Patent Publication Number: US-2022219305-A1

Title: Con rod

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority, under 35 U.S.C. § 119, to UK Patent Application No. 2100313.2 filed Jan. 11, 2021, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to a con rod or a wobble plate for a hammer drill. 
     BACKGROUND 
     A typical hammer drill comprises a body in which is mounted an electric motor and a hammer mechanism. A tool holder is mounted on the front of the body which holds a cutting tool, such as a drill bit or a chisel. The hammer mechanism typically comprises a ram, slideably mounted in a cylinder, reciprocatingly driven by a piston via an air spring, the piston being reciprocatingly driven by the motor via a set of gears and a crank mechanism or wobble bearing. The ram in turn repeatedly strikes the end of the cutting tool via a beat piece. When the only action on the tool bit is the repetitive striking of its end by the beat piece, the hammer drill is operating in a hammer only mode. 
     Certain types of hammer drill also comprise a rotary drive mechanism which enables the tool holder to rotatingly drive the cutting tool held within the tool holder. In such constructions, the cylinder is typically in the form of a rotatable spindle. This can be in addition to the repetitive striking of the end of the cutting tool by the beat piece (in which case, the hammer drill is operating in a hammer and drill mode) or as an alternative to the repetitive striking of the end of the cutting tool by the beat piece by switching off the hammer mechanism (in which case, the hammer drill is operating in a drill only mode). 
     EP1157788 discloses such a hammer drill. 
     Hammer drills typically use one of two types of piston. 
     The first type of piston is known as a flat piston. A flat piston locates inside of a cylinder or spindle. The ram also mounts directly in the spindle or cylinder directly in front of the flat piston. The air spring formed between the ram and piston is contained within a chamber formed by the front end of the piston, the inner side walls of the spindle or cylinder and the rear of the ram. A flat piston makes has no direct contact with the ram. DE4202767 discloses a hammer drill with a flat piston. Typically, a flat piston is driven by a crank mechanism comprising a crank plate and a con rod. The crank plate is rotatably mounted adjacent the rear of the spindle or cylinder and is rotationally driven by a motor. One end of the con rod is pivotally attached to the plate, the pivot axis being eccentric to the axis of rotation of the plate. The other end of the con rod is pivotally attached to the rear of the piston. The rotational movement of the crank plate is converted into a reciprocating movement of the piston. 
     The second type of piston is known as a hollow piston. A hollow piston locates inside of a cylinder or spindle. A tubular recess is formed inside of the front of the hollow piston. The ram mounts directly in the recess of the hollow piston. The air spring formed between the ram and piston is contained within the recess and is formed within a chamber formed by inner walls of the recess of the hollow piston and the rear of the ram. A hollow piston is in direct contact with and provides support for the ram. The ram makes no contact with the spindle or cylinder. EP1157788 discloses a hammer drill with a hollow piston. Typically, a hollow piston is driven by a wobble plate. A wobble plate comprises a circular central plate mounted on a shaft, the plane of the plate being located at an angle relative to a longitudinal axis of the shaft. A circular ring is mounted on the plate and surrounds the periphery of the plate such that plane of the ring is parallel to the plane of the plate. The ring can freely rotate around the periphery of the plate. The ring is prevented from rotating. Therefore, as the shaft rotates, the plane of the plate oscillates back and forth in the direction of the longitudinal axis of the shaft. A finger is attached to the side of the ring and extends radially away from the centre of the ring. The end of the finger remote from the ring is attached to the rear of the piston. As the shaft rotates and the plane of the plate oscillates back and forth in the direction of the longitudinal axis of the shaft, the finger also oscillates back and forth in the direction of the longitudinal axis of the shaft, reciprocatingly driving the piston. 
     Pistons and con rods used in hammer drills are typically constructed from aluminium or plastic. WO03/041915 describes a hammer mechanism with a plastic con rod. 
     A prior art design of hammer mechanism will new de described with reference to  FIGS. 1 to 5 . 
