Patent Publication Number: US-2015075306-A1

Title: Telescoping linear actuator with screw drives

Description:
FIELD OF THE INVENTION 
     This invention relates to telescoping linear actuators, and particularly one with three telescoping sections actuated by two internal screw shafts linearly translated in opposite directions by drive nuts rotated by an internal motor. 
     BACKGROUND OF THE INVENTION 
     A linear actuator is a device that extends along a straight line to provide mechanical force to operate a variable apparatus. Among other applications, a linear actuator can support any lift application that requires controlled vertical motion in a compact envelope, such as medical lifts, packaging applications, and material processing. For example, a vertical actuator may be provided in a hospital gurney to lift and lower the mattress plane with a patient thereon. Telescoping actuators have two or more nested sections that telescopically extend and retract under control of an actuating mechanism such as a hydraulic piston or motor-driven screw drive. One such actuator is described in U.S. Pat. No. 6,026,970. One measure of an actuator design is its extended-to-retracted length ratio. Higher ratios are better for space efficiency. Other measures include energy efficiency, cost, noise, reliability, and safety, including prevention of unintended retraction or collapse of the loaded actuator during a power failure. However, it is difficult to maximize all of these measures concurrently in a single design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a perspective front view of a telescopic linear actuator in accordance with an embodiment of the invention and shown in an extended position. 
         FIG. 2  is a sectional view of the nested telescoping sections of  FIG. 1 . 
         FIG. 3  is a perspective rear view of the actuating mechanism of the telescopic linear actuator of  FIG. 1  shown in a retracted position. 
         FIG. 4  is a top view of a mechanical brake in an embodiment of the invention. 
         FIG. 5  is a perspective rear view of the brake embodiment of  FIG. 4 . 
         FIG. 6  is a circuit diagram of a dynamic brake embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a telescopic linear actuator  20  according to an embodiment of the invention, with three nested telescoping sections -- an outer section  22 , an intermediate section  24 , and an inner  26  section. The sections are actuated to telescopically extend and retract via two internal screw drives  28 ,  30  comprising two screw drive nuts  32 ,  34  mounted in thrust bearings in the intermediate telescoping section  24 , and respective screw shafts  36 ,  38 . The screw drives  28 ,  30  may be embodied as ball screw assemblies, in which ball bearings run in helical races between the drive nut  32 ,  34  and the respective screw shaft  36 ,  38 , minimizing torque for a given axial force. The drive nuts  32 ,  34  are driven via an idler shaft  42  that spans between them and is driven by a reversible motor  40  mounted in the intermediate section  24 , which is powered via power supply leads  41 A,  41 B. The motor  40  may include reduction gears. The top plate  27  of the outer section  22  may be attached to a hospital gurney or other weight to be lifted. 
       FIG. 2  shows a sectional view of the nested sections  22 ,  24 ,  26 , which may be slidably spaced using polymer bearing pads  52 . 
       FIG. 3  shows the actuating mechanisms of  FIG. 1  in a retracted position. A housing  33  may be provided within the intermediate section  24  to support the actuator mechanisms. The housing may include a housing base  23  attached to the bottom end of the intermediate section  24 , an upper support plate  25  at the top end of the intermediate section  24 , and support rods  29  that support the upper support plate  25  from the housing base  23 . The housing may not touch the intermediate section  24  except at the bottom end thereof, so that the upper telescoping section  26  can slide down over the housing and within the intermediate section  24  for retraction of the actuator  20 . Limit switches and/or other travel position feedback devices  31  may be provided to sense the relative positions of the telescoping sections and/or to halt the motor at the limits of travel. For example, a small cluster gear providing a reduction ratio such as 12:1 may be used to sense movement of the actuating mechanism and to drive a potentiometer which provides an analog signal indicative of position over the entire stroke length. 
     The lower screw drive nut  32  may be rotatably mounted on the housing base  23 . The lower or distal end of the lower screw shaft  36  may be attached non-rotatably to the bottom or distal end of the outer telescoping section  22  via a push plate  37  attached to a base plate  21  of the intermediate section  22  or by other means. The lower screw shaft  36  passes through the lower drive nut  32 , and is linearly translated by rotation of the lower drive nut  32 . 
     The upper screw drive nut  34  may be rotatably mounted on the upper support plate  25  at the top end of the intermediate section  24 . The upper or distal end of the upper screw shaft  38  may be attached non-rotatably to the top or distal end of the inner telescoping section  26  via a push plate  39  attached to a top plate  27  of the intermediate section or by other means. The upper screw shaft  38  passes through the upper drive nut  34 , and is linearly translated by rotation of the upper drive nut  34 . The push plates  37 ,  39  transfer and distribute forces between the screw shafts  36 ,  38  and the respective base plate  21  and top plate  27 . 
     One of the screw drives  28 ,  30  may be left-handed while the other one is right-handed, so that turning both drive nuts  32 ,  34  in the same direction translates the respective screw shafts  36 ,  38  in opposite directions  44 ,  46 . This forces the outer telescoping section  22  and the inner section  26  in opposite directions relative to the intermediate section  24 , extending the actuator  20 . Because the two drive nuts  32 ,  34  turn in the same direction, they can each be driven by a simple pulley/belt drive  48 ,  50  at opposite ends of the idler shaft  42  as shown, rather than by gears. Belt drives can be quiet, accurate, and reliable. Some automotive timing belts are designed to last 100,000 miles. Alternately however, other transmission means such as gears or sprocket-and-chain drives may be used. 
