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).
Article discussing side pressure on valve stems and poppet valve design.
Identifier | ExFiles\Box 31\2\ Scan080 | |
Date | 31th October 1914 | |
THE AUTOCAR, October 31st, 1914. 645 Poppet Valve Design in order to get the reamer to cut it might be necessary to cut under that hard surface, and therefore to remove more metal than would be advisable. It is only fair to say that the improper construction of the seat as shown in fig. 2 is usually the result of careless workmanship rather than of improper design. Side Pressure on Valve Stems. The side pressure on the stems of valves is often due to the fact that there is generally no adequate provision made for taking the side pressure which arises from the commonly used type of helical spring. It may not be feasible to alter the spring, but if the valve spring cup or retainer were free to tip in all directions it would compensate for any unequal pressure exerted upon it by the spring. This effect could be obtained if the cup upon which the spring rests were to be allowed to rest on a spherical surface on that part of the retainer which is directly attached to the valve stem. Helical springs are often made so that the ends of the coil do not bear on the spring cup through more than one-quarter of their circumference, thus setting up an unequal pressure; this is further aggravated if both ends of the spring wire ends happen to be on the same side of the valve, one bearing against the spring cup and the other against the cylinder. If one can assume that this unbalanced pressure may be present in a rather large diameter, coarse pitch spring, mounted on a rather small diameter valve which has a maximum amount of clearance in the valve stem guide, one can readily see that after the spring has seated the valve it could actually lift one side of the valve off its seat and either allow gas to pass the valve during the firing stroke of the engine as well as during the compression stroke, or at least release the pressure on one side of the seat so that the oil would remain on the seat. Of course, when either the compression pressure or the explosion pressure exceeded that of the unbalanced spring pressure, the valve would seat all around, but this would not prevent some loss of fresh gas before it took place. There is some such thing occurring is clearly shown by an examination of a valve which shows wear, as in fig. 3, and it is further aggravated by the fact that no amount of grinding will ever keep such a valve tight, because the grinding is done with the stem in a position which it never maintains when the spring is in place. If a proper form of spring retainer were used, with perhaps a similar form of seating for the spring where it bears against the cylinder, there should be no great amount of side pressure applied to the valve stem, and that most annoying leakage of fresh air into the mixture in the inlet passage, which comes from too much wear or too much clearance on the valve stem, would be less apt to cause trouble. The usual result of excessive leakage is that the mixture is so diluted that the engine misses fire frequently when throttled down and in the hands of the average user this is one of the most difficult sources of trouble to find and correct. The usual practice in a case where this misfiring occurs from this cause is to overcome it by enriching the mixture, but as this usually means that it is too rich throughout the entire range of engine speed, it will readily be seen that this remedy is a source of perpetual expense. Fig. 3. -- Dotted lines show valve stem wear due to side pressure. The writer has seen valve stems showing an almost unbelievable amount of wear, in some cases amounting to a good 1/32in., and the fact that this wear was as shown in fig. 3 seems to point to the necessity of some kind of a compensating device to keep extreme side pressure off the valve. Then, again, there are some types of tappets which, when they raise the valve, push it to one side, and if this were pronounced, it might account for a great deal of the side thrust. Attention might be given to this side of the question with profit. If the tappet be either of the direct acting type or of the intermediate pivoted lever type, and is not either equipped with rollers or other means for allowing the lifting effect to be applied vertically and not sidewise, it is almost sure to cause some side wear which could be avoided by slight changes in design. The type illustrated in the June issue of The Automobile Engineer, and which I believe is used on the Hispano-Suiza cars, seems to be one which meets the requirements in this respect. The foregoing considerations are all in the nature of refinements which will lead eventually to greater reliability and to a lessening of the upkeep expenses, and will also go far towards conserving that quietness of action which unfortunately is often absent in cars that have been run a year or so. | ||