New stator mounting method challenges traditional techniques
A British company has developed a new way of mounting stators rigidly in electric motors using tolerance rings. Bristol-based Rencol Tolerance Rings says that its new approach offers a simple, cost-effective alternative to traditional mounting methods, such as heat-to-press and adhesives.
According to Chris Needes, Rencol’s automotive product manager, the heat-to-press approach, which uses an interference fit to keep the stator in position, is "unlikely to be satisfactory".
"While it certainly seems an inexpensive mounting method," he expands, "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."
Such damage could go unnoticed until the electric motor has been in use for a while, when it could lead to premature failure. If this happens, the cost of the replacement stator or motor may well be small, but the cost of fitting it could be considerable. In-service electric motor failures could also have serious safety implications, Rencol says.
Although using adhesives to mount stators is less likely to cause mechanical damage, and is lighter overall and more cost-effective, gluing brings its own problems, according to Rencol. It can be difficult to position the stator concentrically when using adhesives, and once the adhesive has cured, reworking the assembly is almost impossible. It can also take several hours for adhesives to cure and they pose the risk of outgassing of particulates to the electrics. Another potential problem is catastrophic failure of the adhesive bond itself – although becoming rarer with modern epoxy materials.
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.
Tolerance rings are precision-engineered devices made from thin strips of spring-steel into which waves, corrugations or bumps are formed. In the stator-mounting application (shown above), the waves face inwards so that the ends of the ring can be squeezed closed and fitted into a bore of the can. When released, the ring is self-retaining. In the final assembly, each wave in the tolerance ring is deflected elastically and this elasticity provides the holding force.
"When using a tolerance ring there are two methods of mounting the stator within the can," Needes explains, "either putting the ring onto stator and popping it into the can, or putting the ring into the can and inserting stator."