Abstract:
A linear actuator includes a housing formed of a first half shell and a second half shell and defining an accommodation chamber, a front opening in front of the accommodation chamber and a receiving chamber at the bottom side of the accommodation chamber, a driving mechanism having a push rod movably mounted in the accommodation chamber and extending out of the front opening, a motor mounted outside the accommodation chamber and a spindle coupled to and rotatable by the motor for causing the transmission shaft to reciprocate, and limit switches detachably mounted in the receiving chamber for stopping the motor upon movement of the transmission shaft to a front or rear limit position.

Description:
This application claims the priority benefit of Taiwan patent application number 097209674 filed on Jun. 2, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a linear actuator and more particularly, to the structural design of the housing of a linear actuator, which is formed of two half shells, facilitating installation and position adjustment of limit switches. 
     2. Description of the Related Art 
     Many linear actuators are known and intensively used in electric hospital beds, treadmills and to facilitate position adjustment. During application of a linear actuator, limit switches are usually used and respectively mounted in the start end and finish end to control the forward and return strokes of the linear actuator. When a push block of a nut of the linear actuator touches the limit switch at the finish end during the forward stroke, the limit switch is triggered to stop forward movement of the linear actuator. On the contrary, when the push block of the nut touches the limit switch at the start end during the return stroke, the limit switch is triggered to stop backward movement of the linear actuator. Thus, the linear actuator is controlled to reciprocate with the start end and the finish end. 
       FIG. 7  is an exploded view of a linear actuator according to the prior art. According to this design, the linear actuator comprises a housing A, a driving mechanism B, and two limit switches C. The housing A comprises a hollow base A 1 , and an outer tube A 2  that extends from one side of the hollow base A 1  and defines therein a longitudinal receiving chamber A 21 . The driving mechanism B comprises a motor B 1  fastened to one side of the hollow base A 1  of the housing A, a worm B 11  fixedly connected to the output shaft of the motor B 1  and inserted into the inside of the hollow base A 1  of the housing A, a spindle B 2  rotatable mounted in the longitudinal receiving chamber A 21  of the outer tube A 2  of the housing A, a worm gear B 21  fixedly mounted on one end of the spindle B 2  and meshed with the worm B 11 , a nut B 3  threaded onto the spindle B 2  and supported inside the longitudinal receiving chamber A 21  of the outer tube A 2  of the housing A and movable linearly relative to the spindle B 2  during rotation of the spindle—B 2 , and a push rod  4  threaded onto the spindle B 2  and extending out of the outer tube A 2  of the housing A for connection to an external driven member. The nut B 3  has a push block B 31  protruded from the periphery thereof. Further, a rail C 1  is mounted in a longitudinal groove A 22  inside the longitudinal receiving chamber A 21  of the outer tube A 2 . Further, two limit switches C are mounted on the rail C 1  at selected locations at two opposite sides relative to the push block B 31  of the nut B 3  at the spindle B 2 . 
     When started the motor B 1 , the worm B 11  is driven to rotate the worm gear B 21  and the spindle B 2 , causing forward or backward movement of the nut B 3  and the transmission shaft B 4  relative to the spindle B 2 . When the nut B 3  reaches a predetermined position, it will touch one limit switch C, causing the limit switch C to stop the motor B 1 . In actual practice, this design of linear actuator still has drawbacks as described hereinafter. 
     During installation of the linear actuator, the rail C 1  and the limit switches C are mounted in the longitudinal groove A 22  inside the longitudinal receiving chamber A 21  of the outer tube A 2  of the housing A subject to the use of a scale, and then the spindle—B 2  and the nut B 3  are inserted from one end of the longitudinal receiving chamber A 21  into the inside of the outer tube A 2  of the housing A, and then the transmission shaft B 4  is inserted from the other end of the longitudinal receiving chamber A 21  into the inside of the outer tube A 2  of the housing A and threaded onto the spindle—B 2 . During insertion of the spindle B 2  and the nut B 3  into the longitudinal receiving chamber A 21  of the outer tube A 2  of the housing A, the limit switches C may be biased accidentally by the push block B 31  of the nut B 3 . When this problem happens, the user needs to re-install the rail C 1  and the limit switches C. Further, when wishing to adjust the positions of the limit switches C, the user needs to detach all the parts of the linear actuator, solder the limit switches on the other position and rearrange the wires accordingly. 
     Therefore, it is desirable to provide a linear actuator that eliminates the aforesaid drawbacks. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished under the circumstances in view. It is one object of the present invention to provide a linear actuator, which assures positive positioning of the component parts and allows quick detachment of the housing for easy installation and position adjustment of limit switches. 
