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).
Clearance and tolerance issues for the pins in a servo drive mechanism.
| Identifier | ExFiles\Box 152\2\ scan0263 | |
| Date | 20th October 1940 | |
| To Rm.{William Robotham - Chief Engineer} From Rm{William Robotham - Chief Engineer}/HHR. C. to Tarlton wvm. 1294 Rm{William Robotham - Chief Engineer}/HHR.4/LT.20.10.40. SERVO DRIVE. -------------- The trouble here is that the pins are made a very close fit in their mating holes in the driving plate, while their spacing is not correspondingly accurate. The remedy is therefore to increase the clearances on the pins. The servo shaft has already got adequate clearance on the pins. The driving pin has a dimension 0.3124 -1/4 diameter and the hole in the driving plate is 0.3125 -1 diameter; in other words the clearance is 0-1/2 thous. However, the positions of the pins are given as 1.200 +-1/2 P.C. diameter and the holes in the driving plate have the same dimension. Thus the distance between two pins can vary between √3 (R-.001) and √3 (R+.001), and so the difference between max. and min. can be 3 1/2 thous. Under the worst conditions there will be this difference between the pin spacing and that of the holes, so to allow for this, the pin clearances must be made nearly 3 1/2 thous. on diameter. By leaving the holes at 0.3125 -1 diameter and making the pins 0.310 -1/4 diameter, then the clearance will be 2 1/2 - 4 thous. With regard to the idea of using only one driving pin of the present size, this could do the job under normal conditions, but could not stand up to a panic stop, when a driving pin area of 0.003 sq.ins. is required, whereas the area of one pin is 0.0767 sq.ins. Rm{William Robotham - Chief Engineer}/HHR. | ||