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RUNNING TOLERANCE RINGS ROUND EPAS SYSTEMS

With the increased adoption rate of EPAS systems, particularly in the low to mid-range vehicle classes, a range of new and innovative electronic diver-assisted safety enhancements are being developed. Chris Needes of Rencol Tolerance Rings looks at the dynamics of today’s automotive market.

When eSafety was introduced, a joint initiative of the European Commission, industry and other stakeholders that aims to reduce road fatalities by 50% between 2000 and 2010, it created quite a stir within the car manufacturing industry.

The big question asked by the automotive industry was: where do we go from here? The safety enhancements offered by interventions in the braking process by ESP (Electronic Stability Program), ABS (Antilock Braking System) and BAS (Brake Assist System) are, to all intents and purposes, already implemented to all vehicle classes. Similarly, the introduction of numerous airbags and structural improvements designed to enhance crash performance has now boosted passive safety to a high level.

Manufacturers of low to mid-range vehicle classes were particularly concerned about the additional costs of developing further safety enhancements in this more cost conscious area. This is because as a rule, new and innovative features, including safety features, tend to be launched into the high-end cars first before gradually penetrating the mass market.

However, at the same time, the traditional hydraulic steering system is been replaced by the lighter and more cost-effective electric power assisted steering (EPAS) system, particularly in small to mid-range vehicle classes, at an ever increasing rate. So much so, some analysts predict that a staggering 10.7 million EPAS systems will be fitted to vehicles built in Western Europe by 2013.

This trends has spurred car manufacturers to turn their focus on the development of active steering intervention systems for the extra safety enhancements – such as side-wind compensation, micro-split braking (allowing for different friction coefficients on the road surface right and left) and stabilising corrections of steering in combination with ESP (Electronic Stability Program) interventions. As a result, the adoption rate for EPAS systems could potentially speed up.

Tolerance rings are already used extensively within the automotive industry and play an important role in the design and manufacture of safe, dependable steering systems. The increased adoption rate of EPAS systems has now brought the benefits of these special steel fasteners into sharp focus.

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.

When used within the steering column, tolerance rings can allow the column to slide or collapse longitudinally should the steering wheel be struck by the driver during a collision. They are also useful as a slipping element in steering column locks, enabling them to withstand the 100Nm force prescribed by EU regulations without sustaining damage and without the need to adopt expensive heavy-duty construction.

Another steering related application for tolerance rings is bearing mounting, where they guard against problematic (noise, vibration and hardness) resonances and ensure good performance over a range of component tolerances and operating temperatures throughout the car’s lifetime. Already becoming widely accepted in EPAS systems is using tolerance rings for motor mounting, where they help to absorb shock transmitted from the engine, as well as fixing the stators within the drive motors themselves, where they hold the stator securely in place while reducing the levels of vibration to which it is exposed.

The latest generation of rings have modified waveforms, providing a wider contact area, making them particularly well-suited to these installations and can offer extended slip torque from 1Nm up to 200Nm. Typically torque levels of 100Nm to 200Nm are commonly required to protect the drive gears that transmit power to the steering mechanism, since they may encounter repeated abuse from incidents such as wheels hitting kerbs, potholes, and such like.

In addition, without some form of slip clutch device, plastic or lightweight alloy gears and pinions may shear causing the power assistance to fail and calling for expensive repairs. In volume production tolerance rings cost just a few pence, vastly cheaper than alternative slip clutches, and they are proven to accommodate thousands of slip cycles without affecting operating performance under normal load.

Tolerance rings are usually 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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