     To achieve this and other objects of the present invention, the linear actuator comprises a housing, a driving mechanism, which is mounted in the housing and comprises a motor, a spindle-coupled to and rotatable by the motor and a push rod coupled to the spindle for a linear motion upon rotation of the spindle, and at least one limit switch controllable by the nut of the push rod to switch off the motor. The housing is formed of a first half shell and a second half shell. The first half shell and the second half shell are fastened together, defining an accommodation chamber that accommodates the spindle and the push rod of the driving mechanism, a front opening in communication with the front end of the accommodation chamber for the passing of the push rod to the outside of the housing, and a receiving chamber at one side of the accommodation chamber for the positioning of the at least one limit switch. 
     The housing comprises a tubular front coupling portion formed of a front part of the first half shell and a front part fastened to the tubular front coupling portion to reinforce the strength of the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view of a linear actuator in accordance with the present invention. 
         FIG. 2  is an exploded view of the linear actuator in accordance with the present invention. 
         FIG. 3  is an elevational view of a limit switch for the linear actuator in accordance with the present invention. 
         FIG. 4  is a sectional view of the linear actuator in accordance with the present invention. 
         FIG. 5  corresponds to  FIG. 4 , showing the position of the front limit switch adjusted. 
         FIG. 6  is an exploded view of an alternate form of the linear actuator in accordance with the present invention. 
         FIG. 7  is an exploded view of a linear actuator according to the prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1˜4 , a linear actuator in accordance with the present invention is shown comprising a housing  1 , a driving mechanism  2  and at least one, for example, two limit switches  3 . 
     The housing  1  is formed of a first half shell  11  and a second half shell  12 , having an accommodation chamber  13  extending along the length thereof, a front opening  131  located on one end of the accommodation chamber  13 , a rear compartment  132  located on the other end of the accommodation chamber  13 , a bottom receiving chamber  14  located on the bottom side of the accommodation chamber  13 , a longitudinal seat  141  set between the accommodation chamber  13  and the bottom receiving chamber  14 , a series of transverse grooves  1411  located on the longitudinal seat  141  and facing the inside of the accommodation chamber  13 , a tubular front coupling portion  15  extending around the front opening  131 , at least one, for example, and two raised portions  151  protruded from the periphery at two opposite sides. Further, a metal retaining ring  16  is capped on the tubular front coupling portion  15 , having two retaining holes  161  respectively forced into engagement with the raised portions  151 . 
     The driving mechanism  2  comprises a motor  21 , a worm  211  connected to and rotatable by the motor  21 , a push rod  23  having an inner thread  231  axially located on the inside and a nut— 232  protruded from the periphery of one end thereof, a spindle  22  threaded into the inner thread  231  of the push rod  23 , a worm gear  221  fixedly mounted on one end of the spindle  22  and meshed with the worm  211 , and a connector  24  connected to the other end of the push rod  23  remote from the nut  232  for the connection of an external device to be driven by the linear actuator. 
     Each limit switch  3  comprises a switch body  31 , and a positioning device  32  disposed at one side relative to the switch body  31 . The switch body  31  has a plurality of electrode pins  311  located on the bottom side thereof. The positioning device  32  has a button  321  protruded from the top wall thereof and suspending above the switch body  31  and adapted for triggering the switch body  31 , a locating groove  322  located on one lateral side thereof opposite to the switch body  31 , at least one, for example, two protruding blocks  323  suspending in one side, namely, the top side of the locating groove  322 , and a retaining portion  324  located on the other side, namely, the bottom side of the locating groove  322 . 
     During the assembly process of the linear actuator, perpendicularly attach the motor  21  of the driving mechanism  2  to the rear end of the housing  1  to insert the worm  211  of the motor  21  into the rear compartment  132  of the housing  1  and then fixedly secure the motor  21  to the housing  1 , and then thread the spindle— 22  into the inner thread  231  of the push rod  23  and put the push rod— 23  with the spindle  22  in the accommodation chamber  13  of the housing  1  to force the worm gear  221  into engagement with the worm  211  of the motor  21  and to have the push rod  23  extend out of the accommodation chamber  13  to the outside of the housing  1  through the front opening  131 , and then fasten the limit switches  3  to the front and rear sides of the longitudinal seat  141  in the bottom receiving chamber  14  of the housing  1  to aim the button  321  of each limit switch  3  at the nut  232  of the push rod  23 , and then fasten the first half shell  11  and second half shell  12  of the housing  1  together and attach the metal retaining ring  16  to the tubular front coupling portion  15  of the housing  1  to force the two retaining hole  161  of the metal retaining ring  16  into engagement with the raised portions  151  of the tubular front coupling portion  15  respectively and to reinforce the strength of the housing  1 . 
     According to the present preferred embodiment, the first half shell  11  and second half shell  12  of the housing  1  are respectively molded from a plastic material. Alternatively, a metal material can be used to make the first half shell  11  and second half shell  12  of the housing  1 . 
     Referring to  FIG. 5  and  FIGS. 2˜4  again, when started the motor  21  to rotate the worm  211  clockwise or counter-clockwise, the worm  211  drives the worm gear  221  to rotate the spindle  22  in the inner thread  231  of the push rod  23 , causing the push rod  23  to be moved linearly forwards or backwards. Further, the electrode pins  311  of the limit switches  3  are respectively electrically connected to the circuit (not shown) at the start end and finish end of the linear stroke. 
