Note: Descriptions are shown in the official language in which they were submitted.
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ROCK DRILLING TOOL
Background of the invention
The present invention relates to a rock drilling element for percussive rock
drilling, a drill string and a method of transferring impact energy in a drill
string
according to the preambles of the independent claims.
Drilling straight holes to locate the explosives at the right place with good
spacing and burden has always been considered as a must in the industry of
drilling. It is quite easy to drill a straight hole with a down-the-hole
machine
whose percussion piston is located immediately on top of the drill bit. It is
more
difficult to ensure a straight hole with a top hammer machine where the
percussion piston hits a drill string, including from 1 to 10 rods.
When there are particularly high safety demands on blasting, owing to the
proximity of buildings, utility services or other installations, it is crucial
for the
blast holes to be drilled with the greatest possible precision. Indeed, high
drilling
precision is one of the most fundamental ingredients of a safe and accurate
blasting result.
Drill rods are somewhat flexible, which basic specification allows them to
drill through difficult rock at an acceptable deviation, but also with
reasonable low
fatigue stresses in the body and the connecting ends. This feature allows the
drill
rods to achieve a good service life and grants to the top hammer drilling a
low
cost per drilled meter.
The difficulty starts when the commonly achieved deviations are no longer
tolerated. A better location of the explosives inside the rock body becomes
compulsory. A better location of the explosives will also allow decrease of
the
amount of explosives per ton of blasted rock and can lead to substantial
savings
in explosives and in secondary breaking.
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Many means to improve the hole straightness with top hammer drilling
have been developed over the years. The two most common means are: guide
bits and guide tubes. Guide bits are provided with up to 6 or 8 splines on the
external part of the skirt. The splines are the means to improve the guiding
inside
the drilled hole, but they inevitably wear out, by far earlier than the
carbide
buttons crushing the rock into cuttings. After some while, the drill bit is
still able to
drill, but the guiding means have vanished. Guide tubes, such as disclosed in
U.S. Patent No. 6,681,875, have external diameters close to the drill bit
diameter.
The very high rigidity of the guide tube tends to keep the drill bit straight
in line.
Unfortunately, the penetration rate slows down by 10 to 20 %. In addition, a
guide
tube does not withstand the percussion power over a long period of time and
inevitably breaks at the connection to the drill bit, or forces the upper rod
connected to it to break. The resulting drilling costs are usually considered
as
excessive, and guide tubes are not well accepted in the field.
Other common guide systems have short splines that wear out quickly,
and the expected improvement in guiding the drill bit becomes very quickly
ineffective.
Obiects of the invention
One object of the present invention is to provide a drill string having an
efficient and long lasting guide means.
Another object of the present invention is to provide a rock drilling element
that avoids over-stressing of the thread joint connecting to the drill string.
Still another object of the present invention is to provide a rock drilling
element designed with a relatively low linear weight for a high efficiency of
the
shock wave transmission.
These and other objects have been achieved by a rock drilling element, a
drill string and a method of transferring impact energy in a drill string such
as
defined in the subsequent claims with reference to the drawings.
Brief description of the drawings
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Fig. 1A schematically shows a prior art drill string during transmission of a
shock wave.
Figs. 1 B and 1 C schematically show two prior art drill string equipments
during transmission of a shock wave.
Fig. 2A schematically shows a drill string according to the present
invention in an exploded, cross-sectional view comprising an extension rod, a
guide rod and a drill bit.
Fig. 2B schematically shows the guide rod according to the present
invention shown in Fig. 2A.
Fig. 3 schematically shows a drill string comprising a guide rod according
to the present invention during transmission of a shock wave.
Fig. 4A schematically shows an alternative drill string according to the
present invention in an exploded, cross-sectional view having an extension
rod,
an alternative guide rod and a drill bit.
Fig. 4B schematically shows the guide rod according to the present
invention shown in Fig. 4A.
Fig. 5 schematically shows the alternative drill string during transmission
of a shock wave.
Detailed description of the invention
The basic idea for the guide rod or rock drilling element 10 according to
the present invention is to avoid at best any stress concentration at a thread
joint,
which inevitably occurs with the heavy guide tubes as experienced in prior art
solutions such as U.S. Patent No. 6,681,875. The stress concentration in a
conventional guide tube is depicted in Fig. 1 A, and inevitably leads to an
early
breakage inside the thread joint 5 between drill rod 1 and the guide tube 2.
