Fig. 1230.
If after an eccentric strap is bored, and the bolts that hold its two halves together have been slackened, its diameter at a and at c, [Fig. 1230], be measured, it will be found that a is less than c. The cause of this is partly explained under the head of tension of castings; but it is necessary to add that the diameters at a and at c in the figure are equal while the strap is in the lathe, or until the bolts holding the two halves of the strap together are released, yet so soon as this is done the diameter at a will reduce, the bore becoming an oval.[18]
[18] This occurs in all castings of similar form, as brasses, &c.
Fig. 1231.
Now, it is obvious that the eccentric must be turned to the diameter at c, or otherwise it will have lost motion in the strap. If, however, the eccentric be turned to the diameter of c, the strap cannot be tried on, as it will bind at the corners, as shown in [Fig. 1231]. To remedy this evil it is usual to put a piece of sheet tin or metal between the joint faces of the two halves of the eccentric straps before they are chucked to turn them, and to bore them too large to the amount of the thickness of sheet metal so employed. After the straps are bored these pieces of metal are removed, and the strap halves bolted together as in [Fig. 1230], the diameter at c being that to which the eccentric must be turned.
If the sheet metal so inserted were thick enough, the strap bore will measure the same at a as at c, [Fig. 1230]. If it were too thick the diameter at a will be greatest, while if too thin the diameter at a will be the least. There is no rule whereby the necessary thickness for a given size of strap may be known, and the workman is usually governed by his experience on castings of similar metal, or from the same moulding shop.
He prefers, however, to be on the safe side by not putting in too great a thickness, because it is easier to scrape away the bore at the joint than it is to file away the joint faces. The following thicknesses for the respective diameters may be considered safe for castings that have not been reheated after casting.
| Diameter of bore. | Thickness of metal to place between the strap valves. | |
| Inches. | Inch. | |
| 6 | 1⁄64 | |
| 12 | 1⁄32 | |
| 18 | 3⁄64 | |
| 24 | 1⁄8 |