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).
Technical analysis of the causes of brake squeaks and potential cures.
| Identifier | ExFiles\Box 85\1\ scan0269 | |
| Date | 2nd December 1936 | |
| -2- not allow us to give accurate μ figures on this subject. As to outside evidence on this point, it is generally agreed that Bendix brakes using a lining with a μ of about .2 do not readily squeak, and asbestos firms assert that squeaks are often associated with high μ values. (3) We have abundant evidence that over-heated brake linings squeak badly. For one thing any standard 4¼, or even 3½-litre Bentley, if driven hard on a twisty road, the next day squeaks appallingly. On these cars the friction lining is MR. On our rig we can make MR or FBA or S.3805 emit piercing squeals if we run it up to 300°C several times. (These are temperatures easily reached on a Bentley.) (4) We have not much evidence that bad bedding produces squeaks, although Rm{William Robotham - Chief Engineer}/GWH.{George W. Hancock - Head Chateauroux} says it does. The bedding of Bentley and 25/30 H.P. brakes is bad because of the flexible drums, but they do not all squeak. We bed our brakes to match drums under no load, but under brake loads the drums go egg shaped to the extent of some .030" in certain cases and the Ferodo wears itself to a new bedding. It frequent-ly wears away the tip of the tipping shoe, so that instead of touching first on light applications (which is when most squeaks occur), it does not touch at all. The bedding of the Girling brakes on 1.B.IV is bad. This is because we use aluminium drums, aluminium shoes but a steel carrier plate which does not expand as much as the shoes. Bedding on the road takes place under operating temperatures, but on checking the bedding when the drums are cold with red marking, the pressure would appear to be only at one end of each shoe. Nevertheless, this brake system is effective and does not squeak (as a general rule). The only case of squeaks which might be due to bad bedding which we have experienced occurred recently when 6.B.IV came out of the shop with aluminium drums and its brakes just relined. It squeaked quite badly, so we drove it hard for a few miles and the squeaks stopped and did not come back. As it had aluminium drums we could not overheat the brakes by this procedure. This is all we know at present about the causes of brake squeaks. The cures, or possible cures, are as follows:- (1) Rust. With our standard drums no change of material is worth while. Many of them squeak so badly for other reasons that any improvement in this direction would not be noticeable. | ||