     When the nut  232  of the push rod  23  reaches the finish end during a forward linear motion of the push rod  23  subject to clockwise rotation of the spindle  22 , the nut  232  touches the button  321  of the limit switch  3  at the finish end, causing the limit switch  3  at the finish end to switch off the motor  21 , avoiding disconnection of the push rod  23  from the front end of the spindle— 22 . On the contrary, when the nut  232  of the push rod  23  reaches the start end during a backward linear motion of the push rod— 23  subject to counter-clockwise rotation of the spindle  22 , the nut— 232  touches the button  321  of the limit switch  3  at the start end, causing the limit switch  3  at the start end to switch off the motor  21 , avoiding locking of the spindle  22  and preventing worm gear damage. 
     The distance between the two limit switches  3  is determined subject to the designed distance of the linear stroke of the nut  232 . Further, the positioning device  32  of each limit switch  3  is fastened to the longitudinal seat  141  in the bottom receiving chamber  14  of the housing  1  by means of forcing the protruding blocks  323  into engagement with the transverse grooves  1411  on the longitudinal seat  141  to have the longitudinal seat  141  is received in the locating groove  322  and retaining portion  324  be abutted against the bottom side of the longitudinal seat  141 . When wishing to adjust the distance between the two limit switches  3 , remove the metal retaining ring  16  from the tubular front coupling portion  15  of the housing  1 , and then separate the first half shell  11  and second half shell  12  of the housing  1 , and then pull the limit switch  3  away from the longitudinal seat  141 , and then reinstall the limit switch  3  in the longitudinal seat  141  at the selected location. 
     Further, the formation of the series of transverse grooves  1411  on the longitudinal seat  141  constitutes a rack for engagement with the tooth-like protruding blocks  323  of the positioning device  32  of each limit switch  3 . Further, the protruding blocks  323  can be made having a rectangular, dovetail-like or arched profile for positive engagement with the series of transverse grooves  1411  on the longitudinal seat  141  and easy removal of the respective limit switch  3  from the longitudinal seat  141 . 
       FIG. 6  is an exploded view of an alternate form of the linear actuator. According to this alternate form, the housing  1  has a motor chamber  17  perpendicularly connected to the rear compartment  132  for accommodating the motor  21  of the driving mechanism  2 . Therefore, the motor  21  is well protected in the motor chamber  17  and will not be forced to bias during delivery of the linear actuator, avoiding improper engagement between the worm gear  221  and the worm  211  or any possible gear tooth damage. Further when the motor  21  in the motor chamber  17  also prevent it from water and dust. 
     In the aforesaid embodiment, the positioning device  32  of each limit switch  3  is fastened to the longitudinal seat  141  in the bottom receiving chamber  14  of the housing  1  by means of forcing the protruding blocks  323  into engagement with the transverse grooves  1411  on the longitudinal seat  141 , however this arrangement is not a limitation; alternatively the positioning device  32  of each limit switch  3  can be fastened to the longitudinal seat  141  in the bottom receiving chamber  14  of the housing  1  by a screw joint. 
     In general, the invention provides a linear actuator, which has the following advantages and features: 
     1. The first half shell  11  and the second half shell  12  constitute the housing  1 , and the metal retaining ring  16  is fastened to the tubular front coupling portion  15  of the housing  1  to force the two retaining holes  161  of the metal retaining ring  16  into engagement with the raised portions  151  of the tubular front coupling portion  15  and to reinforce the strength of the housing  1 . When wishing to adjust the positions of the limit switches  3 , the user can remove the metal retaining ring  16  from the tubular front coupling portion  15  of the housing  1  and then separate the first half shell  11  and the second half shell  12  for allowing re-installation of the limit switches  3 . Therefore, the invention facilitates adjustment of the positions of the limit switches  3  and avoids displacement of the limit switches  3  due to accidental touching by the spindle  22  or push rod  23  during installation of the driving mechanism  2 . 
     2. The first half shell  11  and second half shell  12  of the housing  1  are molded from plastics for the advantages of ease of fabrication and low manufacturing cost. The use of the metal retaining ring  16  assures tight connection of the first half shell  11  and the second half shell  12  and reinforces the strength of the housing  1 , avoiding vibration of the push rod  23  during operation of the driving mechanism  2 . 
     A prototype of linear actuator has been constructed with the features of  FIGS. 1˜6 . The linear actuator functions smoothly to provide all of the features disclosed earlier. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. For example, any other linear transmission structures may be used to substitute for the worm and worm gear for transmission of rotary driving force from the motor to the spindle; the metal retaining ring can be made having raised portions and the tubular front coupling portion can be made having retaining holes for engagement with the raised portions of the metal retaining ring. Accordingly, the invention is not to be limited except as by the appended claims.