     Referring to  FIG. 1 , a hammer drill comprises a body  2  having a rear handle  4  moveably mounted to the rear of the body  2 . The rear handle  4  comprises a centre grip section  90  and two end connection sections  92 ;  94 , one end connection section being attached to one end of the centre grip section, the other end connection section being connected to the other end of the centre grip section. The handle  4  is connected to the rear of the body  2  by the two end connection sections  92 ,  94 . The rear handle is constructed from a plastic clam shell  100  and a rear end cap  102  which is attached to the clam shell  100  using screws (not shown). The rear of the body is formed by three plastic clam shells  6 ,  70 ,  72  which attach to each other and to the remainder of the body  2  using screws (not shown). 
     An SDS tool holder  8  is mounted onto the front  10  of the body  2 . The tool holder can hold a cutting tool  12 , such as a drill bit. A motor (shown generally by dashed lines  48 ) is mounted within the body  2  which is powered by a mains electricity supply via a cable  14 . A trigger switch  16  is mounted on the rear handle  4 . Depression of the trigger switch  16  activates the motor in the normal manner. The motor drives a hammer mechanism (shown generally by dashed lines  46  in  FIG. 1 ), which comprises a flat piston  204  reciprocatingly driven by the motor via a con rod  206  within a hollow spindle  150 , which in turn reciprocatingly drives a ram  152  via an air spring  170  which in turn strikes, via a beat piece  156 , the end of the cutting tool  12 . The motor can rotationally drive the hollow spindle  150  via a bevel gear  200  and torque clutch  202 . A mode change mechanism (not shown) can switch the hammer drill between three modes of operation, namely hammer only mode, drill only mode or hammer and drill mode. A rotatable knob  18  is mounted on the top of the body  2 . Rotation of the knob  18  changes the mode of operation of the hammer drill in well-known manner. 
     Referring to the  FIG. 2 , the hollow spindle  150  has a longitudinal axis  154 . In side of the hollow spindle  150  is located the ram  152 , forward of the flat piston  204 , a beat piece  156 , forward of the ram  152 , a ram catcher located between the ram  152  and the beat piece  156  and a beat piece support structure. 
     The forward end  162  of the hollow spindle  150  forms part of the tool holder  8 . During normal use, the cutting tool  12  (shown in dashed lines in  FIG. 2 ) is held within the forward end  162  of the spindle  50  by the tool holder. The cutting tool  12  is prevented from rotating relative to the spindle  50  whilst being capable of moving axially over a limited range of movement within the forward end  162  of the hollow spindle  150  in well known manner. 
     The flat piston  204  is mounted directly in the rear of the hollow spindle  150  and comprises an O ring  208  which locates in a groove formed around the main body of the flat piston and which provides an air tight seal between the flat piston and the inner wall of the hollow spindle  150 . 
     The ram  152  is mounted directly in the hollow spindle  150  and comprises a main body  166  attached to an end cap  160 , via a neck  168 , of smaller diameter than the main body  166  of the ram  152 , located at the forward end of the ram  152 . The ram is circular in cross section in any plane which extends perpendicularly from the longitudinal axis  154  (which is co-axial with the longitudinal axis of the hollow spindle  150  when the ram is located inside of the spindle) of the ram  152  along its length. The ram  152  comprises an O ring  158  which locates in a groove formed around the main body  166  of the ram and which provides an air tight seal between the ram  152  and the inner wall of the hollow spindle  150 . During normal operation of the hammer, the ram  152  is reciprocatingly driven by the flat piston  204  via an air spring  170  formed between the flat piston  204  and ram  152  in well known manner along the longitudinal axis  154 . The air spring  170  between the ram  152  and the flat piston  204  is maintained by the air in the air spring  170  being prevented from escaping from (or air external of the air spring entering into) the space between the flat piston  204  and ram  152  due to the two O rings  208 ,  158 . 
