242 HYDRAULIC EQUIPMENT 



water-lubricated lignum vitse bearing, permitting it to be located 

 very close to the runner. 



The lignum vitse is dovetailed into the bearing boxes in the 

 form of strips running parallel to the axis of the shaft and with the 

 end grain of the wood placed normally to the surface of the shaft. 

 Twenty or more of these strips, evenly spaced in a liberal length 

 and separated by spaces for circulation of cooling water, are so 

 proportioned as to present sufficient area to the shaft to insure 

 very satisfactory performance. 



In the case of turbines operated in clear water, the supply for 

 the bearing may be taken through a pipe directly from the wheel- 

 casing. A duplex strainer should be connected in the line to 

 remove any foreign substances which might otherwise reach the 

 bearing and damage it. In installations in which the water car- 

 ries large quantities of foreign matter in suspension, a suitable 

 central filtering system should be provided. 



For a description of the various types of thrust bearings, see 

 page 334. 



Impulse Type. Like the reaction type, impulse turbines are 

 built in many different designs, the controlling factors differing so 

 materially in each installation that they not only affect the general 

 type or arrangement of the design, but also of details. 



Horizontal and Vertical Wheels. Impulse turbines are almost 

 exclusively of the horizontal type. This not only represents the 

 most economical design, but it has many advantages of simplicity 

 of construction and arrangement of parts available for inspection, 

 lubrication, and cleaning. Vertical wheels have, however, been 

 built and operate satisfactorily, and they may be used for com- 

 paratively low-head plants, where the water contains large quan- 

 tities of sand or grit. With this type up to six jets can be installed 

 in a single-wheel runner. 



Runners. There are two general types of wheel-runners, the 

 double-lug bucket type and the chain or triple-lug bucket type. 

 In the former the wheel center consists of a single rim and the 

 buckets have two lugs which are machined to a press fit over the 

 rim of the wheel center and held in position by two bolts. In 

 the latter type, a double or U-shaped wheel rim is required and 

 the buckets have three lugs, a forward center lug and two rear . 

 lugs. The forward center lug is a close fit between the two rims 

 forming the duplex wheel center, and the two rear lugs straddle 



