Fig. 119.—Parts of Sturtevant Aviation Engine. A—Cylinder Head Showing Valves. B—Connecting Rod. C—Piston and Rings.
One of the important conditions in piston design is the method of securing the wrist pin which is used to connect the piston to the upper end of the connecting rod. Various methods have been devised to keep the pin in place, the most common of these being shown at [Fig. 117]. The wrist pin should be retained by some positive means which is not liable to become loose under the vibratory stresses which obtain at this point. If the wrist pin was free to move it would work out of the bosses enough so that the end would bear against the cylinder wall. As it is usually made of steel, which is a harder material than cast iron used in cylinder construction, the rubbing action would tend to cut a groove in the cylinder wall which would make for loss of power because it would permit escape of gas. The wrist pin member is a simple cylindrical element that fits the bosses closely, and it may be either hollow or solid stock. A typical piston and connecting rod assembly which shows a piston in section also is given at [Fig. 118]. The piston of the Sturtevant aëronautical motor is shown at [Fig. 119], the aluminum piston of the Thomas airplane motor with piston rings in place is shown at [Fig. 120]. A good view of the wrist pin and connecting rod are also given. The iron piston of the Gnome “Monosoupape” airplane engine and the unconventional connecting rod assembly are clearly depicted at [Fig 121].
Fig. 120.—Aluminum Piston and Light But Strong Steel Connecting Rod and Wrist Pin of Thomas Aviation Engine.
The method of retention shown at A is the simplest and consists of a set screw having a projecting portion passing into the wrist pin and holding it in place. The screw is kept from turning or loosening by means of a check nut. The method outlined at B is similar to that shown at A, except that the wrist pin is solid and the point of the set screw engages an annular groove turned in the pin for its reception. A very positive method is shown at C. Here the retention screws pass into the wrist pin and are then locked by a piece of steel wire which passes through suitable holes in the ends. The method outlined at D is sometimes employed, and it varies from that shown at C only in that the locking wire, which is made of spring steel, is passed through the heads of the locking screws. Some designers machine a large groove around the piston at such a point that when the wrist pin is put in place a large packing ring may be sprung in the groove and utilized to hold the wrist pin in place.
