Dampe mperr me mecha chanism/ nism/circuit circuit Bench drilling - Tunne Bench unnell driving Roof Roof bolting - Reaming - Production drilling drilli ng drillin g Gunnar Wijk / Maria Pettersson / Andreas Nordbrandt / 2003-08-13
2.2 The dampe damperr mecha mechanis nism m - why? why ?
RECO RECOIL IL DAMPEN DAMPENIN ING. G. Long dril l steel steel life High penetration rate
Most Most of the the Atlas Copco Copco rock drills drills are equipped with with the the so-called so-called double double damper damper system. A built-in built-in recoil dampener: dampener: •Absorbs Absorbs shock reflexes from the the drill steel •Reduces vibrations vibrations of the the drill rods rods •Reduces stress on rock drill feed and boom system. The The damper also also ensur sures that good ood con contact act is main aintain ained betw etween the drill ill bit and and the rock. ck. This This is of great significance to penetration rate and drill steel economy.
2.2 The damper mechanism – more in detail Reflex damper When the impact piston strikes the shank adapter, there is a momentary shock-wave movement of the shank adapter, in the order of 1 mm. -With a high feed force behind the machine, the rock drill is pressed against the drill steel via the rear faces of the shank adapter splines at the moment of impact. But this contact is broken momentarily upon impact. The feed force will then press the rock -drill towards the shank adapter to resume contact. At the same time, however, penetration of the drill bit into the rock causes reflected compressional shock waves in the drill steel to reach the shank adapter, which cause the shank adapter to strike the machine via the rear faces of the splines with impact forces that, theoretically at least, can be nearly as large as those imparted to the drill steel by the impact piston. To protect the rock drill from this kind of impact, the modern rock drill is equipped with a reflex damper to transmit the feed force from the rock drill to the drill steel. Without a reflex damper, the function of the rock drill could be described as follows. When the working pressure drives the impact piston backwards, it drives the rock-drill housing forwards, and vice versa. This means that a rock drill without a reflex damper can be regarded as two pistons which strike the drill steel alternately, i.e. firstly the impact piston with a typical mass m = 5 kg and typical stroke length S = 4 cm, which strikes with a typical impact velocity of v = 10 m/s; and secondly the rock-drill housing with a typical mass of M = 150 kg and typical stroke length mS/A=1.3 mm, which strikes with a typical impact velocity of mv/M - 0.3 m/s. Without a reflex damper, i.e. with only a shock-transmitting axial bearing between the rock drill and e drill steel, all that really happens is that the drill steel is shock-fed against the bottom of the drill hole.
2.2 The damper mechanism – more in detail In this context, the situation could be compared with that of a motor-car. The reflex damper in a rock drill is the equivalent of t he suspension and sh ock absorb ers of a car . A rock drill without a reflex damper is like a car in which the shock absorbers have been replaced with rigid steel rods. If this were the case, it would be of no help at all to keep the suspension. As long as the car is driven on an absolutely flat. asphalted road without bumps, pot-holes or any other irregularities, things would go reasonably well. But sooner or later the car would be subjected to devastating damage, initiated by shocks and vibrations from the wheels contact with the ground, in spite of the fact that pneumatic tyres have a certain shock-damping function. The reflex damper serves not only to protect the rock drill from the impacts from the shank adapter (described above), but also gives improved feeding of the drill steel against the bottom of the hole. In other words, the shock-feeding characteristic is replaced with more continuous feeding. This also makes it easier for the rotation motor to keep the joint in the drill string tight, since they have a tendency to loosen as the impact energy from the piston is transmitted through the joints. Different types of reflex damper are described below in more detail.
