Patent Description:
In many applications, there is a need for linear movement that can be generated with for example a linear electric machine or with hydraulic means. Furthermore, there can be a need to keep the linear movement within an allowed range with the aid of a brake that smoothly stops the linear movement instead of abruptly stopping the linear movement. For example, a brake for linear movement may be useful in a hammer device that is used as an attachment to an excavator or another working machine where the intention is to break up for example stone, concrete, or some other material. A hammer device of the kind mentioned above is often hydraulically driven, allowing it to be connected to the hydraulic system of an excavator or another working machine. The hydraulic hammer device comprises a percussion mechanism capable of delivering impacts to an actuator member, e.g. a chisel, whose end forms a tip which transmits the impacts to material to be broken up. If there are no means for smoothly stopping a linear movement, the hammer device might get damaged in a situation in which an impact is directed to the actuator member when the tip of the actuator member is not against any external material such as stone or concrete, because mechanical structures supporting the actuator receive impacts that would be received by the external material when the external material is being hammered. The impacts directed to the mechanical structures supporting the actuator may cause e.g. material fatigue that may lead to damages.

Braking force generated with a brake for linear movement is advantageously progressive so that the braking force increases along with the linear movement being braked. The progressive braking force makes it possible that a movement range on which the brake is active is short but still the linear movement can be stopped smoothly. For example, in conjunction with a hammer device, a non-progressive brake generating a substantially constant braking force along a linear movement would lead to a compromise between the mechanical length of the construction, smoothness of braking, and power losses which would occur if the movement range on which the brake is active overlaps with the normal operating movement range.

Publication <CIT> describes a mechanical-hydraulic double-acting drilling jar that comprises an inner tubular mandrel telescopingly supported inside an outer tubular housing. The mandrel and the housing each consist of a plurality of tubular segments joined together, preferably by threaded inner connections. Upper and lower pressure pistons are slidably disposed within the housing, respectively closing upper and lower substantially sealed hydraulic chambers. Longitudinal movement of the mandrel engages the collet, which in turn, translates either the upper piston or the lower piston, depending on the direction of mandrel movement. As one of the pistons is moved, fluid pressure builds in the associated hydraulic chamber, retarding further movement of the mandrel, enabling potential energy to build in the drill string. The collet is restricted from expanding until the mandrel reaches a particular point in the housing, at which time the collet expands, releasing the mandrel to rapidly collide a hammer surface thereon with an anvil surface in the housing.

Publication <CIT> describes a motor-driven hammer tool that comprises a casing, a hollow cylinder closed at one end slidable in the casing, and a piston slidable in the cylinder. The cylinder or the piston is formed as an impact body and the other is connected to a motor by a crank and a connecting rod. An intermediate space between the front of the piston and the closed end of the cylinder acts as an air cushion by which the movement of the driven part is transmitted to the impact body.

Publication <CIT> describes a hammer drill that has a piston reciprocated within a cylinder and an impact element pneumatically coupled to the piston via an air chamber to impact against an insert at the front end of the cylinder. A second air chamber is provided between the impact element and the insert, with regulation of an air flow from this air chamber to the outside upon compression during the impact element forwards movement.

Publication <CIT> describes a pneumatic percussive mechanism for a percussion drill and/or a drill hammer. The mechanism comprises, housed in the housing of the percussive mechanism, an axially reciprocating percussion piston. A drive piston is axially reciprocated in a guide cylinder of said percussion piston. The guide cylinder is provided on its inner face with one or more air compensation pockets. At least one idle opening extends through the guide cylinder. In the percussive action, the idle opening does not communicate with the ambience while in the idle mode the idle opening can be displaced via an idle channel that leads to the ambience and thus connects a cavity that receives a pneumatic piston with the ambience by way of the idle opening. The air compensation pockets and the idle openings allow an optimization of the piston return behavior and of the idle operation of the percussive mechanism.

Publication <CIT> describes a powered hammer assembly that comprises:
a housing, a power cell that includes a piston, and a locking valve body assembly that comprises: a valve body that defines a void configured to contain a pressurized fluid, a locking member that is configured to be biased by pressurized fluid into a locking configuration, and a retainer member. The housing defines a first aperture that is configured to receive the locking member, and the housing further defines a retaining slot that is configured to receive the retainer member.

