Calculations and justifications for the lever lengths in an agreed brake system servo.
| Identifier | WestWitteringFiles\L\Jan1924-March1924\ Scan126 | |
| Date | 27th March 1924 | |
| TO R. {Sir Henry Royce} FROM DA. {Bernard Day - Chassis Design} ORIGINAL SECRET DA {Bernard Day - Chassis Design} 1/M27.3.24. Copy to - HS. {Lord Ernest Hives - Chair} " HY. {Tom Haldenby - Plant Engineer} " PM. E. {Mr Elliott - Chief Engineer} A. {Mr Adams} C. 49940 AGREED & RRM. BRAKE SYSTEM. Referring to your remarks re. the length of pressure screw operating levers on N.scheme 1769, and on the agreed scheme which was on a car which came to le Canadel(Henry Royce's French residence), the following were the reasons which caused us to adopt these lever lengths. (1) In your R5/M171223, there are given us instructions as to the working of the slow speed servo, viz:- 750 lbs. ins. of work from pedal:(for 150lbs foot pressure.) 1500 " " " servo. ------- Total 2250 " " " altogether. Length of servo lever to be 4". Length of lever on pressure screw to be 2". Servo lever to go through 60°. Pressure screw lever through 75°. In laying out the agreed scheme we closely followed these instructions except that the servo lever was made 4.2" long to give us a little clearance we wanted, and we found it very difficult to get the pressure screw lever to go through more than 70° because we could not get more than 2.2" linear motion off the pedal lever. But we thought this 10° would be enough to close the servo, and we might still count on the servo lever going through 60°. Then the following figures followed:- Pull from pedal 380 lbs. Torque on pressure screw 380 X 2 = 760 lbs. ins. Load in plane of screw 760 / .55 = 1380 lbs. Taking 55° angle of thread (perpendicular to axis) and an efficiency of 82% - End thrust = 1380 X Tan 35 X 82 / 100 = 793 lbs. Servo work = servo torque = 1500 lbs. ins. M.R. = 2.9". μ = .35 End thrust required - 1500 / (2.9 X 2 X .35) = 740 lbs. (1) | ||
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).