2.2 The hydraulic damper mechanism - principle
Reflected shock waves are absorbed
4
+ drill steel + rock drill feed force
floating position
Drilling position of shank adapter
4
+ controlled drilling force + stable rotation torque
2.2 Different hydraulic damper principles 4
Hydraulic damper, ”fixed”
4
Hydraulic double damper, ”floating”
COP 1032, COP 1238 4
Hydraulic damper, ”floating” COP 1432 COP 1532 COP 1800 COP 2150 COP 2550 COP 4050
COP 1440, COP 1550
2.2 Different hydraulic damper principles 4
Ar rangement 1, Hydraulic damper, ”fixed”
4
Ar rangement 2, Hydraulic damper, ”floating”
4
Arrangement 3, Hydraulic double damper, ” floating”
Features of arrangement 1, 2 and 3
2 and 3
3 only
Protects the rock drill from rock recoils Provides good rock-bit contact Absorbs the energy of the recoils
2.2 The damper mechanism, COP 1238 COP 1238 (single hydraulic damper, see figure): Damper piston area Ad = 12 cm2 The damper is supplied with oil by the percussion pressure p=200 bar, which means that the thrust of the damper piston is pAd =24 kN. The recoil force R during drilling is given by the impact piston's mass m, impact velocity v and impact frequency f, Which means that R =mvf. If m = 5 kg, v = 10 m/s and f = 50 Hz, the impact power would be mv2/2 = 12.5 kW, in which case the recoil force R would be R = 2.5 kN. In horizontal drilling with a normal feed force of F = 8 kN, the force F – R =5.5 kN should be transmitted to the drill steel via the damper piston. Since the ratio of this force to the damperpiston force is 5.5/24 = 0.23, the damper piston will be in contact with the shank adapter during only 23% of the impact cycle time. It is during this contact time that the feed force presses the drill bit against the rock, and therefore enables the torque generated by the rotation motor to tighten the joints in the drill string. The damper piston pressure p is reduced via leakage in the gaps, and thereafter through the restrictor ' just below she front piston-seal housing, which means that the normal pressure on the damper-piston seals is about half the percussion pressure, i.e. p/2. 'With greater clearance between the damper piston and cylinder, greater pressure is obtained on the damper seals. In other words, the real damper-piston seal pressure depends on the production outfall for the gaps in front of the damper-piston seals.
2.2 The damper mechanism, COP 1238 The flow through the damper is between 0.5 and 2.0 dm 3/min depending on the production outfall for the gaps in front of the damper-piston seals. The damper piston is “almost”always pressed against the rotation chuck. When large compressional-wave reflexes come from the drill bit, the damper piston "bounces" backward say by 0.5 mm at the most - after which it returns to strike the shank adapter at practically the same velocity it had when it bounced off the shank adapter, i.e. without significant energy loss. The movements are so small that the damper piston can be housed directly in the steel of the intermediate part, without the occurrence of any scoring tendencies worth mentioning.
2.2 The damper mechanism, COP 1440/1550 COP 1440/1550 (single hydraulic floating damper, see figure) Same damper piston area as for COP 1238 The damper is fed with a constant flow Qd = 7 dm3/min via a constant-flow valve, which is adjusted to give a damper pressure (measured at the rock drill) of pd = 30 bar (which is equivalent to about 35 bar measured on the pressure gauge at the operator's station). The damper pressure is set while the drill bit is NOT pressed against the rock, so that the damper piston is located in the most forward position, and presses the rotation-chuck bushing against the rotation chuck. This position is about 4 mm in front of the impact position, which is shown in the figure, which means that the piston stroke-position controlling holes in the front damperpiston guide are completely open. The damper pressure pd = 30 bar is what is needed to force the damper flow Qd through the restrictor located just beneath the front piston-seal housing. During drilling, the damper-piston flow shares the return hose with the larger percussionmechanism flow. which means that the pressure drop in the hose typically goes up to approx. 5 bar. When the feed force presses the damper piston into the rock drill, the pressure rises when the stroke-position controlling holes are shut off. The machine therefore locates itself in the correct stroke position within an interval less than, say 0.5 mm, forward of the rear edges of the stroke-position controlling holes. This applies to all combinations of recoil force R and feed force F.
2.2 The damper mechanism, COP 1440/1550 Since the machine itself sets the correct damper pressure pd =(F- R)/Ad ,the damper piston will "always" be in contact with the shank adapter, thereby transmitting the. force F - R to the drill bit, thus enabling thread tightening to take place. Contact is only broken immediately after the impact piston has struck the shank adapter, since this causes a shock-wave movement in the order of 1 mm. The damper piston force needs a period of about 0.8 ms to pitch the damper piston and rotation-chuck bushing forward, so that contact with the shank adapter is resumed. This period of time is small compared with the impact cycle time which for f = 70 Hz corresponds to l/f =14 ms. The pressure an the damper-piston seals is less compared with the COP 1238, since the damper pressure itself is less. This gives longer service life to the damper-piston seals, at least in respect of wear caused by the pressure between the seals and the damper-piston surface. The damper-piston movements are greater than in the COP 1238, especially since the damper piston and rotation-chuck bushing follow the shank adapter "immediately after blows from the impact piston. This leads to greater wear. It has not yet been ascertained whether the combined effect of lower seal pressure and larger damper-piston movements gives reduced or increased seal service life, although the indications point strongly in favour of the latter. It must also be mentioned that the larger damper-piston movements in the COP 1440/1550 require the damper piston to be borne in bronze bushings. Reflected compression shock waves from the drill bit pitch the rotation-chuck bushing and the damper piston backward in the same way as in the COP 1238.