The
early breakage has three specific origins, namely compressive shock waves,
torsional waves and static bending stresses.
Regarding compressive shock waves: The heavier linear mass of the
guide tube 2, at least 105-250 % of the linear mass of the drill string
itself, reflects
part of the incident shock wave 4A energy back to the drill string and to the
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drilling machine, as a result of the so-called anvil effect. Therefore, the
first rod 1
connected to the guide tube 2, and more specifically its male threaded spigot,
suffers from a local increase of the compressive stress level. This phenomenon
will be described further in detail below, with reference to Fig. 1.
Regarding rotation mean torque and torsional waves: The lower inertia of
the drill string versus the higher inertia of the tube section of the guide
tube 3 is
sensitive to the constant mean torque and the torsional waves traveling along
the
drill string. The drill rod 1, and more specifically the male thread of the
drill rod,
develops high torsional stresses at each impact, when carbide buttons of the
drill
bit impact on the rock formation.
Regarding static bending stresses: For a given angular deviation of the
hole, not shown, the bending stresses can be smoothened out on both sides of a
thread joint between two adjacent somewhat flexible rods. The case is
fundamentally different when one part is a rigid guide tube 3. Because of the
rigid
guide tube, it can be assumed that the bending stresses induced in the male
spigot of the rod 1 directly connected to a guide tube can almost be doubled
relative to stresses developed in a rod-to-rod thread joint.
All the above-mentioned stresses (percussion, rotation, bending) are
combined into a resulting stress distribution, which local excessive value
will
initiate fatigue failures and will result in full breakage.
Figs. 1 B and 1 C schematically show two prior art drill string equipments
during transmission of a shock wave. In both figures, the piston impacts on a
shank adapter connected to an extension rod. As can be seen from these figures
a shock wave is i.a. dependent on the shape and length L of the piston. In
Fig.
1 B the shock wave has an irregular shape with high maximum amplitude peak A.
In Fig. 1 C the shock wave has a rectangular shape with constant amplitude A.
The length 2L of each shock wave is however always two times the length L of
the piston.
Fig. 1A shows the transmission and reflection of an incident shock wave
4A delivered by a drilling machine piston. The shock wave in Fig. 1A is for
illustrative purposes based on a shock wave as shown in Fig. 1 C. A drill rod
1,
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partly shown at the left-hand side of Fig. 1 A, is tightly threaded to the
female end
of a guide tube 2 via the thread joint 5. A drill bit 3 is connected to the
other end
of the guide tube, and the drill bit is pressed against the rock to be
drilled.
At time t = -1, the shock wave 4A, shown in its entire length, which means
5 twice the length of the impact piston, travels along the rod 1 towards the
right-
hand side of Fig. 1A. The incident shock wave 4A is supposed to travel through
the thread joint 5 without any reflected wave; such an assumption is only made
for sake of discussion. A heavier thread joint with partial reflection when
the
shock wave impacts on it, would slightly decrease the stress level of the
shock
wave transmitted further to the guide tube 3, and slightly increase the stress
level
in the male thread, but would not basically change the explanation. A heavy
joint
would just enhance by a few percent the problem to be described.
At time t = 0, the incident shock wave abuts on the heavier tube section 2.
At time t = 1, because of the heavier linear mass, the incident shock wave
4A is split into a transmitted wave 4C traveling through the tube 2 and a
reflected
wave 4B of same length traveling back towards the drilling machine. The more
or
less dense hatching reflects the stress amplitude in both transmitted 4C and
reflected 4B waves.
The shock wave is defined by its stress level and the pulse length. A high
stress level (typically 200 MPa) is shown with a dense hatching at time t = -1
before the wave abuts on the heavier tube 2. The stress level is slightly
lower,
and thus the hatching is less dense after the wave travels into the tube
section.
The reflected wave 4B is depicted by a very loose hatching, inclined in
another
direction as a symbol of a wave traveling towards the left-hand side in Fig.
1A.
However, the reflected shock wave 4B is additive to the incident shock wave
4A,
and therefore the stress level (shown by density of hatching) is maximum.
At time t = 2, the first half (50 %) of the incident shock wave 4A is
transmitted and reflected.
