Piston rods should be of slightly diminishing diameter at the ends, so that the wear shall not leave a shoulder at each end of the rod.
In determining the diameter of the piston rod, allowance is made for turning it occasionally in the lathe to restore its parallelism, the wear reducing its diameter more in the middle than at the ends. The diameter of a piston rod is found in practice to range between one-sixth and one-tenth the diameter of the cylinder bore.
Steel piston rods wear better than those of wrought iron, being free from scaly seams which are apt to cut the packing and cause the rod to wear in grooves.
The best method of securing a piston rod to a piston head and to the cross head is by a taper seat and a key, so that no nut is needed, and the cylinder cover need not have a recess to receive the nut when the piston is at the end of the stroke, and the amount of clearance is correspondingly reduced.
Piston head key ways are sometimes given so little clearance that the key completely fills the keyway when driven fully home. This prevents the edges of the keys from bulging into the clearance space in the keyway, which action is apt to cause the key to loosen in time. The key should have a safety pin at its small end.
When piston rods are threaded into the cross head, or into the piston, the threads are made an easy fit, and taper seats or split hubs secured by clamping screws are relied upon to keep the rod true to the cross head or piston, it being found that the screw alone cannot be relied upon for this purpose.
PISTON ROD PACKING.
Piston rod packing, of fibrous or similar material, should be cut in rings that will not quite fully envelop the piston rod, and the first ring should be placed with its split upwards. After two or three rings have been inserted, each having its split at a different part of the bore, so as to “break joints,” the gland should be screwed up enough so as to carry the packing home to the back of the stuffing box. This process should be continued until the stuffing box is filled for about two-thirds of its depth, when the gland may be screwed home.
The gland should be screwed up quite evenly, so that the packing in the stuffing box shall be compressed equally all around the rod, and will not cause the gland to bind on the rod or in the stuffing box bore.
The wrench should be applied first to one nut, giving it a turn or two, and then to the other, and after the gland is firmly home the nuts should be eased back about two turns.