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 on crankshaft stiffness, inertia, and frequency related to increasing pin diameter.
| Identifier | ExFiles\Box 6\1\ 01-page141 | |
| Date | 1st December 1927 | |
| X5010 Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}/LGS.12.27. CRANKSHAFT STIFFNESS. With reference to R1/M23.11.27. In an endeavour to obtain a value for increasing the dia. of the 20 HP. pins on the shaft period, we made the following approximation. Total effective inertia concentrated at shaft end is :- Spring drive hub....... .00582 Big ends total......... .00266 Half pistons........... .00187 Crankshaft............. .00561 ------- .01596 Of this the big end contribution is .00266 or 16.8%, the piston pins .0013 or 8.4%. Now if we increase the pin dia. from 1.5" to 2" and increase the hole in the pins from .75" to 1.00". From the tests given in Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}/LGS.12.27 we expect to increase the shaft stiffness by 40% and this alteration, by reference to all the big ends dimensions we already have, will alter the inertia as follows :- WEIGHT | | 1.5" dia.pins. | 2.0" dia.pins. | |-----------------|----------------|----------------| | Big end. | 1.187 | 1.900 | | Shaft pins. | .56 | 1.02 | | | ------- | ------- | | | 1.747 | 2.92 | or 70% total increase on pin and big end. This increase the total inertia of the crankshaft system by 70% of 25.2% or 17.5%. Old shaft frequency = 1/2π √(29900 / .01596) = 193 osc/sec. New shaft frequency = 1/2π √(41860 / .0237) = 1/2π √(1770,000) = 1330 / 2π = 212 osc/sec. or an increase of 10%. | ||