Fig. 458.
For carriage bolts used in woodwork that turn with the nut notwithstanding the square under the head (as they are apt to do from decay of the wood or from the bolt gradually working loose) the form of wrench shown in [Fig. 458] is exceedingly useful, it is driven into the wood by hammer blows at a. The bevelled edges cause the jaws to close upon the head in addition to the handle-pressure.
Fig. 459.
For circular nuts such as was shown in [Fig. 411], the pin wrench or spanner wrench shown in [Fig. 459] is employed, the pin p fitting into the holes in the nut circumference. The pin p should be parallel and slope very slightly in the direction of a, so that it may not meet and bruise the mouths of the pin-holes, a, b, c. The pin must, of course, pass easily into the pin-holes, and would, if vertical, therefore meet the edge of the hole at the top, bruising it and causing the wrench to spring or slip out, as would be the case if the pin stood in the direction of b.
Fig. 460.
It is obvious that to reverse the motion of the nut it is necessary to reverse the position of the wrench, because the handle end must, to enable the wrench to grip the work, travel in advance of the pin end. To avoid this necessity Professor Sweet forms the wrench as in [Fig. 460], in which case it can operate on the nut in either direction without being reversed.