where
p w Gas pressure in loop;
A k Forced area of piston rear chamber;
s
Piston stroke under the action of force P.
Considering various losses and efficiencies, the above equation can be modified
to
e ¼ K a p w A k s
ð11:4Þ
where
K a Conversion coefficient of impact energy.
Only the exact K a is given can the above equation be calculated. The actual K a
value of the impactor can be obtained by experiment and inversion. The conversion
coefficient K a of the relevant DTH hammer impactor can be found in Table 11.6.
(2) Impact frequency
When the cycle time of the impactor is T, the impact frequency is
f ¼ 1=T ¼ 1= at 1
ð Þ
ð11:5Þ
where
t 1 Stroke time. The impact cycle time is expressed by the stroke time t 1 multiplied
by a proportional coefficient a which is at least greater than 2.
Assuming that the initial velocity of piston motion is zero in stroke stage, the
piston moves with uniform acceleration. In stroke s, the motion time of piston is as
follows:
t 1 ¼
2sG k
p w A k g
1=2
ð11:6Þ
where
G k Piston weight;
g Gravity acceleration.
The impact frequency is
f ¼
1
a
p w A k g
2sG k
1=2
ð11:7Þ
11.3 Principle and Parameter Design of Large Diameter …
211