Abstract:
A contamination resistant lead screw or ball screw-driven linear actuator has the lead screw and drive nut enclosed within a specially constructed guide chassis. A carriage is slidably mounted on parallel rails attached to the guide chassis. The carriage is attached to the drive nut through a slot in the guide chassis. An enclosure surrounds the guide chassis. The enclosure and the guide chassis create a labyrinth seal that effectively contains internally generated debris and contamination and excludes environmental dirt, debris and contamination.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to electromechanical linear actuators. More particularly, it relates to a lead screw-driven linear actuator having a specially constructed guide chassis that encloses the lead screw and the drive nut and protects them from dirt, debris and contamination.  
         BACKGROUND OF THE INVENTION  
         [0002]    Linear actuators are used in a great many machine assemblies to provide linear motion between two parts of the machine, for example to provide linear motion between a machine base and a load. Different types of linear actuators include pneumatic cylinders, hydraulic cylinders, rodless cylinders, rack-and-pinion gears, lead screw actuators and ball screw actuators. Lead screw actuators generally have a rotatable lead screw and a drive nut that engages the helical threads of the lead screw. When the lead screw is rotated relative to the drive nut, the drive nut converts the rotary motion to linear motion. In some applications, the rotatable lead screw is held stationary and the drive nut, which is constrained from rotating, moves linearly relative to the lead screw. In other applications, the drive nut is held stationary and the rotatable lead screw moves linearly relative to the drive nut. In yet another variation, the lead screw is constrained from rotating and the drive nut is rotated to create linear motion of the lead screw.  
           [0003]    Ball screw actuators are a special type of lead screw actuator in which the drive nut contains recirculating ball bearings that engage the helical threads of the lead screw. The recirculating ball bearings reduce the friction between the lead screw and the drive nut, providing a highly efficient conversion of rotary motion to linear motion. Lead screw actuators have many advantages including easily controlled speeds, reversibility, precise and repeatable positioning of loads, and high resistance to being backdriven by forces on the load.  
           [0004]    Contamination resistance is important in many applications of linear actuators. For example, in a clean room environment, it is important that the mechanism of a linear actuator does not release debris that would contaminate the operating environment. Conversely, in highly contaminated environments, the mechanism of the linear actuator must be protected from environmental debris and contamination. In nearly all environments, it is desirable to protect the mechanism of the linear actuator from mechanical damage as well.  
           [0005]    One prior art attempt to create a contamination resistant lead screw actuator is described in U.S. Pat. No. 5,915,916. This patent and all other U.S. patents referred to herein are hereby incorporated by reference in their entirety. The lead screw and drive nut are enclosed within a housing that has a slit in it. A moving carriage is attached to the drive nut through the slit. A moving seal belt, which is mounted on pulleys, attaches to the carriage and covers the portion of the slit that is not occupied by the carriage. In other prior art devices, the lead screw and drive nut or other mechanism of a linear actuator is enclosed in a U-shaped channel and telescoping or accordion-folded panels cover the open top of the channel to exclude debris and contamination. These prior art devices tend to be overly complex, expensive and subject to mechanical failure.  
           [0006]    Current manufacturing processes for linear actuators in general, and lead screw actuators in particular, involve the use of very expensive precision manufacturing equipment and very often involve the machining and grinding of hardened steels, which both add significantly to the manufacturing costs. One main source of the expense is the production of the linear rails that are part of the guide assembly in a linear actuator. U.S. Pat. No. 6,052,902 represents one prior art attempt to reduce the complexity and expense of linear motion bearing fabrication. The fabrication method described does not go far enough in eliminating the expensive precision manufacturing processes involved in fabricating a linear actuator assembly. Thus, there is a continuing need for improvements to the current manufacturing processes for fabricating a guide assembly for use in linear actuators.  
           [0007]    It would be desirable, therefore, to provide a contamination resistant linear actuator, particularly a lead screw actuator, that is simple, low cost to manufacture and mechanically reliable.  
