TECHNICAL FIELD

The present disclosure relates to a hammer device comprising an electrically operated piston drive arrangement. The disclosure also relates to an electrically operated piston drive arrangement for a hammer device.

BACKGROUND

In rock drills, drill hammers, and other impact mechanisms, in the following collectively termed hammer devices, a hammer piston performs reciprocating motion in a cylindrical housing and makes repetitive impacts onto a shank adapter or other type of anvil. The hammer device, e.g., a percussion hammer, is configured to perform a repeated process of transferring force to a material by means of high frequency and energy intensive motion. The percussion hammer comprises a piston and a translator/distributor axially arranged relative each other in a housing, the piston being arranged to be moved axially in a reciprocating motion between a first position and a second position. During operation, a large quantity of kinetic energy is transferred to a tool attached to the piston; the kinetic energy being transferred in each stroke. Following each stroke, the piston will be moved back to a position from which it is possible to provide a next stroke, providing the same transfer of kinetic energy over and over again during the operation. Thus, the piston is subjected to high impact and high energy acceleration/deceleration with every hammer strike, and controlled operation of the piston is required regardless of the direction in which the piston travels.

In recent years, linear electric machines have been considered as piston drivers in percussion/drill hammers. The linear electric machine comprises a stator and a translator linearly movable in a longitudinal direction of the stator. The translator and stator are provided with magnetically operating means for converting electric energy into linear movement. W02020/058565 Al discloses a linear electric machine for driving a piston of a hammer device. In the disclosed solution, a mover of the linear electric machine is configured to operate as piston in a hammer device. The mover comprises permanent magnets provided one after another in a longitudinal direction of the mover. The disclosed arrangement provides a reciprocating movement of the piston in the hammer device.

WO2019/068958 Al discloses a hammer device comprising a linear electric machine for linearly moving an actuator member. The mover consists of centre rod surrounded by a plurality of annular elements provided in a stacked structure, one after another in in a longitudinal direction of the mover.

However, when providing joint elements or magnets in the movable part of the linear machine, the joint elements or magnets are subjected to high impact and high energy acceleration/deceleration with every hammer strike. There is also a significant heat exposure to the joint elements or magnets as well as an exposure to vibration. Thus, the linear machine will be operated in a challenging environment that will affect the fixtures between the joint elements and between the magnets and the mover, i.e., the fixture between each element or between each magnet and the mover, exposing the fixtures to wear and fatigue. The magnets are also typically sintered, and may crack due to the mechanical shock waves from repeating hammer strikes. Furthermore, the magnets are sensitive to heat and vibrations that may impair the magnetization of the permanent magnets. Failure in the fixture of an element or even a single magnet may cause significant damage to the hammer device, e.g., by partly blocking movement of the piston; impairments to the magnetization of permanent magnets may reduce the ability to achieve a desired impact when operating a hammer device comprising the linear machine.

SUMMARY

Despite known solutions in the field, it would be desirable to develop a hammer device and a piston drive arrangement that overcomes or alleviates at least some of the abovediscussed drawbacks of presently known solutions.

An object of the present disclosure is thus to provide a hammer device and piston drive arrangement that is less sensitive to wear and exhaustion resulting from the high impact and high energy acceleration/deceleration experienced during operation of the hammer device.

This and other objects are achieved by means of a hammer device and a piston drive arrangement as defined in the appended claims.

According to a first aspect of the present disclosure, this object is achieved by a hammer device comprising a hammer body, a hammer piston and an electrically operated piston drive arrangement. The piston drive arrangement comprises a stator, e.g., tubular stator, fixedly positioned within the hammer body and at least in part encompassing a translator configured to act upon the hammer piston. The stator comprises a plurality of peripheral windings embedded in an interior surface of the stator, e.g., in stator slots, and is configured to induce a reciprocating linear movement in the translator. The translator comprises an elongated stem surrounded by a peripheral, outwardly extending, e.g., circumferential, toothing integrally formed in the elongated stem along a longitudinal extension of the translator. The elongated stem comprises a material of relative magnetic permeability >10 at a magnetic flux density of 0.2 Tesla.

An advantage of the proposed hammer device is that it alleviates the problem of achieving an impact resistant fixture of magnets on the translator as well as the problem of achieving an impact resistant structure of the translator while maintaining electrical properties that enable sufficient impact when operating the hammer device.

