Simple in concept and easy to use, tolerance rings offer convenient and cost-effective solutions for an impressively wide range of automotive applications. Chris Needes of Rencol provides an introduction to tolerance rings and explains some of the ways in which they are being used.
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 and radial loading between the mating components.
Providing exactly the right combination of properties needed for a particular application involves deciding on the number of waves on the ring, the optimum wave design, and the best combination of material from which to manufacture the ring. The design work is, however, undertaken entirely by Rencol; all users need to do is to supply basic information about their application and then, when the rings are supplied, fit them!
Now let’s take a look at some practical examples of where tolerance rings are solving problems in the automotive industry, and at the important benefits they are providing for their users.
Position sensors are widely used in modern cars, not least in 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.
Though not a problem commonly encountered in cars, engine backfiring during starting can cause serious starter motor damage in large diesel vehicles.
Internally, 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 – an expensive component – will eventually be damaged to the point where it has to be replaced or rebuilt.
Rencol has 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.
Those travelling in cars treat the seat adjustment mechanisms surprisingly harshly, and it’s by no means uncommon to find mechanisms that have been damaged by the application of excess force to the adjustment handles. Should this happen to the driver’s seat, it’s likely that the car will no longer be legal to drive.
A tolerance ring used, once again, as an inexpensive slipping clutch provides an attractive solution. If the tolerance ring is fitted to the adjustment handle, all that happens when excess force is used is that the handle is displaced from its normal position. It can easily be pushed back, restoring normal operation.
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.
Tolerance rings are an ideal choice for use in wing mirror assemblies as they provide a rigid joint with a high resonant frequency that helps to prevent the mirror vibrating as a result of wind- and road-induced forces.
In spite of the rigidity of the joint under normal conditions, the tolerance rings allow rotational movement under shock loading, thus protecting the mirror components from damage.
In this short article it has, of course, only been possible to mention a few of the many applications for tolerance rings in the automotive industry. Hopefully, these examples will demonstrate some of the application-specific benefits which the rings offer, but it’s worth noting that they also have benefits that are common to almost every application.
For example, they occupy very little space, which means they can be used in even the most compact of assemblies. They’re easy to assembly and fit, with no special equipment or machines required. And, it’s almost always possible to relax the manufacturing tolerances for components which are joined by tolerance rings, often with big savings on machining costs.
Tolerance rings are convenient, versatile and inexpensive, yet they solve a whole range of design problems and deliver valuable benefits. Many automotive manufacturers are already enjoying these benefits – perhaps it’s time for you to join them!
For further information, please call +44 (0)117 938 1700 or visit www.rencol.co.uk
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