Abstract:
A reciprocating-motion converting mechanism includes a cam that rotates around a rotation center; a reciprocating member that is reciprocated due to rotation of the cam; and a pressing unit that presses the reciprocating member to the cam. A first load torque is generated by a load applied at a point of contact of the cam and the reciprocating motion member. A load applying unit applies a load to the cam to generate a second load torque having a phase substantially opposite to a phase of the first load torque.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2007-063477 filed in Japan on Mar. 13, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, such as a printer, a copier, and a facsimile machine. 
     2. Description of the Related Art 
     A typical image forming apparatus forms an image in the following manner. That is, an electrostatic latent image is first formed on a photosensitive element, the latent image is then developed into a toner image with a toner, the toner image on the photosensitive element is then transferred onto an intermediate transfer body, and the toner image on the intermediate transfer body is then transferred onto a recording medium, such as a paper sheet, and the image is finally fixed onto the recording medium by application of heat. Generally, a roller is used to carry such an image or a recording medium. The image or the recording medium is carried by the rotation of the roller. To drive the roller to rotate, a drive source such as a motor is connected to the roller via a drive-force transmission mechanism such as a gear or a timing belt. 
     There has been developed a retracting mechanism capable of retracting an element when the element is not required to form an image, or to carry the image or a recording medium. For example, in the technology disclosed in Japanese Patent Publication No. 3512307, in a multi-color image forming apparatus, when a black image is to be formed, only the element required to form the black image is activated, i.e., elements that are not required for the formation of the black image are retracted. Moreover, in a manual sheet feeding device disclosed in Japanese Patent Application Laid-open No. 2006-089189, a bottom plate is configured to be retracted downward when the manual sheet feeding device is powered OFF, so that a user can easily handle a recording medium. The retracting mechanism typically includes a cam capable of converting a power transmission from a drive source into a reciprocating motion. 
     Generally, an external force is applied to the reciprocating member by a spring or the like so that the reciprocating member is held in either an operating position or a retracted position as a home position. When the reciprocating member held in the home position moves to some other position due to the rotation of the cam, a force exceeding the external force is applied to the reciprocating member. At this time, a load torque is exerted on a camshaft. Subsequently, when the reciprocating member moves back to the home position from the other position, the external force acts as a drive force to the cam, i.e., the drive force is applied to the cam via the reciprocating member by the action of the external force. At this time, an acceleration torque is exerted on the camshaft. In this manner, because of the reciprocating motion of the reciprocating member between the operating position and the retracted position, the load torque and the acceleration torque are alternately exerted on the camshaft. Such a variation between the load torque and the acceleration torque is referred to as a load variation. 
     The drive source is set to output a power exceeding the maximum load torque of those exerted on the camshaft. Therefore, even if an average torque of the load torque is identical to that of the acceleration torque, when the load variation between the load torque and the acceleration torque is large, it is necessary to use the drive source capable of outputting a power higher than the maximum load torque. One approach is to modify the shape of the cam or the reciprocating member in a manner that leads to reduction in the amount of the load variation. However, this approach could lead to a decrease in the performance reliability of the cam or the reciprocating member. Moreover, the size and the production costs of the apparatus may increase if the modification results in a complicated configuration of the cam or the reciprocating member. 
     For example, when a direct current (DC) brush motor without a rotation control function depending on the load variation is used as the drive source, an angular velocity of the DC brush motor varies in synchronization with the load variation. Namely, as the amount of the load variation increases, the angular velocity of the DC brush motor also increases. Therefore, in such a configuration that a mechanism controls the cam to be driven or stop driving by detecting a rotational position of the cam and a position of the reciprocating member, the angular velocity varies depending on a working position. Therefore, as the load variation is getting larger, it is necessary to detect the positions of the cam and the reciprocating member more precisely and to set a control value more accurately. 
     Furthermore, in a case of the drive source without the rotation control function, a rotation rate of the drive source varies, so that a running sound of the drive source increases and decreases depending on a cycle of the variation of the rotation rate. Thus, a user may feel the running sound as a harsh noise. 
