PELTON WHEEL 



445 



Form of Buckets. Where the dividing ridges of the buckets are 

 straight in profile, these are not fixed radially but are inclined backwards 

 from the direction of rotation at such an angle as to give normal 

 incidence on the first impact of the jet. If placed radially the jet would 

 be deflected into the rim of the wheel during the first half of the period 

 of impact and would tend to produce serious inefficiency. 



FIG. 199. 



A type of construction which is more theoretically correct, and which is 

 found to give better results in practice, is indicated in Fig. 199. 



Let ac = v = velocity of jet. 



Let be = u = velocity of bucket. 



Thenafr = relative velocity of jet and bucket. 



If w = angular velocity of _wheel, and if r = radius at the point of 

 impact, we have u = co r and b c is perpendicular to oc. 



Draw d c parallel to a b. Then d c represents the direction of the jet 

 relative to the moving bucket. 



For the jet to leave the bucket with zero absolute tangential velocity, 

 its final direction must be parallel to, and the component parallel to the 

 plane of the wheel of its final relative velocity must be equal to cb. If 



