Tow bar lift assist system and method
A lift assist system for a tow bar. The system exerts a force exerted on the tow bar to generate an upward moment that counters a downward moment generated by the weight of the tow bar assembly. The greatest upward moments are generated from a lowered configuration through at least a horizontal configuration of the tow bar, thereby providing the greatest degree of lift assistance for the operator when the downward moments are the greatest. A stop assembly may be included that limits how far the tow bar can arc below the equilibrium height in normal operation to prevent the tow bar from reaching the ground during a free fall, thus providing a measure of safety against injury to operating personnel. The lift assist system may depend from flexible suspension members to protect components from excessive stresses in the event of a forced grounding of the tow bar.
1 . A towed vehicle with tow bar lift assist system, comprising:
a chassis supported by a plurality of tire assemblies, wherein lower extremities of said tire assemblies define a ground plane;
a pivot pin coupled to said chassis and defining a pivot axis;
a tow bar assembly coupled about said pivot pin for rotation about said pivot axis, said tow bar assembly including a tow bar that defines a tow bar axis that is perpendicular to said pivot axis and extends through a free end of said tow bar, said tow bar assembly including a linkage arm that extends radially outward from said pivot axis; and
a lift assist system, including
a pivot connector directly connected to said linkage arm of said tow bar assembly,
an actuation shaft coupled to said pivot connector and extending beneath or through said chassis, and
an elastic support that extends beneath or through said chassis and is coupled to said actuation shaft,
wherein:
a reference plane is defined as being coplanar with said pivot axis and parallel to said ground plane;
said tow bar defines a lowered configuration wherein said free end of said tow bar is below said reference plane;
said tow bar defines a horizontal configuration wherein said tow bar axis is coplanar with said reference plane;
said elastic support exerts an assistance force on said actuation shaft to generate an upward moment of said tow bar assembly about said pivot axis, said upward moment A suspending said free end of said tow bar at an equilibrium height above said ground plane in said lowered configuration, said tow bar axis defining an equilibrium angle relative to said reference plane at said lowered configuration; and
said upward moment is generated from said lowered configuration through said horizontal configuration.
2 . The towed vehicle of claim 1 , comprising a support bracket to which said elastic support is coupled for generation of said assistance force.
3 . The towed vehicle of claim 2 , wherein:
a load assembly includes said pivot connector, said elastic support, and said support bracket;
an actuation length of said load assembly is defined from a pivot axis of said pivot connector to a rearward registration surface defined by said support bracket;
said actuation length defines an equilibrium length when said tow bar assembly is in said lowered configuration; and
means for changing said equilibrium length.
4 . The towed vehicle of claim 3 , comprising means for shifting said equilibrium length along said actuation axis.
5 . The towed vehicle of claim 2 , wherein said support bracket coupled to and beneath or within said chassis.
6 . The towed vehicle of claim 1 , wherein:
an elevation angle is defined between said tow bar axis and said reference plane;
an equilibrium angle is defined as said elevation angle in said lowered configuration;
an elevation assistance range is defined as a range of said elevation angle wherein a ratio of an exertion force required for upward rotation of said tow bar assembly with said lift assist system to an exertion force require for upward rotation of said tow bar assembly without said lift assist system is less than 0.5; and
said elevation assistance range includes said lowered configuration and said horizontal configuration.
7 . The towed vehicle of claim 1 , comprising:
a stop assembly coupled to said actuation shaft; and
a stop bracket positioned for engagement of said stop assembly when said tow bar assembly is at an elevation angle that is below said equilibrium angle, said stop bracket being suspended from at least one flexible suspension member connected to said chassis,
wherein:
a load assembly includes said pivot connector, said elastic support, and said support bracket;
when said stop assembly engages said stop bracket and deflects said at least one flexible suspension member, an axial deflection of said at least one flexible suspension member is greater than an axial deflection of said load assembly.
8 . The towed vehicle of claim 1 , wherein application of a downward force of less than 30 pounds force on said tow bar assembly lowers the elevation angle below said equilibrium angle from 10 degrees to 20 degrees inclusive.