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).
Road test report on a 25 HP large pin crankshaft to determine its critical speed and the effects of balance weights.
| Identifier | ExFiles\Box 181\M13\M13.2\ img051 | |
| Date | 9th May 1931 | |
| To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. to OG. Sor. c. to DA.{Bernard Day - Chassis Design} E.{Mr Elliott - Chief Engineer} c. to Ey. Hs{Lord Ernest Hives - Chair}/Thn.3/RT.9.5.31. X5010 LARGE PIN 25 HP. CRANKSHAFT. We have been carrying out road tests on this crankshaft to determine its critical speed without a damper. The principle dimensions of this shaft are as follows - Piece No. S.55819. Balance Weights. sch.1402. Shape of web 3 1/4" oval. Diameter of journal 2.125". Diameter of pin 2.000". Nose of crankshaft prepared for low inertia damper. We first of all tested the crankshaft with 12 balance weights as per sketch. Marked torsional periods were present at 28, 42, and 56 MPH. on accelerating at full throttle, the one at 42 MPH. was also present to a reduced extent on the overrun. This is the most violent of the three and is the half speed one. Without the slipper wheel it only extended over 2-3 MPH. From this we deduce that the peak of the master period should be at 84 MPH. on top gear or 4200 RPM. Actually, however, when running up at 3rd. speed the period becomes very noticeable at 52 MPH. i.e. 3800 RPM. The breadth of the master period appears to be considerable, and in this case 400 RPM. below the critical speed the engine became unpleasant. While conducting this test, we took the opportunity of observing the rattles in the side shaft gears. Since these occurred at 38 - 39 MPH. on top gear and were worse on the overrun that the forward, we deduce that they are some function of the transmission and not of the crankshaft torsional oscillation. On removing the balance weights we found that the half torsional period on top gear had risen to 50 - 51 MPH. or a critical crankshaft speed of 5000 RPM. and appeared to be slightly more spread out with the balance weights fitted. From these figures we deduce that with the eight balance weight scheme on this crankshaft the critical speed should be sufficiently high for this shaft to take care of the increase in revs. with twin carburetters and free exhaust system. Possibly we shall find it necessary to reduce the weight of the pin slightly and correspondingly the dimensions of the balance weights. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||