HOISTING APPLIANCES
SHEAVES
1. Sheaves are grooved iron or steel wheels used to carry or guide a rope. The general method of mounting them on a frame for hoisting light loads is shown in [Fig. 1]. The journal boxes are so constructed as to be easily taken apart for inspection or repair. For hoisting heavy loads, the timbers must be braced, as is explained under the heading Head-Frames in this Section. Sheaves are of two styles—those composed entirely of cast-iron and those with cast-iron hubs and rims and wrought-iron or soft-steel arms or spokes.
Fig. 1
2. The cast-iron sheave, [Fig. 2], has arms with a cross-section, as shown at a b, and with the flanges of the arms tapering from the hub to the rim; that is, d is greater than c and f is greater than e. The bottom of the groove g in the rim should be a circular arc, whose radius is a little larger than that of the rope used over the sheave, to allow for the angling of the rope due to its fleeting on the drum. The flanges h are made quite deep to prevent the rope jumping off.
This sheave is cheaper than a combined cast-iron and wrought-iron or steel sheave, and for many purposes it is entirely satisfactory. Its great weight is an objection, because it adds to the weight on the journals and also offers considerable resistance to being set in motion and stopped.
Fig. 2
If a sheave is merely used to carry the rope or to deflect it only a little, the contact and pressure between the rope and the sheave is small; consequently, the power of the rope to turn the sheave will be slight. In such a case, when the rope starts or stops quickly, as it usually does in modern hoisting plants, the heavier the sheave the more will it lag behind the rope and the greater will be the wear on the rope due to slipping.