Publication <CIT> discloses a hammer device with a frame attachable to a working machine. The hammer device comprising a linear electric machine having a mover.

The following presents a simplified summary to provide a basic understanding of some aspects of various invention embodiments.

In this document, the word "geometric" when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric point, a straight or curved geometric line, a geometric plane, a non-planar geometric surface, a geometric space, or any other geometric entity that is zero, one, two, or three dimensional.

In accordance with the invention, there is provided a new hammer device that is connectable to e.g. an excavator or another working machine. A working machine, such as e.g. an excavator, is typically called an off-road machine. However, to emphasize that an ability for off-road operation is possible but not necessary, the broader term "working machine" is used in this document. A hammer device according to the invention comprises:.

The brake of the hammer device comprises:.

The above-mentioned center element comprises a collar configured to abut to a contact portion of the annular element and to move the annular element together with the center element in a second direction opposite to the first direction and against a spring force of the spring. A portion of the annular element surrounding the contact portion comprises apertures configured to conduct the damper fluid to flow to and from the annular chamber. The above-mentioned spring is a plate spring, i.e. a disc spring, configured to gradually close the apertures in response to a situation in which the annular element is moved in the second direction against the spring force, where a plate of the spring gets gradually parallel with a surface of the annular element around the apertures in response to a situation in which the spring is compressed.

When the above-mentioned plate of the spring gets gradually parallel with the surface of the annular element around the apertures, an ability of the plate to inhibit a flow of the damper fluid via the apertures gradually increases and thereby the braking force increases along with a linear movement. Thus, there is no need for separate valves for controlling the flow of the damper fluid.

In a hammer device according to an exemplifying and non-limiting embodiment, the mover of the linear electric machine is the center element of the brake. In a hammer device according to another exemplifying and non-limiting embodiment, the actuator member is the center element of the brake.

Exemplifying and non-limiting embodiments are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of un-recited features.

Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:.

The invention and the embodiments thereof are not limited to the exemplifying and non-limiting embodiments described below. Thus, the specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims.

<FIG> shows a section view of a brake <NUM> of a hammer device according to an exemplifying and non-limiting embodiment. The geometric section plane is parallel with the geometric yz-plane of the coordinate system <NUM>. The brake <NUM> comprises a frame structure <NUM> and an annular element <NUM> that is linearly movable with respect to the frame structure <NUM>. The brake <NUM> comprises a center element <NUM> that is surrounded by the annular element <NUM> and linearly movable with respect to the frame structure <NUM> and the annular element <NUM>. The frame structure <NUM>, the annular element <NUM>, and the center element <NUM> constitute an annular chamber <NUM> for damper fluid e.g. oil. The center element <NUM> can be for example rotationally symmetric with respect to geometric line <NUM>. It is however also possible that the center element <NUM> has a non-circular cross-sectional shape. The brake <NUM> comprises a spring <NUM> configured to move the annular element <NUM> with respect to the frame structure <NUM> in a first direction, i.e. in the positive z-direction of the coordinate system <NUM>, in which the volume of the annular chamber <NUM> is increasing. The center element <NUM> comprises a collar <NUM> configured to abut to a contact portion <NUM> of the annular element <NUM> and to move the annular element <NUM> together with the center element <NUM> in a second direction, i.e. in the negative z-direction of the coordinate system <NUM>, opposite to the first direction and against the spring force of the spring <NUM>. A portion of the annular element <NUM> surrounding the contact portion <NUM> comprises apertures <NUM> configured to conduct the damper fluid to and from the annular chamber <NUM>. The spring <NUM> is a plate spring configured to gradually close the apertures <NUM> in response to a situation in which the annular element <NUM> is moved in the second direction against the spring force, i.e. in the negative z-direction of the coordinate system <NUM>. A plate <NUM> of the spring <NUM> gets gradually parallel with a surface of the annular element <NUM> around the apertures <NUM> in response to a situation in which the spring <NUM> is compressed. When the plate <NUM> of the spring <NUM> gets gradually parallel with the surface of the annular element <NUM> around the apertures <NUM>, an ability of the plate <NUM> to inhibit a flow of the damper fluid via the apertures <NUM> gradually increases and thereby the braking force increases along with a linear movement in the negative z-direction of the coordinate system <NUM>. Thus, there is no need for separate valves for controlling the flow of the damper fluid.