In some examples the hammer piston is an integrated extension of the elongated stem.

The integration of the elongated stem and the hammer piston, further reduces problems of wear and exhaustion due to the high energy impact exposure on the movable parts in the hammer device.

In some examples, the toothing comprises a plurality of annular protrusions, e.g., homogeneously shaped annular protrusions, extending from the elongated stem.

Thus, a synchronous reluctance piston drive arrangement is achieved having salient poles and a consistent magnetic flux direction having a positive impact on eddy current generation within the translator. In some examples, two adjacent protrusions, e.g., annular protrusions, define an intermediate U-shaped or semi-circular cavity having sidewalls edgelessly extending from the elongated stem.

Consequently, a translator is achieved with improved wear/fatigue resistance, removing the existence of joints and inner corners within the translator.

In some examples, two adjacent protrusions, e.g., annular protrusions, have a mutual distance L and are separated by a gap having a width of 0.6-0.8 L, preferably 0.65-0.75 L when measured at a peripheral top surface of the protrusions.

In some examples, a subset of annular protrusions, wherein the subset may include each annular protrusion, has a tapered shape and a peripheral surface width less than a width closer to the cylindrical stem.

In some examples, the hammer body comprises an open end portion configured to allow a reciprocating movement of the hammer piston in a direction to and fro the hammer body, and a closed end portion comprising a gas-filled chamber configured to receive a first end part of the translator and to force a return action on the translator to accelerate the hammer piston in a direction of the open end portion of the hammer body.

According to a second aspect of the present disclosure, this object is achieved by an electrically operated piston drive arrangement for a hammer device. The piston drive arrangement comprising a stator fixedly positioned within a hammer body and at least in part encompassing a translator configured to act upon a hammer piston. The stator comprises a plurality of peripheral windings embedded in an interior surface of the stator, e.g., in stator slots, and is configured to induce a reciprocating linear movement in the translator. The translator comprises an elongated stem surrounded by a peripheral, outwardly extending, e.g., circumferential, toothing integrally formed in the cylindrical stem along a longitudinal extension of the translator, and the elongated stem comprises a material of relative magnetic permeability >10 at a flux density of 0.2 Tesla.

The improvements in the hammer device, piston drive arrangement and more specifically the translator design allows for reduction in maintenance and down-time of the hammer device due to piston operation failure. Furthermore, wear and fatigue properties are improved for the hammer device. The integration of the translator and hammer piston reduces the complexity of the hammer device as a whole and the piston drive arrangement in particular.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of the example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the example embodiments.

Figure 1 schematically discloses a hammer device according to an example;

Figure 2a-b schematically discloses a piston drive arrangement according to an example;

Figure 3 schematically discloses a section of a hammer body.

DETAILED DESCRIPTION

To achieve a robust and simple solution for electrically driving a percussion unit, a hammer device and piston drive arrangement according to the present disclosure has been developed. The proposed solution is applicable on various sorts of hammer devices, e.g., rock drills, breakers, drill hammers or percussion hammers, including stand-alone equipment, attachment tools on carrier vehicles, and comprised in drilling rigs or down- the-hole equipment.

Aspects of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings. The apparatus and method disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for the purpose of describing particular aspects of the disclosure only, and is not intended to limit the scope of the present disclosure. It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Embodiments of the present disclosure will be described and exemplified more fully hereinafter with reference to the accompanying drawings. The solutions disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the embodiments set forth herein.

