De-coupling effects suppress CVT noise

Saint-Gobain Performance Plastics Rencol discusses how de-coupling effects of tolerance rings help suppress CVT noise.

Improvements in transmission noise have been observed when using elastic bearing couplings in the CVT chain adjuster.

This affirms the research findings, announced by Saint-Gobain Performance Plastics Rencol, that the de-coupling effects of tolerance rings help to reduce the vibration within the bearing and reduces the amount of vibration energy being transferred to noise.

Continuously variable transmissions, or CVTs, are a type of automatic transmission that can be found in cars.

The driver's controls for a CVT are the same as an automatic; but while an automatic transmission has a set number of gear ratios, or speeds, the CVT constantly changes the relationship of engine speed to car speed.

Though there are several types of CVT, most cars use a pair of variable-diameter pulleys with a metal belt or chain running between them.

One pulley is connected to the engine (input shaft), the other to the drive wheels (output shaft).

Adjusting the diameter of the pulleys varies the transmission's ratio as making the input pulley smaller and the output pulley larger gives a low ratio for better low-speed acceleration.

As the car accelerates, the pulleys continuously vary their diameter and the engine speed, or RPM, lowers.

The attraction of this type of transmission for car manufacturers and ultimately the driver is that the CVT provides more useable power, better fuel economy and a smoother driving experience than a traditional automatic.

However, one of the major drawbacks associated with CVTs is chain noise.

This noise, in principle, is caused by the impact of the chain links on the pulley sets and significant acoustic improvements in the chain have already been achieved.

The interruption of the noise-transmission paths within the chain adjuster itself promises further improvements.

Luk has invested considerable development time into the improvement of acoustics in the chain adjuster.

Acoustic tests identified that the impulse between the chain and the pulleys is carried as structure-borne noise - via the pulley set bearings into the transmission housing - and continues from there as airborne noise.

By utilising the de-coupling effects of tolerance rings, the structure-borne noise at the bearings is suppressed significantly.

This concurs with Rencol's findings that as the stiffness of a joint using a tolerance ring is lower than that of a more traditional press fit or glue-bearing mount, the transmitted force through the bearing into the housing is reduced.

With a tolerance ring mount the damping ratio is increased, effectively absorbing more of the vibration energy.

With less vibration, there's less noise produced during running.

Tolerance rings are usually purpose designed for each application, with engineers from Rencol working closely with designers from the early stages of development.

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