780 



THE lilOLL SYSTEM TECHNICAL JOURNAL, MAY 1957 



Therefore, more energy is exerted on accelerating the mass of the 

 plunger during the closing operation than is absorbed in slowing the 

 mass during the opening phase. If the net gain in momentum is larger 

 than can be absorbed by the viscous damping of the oil film in the lapped 

 fit, oscillation^ will occur. In the case of the J-7 this oscillation tended to 

 occur at slightly over 400 cps. 



The Bernoulli force described above was recognized and measured 

 early in the development of this series of valves, but was considered 

 unimportant due to the high stiffness and large damping inherent in the 

 design. The experimental models and the early production models showed 

 no indication of oscillation. Later in the production program, the lapped 

 clearances were increased and high ambient temperatures at the missile 

 test locations were encountered. These tW'O factors combined to reduce 

 the damping, due to the working fluid, to the point where hydraulic 

 oscillation occurred. An external damper was added to alleviate this 

 problem. The damper consisted of an aluminum piston closely fitted to 

 an aluminum cylinder. A viscous fluid (polyisobutylene) between these 

 two parts provided sufficient damping to stabilize operation. This fluid 

 also has the advantage of a relatively small decrease in viscosity with 

 temperature. This type of damper is illustrated on the valve in Fig. 3. 

 (Subsequent improvement in the internal design of the valve reduced 

 the dynamic effect to the point where the need for the external damper 

 has been eliminated.) 



Fig. 14 shows a compensated intake orifice configuration correspond- 

 ing to the insert picture on Fig. 13. It represents the first attempt to 

 balance the dynamic or Bernoulli force. The depth of the annular groove 



P'ig. 14 — A compensated vtilvc orifice. 