Figure 1 schematically discloses an example hammer device 10 according to a non-limiting embodiment. The hammer device 10 is configured to be comprised in a rock drilling machine that may be attachable to a carrier vehicle, e.g., a drill rig or an excavator. The hammer device comprises a hammer body 11, a hammer piston 12 and an electrically operated piston drive arrangement 13. The piston drive arrangement 13 comprises a hollow, e.g., tubular, stator 14 fixedly positioned within the hammer body 11. In some examples, the stator 14 is fixedly positioned within the hammer body 11 by means of a suspension arrangement comprising an impact absorbing material such as a polymer or rubber. The stator 14 encompasses a translator 15, at least partly along a longitudinal extension of the stator 14. The translator 15 is configured to act upon the hammer piston 12 to move the hammer piston 12 in a linear direction to transfer a reciprocating motion to the hammer piston 12. The hammer piston 12 is configured to act upon a shank adapter 12a that in turn may act upon a hammer tool, e.g., a tool comprising a drill bit or chisel, at an extroverted end of the shank adapter 12a. A rotary drive motor of the rock drilling machine may be configured to rotate the shank adapter 12a of the hammer piston 12 during operation, e.g., at a time when the reciprocating motion is induced in hammer piston 12. In some examples, the stator 14 may comprise a central cavity having a noncircular cross section such as a polygon, i.e., an octagonal or a quadratic cross-section. In some examples, the stator 14 may be configured as a double-sided stator 14 with a flat translator 15. The hammer piston 12 may be operatively connected to the translator 15, e.g., mechanically connected to a translator 15 or integrated with the translator 15 as disclosed in Figure 1. The piston drive arrangement 13 may be centred along a longitudinal centre line of the hammer body 11, as exemplified in Figure 1. The stator 14 comprises a plurality of peripheral windings, e.g., copper windings, arranged in stator slots formed within an interior surface of the stator 14. Thus, the peripheral windings may be embedded in the interior surface of the stator 14, e.g., in stator slots. The stator slots may be equidistantly arranged at least over a part of a longitudinal extension of the stator 14. In some examples a non-equidistant arrangement of the stator slots is also conceivable, e.g., arranging the stator slots so that the distance between adjacent stator slots may vary along the longitudinal extension of the stator. Such arrangement of the stator slots may provide improvements when seeking to reduce cogging effects. In some examples, the stator 14 is configured to comprise 20-80 windings, preferably 30-60 windings and more preferred 35-45 windings. Fewer windings are generally used for longer pole lengths. A long pole length may imply a simplified manufacturing process with fewer and thicker windings, but may require a larger material thickness at the translator 15 which may have the drawback of reducing the force per translator mass ratio of the machine and thereby reducing the maximum acceleration of the piston. The windings may be configured as conductor coils, i.e., phase windings, arranged in internal stator slots within the stator 14, i.e., in the interior surface of the stator 14. The windings may be implemented so that each slot comprises only one conductor coil, or a plurality of conductor coils. Thus, the windings may be implemented in a concentrated manner having only one conductor coil/phase winding in each slot, or in a distributed manner where there may be more than one conductor coil/phase winding in each slot. Having a plurality of phases in the stator 14 provides benefits in that it provides improvements in the experienced force ripple, e.g., reducing the effects of force ripple. With three phases or more, it is possible to achieve a constant force and a total sum of the currents being zero, which removes the need for a return conductor. The stator slots may have an internal distance D, each two slots separated by a wall section having a width corresponding to 30-70% of the internal distance D, preferably 40-60% of D, and more preferred 45-55% of D. A protective layer, e.g., a polymer layer, may be provided on the interior surface of the stator 14 as a sealing layer on top of the windings; protecting the windings from mechanical effects of the reciprocating linear movement of the translator 15 and reducing the risk of a short circuit isolation failure in the stator. Typically, the windings are arranged to constitute a multi-phase winding structure, e.g. a three-phase winding structure. The windings may be configured as one-, two- or three- phase windings, that may be wound in opposite directions in adjacent stator slots. It is, however, also possible that each stator slot contains for example two conductor coils which can belong to different phases of the windings or to a same phase of the windings. In some examples, the windings are arranged in an equidistant configuration. When connected to a power grid or another electrical power source such as a battery, e.g., by means of a control system and a power converter, the electrical phase angle of the different phases is controlled using the position of the translator 15 and the desired acceleration direction, so that a suitable magnetic force is achieved. The translator position can either be estimated from the electrical signals using sensorless control or by determining the position by means of a sensor. When the estimation or measurement of the translator position is given in electrical degrees, a start-up procedure may be used where the translator 15 first is moved towards one of the end positions. From this known absolute position, the control system can then keep track of the absolute position during operation by using the position in electrical degrees. By using such control methods, an alternating current feed in the stator windings induces a reciprocating linear movement of the translator 15, i.e. moving the translator 15 in a direction parallel with a longitudinal centre line of the stator 14.

In some examples, the translator 15 has longitudinal length greater than that of the stator 14, having at least a first end part 15a extending beyond the longitudinal extension of the stator 14. A second end part 15b of the translator may also extend beyond longitudinal extension of the stator 14. The second end part 15b is operatively connected to the hammer piston, e.g., by the hammer piston being formed as an integrated extension of the second end part 15b. Thus, the hammer piston 12 may be formed as an integrated extension of the translator.