     The ram catcher comprises a rubber ring  214  which locates against the inner wall of the hollow spindle  150  and is axially held in position inside of the spindle by being sandwiched between a ring retainer, comprising a circlip  216  and metal washer  218 , and a metal tubular insert  210  of the beat piece support structure, both being located inside of the hollow spindle  150 . The rubber ring  214  provides a lip which projects radially inwardly into hollow spindle  150  towards the longitudinal axis  154 . The diameter of the aperture formed by the rubber ring  214  is less than that of the end cap  160  of the ram  152  but similar to that of the neck  168  of the ram  152 . A series of holes  220  are formed around the circumference of the spindle rearward of the circlip  216  which each extend through the wall of the hollow spindle  150 . 
     During the normal operation of the hammer drill, when the cutting tool is engaged with a work piece, the ram  152  is reciprocatingly driven over a range of axial positions (one of which is shown in  FIG. 2 ) inside of the spindle located to the rear of the ram catcher, the ram  152  being prevented from engaging the ram catcher due to the position of the beat piece  156 . The ring  214  has no contact with any part of the ram  152  during the normal operation of the tool. When the ram  152  is able to move forward, due to the position of the beat piece, the end cap  160  engages with the rubber ring  214  and passes through the aperture due to the ring deforming, allowing the lip to flex to enable the cap  160  to pass through it. Once the cap  160  has passed through the ring  214 , the lip returns to its original shape, locating in the neck  168  of the ram to hold the ram  152  stationary (as shown in  FIGS. 3 and 4 ). 
     The beat piece  156  is supported by a beat piece support structure formed in part by the hollow spindle  150  and in part by a support structure inside the hollow spindle  150  comprising a metal tubular insert  210  sandwiched between an O ring  212  and the rubber ring  214  of the ram catcher. The beat piece  156  is circular in cross section in any plane which extends perpendicularly from the longitudinal axis  154  (which is co-axial with the longitudinal axis of the hollow spindle  150  when the beat piece is located inside of the spindle) of the beat piece  156  along its length, the centre of the circular cross section being located on the longitudinal axis. 
     The beat piece  156  comprises a middle section  172 , a front section  174  and a rear section  176 . 
     The middle section  172  has a uniform diametered circular cross section along its length, the centre of the circular cross section being located on the longitudinal axis  154 . 
     The rear section  176  has a uniform diametered circular cross section along its length, the centre of the circular cross section being located on the longitudinal axis  154 . The rear end  240  of the rear section  176  is flat and is impacted by the cap  160  of the ram  152  during normal operation. The rear section  176  is joined to the middle section  172  via a first angled region  242 . The first angled region  242  engages with a correspondingly shaped first angled shoulder  244  formed on the metal insert  210  located inside the spindle when the beat piece is in its most rearward position, limiting the amount of rearward movement of the beat piece  156 . The wall of the angled shoulder  244  is circular in cross section in any plane which extends perpendicularly from the longitudinal axis  154  of the hollow spindle  150 , the centre of the circular cross section being located on the longitudinal axis. When the first angled region  242  is in engagement with the first angled shoulder  244 , there is a uniform amount of contact between the two surfaces around the longitudinal axis  154 . 
     The front section  174  is frusto-conical in shape centred around the longitudinal axis  154  of the beat piece  156 . The front end  246  of the front section  174  is flat and impacts the cutting tool  12  during normal operation. The front section  174  is joined to the middle section  172  via a second angled region  248  which is frusto-conical in shape centred around the longitudinal axis  154  of the beat piece  156 . The second angled region  248  engages with a correspondingly shaped second angled shoulder  250  formed on the inner wall of the hollow spindle  150  when the beat piece is in its most forward position, limiting the amount of forward movement of the beat piece  156 . The wall of the second angled shoulder  250  is circular in cross section in any plane which extends perpendicularly from the longitudinal axis  154  of the hollow spindle  150 , the centre of the circular cross section being located on the longitudinal axis  154 . When the second angled region  248  is in engagement with the second angled shoulder  250 , there is a uniform amount of contact between the two surfaces around the longitudinal axis  154 . 