         SUMMARY OF THE INVENTION  
         [0008]    In keeping with the foregoing discussion, the present invention provides a contamination resistant lead screw-driven linear actuator in which the lead screw and drive nut are enclosed within a specially constructed guide chassis that protects them from dirt, debris and contamination. The guide chassis also serves to effectively contain any debris or contamination produced by the linear actuator mechanism so that it does not contaminate the operating environment of the linear actuator.  
           [0009]    The linear actuator is constructed around a guide assembly that includes a guide chassis to which are attached a pair of parallel guide rails. A carriage is slidably mounted on the guide chassis by bearing assemblies that are supported on the parallel guide rails. One or more magnets are attached to the carriage to facilitate non-contact position sensing. A drive nut, which preferably includes an anti-backlash mechanism, is driven back and forth by a lead screw, which is in turn driven by a reversible electric motor. The lead screw and the drive nut are enclosed within an approximately cylindrical central passage within the guide chassis. A narrow slot through the wall of the guide chassis extends along the length of the central passage. The drive nut is attached to the carriage through the slot in the guide chassis by a nut flange. The slot in the guide chassis is preferably angled downward away from the top of the guide assembly where the carriage is mounted. This configuration helps to resist environmental debris from entering the central passage and contaminating the lead screw and drive nut.  
           [0010]    The linear actuator has an enclosure that includes a side cover plates and an end cover plate, which cover the sides and end of the guide subassembly, and a molded plastic motor housing, which covers the electric motor. Hall effect position sensors are repositionably mounted on the exterior of the enclosure to sense the position of the carriage. The enclosure, together with the guide chassis, creates a labyrinth seal that effectively contains internally generated debris and contamination and excludes environmental dirt, debris and contamination.  
           [0011]    In a preferred embodiment, the linear actuator is provided as a fully assembled integrated module ready to install for a variety of linear motion applications. In a typical application, the guide chassis of the linear actuator is affixed to a base, such as a work surface or a machine, and a load, which may be a mechanical or electromechanical device, a tool, a fixture, an optical component, etc., is mounted on the carriage of the linear actuator. The carriage is driven back and forth along the length of the parallel guide rails by the electric motor to position the load with respect to the linear actuator.  
           [0012]    The present invention also provides an improved method of manufacturing a screw-driven linear actuator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a perspective view of a linear actuator constructed in accordance with the principles of the present invention.  
         [0014]    [0014]FIG. 2 is a top view of the linear actuator of FIG. 1.  
         [0015]    [0015]FIG. 3 is a side view of the linear actuator of FIG. 1.  
         [0016]    [0016]FIG. 4 is an end view of the linear actuator of FIG. 1.  
         [0017]    [0017]FIG. 5 is a cutaway perspective view of the linear actuator showing the internal components.  
         [0018]    [0018]FIG. 6 is an exploded view of the linear actuator.  
         [0019]    [0019]FIG. 7 is an exploded view of the guide subassembly of the linear actuator.  
         [0020]    [0020]FIG. 8 is an end view of the guide assembly of the linear actuator.  
         [0021]    [0021]FIG. 9 is a cross section of an alternate embodiment of the linear actuator with integral side covers.  
         [0022]    [0022]FIG. 10 is a perspective view of a recirculating-ball linear bearing used in the linear actuator.  
         [0023]    [0023]FIG. 11 is an end view of the recirculating-ball linear bearing.  
         [0024]    [0024]FIG. 12 is an exploded view of the recirculating-ball linear bearing.  
