202 MOUNTED INSTRUCTION 



(j) Men should be trained to pull together silently. At the caution 

 "fast" the slack is taken in, and at the command '"heave" they should 

 pull together and keep what they gain. 



(k) To prevent long tackles from twisting it is a good plan to leave 

 them so that the running end comes from the center sheave. 



(1) Never trust to hold a suspended Aveight directly by man power 

 alone. If it is possible, always take a turn around some object and in 

 this way hold securely what has been gained. 



(m) Always use blocks that are large enough so that the fall will 

 run freely through the block and not ride upon the edge of the sheaves. 

 The rope should not quite fill the grooves on the sheave. 



(n) When the falls are new it will be found that the tackles have 

 a tendency to twist. This can be prevented by placing a smooth picket 

 at right angles between the leads as close to the moving block as pos- 

 sible, the picket being kept in place by a lashing or drag rope at one 

 end while the other end is held by two men or made fast to some fixed 

 object. 



(o) Care must be taken that the men can apply their strength in 

 the direction in which it would be most effective. This is done by 

 using a single block made fast at some point so as to lead the fall in 

 the direction desired. 



WHiere the necessary equipment is available the following maneuvers 

 should be practiced with tackles (see Pages 185-186, Engineers' Field 

 Manual) : 



(a) Overhaul, round in and chock-a-block. 



(b) Whip tackle. 



(c) Gun tackle. 



(d) Luff tackle. 



(e) Double tackle. 



(f) Triple tackle. 



(g) Single Burton, 

 (h) Double Burton. 



By the power of a tackle we mean its mechanical advantage or the 

 ratio of the force exerted by it to that applied to the fall. 



A whip tackle doubles the power. 



A whip upon a whip quadruples the original power or multiplies it 

 by four. 



The power of a gun tackle is 2. 



The power of an inverted gun tackle is 3. 



The power of a luff tackle is 3. 



The power of any similar combination of two blocks consisting of 

 two or more pulleys each, over which a continuous rope passes, is 

 equal to the number of parts of the rope that act onthe block to which 

 the weight is attached. The force or pull necessary to support a given 

 weight using this kind of a tackle is therefore equal to the weight 

 divided by the power of the tackle. 



A single Burton has a power of 3. Inverted, this tackle is a whip 

 upon a whip and has a power of 4. 



