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).
The calculation of bearing loads for a 3 bearing, 4 throw crankshaft in a 4-cylinder engine.
| Identifier | ExFiles\Box 115\2\ scan0497 | |
| Date | 26th October 1939 | |
| 1020A 111b Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden}9/JH.26.10.39. BEARING LOADS FOR A 3 BEARING 4 THROW CRANKSHAFT. The ensuing calculations have been made for a 12 H.P. 4 cylinder engine of known dimensions. Weights of the piston, connecting rod etc. have been deduced from known parts of similar size, and for the calculation of loading due to gas pressure a typical indicator diagram with explosion pressure 500 lb.sq.in. (gauge) has been constructed. A Polar loading diagram at 4500 r.p.m. for a big end bearing was constructed, taking the resultant of inertia and gas pressure loading. By resolving these loads parallel to the crank and having knowledge of the firing order polar loading diagrams for the main bearings were obtained. The load from each piston was assumed to be taken by the nearest bearing on either side, in quantity indirectly proportional to the distance between centre lines of piston and bearing. Thus the centre main bearing takes about 2/3 of its loading from Nos. 2 and 3 cylinders and 1/3 from Nos. 1 and 4. Whereas the front bearing takes 2/3 from No.1 and 1/3 from No.2. This results in the centre bearing having to take about twice as much load for most of the revolution, and incidentally means that the centre bearing has a loading cycle of 360° compared with 720° for each outer bearing. The big end polar diagram at 4500 r.p.m. is interesting due to the high inertia forces, inertia load being 75% of gas load at T.D.C. on the firing stroke. The greatest load on the bearing is 1500 lbs.sq.in. due soley to inertia at the end of the exhaust stroke, and the load is at a minimum just before the maximum explosion pressure, when the rise in pressure has overcome the inertia force. For the sake of comparison a Bentley V big end loading diagram at 4250 r.p.m. was drawn. This shows similar characteristics and loading save in the neighbourhood of exhaust T.D.C. where the pressure on the bearing reaches 1750 lb.in.sq. The main bearing diagrams at 4500 r.p.m. also show great reduction of loading due to the explosion. This effect is most noticeable on the outer bearing diagram due to the cylinder nearer the bearing, and a kink shows at a point 540° further round due to the firing of the further cylinder. On | ||