At time t = 3, the incident shock wave 4A travels to the right and the length
of the rod subjected to high stress level is shorter than before. For sake of
simplicity and in order to minimize the number of figures, the transmitted
wave
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4C traveling to the drill bit now abuts on the rock. The reactions at the
drill bit 3
are very variable, depending on the drill bit weight and rock hardness. It is
assumed that the drill bit of same linear mass as the tube section is used,
and
that the rock to be hard enough to withstand the drill bit motion and bit
pressure.
A second reflected compressive shock wave will then be initiated.
At time t = 4, the incident shock wave 4A is fully transmitted into the tube
section, and therefore, the creation of the stress wave 4B comes to an end.
At time t = 5, the reflected wave 4B is completed and travels to the left-
hand side towards the drilling machine.
The most unfavorable time for the thread joint 5 is from time about t = 0 to
about t = 4, when incident 4A and reflected 4B stress waves are superimposed.
Fig. 1A shows a triangle depicting position and time for the overlap
between incident and reflected waves. As can be seen in Fig. 1A, the thread
joint
5 is subject to the increased stress levels from time t = 1 to t = 3. The time
periods in this context are very short, since the shock wave travels in steel
with a
speed of about 5200 m/s, and a usual time period for a shock wave to pass a
drill
steel cross-section is about one third of a millisecond (0.33 ms). This short
time
corresponds to the increase of stress shown by the triangle vertical base line
in
Fig.lAfromt=0tot=4.
For example, if an incident shock wave is 200 MPa, and the reflected
shock wave 4B is 40 MPa (only 20 %) resulting from the higher tube impedance,
then the resulting stress level is 240 MPa. As a matter of comparison, the 240
MPa stress level would develop in a regular rod-to-rod thread joint not shown,
at
a drilling machine with a 44 % higher energy per impact (E), as a result of
the
formula:
E = jU2.dt
where a is the compressive stress.
The conventional drill string cannot withstand a 44 % increase in energy
per impact. The thread joint between drill rod and guide tube, subjected to a
44 %
higher stress, has proven to be the weak point of the drill string. The
fatigue
cracks usually develop in the thread joint 5, and more precisely in the male
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thread, limiting the life of the two components to the range of 800 to 2500
drilled
meters. It should be noted that a standard rod-to-rod thread joint is able to
drill
from 10 000 to 20 000 drilled meters. Such rod and guide tube lives are
commonly recorded on jobsites.
The object of the present invention is to avoid the higher stress level that
currently occurs in any thread joint between the drill rod and the guide rod.
An embodiment of a drill string according to the present invention for
percussive rock drilling comprising a guide rod 10 according to the present
invention is described hereinafter with reference foremost to Figs. 2A and 2B.
The guide rod 10 comprises an elongated first or slender portion 10A with a
substantially cylindrical basic shape of a diameter Dl and a length L1 and a
second or guide portion 10B with a substantially cylindrical basic shape of a
diameter D2 and a length L2. The guide rod further comprises a first or upper
end
11 defined by a preferably welded-on sleeve or female portion 12 and a second
or lower end 13 defined by a spigot or male portion 14. The spigot 14 has a
substantially cylindrical external thread 15 and the sleeve 12 has a
substantially
cylindrical internal female thread 16. The first portion 10A has an outer
diameter
Dl approximately equal to the major diameter of the female thread 16.The
female thread 16 is provided in a recess in the sleeve having an abutment
surface or bottom 18. The slender portion 10A outer diameter Dl is
approximately equal to the major diameter of said thread 16. The length L1 can
be defined as the distance from the bottom 18 to the closest position where
the
guide portion 108 has a full diameter D2. The length L1 is greater than the
length
of the piston used in the drilling machine, i.e. at least 500 mm. The length
L2 can
be defined as the distance between the ends of the guide portion 10B, which
ends have full diameters D2. The guide portion diameter D2 is 105-250 % of the
slender portion 10A diameter. When it comes to the cross-sectional areas (in
mm2) or linear mass (in kg/m) the guide portion 10B is maximum 250% of the
slender portion 10A.
A flushing channel which is generally depicted 19 extends internally of the
guide rod 10, through which a flush medium, usually air or water, is
transferred.
