In several model experiments, the ratio of plunger area to piston return area proved to 

 be an important variable for achieving stronger impacts. In the first example, the model showed 

 that for a 58 percent decrease in the piston return area, to 0.0340 square inches, the impact 

 energy increased by approximately 40 percent in comparison to the increased damping coefficient 

 model results. Since further reduction in piston return area was not desirable, the ratio was 

 adjusted by an increase in the plunger area. 



In a model experiment, the plunger area was doubled to 0.34 square inches. The drill 

 cycled evenly, with impact energies averaging 6 foot-pounds (this was close to the desired result 

 of 7 foot-pounds). Despite the larger plunger area causing larger actuator forces, the drill body 

 displacement was less than 0.15 inch and there were no short cycles or missed impacts. The 

 shape of the plunger/piston motion curve has a sawtooth shape, with a long return run, allowing 

 gravity to act on the drill body to slow its upward speed and begin to return it to its starting 

 position. Each subsequent drive stroke imparted an upward displacement, but gravity mitigated 

 drill body displacement during a longer cycle time of 40 ms, or 1 ,500 beats per minute. Figure 

 13 shows the impact energy, body motion, and piston stroke relative motion plots for the model 

 with the doubled plunger area. 



Finally, hardware tests showed the impact mechanism to be sensitive to flow restriction 

 at the exhaust orifice. When the rotary drive motor was connected to the circuit in series to the 

 impact mechanism, the impact cycles became erratic and the number of cycles per second 

 decreased. An externally applied force to the drill caused the rotary motor to stall, causing the 

 impact cycle to become irregular or stop. 



Because the stalled motor increases the back pressure at the exhaust orifice, flow through 

 the impact mechanism is reduced to the point where the pressure across the impact mechanism 

 is insufficient to produce a cycle. A pressure relief valve installed on the motor supply line and 

 set to 350 psi produced no change in behavior. When the relief valve was set at 50 psi, the 

 impact cycles became more regular. In this condition, an externally applied force stalled the 

 motor but had no affect on cycle performance. Intermediate pressure settings produced a 

 corresponding reduction in cycle performance. 



CONCLUSIONS 



The following conclusions are presented as a summary of the development and validation 

 process for the computer model of the linear impact mechanism used in the seawater hydraulic 

 rock drill: 



1 . The computer model of the single poppet-kicker port linear impact mechanism has 

 been validated for both the baseline (using production parts), and the redesign rock drills. The 

 damping coefficients, bulk modulus, and other model parameter values have been determined 

 such that the model results closely match the test data for cycle time, body motion, and impact 

 energy. The validated model shows a reliable 6 foot-pound blow energy, just short of the 

 required 7 foot-pound level necessary for the rock drill design. 



2. The linear impact mechanism model provides a cost effective tool for evaluating 

 design modifications. The predictions of drill performance for an applied force, a reduced 

 supply pressure, a change in poppet area, and a reduced plunger and piston size were 

 experimentally verified. The pressure and motion curves predicted by the model matched the 



10 



