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).
Valve lift curves, cam timing adjustments, and the design considerations for making camshafts interchangeable.
| Identifier | ExFiles\Box 43\3\ Scan137 | |
| Date | 11th March 1927 | |
| X4003 BYL/C.11.3.27. Hs{Lord Ernest Hives - Chair} RECEIVED MAR 11 1927 R.R.F.10. VALVE LIFT CURVES FOR ROCKERS WITHOUT ROLLERS. With reference to E5/M7.3.27. the time period of the sharp acceleration was decreased as suggested in your E1/M.15.2.27. It appeared to us that the object was to reduce the total length of the exhaust valve period by the same amount and thus obtain the timing suggested by R.{Sir Henry Royce}, i.e. exhaust to open 60º before B.D.C. We have discussed this matter with Rg.{Mr Rowledge} who is of the opinion that we should aim at this timing, as the present position of exhaust opening at 68º is rather early. In regard to the size of grinding wheel, we have made large layouts of both the inlet and exhaust cams, which we have produced to give the valve curves shown on Schs.888 (inlet) and 889 (exhaust) from which we find that the grinding wheel for the inlet could be as much as 4 1/4" diameter, and 4" diameter for the exhaust, but since the existing grinding machines are only suitable for grinding wheels up to 3" diameter this size will be used. There should be no difference in timing between 'A' unit and 'B' unit, as the angle between the inlet and exhaust cams is adjusted by twice the offset of the cam follower centre, as shown on Scheme 881 - this amount being added to or subtracted from the theoretical angle between the cams according to the position of the curved follower relative to the direction of camshaft rotation. The point you are referring to is, we think, a suggestion put forward by the Experimental Department, that since the camshafts rotate in the same direction the cams strike the rockers on 'B' unit at a greater distance from the rocker pivot than is the case on 'A' unit, which has the effect of displacing the valve lift curve slightly on one side as compared with the other, although the actual points of opening and closing of the valves are not affected. Although this view appears to be confirmed by the actual valve lift curves plotted for both units from actual measurements, we are so far unable to see that this small displacement can be due to the fact that the cams strike the rockers on one side on 'B' unit, and on the opposite side on 'A' unit. Regarding the question of making the camshafts interchangeable, we are allowing the Experimental Department to produce the first set in accordance with the present scheme, but we propose to look into the question of arranging the pads to line up, and altering the rocker radius from 2.225" to 2.200" as it would certainly be an advantage to have interchangeable camshafts. BY.{R.W. Bailey - Chief Engineer} | ||