Turning to Figures 2a and b, a piston drive arrangement is exemplified. The translator 15 comprises an elongated stem 16, e.g., a cylindrically shaped stem/core structure or an equilateral rod, surrounded by a toothing 17 integrally formed in the translator stem. The translator 15 is made of a magnetic material, i.e., a highly permeable material of relative magnetic permeability >10 at a flux density of 0.2 Tesla. Examples of materials that may be used for the translator comprise hardened steel, including ferromagnetic steel and martensitic steel. The stem 16 is surrounded by a peripheral, outwardly extending, e.g., circumferential, toothing 17 integrally formed in the translator along a longitudinal extension of the translator, i.e., a longitudinal extension parallel with a longitudinal centre line of the translator. Thus, in the disclosed example configuration, the translator has a geometry comprising a plurality of distinct, integrated, equally shaped protrusions 17a, also known as ridges, that rise from the stem 16, preferably with a disc-shaped geometry in the radially outwardly directed parts of the protrusions.

In some examples, adjacent protrusions 17al, 17a2, e.g., each two adjacent protrusions, define an intermediate U-shaped or semi-circular cavity having sidewalls edgelessly extending from the elongated stem. In some examples, two adjacent protrusions have a mutual distance L; the distance being determined with respect to a corresponding measurement points on each protrusion and corresponding to a pole length. In some examples, the cavity defined by two adjacent protrusions has a width W of 0.6-0.8 L, preferably 0.65-0.75 L when measured at a peripheral surface of the protrusions.

An electrical pole is formed over each protrusion and adjacent cavity when powering the piston drive arrangement 13. A magnetic flux is induced in a closed loop extending over a pole length L, i.e., induced over a translator protrusion/cavity pairand overa corresponding section of the stator 14 comprising one or more conductor coils as exemplified in Figure 2b. When configuring the piston drive arrangement 13 with a short pole length, it is possible to produce a high magnetic force with a light translator, e.g., a translator 15 having a hollow stem 16. The wall thickness of a protrusion L-W, i.e. the difference between a pole length and a width of a cavity, is selected based on an electro-magnetic perspective. Thus, the wall thickness (protrusion width) is selected to be thick enough to carry the magnetic flux from one protrusion to another without saturating the material in the translator stem. The shorter the pole length is, the smaller the flux that needs to be carried by the translator stem becomes. Since the magnetic shear stress in the air gap can be maintained with shorter poles, more magnetic force per unit mass of the translator can be achieved since more airgap surface per unit mass ofthe translator can be fitted in. However, the drawback of short pole lengths is the increased manufacturing difficulty. In the present disclosure, example pole lengths of 8-50 mm, preferably 12-35 mm and more preferably 15-25 mm are applicable.

As previously disclosed, two adjacent protrusions 17al, 17a2, e.g., annular protrusions, may define an intermediate U-shaped or semi-circular cavity having sidewalls edgelessly extending from the elongated stem. The two adjacent protrusions may be separated by a distance L; a distance that may correspond to a pole length. The semi-circular cavity may have a diameter corresponding to the outer width of the cavity. In some examples, the U- shaped cavity is delimited by two symmetrically shaped curved corners having respective inner radius greater than 2.5% of a distance between two adjacent protrusions, and preferably greater than 10%. In some examples, an inner radius greater than 20%, and more preferably greater than 30% of the distance between two adjacent protrusions may be selected, providing for a smooth, edgeless cavity that provides benefits with regards to withstanding fatigue due to the repeated strokes.

