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Down-Hole Hammering Mechanism (DHM)

A co-project with European Space Agency, Selex-Galileo and Department of Automation and Systems Technology

ESA Contract N°: 19294/05/NL/CP
GA Sub-contract N°: 5100018172

In 2006-2008 Automation technology group conducted with the Italian space company GalileoAvionica –later Selex-Galileo, a project for European Space Agency (ESA) to developand demonstrate a small-scale hammering system to be placed inside a drill string of a futureplanetary drilling system. The Automation technology group conducted a technology survey and performed preliminary drilling tests with an updated version of the Miranda drilling system. According to the test results the TKK designed a couple ofconceptual designs for the hammering mechanism and after ESA approval also made the detailed design of the system. The hammering mechanism was partly manufactured and fully assembled in the facilities of Automation technology group. Testing of the mechanism took place in premises of Selex-Galileo in Milan, Italy.

Executive summary (2.2 Mb)


Project personnel

Program manager at ESA: Lucio Scolamiero
Mechanisms Section, Mechatronics and Optics Division, TEC-MMM
European Space Agency, ESA ESTEC
Noordwijk, The Netherlands

Personnel in Italy

Selex-Galileo Edoardo Re, Program manager
Pier-Giovanni Magnani, Project leader
Mirko Izzo, Project engineer

Personnel in Finland

Literature survey, theory of rock drilling:
Paavo Jääskeläinen, Visa-Matti Myllymäki, Panu Kaukinen,
Helsinki University of Technology, Laboratory of rock engineering.

Technology survey, concept development, conceptual testing, detailed design and modeling, mathematical analyses, parts procurement, assembly, reporting, subcontractor representative and sub-project leader at TKK:
Tomi Ylikorpi, Helsinki University of Technology, Faculty of Electronics, Communications and Automation, Department of Automation and Systems Technology.

Manufacturing of conceptual models’ parts and DHM parts (except drill tools):
Tapio Leppänen, Helsinki University of Technology, Faculty of Electronics, Communications and Automation, Department of Automation and Systems Technology.

Manufacturing of DHM parts (except drill tools):
Protoshop Oy, Otaniemi, Espoo, Finland.

Literature survey

Sometimes materials are so hard that they can’t be easily penetrated by normal rotary drilling. This applies to the case of hard rock. The hardness of the rock gets so high, that the hardmetal bits start to wear more rapidly. The most common method of rock drilling is percussive drilling. Percussive drilling is usually divided to four basic functions – percussion to indent the button, feed to ensure bit-rock contact, rotation to index bit indentation and flushing to remove cuttings and cool the drilling tool. In percussive drilling impacts are delivered by a rock drill to a drill bit, which forms a heterogeneous stress field to the rock. The stress waves propagate to the rock causing stress-initiated cracks. If these cracks expand to the free surface at the borehole bottom, a chip loosens. This chipping is the main objective of percussive drilling. The chipping is depending on the proportion of the stress levels, the shape and size of the intender and the rock properties. The rock textures, dominant minerals and their bonds define the hardness and the drillability of the rock. Even the foliation in metamorphic rocks is a factor to be noticed. The figure illustrates chip formation under pressure of an intender. (Liu et al. 2002)

The figure on right shows measured intending force in granite and two models describing the maximum intending force. The peaks in experimental results indicate the moment of rock fracture that is followed by decrease in force. As the intender is pushed further against the rock the force increases again until the rock breaks again. In DHM we aim to reach the first peak for each strike, that is roughly 6-7 kN for Sierra Granite with a 60 degree 1 inch conical indenter. (Pang et al. 1989)

Percussion action generates stress waves that travel through the drill string with a speed of sound velocity in drill material. (Tamrock 2002)

Stress wave amplitude in the rod, σ, produced by a piston may be calculated as a function of piston velocity, cross-sectional areas of piston and rod, rod density and sound velocity in the rod. (Graff 1975) The dynamic force, F, produced by the stress wave may be given as F= σAr. (Mänttäri 1997)


Liu, H.Y., Kou, S.Q., Lindqvist, P.-A. and Tang C.A., 2002: Numerical simulation of the rock fragmentation process induced by indenters, Int. J. Rock Mech. Min. Sci., 39, pp. 491-505

Mänttäri, M 1997: Laboratory scale rock drillability tests. Licentiate thesis Helsinki University of technology

Pang, S.S., Goldsmith, W., Hood, M., 1989: A Force-Indentation Model for Brittle Rocks, Rock Mech. Rock. Eng. 22, pp. 127-148

Tamrock Corp., 1997: Underground drilling and loading handbook.

