LIFTS AND HOISTS 747 



and this to a certain extent counterbalances the difference in effective 

 weight of the lift ram. 



A second type of balance cylinder, which has the advantage that all 

 packings are external, is illustrated in Fig. 362. Here pressure water is 

 admitted through the hollow ram A to the movable cylinder B, and forces 

 water at a reduced pressure out of the fixed cylinder C into the lift cylinder. 

 The cylinder B is weighted, this weight being proportioned so as to pro- 

 duce a pressure in C sufficient to approximately balance the weight of the 

 lift ram and cage. No attempt is made to allow for the varying effective 

 weight of the lift ram. Sufficient of the weight is left unbalanced to 

 cause the lift to descend with sufficient rapidity on the down stroke 

 even with an empty cage. A lift of this type, to lift 20 cwt. through a 

 distance of 90 feet, carries a six-inch ram and uses 24J gallons of water 

 at 700 Ibs. pressure per trip, as against 109 gallons without balance 

 cylinder. With the direct-acting or ram-supported lift, hoisting speeds 

 up to 180 feet per minute are common, 240 feet per minute being about 

 the maximum. 



The second type of lift the suspension type is manipulated from a 

 hydraulic ram having a comparatively short stroke. The requisite travel 

 in the wire rope or ropes by which the cage is suspended, is obtained by 

 multiplying this by means of a jigger. 



The weight of the cage may be balanced by hanging weights, the vary- 

 ing immersion of the ram in this case being unimportant. A hoist on 

 this principle, but with unbalanced cage, is shown in Fig. 363, while a 

 balanced lift is shown in Fig. 364. Here two wire ropes are employed 

 for lifting and two for carrying weights which partly counterbalance the 

 cage. As the cage of a suspended lift rises, a portion of the weight of the 

 suspending rope is transferred to the plunger side of the supporting 

 pulley, and the effective weight transferred to the plunger consequently 

 varies throughout the whole of its stroke. Fig. 365 shows one, and 

 Fig. 366 a second method of compensating for this rope variation. In 

 the former a double- balance chain is suspended from the cage as shown, 

 so that if R be the travel of the cage, the length of each chain is R -f- 2. 

 Let m be the multiplying factor for the jigger ; W the weight of 

 unbalanced portion of cage ; w the weight of the suspending cable per 

 foot run ; w' the weight of each balance chain per foot run. 



Then with cage at bottom, the pull on plunger = m { W + w R } 



top, =m { W + w' R} iv R 



And for these to be equal, w' = w ( 1 + - - j . 