         [0025]    [0025]FIG. 13 shows two recirculating-ball linear bearings installed in the linear actuator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    [0026]FIG. 1 is a perspective view of a fully assembled linear actuator  100  constructed in accordance with the principles of the present invention. FIG. 2 is a top view, FIG. 3 is a side view and FIG. 4 is an end view of the linear actuator  100  of FIG. 1. FIG. 5 is a cutaway perspective view showing the internal components of the linear actuator  100 . The components of the linear actuator  100  are shown in an exploded view in FIG. 6. At the heart of the linear actuator  100  is the guide assembly  102 . The components of the guide subassembly  102  are shown in an exploded view in FIG. 7. The guide subassembly  102  includes a guide chassis  104  to which are attached a first guide rail  106  and a second guide rail  108 . Alternatively, the guide rails  106 ,  108  may be formed integrally with the guide chassis  104 . A carriage  110  is slidably mounted on the guide chassis  104 . The carriage  110  is supported on the parallel guide rails  106  and  108  by a first bearing assembly  112  and a second bearing assembly  114 . The bearing assemblies  112 ,  114  may be configured as recirculating-ball linear bearings or, alternatively, sliding bearings may be used. A drive nut  116 , which in a preferred embodiment includes an anti-backlash mechanism as described in U.S. Pat. No. 5,839,321, is attached to the carriage  110  by a nut flange  118 . A first magnet  120  and a second magnet  122  are attached to the carriage  110 . The guide subassembly  102  is terminated on one end by a first end plate  130  and on the other end by a second end plate  132 . Mounting holes  278  are drilled in each of the first and second end plates  130 ,  132 . The mounting holes  278  provide a standardized way of mounting the linear actuator to a work surface.  
         [0027]    The drive nut  116  is driven back and forth by a lead screw  124 , which is in turn driven by a reversible electric motor  126 . In a preferred embodiment, the drive nut  116  and the lead screw  124  are configured as a ball screw actuator in which the drive nut  116  contains recirculating ball bearings that engage the helical threads of the lead screw  124 . The electric motor  126  is attached to the first end plate  130  and the end of the lead screw  124  is supported by a radial bearing  128  mounted in the second end plate  132 . In a preferred embodiment, the electric motor  126  is a stepper motor with an integrated motor driver, such as a NEMA  17  stepper motor, which provides precise control over the position of the carriage  110  along the length of the guide chassis  104 . Alternatively, a separate motor driver for the electric motor  126  may be provided as part of linear actuator  100  or separate from it. In other embodiments, another type of AC or DC reversible electric motor could be used in place of the stepper motor.  
         [0028]    In a preferred embodiment, the electric motor  126  and lead screw  124  are permanently joined together to form a unitary or integral motor-lead screw assembly  125 . The integral motor-lead screw assembly  125  is constructed by forming a hole in the rotor of the electric motor  126  to receive the lead screw  126  and forming a short cylindrical stub at one end of the lead screw  126  to mate with the hole in the rotor by means of a clearance fit. A bonding agent or cement is applied to the hole and/or the stub and the stub is inserted into the hole in the rotor. Once the bonding agent or cement has cured, the electric motor  126  and lead screw  124  are permanently joined together to form an integral motor-lead screw assembly  125 . This arrangement decreases the overall length of the motor-lead screw assembly  125 , while providing direct-drive performance. It also simplifies the construction of the linear actuator  100  by eliminating the need for separate motor mounts and flexible couplings.  
         [0029]    The linear actuator  100  is completed with a first side cover plate  134 , a second side cover plate  136  and an end cover plate  138 , which cover the sides and end of the guide subassembly  102 , and a molded plastic motor housing  140 , which covers the electric motor  126 . A first Hall effect sensor  142  and a second Hall effect sensor  144  are repositionably mounted on the second side cover plate  136 . In a preferred embodiment, all of the electrical connections for the linear actuator  100  are centralized in a single communications port  300  mounted on the exterior of the motor housing  140 . In addition, one or more sensor input ports  302  are provided for making connections with the Hall effect sensors  142 ,  144 .  
         [0030]    In a preferred embodiment, the linear actuator  100  is provided to the user as a fully assembled integrated module ready to install for a variety of linear motion applications. In a typical application, the guide chassis  104  of the linear actuator  100  is affixed to a base, such as a work surface or a machine, and a load, which may be a mechanical or electromechanical device, a tool, a fixture, an optical component, etc., is mounted on the carriage  110  of the linear actuator  100 . The carriage  110  is driven back and forth along the length of the parallel guide rails  106  and  108  by the electric motor  126  to position the load with respect to the linear actuator  100 .  