     Moreover, as described above, in an area in which the acceleration torque is exerted on the camshaft while the cam makes one revolution around the shaft, an acceleration for accelerating the cam or the reciprocating member to move is generated depending on a degree of the acceleration torque or the external force. When the reciprocating member moves back to the home position, the reciprocating member may make a relatively loud impact sound due to the acceleration. Thus, the user may feel the impact sound as a harsh noise. Therefore, for example, an impact absorbing material is provided in a collided portion to reduce the impact sound, or a decelerating material for applying a frictional load or the like to the cam or the reciprocating member in the area in which the acceleration torque is exerted on the camshaft is provided so that the acceleration torque can be reduced. However, when the impact absorbent material or the decelerating material is provided to the apparatus, a configuration of the apparatus becomes complicated. In addition, it is necessary to consider a time degradation of the impact absorbent material or the decelerating material. Moreover, when the decelerating material is provided to the apparatus, the decelerating material is set up to apply the load to the cam or the reciprocating member in such a direction that the load torque is exerted on the camshaft even when the acceleration torque is exerted on the camshaft, so that the average load torque increases, and thus a power consumption also increases. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided an image forming apparatus that employs electrophotographic technique to form an image on a recording medium. The image forming apparatus includes a reciprocating-motion converting mechanism that includes a cam that rotates around a rotation center; a reciprocating member that is reciprocated due to rotation of the cam, wherein a first load torque is generated by a load applied at a point of contact of the cam and the reciprocating motion member; and a pressing unit that presses the reciprocating member to the cam; and a load applying unit that applies a load to the cam to generate a second load torque, wherein a phase of the second load torque is substantially opposite to a phase of the first load torque. 
     The above and other objects, features, advantages and technical and industrial-significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a color image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a reciprocating-motion converting mechanism employed in the color image forming apparatus according to the embodiment; 
         FIG. 3  is a schematic diagram for explaining a load torque exerted on a rotation shaft of a cam shown in  FIG. 2 ; 
         FIG. 4  is a graph for explaining a transitional change in the load torque exerted on the cam depending on a rotation angle of the cam; and 
         FIG. 5  is a perspective view of a manual sheet feeding unit including the reciprocating-motion converting mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of a color laser printer as an example of an image forming apparatus according to an embodiment of the present invention. 
     As shown in  FIG. 1 , the color laser printer according to the embodiment is a tandem color laser printer. A main body  1  of the color laser printer includes four image forming units  2 , an intermediate transfer belt  4  as an intermediate transfer body, supporting rollers  5  and  6 , a laser scanning unit (LSU)  8 , four transfer rollers  10 , a secondary transfer roller  12 , a pair of registration rollers  13 , a fixing unit  14 , a sheet discharging unit  15 , a belt cleaning unit  16 , a sheet stacking unit  17 , a sheet feeding unit  20 , and a manual sheet feeding unit  40 . The image forming units  2  are arranged in the substantially center of the main body  1 , and form yellow (Y), cyan (C), magenta (M), and black (K) color images, respectively. The sheet feeding unit  20  is arranged below the image forming units  2 , and feeds a sheet on which an image is to be formed by the image forming units  2 . The intermediate transfer belt  4  is a loop transfer belt supported by the supporting rollers  5  and  6 , and is driven to rotate in a counter-clockwise direction in  FIG. 1 . Incidentally, the intermediate transfer belt  4  is used as the intermediate transfer body in the embodiment. Alternatively, a drum can be used instead of the intermediate transfer belt  4 . The secondary transfer roller  12  is arranged to be opposed to the supporting roller  6  across the intermediate transfer belt  4 . 
     Each of the image forming units  2  includes a photosensitive drum  3  ( 3 Y,  3 C,  3 M, and  3 K, respectively) as an image carrier, a charging unit  7 , a developing unit  9 , and a cleaning unit  11 . The photosensitive drums  3 Y,  3 C,  3 M, and  3 K are tandemly-arranged above the intermediate transfer belt  4  with keeping a predetermined interval between each two of them, and Y, C, M, and K toner images are formed on surfaces of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K, respectively. 
