values are 1.04 for roller bearing sheaves and 1.09 for plain bearing sheaves (with wire rope). Situations in which reeving to disadvantage may be more desirable include lifting from a fixed point overhead-the additional pulley allows pulling downwards instead of upwards so that the weight of the lifter can offset the weight of the load, or allows pulling sideways, enabling multiple lifters to combine effort.Įff = K N − 1 K S N ( K − 1 ). Reeving to disadvantage adds an extra sheave to change the direction of the pulling line to a potentially more ergonomic direction, which increases friction losses without improving the velocity ratio. For example, if the load is to be hauled parallel to the ground, reeving to advantage enables the pulling force to be in the direction of the load movement, allowing obstacles to be managed more easily. Reeving to advantage is the most efficient use of equipment and resources.
The decision of which to use depends on pragmatic considerations for the total ergonomics of working with a particular situation. This is an example of the Luff tackle.ĭiagram 3a: The Luff tackle adds a fixed pulley "rove to disadvantage." The tension in the rope remains W/3 yielding an advantage of three.
Thus, the mechanical advantage is three-to-one.īy adding a pulley to the fixed block of a gun tackle the direction of the pulling force is reversed though the mechanical advantage remains the same, Diagram 3a. ĭiagram 3 shows three rope parts supporting the load W, which means the tension in the rope is W/3. The hauling part is pulled from the fixed block. "Rove to disadvantage" – where the pull on the rope is in the opposite direction to that in which the load is to be moved.The hauling part is pulled from the moving block. "Rove to advantage" – where the pull on the rope is in the same direction as that in which the load is to be moved.
In this case the block and tackle is said to be "rove to advantage." The mechanical advantage of any tackle can be increased by interchanging the fixed and moving blocks so the rope is attached to the moving block and the rope is pulled in the direction of the lifted load. Therefore, the mechanical advantage of a double tackle is 4. In other words, to raise a load at 1 metre per second, the hauling part of the rope must be pulled at 4 metres per second. A line with a mechanical advantage of 4 has a velocity ratio of 4:1. The velocity ratio of a tackle is the ratio between the velocity of the hauling line to that of the hauled load. Ideal mechanical advantage correlates directly with velocity ratio. Separation of the pulleys in the double tackle show the force balance that results in a rope tension of W/4. Note that the gun tackle, double tackle and threefold purchase all have the same number of pulleys in both blocks (one, two and three, respectively) whereas the Luff tackle and Gyn tackle have mis-matched blocks with differing numbers of pulleys. In the diagram shown here, the number of rope sections of the tackles shown is as follows: They are common on boats and sailing ships, where tasks are often performed manually, as well as on cranes and drilling rigs, where once rove, the tasks are performed by heavy equipment. A block and tackle system amplifies the tension force in the rope to lift heavy loads. The process of threading ropes or cables through blocks is called " reeving", and a threaded block and tackle is said to have been "rove". An assembly of blocks with a rope threaded through the pulleys is called tackle. All these are "rove to disadvantage" (see below).Ī block is a set of pulleys or sheaves mounted on a single frame.
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