         [0031]    [0031]FIG. 8 is an end view of the guide assembly  102  of the linear actuator  100  with the first side cover plate  134  and second side cover plate  136  installed. In this view, the cross-sectional profile of the guide chassis  104  and the carriage  110  can be clearly seen. The guide chassis  104  has a base  150  that is shaped to facilitate alternative mounting options. The base  150  has dovetail shaped lateral edges  154  to facilitate inserting the base  150  into a dovetail slot or for clamping the linear actuator  100  down to a base. A T-shaped slot  152  is also provided in the center of the base  150  to facilitate clamping the linear actuator  100  down to a base. The guide chassis  104  is formed around an approximately cylindrical central passage  160  with a first vertical side wall  156  that extends upward from the base  150 , an approximately horizontal upper wall  158  extending from the top of the first side wall  156  and a second vertical side wall  162  depending from the upper wall  158 , leaving a gap or slot  164  connecting with the central passage  160  between the second side wall  162  and the base  150 . Thus, the lead screw  124  and the drive nut  116  are effectively enclosed within the central passage  160  of the guide chassis  104 . The slot  164  in the guide chassis  104  is preferably angled downward away from the top of the guide assembly  102  where the carriage  110  is mounted. In a preferred embodiment, the slot  164  is located on the guide chassis  104  below the second guide rail  108  and angled downward away from the top of the guide assembly  102  at an angle from approximately 90 to 180 degrees. This configuration helps to resist environmental debris from entering the central passage  160  and contaminating the lead screw  124  and drive nut  116 .  
         [0032]    A first approximately semicircular channel  166  is formed in the first side wall  156  to receive the first guide rail  106 . A second approximately semicircular channel  168  is formed parallel to the first semicircular channel  166  in the depending second side wall  162  to receive the second guide rail  108 . The first and second guide rails  106 ,  108  are preferably made from cylindrical steel rods, for example from 5 mm diameter steel rods.  
         [0033]    In a preferred embodiment, the first and second semicircular channels  166 ,  168  are formed with crush zones  170  that deform so that the rails  106 ,  108  can be press fit into the semicircular channels  166 ,  168 . The crush zones  170  are areas along the length of the guide chassis  104  where the material is designed to be weaker than the surrounding material to allow for controlled deformation of the material in the crush zones  170  as the rails  106 ,  108  are pressed into the semicircular channels  166 ,  168 . Alternatively or in addition, a bonding agent, such as adhesive or cement, may be used to bond the rails  106 ,  108  into the semicircular channels  166 ,  168  and/or to fill in any gaps in the assembly.  
         [0034]    Compared to conventional guide rail fabrication processes, the use of crush zones  170  can reduce the overall cost of manufacturing and assembly, without loss in precision. Predefined failure points, i.e. crush zones  170 , allow for controlled positioning and alignment of the rails  106 ,  108  without requiring the channels  166 ,  168  to be precision machined or ground, as is done in the conventional fabrication process. Additionally, a relatively low cost linear extrusion can be utilized rather than a machined part for fabricating the guide chassis  104 . These attributes of the crush zones  170  can significantly reduce the overall cost of manufacturing and assembly of the linear actuator  100 , while maintaining the precise alignment of the linear rails  106 ,  108  that contributes to smooth operation.  
         [0035]    Optionally, the guide chassis  104  may also include a first upper flange  172  and a second upper flange  174  that extend laterally from the edges of the horizontal upper wall  158 . The first and second upper flanges  172 ,  174  cover the guide rails  106 ,  108  and shield them from debris and contamination. Slotted holes  176 , which are sized to accept self-tapping screws, are formed in the ends of the first and second upper flanges  172 ,  174 . Similarly, slotted holes  178  for self-tapping screws are also formed near the lateral edges  154  of the base  150 . Corresponding holes  186 ,  188  are provided in each of the end plates  130 ,  132  for attaching the end plates  130 ,  132  to the guide chassis  104  with self-tapping screws (see FIG. 6.)  
