LIFTS AND HOISTS 



745 



Since the volume of water displaced by the ram diminishes as the lift 

 rises, the effective weight of the ram, which is its own weight less that of 

 the water displaced, increases. In lifts for large powers the effect of this 

 variation is in general unimportant, 

 and becomes of less importance as the 

 working pressure increases. 



EXAMPLE. 

 Pressure = 250 Ibs. per square inch. 



W = 5 tons. 

 Lift = 60 ft. 



5 x 2,240 



From Pressure Supply '_'- 



Area of ram = 



250 



44-8 square inches. 

 Weight of a column of 



/. Difference in \ / 



apparent weight of I I water 60 feet long and 

 ram at top and | 1 44-8 square inches sec- 

 bottom of its stroke > \ tional area, 



x 62 . 4 





= 1,164 Ibs. 



If working pressure = 500 Ibs. per square inch. 

 the area of ram = 22-4 square inches and its dif- 

 ference of weight = 583 Ibs., a value which is 

 small in comparison with the weight of five tons. 



Various devices have been adopted to 

 overcome these difficulties. 



The weight of the ram and cage may 

 be balanced by a counterbalance weight 

 attached to the cage by chains passing 

 over a series of pulleys at the top of 

 the lift shaft, and since, as the lift rises, 

 the length of chain on the balance- 

 weight side of the pulleys increases, 



To Li ft Cylinder 



FIG. 362. Balance Cylinder for 

 Hydraulic Lift. 



this may be made to counterbalance the increasing effective weight of the 

 ram by making the chain of such dimensions that its weight per foot run 

 is half that per foot run of the column of water displaced by the ram. 

 This method suffers from the disadvantage that the upper part of the 

 ram is in tension, and a fracture would cause the cage to be dashed 

 against the top of the shaft. It thus detracts from the otherwise essen- 

 tially safety features of this type of lift, and also increases the mass to be 

 accelerated at the beginning of the travel. 



A second device, which is more common in high-class work, is that of 

 the balance cylinder, one type of which is illustrated in Fig. 361. Here 

 pressure water is admitted to the interior of the hollow ram B. The 



