Spring washer anti-loosening principle
The anti-loosening principle of the spring washer is that after the spring washer is flattened, the spring washer will generate a continuous elastic force, so that the threaded connection pair of the nut and the bolt will continue to maintain a frictional force, generating a resistance torque to prevent the nut from loosening. At the same time, the sharp corners of the opening of the spring washer are respectively embedded in the surfaces of the bolt and the connected part, thereby preventing the bolt from rotating relative to the connected part.
What is the anti-loosening effect of spring washers?
Spring washers are widely used in load-bearing and non-load-bearing structures of general mechanical products. They are characterized by low cost, easy installation, and are suitable for parts with frequent assembly and disassembly. But the anti-loosening ability of the spring washer is very low! Especially in Europe and the United States, the adoption rate of products requiring high reliability is extremely low, especially the important load-bearing structural connection parts that have been abandoned for many years. Our country still has some applications in military industry, but they have been improved to stainless steel materials. It is said that the use of steel spring washers has long been banned in CASC! It is also said to be very unsafe for two reasons: one is swelling and the other is hydrogen embrittlement.
Spring washer anti-loosening case
The connection between the axle main reducer and the axle housing uses 10.9 grade M16X100 bolts, the tightening torque is 280+20 N·m, and a high-precision electric tightening machine is used to tighten. The change of torque with rotation angle during bolt tightening is detected under two conditions: with spring washer and without spring washer. Comparing the torque angle curve, it is found that with a spring washer, there is always a pretightening torque of about 10 N·m; while without a spring washer, the bolt's torque is at 0 N·m before the torque rises significantly.
It can be inferred that the spring washer can be completely flattened with a bolt pretightening torque of about 10 N·m. After testing with a digital torque wrench, it was found that the bolt torque had not reached 20 N·m and the spring washer had been completely flattened, which verified the correctness of the above inference.
Analysis of the above two points shows that the spring washer can only provide an elastic force of 10 N·m, and the elastic force of 10 N·m can be ignored for the bolt pretightening torque of 280 N·m. Secondly, such a small force is not enough to The sharp corners of the spring washer cutout are embedded in the surface of the bolt and the connected parts. After disassembly and observation, there are no obvious indentations on the surface of the bolts and the connected parts. Therefore, the anti-loosening effect of spring washers on bolts can be ignored.
In addition, a washer is added between the bolt and the connected part. If the quality of the washer is defective, it is equivalent to adding another safety hazard to the bolt connection.
To sum up, when the bolt torque is large (greater than 200 N·m), using spring washers to prevent the bolts from loosening will do more harm than good. For this reason, under the action of impact, vibration and load change, the preload force may disappear at a certain moment, and the connection may become loose.
At the same time, NASA also discovered the problem of open spring washers. In the NASA standard, in the chapter on locking nuts, it is written: "Typical spiral spring washers... act as springs when tightening bolts. Function. However, when the bolt is fully tightened, the washer is generally flat. At this time, it can be regarded as an integral flat washer, and its locking function does not exist at all. The lock washers are useless for locking." Excerpt from the original article, see picture below.
