From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of brake judder and squeak, relating friction, pressure, and oscillation, with proposed solutions.
| Identifier | ExFiles\Box 140\4\ scan0142 | |
| Date | 20th January 1938 | |
| Messrs. Ferodo Ltd. -2- 20th. January 1938. where F₁ = Friction force when lining moves with drums. F₂ = Friction force when lining moves against drum. dx = amplitude of oscillation. Thus F₁ must be greater than F₂ if any vibration occurs. Now F = μP where P is the pressure between lining and drum, and F can change if either μ or P changes. In the case of wet judders we were able to show conclusively that μ changed. When the shoes moved against the drum direction a water film was built up between lining and drum and μ dropped to zero (very nearly), but when the shoes moved with the drum the water film broke down and μ reached its normal dry value. Thus the energy supplied per cycle was μ.dx. very nearly. In the case of dry judders (which were of sufficient power to twist axles) we found that both μ and P varied. In this case the variation of μ with speed assisted the judder, but this variation is a characteristic of the lining and it was not in our power to alter it. We also found, however, that P varied in such a way as to assist the judder, by increasing when the brake carrier swung forward and decreasing when it swung backwards, thus supplying the following energy to the cycle :- Σ P₁ x μ dx - Σ P₂ x μ dx. As we are a firm of Engineers, and as this effect was purely mechanical, we were able to alter it and every Bentley car is now fitted with a device to prevent this cyclic variation of P. Now as regards squeaks. Here the variation of P (if any) is very small and the controlling factor is the variation of μ with speed. If μ rises when the speed falls, then a squeak can occur and the energy supplied per cycle is :- Σ μ₁ x P dx - Σ μ₂ x P dx. but if only we can make μ₂ greater than μ₁ then energy is cont'd.{John DeLooze - Company Secretary} | ||