At the head of the plane is mounted a drum or sheave, and around it passes a rope, one end of which is attached to the loaded cars at the top, the other to the empty cars at the foot.
The loaded train is coupled to the main rope, and to the rear end is attached the tail-rope; which reaches to the end of the line, passing there around a large grooved sheave and thence back to the engine.
In the endless rope system the rope runs from a grip wheel on the driving engine to the end of the line, round a return sheave, and thence back to the engine.
To prevent excessive bending stresses the diameter of drum and sheave must bear a proper ratio to that of the rope.
Arrived at the farther sheave C, the now cool pigs are dumped into a railway car.
The pulley or sheave FG has a weight W
A cord has one end made fast a to and wrapped round the barrel AE; it passes from A under the sheave FG, and has the other end wrapped round and made fast to the barrel BD.
Transmitted by wrapping connectors to FG, and combined by that sheave so as to act on the fol w lower W, whose motion is the same with that of the centre of FG.
Hence the instantaneous axis of the sheave FG is in the diameter FG, at the distance FG ACBC
From the centre towards G; the angular velocity of the sheave is AC + BC
In practice the full advantage of this or any other similar combination is not realized, because of the friction of the sheaves against the pin or shaft, and more important still is the stiffness of the rope, which requires work to be done upon it to bend it round the sheave and straighten it again.
- Sheave the effort E and to increase the resistance R by Pulley Block.
With this arrangement a single revolution of the upper sheave causes the endless chain to wind up the chain on one side by an amount irD, and to unwind an amount Ord on the other side, and in consequence the lower sheave is raised by 7r(D - d)/2.
The value d/D usually lies between the limits io/i i and 15/16, and, if a greater difference of E from R is required, a further mechanical advantage can be obtained by employing a separate hand-wheel and chain, or by forming the upper sheave with an annular spurwheel gearing with a pinion driven by a hand-wheel and chain, as in the Tangye form of Weston pulley-block.
The motion obtained is divided between the two vertical parts of the chain H, which is wrapped round each sheave in opposite directions, with a free loop I between, while the ends are attached to the lifting hook.