         [0036]    The carriage  110  has a shape that conforms closely to the upper part of the guide chassis  104 . The carriage  110  has a horizontal upper surface  190  that is connected to a first depending vertical leg  192  and a second depending vertical leg  194 . A dovetail slot  182  is formed in the upper surface  190  as one means of attaching a load to the carriage  110 . The first and second depending vertical legs  192 ,  194  are spaced apart to form an internal channel  200  that is sized and shaped to slide telescopically over the upper part of the guide chassis  104 . If the guide chassis  104  is constructed with first and second upper flanges  172 ,  174  as shown, then corresponding slots  196 ,  198  are formed in the first and second depending vertical legs  192 ,  194  to provide clearance for the flanges  172 ,  174 . A tongue  202  extends from the second depending vertical leg  194  through the slot  164  between the second side wall  162  and the base  150  of the guide chassis  104 . The tongue  202  is configured to interlock with an arm  208  extending from the nut flange  118  without the need for any additional fasteners. This arrangement effectively attaches the carriage  104  to the drive nut  116 . In another preferred embodiment, the nut flange  118  is integrated into the tongue  202 , thereby reducing the part count. Alternatively, one or more screws or other fasteners may be used to secure the tongue  202  that extends from the second depending vertical leg  194  of the carriage  104  to the arm  208  that extends from the nut flange  118 .  
         [0037]    A first internal semicircular channel  204  is formed in the first depending vertical leg  192  to hold the first bearing assembly  112  in alignment with the first guide rail  106  and a second internal semicircular channel  206  is formed in the second depending vertical leg  194  to hold the second bearing assembly  114  in alignment with the second guide rail  108 . First and second external slots  212 ,  214  are formed in the first and second depending vertical legs  192 ,  194 , respectively, to receive the first and second magnets  120 ,  122  (see FIG. 7.)  
         [0038]    In a preferred embodiment, the guide chassis  104  is made from aluminum and extruded with the cross-sectional profile shown, the extrusion is cut to length to produce the final part. Similarly, the carriage  110  is made from aluminum and extruded with the cross-sectional profile shown, the extrusion is cut to length and the holes are drilled in a single operation to produce the final part. This minimizes the amount of machining necessary to produce these parts. It also reduces the inventory of parts needed to produce different sizes of linear actuators  100  with a range of stroke lengths. The guide chassis material can be stocked in as-extruded lengths and the extrusion cut to length to produce a guide chassis  104  with any desired stroke length. Typically, the guide chassis  104  will be sized to provide a linear actuator  100  with a stroke length of approximately 100 mm to 600 mm, but virtually any stroke length is possible with this manufacturing technique.  
         [0039]    Preferably, the first and second side cover plates  134 ,  136  are also made from an aluminum extrusion and cut to length. Dovetail slots  224 ,  226  are formed in the first and second side cover plates  134 ,  136  for mounting the first and second Hall effect sensors  142 ,  144 . Because of their symmetry, the first and second side cover plates  134 ,  136  may be made from a single aluminum extrusion. Slots  184  are provided in the first and second end plates  130 ,  132  to hold the first and second side cover plates  134 ,  136  in place (see FIG. 6.) Bosses  228 ,  230  on the lower edges of the first and second side cover plates  134 ,  136  interlock with the slots  184  in the first and second end plates  130 ,  132 . Grooves  220 ,  222  may be provided on each side of the base  150  of the guide chassis  104  for alignment of the first and second side cover plates  134 ,  136  when the linear actuator  100  is assembled.  
         [0040]    It can be readily seen from FIG. 8 that the hollow configuration of the guide chassis  104  substantially encloses the lead screw  124  and the drive nut  116 . This protects these components from dirt, debris and contamination. The first and second side cover plates  134 ,  136  along with the first and second upper flanges  172 ,  174  of the guide chassis  104  provide additional protection by effectively creating a labyrinth seal with a long path length that prevents dirt, debris and contamination from entering the central passage  160  of the guide chassis  104 . In addition, the guide rails  106 ,  108  and the bearing subassemblies  112 ,  114  are protected from dirt, debris and contamination. This advantage is important for operating the linear actuator  100  in dirty environments where dirt, debris and contamination could damage an unprotected lead screw linear actuator. Furthermore, the configuration of the linear actuator  100  also prevents oil, grease or debris originating from within the linear actuator  100  from escaping and contaminating the environment.  
