When the rod requires repairing a more serious difficulty arises. Suppose, for example, that the strap requires refitting to the rod, then it must evidently be closed between the jaws, especially if the rod end requires filing up, as it usually does. Now the jaws being parallel cannot be closed without being taken to the blacksmith shop and closed across the crown, as at a in [Fig. 2357]; for if the jaws are closed (as they might be) by pening the corners b, c the jaws would close as denoted by the dotted lines. The brasses will have to be made larger than the diameter at d, in order to fill the space at a, and will be an easier fit as they pass from d to a, whereas the opposite should be the case. The strap must therefore be closed across a in the blacksmith’s fire; this will scale the crown end and render it necessary to file down the whole of the surface on each of the side faces of the strap and rod in order to make them parallel, as they must be to have the flanges of the brasses fit when home in the strap.
Fig. 2357.
The blacksmithing will in most cases render it necessary to file out the keyways, and this again entails the making of a new gib and key. All this extra work may be avoided by making the block and strap a little taper. But before proving this it may be noted that when the rod is made parallel the strap may be made to fit tightly by making the jaws taper, as shown by dotted lines in [Fig. 2357]; so that when the strap is on the rod, and the jaws spring open by reason of the close fit, the fitting surfaces will be parallel. Such a construction would be faulty however, for the brasses would fit too tight when entering the strap, and get easier as they passed to their places, whereas, as already stated, the exact opposite should be the case.
Fig. 2358.
Let us now observe the advantages of a strap, whose inside faces are made as in [Fig. 2358]; smaller at a than at b, and also at c than at d, while the thickness from a to b is greater than that from c to d, while the widths c d are less than the corresponding width of the rod.
First, as to fitting the strap to the rod. It may be made so tight to the rod that it will only just pass on when pushed by the hand.