Abstract:
A load directing trunnion mount for a linear actuator is constructed to receive all of the tensile forces exerted on the actuator shaft or lead screw of the linear actuator. In this manner, the load directing trunnion mount relieves a transmission drive element (for example, a drive gear or a sprocket or pulley) and the transmission housing enclosing the drive element from tensile forces exerted on the actuator shaft during use of the linear actuator.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention pertains to a load directing trunnion mount for a linear actuator that is constructed to receive all of the tensile forces exerted on the actuator shaft or lead screw of the linear actuator. In this manner, the load directing trunnion mount relieves a transmission drive element (for example, a drive gear or a sprocket or pulley) and the transmission housing enclosing the drive element from tensile forces exerted on the actuator shaft during use of the linear actuator. 
     (2) Description of the Related Art 
     A linear actuator of the type with which the present invention is concerned is basically a mechanism that converts rotational movement into linear movement. The mechanism includes a drive assembly that can be controlled to selectively rotate a screw threaded shaft or lead screw of the actuator in opposite directions. The drive assembly typically includes a motor, for example an electric motor, and a transmission coupling the motor output shaft to the lead screw. The transmission assembly can be a gearing assembly, a sprocket and chain assembly or a belt and pulley assembly. The linear actuator also includes a nut assembly that is mounted on the lead screw of the actuator for linear movement of the nut assembly along the lead screw in response to rotation of the lead screw. By rotating the lead screw in opposite directions of rotation, the nut assembly moves in opposite linear directions along the length of the lead screw. 
     FIG. 1 shows one operative environment of a linear actuator of the type described above. It is emphasized that the particular use made of the linear actuator in FIG. 1 is illustrative only. Linear actuators of the type shown are used in a variety of different environments where it is desired to convert reciprocating rotary movement to reciprocating linear movement. 
     FIG. 1 is a schematic representation of a cross section through a supporting frame of an adjustably elevating exercise treadmill of the prior art. The figure shows a cross section through a frame member  12  of the treadmill frame that supports the running deck (not shown) of the treadmill. The right-hand end of the frame member  12  is shown broken away, but the right-hand end of the frame member would rest on the supporting surface  14  on which the exercise treadmill is placed. The left-hand end or the elevating end of the treadmill frame is supported on a pair of bell cranks  16 , only one of which is shown in FIG.  1 . The bell cranks  16  are mounted to opposite frame members  12  of the frame by a pivot shaft  18 . One arm  22  of each bell crank extends downwardly from the pivot shaft  18  to a cylindrical roller  24  mounted on the distal end of the arm. The roller  24  supports the forward or left-hand elevating end of the treadmill frame on the support surface  14 . The second arm  26  of each bell cranks extends upwardly from the pivot shaft  18 . The distal end of each second arm  26  is mounted by a pivot connection  28  to a nut assembly  32  mounted on the lead screw  34  of the linear actuator. The nut assembly  32  can have internal screw threading that is complementary to the external screw threading of the lead screw, or can be a recirculating ball type nut assembly or other type of nut assembly commonly employed with linear actuators of this type. The lead screw  34  is mounted for rotation inside a transmission housing  36  by a pair of bearings mounted in opposite walls of the transmission housing. The transmission housing  36  contains a transmission mechanism that includes a drive element, for example a gear, sprocket or pulley, that is secured to the lead screw  34  for rotation therewith. The drive element is driven by the transmission contained in the transmission housing  36  which in turn is driven by an electric motor  38 . Exercise treadmills of this type commonly have controls (not shown) that can control the electric motor  38  to drive the transmission and ultimately the lead screw  34  in opposite directions of rotation. By controlling the rotation of the lead screw  34  in two directions, linear movement of the nut assembly  32  across the lead screw  34  is also controlled. The linear movement of the nut assembly  32  across the lead screw  34  controls pivoting movement of the bell cranks  16  about their pivot shaft  18  which in turn controls elevating movement, of the left-hand end of the treadmill shown in FIG.  1 . For example, operation of the electric motor  38  to rotate the lead screw  34  causing the nut assembly  32  to move to the left as viewed in FIG. 1 will result in the bell cranks  16  rotating in a counterclockwise direction about its pivot shaft  18  and thus elevating the left-hand or forward end of the treadmill frame shown in FIG.  1 . Controlling the electric motor  38  to rotate the lead screw  34  in the opposite direction causing the nut assembly  32  to move to the right as shown in FIG. 1 will cause the bell cranks  16  to move in a clockwise direction about their pivot shaft  18  resulting in the lowering of the treadmill frame shown in FIG.  1 . 