In the most outwardly directed part, the plurality of protrusions may have outwardly directed external surfaces of a same diameter along a longitudinal direction of the protrusions, e.g., surfaces orthogonal to the longitudinal extension of the translator. Thus, each protrusion may have an outwardly facing, cylindrical surface and the outer surfaces of the plurality of protrusions define a cylinder shape. In some examples, a subset of the annular protrusion may have a tapering shape, and a peripheral surface width less than a width closer to the elongated stem. In some example, all annular protrusions may have an equal shape that may be a tapering shape. An outmost diameter of the translator, i.e., protrusion, and intermediate air gaps formed between opposite sides of the translator and stator 14, correspond to the inner diameter of the stator 14. At the outermost part of the protrusion, an air gap G to the inner surface of the stator 14 may be in the range of 0.25 to 1 mm, preferably 0.25-0.5 mm. At the bottom of the cavity formed between each two protrusions, an air gap to the stator 14 may be 1-15 mm, preferably 2-8 mm, more preferably 3-7 mm and most preferably 3-5 mm. The translator material is configured to experience a high degree of magnetization when operated in the piston drive arrangement 13. In some examples, the translator 15 may be at least partly hollow, e.g., a cylindrically shaped hollow stem/core structure embodied in a homogeneous material. In other examples, the elongated stem 16 may comprise two or more materials, e.g., a low density inner core material, and a high strength outer core material; i.e., the outer core comprising an integrated toothing. In some examples, the translator 15 is made of a homogeneous material, providing for a constant elasticity module and density throughout the translator. In some examples, the translator 15 may have a transversal cross section of the same dimensions along the longitudinal extension of translator. In some examples, the translator 15 has a cross section designed to generate a stress wave with gradually increasing force, e.g., a cross section that gradually increases or decreases along the longitudinal direction.

When the translator and hammer piston 12 are configured as a single integrated unit, i.e., by integrating the cylindrical stem and the piston, a highly permeable magnetic material used for the integrated unit may be hardened in a piston portion of the unit to ensure sufficient wear/fatigue resistance at the piston portion configured to act upon a hammer tool.

The toothing 17 comprises a plurality of shaped teeth, i.e., a plurality of protrusions extending from the stem. Each protrusion 17a is magnetic, preferably having homogeneous magnetic properties. When powering the piston drive arrangement 13, a magnetic flux may travel in closed loops extending over one or more pole length, i.e., one or more translator protrusions and corresponding sections of the stator 14 comprising one or more conductor coils/phase windings. The pole length L of the translator 15 can be made to match the corresponding periodicity on the stator 14, i.e., periodicity of the stator windings 14a, so that each pole along the translator experiences an equal stator magnetic circuit. The configuration of the stator windings 14a is selected to maximise the average force produced. In some examples, the translator toothing 17 and periodicity of stator windings 14a will be configured with a slight mismatch; providing the benefits of reducing force ripple and voltage/current ripple substantially. There are various ways to achieve such a configuration. In some examples, the translator 15 is arranged to have a toothing 17 that provides for a slight increase or decrease of the pole length. In some examples, the translator protrusions are arranged with varying distances, i. e., implying varying individual pole distances along the movement direction of the translator to form a suitable pattern that can be optimized to yield and optimum balance between average force and force/voltage/current ripple.

Figures 2a and b bring forward further details of the piston drive arrangement as explained in the description of Figure 1. The piston drive arrangement 13 comprises a hollow, e.g., tubular, stator 14 fixedly positioned within a hammer body 11, and a translator configured to act upon a hammer piston 12. The stator 14 comprises a plurality of peripheral windings 14a embedded in an interior surface of the stator 14, i.e., within stator slots 14b, and is configured to induce a reciprocating linear movement in the translator. Thus, the piston drive arrangement 13 is a linear electric machine. The translator comprises the elongated stem 16 surrounded by the peripheral, outwardly extending toothing 17 that is integrally formed in the translator 15 along a longitudinal extension of the translator. The translator is made of a material having magnetic properties, i.e., having relative magnetic permeability >10 at a magnetic flux density of 0.2 Tesla. In some examples, the translator comprises hardened steel, including ferritic steel and martensitic steel. In other examples, the translator may comprise two or more materials, e.g., a low density inner core material and a high strength material used at least in part in the toothing. The translator and the electromagnetically active parts of the stator 14 are preferably rotationally symmetric with respect to a symmetry line. Figure 2a and b, represents a three-phase configuration of the piston drive arrangement 13; wherein the translator protrusions/teeth are distanced from one another in a longitudinal direction by a distance L corresponding to a distance comprising 2 to 4, preferably 2.5 to 3.5, and most preferably 2.8 to 3.2 windings 14a provided on the stator 14. Thus, in the three phase configuration, supporting three phases each having 2 winding directions, the stator 14 is configured for six current variants separated by 60 degrees electrically. In some examples, not disclosed, a two-phase configuration may be employed as well as a one-phase configuration. In the two-phase configuration, the distance L between two adjacent translator teeth/protrusions will correspond to a distance comprising two adjacent windings provided on the stator. For the one-phase configuration, the distance L between two adjacent translator teeth/protrusions will correspond to a distance comprising a single winding on the stator. Note, however, that an implementation with multiple phases does not necessarily need to have two different winding directions in the stator slotsl4b. Any of the phase configurations mentioned can be implemented with half the number of slots and windings per electrical period, and with twice as many electrical degrees between each stator slot 14b. Then, there is also only half as many slots in the stator 14 for one distance between the protrusions on the translator. Such a configuration has the advantage that fewer slots and windings are required which makes manufacturing simpler, but on the other hand it increases the force variation which generates more vibrations. The piston drive arrangement can of course be implemented with more than three phases as well, and the more phases that are employed, the smoother the force will be but the more complex the machine will be to manufacture and assemble.