         [0041]    This advantage is important for operating the linear actuator  100  in clean environments where contamination from within the linear actuator  100  would be undesirable.  
         [0042]    In an alternate embodiment shown in FIG. 9, the linear actuator  100  may be made with side covers  134 ′,  136 ′ that are integral to the guide chassis  104 .  
         [0043]    [0043]FIG. 10 is a perspective view of a recirculating-ball linear bearing  240  used in the linear actuator of the present invention. FIG. 11 is an end view of the recirculating-ball linear bearing  240  and FIG. 12 is an exploded view of the recirculating-ball linear bearing  240 . The recirculating-ball linear bearing  240  utilizes a plurality of ball bearings  242  enclosed within a ball retainer  244 . The ball retainer  244  is preferably injection molded of plastic with an inner portion  246  and an outer portion  248 . A tongue  282  protruding from each end of the outer portion  248  forms a nesting joint with a corresponding slot  284  on each end of the inner portion  246  to hold the ball retainer  244  together. The inner portion  246  of the ball retainer  244  has two elongated oval tracks  252  molded within it. In one preferred embodiment, there are 19 ball bearings  242  enclosed within each track of the  252  ball retainer  244 . On one side of each elongated oval track  252 , an open slot  254  allows the ball bearings  242  to protrude from the ball retainer  244  and contact the rail guide  106  or  108 . The opposite side  256  of each elongated oval track  252  is closed to prevent the ball bearings  242  from contacting any bearing surfaces as they recirculate to the open slot  254 . Openings  258  in the outer portion  248  of the ball retainer  244  are configured to receive load bearing plates  260 . The load bearing plates  260  are preferably made of stainless steel or another hard material. Ridges  262  molded along the inside edges of the openings  258  engage grooves  264  in the load bearing plates  260  and retain them in place.  
         [0044]    The inner portion  246  of the ball retainer  244  is molded with a concave inner surface  268  with a radius of curvature slightly larger than the radius of curvature of the guide rails  106 ,  108  to provide a small amount of radial clearance. A pair of wipers  270  having a radius of curvature approximately the same as the radius of curvature of the guide rails  106 ,  108  that protrude from the inner surface  268  are molded integrally with the inner portion  246  of the ball retainer  244  and serve to capture lubricating grease within the linear bearing  240  and prevent dirt and debris from entering the linear bearing  240 .  
         [0045]    The outer portion  248  of the ball retainer  244  is molded with a convex outer surface  272  with a radius of curvature approximately the same as the radius of curvature of the internal semicircular channels  204 ,  206  in the carriage  110 . Flexible protrusions  274 ,  276  protrude from the outer surface  272  to retain the recirculating-ball linear bearing  240  in the internal semicircular channels  204 ,  206  in the carriage  110 .  
         [0046]    As described above in connection with FIG. 7, the preferred embodiment of the linear actuator  100  utilizes two bearing assemblies  112 ,  114  to support the carriage  110  on the parallel guide rails  106 ,  108 . Each bearing assembly  112 ,  114  utilizes two recirculating-ball linear bearings  240  with two tracks  252  containing ball bearings  242  in each one. FIG. 13 shows a bearing assembly  112  with the two recirculating-ball linear bearings  240  installed within the first internal semicircular channel  204  in the first depending vertical leg  192  of the carriage  110 . The bearing assembly  112  encompasses slightly less than half of the guide rod  106 . The ball bearings  242  protruding through the open slots  254  in the ball retainers  244  bear against the surface of the guide rail  106 . The load bearing plates  260  transfer the force from the ball bearings  242  to the carriage  110 . The bearing assemblies  112 ,  114  allow the carriage  110  to move along the guide rails  106  with relatively little resistance, but the bearing assemblies  112 ,  114  effectively resist vertical and lateral forces on the carriage  110 .  
         [0047]    While the present invention has been described herein with respect to the exemplary embodiments and the best mode for practicing the invention, it will be apparent to one of ordinary skill in the art that many modifications, improvements and subcombinations of the various embodiments, adaptations and variations can be made to the invention without departing from the spirit and scope thereof.