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).
Detailed description of the Spontan braking system, its operation, cooling, and interaction with other components.
| Identifier | ExFiles\Box 41\4\ Scan049 | |
| Date | 21th September 1931 guessed | |
| SPONTAN a real emergency. With a view to clearing the car floor as far as possible the brake handle is fitted on the dash board in the middle of the car, this position having the advantage of being easily accessible from both front seats. The Spontan braking system is shown diagramatically by Fig. 19, and the design of the propeller shaft brake is indicated by Fig. 20. Referring to the latter Fig., the pair of light-alloy shoes 49 expand inside the brake drum 39. The brake is operated from the manoeuvre pedal by the lever 47 which lowers the ring 40 pivoted at 41, thus by means of the roller 45 and the links 46 forcing the brake shoes apart. The brake shoe pivots 48 are not fixed but supported on the ring 42 which can turn on the ball bearing 43. The shoes when forced against the drum, are therefore at first carried with the drum, but this movement is soon checked by the links 44 attached to the brake shoes. These links transfer a couple of parallel forces which produce the braking effect in an ideal way without applying any unbalanced forces on the shaft or its bearings. During the turning movement of the brake shoes the roller 45 travels along the circumference of the ring 40. As the shaft and the ring are no longer concentric, the pressure of the ring against the roller becomes less the further the roller travels from its bottom position. At the same time the checking power of the link forces increases until the brake gear finally arrives in a state of equilibrium, corresponding to the momentary position of the ring. An increased braking effect is then possible only if the ring by means of the manoeuvre pedal of the car is moved further away from its central position. A sudden increase of the coefficient of friction between the brake lining and the drum causes the shoes to turn a little further, resulting in an automatical and instantaneous release of the brake pressure. Thus sudden jerks are prevented, the braking effect is to a great extent independent of the varying frictional coefficient, and the driver will find that a certain position of his manoeuvre pedal always corresponds to a certain braking effect. Brake adjustment is effected by moving the brake shoe pivots 48 by means of a wing screw, easily accessible from the driver's seat through a hole in the car floor. Experience has proved the wear of the brake linings to be unexpectedly small. A matter of vital importance in connection with the propeller shaft brake is the provision of a cooling efficient enough to prevent overheating of the brake even under the severest working conditions. A simple and satisfactory solution of this problem has been arrived at by providing the outside of the brake drum with numerous helical cooling ribs, as shown by Fig. 14. The ribs are so arranged as to screw the air between the ribs in the direction of the natural draught caused by the movement of the car. Due to the high speed of the drum circumference, a vigorous current of air is created between the ribs, causing a very efficient cooling of the drum. Fig. 19 shows the gear that enables the propeller shaft brake to apply the front wheel brakes when greater braking effect is required. 65 is the pair of links referred to on page 28 as forming the anchorage for the propeller shaft brake shoes. The tension in these links tends to turn the bell-crank levers 67 against the resistance of the check springs 66. When the braking effect exceeds a certain value, the tension of the check springs is overcome by the links. The bell-crank levers then begin to turn, thus operating the front wheel brakes by means of the brake rods 64. The front wheel brakes are of orthodox design with the exception that the principle of the yielding anchorage is employed, as shown diagramatically by Fig. 21. The front brake operating lever 54 forces the primary shoe 53 against the drum 52, and the rotation of the drum tends to carry with it the primary shoe, thus forcing the secondary shoe 57 against the drum. By means of the coil spring 56 the secondary shoe is flexibly anchored to the fixed pin 55, and | ||