Tolerance rings

The number of electric motors used in car applications is growing at an astounding rate, with industry analysts predicting this figure will continue rise. Chris Needes of Rencol Tolerance Rings looks at the common applications of these motors and explains some of the ways in which tolerance rings are being used.


 


With the number of electric motors in automotive applications set to increase more dramatically, car manufacturers are now pushing technology boundaries to simplify each of the designs in order to reduce size and weight of the overall assembly and making it more cost-effective.


 


Electric motors are found in anything that has an electrical movement or solenoid function; familiar examples include window lifts, fuel pumps, mirror and headlamp adjusters, clutches, transmissions, windscreen wipers and seat adjusters. While there is steady growth for electric motors in powered interior applications, such as seat adjusters, window lifts and wing mirror adjusters, rapid growth is being driven by new technologies, primarily electric power steering (EPS) systems and active suspension and brake systems.


 


As the number of electric motors used in cars increases, so does the overall weight of the car. This increased weight is gradually becoming more of a concern, for those manufacturers keeping a close eye on overall costs and fuel economy, and is pushing electric motor suppliers to reduce the size of their products in a bid to save cost and reduce weight.


 


Already some of the major electric motor suppliers to the automotive industry have introduced a number of new innovative products. However, size and weight are not the only factors to think about when specifying an electric motor. There are also reduction in power input and the improvement of efficiency to consider. 


 


Some novel advances in motor control are already influencing wiper designs. Although there isn’t always an obvious cost advantage of using an electronic control unit (ECU) in these systems, there is a gradual increase in x-by-wire systems in development, for high-end cars in particular, and some are already coming into production.


 


Today however automotive manufacturers are not only looking to advances in electric motor design but also the system itself – any design improvements that can help reduce the overall size, weight or production costs of the assembly. Here the innovative use of tolerances rings is already proving invaluable for an impressively wide range of applications.


 


The idea behind tolerance rings is very straightforward – they’re 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 frictional fastener.


 


Typically manufactured from high quality spring steel, stainless steel or specialist spring materials, tolerance rings are invariably custom designed to suit a particular application. All types, however, have one essential characteristic in common – a series of protrusions or ‘waves’ around their circumference. Each of these waves acts as an individual radial spring which, when the tolerance ring is in situ, transfers forces between the mating components. This arrangement means that tolerance rings are capable of handling direct torque transfer, torque slip, axial retention, controlled collapse, radial loading and differences in thermal expansion between the mating components.


 


Steering systems


Tolerance rings have a big role to play in the design and manufacture of safe, dependable steering systems. For example, when used within the steering column, they can allow the column to slide or collapse longitudinally should the steering wheel be struck by the driver during a collision.


 


Tolerance rings are also useful as a slipping element in steering column locks, enabling them to withstand the 100Nm force prescribed by European legislation without sustaining damage and without the need to adopt expensive heavy-duty construction.


 


Other steering related applications for tolerance rings include bearing mounting, where they guard against problematic resonances; motor mounting in electric steering systems, where they help to absorb shock transmitted from the engine; and the fixing of the stators within the electric steering drive motors themselves, where they hold the stator securely in place while reducing the levels of vibration to which it is exposed.


 


Position sensors


Position sensors are widely used in modern cars, not least in conjunction with the electric motor in a power steering system, and these sensors frequently incorporate magnets manufactured from powdered metal. These have excellent magnetic properties but can be brittle, which makes them difficult to fix in place.


 


Conventional press fitting is unsatisfactory, as it produces a high breakage rate. Fixing the sensors with adhesive looks like an attractive alternative, but in practice it’s difficult to consistently achieve accurate sensor positioning and alignment of the poles.


 


A sprung tolerance ring around the magnet assembly provides a complete solution, allowing the assembly to be press fitted without risk of damage, since the fitting force can be accurately determined by the design of the ring and no longer depends heavily on component tolerances. Because of the resilient nature of the tolerance ring, this arrangement also provides the sensor with useful degree of protection against vibration and shock.


 


Starter motors


Though not a problem commonly encountered in cars, engine backfiring during starting can cause serious starter motor damage in large diesel vehicles.


 


Internally, electric starter motors have a gear drive with a one-way clutch. When a backfire occurs, the vehicle engine momentarily stops or even reverses. If this happens while the starter is engaged, a severe shock load is imposed on the gears and the clutch, which can cause damage. It is even possible for the commutator and the motor windings to slip on the shaft, resulting in broken wires. In any of these situations, the starter motor will eventually be damaged to the point where it has to be replaced or rebuilt.


 


Rencol solved this problem by fitting a tolerance ring between the gear and the internal clutch cam. With this arrangement, when the engine backfires the starter continues to drive but the gear slips, preventing damage. In most cases, the tolerance ring can be added without the need to redesign or otherwise modify the starter assembly.


 


Tolerance rings are normally purpose-designed for each application, with Rencol’s engineers working closely with designers from the early stages of development. For further information on how tolerance rings can be used in your next design project, please call Rencol Tolerance Rings on +44 (0)117 938 1700 or visit www.rencol.co.uk.


 


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