     Each of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K is surrounded by the charging unit  7 , the LSU  8 , the developing unit  9 , the transfer roller  10 , and the cleaning unit  11 . Each of the charging units  7  charges the surface of each of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K. The LSU  8  exposes the surface of each of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K to a laser beam corresponding to image data. Each of the developing units  9  develops an electrostatic latent image, which is formed on the surface of each of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K by the exposure, into a toner image. The transfer rollers  10  are respectively arranged to be opposed to the photosensitive drums  3 Y,  3 C,  3 M, and  3 K across the intermediate transfer belt  4 . Each of the cleaning units  11  removes a residual toner from the surface of each of the photosensitive drums  3 Y,  3 C,  3 M, and  3 K after the toner image is transferred onto the intermediate transfer belt  4 . 
     A process of forming an image performed by the color laser printer is explained below. When the photosensitive drum  3  is driven to rotate in the clockwise direction in  FIG. 1 , the surface of the photosensitive drum  3  is charged to a predetermined polarity by the charging unit  7 . The charged surface of the photosensitive drum  3  is exposed to a laser beam corresponding to image data by the LSU  8 , whereby an electrostatic latent image is formed thereon. The electrostatic latent image formed on the surface of the photosensitive drum  3  is developed into a toner image by the developing unit  9 . The toner image on the surface of the photosensitive drum  3  is transferred onto the intermediate transfer belt  4  by the application of pressure between the photosensitive drum  3  and the transfer roller  10 . 
     In a case of forming a color image, the above process is performed by all the image forming units  2 . Namely, Y, C, M, and K toner images on the photosensitive drums  3 Y,  3 C,  3 M, and  3 K are transferred onto the intermediate transfer belt  4  in such a manner that the Y, C, M, and K toner images are sequentially superimposed on the intermediate transfer belt  4 . 
     The sheet feeding unit  20  includes a sheet tray  21  as a sheet containing unit, a sheet feeding roller  22 , and a friction pad (not shown) as a separating unit. A recording medium, such as a transfer sheet or a resin film, (hereinafter, just “a sheet”) is contained in the sheet tray  21 . The sheet feeding roller  22  feeds the sheet contained in the sheet tray  21 . If a plurality of sheets is fed by the sheet feeding roller  22 , the sheets are separated by the friction pad to be fed one by one. The manual sheet feeding unit  40  includes a manual sheet feeding roller  41  and a bottom plate  42 . 
     A sheet fed from the sheet feeding unit  20  or the manual sheet feeding unit  40  is conveyed toward the registration rollers  13 . A leading end of the sheet is struck on the registration rollers  13  that are not driven to rotate at this time, so that a skew of the sheet is corrected. After that, the registration rollers  13  are driven to rotate at such a timing that the leading end of the sheet and the toner images transferred onto the intermediate transfer belt  4  get to the secondary transfer roller  12  at the same time. When the registration rollers  13  are driven to rotate at the timing, the sheet is conveyed toward the secondary transfer roller  12 . 
     When the sheet passes between the supporting roller  6  and the secondary transfer roller  12 , the toner images on the intermediate transfer belt  4  are transferred onto the sheet by the application of pressure between the supporting roller  6  and the secondary transfer roller  12 . The sheet onto which the toner images are transferred is conveyed to the fixing unit  14 , and the unfixed toner images are fixed on the sheet by the fixing unit  14 . The sheet on which the toner images are fixed is conveyed to the sheet discharging unit  15 , and discharged onto the sheet stacking unit  17  arranged on top of the main body  1  by the sheet discharging unit  15 . After the toner images on the intermediate transfer belt  4  are transferred onto the sheet, the belt cleaning unit  16  removes transfer residual toners from a surface of the intermediate transfer belt  4 . 
