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).
Mathematical investigation into the benefits of heavy crankshafts and stiff crankcases for the 25 HP engine.
| Identifier | Morton\M13.2\ img039 | |
| Date | 11th February 1931 | |
| ORIGINAL To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} c. to Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} c. to Da.{Bernard Day - Chassis Design} E.{Mr Elliott - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}1/MJ.11.2.31. X. 5000; X. 634. 25 HP. CRANKCASE Replying to your RS{Sir Henry Royce's Secretary}/M3.2.31. We have carried out some mathematical investigations into the benefits of heavy crankshafts and stiff crankcases. We had suggested that the reason for the small local crankcase distortions that occur when an engine is running, as opposed to the large ones we should expect from the experiments we have carried out on static deflections of bending, was the reciprocating force not having time to accelerate the mass of the crankshaft through more than a very small distance. We therefore investigated mathematically the behaviour of the crankshaft entirely unsupported and acted upon by the centrifugal forces of the crankpin and big end. We found that the centre of the crankshaft would run out of truth in a circle of 1" radius being inversely proportional to the mass of the crankshaft. Obviously therefore, we were wrong in suggesting that this might be the controlling factor. The stiffness of a crankshaft is negligible compared with that of the crankcase, hence we consider that it is the crankcase prevents local excessive distortion in six cylinder engines. This view does not contradict our report on static deflection where we say that the crankcase as a whole would deflect considerably under the calculated loads. If we now assume that the stiffness of a single web in compression and combined with the inertia of the crankcase is so small the only thing that could be responsible for controlling the crankshaft. We are therefore very hopeful that the Nitro Alloy balance weight shaft in the new 25 HP. will greatly prolong the bearing life and together with a stiff crankcase give engine stiffness that will be maintained better than the typical American cars, which start by being excellent but deteriorate rather rapidly. They have the stiffness but not the bearing life. As to the reduction in bearing loads due to balance weights, we find that the alternating forces at T.D.C. are only halved by the complete balance of the crankshaft and big ends so as the inertia force is about equal to the centrifugal force. Therefore we still need the stiff crank-case to give smoothness to our balanced crankshaft. Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} | ||