AN ELECTRICALLY OPERATED HYDRAULIC CONTROL VALVE 729 



where 



V = fluid velocity at the vena contraeta, ft/sec 



h = pressure drop across orifice, feet 



g = acceleration of gravity, ft/sec^ 



Von Mises' has shown that the departure angle of the jet from a small 

 orifice, such as in the configuration shown in Fig. 18, is 69° from the 

 longitudinal axis. Tests made with an orifice, shaped like those in a 

 simple valve, showed that the jet continues at this angle for a short 

 distance only. Further downstream the jet turns to hug the radial sur- 

 face on the plunger. This action is depicted by the dotted lines in the 

 insert on Fig. 13. Bernoulli's equation explahis that pressure is ex- 

 changed for velocity. The low pressure within the jet stream pulls it 

 toward the nearest wall of the cavity. The flow of oil over this surface 

 reduces the pressure on that wall and unbalances the distribution of 

 forces on the surface of the annular grooves in the plunger. The area 

 of reduced pressure causes a net longitudinal force in the direction to 

 center the plunger or close the valve. 



Examination of the situation around the exhaust orifice shows that a 

 similar action will occur, but will not result in a comparable force on the 

 plunger. The area of high velocity lies along a surface in the valve body 

 rather than acting on the plunger. The velocity upstream from the 

 orifice is not localized and hence produces forces that are small with 

 respect to those downstream. The fact that the exhaust port forces on 

 the plunger are small compared to those of the intake was confirmed by 

 tests.* 



There is a small time lag between the opening of the ports and the 

 dynamic centering force which is proportional to the rate of change of 

 oil flow. This lag results from the fact that finite time is required to 

 change the velocity of the oil mass in the system. At high frequencies, 

 the delay results in a considerable phase lag between the plunger posi- 

 tion and the dynamic force. This delay means that fluid velocity, and 

 hence the force, is higher during the quarter cycle in which the valve is 

 closing than it is during the cjuarter cycle in which the valve is opening. 



* Considerable work has been conducted on valve theory and design elsewhere 

 since the J-7 valve was developed. Reference 4 is an excellent example of a thor- 

 ough analysis of valve d3'namics with an approach to the i)rol)lem from a thtferent 

 viewpoint. This reference reports on tests and theories which show the secondary 

 forces from the exhaust and intake orifices to be equal. This is in direct contradic- 

 tion to the experience with the J-7 valve and remains as an unresolved problem 

 in the mind of the writer. 