Turning to Figure 2b, details of the translatortoothing are disclosed. In some examples, the toothing comprises a plurality of homogeneously shaped protrusions, e.g., teeth, extending from the stem and having a base width at a base portion joined to the stem and distanced from one another by a gap distance. In some examples, the width of the translator teeth was set to 30% of the distance L between two adjacent protrusions, i.e., 30% of a pole length. The protrusions may have a tapered shape and a peripheral surface width less than the base width, e.g., a root width representing 30-50% of the pole width. The tapering width of the translator teeth provides the advantage of reducing the magnetic saturation effects and thereby the magnetic reluctance in the innermost portion of the toothing, i.e., the portion integrated with the elongated stem. Furthermore, the tapering width of the protrusions reduces mechanical stress in said innermost portion. In some examples, the stem of the translator has a cylindrical shape with a diameter d, and the toothing integrally formed in the translator. The diameter of the elongated stem will vary with the size of the hammer device.

Figure 2b also discloses a portion of the stator 14 and its plurality of peripheral windings 14a, e.g., copper windings, arranged in stator slots 14b within an interior surface of the stator 14. In some examples, the stator 14 is configured to comprise 20-80 windings, preferably 30-70 windings and more preferred 35-65 windings. The windings may be configured as conductor coils arranged in internal stator slots 14b within the stator 14. The stator slots may have an average internal distance D, each two slots separated by a wall section having a width corresponding to 30-70% of the internal distance D, preferably 40- 60% of D, and more preferred 45-55% of D. In some examples, there is at least one tooth per winding 14a arranged in the stator 14, i.e., the number of peripheral windings embedded into the interior surface of the stator 14 corresponds to a multiple of the number of teeth comprised in the peripheral, outwardly extending, e.g., circumferential, toothing of the translator. In a three phase configuration, a number of windings arranged along the stator 14, may correspond to a multiple of 2 to 4, preferably 2.5 to 3.5 and most preferably 2.8-3.2 times the number of teeth provided along a corresponding longitudinal extension of the translator. Another option is to provide half that number of windings, and only use windings wound in the same direction.

Turning back to Figure 1, open and closed end portions 11a, lib of the hammer body 11 are disclosed. The hammer body 11 comprises an open end portion 11a configured to allow a reciprocating movement of the hammer piston 12 in a direction to and fro the hammer body 11, and a closed end portion lib comprising a gas-filled chamber that will be described in more detail in Figure 3. The gas-filled chamber is configured to receive a first end part 15a of the translator and to force a return action on the translator to accelerate the hammer piston 12 in a direction of the open end portion 11a of the hammer body 11. The toothed part of the translator 15 may be longerthan the stator 14 so that at least some of the teeth are moved into and out of the interior of the stator 14 during the reciprocating linear motion. In some examples, the toothed part of the translator 15 may also be arranged to be shorter than the stator 14, which may imply the drawback that the electrical machine becomes heavier and less efficient, but which maximizes the force per unit translator mass since all the translator teeth then are active during the entire motion. An outer diameter of the toothed part of the translator 15 essentially corresponds to the inner diameter of the stator 14, leaving an air gap G, as indicated in Figure 2b. The air gap may be less than < 1mm, preferably <0.5mm, and more preferably <0.35mm. The electromagnetic performance of the machine increases when the air gap is made smaller, but the smaller the air gap is the harder it is to construct and manufacture the hammer device and finer manufacturing tolerances of the machine parts are required. Turning again to the disclosure in Figures 2a-b, the hammer body 11 may comprise bearings 18, e.g., positioned to support the translator 15 at said open and closed end portions 11a, lib of the hammer body. The bearings 18 having a diameter corresponding to a diameter of the translator. In some examples, a bearing arranged in the vicinity of the first open end portion 11a of the hammer body 11 has an inner diameter corresponding to an outer diameter of the toothing 17 of the translator 15. In some examples, a bearing arranged in the vicinity of the closed end portion lib has an outer diameter corresponding to a cavity in the second part of the stem of the translator or an inner diameter corresponding to the outer diameter of the toothing. The bearings 18 will be arranged to bear on the translator, thereby supporting movement of the translator within the stator 14. In some examples, the bearings 18 are made of a low friction material, e.g., brass or plastic. This provides the advantage that the electromagnetic performance is not dependent on the electrical conductivity and the magnetic permeability of the bearing material, which would be a problem when using a translator having permanent magnets.