Technology Survey

Technology survey searched for different mechanical solutions to generate impacts for rock drilling. Survey extended through conventional hammer drills and percussive drills into nailing guns and magnetic actuators.

Percussive and hammer drills

Percussive drilling tools contain a ratchet mechanism that upon drill rotation and axial push force generate impacts on drill mandrel. The image below right presents a ratchet mechanism giving 16 impacts per revolution. Percussive drills are designed to rotate with high speed and thus generate a high number of impacts per minute. Typical performance can be with 500-800 W drill 800-2700 RPM 19 000 - 42 000 bpm (blows per minute). A typical percussive drill and an example of a ratchet mechanism (different makes, not related to each other). (Images: DeWalt, TKK)

Hammer drills use a separate cam-driven piston that generates impacts in the end of a rotating mandrel. There may be a piston and an air filled cylinder between the cam and mandrel, then were are talking about pneumatic or electropneumatic hammer drill. Hammer drill rotation velocity and impact frequency are lower than those for the percussive drills, but it’s impact energy is much higher. Typical performance of a 800 W hammer drill can be 800 RPM, 3000 bpm and 1-3 J (even 10 J) energy per impact. (Images: DeWalt, Chronis& Slater, 1996)

Illustration presents a hammer design with an axial cam and a spring generating the impact energy. (Chronis& Slater, 1996)

Nailing Guns

Nailing guns hit the nail in with the aid of a spring triggered by a motor or a solenoid. They may also utilize gas pressure of compressed air or combustion gas. The spring operated guns provide a moderate impact rate and energy, the other ones utilize consumable gas that should be exported along with a space craft.

Mechanical moles

There exist cylindrical moles intended to penetrate in permafrost on Earth or in sand on Mars. Their operation relies on impacts generated by internal cam and spring. Units designed for permafrost penetration are powerful and generate strong impacts, but hey are also large in size. Units intended for Mars exploration are small providing small impact force sufficient for penetrating into loose sand, not to break a rock. (marstech.jpl.nasa)

Sonic and ultrasonic devices

Sonic vibration is being used for terrestrial drilling. (bowser-morner) Ultrasonic devices have been developed also for Mars rock drilling. (ndeaa.jpl.nasa) Ultrasonic devices are efficient when hole diameter to be drilled is relatively small compared to actuator diameter, DHM aims to fit all needed components inside the drill rod. (Parallel to DHM development also an ultrasonic drill was being demonstrated elsewhere under another ESA-contract.)

Preliminary concept selection

After evaluation of possible mechanical solutions weight was put on reliability, effectiveness and simplicity of construction. It was decided to concentrate on percussive and hammer drilling methods that show long heritage in everyday use, and on electromagnetic resonating system that seems promising with respect to effectiveness and mechanical simplicity.


Chronis, Slater, “Mechanisms and mechanical devices sourcebook”, 2.ed. McGraw-Hill, 1996

Preliminary testing

In order to make final decision of the DHM operation selected five different impacting methods were tested. TKK conducted preliminary development and testing of percussive and hammering concepts, Selex-Galileo conducted development and testing of resonant tool concepts (see details in executive summary).

Miranda drilling test jig.

For testing TKK utilized the existing Miranda-drilling test jig that was build earlier to participate in ESA Student contents. It constitutes of a vertical roller guide equipped with a ball nut and a feed motor. Drill rotation motor and drill mandrel mount on a platform that travels along the vertical guide. Coupling between the ball nut and mandrel platform is realized with the aid of springs so that approximately constant axial drilling force can be adjusted with the ball screw (instead of providing constant axial feed velocity that is often the case). Total drilling force is then the mass load of the drill added with the tension from the springs. When the drill is resting against a rock to be drilled, drilling force can be increased by driving the lead nut downwards extending the springs. A drill string exceeding 1 m in length can be mounted on the mandrel and it penetrates into a deep sand-box placed just below the drill rig. For these tests the drilling depth data was collected visually with the aid of rulers; drill rotation speed, input voltage and current were measured with external metering devices. Also an additional sample holder was built on top of the sandbox to hold rock samples being used in tests.The jig has been also used for Dr. Matti Anttila’s doctoral research on Mars drilling system development.

