The decrease in plunger area from 0.24 square inch to 0. 17 square inch was accomplished 

 with a plunger of the smaller diameter without an end cap feature. The plunger mass and travel 

 limits were adjusted in the model to represent this redesign. The single diameter plunger 

 traveled in contact with the piston throughout the piston stroke. To maintain cycle timing at the 

 lower end of the stroke, the plunger length was extended 0.31 inch to position the plunger cutout 

 between the kicker port and the supply pressure port. This plunger configuration eliminated the 

 "dead band" in piston stroke inherent in the 3P drill. The portion of the piston stroke in the 3P 

 drill where the plunger is no longer in contact and driving the piston is referred to as the "dead 

 band" portion of the piston stroke. The dead band occurred when the plunger came to rest in 

 the sleeve at the stop for the end cap. During this portion of the stroke, the piston undergoes 

 deceleration caused by piston seal friction and by a continuously energized piston return. 

 Eliminating the dead band by driving the piston continuously to anvil impact improved impact 

 mechanism efficiency. 



A reduced upper piston diameter provided a net piston return area of 0.05 14 square inches 

 giving a piston return force of 51 pounds at 1,000-psi supply pressure. The piston mass was 

 adjusted to represent the change. A spacer was added to the top of the piston sleeve to prevent 

 over stroking the lengthened plunger. This spacer did not decrease the operating stroke of the 

 impact mechanism as the stroke length is bound by the timing of the kicker port actuation. The 

 piston hard stop was modified to reflect the new location between the piston housing and the top 

 of the piston. 



Referring to Table 2, testing of the baseline impact mechanism using the 3P drill parts 

 produced the results shown as Tests 1, 2, and 3 (direct measurement of body motion was not 

 available for these tests). The data showed erratic impacts and wide variability. No weight was 

 added to the drill for the first test set. The results of the baseline model, adjusted to match the 

 configuration of Test 1 , are included in the lower portion of the table. The data for all 1 ,000-psi 

 supply pressure sets is in bold typeface for comparison. 



A comparison of individual data for Tests 1 through 3 shows a large spread in the range 

 of impact energy. A closer review of the test data indicates a pattern of short cycling producing 

 inconsistent impact energy. Comparing Test 1 to the baseline model shows general agreement 

 in cycle time but the model predicted larger impacts with less variation. The addition of 100 

 pounds in the model had the expected increase in impact energy but also increased the variability 

 of the data. 



Tests 4, 5, 6 were conducted with the 36 percent reduced area piston installed. This 

 resulted in longer cycle times and an order of magnitude less drill body displacement than 

 predicted by the baseline model. Tests at 1,000- and 1,500-psi pressures showed a double 

 impact with the second impact being far smaller than the first. The reason for the double impact 

 is discussed in the next section on damping coefficients. It is clear, however, that the double 

 impacts are responsible for the unexpected longer cycle times at the higher pressures. An 

 example of this double impact is shown in Figure 9 for the Test 5 data. 



Table 3 presents the results from seven tests conducted with the 30 percent reduced 

 plunger and 36 percent reduced piston installed. For Test A2 and Test A7, a test operator leaned 

 full weight on the drill, applying an estimated 100 pounds of force downward on the drill. The 

 baseline model was updated to match the configuration of Test A2. The redesign model results 

 are displayed in the lower half of Table 3. The data for all 1,000-psi supply pressure sets is in 

 bold typeface for comparison. 