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The through-going flush channel 19 is provided to lead flush medium to the
rock
drill bit 3 for percussive top hammer drilling. This channel is suitably
centrally
positioned in the guide rod.
The slender portion 10A and the guide portion 10B are preferably
essentially cylindrical. A first shoulder 25 and a second shoulder 26 border
the
cylindrical part of the slender portion 10A at respective axial ends thereof.
The
first shoulder 25 is provided in the vicinity of the female thread 16.
Fig. 3 shows the transmission of a shock wave similar to Fig. 1A with
identical shock wave transfer and reflection, applied to drill string
according to the
present invention comprising the guide rod 10 according to the present
invention.
The guide rod 10 includes a sufficiently long slender rod section 10A, defined
in
such a way that the thread joint 5 is definitely located outside of the
triangle
where incident 4A and reflected 4B shock waves overlap.
At time t = -1, t = 0 and t = 1, the thread joint 5 is subjected to the
incident
shock wave 4A, similar to any thread joint between two standard rods.
At time t = 2, the incident shock wave 4A has already ended and the
stress level is close to zero. This feature occurs before the reflected shock
wave
4B reaches the thread joint 5 in opposite direction.
At time t = 3, t = 4 and t = 5, a harmless reflected wave 4B travels across
the thread joint 5, having no noticeable influence on the guide rod 10 life.
The basic idea for the guide rod 10 according to the present invention is to
keep the end part or the part of the guide rod facing away from the drill bit
3 as
identical as possible to the drill rod 1 connected to it, and therefore to
avoid the
negative influence of a 105 % to 150 % heavier linear mass of the conventional
guide tube, enhancing locally compressive, rotation and bending stresses. In
order to avoid any increase of the compressive stresses resulting from the
impact
pulses in the thread joint 5, which is the most sensitive portion, this
slender
portion 10A of the guide rod 10 should have a length equal to or preferably
longer than the impact piston, which means the length of the slender portion
10A
should be at least 500 mm. This slender portion 10A simultaneously smoothens
the torque pulses and the bending stresses before those are conveyed towards
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the thread joint 5 and conveyed into the very sensitive male thread of the rod
1
connected to it.
The guide portion 10B of the guide rod is a tubular section acting as a
bearing in contact with the hole wall to improve the guidance of the drill bit
3. The
major reason for defining a tubular section instead of for example six long
splines
is deducted from field experience, that is guide tubes are considered to be
less
aggressive in overburden drilling and in soft rock drilling, while six splines
may
deteriorate the wall and drive the hole to collapse. This second portion 10B
is
preferably carburized or heat-treated, to withstand high wear because of heavy
friction against abrasive rock, to a surface hardness between 48HRC and
62HRC. The second portion 10B may comprise external shallow splines in order
to increase the flushing area and simultaneously decrease the clearance
between splines and hole wall, for an improved guidance.
The method according to the present invention for transferring impact
energy from a top hammer unit to a drill bit can be summarized as follows. The
top hammer unit has a piston that provides shock waves 4A. Each shock wave
has a length 2L. The method comprises the steps of:
- providing a drill string comprising one or more extension rods 1 or
extension
tubes, a rock drilling element 10 as defined above, and a drill bit 3 or one
or more
guide tubes connected to a drill bit 3,
- connecting an end 11 of said rock drilling element 10, 10' facing towards
the
piston via a thread joint 5 to an extension rod 1 or an extension tube,
- accelerating the piston,
- impacting an end of the drill string to create the shock wave 4A,
- allowing more than half of the shock wave 4A to pass the thread joint 5
before
any reflected wave 4B is allowed to be created, and
- rotating and impacting said drill bit against a rock material for making a
hole
therein.
The second portion 10B can in addition be altered in length, in order to
optimize the shock wave transmission to the drill bit and to the rock. An
alternative guide rod 10' according to the present invention is shown in Figs.
4A,
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4B and 5. Contrary to our previous description, the drill bit 3 linear mass is
often
not equal to the tube linear mass. The drill bit 3 is much heavier, and so is
the
thread joint from the second portion 10B to the drill bit 3. Because of this
observation, more energy is reflected backwards to the drilling machine and
not
5 transferred to the rock.