Drilling tests were conducted in four different rocks using a commercial 25 mm rock coring tool. Image: Preliminary testing rocks and tool. (TKK)

Percussive drilling

Image: Percussion system mounted on drilling test jig. (TKK) In the first test set-up the test jig was modified to carry a ratchet mechanism detached from a commercial percussive drill. The drilling tests were run in calcite and granite with 115 and 180 rpm using 164 N axial force. The tests indicated no benefit at all from percussion with respect to pure rotary drilling. Further testing with the original percussive hand drill with variable drill rotation velocity showed that percussive drilling rate was strongly depending on drill rotation velocity. At low velocities there is little effect from percussion. Percussion starts to show some effect at 1000 RPM giving double penetration speed at 1500 RPM and triple at 2000 RPM (in calcite). Clearly this kind of percussion system is not suitable for low-speed and low-power DHM-drill.

Commercial percussive drilling test results with variable rotation velocity. (TKK)

Cam hammer testing

Image: Cam hammer test assembly. (TKK) For testing a cam hammer concept substantial changes were made for the test jig. The original drill rotation motor was detached from the drill rod and was modified to rotate a striker through a spline shaft. A cylindrical cam-profile was manufactured and placed on carriage carrying the drill rod. Drill mandrel bearing assembly was modified to allow small axial motion of the rod under hammer impacts. The striker rotates along cam ramp supported by a roller. Two axial springs collect energy during striker rise and at the drop they accelerate the striker to hit the upper end of the drill rod. Impact shock travels through the drill rod into drill tool and rock sample below. A separate indexing motor was placed aside of drill rod to provide indexing between each strike.

The tests were conducted with 59 impacts per minute, 48 impacts per revolution, 1.88 J energy per impact and 198 N thrust force. Striker motor took power 28 W and indexing motor 0.5 W.

The test results are interesting and promising. It is evident that for drilling in soft materials like calcite and marble hammering does not provide any benefit. Hard and fragile materials, like granite, benefit from hammering the most. Hard and ductile mafurite is the most difficult material to drill into. Rotary drilling into hard materials is practically impossible with given 30 W power. In addition rotary drilling would cause strong wear of the tool and severe heating of the rock sample enough to jeopardise its scientific value.

Expected rate of penetration at given power could be around 0.2 mm/min in soft rock with or without hammering, 0.1 mm/min in hard but fragile materials –like granite- when utilizing hammering and around 0.05 mm/min in hard and ductile materials like mafurite. Rotary drilling can be recommended up to materials similar to marble (100 MPa class compressive strength).

Cam hammer test results. (TKK)

Long-stroke cam hammer

Image: Long-stroke cam hammer. (TKK) The cam-hammer used in previous tests presented a one-revolution cam. Rise of the cam must be kept reasonable and available impact energy is a compromise between cam diameter, cam rise, motor torque and spring load. In the following concept a multi-revolution cam was used in an attempt to increase available impact energy while keeping motor torque low. A bi-directional screw (diamond screw) and a specific magnetic catch-and-release –mechanism were demonstrated to move the striker against spring load. Testing indicated that available impact energy may indeed be increased with very low power, but operation cycle is long and impact frequency becomes unbearable slow. The system provided 3.4 J impacts, one impact in 16 seconds, using 4.5 W average power (8.4 W max.)


Image: Schematic representation of the second resonator hammer concept. (Selex-Galileo) Selex-Galileo in Milan built and tested two concepts based on mechanically actuated resonating mass with mechanical springs. Impact rate of the resonating mass was significantly higher than for TKK cam hammers and thus also efficiency of the system was quite promising.

3-D model figures of second resonator hammer concept. (Selex-Galileo) The resonator hammer demonstration model operated with 38W input power producing 20-30 Hz impact frequency. In tests performed in granite the tool removed up to 3 cubic-mm material per second. Tuning of the distance between the tool and sample rock is very sensitive. See details in executive summary.

Detailed design and operation of DHM

DHM detailed design. (TKK) The final conceptual solution was agreed together with ESA, Selex-Galileo and TKK, and it was to be based on single revolution cam hammer. Design goal was to produce 1 J impacts (6.4 kN dynamic force) at 1 Hz using power less than 50 W. Everything was to be fitted inside a 29 mm outer diameter drill rod. A driving factor became from motor diameter; the smallest suitable motor was a brushless Maxon Motor EC 22 providing 50 W of power. Friction losses were minimized and durability maximized by using rolling components where ever possible. System contains two ball bearings, one linear ball spline transferring torque and one ball bushing guiding the striker. Two plastic sliders and two Teflon seals are used. Demonstration model was calculated to weigh 1.1 kg while flight model could weight 0.75 kg with extensive use of aluminium and titanium.


