Investigation on the requirements of shims without metal carriers. Typically, a shim is used for dampening reasons. The adhesive serves to bond the shim to the backplate of the brake pad. This is essential to maximize the dampening effects of the shim. Due to its excellent dampening properties as well as its elasticity, different kind of rubbers may be used. Some kind of steel layer typically serves both as a carrier for the rubber as well as adding strength to the product in the XY-plane. Typically, the rubber does not bleed out, due to the strong bond to the steel carrier material of the shim. Therefore, only a deformation in z of the rubber is typical in brake applications. Nevertheless, there are some applications, which use an additional dampening foil made of rubber and an adhesive, where also the rubber squeezes out in XY direction. This indicates, that the glue of such an application without a strong carrier does not provide enough bonding to prevent the movement of rubber in XY. Similar to grease such NVH solutions lose their functionality after a period of time. Forces during breaking To better understand the potential failure modes of a shim inside of a calliper one needs to understand the basic forces inside of a calliper during a brake application. Where the piston creates a normal force in Z-direction, the friction force is in radial (XY) direction. This leads to both a shear force as well as a normal force inside of the shim. Due to this shear force, as a 3-dimensional tensor, the elasticity components come into play. Assumptions For reasons of simplification in this evaluation, it will be assumed that the main stiffness in XY-direction of a shim is created by the steel layer. Also, it will be assumed, that the steel will only have elastic deformation due to external forces. the different external forces usually applied onto a shim are: - Piston (Force z, x and y) - Frictional force related to elongation - Temperature According to above assumptions, we arrived at following Conclusion: - the effect of the frictional force and temperature is 100 times higher compared to the elasticity due to the compression force Fz. - An alternative base material as a replacement for steel in a shim should therefore show a similar behaviour to not impact the functionality of the shim. - Since the elasticity of rubber is significantly higher compared to the steel layer, a sufficient safety margin should be available in this system. In other words: It has been shown, that the standard steel carrier material of a shim shows insignificant deformation due to the forces applied during breaking alone. The bonding forces of the rubber to the steel carrier therefore prevent rubber squeeze out in XY direction successfully. Any replacement material to this steel carrier therefore needs to provide a sufficient stability in XY-direction as well as equal or better rubber bonding properties to also prevent rubber squeeze out and thus the destruction of the shim. The most promising alternative materials based on these key parameters are GFK and Steel fibre rovings, but also other similar substrates could be used. Even though GFK only has 1/4th of the mechanical stiffness vs. deformation (Youngs Modulus) compared to steel, this would still theoretically only result in 4 times the elongation, being below the derived threshold of 0.02%-dimensional change. Thermal and corrosion properties of GFK, better suit the product application, in comparison to a steel carrier. Permanent thermal deformation due to the relaxation of inner stresses of the steel layer may not apply to flexible base layers made of GFK, Steel fibre roving, or other similar webs.
Mr. Gino Fronzoni, CEO, Omnia Advanced Materials; Dr. Axel Stenkamp, consultant, Omnia Advanced Materials