Automotive Engineer

Electric power assisted steering (EPAS) is rapidly growing in popularity, not least because it cuts costs and helps to reduce vehicle weight. But, says Chris Needes of Saint-Gobain Performance Plastics, many manufacturers of EPAS systems are missing opportunities to maximise the benefits that their products offer.

After safety, the most important concern for automobile manufacturers today is vehicle weight, since lower weight means lower fuel consumption and, therefore, improved environmental performance. The drive to find ways of saving weight has led to conventional hydraulic power steering systems falling from favour, to be replaced by lighter and more cost-effective EPAS systems. In fact, so great is the attraction of EPAS that 10.7 million systems are expected to be fitted to vehicles built in Western Europe by 2013.

The current generation of EPAS systems undoubtedly delivers significant weight savings, while also providing a good driver interface and low noise levels, but there are now opportunities to further enhance these benefits. For example, tolerance rings and self-lubricating low-friction plain bearings can be used in many EPAS applications where the alternatives would be bulkier, heavier and more costly.

Let’s start by looking at tolerance rings. The idea behind these versatile components is very straightforward – they are simply radially sprung steel rings that are designed to be press fitted between two mating components, such as a motor shaft and a take-off pulley. They are, in other words, a special form of spring fastener that can compensate for thermal expansion, reduce noise and vibration, offer predictable and repeatable slip, and provide overload protection. Further benefits are that they’re easy to use, and that they’re inexpensive; in production quantities, they cost just a few pence each.

In EPAS systems, there are many possible applications for tolerance rings. Some manufacturers are, for example, already using them as a slipping element in steering column locks. This allows the locks to withstand the 100 Nm force prescribed by European legislation without the need to adopt weighty and costly heavy-duty construction.

Used to mount the drive gear in a column-mounted EPAS, a tolerance ring will provide effective torque control by acting as a simple “fuse”, while a tolerance ring built into a steering column will provide the axial resistance necessary to ensure the controlled collapse of the column if the vehicle is involved in a collision.

In fact, tolerance rings provide a simple and effective way of mounting all of the bearings, gears and pulleys used in EPAS systems. One application that is, however, slightly less obvious is the mounting of sensors.

These often incorporate brittle magnets manufactured from powdered metal. Conventional press fitting produces a high breakage rate, and it is difficult to achieve accurate positioning and alignment if adhesive is used. A tolerance ring around the magnet provides a complete solution, and allows the assembly to be press fitted without risk of damage, since the fitting force is controlled by the design of the ring, rather than by component tolerances.