SHRINKAGES OF COILS OF 12.5 INCH R. M. L. GUNS.
| Coils. | Shrinkages. | Remarks. | |||||||
| In Inches. | In terms of diameter. | ||||||||
| Rear. | Front. | Rear. | Front. | ||||||
| Breech‑piece | .02 | 2 | .02 | 6 | D | D | Shrunk on | A tube. | |
| 857 | 807 | ||||||||
| B coil | .05 | 5 | .01 | D | D | „ | „ | ||
| 561 | 190 | ||||||||
| B tube | .03 | 5 | nil. | D | nil. | „ | „ | ||
| 668 | |||||||||
| C coil | .03 | .06 | D | D | Shrunk on to breech piece and rear end | ||||
| 1134 | 729 | ||||||||
| of 1 B coil.” | |||||||||
The objections to fitting work by contraction where accuracy is required in the work are, that if the enveloping piece is of cast iron its form is apt to change from being heated. Furthermore, if the enveloping piece, which is always the piece to be heated, is of unequal thickness all round the bore, the thin parts are apt to become heated the most, and to therefore give way to the strain induced by contraction when cooling, which, while not, perhaps, impairing the fit, may vitiate the alignment of parts attached to it. Thus, a crank pin may be thrown out of true by the alteration of form induced first by unequal heating of the metal round the crank eye, enveloping the shaft; and secondly, because of the weakest side of the eye giving way, to some extent, to the pressure of the contracting strain. To counteract this, the strongest part of the enveloping piece should be heated the most, or if the enveloping piece be of equal strength all round its bore, it should be heated equally all round. To effect this object heated liquids, as boiling water, or heated fluids, as melted lead, may advantageously be employed.
In some practice, locomotive wheel tires are heated for shrinking in boiling water. The allowance for shrinkage is from .075 millimètre to every mètre in diameter, which is .02952 inch to every 39.37079 inches of diameter.
The employment of hot water, however, necessitates that the tires be bored very smoothly and truly, and that the wheel rim be similarly true and smooth, otherwise the amount of expansion thus obtained will be insufficient to maintain a permanent fit under the duty to which a wheel tire is submitted.
Shrinking is often employed to strengthen a weak place or part, or one that has cracked. The required size is, in this case, a cylindrical surface that is not a true cylinder, obtained by a rolling wheel rotated by friction over the surface to be enveloped by the band. Or if the surface is of a nature not to admit of this, a strip of lead or piece of lead wire may be lapped round it to get the necessary measurements.
The bands for this purpose are usually of wrought iron, and require in the case of irregular surfaces to be driven on by hammer blows, so that the fit may be correct. As the band is forced on a heavy hammer is held against it, to prevent its moving back and off the work as the other parts are forced on.
Fig. 1428.