672 HYDEAULICS AND ITS APPLICATIONS 



Pump 10 in. to 20 in. diameter ; 6 vanes 



y from 15 to 30 . . -81 



30 to 60 . . -78 



60 to 90 . . -75 



The values point to the absolute necessity for providing a sufficient 

 number of wheel vanes where a diffuser ring is fitted, and where, in con- 

 sequence, serious loss by shock may occur if the guide vane angle is 

 designed to suit what may be a fictitious direction of outflow from the 

 impeller. 



ART. 183. BALANCING OF END THRUST. 



End thrust on a pump shaft having one or more impellers with single 

 axial inlets is due to a number of causes. 



(a) To the difference of pressure at a given radius on the two sides of 

 the vane shrouding, due to the fact that while the water in the impeller 

 is rotating in a forced vortex, that outside the shrouding is in a state of 

 comparative rest, and therefore exists under different pressure conditions. 



This is by far the most important factor in producing end thrust. 



(b) To the fact that if the discharge diameters of the shroudings in a 

 double cased pump are equal, there is an unbalanced pressure on the 

 portion of the shrouding opposite to the inlet opening. This produces a 

 thrust in the direction of inflow and so tends to balance (a). 



(c) Since the water is taken in axially and diverted radially, it suffers 

 a change of momentum in an axial direction, and this change of momen- 

 tum can only be produced by an axial force transmitted through the shaft 

 to the impellers. 



Thus if v is the velocity of axial flow through the supply passages ; Q 

 the discharge per second ; and n the number of impellers, this change of 

 momentum takes place n times in the pump, and the total end thrust on 

 the shaft due to this cause is given by 



62-4 Q , 

 n' -- ' v Ibs. 

 9 



This thrust also acts in the direction of inflow. 



Apart from the system of arranging the impellers in pairs placed back 

 to back, each pair thus being in balance, four methods of balancing are in 

 use : 



(a) By radial balancing vanes mounted on the rear face of the 

 shrouding (Fig. 305). 



(b} By making the diameter of shrouding on the entrant side greater 