     When the hammer drill is operating in the normal manner with the cutting tool  12  cutting a work piece, the ram strikes the beat piece  156  which in turn strikes the end of cutting tool  12  in the tool holder  8 . The ram  152  is reciprocatingly driven over a limited range of axial movement within the spindle, the maximum distance from the flat piston being limited by the position of the beat piece  156  which it impacts, the position of which in turn is controlled by the end of the cutting tool  12 . Whilst traveling within this range of axial movement, the O ring  158  of the ram  152  does not pass the holes  220 . As such, the air spring  170  between the flat piston  204  and ram  152  is maintained. The rear section  176  projects rearwardly through the aperture of the ring  214  of the ram catcher, to enable the cap  160  of the ram  152  to strike it as shown in  FIG. 2 . 
     When the cutting tool  12  is removed from the work piece, the beat piece  156  is able to move forward as the cutting tool  12  can extend out of the tool holder  8  to its maximum position. If the motor is still running, the flat piston  204  is able to drive the ram  152  via the air spring  170  further along the hollow spindle  150 , as the beat piece  156  can move forward, passing the air holes  220 . Once the O ring  158  of the ram  152  has passed the air holes  220 , the air is able to freely pass into and out of the hollow spindle  150  in the space between the flat piston  204  and ram  152 , causing the air spring  170  to be broken and thus disconnecting the drive between the flat piston  204  and ram  152 . As the air spring  170  is broken, the ram  152  is able freely continue to travel along the length of the hollow spindle  150 . The ram  152  engages with the ram catcher, the cap  160  passing through the ring  214  allowing the neck  168  to engage with the ring, to secure the ram in the ram catcher, as seen in  FIGS. 3 and 4 . The reciprocating movement of the flat piston  204  has no effect on the ram  152  as the air spring  170  is broken due to the holes  220  which allow air in and out of the spindle  170  in the space between the flat piston  204  and ram  152 . The beat piece  156  is pushed forward in the hollow spindle  150  by the ram  152  in the ram catcher. In order to release the ram  152  from the ram catcher, the cutting tool  12  is pressed against a work piece causing it to be pushed into the tool holder  8 , which in turn pushes the beat piece  156  rearwardly into engagement with the cap  160  of the ram  152 , pushing it out of the ram catcher and past the holes  220 . In such a position, the air spring  170  is reformed and the flat piston  204  is able to reciprocatingly drive the ram  152  again. 
     In existing designs of hammer mechanism which use a con rod, the con rod is made from plastic or aluminium. If it is made from plastic, it can deform particularly when exposed to heat due to the operation of the hammer drill. If the con rod is made from aluminium, it is subject to failure if it is not lubricated properly with oil and/or grease. 
     SUMMARY 
     Accordingly, there is provided a con rod for a hammer drill characterized in that the con rod is made from sintered steel. 
     The used of sintered steel to manufacture a con rod enables the density of the steel in the con rod to be controlled which in turn allows for the weight of the con rod to be adjusted and optimized when compare with other components of the hammer mechanism such as the piston. Optimizing the weight of the cranks shaft is important as it effects the forces experienced by the reciprocating drive mechanism for the piston as it reciprocatingly drives the piston within the cylinder. This in turn effects the amount of vibration generated by the hammer mechanism. Furthermore, manufacturing the con rod from sintered steel provides a higher compressive strength than a con rod made from either aluminum or plastic and provides better resilience to higher temperature than a con rod made from either aluminum or plastic, and is subject to fewer mechanical failures than a con rod made from either aluminum or plastic. 
     Manufacturing a con rod from sintered steel also has the advantage of providing a sinter effect with a porosity for accommodating grease and/or oil for improved lubrication. The con rod may be impregnated with the lubricant such as grease and/or oil. The porosity of the sintered steel con rod allows lubricants to flow through the con rod and/or remain captured within the con rod. 
