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).
To Mr. Claude Johnson discussing piston skirt design, thermal expansion, clearances, and issues with gudgeon pin locking screws.
| Identifier | ExFiles\Box 148\3\ scan0006 | |
| Date | 24th March 1921 | |
| Oy10 - G 24321 Mr. Claude Johnson - Sheet 2. The expansion of the skirt due to its temperature coupled with the contraction in the large diameter of the skirt due to the expansion of the head, as explained above, will give an approximately constant clearance on the large diameter of the skirt. In addition to this, the skirt being largely separated from the head and being of oval shape, will, if it expands considerably, adapt itself to the circular cylinder bore by pushing out the small diameter of the skirt without attaining such a high surface pressure on the fitting diameter of the skirt as to cause seizing. In short, the skirt is made as flexible as possible and is so attached to the head that expansion of the head tends to increase the clearance of the skirt. We should be glad to know whether you have made any experiments at Derby on this type of piston, because it appears to us to offer advantages over our existing type, in which we rely for close fit of the skirt on the use of the skirt virtually as a piston ring, although we know that an aluminum alloy is not a suitable material of which to make a piston ring. The attached sketch will illustrate the type of piston referred to. In putting our trial set of pistons through the shop, we found that one of the pistons was .006 oversize on the large diameter and .020 undersize on the small diameter, and traced this to the fact that the gudgeon pin locking screw which goes in the clearance hole in the piston boss was actually binding on the inner side of the hole due to a slight inaccuracy in locating the hole either in the gudgeon pin or the piston. On taking out this screw the piston returned to its normal shape. The clearance hole allows nominally .016 of clearance over the diameter of the locking screw, but we feel that this clearance is not enough, because we find that in many of our present pistons the locking screw has been actually cramping the piston, due to distortion of the piston in drilling or to distortion of the gudgeon pin in hardening. The effect of this must be to prevent the piston expanding on its small diameter when it gets hot, so that the whole of its expansion must act on the large diameter of the skirt in such a way as to close the longitudinal slot. Even if the locking screw were exactly central in the clearance hole, we think that the .016 clearance provided would be only barely sufficient to take care of the difference between the coefficients of expansion of the aluminum piston and the steel gudgeon pin. Although we have not been able to thoroughly investigate this matter as yet, we believe that this cramping of the piston on the small diameter may account for some of the collapsing of pistons when running the engine. | ||