In this paper, through the force analysis of the wave spring washer, it is concluded that the root cause of the expansion of the spring washer is that the outer chamfer of the nut bearing surface is too large. To prevent the occurrence of ring expansion, the cause should be found from the matching nut and flat washer.
During the assembly process of the wave spring washer, the diameter of the spring washer becomes larger and expands outward. This phenomenon is called the “expansion ring” of the spring washer. The phenomenon of expansion of spring washers has a long history. As early as more than half a century ago, there were relevant reports. As for the reason for the expansion of the ring, there are different opinions, and there is still no clear explanation. So far, there is no conclusion, and the quality dispute of the spring washer caused by the expansion ring has not stopped.
This paper makes an in-depth analysis of the reasons for the occurrence of ring expansion.
It is also a common assembly form to add a flat washer between the wave spring washer and the body or between the spring washer and the nut.
The standard specifies the opening width (m) of the wave spring washer after it is flattened, which cannot be greater than a specified value. This is the minimum requirement for the manufacture of spring washers, which can be easily achieved as long as the normal cutting process is used. When we tighten the nut, the spring washer is flattened first. Under normal circumstances, the torque required to flatten the spring washer is very small, only a fraction of the torque of the nut.
The m value specified by the standard should be measured at this time. However, the expansion phenomenon of the wave spring washer occurs after the tightening torque is continuously applied, and its opening width increases with the increase of the tightening torque. When it reaches a certain level, the value of the opening width increases rapidly, and the tightening torque is difficult to increase until the nut cannot be tightened, resulting in the loss of the use effect of the spring washer.
For this phenomenon, people often look for the reason from the spring washer itself, from the aspects of manufacturing quality, design features, manufacturing process and even materials, thermal surface treatment and so on.
For example, some people once thought that light-duty wave spring washers are more prone to expansion than standard spring washers, and some people once thought that the trapezoidal cross-section of the washer is an important factor in causing the expansion ring, and some people think that the cutting angle of the spring washer and the broken edge angle are opposite to the expansion ring. Some people even regard the material of the spring washer as an important reason for the expansion of the ring, and think that the stainless steel spring washer is more prone to the expansion of the ring than the spring steel spring washer.
The root cause of wave spring washer open expansion ring and hydrogen embrittlement rupture:
- The expansion of the spring washer is usually not the problem of the spring washer itself
- Most of the spring washers with expanding rings are subjected to a radial external tension, and the external tension is derived from the tightening axial force formed by the tightening torque.
- The outer chamfer of the support surface of the nut makes the axial locking force form a radial component, thus causing the opening of the spring washer to bulge. The smaller the chamfer aperture, the greater the probability of bulging rings.
- Adding a flat washer between the nut and the wave spring washer is beneficial to reduce or avoid the expansion of the ring, but the flat washer is too thin or too soft to avoid the expansion of the ring.
- The root cause of the hydrogen embrittlement rupture of the spring washer is usually the unreasonable heat treatment method and the rupture caused by the lack of immediate dehydrogenation treatment after electro-galvanizing.
In all kinds of mechanical connectors, the anti-loosening of bolts is a very important link. There are many ways to prevent bolt loosening in life, including the use of spring washers to prevent loosening. Spring washers are often called spring washers in the screw industry. So why is it used frequently? This has to be explained from the function and principle of the spring washer.
The role of the wave spring washer:
The basic function of the spring washer is to give a force to the nut after the nut is tightened, increasing the friction between the nut and the bolt! That is to say, a protective measure is added to prevent the loosening of the fastening bolts caused by the vibration of the equipment in operation.
Anti-loose principle of wave spring washer:
The anti-loosening principle of the spring washer is that after the wave spring washer is flattened, the spring washer will generate a continuous elastic force, so that the threaded connection pair between the nut and the bolt will continue to maintain a frictional force and generate a resistance torque, thereby preventing 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, thereby preventing the bolt from rotating relative to the connected part.
However, it has been proven that wave spring washers can only provide a spring force of 10 N/m, which is negligible for a bolt preload of 280 N/m. Secondly, such a small force is not enough to make the sharp corners of the spring washer cut into the surface of the bolt and the connected part. After disassembly, there is no obvious indentation on the surface of the bolt and the connected parts. Therefore, the anti-loosening effect of the spring washer on the bolt can be ignored.
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