2.2 The damper mechanism, COP 1440/1550 These components are then turned around to strike at roughly the same velocity which they received in the first place, i.e. without significant energy loss. This means that any shock wave energy in the drill steel that has not been utilized for rock destruction will remain in the drill steel, gradually being converted to heat in the friction surfaces of the drill steel, i.e. at the threads in the joints.
2.2 The damper mechanism, COP 700 / 900 COP 700 / 900 (floating pneumatic damper):
In terms of function-, this damper is identical to the damper in the COP 1440/1550, but instead of using hydraulic oil and a diaphragm to connect the gas pressure in the hydraulic machine's reflex-damper accumulator to the damper piston area Ad, the gas pressure in the "reflex-damper accumulator" of the pneumatic machine acts directly on the damper-piston area. The reflex damper is permanently connected to the lubricating-air pressure (6bar) via a pressure regulator. If the pressure in the reflex damper falls below 4 bar, the lubricating air fills the shortfall in the damper immediately. In normal drilling work, the feed force compresses the reflex-damper gas, so that the damper pressure is typically 10 bar. If some of the-air leaks out of the damper during drilling (even though such leakage seems to be extremely rare), the damper pressure will momentarily fall below 4 bar when the feed force is released on completion of the drill hole. When and if this happens, however, the lubricating air will immediately recharge the damper to the correct pressure. In underground mines, the air pressure can sometimes be considerably higher than normal, for example at the start of a shift when very few machines are working and the air flow through the network is low. At times like this, the reflex damper in COP 700/900 would receive an excessive charging pressure. To prevent this from happening, the lubricating air to such machines must be taken via a pressure regulator, adjusted to 6 bar.
2.2 The damper mechanism, hydr. double damper COP 1800/2000/4050 (hydraulic double damper, see figure):
The damper in the COP 1838 is similar to the one in the COP 1440/1550, but has an "extra cylinder" and corresponding diameter change on the damper piston. When reflected compressional shock waves from the drill bit pitch the rotation-chuck bushing and damper piston backward, the pressure rises rapidly in the "extra cylinder". When this happens, the oil volume in the "extra cylinder" is jetted out, mainly forward, around the edges of the "extra damper piston". This oil jetting consumes the movement energy of the rotation-chuck bushing and damper piston. When the damper piston and rotation-chuck bushing are then pressed forward again by the pressure on the damper piston's outer ring area, the components return to the shank adapter at a velocity that is typically around 30% of the velocity they received from the compressional shock-wave reflexes from the drill bit. This means that about 1 - 0.32 =90% of the energy in the compressional shock-wave reflexes will have left- the drill steel. to be "cooled away" in the oil cooler on the rig. The temperature of the return oil is typically 6 ‘C higher than the temperature of the incoming pressure oil in the COP 1440/1550, because the efficiency is not 100%. The efficiency of the COP 1838 is about the same. The extra "power" that has to be cooled away due to the double damper in the COP 1838 corresponds, therefore, to a return-oil temperature that is typically max. 8 °C higher than the temperature of the incoming pressure oil.
2.2 The damper mechanism, hydr. double damper When the damper piston is "pulled" out of a "extra cylinder" by pressure on the damper piston's outer ring area, cavitation damage would occur in the "extra cylinder" if it were not filled with oil at exactly the same rate as the piston is pulled out. This filling function is achieved by means of a "super-fast" check valve with seat sealing to prevent cavitation. The damage on the rear damper-piston seal, the seal is pressurized to the damper pressure. Leakage in the rear annulus around the damper piston therefore takes place in the opposite direction compared with the leakage in COP 1238/1440/1550. This means that the damper flow Od is smaller in the COP 1838 than in COP 1440/1550, but the difference is marginal. In principle, the damper in the COP 4050 is identical to the double damper in COP 1838.
