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).
Misunderstandings of 100% balance in four-bearing crankshafts.
| Identifier | ExFiles\Box 138\1\ scan0078 | |
| Date | 21th November 1933 | |
| X634 To E: from Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}14/KT.21.11.33. X-459 FOUR BEARING CRANKSHAFTS. There seems to have been some misunderstanding over what is meant by 100% balance of a crankshaft: We have always considered that for a seven bearing shaft this meant that one large weight exactly balanced one slab, half a crankpin, and half a connecting rod, the end and centre bearings then being relieved of all centrifugal loading. We agree that the other bearings have less balance except in the Chrysler scheme. On the four bearing shaft we consider the three units of pairs of crankpins. 100% balance then means that each unit is exactly balanced. It does not necessarily follow that the end slab balances itself and a half crankpin, but it will be very nearly so. The three end slabs must, however, balance themselves, two pins, and two connecting rods. On this understanding the four bearing shaft shown on LeC.3660 has only 60% balance, the same as the seven bearing shaft. The crankshaft originally submitted by us we find was not in static balance, being overbalanced in the middle and underbalanced at the ends. By the modifications shown this shaft balances the crankshaft and big ends on all bearings. This shaft does not possess the easy machining features of that to LeC.3660 and has only 2" instead of 2 1/4" pins. The varying web shape is necessary to obtain greater balance. From the point of view of balance and total inertia a small pin shaft gains greatly. There is less unbalance and more can be taken off the disc web on the side of the pins giving greater balance. Comparing the two shafts we get:- | ||