The movie shows internal structure and operation of the DHM. (Switch on loop/repeat on your player to see continuous motion.) The impact force generating spring is located inside the moving cam/hammer and is not visible.

Drill rod outer tube and DHM ready to be placed inside. (TKK)

Analytical calculations

Critical properties of DHM-design were checked with appropriate analyses at TKK.

Needed motor torque: ESA Space Mechanisms Standard ECSS--E--30 Part 3A, tells how to define needed motor torque: - Tmin = 2,0 × ( 1,1 I T + 1,2 S + 3 FR + 3 HY + 3 HA+ 3 HD ) + TL According to standard DHM needs 532 mNm from gearbox output shaft. Gearbox allows 1300 mNm and motor with gearbox can provide 3108 mNm continuous torque.

Sensitivity analysis: DHM sensitivity to changes in components’ properties was calculated. Spring load increase +20%, friction increase +20% and motor torque reduction of -50% all together still leave remaining torque ratio 2.02 for the motor. (Yet including all safety factors and torque factor)

Reduction of spring constant by -20% reduces impact energy from 1.00 J to 0.796 J and dynamic force from 6.40 kN to 5.72 kN.

Thermal analysis:

A mathematical model was built presenting the DHM buried in sand assuming conduction + radiation into 2 cm thick sand layer. With -80C external temperature 90 minutes operation cycles with full 50 W power may be conducted; no sample melting or motor overheat will take place. At room temperature (terrestrial testing) using 30 W power there is a need to limit running time in 2 hrs. to protect the motor from overheating. At 54 W power maximum operation time is only 5 minutes.

Images: Thermal flow in DHM thermal analysis. (TKK). Thermal flow block-diagram and analysis results. (TKK)

Operational clearances: It was checked that the moving parts would not get jammed in any operational cases due to thermal expansion of materials. A worst case assumption was made assuming aluminium parts and steel parts assembled in 23 C room temperature with clearance of 0.01 mm. A temperature difference of 40 degrees between the parts was assumed; this was the maximum gradient seen in the thermal simulation. Both a cold case (-100 centigrade) and a hot case (+150 centigrade) of external operation temperature were checked. Minimum clearance in hot case with Al shaft + St housing was 0,00690 mm. For plastic parts 0.1 mm clearance was shown sufficient.

Vibration analysis: DHM capability to withstand spacecraft launch vibrations without any additional launch locks was examined. Load case consisted of a 20 g sinusoidal 3-axis vibration and 30 g omnidirectional quasistatic acceleration. Load carrying capacity of ball bearings, gearbox shaft, and contact forces of spring preloaded parts were checked.

Breadboard verification

Breadboard performance was checked by measuring the actual hardware. Impact energy was calculated from measured spring load and it is 0.98 J while theoretical calculation expected 1 J. Measured maximum torque for the gearbox is 368 mNm. Theoretical calculations indicated 266 mNm including uncertainty factor 3 for friction.

Tests and results

Image: The test-setup at Selex-Galileo in Milan (Selex-Galileo). Extensive testing of DHM was conducted at Selex-Galileo in Milan. See details in executive summary. Selex-Galileo procured several different tools and performed several tests with those. One specific tool is able to collect a core sample from rock. With the combination DHM-new thin corer it was possible to drill into granite with a penetration speed of 0.02 mm/minute with 100 N thrust and with 7.2 W power.

Images : The holes drilled into granite, the drill tools used during the DHM test campaign, schematics and pictures of commercial drill tools. (Selex-Galileo)

Only a small fraction of available 50 W power of DHM has been used and mechanical durability of DHM structure was not yet challenged. At the moment input velocity of the gearbox became a limiting factor for impact rate. Since available motor torque exceeds requirement the gearbox ratio can be decreased and impact frequency and penetration rate can be increased with somewhat higher input power.

The Down-hole Hammering Mechanism has proven its effectiveness to allow drilling into very hard material like granite and getting samples with low thrust applied and with very low power consumption in the range of 5-10W.


Tomi Ylikorpi, Lic.Sc.(Tech.)
Aalto University, School of Science and Technology
Department of Automation and Systems Technology
PO BOX 15500, 00076 AALTO
Tel. +358-50-400 6300,