Fig. 121
Another form of jib known as the ‘mason’s’ is shown in [fig. 121]. It is of framed timber from 9 in. by 3 in. to 11 in. by 3 in., leaving a 4-inch opening down the centre, and rests across the ledgers. It allows the pulleys which are hung on to the iron movable axle, to be shifted horizontally throughout its length. For heavy material this is invaluable, as the load can be raised, moved to its position for fixing, and lowered as required.
Fig. 122
Shears.—The shears or shear legs is another contrivance for supporting heavy weights. It consists of two legs forming two sides of a triangle, and may carry a pulley at the apex as shown in fig. 122, or a jib as shown in [fig. 123]. In the first case the legs are not rigidly fixed, but are kept in position by ropes, A and B, which, on being slackened, allow the shears to move from the perpendicular ([fig. 124]). In this manner loads can be lifted and placed in a different position other than that immediately over the one they first occupied. The range is, however, limited, as in practice the shears should not move more than 18 inches to 2 feet from the perpendicular.
Fig. 123
Fig. 124
Shears are useful for raising and lowering the machinery on Scotch derricks, and are often fixed on gantries to carry jibs.