     On the other hand, in a case of forming a K monochromatic image, only a K toner image is formed on the photosensitive drum  3 K, and the K toner image on the photosensitive drum  3 K is transferred onto the intermediate transfer belt  4 . Therefore, the photosensitive drums  3 Y,  3 C, and  3 M are not required for forming the K monochromatic image, so that the intermediate transfer belt  4  is moved away from the photosensitive drums  3 Y,  3 C, and  3 M by a reciprocating-motion converting mechanism. Specifically, the reciprocating-motion converting mechanism includes a link mechanism  18  and a cam  19 . The link mechanism  18  is interlocked with the transfer rollers  10  that are opposed to the photosensitive drums  3 Y,  3 C, and  3 M. The link mechanism  18  moves up and down in accordance with a rotation of the cam  19 , so that the intermediate transfer belt  4  has contact with or moves away from the photosensitive drums  3 Y,  3 C, and  3 M in accordance with the movement of the link mechanism  18 . In this manner, the photosensitive drums  3 Y,  3 C, and  3 M can avoid performing any unnecessary process, and it is possible to prevent the K toner image transferred onto the intermediate transfer belt  4  from having contact with the photosensitive drums  3 Y,  3 C, and  3 M. 
     Such a reciprocating-motion converting mechanism is also provided in the manual sheet feeding unit  40 . The manual sheet feeding unit  40  includes the manual sheet feeding roller  41 , the bottom plate  42 , and a cam  43 . When a sheet is fed from the manual sheet feeding unit  40 , the bottom plate  42  has contact with the manual sheet feeding roller  41 , and the sheet is fed toward the secondary transfer roller  12  by passing between the bottom plate  42  and the manual sheet feeding roller  41 . On the other hand, except when a sheet is fed from the manual sheet feeding unit  40 , the bottom plate  42  is pressed to be retracted to a position as indicated by a dashed-two dotted line shown in  FIG. 1  by a rotation of the cam  43  so that the bottom plate  42  is away from the manual sheet feeding roller  41 . Therefore, it is possible to improve the user-friendliness in such a way that the user can easily take out a sheet from the manual sheet feeding unit  40 , or insert a sheet into the manual sheet feeding unit  40 . 
       FIG. 2  is a schematic diagram of the reciprocating-motion converting mechanism employed in the color laser printer according to the embodiment. 
     The reciprocating-motion converting mechanism shown in  FIG. 2  includes a cam  31 , a reciprocating member  32 , a load applying unit  33 , and a load applying unit  34 . The cam  31  is supported by a rotation shaft  30 , and rotates around the rotation shaft  30 . The reciprocating member  32  is interlocked with the cam  31 . 
     The load applying unit  33  is composed of a spring and the like, and applies a load to the cam  31  via the reciprocating member  32  so that the reciprocating member  32  is held in a predetermined position within a movable area M of the reciprocating member  32  as a home position. In a case shown in  FIG. 2 , the load applying unit  33  is composed of a compression coil spring, and a load of the compression coil spring is exerted in such a way that the reciprocating member  32  is pressed to the side of the cam  31 . 
     The load applying unit  34  is composed of a coil spring, and one end of the load applying unit  34  supports a point on the cam  31  as a load applied point A, and the other end of the load applying unit  34  is attached to a point on an infinite line L 2  as a support point B. The load applied point A moves in a circle around the rotation shaft  30  in synchronization with a rotation of the cam  31 . The infinite line L 2  extends from the rotation shaft  30  and is tilted at an angle θ with respect to an infinite line L 1  that extends from the rotation shaft  30  by passing through the load applied point A when the reciprocating member  32  is in a position X 1 , i.e., the farthest position from the rotation shaft  30  within the area M by the rotation of the cam  31 . A load F is exerted in a direction from the load applied point A to the support point B as indicated by an arrow shown in  FIG. 2 . 
     When the cam  31  rotates by the application of a drive force from a drive source (not shown), a contact point between the cam  31  and the reciprocating member  32  moves depending on a shape of the cam  31 , and the reciprocating member  32  is reciprocated between the position X 1  and a position X 2  where the reciprocating member  32  is in the nearest position to the rotation shaft  30  within the area M by the rotation of the cam  31 . In this manner, the cam  31  converts the drive force into the reciprocating motion of the reciprocating member  32 . Incidentally, an arrow R shown in  FIG. 2  indicates a rotating direction of the cam  31 . 
