The sheave is generally made of cast iron. The strap may be of brass, cast iron, or wrought iron; when the strap is made of wrought iron it is commonly lined with brass.
Exercise 40: Locomotive Eccentric.—In fig. 40 D E is the sheave, F H the strap, and K the eccentric rod. The sheave and strap are made of cast iron, and the eccentric rod is made of wrought iron. (a) is a vertical cross section through the oil-box of the strap; (b) is a plan of the end of the eccentric rod and part of the strap. All the nuts are locked by means of cotters. Draw first the elevation, partly in section as shown. Next draw two end elevations, one looking each way. Afterwards draw a horizontal section through the centre, and also a plan. Scale 4 inches to a foot.
Fig. 40.
XII. CONNECTING RODS.
The most familiar example of the use of a connecting rod is in the steam-engine, where it is used to connect the rotating crank with the reciprocating piston. The rod itself is made of wrought iron or steel, and is generally circular or rectangular in section. The ends of the rod are fitted with steps, which are held together in a variety of ways.
Strap End.—A form of connecting rod end, which is not so common as it used to be, is shown in fig. 41. At (a) is shown a longitudinal section with all the parts put together, while at (b), (c), (d) and (e) the details are shown separately. A B is the end of the rod which butts against the brass bush C D, which is in two pieces. A strap E passes round the bush and on to the end of the rod as shown. The arms of the strap have rectangular holes in them, which are not quite opposite a similar hole in the rod when the parts are put together. If a wedge or cotter F be driven into these three holes they will tend to come into line, and the parts of the bush will be pressed together. To prevent the cotter opening out the strap, and to increase the sliding surface, a gib H is introduced. The gib is provided with horns at its ends to keep it in its place. Sometimes two gibs are used, one on each side of the cotter; this makes the sliding surface on both sides of the cotter the same. The cotter is secured by a set screw K. The unsectioned portion of fig. (a) to the right of the gib, or to the left of the cotter, is called the clearance or draught.
Exercise 41: Connecting Rod End.—Make the following views of the connecting rod end illustrated by fig. 41. First, a vertical section, the same as shown at (a). Second, a horizontal section. Third, side elevation. Fourth, a plan. Or the first and third views may be combined in a half vertical section and half elevation; and the second and fourth views may be combined in a half horizontal section and half plan.