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).
Calculations and analysis of a servo-assisted braking system, focusing on friction, load, torque, and thread angles.
| Identifier | WestWitteringFiles\L\Jan1924-March1924\ Scan127 | |
| Date | 27th March 1924 guessed | |
| (2) So we appeared to have a margin even with this angle of thread. But we realised that the co-efficient of friction was the doubtful point, and so on LeC.1628. Mr. Stent put a note pointing out that threads with less angle would probably have to be tried. (In this type of arrangement it is necessary that the pressure screw lever should go through the working angle of the servo, and thus must be about 2" long.) In REM. scheme rather more braking is asked for from the servo. You instructed us to divide the brake work into 8 parts - viz, 2 parts due to pedal and 6 parts due to servo. Allowing about 8% off the pedal load for servo operation this would make 1 part equal 350 in. lbs., and 6 parts 2100.in lbs. of work. On N.sch. 1769. servo lever goes through 75° or 3.65" travel. Load = 2100 / 3.65 = 575 lbs. Servo torque = 575 X 3 = 1725 lbs. ins. M.{Mr Moon / Mr Moore} R.{Sir Henry Royce} = 2.9 End pressure required = 1725 / (2.9 X .2 X .35) = 850 lbs. Pedal pull = 380 lbs. Torque on pressure screw = 380 X 1.8 = 684 lbs. ins. Load in plane of thread = 684 / .575 = 1190 lbs. At 82% efficiency and 45° angle of thread end pressure = 975 lbs. So we still appeared to have a margin, and considered that if this was not enough, the thread could be dropped to 30° giving us 1700 lbs. or twice the pressure, or half the co-efficient we were reckoning on. But the lever length which gave us a 45° thread (viz 1.8) seemed tobe the one to be aimed at, and had the advantage of keeping the pieces more cobby and giving us a nearly straight run of the pull rods. But the co-efficient of friction with the flat plates seems to be less for some reason, than with drums. One point we have thought might be causing an apparent drop in the co-efficient of friction, and that is the jaws tying the two pressure plates together at such a small radius, and also sliding bearing friction. | ||