Tension driven scissor lift

1. A lift device, comprising:

a base;

a platform that is repositionable relative to the base between a fully raised position and a fully lowered position;

a scissor assembly coupling the base to the platform, the scissor assembly including:

a scissor layer including a first scissor arm pivotally coupled to a second scissor arm;

a pulley coupled to the scissor layer;

a tensile member wrapped around the pulley; and

an actuator configured to vary a working length of the tensile member such that the first scissor arm rotates relative to the second scissor arm to move the platform relative to the base; and

a booster positioned to engage a portion of the tensile member and apply an upward force on the scissor assembly when the platform is in the fully lowered position.

2. The lift device of claim 1 , wherein the booster does not engage the portion of the tensile member when the platform is in the fully raised position.

3. The lift device of claim 1 , wherein the booster does not apply the upward force on the scissor assembly when the platform is in the fully raised position.

4. The lift device of claim 1 , wherein the pulley is a first pulley, wherein the scissor assembly further includes a second pulley and a third pulley, wherein the portion of the tensile member extends directly from the second pulley to the third pulley, and wherein the booster engages the portion of the tensile member between the second pulley and the third pulley when the platform is in the fully lowered position.

5. The lift device of claim 1 , wherein the booster is coupled to the first scissor arm.

6. The lift device of claim 5 , wherein the tensile member extends below the booster such that the upward force of the booster lifts the first scissor arm away from the portion of the tensile member.

7. The lift device of claim 5 , wherein the pulley is a first pulley, wherein the scissor assembly further includes a second pulley coupled to the second scissor arm, and wherein the tensile member wraps around the second pulley.

8. The lift device of claim 7 , wherein the scissor layer is a first scissor layer, wherein the scissor assembly further includes a second scissor layer including a third scissor arm pivotally coupled to a fourth scissor arm, and wherein an upper end of the first scissor arm is pivotally coupled to a lower end of the fourth scissor arm about an end axis.

9. The lift device of claim 8 , wherein the pulley is coupled to the first scissor arm along the end axis such that the tensile member extends around the end axis.

10. The lift device of claim 9 , wherein the first scissor arm is configured to rotate relative to the second scissor arm about a middle axis that extends laterally, and wherein the second pulley is positioned along the middle axis such that the tensile member extends around the middle axis.

11. The lift device of claim 1 , wherein the pulley is a first pulley, wherein the scissor assembly further includes a second pulley coupled to the second scissor arm, wherein the tensile member wraps around the second pulley, and wherein the tensile member forms at least two loops around the first pulley and the second pulley.

12. The lift device of claim 1 , wherein the first scissor arm is configured to rotate relative to the second scissor arm about a middle axis, and wherein the pulley is coupled to the first scissor arm at the middle axis.

13. The lift device of claim 12 , wherein the pulley is a first pulley, wherein the scissor assembly further includes a second pulley coupled to the second scissor arm, wherein the first pulley is longitudinally offset from the second pulley, and wherein the actuator is configured to vary the working length of the tensile member to vary a longitudinal distance between the first pulley and the second pulley.

14. The lift device of claim 1 , wherein the tensile member is fixedly coupled to the second scissor arm.

15. The lift device of claim 1 , wherein the actuator includes a drum coupled to the tensile member and an electric motor coupled to the drum, and wherein the electric motor is configured to rotate the drum such that the tensile member wraps around the drum to vary the working length of the tensile member.

16. A lift device, comprising:

a base;

a platform that is repositionable relative to the base between a fully raised position and a fully lowered position;

a scissor assembly coupling the base to the platform, the scissor assembly including:

a scissor layer including a first scissor arm pivotally coupled to a second scissor arm;

a pulley coupled to the scissor layer;

a tensile member wrapped around the pulley; and

an actuator configured to vary a working length of the tensile member such that the first scissor arm rotates relative to the second scissor arm to move the platform relative to the base; and

a booster positioned to apply an upward force on the scissor assembly when the platform is in the fully lowered position, wherein the booster is configured such that a magnitude of the upward force decreases as the platform moves from the fully lowered position toward the fully raised position.

17. The lift device of claim 16 , wherein the booster does not apply the upward force on the scissor assembly when the platform is in the fully raised position.

18. The lift device of claim 16 , wherein the booster is positioned to engage at least one of (a) the first scissor arm or (b) a portion of the tensile member to apply the upward force.

19. The lift device of claim 16 , wherein the booster is coupled to the first scissor arm.