Belt tensioner minimises tensional vibrations

NSK has developed a new auto-tensioner with viscous damper, which can reduce high frequency tensional variations and belt noise.

The high strength, low weight benefits of timing (or cam) belts, combined with their maintenance-free and low noise characteristics means that they are used extensively for driving the camshafts of modern automotive engines.

This is a demanding task, requiring correct tensioning to ensure that belt performance is maintained at the optimum level over time.

The tensioning itself is carried out by a mechanism integrated into the belt drive, comprising a dedicated pulley, which is either a fixed or self-adjusting type.

Experience shows that engines using a fixed-type tensioner are liable to suffer from tension variations during running.

These are due to elongation and wear over the lifetime of the belt, and dimensional changes resulting from the engine block and the belt having different coefficients of thermal expansion.

The result of these two conditions is that the drive is prone to noise problems and provides a shorter belt life under high tension.

Unfortunately, reducing the tension does not provide a solution.

This can result in belt flapping which gives rise to unwanted noise and possible shorter life due to failure of the belt corrugations.

A better solution is to employ an auto-tensioner, which can be used to maintain a constant tension.

Recently, though, units of this type with hydraulic or friction-type damping systems have come into sharp focus.

Experience has shown that they can exhibit unstable damper characteristics and are not fully capable of ensuring optimum tension.

In view of these problems a new approach was called for and this has now been provided by NSK with its development of a new auto-tensioner with viscous damper, which can reduce both high frequency tensional variations and belt noise.

The centre of the tensioner bearing is offset from the centre of displacement and this positional change acts to adjust the belt tension.

The tension itself is produced by a tensile coiled spring or a twisted coil spring.

One end of the spring is attached to the adjustable assembly and the other end to a fixed point such as the engine block.

A multi-plate damper is located to encompass the centre of rotation and provides resistance proportional to the speed of the sub-assembly's movement.

Because of this arrangement, the tensioner follows, without resistance, the slow tensional variations such as those due to belt elongation and temperature change.

It also resists movements caused by the high-frequency tensional variations from the engine.

The multi-plate damper, which is key to the tensioner's operation, uses the viscous resistance of oil contained between the inner and outer plates, which are fixed to the shaft and sleeve respectively.

The oil is a highly viscous silicone type whose viscosity varies only slightly with temperature.

The gap between the plates is determined by the difference between outer plate thickness and the inner plate spacer.

The damper capacity depends on the number of these plates chosen, the set gap between the plates and the oil viscosity.

The innovative design of the new tensioner offers a number of significant advantages, these include: * Space saving: with the centre of oscillation within the tensioner bearing itself the new unit requires only a small space envelope.

* Replacement of existing tensioners: this is easily achieved because the new auto-tensioner requires only a simple screw attachment and a spring locating point.

* Large resistance to high-frequency tensional variations and belt flapping: these are ensured by using the viscous resistance of a fluid.

In addition, since the tensioner is free from sharp changes in resistance and damper position, it can effectively control belt tension variations even when the engine is running.

* The required damper capacity is easily attainable from the number of plates incorporated.

* The damper capacity does not significantly vary with age or usage.

Actual tests of the new auto-tensioner when installed on vehicle engines indicate that timing belt noise originates chiefly from the engagement of the timing belt teeth with the pulley teeth.

This is closely related to the prevailing belt tension during engine operation i.e the higher the belt tension, the larger the teeth engagement noise.

To minimize belt noise, therefore, it is necessary to control the belt tension to the lowest possible level at which the belt will not flap throughout the engines operational range.

The new auto tensioner design enables improved control of the dynamic reaction by presetting the belt tension to within a much narrower range than when a fixed tensioner is used.

Further tests were also undertaken by NSK to determine how the damper in the auto tensioner would perform over time on vehicles, in terms of distance travelled.

The tests were performed on a unit with four sets of damper plates, the optimum specification to ensure minimal belt engagement noise and belt flapping.

From the test results it is evident that the damper capability remains almost constant throughout operation over long distances, showing little degradation due to ageing.

This is due, in no small part to a design which uses the shearing force of oil to prevent direct contact of the damper's moving parts.

With its new auto-tensioner unit NSK has once again succeeded in providing a package that gives tangible benefits to both car manufacturers and car users, and also benefits the environment through reduced noise, lower weight and longer life.

These twin objectives are the cornerstones of NSK's global product strategy and the foundation of its dynamic partnerships with the world's leading automotive manufacturers.

Back to top