Fig. 3350.
The importance of a proper adjustment of the axle boxes, coupling boxes, and connecting rods cannot be overestimated, and it is necessary therefore to explain it thoroughly. In [Fig. 3350], then, s, s represent two wheel axles, whose boxes are between their wedges. At s, s′ are the screws for setting up the shoes or wedges w and w′ respectively. The axles are shown on the line of centres c, c of the engine, the piston being at the head end of the cylinder, and the crank pins on the line of centres as denoted by the small black circles. The wedges w and w′ are shorter than the leg of the pedestal, so that they may be set up by the set screws s and s′, and take up the wear.
In some engines the wedges v and v′ are also shorter for the same purpose. Now it is clear that setting up the screws s and s′ will move the axles s, s′ to the left, and this will alter the clearance between the piston when it is at the end of the stroke and the cylinder cover.
It is clear that the distance between the centres of the two axles must be the same as the distance between the centres of the two crank pins, or else the frame will be subjected to a great strain, tending to break the crank pins and the side rods.
In order to keep the clearance equal and to know when it is equal, it is necessary, at some time when the cylinder cover at the head end is off, to disconnect the connecting rod and push the piston clear up against the left hand cylinder cover, and from the cross head as a guide, make on the side of the guide a line l′. Then put on the cylinder cover at the head end and push the piston up against it and mark a line l. Then when the connecting rod is put on again, the wheels may be moved around if the engine is jacked up, or, if not, the engine may be moved along the rails with a pinch bar, and the clearance will be equal when the cross head (at the ends of the stroke) comes within an equal distance of the respective lines l′, l when the crank is on the dead centres, and it is well to adjust the wedges w w′ so that the cross head does travel within an equal distance, and mark on the guide bar two more lines, one at each end of the bar.
These lines are a permanent guide in setting up the shoes or wedges, and lining up the connecting rods, and coupling or side rods, because it is clear that from the method employed in marking them the distance between the end of the cross head, when at the end of its stroke, and the line l, and that between the face of the piston and the cylinder cover, will be equal.
A proper adjustment, therefore, should be made as follows: The piston should be at the end of its stroke, the crank pins being on the line of centres.
Screw s should be operated to set up the wedge w, taking up the wear of the sides of the box, and bringing the edge of the cross head the proper distance from the line l. The connecting rod brasses should then be set up to fit the pins, and the screw s′ operated to set up wedge w′ to have easy contact with the side of its axle box. If, however, there has been so much wear on the axle boxes that they are still too loose between the wedges, both wedges may be set up to take up this wear, since it is more important to have the axle boxes a proper fit between the wedges than it is to maintain an exactly equal amount of clearance at each end of the cylinder.