     Subsequently, a load torque exerted on the rotation shaft  30  of the cam  31  is explained below with reference to  FIG. 3 . When the cam  31  rotates around the rotation shaft  30  by a rotation angle ζ in the direction of the arrow R by the application of the drive force from the drive source (not shown), the reciprocating member  32  moves by a moving amount X from the position X 1  to a position where the reciprocating member  32  has contact with the cam  31  depending on the shape of the cam  31 . At this time, an external force P is applied to the cam  31  via the reciprocating member  32  by the load applying unit  33 . The external force P and a frictional force Q are not directed toward the center of the rotation shaft  30  regardless of the shape of the cam  31 , so that the external force P and the frictional force Q act as a moment around the rotation shaft  30 . When a direction of application of the moment is opposite to the rotating direction of the cam  31 , the moment becomes the load torque. On the other hand, when the direction of application of the moment is identical to the rotating direction of the cam  31 , the moment becomes the acceleration torque. When an angle between the infinite line L 1  and the infinite line L 2  is “θ+ζ” in accordance with the movement of the load applied point A in synchronization with the rotation of the cam  31 , the load F applied by the load applying unit  34  is exerted in the direction from the load applied point A to the support point B. At this time, the load F acts as a moment around the rotation shaft  30 . When the direction of application of the moment is opposite to the rotating direction of the cam  31 , the moment becomes the load torque. On the other hand, when the direction of application of the moment is identical to the rotating direction of the cam  31 , the moment becomes the acceleration torque. 
     A transitional change in the load torque exerted on the cam  31  depending on the rotation angle ζ is explained below with reference to a graph shown in  FIG. 4 . When the cam  31  rotates by the rotation angle ζ of 0° or 360°, the reciprocating member  32  is in the farthest position from the rotation shaft  30  within the area M. When the load torque is a positive value, the moment is exerted on the cam  31  as the load torque. On the other hand, when the load torque is a negative value, the moment is exerted on the cam  31  as the acceleration torque. First, a transitional change in a load torque that is caused by the external force P and the frictional force Q and exerted on the contact point between the cam  31  and the reciprocating member  32  (hereinafter, “load torque Tp”) is explained below. It is assumed that the cam  31  starts rotating when the reciprocating member  32  is in the farthest position from the rotation shaft  30 . As the cam  31  rotates, the load torque Tp is on a decreasing trend. When the load torque Tp falls to a negative value, the acceleration torque is exerted on the contact point. As the cam  31  further rotates, when the load torque Tp reaches the minimum point, the load torque Tp is on an increasing trend. Then, when the load torque Tp reaches the maximum point, the load torque Tp is again on a decreasing trend. When the cam  31  rotates by the rotation angle ζ of 360°, the load torque Tp indicates the same point as that is when the cam  31  rotates by the rotation angle ζ of 0°. When the load torque Tp indicates the maximum point, the maximum load torque is exerted on the contact point. When the load torque Tp indicates the minimum point, the minimum load torque is exerted on the contact point. Such a variation between the maximum load torque and the minimum load torque is referred to as a torque variation. Although a waveform indicating the torque variation of the load torque Tp has a different shape depending on the shape of the cam  31  or surface characteristics of the cam  31  at the contact point, in general, the maximum load torque is obtained when the reciprocating member  32  is in the farthest position from the rotation shaft  30  in accordance with the rotation of the cam  31 , and the minimum load torque is obtained while the reciprocating member  32  moves away from the farthest position from the rotation shaft  30  in accordance with the rotation of the cam  31 . Next, a transitional change in a load torque caused by the action of a load applied by the load applying unit  34  (hereinafter, “a load torque Tk”) is explained below. It is assumed that the angle θ between the infinite line L 1  and the infinite line L 2  when the reciprocating member  32  is in the position X 1 , i.e., in the farthest position from the rotation shaft  30  is referred to as an initial phase displacement. In a waveform indicating a torque variation of the load torque Tk, the maximum load torque is obtained while the reciprocating member  32  moves away from the position X 1  in accordance with the rotation of the cam  31 , and the minimum load torque is obtained while the reciprocating member  32  moves towards the position X 1  in accordance with the rotation of the cam  31 . Therefore, the angle θ is set up within a range of −90° to 90°. In other words, when the reciprocating member  32  is in the position X 1  in accordance with the rotation of the cam  31 , the support point B is set up to be located on the infinite line L 2  tilted at the angle θ around the rotation shaft  30  with respect to the infinite line L 1 , and an mount of the tilt is set up within a range of 0° to 90°. As a result, a phase of the torque variation of the load torque Tk is substantially opposite to that of the load torque Tp. A camshaft torque T is a composition of the load torque Tp and the load torque Tk. The load torque Tp and the load torque Tk are balanced out against each other, so that it is possible to reduce the torque variation and the maximum load torque. 
