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).
Technical memo discussing the effects of crankshaft pin size and balance weights on bearing loads and critical speed.
| Identifier | ExFiles\Box 182\M19\ img008 | |
| Date | 15th January 1931 | |
| (2) Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}2/KT.15.1.31. cont. how much pins of this size increase the bearing loads unless balance weights are fitted. This trouble did not occur on the smaller pin crankshaft when run at the same speed. It will be noted that our recent figures do not agree exactly with those taken in 14.11.29; however the earlier figures were obtained by running the crankshafts on the half speed periods without the slipper wheels and calculating the critical speed from the results, a method which is obviously inferior to that adopted in the present tests, further-more the previous tests referred to the old type slipper wheel. Clearly the critical speed the customer worries about is the one which takes place on the car as he will drive it i.e. slipper wheel fitted and this is the figure we give. Summarising our results:- The big pin 40/50 crank is 11 lbs. heavier than the standard crank, if it was balanced this difference in weight would be still greater. We cannot say that any advantage has been gained from this 10 lbs. as the crankshaft stands, one reason being that the pins are too heavy, it will be remembered that they were only bored with 1.00" dia. holes in this case. Our only objection to large pins are, (1) That the additional con. rod big end inertia necessitated by the larger dia. may more than counter-balance the increased shaft stiffness. (2) That if the pin is made too big it will not be feasible to drill a sufficiently large hole through it for lightening purposes. Thus in the crank sketch Tsm.16/12/30, it would be difficult to drill the pins larger than 1.250 whatever their outside dia. (3) In balanced crankshafts, big ends count nothing in stiffness but twice in inertia. We have a clear recollection of the bad results obtained on the 40/50, when we used 1.875" dia. pins bored 1.125 and heavily chamfered, (we consider the over chamfering was partly responsible for the bad characteristics) but we think our results shew that there may likewise be an uneconomical size in the other direction for a given journal dia. and stroke. It will be noted that the balance weights have lowered the critical speed on the 2.125" dia. crank by 660.R.P.M. this may be of some assistance in determining the critical speed of the other balanced crankshafts. We will run the 2.375" dia. pin crankshaft with bored out pins to see if we gain more inertia than we lose in stiffness. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||