In some examples, the cavities between each two protrusions 17al, 17a2 of the translator may be filled with a non-magnetic and electrically non-conducting material, e.g., a nonmagnetic material having an elastic modulus >1 GPa, while the translator may have an elastic modulus 180-220 GPa, preferably 190-210 GPa. In order to alleviate the known problems of wear and fatigue, a protective layer, e.g., a polymer layer, may be provided on surface of the translator as a sealing layer; protecting the non-magnetic filling from mechanical effects of the reciprocating linear movement of the translator.

In order to alleviate the known problems of wear and fatigue, a protective layer, e.g., a polymer layer, may be provided on surface of the translator as a sealing layer; protecting the non-magnetic filling from mechanical effects of the reciprocating linear movement of the translator.

Further, the translator may experience eddy currents which arise from a time-varying magnetic flux in the translator. In some examples, longitudinal cuts are provided in the translator toothing and/or in the stem along the longitudinal extension of the translator, i.e., in the direction of movement. This provides the benefits of reducing eddy currents in the toothing and partly in the stem and thereby increases the efficiency of the electrical machine.

Figure 3 discloses a section of an example hammer body 11 comprising the above described piston drive arrangement 13. The translator 15 is configured so that at least a first end part 15a of the translator 15 extends outside the encompassing stator 14 throughout the reciprocating movement of the translator 15. The hammer body 11 further comprises a chamber 19, configured to receive and accelerate the translator at a turning position of reciprocating motion.

In the disclosed example, the translator 15 is at least partially hollow and configured to receive a cylinder shaped piston 20 extending from the upper closed end portion lib of the hammer body 11 into a corresponding cylindrical cavity formed in the translator 15. Consequently, the cylindrical cavity of the translator may be configured to represent a further gas-filled chamber contributing to the acceleration of the translator stroke when reaching a turning point in the reciprocating movement of the translator. This optional configuration of the upper closed end portion lib of the hammer body 11 provides radial support during the reciprocating movement of the translator and contributes to the acceleration of the translator stroke from the return position of the translator 15. As a result, a higher gas pressure can be achieved, so that when the chamber acts as a gas spring, the force of the spring will be larger, the stroke shorter and the bounce time considerably shorter.

In some examples, a bearing 18 arranged on the cylindrical shaped piston 20 has an outer diameter corresponding to an inner diameter of the cavity of the translator. The bearing 18 is arranged to bear on the translator, radially supporting movement of the translator in the longitudinal direction of the piston drive arrangement 13. As mentioned, the space formed between the cylindrical shaped piston and the translator represents a further gas- filled chamber 19a capable of forcing a return action on the translator. In some examples, the bearing is made of a low friction material, e.g., brass or plastic.

Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and/or is implied from the context in which it is used. Reference has been made herein to various embodiments. However, a person skilled in the art would recognize numerous variations to the described embodiments that would still fall within the scope of the claims.

Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, wherever suitable. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa.

In the drawings and specification, there have been disclosed exemplary aspects of the disclosure. However, many variations and modifications can be made to these aspects without substantially departing from the principles of the present disclosure. Thus, the disclosure should be regarded as illustrative rather than restrictive, and not as being limited to the particular aspects discussed above. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

Hence, it should be understood that the details of the described embodiments are merely examples brought forward for illustrative purposes, and that all variations that fall within the scope of the claims are intended to be embraced therein.