Multiple stage, multiple extend, speed reducing ball screw linear actuator and method of constructing and operating the actuator

Abstract

A multiple extend ball screw and nut linear actuator incorporates a revolvable axially restrained ball screw with a ball nut thereon. A constant speed reversible drive revolves the screw to move the ball nut forwardly and rearwardly. External helical threads of the same hand as the ball nut threads are provided externally on the ball nut but have a substantially reduced lead. A second rotatably restrained nut having threads matching the external threads on the ball nut mounts on the ball nut and connects to the system to be actuated.

We claim:  
     
         1 . A multiple stage, multiple extend, end of stroke speed reducing, ball screw and nut linear actuator comprising, in combination: 
 a. a ball screw having helical groove and land portions with a first nut mounted thereon having complemental helical groove portions defining a ball raceway system between the screw and first nut connected with recirculant ball passages;    b. said raceway system having a predetermined hand and lead, and incorporating load bearing balls;    c. a mount system for said screw restraining its axial movement while permitting its rotation;    d. a constant speed reversible drive connected to revolve said screw in one direction at a constant speed and in the reverse direction, and move said first nut forwardly and rearwardly respectively;    e. external helical groove and land portions of the same hand as said nut groove portion on said first nut but of a substantially reduced lead;    f. a second rotatably restrained nut having helical internal groove and land portions complementing said external groove and land portions on said first nut; and    g. yieldable members on said screw in the path of said first nut for halting travel of said first nut axially and causing coupling of said ball screw and first nut and rotation of said first nut at said constant speed to effect axial movement of said second nut in the same direction of travel at a significantly reduced speed.    
     
     
         2 . The actuator of    claim 1    wherein said stop members are provided at each end of said screw and comprise resilient energy absorbing members providing a speed deceleration zone of travel for said actuator.  
     
     
         3 . The actuator of    claim 2    wherein said stops comprise spring washers providing a resiliently resisted zone of travel for said first nut.  
     
     
         4 . The actuator of    claim 1    wherein said external groove portions on said first nut and said internal land groove portions on said second nut are acme threads.  
     
     
         5 . The actuator of    claim 1    wherein said land and groove portions on said first nut and second nut are of a pitch on the order of one quarter of the pitch of the land and groove portions of said ball screw.  
     
     
         6 . The actuator of    claim 1    wherein said groove and land portions on said second and first nuts mount adjustable friction increasing members.  
     
     
         7 . The actuator of    claim 1    wherein said friction increasing members comprise set screws.  
     
     
         8 . The actuator of    claim 1    wherein resilient stop members are provided opposite each end of said second nut in the path thereof.  
     
     
         9 . The actuator of    claim 1    wherein said reduced lead of the land and groove portions of said first and second nuts is at least half the lead of said land and groove portions of said ball screw.  
     
     
         10 . A two stage linear actuator comprising: 
 a. a first stage threaded elongate ball screw member with a ball nut member mounted thereon, there being first threads of a predetermined hand and lead provided to couple said members for relative translatory movement;    b. mechanism resiliently halting said travel of said members to provide a speed reduction zone of travel and then coupling said members for conjoint rotary travel; and    c. a second rotatably restrained threaded nut member mounted on said ball nut member for axial travel thereon, there being second threads between said nut members of the same hand as said first threads, but a significantly reduced lead relative to said first threads.    
     
     
         11 . A method of constructing a two stage actuator incorporating a first ball screw with a ball nut thereon, said ball screw and ball nut having threads of a first hand and lead providing a ball raceway; there being a second rotatably restrained nut threaded on said ball nut; comprising: 
 a. loading balls to said raceway;    b. providing mechanism revolvably mounting but axially restraining said ball screw;    c. providing resilient stops for said ball nut for first decelerating travel of said ball nut and them coupling said ball screw and ball nut for conjoint rotation; and    d. attaching a member to be actuated to said second nut for linear axial travel therewith.    
     
     
         12 . The method of    claim 11    comprising providing friction adjusting elements on said second nut to frictionally resist its travel on said ball nut and adjusting the position of said elements.  
     
     
         13 . A method of operating a two stage actuator comprising a ball screw with a ball nut threaded thereon to provide a helical raceway between them having load bearing balls therein, the threading having a hand and lead; and there being an actuator nut attached to the system to be operated threaded on said ball nut for axial travel thereon, the threading of said actuator nut being of the same hand but of a significantly reduced lead relative to said ball nut threading; comprising: 
 a. revolving said ball screw at a constant speed in one direction of rotation while restraining its axial travel to move said ball nut axially in a forwarding direction;    b. resiliently blocking axial travel of said ball nut after a predetermined movement thereof and decelerating its travel before coupling it to said screw for conjoint rotation therewith;    c. utilizing the rotation of said ball nut to drive said actuator nut axially forwardly in the same direction of axial travel at a significantly reduced speed.    
     
     
         14 . The method of    claim 13    wherein said actuator nut is moved at a speed reduced at least in half.  
     
     
         15 . The method of    claim 13    wherein steps a-c are followed in a reversing direction when said ball screw is rotated in the reverse direction.