60 TUNNELING 



As already stated, the most efficient and most commonly 

 used device for hoisting at tunnel shafts is the steam hoisting 

 engine. There are numerous builders of hoisting engines, each 

 ol \\hk-h manufactures several patterns and sizes of engines. 

 In each case, however, the apparatus consists of a boiler supply- 

 ing steam to a horizontal engine which operates one or more 

 rope drums. The engines are always reversible. They may 

 be employed to hoist the skips directly, or to operate elevators 

 upon which the skips or cars are loaded. In either case the 

 hoisting ropes pass from the engine drum to and around ver- 

 tical sheaves situated directly over the shaft so as to secure the 

 necessary vertical travel of the ropes down the shaft. Where 

 the shaft is divided into two compartments, each having an ele- 

 vator or hoist, double-drum engines are employed, one drum 

 being used for the operations in one compartment, and the other 

 for the operations in the other compartment. Where the work 

 is to be of considerable duration, or when it is done in cold 

 weather, more or less elaborate shelters or engine houses are 

 built to cover and protect the machinery. 



Choice between the method of hoisting the skips directly, 

 and the method of using elevators, depends upon the extent and 

 character of the work. Where large quantities of material are 

 to be hoisted rapidly, it is generally considered preferable to 

 employ elevators instead of hoisting the skips directly. In 

 direct hoisting at high speed, oscillations are likely to be pro- 

 duced which may dash the skips against the sides of the 'shaft 

 and cause accidents. The loads which can be carried in single 

 skips are also smaller than those possible Avhere elevators are 

 used ; and this, combined with the slower hoisting speed required, 

 reduces the capacity of this method, as compared with the use 

 of elevators. Where elevators are employed, however, the plant 

 required is much more extensive and costly ; it comprising not 

 only the elevator cars with their safety devices, etc., but the 

 construction of a guiding framework for these cars in the tun- 

 nel shaft. For these various reasons the elevator becomes the 



