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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



center the annature. Tlii.s force creates a stilTiie.ss tluit adds to the elTecl 

 of the mechanical centering spring. The; magnitude of the force is (juite 

 nonhnear, it varies with pressure drop, and hence, with load as well as 

 with armature displacement. Such variations tend to upset the stability 

 of the servo loop in which the valve is used. 



Fig. 13 is a simplified drawing of a valve w^hich can be used to explain 

 the dynamic effect. It will be noted that when the valve plunger is 

 displaced in either direction, the fluid flow is metered by two control 

 orifices in series. The oil flows from oil supply, through the valve body, 

 into a groove in the plunger via the first of the two orifices. The second 



SUPPLY 



ENLARGEMENT OF 

 SUPPLY ORIFICE 



Fig. 13 — Simple valve with rectangular lands. 



orifice meters the flow from the other groove in the plunger into the 

 exhaust part of the valve body. Most of the pressure drop in the valve 

 appears across the two orifices and is equally divided between them. The 

 maximum fluid velocity occurs immediately downstream from the ori- 

 fices at the vena contracta of the jet. This point is labeled "A" in the 

 enlarged insert on Fig. 13. The velocity at this point can be computed 

 by use of Bernoulli's Theorem. In the case of the J-7 valve, where the 

 valve opening is small compared to other passage dimensions, many of 

 the terms of the equation that formulates this theorem can be neglected. 

 In this way the eciuation becomes 



h = 



V 



29 



V = V2gfi 



(0) 



(7) 