In the exemplifying brake <NUM> illustrated in <FIG>, the contact portion <NUM> of the annular element <NUM> comprises other apertures <NUM> which are configured to be closed by the collar <NUM> of the center element <NUM> in response to a situation in which the collar <NUM> moves the annular element <NUM> in the second direction in which the volume of the annular chamber <NUM> is decreasing. The apertures <NUM> are configured to conduct the damper fluid into the annular chamber <NUM> in response to a situation in which the collar <NUM> is a distance away from the contact portion <NUM> and the spring <NUM> moves the annular element <NUM> in the first direction in which the volume of the annular chamber <NUM> is increasing. Thus, the apertures <NUM> do not reduce the braking force since they are closed by the collar <NUM> when the collar <NUM> moves the annular element <NUM> in the second direction. On the other hand, the apertures <NUM> allow the damper fluid to flow into the annular chamber <NUM> when the collar <NUM> is a distance away from the contact portion <NUM>. Thus, the apertures <NUM> make the brake <NUM> faster to recover into a position in which the spring <NUM> is not compressed and the brake <NUM> is ready to operate.

<FIG> shows a section view of a brake <NUM> of a hammer device according to an exemplifying and non-limiting embodiment. The geometric section plane is parallel with the geometric yz-plane of the coordinate system <NUM>. The brake <NUM> comprises a frame structure <NUM> and an annular element <NUM> that is linearly movable with respect to the frame structure <NUM>. The brake <NUM> comprises a center element <NUM> that is surrounded by the annular element <NUM> and linearly movable with respect to the frame structure <NUM> and the annular element <NUM>. The annular element <NUM> constitutes a wall of an annular chamber <NUM> for damper fluid e.g. oil. The center element <NUM> can be for example rotationally symmetric with respect to geometric line <NUM>. It is however also possible that the center element <NUM> has a non-circular cross-sectional shape. The brake <NUM> comprises a spring <NUM> configured to move the annular element <NUM> with respect to the frame structure <NUM> in a first direction, i.e. in the positive z-direction of the coordinate system <NUM>, in which the volume of the annular chamber <NUM> is increasing. The center element <NUM> comprises a collar <NUM> configured to abut to a contact portion <NUM> of the annular element <NUM> and to move the annular element <NUM> together with the center element <NUM> in a second direction, i.e. in the negative z-direction of the coordinate system <NUM>, opposite to the first direction and against a spring force of the spring <NUM>. A portion of the annular element <NUM> surrounding the contact portion <NUM> comprises apertures <NUM> configured to conduct the damper fluid to and from the annular chamber <NUM>. The spring <NUM> is a plate spring configured to gradually close the apertures <NUM> in response to a situation in which the annular element <NUM> is moved in the second direction against the spring force. A plate <NUM> of the spring <NUM> gets gradually parallel with a surface of the annular element <NUM> around the apertures <NUM> when the spring <NUM> is compressed. When the plate <NUM> of the spring <NUM> gets gradually parallel with the surface of the annular element <NUM> around the apertures <NUM>, an ability of the plate <NUM> to inhibit a flow of the damper fluid via the apertures <NUM> gradually increases and thereby the braking force increases along with a linear movement in the negative z-direction of the coordinate system <NUM>.

<FIG> shows a hammer device <NUM> according to an exemplifying and non-limiting embodiment. <FIG> and <FIG> show section views taken along a line A-A shown in <FIG> in two different situations. The geometric section plane is parallel with the yz-plane of a coordinate system <NUM>. <FIG> shows a magnification of a part B of <FIG> and <FIG>. The hammer device <NUM> comprises a frame <NUM> attachable to a working machine, e.g. such as to the boom of an excavator in place of a bucket. The frame <NUM> comprises attachment members <NUM> for attaching to the working machine so that the frame is nondestructively detachable from the working machine. The hammer device <NUM> comprises an actuator member <NUM>, e.g. a chisel, linearly movably supported by the frame <NUM>. An end of the actuator member <NUM> is shaped to constitute a tip for breaking material e.g. stone or concrete.

