Suppose the joint to be at an angle as at a, a, [Fig. 2401], instead of square across, as denoted by the dotted lines b, b, then the respective brasses will be forced by the key-pressure in the direction of the respective arrows, and there will be a tendency to twist the brasses in the strap. Or suppose the joint faces to be out of square as at c, c, instead of square as at d, then there will be a tendency to twist the respective brasses in the direction of e, f, and therefore to cause these to move in the direction of g, h, and as a result the brasses will spring the strap away from the rod, as shown at i, j.
Fig. 2402.
To line up the brasses for length we proceed as follows: One of the liners adjusts the length of the rod and the other simply serves to set the key back to its proper height, so that it shall not pass too far through the keyway, as the wear of the brasses lets it down. Which of the liners will be the one by which to alter or adjust the length of the rod depends upon the design of the rod itself; but, in the case of all solid-ended rods, or those in which the position of the strap is fixed by means of bolts, it is the liner behind the end brass, as d, in [Fig. 2402], as stated in the opening of this discussion, and it is the first one, therefore, to be fitted. The space at e is where the second liner requires to be placed, its thickness being that necessary to lift up the key from its bottom or lowest position, as shown in the cut, to the highest position.
Fig. 2403.
In strap-ended rods in which the strap is not bolted to the rod, but moves farther upon the rod as the key passes farther through the keyway, it is the brass next to the rod end, as b, in [Fig. 2403], by which to adjust the length of the rod, and its liner l is, therefore, the one to be fitted first; the space e is, in this case, the one to be fitted with a liner of sufficient thickness to lift the key up. It will now be noted that the thickness of l in both figures requires to be exact, so that the rod may be of correct length, which is necessary, so that there may be the same amount of clearance or space between the piston head and the cylinder cover when the piston is at the respective ends of the stroke. But the liners to fill the respective spaces e need not necessarily be of the exact thickness (although it is better that they should be), because if too thin the only effect will be that the key will pass farther through the keyway than otherwise. In considering in any form of rod which is the liner to be put in first to bring the rod to length, we have the general rule that the brass that moves in the strap or rod end when the key is moved farther through the keyway is the one to be lined last. The method of obtaining the proper thickness of the liners l, [Figs. 2402] and [2403], are as follows: If the rods have been correctly made at first, the centre of the brass bores will be midway in the spaces for the brasses (denoted by f in the two figures). If the oil-holes in the strap or rod end (as the case may be) have been drilled in the centre of this space f as they should be, then the line g will represent the centre of f and the centre of the oil-holes, and all that will be necessary will be to place behind d and b respectively a liner of sufficient thickness to bring the joint face of these brasses (d and b) even with the line g. To ascertain the thickness of liner necessary for this purpose, suppose the case of a rod end of the design shown in [Fig. 2402], then, with the strap off the rod, drive the brass d down until its crown face beds fairly against the strap c, and with a scriber mark on the inside face of the jaw of the strap a line coincident with the joint face of the brass, then set the brass up the strap until its joint face comes fair with the centre of the oil-hole or the central line g, and then mark a second line so that on taking the brass out of the strap there will appear two lines, and the distance between these two lines is the necessary thickness of liner. In the case of the form of rod end shown in [Fig. 2403], the process would be as follows: Let the strap have placed in it the brass b only, place it upon the rod, and set it so that it binds the gib and key, when the key is lifted up to its required position, then, with the brass b bedding fairly against the rod end, mark on the strap a line coincident with the joint face of the brass as before. Then move the brass in the strap until its face comes fair with the centre of the oil-hole or line g, and mark another line, and the thickness between these lines is the thickness of liner required at l.