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).
Letter from Gandy's discussing the causes and theories of squeaking in 'Safeguard' brake linings.
| Identifier | ExFiles\Box 140\4\ scan0040 | |
| Date | 21th August 1937 | |
| GANDY'S "SAFEGUARD" BRAKE LINING CONTINUED - 2 - Messrs. Rolls-Royce Ltd. 21st August,1937. how many you require, we will be pleased to make up the necessary samples. We are surprised to have your report that the "400" N.M. squeaks, as we did not find this to be the case on our Testing Machine, although a slight eccentricity of our Test Drum usually accentuates any tendency which a lining might have to produce a squeak, but the particular construction of your test equipment might be a contributory factor in causing the squeak. Reviewing your problem broadly, it would appear to be divisible into two parts (a) where the braking is light, and (b) where the braking is severe. These two sets of conditions are so diverse that it is difficult to detect a common factor which would account for squeaking arising in both cases.(a) Where the braking is light, wear on the lining is so slight that glazing of the friction surface tends to occur often accompanied by a reduction in the co-efficient of friction, and hence squeak would not be expected to arise. We have noticed with older types of lining, a tendency for the impregnating material to be transferred to the drum face, at low temperatures, but the deposit disappears as the temperatures rise. On the "Bow and String" theory, this deposit might be responsible for squeak arising under very light braking conditions. Under the division (b) where severe braking conditions obtain, entirely different factors come into play. Temperatures well in excess of normal arise and tend to raise permanently the frictional value of the lining, a condition favourable to the production of squeak. At very high temperatures, there is a possibility of destructive distillation of the bonding materials occurring, and thus producing a deposit on the drum, which again tends to support the "bow and string" explanation of brake squeak, and provides a factor common to both diverse sets of conditions. The effects under (b) can only be overcome by increasing the braking area or by improving the heat dissipation from the drum, thus preventing such high temperatures arising. Seeking further common factors, it occurs to us to enquire whether there is a possibility for torsional vibration to be transmitted to the brake assembly from the friction-servo, which would produce squeak according to | ||