The hammer device <NUM> comprises a linear electric machine <NUM> having a mover <NUM> and a stator <NUM>. The mover <NUM> is configured to direct impacts to the actuator member <NUM> in the negative z-direction of the coordinate system <NUM>. <FIG> shows a situation in which the mover <NUM> is in its lowest position and in contact with the actuator member <NUM>. <FIG> shows a situation in which the mover <NUM> is in an upper position. The stator <NUM> is attached to the frame <NUM> and comprises windings for generating a magnetic force directed to the mover <NUM> in response to electric current supplied to the windings. The windings may constitute for example a multi-phase winding, e.g. a two- or three-phase winding. In the exemplifying hammer device <NUM> illustrated in <FIG>, the linear electric machine <NUM> is a tubular linear electric machine in which the conductor coils of the windings are configured to surround the mover <NUM>. <FIG>, <FIG>, and <FIG> show cross-sectional views of the conductor coils of the windings. In <FIG> and <FIG>, the cross-sections of the conductor coils are depicted by black rectangular patterns. In <FIG>, two of the conductor coils of the windings are denoted with references <NUM> and <NUM>. The mover <NUM> can be, for example, substantially rotationally symmetric with respect to a geometric line <NUM> shown in <FIG>.

The hammer device <NUM> comprises a brake <NUM> according to an exemplifying and non-limiting embodiment of the invention. The brake <NUM> can be for example like the brake <NUM> illustrated in <FIG>. In this exemplifying hammer device <NUM>, the mover <NUM> of the linear electric machine <NUM> is the center element of the brake <NUM>. The brake <NUM> inhibits a linear movement of the mover <NUM> in the negative z-direction of the coordinate system <NUM> when a collar <NUM> of the mover <NUM> is in contact with an annular element <NUM> of the brake <NUM>.

In the exemplifying hammer device <NUM> illustrated in <FIG>, the mover <NUM> comprises annular permanent magnets provided one after another in the longitudinal direction of the mover <NUM>, i.e. in the direction of the z-axis of the coordinate system <NUM>. The axial direction of the annular shape of each permanent magnet coincides with the longitudinal direction of the mover <NUM>. In <FIG>, two of the annular permanent magnets are denoted with references <NUM> and <NUM>. The magnetizing directions of the permanent magnets coincide with the longitudinal direction of the mover <NUM>, and the magnetizing directions of successive permanent magnets are opposite to each other. The magnetizing directions of the permanent magnets are indicated with arrows in <FIG>. Exemplifying magnetic flux lines are depicted with dashed lines. In this exemplifying case, the mover <NUM> comprises a center rod <NUM> and annular ferromagnetic elements provided around the center rod <NUM> to form a ferromagnetic core structure of the mover <NUM>. In <FIG>, two of the annular ferromagnetic elements of the mover <NUM> are denoted with references <NUM> and <NUM>. As shown in <FIG>, each annular permanent magnet is situated between two successive annular ferromagnetic elements.

Advantageously, the center rod <NUM> of the mover <NUM> is made of non-ferromagnetic material in order to maximize a magnetic coupling between the permanent magnets and the windings of the stator <NUM>, i.e. to minimize a leakage flux via the center rod <NUM>. The center rod <NUM> can be made of for example austenitic steel or some other suitable non-ferromagnetic and sufficiently strong material.

In a hammer device according to an exemplifying and non-limiting embodiment, the above-mentioned annular ferromagnetic elements of the mover <NUM> are made of solid steel and provided with slits extending from an air-gap surface of the mover towards, but without reaching, a surface of a yoke section of a ferromagnetic core-structure of the mover and extending longitudinally through the annular ferromagnetic elements. <FIG> shows the annular ferromagnetic element <NUM> when seen along the negative z-direction of the coordinate system <NUM>. In <FIG>, one of the slits is denoted with a reference <NUM> and the air-gap surface of the mover is denoted with a reference <NUM>. The solid steel provided with the slits emulates a laminated structure and thus eddy currents induced in the ferromagnetic core-structure are reduced. Despite the slits, each annular ferromagnetic element is a single piece of material since the slits do not reach the surface of the annular ferromagnetic element constituting a part of the surface of the above-mentioned yoke section. The strongest magnetic flux variations take place near the airgap and thus the slits are more important in areas near the airgap than in areas near the surface of the yoke section.