2.2 The hydraulic double damper - function 4
Reference position Constant flow
Feed force
Impact position
”floating” position
2.2 The hydraulic double damper - function
The Impact - The shank adapter is pushed towards the rock
4
Constant flow
Feed force
Impact position
2.2 The hydraulic double damper - function
The damper piston and the rotation chuck bushing are pressed towards the shank adapater, The impact piston starts a new stroke
4
Constant flow
Feed force
Impact position
2.2 The hydraulic double damper - function
The rock drill body moves forward due to the feed force. The ”equilibrium”is reestablished.
4
Constant flow
Feed force
Next impact position
2.2 The hydraulic double damper - function 4
A recoil comes and forces the damper piston to the right. High inner stage damper pressure. The recoil energy is absorbed and transferred into heat.
Constant flow
Feed force
Next impact position
2.2 The hydraulic double damper - function
The damper piston is pushed forward. The check valve opens.
4
Constant flow
Feed force
Next impact position
2.2 The hydraulic double damper - function
The damper piston stops at the ”equilibrium” point. After the stroke, see slide ”no 1”again.
4
Constant flow
Feed force
Next impact position
2.2 The hydraulic double damper - Advantages
The oil in the damper is an axial bearing
4
+the service life of the rock drill, vibrations from 40 G ⇒ 4G
Efficient contact between rock and bit
4
+penetratin rate + good joint-thread tightening + reduced need of feed force (see next slide) + the service life of the shank adapter + the service life of the drill steel + hole straightness
Correct stroke position
4
+ controlled impact velocity
Energy from the recoils is transferred into heat
4
+ the service life of the shank adapter + the service life of the drill steel
2.2 Examples of service life Surface
Customer 1
Customer 2
Customer 3
shank adapter life [m]
typical rod service life rod 1 [m]
COP1238
2500 - 4000
2000 - 3000
COP1838
~12000
7000 - 8000
COP1238
~ 3000
x
COP1838
~ 9000
x
COP1238
3500 - 4000
2000 - 3000
COP1838
> 8000
> 4000
COP1238
2000 - 3500
600 - 1000
COP1838
3500 - 4500
1000 - 1500
Underground
Customer 4
2.2 The hydraulic double damper – Feed force
Button bit Ø51
COP1838ME II 230 bar HL550 Super 210 bar HL550 Super 230 bar
2,6 2,5 2,4 m / 2,3 m2,2 [ 2,1 e t 2,0 a r 1,9 n 1,8 e P1,7 1,6
2,3 ] 2,2 e l 2,1 u 2,0 o 1,9 J / ³ 1,8 m1,7 m1,6 [ n 1,5 i 1,4 K 1,3 1,2
] n i
4
5
6
7
8
9
10
11
12
13
14
Feed for ce [kN]
In this chart, COP1838, was run at lower Impact power than HL 550. No correct comparisation.
15
4
5
6
7
8
9
10
11
12
13
14
Feed force [kN]
This is a more fair comparisation as the ”rock penetration efficiency” vs the feed force is plotted.
15
2.2 The damper mechanism, soft/hard damper The figures to the right show the results of shock-wave measurements taken on a prototype version of the COP 1440 (which corresponds to the "soft damper”case), as well as the results from a modification of the machine to make its reflex damper the same as that in the COP 1238 ME, i.e. the “hard damper” case. The arrows in the figures indicate the primary shock waves generated by blows from the impact piston to tide shank adapter. The reflected primary shock waves from the drill bit come about 1 mm later in' the figure's time scale. In the case of the 'hard damper", the reflected primary shock waves are often (but not always) almost pure tensile waves, in contrast to the primary compressional shock waves that are sent to the drill bit. A reflected tensile shock wave indicates that the drill bit was not in contact with the rock when the primary compressional shock wave arrived at the drill bit. The consequence of this is not only a reduced penetration rate, but also reduced drillsteel life. With the "soft damper", the reflected primary shock wave always has only a small initial tensile element, and terminates with a small compressional element, which indicates that the drill bit has always been in good contact with the rock.
2.2 The damper mechanism, soft/hard damper This in turn indicates that most of the energy in the primary shock waves has been consumed in the penetration of the drill bit into the rock. Since the rock-drill operation is a little uneven, the amplitudes of the primary compressional shock waves vary quite a lot. This is because the measurements were taken on prototype machines, i.e. not fully trimmed rock drills.