     The captured grease and/oil within the con rod improves the lubrication of the con rod where it pivotally connects to a crank plate and piston by reducing the frictional contact which in turn provides a smoother movement. This reduces heat and vibration generated by the operation of the con rod. 
     Similar benefits and advantages can be gained when a wobble plate is used within a hammer mechanism and some or all of the component parts are made from sintered steel. 
     Accordingly, in an embodiment of the invention, a hammer drill is provided including a housing; a tool holder mounted on the housing and configured to hold a cutting tool; a motor mounted within the housing; and a hammer mechanism. The hammer mechanism in turn includes a crank plate; a con rod pivotally connected at a first end to the crank plate, the con rod being made of sintered steel in a one-piece construction; a piston slidably mounted in the housing and reciprocatingly driven along a longitudinal axis by the motor via the crank plate and con rod, wherein a second end of the con rod is pivotally connected to the piston; a ram mounted in the housing forward of the piston that is reciprocatingly driven along the longitudinal axis by the piston via an air spring; and a beat piece supported in an axially sliceable manner along the longitudinal axis within a beat piece support structure, wherein during the normal operation of the hammer mechanism, the beat piece is repetitively struck by the ram and transfers impact energy to the cutting tool. 
     Alternatively, in an embodiment, a hammer drill is provided including a housing; a tool holder mounted on the housing and configured to hold a cutting tool; a motor mounted within the housing; and a hammer mechanism. The hammer mechanism in turn includes a wobble plate driven by the motor, the wobble plate being at least partially made of sintered steel; a con rod pivotally including a first end coupled to the crank plate and a second end; a piston slidably mounted in the housing and coupled to the second end of the con rod, the piston being reciprocatingly driven along a longitudinal axis by the motor via the wobble plate and the con rod; a ram mounted in the housing forward of the piston that is reciprocatingly driven along the longitudinal axis by the piston via an air spring; and a beat piece supported in an axially sliceable manner along the longitudinal axis within a beat piece support structure, wherein during the normal operation of the hammer mechanism, the beat piece is repetitively struck by the ram and transfers impact energy to the cutting tool. 
     Other embodiments and optional details of the invention are described in the detailed description below and in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure. 
         FIG. 1  shows a sketch of a side view of a prior art hammer drill; 
         FIG. 2  shows a cross sectional view of the hammer mechanism with the ram in a position where it can freely slide within the spindle; 
         FIG. 3  shows a cross sectional view of the hammer mechanism with the ram in the ram catcher and the beat piece sliding in the spindle; 
         FIG. 4  shows a cross sectional view of the hammer mechanism with the ram in the ram catcher and the beat piece in its furthest forward position in the spindle; 
         FIG. 5  shows the beat piece; 
         FIG. 6  shows a vertical cross-sectional view of a hammer drill in accordance with a first embodiment the present invention; 
         FIG. 7  shows a rear view of the piston shown in  FIG. 6 ; 
         FIG. 8  shows a side view of the piston shown in  FIG. 6 ; 
         FIG. 9  shows a rear perspective view of the piston shown in  FIG. 6 ; 
         FIG. 10  shows a front perspective view of the piston shown in  FIG. 6 ; 
         FIG. 11  shows a side view of the con rod of  FIG. 6 ; 
         FIG. 12  shows a cross sectional view of the con rod in the direction of Arrows A in  FIG. 11 ; 
         FIG. 13  shows a cross sectional view of the con rod in the direction of Arrows B in  FIG. 11 ; and 
         FIG. 14  shows a second embodiment of the present invention. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Two embodiments of the present invention will now be described with reference to  FIGS. 6 to 14 . 
       FIG. 6  shows a cross section view of hammer drill having a con rod in accordance with a first embodiment of the present invention. Where the same features in the embodiment shown in  FIG. 6  are shown in the prior art example described above, the same reference numbers are used and the same description is applicable. The main difference between the prior art design and the embodiment is the design of the con rod  206 , the piston  204  and hollow spindle  150 . 
