accumulators; valves; actuators; line elements; and control functions. Figure 3 shows the basic 

 hardware components. 



The control and hydraulic functions in the model were divided into four sections: 



1 - Supply Pressure Activation 



2 - Poppet Motion 



3 - Plunger Motion 



4 - Piston Motion 



Detailed descriptions of each section are provided in Appendix A. 



MODEL VALIDATION 



Parametric studies were conducted using the computer model as a tool to optimize linear 

 impact mechanism performance. Model features were added or adjusted to simulate changes to 

 the linear impact mechanism design. Drill hardware testing validated predicted model results. 

 Since some of the model parameters were approximated rather than measured values, a precise 

 matching of the results was not expected nor necessarily desired. The focus of the effort was 

 to evaluate model trends and relate findings to specific hardware parameters. The first finding 

 dealt with the sensitivity of the model to small changes in supply poppet valve flow coefficient. 



Poppet Valve Coefficient 



As reported in Reference 2, the shape of the supply poppet seat affected drill operation. 

 It was empirically found that a grooved valve seat produced desirable results. This annular 

 groove had the effect of increasing the valve's flow coefficient. The model showed weak 

 impacting from low piston velocities with smaller valve flow coefficients. Conversely, greater 

 impact energies from high piston velocities were modeled using larger valve flow coefficients. 



An experiment was conducted to quantify the differences in behavior between the grooved 

 and the nongrooved poppet. It was also an objective to determine the validity of the valve 

 coefficient used in the model. Steady flow and dynamic flow conditions were evaluated. Figure 

 4 is a schematic of the test fixture. 



During the static tests, the displacement of the poppet was fixed either full open or nearly 

 closed. The pressure drop across the valve was measured for several flow rates. The valve flow 

 coefficient obtained for the grooved and nongrooved poppet are shown in the graph of Figure 

 5. 



No significant difference between poppets was observed for a nominal displacement of 

 0.050 inches. The value of the open poppet flow coefficient was close to the modeled value of 

 5. The smooth poppet yielded a higher valve flow coefficient for a nominal displacement of 

 0.015 inches. This was opposite of what was expected and has not been explained. The curves 

 also show that as the valve opens, the grooved poppet flow coefficient increases, whereas the 

 smooth poppet flow coefficient decreases. 



During dynamic tests, poppet valve configurations for grooved and nongrooved poppets 

 were evaluated with and without the biasing spring. Evaluation of one open-close cycle was 

 representative of valve operation, however, manual control of the flow to the kicker port limited 