     Transmission housings  36  of the type shown in FIG. 1 are commonly connected to the frame members  12  of the treadmill by a pivot pin  42  extending through a hole in a flange or flanges  44  of the transmission housing and a hole in a flange  46  mounted on the treadmill frame. When a load is placed on the treadmill, for example, by a jogger on the treadmill, the load is transmitted through the bell crank  16  to the lead screw  34  as a tensile force on the lead screw. This tensile force exerted on the lead screw  34  is transmitted to the transmission housing  36  and ultimately to the flanges  44  of the transmission housing that are connected by the pivot pin  42  to the flange  46  of the frame. 
     FIG. 2 shows a detailed view of a prior art linear actuator of the type employed in a treadmill such as that shown in FIG.  1 . In FIG. 2, like parts of the linear actuator described in reference to FIG. 1 have the same reference numbers. FIG. 2 shows a distal portion  52  of the lead screw that extends outside the transmission housing  36  and has the nut assembly  32  mounted thereon. An opposite proximal portion  54  of the lead screw extends into the transmission housing  36 . FIG. 3 is a partial view showing the prior art transmission housing  36  in cross section and the proximal portion  54  of the lead screw mounted in the transmission housing as well as the drive element mounted on the lead screw proximal portion. 
     Referring to FIG. 3, the transmission housing has a first end wall  56  with a first shaft opening  58  passing therethrough. An opposite second end wall  62 , shown to the left in FIG. 3, encloses an interior volume  64  of the transmission housing with the first end wall  58 . A cylindrical recess  66  is formed into the second end wall  62 . The recess  66  is concentric with the first opening  58  through the housing first end wall  56 . The proximal portion  54  of the lead screw extends through the first opening  58  of the housing and into the cylindrical recess  66  of the housing second end wall  62 . Beginning at the right hand end of the lead screw proximal portion  54  shown in FIG. 3, the proximal portion is mounted for rotation in the first opening  58  by a bearing or bushing  68  mounted in the opening. A circular washer  70  is then mounted on the proximal portion  54  of the lead screw. The washer  70  seats against the bushing  68  and an annular shoulder  72  formed in the interior of the transmission housing first end wall  56 . A thrust bearing  74  is then mounted on the lead screw proximal portion  54  seating up against the washer  70 . The drive element is then mounted on the lead screw proximal portion  54 . In FIG. 3, the drive element is a gear  76 , but the drive element could be a sprocket for a chain drive or a pulley for a belt and pulley drive, depending on the particular transmission employed. The gear  76  has a circular recess  78  formed into the left-hand side of the gear as shown in FIG. 3, and then a slot  82  that is further recessed into the gear from the circular recess  78 . The slot  82  extends across the center of the gear. A pin  84  is inserted through a pin hole  86  that passes through the lead screw proximal portion  54 . The pin  84  is received in the slot  82  and thereby secures the gear  76  to the lead screw proximal portion  54  for rotation therewith. A first circular spacer  92  is then mounted on the shaft and is partially positioned in the circular recess  78  of the gear. A second spacer  94  or thrust washer is mounted on the shaft between the first spacer  92  and an interior surface of the transmission housing second end wall  62 . The proximal portion  54  of the lead screw then extends into a bearing or bushing  96  that mounts the proximal portion  54  of the lead screw adjacent its proximal end  98  in the cylindrical recess  66  of the transmission housing second end wall  62 . 
     The two bearings  68 ,  96  mount the proximal portion  54  of the lead screw for rotation in the respective first  56  and second  62  end walls of the transmission housing. The thrust bearing or thrust washer  74  transmits any tensile forces exerted on the lead screw  34  from the gear  76  to the thrust washer  70  and ultimately to the annular shoulder  72  of the transmission housing first end wall  56 . In FIG. 3, the path of tensile forces exerted on the lead screw  34  is represented by the darkened line  102 . As shown in FIG. 3, the tensile forces are first transmitted from the lead screw  34  to the pin  84  that secures the gear  76  to the proximal portion  54  of the lead screw. The pin  84  transmits the tensile forces to the gear  76  which then transmits the tensile forces through the thrust bearing  74 , the thrust washer  70  to the annular shoulder  72  in the first end wall  56  of the transmission housing. The tensile forces transmitted to the transmission housing are then transmitted through the first end wall  56  of the housing to the second end wall  62  and ultimately to the pair of flanges  44  that are mounted by the pivot pin  42  to the frame flange  46 . 
