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General
The down-the-hole hammer is a percussion hammer drill. As
the name implies, the hammer works down the hole at the
end of the drill string, where the impact piston strikes the
drill bit directly.
Compressed air is led to the hammer via the rotation spindle
and drill pipes. Exhaust air from the hammer is discharged
through holes in the drill bit and used to flush clean the
drill hole. Rotation is provided by a rotation unit on the feed
beam and transmitted to the hammer via the drill pipes. The
drill pipes are threaded so that the drill string can be ex
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tended as drilling progresses and the hole becomes deeper.
Feed force is also transmitted to the hammer via the rotation
unit and drill pipes. One of the main advantages of DTH
hammers is that the drilling rate is not affected very much by
the length or depth of the drill hole.
DTH hammers are very productive and have many applica
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tions in the mining, quarrying, civil-engineer ing and water-
well drilling industries.
Application (drill rigs)
A= Drill pipe
B= Down-the-hole hammer
C= Drill bit
D= Rotation unit
E= Feed
F= Drill rig
Secoroc COP down-the-hole hammers are designed for use
on DTH or ITH drill rigs. They can also be used on rotary and
auger type drill rigs, provided that such rigs meet the specifi
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cations for DTH applications. The main demands on the drill
rig are as follows:
• It should be equipped with a rotation unit that has a vari
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able rotation speed of 0–90 r/min and a rotation torque of
750–3000 Nm (75–300 kpm). Naturally, the torque demand
for a recommended rotation speed will depend on the ham-
mer size and bit diameter.
• A variable feed force of 3–43 kN (300–4300 kp) for shallow
holes (less for deeper holes, bearing in mind the weight of
the drill string). Obviously, the feed must be strong enough
to pull the hammer and drill string out of the drill hole. This
is an especially important consideration when drilling deep
holes. The weight of the drill string varies between 9 and 34
kg/m, depending on the pipe- and bit diameters.
Technical description
The Secoroc COP down-the-hole hammer and drill bit operate at
the bottom of the hole as a unit.
The COP 32/42 consists of a long cylinder G,
which houses a check valve B, compression
ring C, piston E, control tube E, control tube D,
bushing H, stop ring J and drill-bit shank K� The
rear end of the cylinder is closed by a threaded
top sub A� The top sub has a male thread for
connection to the drill pipes, and also a spanner
flat. (For the COP 32, a top sub with female
thread is also available.)
A driver chuck L threads into the front end
of the cylinder. A splined union between the
driver chuck and the drill bit shank K seves to
transmit rotation to the drill bit. The front end of
the driver chuck transmits feed force to the drill
bit. The check valve B prevents impurites from
entering the hammer when the working air
pressure is switched off.
When feed force is applied, the drill bit is
pushed into the hammer, where it presses
against the bushing H. The piston E strikes the
shank K of the drill bit directly. The path of the
working air through the hammer is controlled
by the piston, with the aid of the control tube D.
Both components contain regulation ducts. A
built-in damping chamber cushions the return
stroke of the piston and increases the impact
frequency. After the working air has imparted
most of this pressure energy to the impact
piston, it is led into the flushing galleries in the
drill bit and cut through the flushning holes in
the head of the drill bit. From here it expands
upwards out of the hole, carrying the drill cut
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tings with it.
When the hammer is lifted off the bottom of the
drill hole, the drill bit drops down to its outer
position in the machine. When this happens,
the piston fails into the blowing position. The percussion mecha
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nism then stops working and only air blowing takes place, i.e. a
large volue of air flows straight through the hammer and drill bit.
Blowing also starts if the drill bit loses contact with the bottom of
the hole while drilling is in progress. The hammer then restarts as
soon as the drill bit is pressed back against the driver chuck. Blow-
ing is used when extra powerful cleaning of the hole is required,
and also in certain difficult conditions.
In difficult drilling conditions, extra flushing can be obtained by
exchanging the plug F in the control tube for a special choke plug,
which has an air duct through it. The duct enables extra flushing
air to flow directly to the flushing galleries in the drill bit. This can
be desirable, e.g. when large volumes of water flow into the drill
hole, or when there is a large difference between the diameter of
the drill bit and the diameter of the drill pipes, or when the pen
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etration rate is abnormally fast.
Friction between the drill pipes and the hole wall can sometimes
reduce the penetration rate. This can often be counteracted by
increasing the air pressure, which gives greater impact enenrgy
and faster penetration.
With the aid of Epiroc ODEX equipment, COP down-the-hole
hammers can be used to simultaneously drill and case holes in
overburden. Secoroc equipment for the precision dillilng of long
straight holes is also available.
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