     The idler shaft  42  may be mounted rotatably in the housing  33  in the intermediate section  24 , and extends between the two drive nuts  32 ,  34 . The motor  40  drives the idler shaft  42  via a belt drive  35  or other means. The Idler shaft in turn drives the drive nuts  32 ,  34 . Rotating the idler shaft  42  in a first direction translates two screw shafts  36 ,  38  in opposite directions relative to the intermediate section  24 , extending the outer section  22  and the inner section  26  in opposite directions relative to the intermediate section  24 . Rotating the idler shaft  42  in the opposite direction retracts the inner  22  and outer  26  sections. A mechanical brake  43  may be provided as later described. The two screw shafts  36 ,  38  may both have the same diameter and length, thus having the same maximum force capacity and drive parameters except for handedness. This reduces engineering complexity and maximizes space efficiency. 
       FIG. 4  shows an embodiment of a mechanical brake comprising a brake drum  54  with a cylindrical surface  56  mounted to the idler shaft  42  for co-rotation therewith. The brake drum may be located on an intermediate position of the idler shaft  42  as shown in  FIGS. 1 and 3 , or on an end of the idler shaft  42 , such as the top end above the upper support plate  25 , as shown in  FIGS. 4 and 5 . A brake spring  43  may be wrapped circumferentially around the cylindrical surface  56  of the brake drum  54 . The brake spring  43  has a first end  43 A that is fixed relative to the housing. For example, it may be fixed to the upper support plate  25 . The spring  43  is wrapped around the cylindrical surface  56  of the brake drum  54  and has a second end  43 B that is free to move with the cylindrical surface  56  of the brake drum  54 . When the idler shaft  42  rotates in a direction  60  to extend the actuator  20 , the cylindrical surface of the brake drum  54  rotates circumferentially away from the second end  43 B of the brake spring  43  toward the first end  43 A thereof, causing the second end  43 B to move toward first end  43 A, thereby loosening the brake spring  43  on the brake drum  54  so that it provides only slight drag and free rotation is achieved. Once motion has ceased, the spring diameter collapses around the brake drum  54  and provides friction which maintains the position of the idler shaft  42 . When the lifted weight causes the idler shaft  42  to begin rotate to retract the actuator  20 , the cylindrical surface of the brake drum  54  rotates the second end  43 B of the brake spring  43  away from the first end  43 A, thereby tightening the brake spring  43  on the brake drum  54  and causing the spring  43  to grab the cylindrical surface  56  of the brake drum  54 , locking its rotation and preventing retraction or collapse. This braking occurs for example when power to the motor  40  is switched off after an extension operation, and the weight being lifted by the actuator  20  tries to collapse it. 
     Because the brake spring  43  operates to resist the collapse of the actuator  20  under the influence of gravity on the hospital gurney or other weight being lifted, it is necessary for the drive motor  40  to overcome the braking effect of the brake spring  43  when retraction of the actuator  20  is desired. Optionally, an electrically-operated brake release linkage  62  may be used in some embodiments to pull the second end  43 B of the brake spring  43  away from the circumferential surface  56  of the brake drum  54  to release the spring  43  from the brake drum  54  when retraction of the actuator  20  is desired. This reduces the load on the motor  40  during downward movement. A solenoid  64  may operate the linkage  62  to release the brake spring  43  whenever the motor  40  is powered to retract the actuator  20 . The brake  43  may default to the engaged (non-released) condition when the solenoid is inactive during a power failure, thus preventing collapse of the actuator during a power failure. 
       FIG. 6  shows a circuit  70  for an electromagnetic brake embodiment of the invention. When the motor  40  is inactive, its leads  40 A,  40 B are disconnected from the power supply leads  41 A,  41 B and are connected instead by default to a resistor R 1  (“NC” means normally closed). The motor  40  then becomes a regenerative resistance brake that opposes turning of the idler shaft  42 . When the motor is activated, a relay  72  disconnects the resistor circuit  71 , and connects the power supply  41 A,  41 B to the motor  40 . A fly-back diode  74  may be provided in the relay circuit to damp voltage spikes therein. In the circuit state shown, the motor  40  resists rotation of the idler shaft  42 , thus braking collapse of the actuator. Both a mechanical brake  43  and an electromagnetic brake  70  may be provided so that when the motor is de-energized after a lifting operation, it immediately begins its dynamic braking function, and once motion has stopped, the spring brake immediately engages. Both types of brakes may engage by default during power failure, and the electromagnetic brake may contribute up to 35% of the overall braking capacity in some embodiments. 
     A linear actuator based on an embodiment of the present invention may have an extension to retraction ratio such as 2.5:1 or more, due to the space efficiency of the drive mechanisms. The two ball screw assemblies occupy and same plane in space and are driven in the same direction, yet extend in opposite directions, allowing the actuator to achieve a low retraction height. There is no requirement for a transmission to produce counter rotating shafts since the opposite hand configuration eliminates this need. The two-stage, belt drive, ball bearing supported transmission configuration supports quiet uniform motion by eliminating a requirement foe meshed gears. Among other applications, the actuator can support any lift application that requires controlled vertical motion in a compact envelope, such as medical lifts, packaging applications, and material processing. For example, a single vertical actuator may be provided in a hospital gurney to lift and lower the mattress plane with a patient thereon. In such application, the dynamic axial force capacity of the unit may be for example about 4400 N or about 1000 lbs, and the static axial support capacity may be for example about 5400 N or about 1200 lbs. 
     An advantage of rotating the drive nuts  32 ,  34  instead of rotating the screw shafts  36 ,  38  is a reduction in the number of bearings. A rotatable screw shaft requires two bearings per shaft—one at each end—while a rotatable nut requires only one bearing. The present invention provides a mechanism for a telescoping linear actuator that maximizes the extended-to-retracted ratio, payload capacity, energy efficiency, reliability, and safety, while minimizing cost and noise. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.