A Review of Theories on Constrained Layer Damping and Some Verification Measurements on Shim Material

Introducing a rubbercoated steel plate - a shim - on the backing plate of a brake pad often solves squeal problems in disc brakes. This may be attributed to one or more of several effects exerted by the shim:

• Isolation or decoupling of brake components
• Alteration of the force configuration
• The addition of damping

The effect of damping may be active in either or both of the following ways:

• In the direction of the normal force exerted by the piston
• As a constrained layer damping of the bending vibration of the pad

This paper will deal with the constrained layer damping effect introduced by applying a viscoelastic material and an additional steel plate on top of the backing plate. This paper reviews some theories on constrained layer damping and extracts information on the effect of varying important parameters such as layer thickness, stiffness of the constrained material etc. Measurements on specially designed damping materials are performed in order to verify the predictions of the theories, and finally measurements on a commercial available shims material are presented. The measurements are made with the conventional ‘half power bandwidth method’ on a specially designed plate that behaves like a beam in the frequency range of interest. The theory on constrained layer damping predicts important properties that are relevant for the application of shims material and the measurements shows a qualitative agreement with the predictions.