for the friction coefficient. For example, in the bolt connection of wind power, bolts, nuts and washers are generally purchased.
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 the friction coefficient between the nut and the washer. At the same time, the supplier is required to test the torque coefficient to meet the technical requirements.
When the bolt strength is high and the allowable compressive stress of the connected parts is low. For example, now the requirement of automobile lightweight is higher and higher. In the body, even chassis structural parts, lightweight connection technology and structure are widely used. In order to reduce the compressive stress of the pressure bearing surface of the connected parts in the bolt connection joint and prevent collapse and permanent plastic deformation, it is generally necessary to add flat washer.
In addition, in the bolt connection, the pressure transmission on the bearing surface of the bolt head or nut is gradually expanded 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 on the contact surface of the connected parts and ensure the effectiveness of the bolt connection.
For the joints that need to be sealed, the stress distribution diameter of the stress cone on the joint surface is increased. In this way, adding washer can ensure the sealing effect, as shown in Fig. 4.

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Fig. 4.

In some cases, for example, stamping parts and welding parts are mostly used in automobiles, and the position of holes between the two parts can not be matched accurately. In order to ensure that the bolt holes between the two connectors can be one-to-one correspondence, 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, 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 meter, 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. When the long hole is used, the thickened flat washer should be used, more specifically, special washer or multiple washers should be used.
When used with bolts and nuts, the selection standard of flat washer is as follows:

  • 1. When choosing washer materials, attention should be paid to the electrochemical corrosion of different metals in contact. The material of flat pad is generally the same as that of the connected parts, usually steel, alloy steel, stainless steel, aluminum alloy, etc. Copper and copper alloy can be used when there is a requirement for electrical conductivity.
  • 2. The inner diameter of the flat washer shall be selected according to the larger diameter of the thread or screw. The outer diameter of the connected material (such as composite material) or matching with the spring washer should be larger.
  • 3. When w-washer is placed on the bolt or screw head, flat washer with inner hole chamfer can be selected to avoid interference between fillet under head and washer.
  • 4. For important bolts with large diameter or in order to increase the anti extrusion ability, steel washer should be used. Steel washer shall be used for tension bolt or tension shear composite bolt connection.
  • 5. Special washer can be used in special requirements, such as copper washer for conducting electricity and sealing washer for air tightness.

Spring washer

The main purpose of spring washer is to relax. It is generally used in the situation where the requirement of bolt preload is not high and the dynamic load is small. At present, a large number of tests have confirmed that spring washer has little effect on anti loosing. Therefore, spring washer is not recommended for tightening occasions with high preload and important connection.
However, for the electrical connection joint, the spring washer has certain effect on the final complete loosening, especially when the looseness reaches a certain degree, there is still a certain preload when the preload is small, at this time, the spring washer still can play a certain role than the effect without spring washer.
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. Of course, disc spring can be used instead of spring washer at this time. Disc spring can be more evenly distributed in axial pressure.
Some experiments show that under dynamic load, the spring washer of high strength bolt connection has almost no anti loosing effect, and will cause the rapid attenuation of bolt clamping force. Therefore, spring washer is rarely used in high strength bolt connection abroad.

When to use flat washer and when to use spring washer?

1. In general, flat pad can be used only when the load is small and does not bear vibration load.
2. Under the condition of large load and bearing vibration load, flat cushion and spring cushion must be used.
3. Basically, spring washer is not used alone, but combined.
In the actual use process, due to the different emphasis of flat washer and spring washer, the two are used together in many occasions, which can protect parts, prevent nut loosening and reduce vibration at the same time, which is a great choice.

What can I do without a washer?

1. Generally speaking, no washer is required for the connection of shear stress bolt (reamed bolt). First of all, shear stress bolt does not need too much pre tightening force and does not bear axial force during operation. In this case, bolt hole is generally small, and bearing surface of nut or bolt head is generally large, so the connected parts can bear it without reducing the compressive stress.

2. When the flange nut or bolt is used, because the contact area of flange bolt or nut is large, the washer is not needed to increase the contact area and reduce the compressive stress.

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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