Can the Spring Washer Prevent Loosening?

Can the spring washer prevent loosening? Let’s discuss it today.

Locking principle of spring washer

The anti loosening principle of the spring washer is that after the spring washer is flattened, the spring washer will produce a continuous elastic force, so that the threaded connection pair of the nut and the bolt will continue to maintain a friction force, generate a resistance torque, and prevent the nut from loosening.

At the same time, the sharp corners at the opening of the spring washer are respectively embedded in the surface of the bolt and the connected part, so as to prevent the bolt from rotating relative to the connected part.

spring washer

How about the anti loosening effect of the spring washer?

Spring washers are widely used in load-bearing and non load-bearing structures of general mechanical products.

They are characterized by low cost and convenient installation, and are suitable for parts with frequent assembly and disassembly.

But the anti loosening ability of spring washer is very low!

Especially in the current European and American countries, the adoption rate of products requiring high reliability is very low, especially the important connecting parts of load-bearing structures have been abandoned for many years.

Our country still has some applications in military industry, but it has been improved to stainless steel.

It is said that steel spring washers have long been banned in CASC!

It is also very unsafe for two reasons: one is the rising circle, and the other is hydrogen embrittlement.

Spring washer locking case

The connection between the axle final drive and the axle housing uses grade 10.9 M16 × 100 bolts, with a tightening torque of (280+20) N · m, tightened with a high-precision electric tightening machine.

The change of torque with the angle of rotation during the tightening process of the bolt is detected under the conditions of having spring washer and without spring washer.

By comparing the torque angle curve, it is found that there is always a pre tightening torque of about 10N · m when there is a spring washer;

Without the spring washer, the torque of the bolt is in the state of 0N · m before the torque increases significantly.

It can be inferred that the spring washer can be completely flattened with a bolt pre tightening torque of about 10N · m.

Through the inspection with a digital torque wrench, it is found that the bolt torque has not reached 20N · m, and the spring washer has been completely flattened, which verifies the correctness of the above inference.

Analysis of the above two points shows that the spring washer can only provide 10N · m elastic force, and the 10N · m elastic force can be ignored for the 280 N · m bolt pre tightening torque.

Secondly, such a small force is not enough to make the sharp corner at the notch of the spring washer embedded in the bolt and the surface of the connected part.

After disassembly, there is no obvious embedded mark on the surface of the bolt and the connected part.

Therefore, the anti loosening effect of spring washer on bolt can be ignored.

In addition, add a washer between the bolt and the connected part.

If there is a problem with the quality of the washer, it is equivalent to adding another potential safety hazard to the bolt connection.

To sum up, when the bolt torque is large (greater than 200N · m), the method of locking the bolt with spring washer does more harm than good.

Therefore, under the action of impact, vibration and variable load, the preload may disappear at a moment, and the connection may loosen.

At the same time, NASA also found the problem of open spring washers. In the lock nut chapter of NASA standard, it is written as follows:

“The typical helical spring washer… serves as a spring while the bolt is being tightened. However the washer is normally flat by the time the bolt is fuly torqued. At this time it is equlalent to a solid flat washer, and its locking abllty is non existent. In summary, lockwasher of this type is useless for locking.”

See the following figure for excerpts from the original text.

However, some professionals disagree with this statement. What do you think?

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