     Prior art linear actuator mountings of the type shown in FIGS. 1-3 have experienced several different types of failures when subjected to the repeated poundings of a relatively heavy jogger running on the treadmill supported by the linear actuator. The repeated pounding of the jogger on the treadmill produces repeated tensile forces exerted on the lead screw  34  that are transmitted through the transmission housing  36  to the housing flanges  44  in the manner described above. This has resulted in the transmission flanges  44  pulling and bending the second end wall  62  of the transmission housing away from the first end wall  56  of the transmission housing or to the left as viewed in FIG.  3 . This can result in a bowing of the transmission housing second end wall  62  where it joins with the transmission flanges  44 . In severe cases, the bowing of the second end wall  62  can result in the lead screw proximal portion  54  disengaging from inside the bearing  96  mounted in the second end wall  6  or in a cracking or splitting of the second end wall  62  in the area of the transmission flanges  44 . Reinforcing the connection of the flanges  44  to the second end wall  62  of the housing, for example by adding gussets between the flanges and the end wall or by thickening the material of the end wall, would overcome this problem but would also increase the cost of manufacturing the trunnion mount for the linear actuator. 
     Prior art actuator mounts of the type disclosed in FIGS. 1-3 have often been constructed with plastic drive gears  76  to reduce their cost. However, the plastic drive gears  76  have also been known to fail as a result of repeated tensile forces exerted on the lead screw. A series of repeated tensile forces exerted on the lead screw  34  and transmitted through the pin  84  to the plastic gear  76  has resulted in cracking and splitting of the gear. This problem could be overcome by replacing the plastic gear  76  with a gear constructed of metal, however this would also increase the cost of manufacturing the trunnion mount. 
     In prior art trunnion mounts of the type disclosed in FIG. 3 that employ a metal drive gear  76  to avoid the problems of the splitting of plastic drive gears, the repeated tensile loading of the lead screw  34  is still transmitted to the pin  84 . This often results in the pin  84  cracking in the slot  82  of the gear  76  resulting in a failure of the driving connection between the gear  76  and the lead screw  34 . 
     What is needed to overcome the problems of the actuator mount described above is a way of transmitting the repeated tensile loading of the lead screw  34  to the frame flange  46  of the treadmill frame while avoiding transmission of the tensile forces to the drive element of the actuator transmission or to the transmission housing enclosing the drive element. 
     SUMMARY OF THE INVENTION 
     The present invention provides a trunnion mount that overcomes the problems associated with the prior art actuator mount by transmitting tensile forces exerted on the lead screw completely through the transmission housing to a trunnion mount that connects the lead screw to the frame flange of the treadmill frame, thus bypassing the transmission housing and the drive elements contained in the housing. The trunnion mount of the invention employs a transmission housing having axially aligned openings in opposite first and second end walls of the housing. The proximal portion of the lead screw extends completely through the housing through the axially aligned openings to a trunnion mount mounted on the proximal portion of the lead screw outside the transmission housing. The trunnion mount provides a pivot connection to the frame flange of the treadmill frame that is completely separate from the transmission housing. The proximal portion of the lead screw is mounted to the trunnion by a nut screw threaded on the proximal portion and a thrust bearing mounted on the proximal portion between the nut and the trunnion. Thus, the proximal portion of the lead screw is free to rotate relative to the trunnion. However, the nut mounted on the proximal portion of the lead screw and the thrust bearing mounted on the proximal portion of the lead screw between the nut and the trunnion transmit any tensile forces exerted on the lead screw through the nut and the thrust bearing directly to the trunnion. The proximal portion of the lead screw is mounted in the two housing end walls by bearings or bushings. The drive element or drive gear is mounted on the proximal portion of the lead screw inside the transmission housing by a pin, just as in the prior art. However, the drive gear is held in engagement with the pin by a pair of spring washers or Belleville springs mounted on the proximal portion of the lead screw between the gear and the first end wall of the transmission housing. Thus, any tensile forces exerted on the lead screw are transmitted through the proximal portion of the lead screw completely through the transmission housing and to the trunnion connecting the lead screw to the frame flange of the treadmill frame. In this manner, the opposed end walls of the transmission housing as well as the gear and the pin connecting the gear to the lead screw are completely separated from any tensile forces exerted on the lead screw. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and features of the present invention are revealed in the following detailed description of the preferred embodiment of the invention and in the drawing figures wherein: 
     FIG. 1 is a schematic representation of one operative environment of a prior art linear actuator and a pivoting mount for the linear actuator; 
     FIG. 2 is a partial view of the linear actuator and pivoting mount of FIG. 1; 
     FIG. 3 is a partial view showing the detail of the pivoting mount of FIG.  1  and the drive element of the linear actuator; and 
     FIG. 4 is a partial view similar to that of FIG. 3 but showing the improved trunnion mount of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 4 shows the trunnion mount of the present invention that may be employed in replacing the prior art actuator mount of FIGS. 1-3. However, it should be understood that the operative environment of the linear actuator trunnion mount shown in FIG. 1 is relied on herein only in explaining how tensile forces exerted on a lead screw of a linear actuator can be transmitted to an actuator mount resulting in the failure of the prior art actuator mount. It is not intended that the operative environment of FIG. 1 in any way limit the trunnion mount of the invention shown in FIG.  4 . The trunnion mount of the invention shown in FIG. 4 can be used in any application of a trunnion mount for a linear actuator. 