     Referring to  FIG. 6 , the hammer mechanism comprises a first gear  400  rotationally driven by an electric motor (not shown). The first gear  400  is rigidly attached to a first spindle  402  such rotation of the first gear  400  results in rotation of a spindle  402 . 
     A second gear  406  is mounted on the first spindle  402  adjacent the first gear. The second gear  406  is axially fixed on the first spindle  402  but can freely rotate around the first spindle  402 . A crank plate  408  is mounted on a top end of the first spindle  402 . The crank plate  408  is axially fixed on the spindle but can freely rotate around the end of the spindle  402 . 
     A sleeve  404  is mounted on the spindle  402  and surrounds a splined section  410  of the first spindle  402 . The inner part of the sleeve  404  comprises corresponding splines which engage with the splines of the spindle  402 . The sleeve  404  can axially slide along the first spindle  402  but is rotationally fixed to the first spindle  402  via the meshing splines so that rotation movement of the sleeve  404  always results in rotational movement of the spindle  402 . The sleeve  404  can slide vertically between three positions; a lower position where it in driving engagement with spline section  410  and the second gear  406  only; a middle position where it is driving engagement with the spline section  410 , the second gear  406  and the crank plate  408 ; and an upper position where it in driving engagement with the spline section  410  and the crank plate  408  only. The sleeve  404  is moved between its three positions via a mode change mechanism  412  which is operated using a mode change knob  414 . 
     The second gear  406  is in driving engagement with a third gear  416  which is mounted on a second spindle  418 . Rotation of the second gear  406  results in rotation of the third gear  416 . The third gear  416  is axially fixed on the second spindle  418 . The third gear  416  is rotationally fixed to the second spindle  418  via a torque clutch  420  so that rotation of the third gear  416  results in rotation of the second spindle  418  if the torque across the torque clutch  420  is below a pre-set value and that rotation of the third gear  416  results in rotation of the third gear  416  around the second spindle  418  if the torque across the torque clutch is above a pre-set value with the second spindle remaining stationary. 
     A first bevel gear  422  is formed on the top end of the second spindle  418 , the first bevel gear  422  is in driving engagement with a second bevel gear  424  which surround and is rigidly connected to the hollow spindle  150 . Rotation of the second spindle  418  results in rotation of the hollow spindle  150  via the bevel gears  422 ,  424 . 
     The crank plate  408  has an eccentric pin  426  integrally formed on the top of the crank plate  408 . The longitudinal axis of the eccentric pin  426  is parallel to but offset from longitudinal axis of the first spindle  402  such rotation of the first spindle  402  results in the eccentric pin  426  rotating around the longitudinal axis of the first spindle  402 , the eccentric pin  426  moving back and forwards as well as side to side as it does so. A con rod  206  connects between the eccentric pin  426  and the piston  204  inside of the hollow spindle. Rotation of the crank plate  308  results in the reciprocation of the piston  204  within the hollow spindle  150 . 
     Referring to  FIGS. 7 to 10 , the piston  204  is a flat piston and comprises a front circular disk  300  having flat front surface  302 . A circumferential groove  304  extends around the edge of the circular disk  300 . A circular peripheral wall  306  extends rearwardly from the edge of the circular disk  300 , perpendicularly to the plane of the circular disk  300 . Two straight sections  308  are formed on two opposite sides of the wall  306 . A frame  310  is formed on each straight section  308 . An aperture  314  is formed through each frame  310  and straight section  308 . The piston  204  is manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as grease and/or oil. 
     The rubber O ring  208  locates in the groove  304 . The piston  204  is mounted inside of the hollow spindle  150  and connected to the con rod  206  via a cross pin  312 . 
