Rigid Stator Mount Employs Ring of Steel

Chris Needes, Automotive Product Manager at Rencol, said: “It's 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.

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 hitch 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’s also the overall assembly time to consider – curing can take several hours – and the potential for outgassing of particulate to the electrics.

To overcome the problems associated with the traditional heat to press and gluing methods, Rencol’s solution is the novel use of 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.

“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,” Mr. Needes explained.