Friction grip joints - A calculation example

Friction grip joints

A Calculation Study

In a previous article (read it here) the importance of the value of the interface friction in friction grip joints was discussed. As mentioned in the article, there are ways in which the coefficient of friction can be increased. This article further explains this topic and puts some numbers to it based upon an adapted service problem with some specific details changed. Prior to the bolt being tightened, any shear loading would cause the joint plates to slip resulting in wear issues, amongst others. By tightening, a preload is induced into the bolt that is reacted by a clamp force acting on the joint. The clamp force in conjunction with the friction that is present between the plates generates a friction grip that resists the shear force. The importance of the value of friction coefficient between the joint plates is often underestimated.

Bolt Science has been consulted on a number of occasions in which bolts were coming loose on joints. In some instances, such issues can be resolved by the use of an effective locking method, such as for example, wedge-lock washers. However, this is not always the case. The locking approach works if the joint only experiences an occasional overload that results in the joint slipping. If the joint experiences frequent loading that causes joint slip, a bolt fatigue issue substitutes for the loosening problem. Usually, some time passes before the bolt fails by fatigue, but failure is likely if the joint experiences repeated slip.

The fatigue is the result of bending that the bolt experiences when the joint slips. This was explained on another page. It is not that a better locking method is needed, but increasing the clamp force, or a higher friction value is needed to prevent slip in the first place. If you prevent joint slip, you prevent self-loosening and the fatigue failure that joint slip can cause.

To resolve a joint slip issue, you could try a 'suck-it-and-see' approach. For example, increase the tightening torque, if feasible, and keep your fingers crossed that the issue is resolved. If fatigue is involved, either expensive testing is needed, or alternatively wait, potentially months, and hope that the problem goes away. In many problems of this type, not all joints fail due to the combination of the several factors involved (loading, friction variation, bolt preload variation etc.) A more satisfactory approach is to look at the problem from an analytical perspective. That is complete a series of joint analyses.

As example consider a joint consisted of several M10 property class 10.9 bolts. The applied force is resolved so that an individual bolt region sustains a shear force of 5 kN. The joint plates have an interface coefficient of static friction of 0.15. To prevent joint slip a clamp force of at least 33.3 kN (5 / 0.15) is needed plus any preload loss from embedding. Using the BOLTCALC software to complete the calculations, allows a Preload Requirement Chart to be created. This is shown below:

The chart shows that the bolt preload varies between 27 kN to 43 kN. Allowing for the clamp force needed to prevent joint slip plus anticipated losses due to embedding, the joint requires a minimum of 39 kN. As a result, a significant proportion of the joints can be anticipated to sustain slip with the loosening/fatigue issues normally associated with such movement. Based upon the Preload Requirement Chart, it would be anticipated that over half the assemblies would sustain slip and fail by the bolt self-loosening or fatigue. These issues can arise due to an underestimate of the loading or assuming too high a friction value coupled with inadequate testing.

In this application, the tightening torque can't be significantly increased since the bolt is being tightened to 90% of minimum yield (allowing for both axial and torsional stresses). Increasing the bolt strength to a 12.9 and subsequently increasing the tightening torque are deemed undesirable due to concerns over hydrogen embrittlement. The bolt diameter could be increased to M12, that could provide a minimum preload of 40 kN giving little design margin. An M14 would provide a larger margin but both are not well suited to a service fix since the hole size would need to be increased.

For a service fix it was decided to look at the use of a friction shim. With a shim at the joint interface, the coefficient of static friction is increased to 0.7 reducing the preload requirement from 39 kN to 18 kN. There is now a design margin of 9 kN. Whether this is deemed sufficient is an engineering judgement that would consider the likelihood of the loading being as assumed etc. When a friction shim is included, it can be expected that the preload loss from embedding will increase due the extra interface being present and the surface condition. The increase in embedding loss is more than compensated by the reduction in the clamp force needed to prevent slip.

The example here is adapted from an actual service problem with specific details changed. The proposed friction shim proposal was accepted and the product in service modified. Being able to show using a joint analysis that, in theory at least, the proposal would solve the issue provided the confidence needed to approve the change. The costs of the having someone disassemble the joint, insertion of the shim and the re-tightening is more expensive than the cost of the friction shim.