     The con rod  206  is shown in more detail in  FIGS. 11 to 13 . The con rod comprises a central section  440  which interconnects two end ring sections  442 ,  444 . The central section  440  is of rectangular cross section with an elongate groove  446 ,  448  extending in a lengthwise direction along each of the sides of the central section  440 . Each end ring section comprises a circular aperture  450 ,  452 . One end section  442  connects to the eccentric pin  426 , the eccentric pin  426  locating inside of the circular aperture  450  of the end section  442 . The other end section  444  connects to the cross pin  312  for the piston  204 , the cross pin  312  locating inside of the circular aperture  452  of the end section  444 . A semi-circular groove  460  is formed in the side wall of each aperture  450 ,  452  along the length of the apertures  450 ,  452 . The con rod  206  is manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as grease and/oil. The impregnated lubricant reduces the friction between the con rod  206  and the eccentric pin  426  and cross pin  312  as the eccentric pin  426  and cross pin  312  pivot within the apertures  450 ,  452  during the operation of the hammer mechanism. The semi-circular grooves  460  enable addition additional lubrication to enter the apertures  450 ,  452  and engage with the surfaces of the apertures  450 ,  452 , eccentric pin  426  and cross pin  312  making friction contact. 
     It will be appreciated that the eccentric pin  426  and/or cross pin  312  could be manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as grease and/or oil. This would further help lubrication to reduce the frictional contact. If the eccentric pin  426  and crank plate  408  are manufactured in one-piece construction, then both of these can be manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as grease and/or oil. 
     The design of the hollow spindle  150  is manufactured from steel. The coefficient of expansion of the steel hollow spindle  150  is the same as that of the sintered flat piston  204 . 
     Alternatively, the hollow spindle  150  is manufactured from sintered steel. Ideally, it would be manufactured in a one-piece construction. The coefficient of expansion of the sintered steel hollow spindle  150  is the same as that of the sintered flat piston  204 . The sintered steel hollow spindle  150  can impregnated with a longitudinal axis of the first spindle lubricant such as grease and/oil. 
     The sintered con rod  206 , the sintered steel piston  204  and/or the sintered steel hollow spindle  150  can be manufactured by using a sintering process and then submersing them in a lubricant, such as a grease and/or oil, to impregnate the con rod and/or piston and/or spindle with the lubricant. 
     A second embodiment of the present invention will now be described with reference to  FIG. 14 . Where the same features in the second embodiment are present in the first embodiment, the same reference numbers have been used. The difference between the first embodiment and the second embodiment is that the crank plate and eccentric pin have been replaced with a wobble plate. 
     The wobble plate comprises a circular central plate  500  mounted on a shaft  502 , the plane of the plate  500  being located at an angle  504  relative to a longitudinal axis  506  of the shaft  502 . The shaft  502  is driven by the first spindle  402  via set of bevel gears  508 . A circular ring  510  is mounted on the plate  500  via a bearing  512  and surrounds the periphery of the plate  500  such that plane of the ring  510  is parallel to the plane of the plate  500 . The ring  510  can freely rotate around the periphery of the plate  500 . The ring  510  is prevented from rotating. Therefore, as the shaft  502  rotates, the plane of the plate  500  oscillates back and forth in the direction of the longitudinal axis  506  of the shaft  502 . A finger  514  is attached to the side of the ring  510  and extends radially away from the centre of the ring  510 . The end of the finger  514  remote from the ring  510  is attached to the rear of the piston  204  via a con rod  206 . As the shaft  502  rotates and the plane of the plate  500  oscillates back and forth in the direction of the longitudinal axis  506  of the shaft  502 , the finger  514  also oscillates back and forth in the direction of the longitudinal axis  506  of the shaft  502 , reciprocatingly driving the piston  204 . 
     The ring  510  and finger  514  is manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as oil. The impregnated lubricant reduces the friction between the ring  510  and the bearing  512  and between the finger  514  and the con rod  206 . The plate and shaft can also be manufactured in a one-piece construction from sintered steel which has been impregnated with a lubricant such as oil. With the reduction in friction, it will be appreciated that the bearing  512  can be omitted, with the ring  510  being directly rotationally mounted on the plate  500 . 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.