depending on the element. The damping in each hard stop was changed individually to a value 

 representing 38 percent of critical damping (corresponding to a coefficient of restitution of 0.3, 

 a typical value for metal-to-metal contact). The redesign model was adjusted for increased 

 damping coefficients, with the results shown in Table 4. 



Table 4 



Comparison of Adjusted Model Results 



[Pressure = 1,000 psi, Added Weight = 100 lb] 



Test 



Cycle Length (ms) 



Impact Energy 

 Range (ft-lb) 



Body Motion (in.) 



Test A2 



24 



1.2 to 3.0 



0.01 



Redesign Model 



19 



2.3 to 3.3 



0.27 



38% Damping 

 Model 



25 



1.2 



0.004 



30% Damping 

 Model 



25 



1.4 



0.007 



Increased damping produced an order of magnitude reduction in predicted drill body 

 displacement matching test data under similar conditions. The model predicts a more regular 

 impact energy with little spread in the value. As expected, there was a corresponding decrease 

 in the impact energy for the model. Compared to Test A2, the 38 percent damping model 

 yielded an increased cycle length to slightly longer than that in the test data (25 ms versus 24 

 ms), but a decreased impact energy to the bottom of the test data range (1.2 foot-pounds versus 

 1.2 to 3 foot-pounds). A small reduction to the damping coefficients (30 percent of critical 

 damping) increased both the impact energy and body motion to values closer to those measured 

 in Test A2. Figure 12 presents the impact energy, body motion, and piston stroke relative 

 motion plots, respectively, for the model with the 30 percent damping coefficients. 



RESULTS 



A common occurrence during model development was the discovery of several variables 

 that influenced an observed parameter of impact mechanism performance. One such variable 

 involved the length of the plunger cutout which was known to control cycle timing. While 

 adjustment of the damping coefficients eliminated the double impact, the model showed that 

 lengthening the plunger cutout from 0.75 inches to 0.95 inches also eliminated the double impact 

 phenomenon even for higher pressures and impact energies. Though appropriate correction of 

 the damping coefficients achieved the desired effect, it is worth noting the possibility that impact 

 mechanism sensitivity to one variable may be reduced by smaller adjustments to several 

 variables. 