Figs. 4A and 4B schematically show an alternative drill string according to
the present invention and an alternative guide rod 10' according to the
present
invention, respectively, wherein like numerals depict like features as in the
previously described embodiment. The alternative guide rod 10' comprises the
10 advantages of the guide rod 10 and thus has a slender portion 10A' and a
guide
portion 10B'. The major difference from the guide rod 10 is that the length
L2' of
the guide portion 10B' has been reduced. The length L1' of the slender portion
10A' is greater than the length of the piston used in the drilling machine,
i.e. at
least 500 mm. The alternative guide rod 10' further has a possibility to
provide
some more energy to the rock through the second portion 10B' of the guide rod
10' and the drill bit 3 considered as a whole. The overall length of the
second
portion 10B' and the drill bit 3 is designed as substantially half of the
length of the
piston of the drilling machine, which means that their overall length is
substantially one quarter of the incident shock wave 4A. In such a
configuration,
the first half of the shock wave 4A creates a first level of stress in the
guide
portion plus bit assembly, while the second half of the shock wave further
increases the first level of stress to a higher value. The enhanced stress
level is
then able to push the carbide buttons some further into the rock. This process
can improve in fact the overall energy transfer to the rock and the overall
efficiency.
In light of the description related to Figs. 4A, 4B and 5, the ideal length
for
the guide portion 10B' plus bit 3 is theoretically substantially equal to half
of the
piston length. In fact, optimization by finite element analysis shows that the
overall guide portion plus bit length should be approximately one third of the
piston length. This value is only an indication considering the finite element
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analysis being the only way to optimize the shock wave transfer to the rock
while
considering the true mass distribution along the tube and bit.
In a computer simulation test, the efficiency of shock wave transmission
has improved from 0.7245 (with a conventional full length guide tube) to
0.7677
(with optimized guide portion 10B' length), which is almost a 6% improvement
of
energy transfer.
It should be noted that optimizing the shock wave transmission is not
compulsory. A somewhat or even a much longer guide portion for improved
guiding inside the hole (but not optimized with regard to energy transfer) can
be
designed to solve different drilling situations. For example, when the
straightness
of the hole is of a higher priority than the penetration rate. Such a guide
rod
would still take advantage of the lower compression stresses, more even
rotation
stresses and more even bending stresses in the thread joint 5 that will highly
benefit the drill string life.
Such a guide rod 10 and 10' is designed to accept conventional drill bits
with a skirt and a female thread. The drill bit 3 can have either a standard
skirt or
a guide skirt. The above-mentioned embodiments of a guide rod according to the
present invention preferably has a peripheral contact (also called shoulder
contact) between the guide rod 10, 10' and the drill bit 3. The major reason
for
having the shoulder contact around a large thread is to provide the shock
energy
precisely where it is going to be useful at the peripheral buttons of the
drill bit.
The drill bit 3 can alternatively be designed with a male threaded spigot to
be inserted inside the guide rod having a corresponding female thread. A
carburized guide portion able to withstand high wear can in this context be
the
sole means for guiding inside the hole, such that the drill bit 3 needs no
integral
guiding devices.
The guide rod has been shown up to now with a bit directly connected to
it. It is also possible to use the guide rod as an intermediate element
connecting
together two drill string sections of different cross-sectional area (in mm2)
or
linear mass (in kg/m). For example, a 60 mm drill rod delivers the impact
pulses
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to the guide rod, which in turn is connected to one or more guide tubes with
heavier linear mass. The drill bit 3 is finally connected to the last guide
tube.
The drill string of rods could alternatively be a string of drill tubes
wherein
the guide rod 10, 10' then is replaced by a guide tube of similar geometry but
with
greater dimensions. Such a guide tube should be essentially identical to the
drill
tubes in its upper end, and have a larger and heavier tube at its lower end to
suit
the drill bit diameter.
The present invention proposes a guide rod where the thread joint is
moved away from the unfavorable reflection area. Thereby, several advantages
are obtained, namely an efficient and long lasting guide means that avoids
over-
stressing of the thread joint connecting to the drill string and a high
efficiency of
the shock wave transmission.
Although the present invention has been described in connection with
preferred embodiments thereof, it will be appreciated by those skilled in the
art
that additions, deletions, modifications, and substitutions not specifically
described may be made without departing from the scope of the invention as
defined in the appended claims.