In a hammer device according to an exemplifying and non-limiting embodiment, the above-mentioned slits are filled with electrically insulating solid material that can be for example resin. The electrically insulating solid material in the slits can be useful e.g. for dampening vibrations of the steel portions between the slits.

In a hammer device according to an exemplifying and non-limiting embodiment, the slits of the annular ferromagnetic elements of the mover comprise first slits and second slits so that the first slits are shorter in a radial direction than the second slits. In the exemplifying case illustrated in <FIG>, an angle between adjacent slits is <NUM> degrees on the area where there are both the longer slits and the shorter slits and an angle between adjacent slits is <NUM> degrees on the area where there are only the longer slits.

It is to be noted that hammer devices according to exemplifying and non-limiting embodiments may have different ferromagnetic core structures of the mover, and thus hammer devices according to exemplifying and non-limiting embodiments are not limited to any specific ferromagnetic core structures of the mover.

In the exemplifying hammer device <NUM> illustrated in <FIG>, the ferromagnetic core structure (<NUM>) of the stator <NUM> comprises annular ferromagnetic elements which surround the mover <NUM> and which are stacked one after another in the longitudinal direction of the mover and form slots for conductor coils of the stator windings. In <FIG>, two of the annular ferromagnetic elements of the stator <NUM> are denoted with references <NUM> and <NUM>. An exemplifying way of implementing the windings of the stator <NUM> is that each slot is provided with only one conductor coil belonging to one phase of the windings. It is also possible to provide each slot, for example, with two conductor coils belonging either to a same phase of the windings or to two different phases of the windings. The stator <NUM> comprises also a stator frame <NUM> having cooling channels for conducting cooling fluid, e.g. water or air. In <FIG>, one of the cooling channels is denoted with a reference <NUM>.

In a hammer device according to an exemplifying and non-limiting embodiment, the above-mentioned annular ferromagnetic elements of the stator <NUM> are made of solid steel and provided with slits extending from an air-gap surface of the stator towards, but without reaching, a surface of a yoke section of the ferromagnetic core-structure and extending longitudinally through the annular ferromagnetic elements. <FIG> shows the annular ferromagnetic element <NUM> when seen along the negative z-direction of the coordinate system <NUM>. In <FIG>, one of the slits is denoted with a reference <NUM> and the air-gap surface of the stator is denoted with a reference <NUM>. The solid steel provided with the slits emulates a laminated structure and thus eddy currents induced in the ferromagnetic core-structure are reduced. Despite the slits, each annular ferromagnetic element is a single piece of material since the slits do not reach the surface of the annular ferromagnetic element constituting a part of the surface of the above-mentioned yoke section. The strongest magnetic flux variations take place near the airgap and thus the slits are more important in areas near the airgap than in areas near the surface of the yoke section.

It is to be noted that hammer devices according to exemplifying and non-limiting embodiments may have different ferromagnetic core structures of the stator, and thus hammer devices according to exemplifying and non-limiting embodiments are not limited to any specific ferromagnetic core structures of the stator.

The mover <NUM> can be moved in a controlled way for example with a power electronic converter coupled to the windings of the stator <NUM>. In many cases, it is advantageous for the control by the power electronic converter to know the position of the mover <NUM> with respect to the stator <NUM>. For example, the position of the mover <NUM> can be measured with a mechanical position sensor comprising a sensor rod fixed to the mover. The position of the mover <NUM> can be measured also in a contactless way, for example with a laser measurement arrangement. It is also possible to provide the mover <NUM> and the stator <NUM> with structures operable as an inductive or capacitive position sensor. Hammer devices according to certain embodiments may comprise any suitable means for position measurement and/or estimation, and hammer devices according to other embodiments can be without any means for position measurement and/or estimation.

