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).
Engine boom issues in 40/50 & 25 HP models, detailing tests on balanced crankshafts and flywheels.
| Identifier | Morton\M19\ img089 | |
| Date | 15th December 1930 | |
| To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. to Sg.{Arthur F. Sidgreaves - MD} c. to DA.{Bernard Day - Chassis Design} E.{Mr Elliott - Chief Engineer} c. to EY. ORIGINAL ENGINE BOOMS 40/50 & 25 HP. We have been running chassis 24-EX. on the road with a balanced crankshaft and the engine single point mounted at the front and rear. We consider that the fully balanced crankshaft has an appreciable effect on the 60 m.p.h. boom period which is so troublesome in customers' cars. In our opinion the new engine mounting gets rid of some 60% of this boom and the balanced crank perhaps another 15%. We rather anticipate that we may have a little left in the exhaust and a little in the flywheel, and shall conduct tests to find out as soon as we have a test rig available whereby we can drive the car by - means of drums. The crank tested has 2.625 journals and 2.123 pins. Balance weights to drawing LEC.2616. It is noticeable that the critical speed of the crankshaft was lowered from about 100 m.p.h. with the plain shaft to 82/4 m.p.h. with the balanced shaft, shewing that if we are to fit balance weights the dimensions of the crank must again be increased to keep the critical speed outside the running range. It is suggested by Tsm. that the radius at which the mass of the balance weights operate should be as small as practicable as this keeps their moment of inertia as low as possible for any given balancing effect, thus keeping up the critical speed of the shaft. We have an optical set up working which indicates the movement of the 20 HP. flywheel by means of a beam of light reflected from a mirror in the flywheel centre. The standard flywheel shews a subsidiary period at 1900 R.P.M. which is 40 M.P.H. on top gear or about the speed where our booming troubles commence. From this speed upwards the flywheel never runs steadily, its second or main period at 2800 to 2900 R.P.M. being most clearly evident. This set up should enable us to get a quantitative value for the new lightened flywheel, balanced shaft, tail bearing etc., and we hope to let you have photographic records shortly. Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer} | ||