Vehicle transporter with screw actuators

1. A vehicle transporter comprising:

(a) a vehicular frame;

(b) a vehicle support member movable relative to said vehicular frame; and

(c) a screw actuator comprising:

(i) an elongate tubular first member connected to one of said vehicular frame and said vehicle support member;

(ii) an elongate tubular second member connected to the other of said vehicular frame and said vehicle support member and slidable axially in an interior of said first member to vary a length of a portion of said second member coextensive with said first member;

(iii) a screw rotatable in an interior of said second member to force said tubular members to slide relative to each other; and

(iv) a clutch resisting rotation of said screw until a minimum torque is exerted on said screw.

2. The vehicle transporter of claim 1 wherein said minimum torque is a torque greater than a torque required to rotate said screw when no force resists relative movement of said first and said second members.

3. The vehicle transporter of claim 1 wherein said clutch comprises:

(a) a friction element engageable with a surface movable with rotation of said screw, said friction element movable substantially normal to said surface; and

(b) a loading element to variably force said friction element against said surface.

4. The vehicle transporter of claim 1 wherein said clutch comprises:

(a) a bolt in threaded engagement with said first member and projecting substantially radial to said screw; and

(b) a friction element including a surface slidably engageable with a surface movable by rotation of said screw and substantially constrained axially and circumferentially relative to said screw by said bolt but movable substantially radially relative to said screw by rotation of said bolt.

5. The vehicle transporter of claim 1 wherein said screw actuator further comprises a motor including a motor frame affixed to said first member and a motor shaft rotatable in said motor frame and drivingly connected to rotate said screw.

6. The vehicle transporter of claim 5 wherein said clutch comprises:

(a) a housing secured between said motor frame and said first member;

(b) a bolt in threaded engagement with said housing and projecting substantially radial to said screw; and

(c) a friction element including a friction surface slidably engageable with another surface movable by rotation of said screw and substantially constrained in a direction parallel to said another surface by said bolt but movable substantially normal to said another surface by rotation of said bolt.

7. The vehicle transporter of claim 1 wherein said screw comprises a screw thread that is self-locking to prevent forces tending to slide said tubular members relative to each other from rotating said screw.

8. The vehicle transporter of claim 1 wherein said screw comprises an Acme screw thread having a lead angle not exceeding five degrees.

9. The vehicle transporter of claim 1 wherein said connections of said screw actuator to said vehicular frame and to said vehicle support member enable said screw actuator to pivot relative to at least one of said vehicular frame and said vehicle support member.

10. The vehicle transporter of claim 9 wherein said screw actuator further comprises a motor including a frame connected to said first member enabling said motor and said first member to pivot in unison and a motor shaft rotatable in said frame and drivingly connected to rotate said screw.

11. The vehicle transporter of claim 10 wherein said motor shaft is rotatable on a common axis with said screw.

12. A screw actuator comprising:

(a) an elongate tubular first member;

(b) an elongate tubular second member slidable axially in an interior of said first member to vary a length of a portion of said second member coextensive with said first member;

(c) a screw rotatable in an interior of said second member to force said members to slide relative to each other; and

(d) a clutch resisting rotation of said screw until a minimum torque is exerted on said screw.

13. The screw actuator of claim 12 wherein said minimum torque is a torque greater than a torque required to rotate said screw when no force resists relative movement of said first and said second members.

14. The screw actuator of claim 12 wherein said clutch comprises:

(a) a friction element engageable with a surface movable with rotation of said screw, said friction element movable substantially normal to said surface; and

(b) a loading element to variably force said friction element against said surface.

15. The screw actuator of claim 12 wherein said clutch comprises:

(a) a bolt in threaded engagement with said first member and projecting substantially radial to said screw; and

(b) a friction element including a surface slidably engageable with a surface movable by rotation of said screw and substantially constrained axially and circumferentially relative to said screw by said bolt but movable substantially radially relative to said screw by rotation of said bolt.

16. The screw actuator of claim 12 wherein said screw actuator further comprises a motor including a motor frame affixed to said first member and a motor shaft rotatable in said motor frame and drivingly connected to rotate said screw.

17. The screw actuator of claim 16 wherein said clutch comprises:

(a) a housing secured between said motor frame and said first member;

(b) a bolt in threaded engagement with said housing and projecting substantially radial to said screw; and

(c) a friction element including a friction surface slidably engageable with another surface movable by rotation of said screw and substantially constrained in a direction parallel to said another surface by said bolt but movable substantially normal relative to said another surface by rotation of said bolt.

18. The vehicle transporter of claim 16 wherein said motor shaft is rotatable on a common axis with said screw.

19. The screw actuator of claim 12 wherein said screw comprises a screw thread that is self-locking to prevent forces tending to slide said members relative to each other from rotating said screw.

20. The screw actuator of claim 12 wherein said screw comprises an Acme screw thread having a lead angle not exceeding five degrees.