     With such a configuration of the reciprocating-motion converting mechanism, the intermediate transfer belt  4  is configured to have contact with or move away from the photosensitive drums  3 Y,  3 C, and  3 M, and also the bottom plate  42  is configured to have contact with or move away from the manual sheet feeding roller  41 . Therefore, with such a simple configuration, it is possible to prevent an occurrence of a noise such as an impact sound, and also to provide an image forming apparatus capable of preventing an increase of a power consumption. 
       FIG. 5  is a perspective view of the manual sheet feeding unit  40  in which the reciprocating-motion converting mechanism according to the embodiment is employed. The manual sheet feeding unit  40  includes the manual sheet feeding roller  41 , the bottom plate  42 , the cam  43 , a rotation shaft  44 , a rotating body  45 , a load applying unit  46 , and a load applying unit  47 . The load applying unit  47  is set up to apply a load to the bottom plate  42  so that the bottom plate  42  has contact with the manual sheet feeding roller  41  firmly. When a recording medium is fed by the manual sheet feeding unit  40 , the bottom plate  42  and the manual sheet feeding roller  41  have contact with each other firmly by the action of the load applied by the load applying unit  47 . When the manual sheet feeding roller  41  is driven to rotate by a drive source (not shown), the recording medium is fed along a rotating direction of the manual sheet feeding roller  41 . On the other hand, when a recording medium is not fed by the manual sheet feeding unit  40 , the cam  43  is driven to rotate by a drive source (not shown). The bottom plate  42  rotates around a rotation shaft  50  of the bottom plate  42  by being pressed downward by the rotation of the cam  43 , so that the bottom plate  42  moves away from the manual sheet feeding roller  41 . Thus, it is possible to improve the user-friendliness in such a way that the user can easily take out a recording medium from the manual sheet feeding unit  40 , or insert a recording medium into the manual sheet feeding unit  40 . 
     As shown in  FIG. 5 , one end of the load applying unit,  46  supports an arbitrary point on the rotating body  45  (hereinafter, “a supported point”), and the other end of the load applying unit  46  is fixed to a point within an area other than a moving area of a sheet holding unit (not shown) or the like so that an angle between the rotation shaft  44  and the supported point is within a range of 0° to 90° when the bottom plate  42  is in the farthest position from the rotation shaft  44  in accordance with the rotation of the cam  43 . With such a configuration, the load applying unit  46  can achieve an effect equivalent to that of the reciprocating-motion converting mechanism according to the embodiment. In addition, the load applying unit  46 , which causes a camshaft torque, supports the rotating body  45  that rotates in synchronization with the cam  43  fixed to the rotation shaft  44 , so that the effect of the load applying unit  46  can be exercised regardless of a shape and a size of the cam  43 , an installation position of the cam  43  with respect to the rotation shaft  44  in a longitudinal direction, and a shape of the bottom plate  42 . 
     Moreover, when the rotating body  45  is composed of a power transmission member, such as a gear or a timing pulley, the rotating body  45  not only can support the load applying unit  46 , but also can transmit a drive force from a drive source (not shown) to the rotation shaft  44 . Therefore, it is possible to make the configuration easier. 
     According to an aspect of the present invention, it is possible to obtain a substantially opposite phase to a phase of a variation of a load torque caused by a rotation of a cam by the use of a load applying unit. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.