Sheaves are used in pulley systems to gain a mechanical advantage.
The groove depth of a sheave should not be less rope than 1.5 times the rope diameter. However, if the rope Is positively prevented from leaving the groove the minimum depth of the groove can be equal to the rope diameter.
The sheave groove sides should have a flare angle of a minimum of 42 and a maximum of 52.
The round grooves should be slightly larger than the nominal diameter of the rope. Grooves which are too large will flatten the rope. Grooves too small will pinch the rope and the extra friction can cut It to pieces. Sheaves should have a smooth finish with flared edges which are rounded-off.
Caution: Modern cranes and hoists are complex engineering equipment, and many have special construction luff and hoist wires. It Is essential that the sheaves which were designed for a particular crane or hoist are used for that purpose.
It Is also essential that when a rope Is replaced, the replacement Is the same diameter and construction and that the sheave system Is thoroughly checked to ensure that any damaged or worn grooves likely to ruin the new rope are repaired or replaced.
Sheaves should be Inspected regularly. Pay particular attention to the sheave groove and flange. Any cracks or chips on the flange can cut the wire as It lays Into the groove.
The groove should be checked for wear which has reduced the groove diameter, giving an uneven bearing surface for the wire.
All sheaves should be checked for lubrication. Badly lubricated sheaves cause extra friction In the system and wear on the sheave pin and bearing.
The pin should be prevented from rotating with the sheave. Some sheave pins only have a small cotter pin which fits Into a recess on the cheek plate. The cotter pin sometimes shears and allows the pin to turn with the sheave.
Rotating pins are dangerous as they turn and can cut through the cheek plate.
A 'Jockey sheave' is sometimes used as the first diverting sheave to reduce the fleet angle. This sheave fits on an extended pin to allow It to slip from side to side reducing the fleet angle. The jockey sheave pin should be kept well greased and free from grit and dirt to allow the sheave to slide across the pin.
Drums are the pulling mechanism which rotates, hauls In and stores surplus rope. The braking mechanism Is connected to either the drum or the gearing which Is joined to the drive mechanism. The rope should lay neatly on the drum and not be bunched up. There should be a minimum of two full turns on the drum at all times.
The rope must be anchored to the drum with a fixed mechanical anchorage.
Be aware of the danger of not properly tightening an anchorage. Do not rely on the frictional grip from the two turns on the drum.
The top layer on a multl-layered drum must not be closer than two rope diameters to the top of the flange when the drum is full.
The maximum fleet angle is measured from the centre of the drum to the centre of the first diverting sheave then back to the inside flange at the middle of the drum.
The fleet angle for a grooved drum is 5Â° and for an ungrooved drum is 3Â°. To achieve these angles the distance from the drum to the first diverting sheave must be a minimum of:
If the fleet angle is too large or the distance between the drum and the first lead or diverting sheave Is too short, the rope will not lay neatly on the drum and will create severe wear on the rope and the sheave flange.