Tolerance rings find new application in electric motors

Rencol Tolerance Rings has developed an innovative way of mounting the stator rigidly in the can of an electric motor using a tolerance ring. The new approach is a simple and cost-effective alternative to the more traditional mounting methods, such as heat-to-press and the use of adhesives.

Chris Needes, Automotive Product Manager at Rencol, says: "It is not hard to see that the seemingly straightforward heat-to-press approach, which will achieve an interference fit to retain the stator in position in the can, is unlikely to be satisfactory. While it certainly seems an inexpensive mounting method, the forces needed to fit the component are extremely high – in the region of a couple of metric tonnes – and may result in the laminate stator falling apart."

At worst, such damage may be sufficiently minor to go unnoticed until the electric motor has been in use for some time, when it can lead to premature failure. Should this happen, the cost of the replacement stator or the electric motor itself may well be relatively small, but the cost of fitting it is likely to be considerable. Plus in-service electric motor failures could have serious safety implications, particularly in electric power-assisted steering (EPS) systems, for instance, or cause a line stop situation.

Sticky situation

An alternative stator mounting method that is less likely to cause mechanical damage, as well as being lighter overall and more cost-effective, is the use of adhesives; unfortunately, gluing brings its own problems. The first is that is can be very difficult to position the stator concentrically when using adhesives. The second drawback is that once the adhesive has cured, reworking the assembly is practically impossible. Another potential problem, though perhaps becoming rarer with modern epoxy materials, is catastrophic failure of the adhesive bond itself. There is also the overall assembly time to consider – curing can take several hours – and the potential exists for outgassing of particulates to the electrics.

To overcome the problems associated with heat-to-press and adhesive methods, Rencol has developed a way to use a tolerance ring to achieve a rigid stator mount. The tolerance ring is a precision-engineered device made from thin spring-steel strips of material into which waves, corrugations or bumps are formed. In this particular stator mount application, the waves face inward so that the ends of the ring can be squeezed closed and fitted into a bore of the can; when released, it is self-retaining. In the final assembly, each wave in the tolerance ring is elastically deflected and it is this elasticity that provides the holding force.

Needes explains: "When using a tolerance ring there are two methods of mounting the stator within the can – either putting the ring onto stator and popping it into the can or putting the ring into the can and inserting stator."

Overall the cost savings are claimed to be significant.