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 analysis of cam contours, clearances, and their effects on tappet noise and valve velocity.
| Identifier | ExFiles\Box 102\5\ scan0154 | |
| Date | 11th December 1933 | |
| -3- Ha/EJH.11/KT.11.12.33. If the cams conformed perfectly to 1eG.3292 these velocities would correspond to clearances of 0.004" and 0.0075" respectively. Assuming a limit of accuracy of .002" for the base circles then under the worst conditions the max. permissible clearance would be .002" and .0055". Sheet 7. shows the toe contours to 1eG.3292. and the corresponding velocity diagram. This toe contour produces a uniformly increasing velocity so that as the clearance increases the tappet noise increases. Suggested alternative opening and closing toe contours are also shown. These produce a heavy initial acceleration and then constant velocity until the lift reaches .010". The values of this constant velocity are respectively .0003" per degree and .0005" per degree. The closing toe would be longer by about 5 crankshaft degrees and the opening toe shorter by 7 degrees. These toes should give a constant small degree of noise for any clearance up to .010". It is essential that cams should not have any irregularities such as measured on Unit. B.7. The presence or otherwise of these irregularities should be definitely established by more accurate measurement. Some of the cams on B.7 show also large deviations from the designed shape, and these have the effect of reducing the closing velocity for clearances up .0025" and increasing it for larger clearances. It is possible that the impact of an exhaust valve on its seat plays a large part in breaking off the head at high temperatures and lower closing velocities would help in this respect. Ha/F.J.Hardy. | ||