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).
Engine tappet design, spring washers, and oil pressure application.
| Identifier | ExFiles\Box 147\1\ scan0036 | |
| Date | 27th March 1935 guessed | |
| - 2 - about 0.040 inch depending on the accuracy with which the individual parts have been machined in their lengths. This must be corrected by unscrewing the tappet screw by 0.040 - 0.004 = 0.036 inch or less than one turn of the screw. The spring washer will then not be as closely coiled as before but its proportions are such that this amount of easing produces no appreciable diminution in its strength. It is essentially my aim that any movement of these parts relatively to one another shall be limited to the amount of the tappet clearance, or slightly more so as to be on the safe side, and that the spring washers should be designed accordingly. I would therefore be much obliged if you would consider the "choc-a-bloc" criticism in the light of what I have just said. As regards another point, aggravated wear of the bottom tappet arising out of a constantly applied spring pressure instead of pressure applied intermittently. Theoretically the valve spring need only be so much stronger than the outer spring washer as may be necessary to overcome it and close up the clearance gap; in that case there would be practically no extra load on the bottom tappet and consequently no extra wear. To satisfy this rather theoretical condition is doubtlessly not feasible in practice, but I see no reason why the valve spring should not be designed so as to be only several pounds stronger than the washer and so produce an extra loading of the bottom tappet which can induce very little extra wear but which will nevertheless exert a quieting effect on the valve gear and be conducive to silent working. In this connection I would like to suggest that when once the dimensions of the spring washers and valve spring have been determined for any particular engine it is a matter of spring manufacturers' ordinary routine to reproduce them accurately and cheaply in large quantities. Referring to the whole question of applying oil pressure to motor car tappets which we also discussed I would like to suggest that in order to do this in the literal sense quite prohibitive oil pressures would have to be employed unless the diameters of the oil pressure chambers within the tappets were made equal to and perhaps even greater than the diameters of the valve heads, especially in the case of exhaust valves. I think it is correct to say that any of the so-called hydraulically adjusted tappets about which one has heard rely principally on some form of spring loading supplemented by oil injected under pressure. As far as I know the effect of the latter can never be other than negligible in a commercial motor car engine for the reason I have just given except in | ||