It is to be noted that hammer devices according to embodiments of the invention are not limited to any specific type of a linear electric machine. For example, the linear electric machine of a hammer device according to an exemplifying and non-limiting embodiment can be a flux switching permanent magnet synchronous machine "FSPMSM" where permanent magnets are located in a stator. It is also possible that a hammer device according to an exemplifying and non-limiting embodiment comprises a reluctance linear electric machine in which no permanent magnets are needed. In a reluctance linear electric machine, all magnetic flux is produced by electric currents and the magnetic force directed to the mover is generated by reluctance variation based on the design of the mover. In comparison to a permanent magnet machine, the drawbacks of a reluctance machine are lower power density, i.e. a larger machine is needed to produce the same power, and lower efficiency because all magnetic flux is produced by electric currents, resulting in higher resistive I<NUM>R losses in the electric machine, and, in addition, higher losses in the power electronics feeding the electric machine.

<FIG> shows a section view of a hammer device <NUM> according to an exemplifying and non-limiting embodiment. The geometric section plane is parallel with the yz-plane of a coordinate system <NUM>. The hammer device <NUM> comprises a frame <NUM> attachable to a working machine. The frame <NUM> comprises attachment members <NUM> for attaching to the working machine so that the frame is nondestructively detachable from the working machine. The hammer device <NUM> comprises an actuator member <NUM>, e.g. a chisel, linearly movably supported by the frame <NUM>. An end of the actuator member <NUM> is shaped to constitute a tip for breaking material e.g. stone or concrete. The hammer device <NUM> comprises a linear electric machine <NUM> having a mover <NUM> and a stator <NUM>. The mover <NUM> is configured to direct impacts to the actuator member <NUM> in the negative z-direction of the coordinate system <NUM>. The stator <NUM> is attached to the frame <NUM> and comprises windings for generating a magnetic force directed to the mover <NUM> in response to electric current supplied to the windings. The windings may constitute for example a multi-phase winding, e.g. a two- or three-phase winding.

The hammer device <NUM> comprises a brake <NUM> according to an exemplifying and non-limiting embodiment of the invention. The brake <NUM> can be for example like the brake <NUM> illustrated in <FIG>. In this exemplifying hammer device <NUM>, the actuator member <NUM> is the center element of the brake <NUM>. The brake <NUM> inhibits a linear movement of the actuator member <NUM> in the negative z-direction of the coordinate system <NUM> when a collar <NUM> of the actuator member <NUM> is in contact with an annular element <NUM> of the brake <NUM>.

Claim 1:
A hammer device (<NUM>, <NUM>) comprising:
- a frame (<NUM>, <NUM>) attachable to a working machine, the frame comprising attachment members (<NUM>, <NUM>) configured to attach to the working machine so that the frame is nondestructively detachable from the working machine,
- an actuator member (<NUM>, <NUM>) linearly movably supported with respect to the frame, and
- a linear electric machine (<NUM>, <NUM>) comprising a mover (<NUM>, <NUM>) configured to direct impacts to the actuator member and a stator (<NUM>, <NUM>) connected to the frame and provided with windings configured to generate a magnetic force directed to the mover in response to electric current supplied to the windings,
the hammer device (<NUM>, <NUM>) being characterized in that it further comprises a brake (<NUM>, <NUM>) configured to brake movement of the mover of the linear electric machine,
wherein the brake comprises:
- a frame structure (<NUM>, <NUM>),
- an annular element (<NUM>, <NUM>) linearly movable with respect to the frame structure, the annular element constituting a wall of an annular chamber (<NUM>, <NUM>) for damper fluid,
- a center element (<NUM>, <NUM>) surrounded by the annular element and linearly movable with respect to the frame structure and the annular element, and
- a spring (<NUM>, <NUM>) configured to move the annular element with respect to the frame structure in a first direction (+z) in which a volume of the annular chamber is increasing,
wherein the center element comprises a collar (<NUM>, <NUM>) configured to abut to a contact portion (<NUM>, <NUM>) of the annular element and to move the annular element together with the center element in a second direction (-z) opposite to the first direction and against a spring force of the spring, and a portion of the annular element surrounding the contact portion comprises apertures (<NUM>, <NUM>) configured to conduct the damper fluid to flow to and from the annular chamber, wherein the spring is a plate spring configured to gradually close the apertures in response to a situation in which the annular element is moved in the second direction against the spring force, a plate (<NUM>, <NUM>) of the spring getting gradually parallel with a surface of the annular element around the apertures in response to a situation in which the spring is compressed.