     The linear actuator trunnion mount of the invention shown in FIG. 4 also employs a lead screw having a first portion or distal portion  112  that extends outside of a transmission housing. A nut assembly  32  of the type shown in FIGS. 1 and 2 is mounted on the lead screw distal portion  112 . The second portion of the lead screw, or the proximal portion  114 , extends into a transmission housing  116 . The proximal portion  114  of the lead screw extends through a first opening  118  in a first end wall  122  of the transmission housing. A bearing or bushing  124  mounts the proximal portion  114  of the lead screw for rotation in the end wall. Instead of a thrust washer, a spring assembly comprised of a pair of spring washers or Belleville springs  126  is then mounted on the proximal portion  114  of the lead screw. The drive element, in this case a drive gear  128 , is then mounted on the proximal portion  114  of the lead screw and is secured thereto by a pin  132  in the same manner as the prior art drive element described earlier. First  134  and second  136  spacers are then mounted on the lead screw proximal portion  114  in the same manner as the prior art actuator mount, with the first spacer  134  being received in the circular recess in the gear  128 . The lead screw proximal portion  114  then passes through a bearing or bushing  142  mounted in a second opening  144  in the second end wall  146  of the transmission housing. The bushing  142  mounts the lead screw proximal portion  114  for rotation in the second opening  144  of the second end wall. However, unlike the prior art actuator mount, the lead screw proximal portion  114  extends completely through the second end wall  146  of the transmission housing. 
     The lead screw proximal portion  114  extends through a shaft hole  148  in the trunnion  152  of the invention. The trunnion  152  includes a base portion  154  through which the shaft hole  148  extends and a pair of projecting arms  156  that extend at right angles from the base portion  154 . The pair of arms  156  have coaxial holes  158  that pass therethrough and receive a pivot pin  42  that mounts the arms to the frame flange  46 , thereby securing the trunnion  152  for pivoting movement relative to the frame flange  46 , but preventing any axial or linear movement of the trunnion  152  relative to the frame flange  46 . The lead screw proximal portion  114  passes through the trunnion shaft hole  148  and has a section of external screw threading  164  formed thereon. The external screw threading  164  extends to the proximal end  166  of the lead screw. A thrust bearing  168  is mounted over the screw threading  164  of the lead screw proximal portion and a complementary internally threaded nut  172  is screw threaded over the external screw threading  164 . The nut  172  functions as an enlarged head of the lead screw proximal portion  114  adjacent is proximal end  166  that cannot be pulled through the shaft hole  148  of the trunnion. A hole  174  for a cotter pin (not shown) passes through the lead screw adjacent its proximal end  166  to prevent the nut  172  from backing off the lead screw. The thrust bearing  168  positioned between the nut  172  and the trunnion  152  permits the lead screw to rotate freely in opposite directions of rotation relative to the trunnion, but prevents any axial movement of the lead screw relative to the trunnion, in particular axial movement to the right which would be caused by tensile forces exerted on the lead screw. 
     With the construction of the trunnion mount of the invention shown in FIG.  4  and described above, the path of tensile forces represented by the darkened line  176  in FIG. 4 passes completely through the transmission housing  116  bypassing the drive gear  128  and the pin  132  securing the gear to the lead screw proximal portion  114 . The tensile forces are transmitted to the nut  172  screw threaded on the lead screw proximal portion  114  which in turn, through the thrust bearing  168 , transmits the tensile forces to the trunnion  152 . The trunnion arms  156  transmit the tensile forces to the frame flange  46  by the pivot pin  162 . 
     Thus, the particular construction of the trunnion mount of the invention described above and shown in FIG. 4 overcomes the problems associated with the prior art actuator mount by providing a trunnion mount that transmits tensile forces exerted on the lead screw directly to the trunnion  152  connecting the lead screw to the frame flange  46  and avoiding any transmission of tensile forces to the transmission housing  116  or the drive gear  128  and pin  132  mounting the gear on the lead screw proximal portion  114 . 
     While the present invention has been described by reference to a specific embodiment, it should be understood that modifications and variations of the invention may be constructed without departing from the scope of the invention defined in the following claims.