Other Industries

By Dr Bill Eccles of Bolt Science Ltd

I was recently asked under what circumstances would I recommend the use of helical spring washers – the type of spring washers which are split. My reply was that I would not and I thought that I would review the reasons why I believe all the evidence points towards helical spring washers being ineffective.  

The helical spring washer was invented over one hundred years ago, with billions being made in the intervening period. There are two claims traditionally made for this washer – the first being that using this washer facilitates the locking of an assembly so that self-loosening is prevented and the second being that compression on the washer by a nut or bolt will affect the spring rate of the joint so that it will become more resistant to relaxation and fatigue. Both of these claims can be misleading, at best.

In 1969, Gerhard Junker (founder of Junker test) wrote a technical paper on his theory of self-loosening that showed experimentally that helical spring washers failed to prevent bolts from self-loosening. In his paper he demonstrated that preloaded fasteners self-loosen when relative movement occurs between the mating threads and the fastener bearing surface under the action of transverse joint loading. Helical spring washers fail to prevent fastener rotation arising from such movement. Since then, tests have shown that helical spring washers can result in the loosening rate of nuts being increased rather than prevented.

On some installations a plain washer is placed between the joint and the helical spring washer. The intention of this practice is to attempt to use the spring rate of the washer to beneficially change the stiffness characteristics of the joint. Having a greater amount of elastic deformation in a joint is beneficial since it will tend to reduce the effects of relaxation that can be particularly pronounced on small grip lengths. Such stiffness changes can also be beneficial in reducing the proportion of any axial applied load that is sustained by the bolt. Such reduction in bolt loading will enhance its fatigue resistance.  

However, one issue of using helical spring washers in this manner is that they tend to flatten out at a relatively low bolt preload. The deflection characteristic of a joint that includes a helical spring washer is shown in Figure Two. Typically, such washers flatten out between 10% – 20% of the target preload when fasteners of property Class 8.8 and above are being used. Any embedding or relaxation that occurs to the joint will reduce the target bolt preload by a similar amount to an application where a washer had not been fitted.

It is the stiffness rate at the preload that is of crucial importance. Normally it is desirable to have a low stiffness bolt so that the proportion of any applied load that will be sustained is reduced and hence the bolt’s fatigue performance is improved. However, it is again the stiffness at the target preload value that is crucial and the fitment of a helical spring washer plays no part in affecting this and hence is of no benefit.  

There are washers that are specifically designed to change the bolt or joint spring rate. For example, one such type is the conical spring washer which, when complying to the appropriate standard such as the German standard DIN 6796, are designed to prevent loosening of the assembly by relaxation or embedding. Conical spring washers are designed to flatten out close to the target preload and so, unlike helical spring washers, have a beneficial effect in reducing losses from such settlement.  

During the testing of the helical spring washer it has also been noted that some washers start to split apart during the tightening process. This seems to occur when the washer is flattened, and it partially, or fully, digs into the nut or bolt surface. It is not only on the test bench that this type of washer is shown to be ineffective. By failing to prevent self-loosening, helical spring washers have contributed to accidents occurring.

One such accident occurred on 23^rd February 2007 when a high-speed train derailed on points in the UK. Helical spring washers had been used on joints holding the points together that resulted in the nuts becoming completely detached from bolts which ultimately led to the train being derailed whilst travelling at high speed. Bolt Science have investigated other accidents in which helical spring washers have failed to prevent loosening of bolts occurring.

In a publication in 2004, the German standards body Deutsches Institut fur Normung (DIN) published a document explaining why a range of standards covering so called lock washers – including helical spring washers – had been withdrawn. The withdrawal of the standards was done so that designers would be discouraged from specifying them since they had been shown to be ineffective when resisting self-loosening or effectively altering the spring rate of the joint.  

At the time the standars body received criticism from many quarters which led them to conclude that many people were unaware of the state of technology for locking devices. Hence, considering all the above reasons, you can appreciate why I will not recommend the use of such washers.

The most effective way to prevent loosening is to design the joint and then assemble it correctly. Although there are effective locking methods, if the joint repeatedly sustains forces that encourage the joint to move, the bolts may not self-loosen but they will eventually fail as a result of fatigue or wear. Therefore, there is no substitute for designing the joint correctly.

Training services from Bolt Science

Bolt Science is a provider of training courses on bolting – which includes detailed analysis of circumstances in which bolts can self-loosen, as well as the loosening resistance of the major locking methods available (this includes videos of Junker tests). Bolt Science training also covers how to complete a joint analysis that will allow the elimination of the tendency for bolts to self-loosen.