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).
Analysis page on aircraft valve mechanisms, with graphs illustrating cam lift curves and seating velocities.
| Identifier | ExFiles\Box 158\5\ scan0021 | |
| Date | 1st March 1939 | |
| March, 1939 AIRCRAFT-VALVE MECHANISMS 111 Fig. 4 - It will be noted from this enlarged section of the closing event of Fig. 3 that the valve contacts the seat at very close to the desired velocity. Fig. 5 - This enlarged section of the closing side of both types of cam brings out the higher deflection of the high-acceleration type artificial conditions and are open to criticism, but are indicative, we believe, of the effect of high seating velocities. To compensate for the variable clearance between tappet and valve under different operating conditions, the designer makes the cam with a so-called ramp that permits reduced velocities at the time of valve closure. The height of this ramp is accepted generally as being equivalent to the amount of clearance required under the conditions where this clearance is maximum. However, other variables affect the requirements and, to design properly for them, a formula has been worked out, taking into consideration the particular items that affect this condition. Fig. 7 illustrates this point, and it will be noted that, with certain designs, the required ramp to give calculated seating velocities must be considerably over that required for clearance take-up only. The relation of the valve to its seat is influenced by the amount of distortion that occurs in the port shape proper and also in the out-of-square condition that can exist between the seat and the valve guide, either due to valve-seat deck or guide change. Unequal stresses set up by mechanical as well as thermal loads can cause these distortions to occur, and the net result is the valve striking the higher side of the seat well up on the flank of the cam. A condition could exist where the velocity would far exceed that calculated and would be very difficult to detect. Assuming that the clearance allowed was 0.020 in. cold without allowing for the necessary addition required to take care of tipping that should be calculated according to formula on Fig. 7. (This, we will assume, would be 0.006 in.), during operation the relation between the guide and cylinder seat shifts to the extent that the result would be equivalent to another 0.004-in. clearance. Now, if the load were reduced so that the valve cooled and tended to increase its clearance, approaching the cold condition, a valve would strike the seat at a point equivalent to 0.025 to 0.030 in. clearance. At this clearance it will be noted on Fig. 2 that the velocity is approximately 5.5 fps which would certainly be far beyond anything that the designer had allowed for. The result of out-of-squareness and high velocity is illus- Fig. 3 - This lift curve is of a cam with approximately one-third the acceleration of that shown on Fig. 1. Graph Labels (Fig. 4): Lift in Thousandths Camshaft Deg Closing Side of Low Acceleration Cam 500 rpm 2050 rpm 2400 rpm Graph Labels (Fig. 5): Lift in Thousandths Camshaft Deg Low Acceleration High Acceleration At 2400 rpm Graph Labels (Fig. 3): Lift in Thousandths Camshaft Deg Lift Curve of Low Accelerated Cam 500 rpm 2050 rpm Low Speed High Acceleration Low Speed Low Acceleration | ||