3. The sheave with a cast-iron hub and rim and wrought-iron or soft-steel spokes, [Fig. 3], is an excellent and extensively used sheave, especially the larger diameters. The spokes are screwed into the hub and rim and are carried to the right and to the left of the hub alternately, as shown in [Fig. 3 (b)], so as to take hold of the opposite ends of the hub, thereby giving stiffness to the sheave against any side stress.
Fig. 3
With a sheave having cast-iron arms, the load from the rope is transmitted to the shaft by a compressive stress through the arms directly under the load; that is, if a rope runs over the sheave, [Fig. 2], putting a load on it from j to k, this load will be transmitted as a compressive stress through the arms l and m to the hub and the shaft. Of course, a part of this load is carried around the rim to the lower arms and is supported by them in tension, but these lower arms are not considered in designing the sheave because cast-iron is of comparatively little value in tension, whereas it is of great value in compression. In the case of the sheave with wrought-iron arms, or spokes, [Fig. 3], the load is transmitted around the rim to the side opposite its point of application and is carried from there to the hub and shaft by the tension of the spokes; in fact, from the method of construction, the spokes in this sheave act only by tension. The sheave is strong and rigid, and much lighter than a cast-iron sheave of the same strength, so that there is less wear between it and the rope due to any slipping action when it is started or stopped.
Fig. 4
Fig. 5
4. Sometimes, the spokes, instead of being radial as in [Fig. 3], are made tangent at the center of the wheel, [Fig. 4], to an imaginary circle, which is about 2 inches in diameter for a 10-foot sheave. Alternate pairs of spokes are made tangent to the opposite sides of the circle, so that they pull against each other, and this makes the sheave rigid in both directions. That is, spoke A is tangent to the right side of the tangent circle and A′ to the left side, while spoke B is tangent to the right side of the circle and B′ to the left side. The pair B B′ is joined to one end of the hub, while the pair A A′ is joined to the other end, thus giving lateral stiffness to the sheave. This arranges the spokes in groups of four, so that the total number must be some multiple of four. The tangential direction of the spokes is often necessary in very large sheaves carrying heavy loads, because with such a sheave it requires considerable force to turn the shaft in its bearings, and while radial spokes act only as long levers in turning the shaft, with tangential spokes there is also a direct pull to do it.
5. Wood-Lined Sheaves.—The rims of all sheaves are made either solid or with wooden lining, as shown in section in [Fig. 5]. One flange a of the rim is a separate piece that is held on by bolts b. The wooden lining is in the form of blocks placed with the grain of the wood running radially and held securely by clamping together the two flanges with bolts, as shown. With such a sheave, there is much less wear on the rope than there is with one that has a plain cast-iron rim. The wear of the sheave proper is also avoided, because as the blocks wear down they are taken out and replaced by new ones.