As vehicle manufacturers move towards tubular steering systems, they need to consider compliance with EU anti-theft regulations. Tolerance rings are helping to provide a cost-effective solution. Chris Needes, global market manager automotive chassis, Saint-Gobain Performance Plastics, explains why

In recent years, automotive steering systems have evolved far beyond their primary function. They have now become a tool for car manufacturers to provide their customers with even more sophisticated features.

In the 1990s, the need for anti-theft protection became more urgent. Thus, in 1995, EU regulatory bodies mandated that cars be fitted with security devices. Tolerance rings in steering systems allow car manufacturers in Brazil, Russia, India and China (BRIC) to offer anti-theft capabilities in order to meet EU regulations, while at the same time imparting additional benefits such as cost reduction.

Traditionally, car steering columns were constructed using a solid shaft, but this was a simple, limited way of connecting the steering system to the vehicle. In the early 1990s, however, we saw the emergence of hollow shaft steering columns in high-end vehicles. Being lightweight, these resulted in both material and energy savings in the car production stage, less noise, reductions in CO2 emissions due to lighter car loads, and, most importantly, it facilitated the easy incorporation of additional features.

In most new cars, the steering system and adjoining steering column are developed to house mechanical features that improve functionality, safety, carbon footprint and aesthetics. Now, this is the main interface between drivers and the technological performance of their automobiles, being used to change a radio station or control car speed, for example.

In Europe, the potential of the hollow (or tubular) steering system has long been recognised. Unlike their European counterparts, BRIC (Brazil, Russia, India and China) car manufacturers have been slower to make changes to traditional manufacturing methods and transition to tubular shafts. Now, more and more regional producers are contemplating the incorporation of hollow shafts.

As they transition to tubular steering systems, BRIC manufacturers must consider compliance with EU anti-theft regulations to be able to export to Europe. Since November 1995, the EU commission has been prompting car manufacturers to provide features for vehicles to ensure they are operated safely and, at the same time, ensure that an essential level of security is provided (Directive 95/56/EC).

This regulation dictates that steering locks must be able to withstand forces of 100Nm applied via the steering wheel without failing. Thus, it became law that cars exported to European markets be fitted with security devices. Additionally, the recently launched Chinese directive GB15740-2006 strengthens the need for an anti-theft mechanism.

Tolerance rings

The current most popular anti-theft method in the BRIC region uses a manual process whereby the shaft is welded to the steering column. While this alternative allows manufacturers to meet international regulations, it is labour-intensive, unable to be used in mass production, and only applicable for solid shaft steering systems. Additionally, theft attempts can completely damage the steering system.

But, many European manufacturers are using a new solution – tolerance rings – which can be used within hollow steering designs to comply with the EU and Chinese anti-theft regulations whilst also providing cost reductions in manufacturing. Car manufacturers leveraging the benefits of this technology include Audi, BMW, Citröen, Seat and Volkswagen, who all use tolerance rings in their rack and column mounted electric power steering systems.

Meeting requirements

Tolerance rings are high-quality steel, radially sprung fasteners, that allow for optimal joining between two annular (ring-shaped) components. These rings are small in scale, but enormous in impact.

Not only do these withstand the force requirements prescribed by the EU directive, proving their ability to perform as an anti-theft mechanism, but from a production standpoint, tolerance rings are easy to apply. In the standard three-component system, the tolerance ring is simply placed on the steering shaft and the lock column is pressed over the ring to provide interference pressure. This allows the ring to protect the steering column assembly from potential damage as a result of attempted theft. Unlike manual welding techniques, tolerance rings allow for standardisation by providing consistency from part to part.

The key to the tolerance ring’s effectiveness, and also its adaptability, is to be found in the radial protrusions that run around its circumference. The way that these ‘waves’ transfer force between the joining components means that direct torque transfer and torque slip can be effectively handled.

Other important factors of a tolerance ring are its shape, size, number of radial protrusions and its thickness and hardness, which can be combined to provide the ideal interference fit. The ideal parameters depend on the components used in the car’s steering shaft, and manufacturers should seek a solution customised for their operations. When joining a steering shaft and column comprised of soft steel components, using a very heavy tolerance ring with numerous stiff waves and a thick material can cause damage during torque slip. Ideally, the ring’s hardness should match the hardness of the other components in the system.

Saint-Gobain, for example, customises its RENCOL tolerance rings to meet the exact needs of the automobile manufacturer or engineer.

By adopting a combination of hollow steering shafts and tolerance rings, BRIC car manufacturers can provide a superior anti-theft solution to satisfy consumers and regulatory bodies alike with high-end features and anti-theft functionality. These benefits, coupled with production efficiencies and standardisation, illustrate the enormous potential of tolerance rings in the automotive industry.