Surface mounted vehicle lift

Details Surface mounted vehicle lift Surface mounted vehicle lift

2000-11-20

2002-09-03

Watson, Robert C. (Department: 3723)

Implements or apparatus for applying pushing or pulling force

Multiple lifters

Track-straddling platform

C254S01000C, C254S08900R

Reexamination Certificate

active

06443429

ABSTRACT:

FIELD OF INVENTION
This invention relates to load elevating machines and more particularly to load elevating machines employing pivoting leg elements and hydraulic cylinders as the elevating means.
BACKGROUND OF THE INVENTION
The prior art teaches load elevating machines which use parallel longitudinal platforms fitted with hydraulic cylinder powered articulating legs whereby a vehicle or load may be positioned on the platforms of the lift and then be elevated to a desired level above the supporting floor. When two or more platforms are used, one platform, two or more lift legs, and a base element act as a kinematic assembly to elevate and support one side of a load, and a second assembly supports an opposed side of the load. The prior art is generally directed to machines which exploit a fundamental design feature of parallelogram linkages, which are also known as classic four bar mechanisms. This fundamental design feature results in an elevating motion of upper horizontal links (the load bearing platforms) which necessarily comprises concurrent vertical and horizontal translation of the upper horizontal links. The loads elevated may induce significant tensile or compressive loads in both upper and lower horizontal links, the platforms and bases, respectively. The bases may be discrete structural members, or alternatively, employ a substrate supporting the machine as the required base structural element by suitable mechanical attachment of the lift legs to the supporting substrate.
For conditions of non-uniform platform loading at or below the uniformly distributed load rated lifting capacity of a given platform, parallelogram machines necessarily develop and transmit longitudinal forces in the platform and the base structures. This characteristic is the underlying reason that parallelogram lifts may sometimes lift a runway-rated-load independent of the distribution of the load along the platform length. Dependent on the direction of rotation of the lifting legs with respect to the location of the center of gravity location, and also dependent upon the magnitude of the non-uniform load with respect to the lifting leg platform pivot points, the longitudinal stress resulting from this effect is tensile in one structure and compressive in the other. Parallelogram machines develop maximum cylinder pressure, and thus maximum cylinder rod force, at the beginning of elevation from the lowered position.
Typically, the lift leg hydraulic cylinders of parallelogram lifts operate at a mechanical disadvantage of about ten to one with respect to the vertical loading as a result of the height restrictions of the lowered platform.
For example, a particular platform lift leg pivot point of a two leg platform assembly that is loaded to a design maximum platform capacity of, for example, 20,000 pounds, and that the other platform lift leg pivot point is not loaded, the loaded arm cylinder can supply 10,000 pounds force of the design lift effort (50% of platform lifting capacity). However, to do so, the loaded arm cylinder must develop a cylinder force of ten times the platform lifting force. In this example, the cylinder rod force required is thus 100,000 pounds. Furthermore, the cylinders of the platform are hydraulically connected in parallel. Therefore, the unloaded cylinder must be at the same pressure as the loaded cylinder, and accordingly will also generate a rod force of 100,000 pounds. At the same mechanical disadvantage, this rod force would result in an elevating force of 10,000 pounds, and the 20,000 total platform load capacity would be provided, but the lifting force of the unloaded leg must be transmitted to the loaded leg.
When rod forces are applied, a reacting force to the unloaded leg cylinder force must be transmitted along the platform structure to the loaded leg to provide an additional loaded leg rotation couple. By first principles of statics and dynamics, this couple requires in an equal but opposite-in-direction force in the base structure to maintain the necessary force equilibrium. For the lift leg geometry described, the platform and base structure longitudinal forces are essentially the cylinder-rod-generated force of 100,000 pounds. Thus a two lifting leg platform assembly rated at 20,000 pounds design capacity can have non-intuitive longitudinal forces of 100,000 pounds, or five times the design lifting capacity of the structure.
A physical manifestation of the possible large hidden forces inherent in parallelogram lifts for non-uniform platform loadings is de-synchronization of one or both of the platform lifting legs. If the two parallel platforms of a parallelogram lift are also laterally non-uniformly loaded, then the lift leg pair(s) without control system regulation will contribute additional lateral inclination of the platform support plane. Installation of additional lift leg lateral pair synchronization controls would appear to correct this undesirable development, but to do so essentially reduces the capacity of the entire parallelogram lift to the capacity of a single lifting leg. This limitation occurs because the most loaded lift leg then establishes the maximum differential height of any other lifting leg, and the surplus lift capacity of other legs cannot be transferred to the over loaded leg. There are additional significant parallelogram lift design factors, including mechanical instability, hydraulic instability and platform loading transfers due to the center of gravity migration of a canted three dimensional load which were excluded in this discussion. These factors in general contribute further adverse effects, and do not beneficially alter the characterization of parallelogram lifts.
U.S. Pat. No. 4,848,732 to Rossato teaches a medium range capacity parallelogram machine with runway mounted cylinders and cylinder engaging latches and a passive control system. A laterally adjacent lift leg pair is connected by a torque tube between a pair of parallel base members. A predetermined torsional deflection of the torque tube actuates a switch which immobilizes the lift. Manual control procedures are employed to restore acceptable lateral synchronization of the lift legs and to reactivate the lift.
U.S. Pat. No. 5,040,637 to Hawk teaches a low range capacity parallelogram machine with a single cylinder and a cylinder independent latching load path with automatic latching at ascent. Unlatching is accomplished by a deliberate small elevation of the lift platform prior to lowering.
U.S. Pat. No. 5,050,844 to Hawk teaches a medium to high range capacity parallelogram machine with base mounted cylinders and an active, interlocked and control panel enunciated automatic control system. Lifting leg angular position synchronization is accomplished by detection of lifting leg differential angular position, processing of the detected error with a logic algorithm and resultant modulation of flow to the hydraulic cylinders of the lifting legs to correct the detected error.
U.S. Pat. Nos. 5,096,159; 5,190,122; and 5,199,686 to Fletcher, teach a medium to high range capacity parallelogram machine with runway mounted cylinders and an active automatic control system. Lift leg angular position synchronization is obtained by detection of differential lifting leg cylinder extension position, with logic processing and modulation of flow to the hydraulic cylinders of the lifting legs to correct the detected error. The lift equipment installation supporting substrate is employed as the necessary base element by structural attachment of each lifting leg lower pivot plate into that substrate.
Parallelogram machines have inherent limitations that become intractable for specified non-uniformly distributed loads with arbitrary and non-symmetrical runway loading point locations. Thus, there is a need for a surface mounted vehicle lift which overcomes the inherent limitations of known parallelogram machines.
SUMMARY OF THE INVENTION
The present invention is directed to hydraulic cylinder powered lift leg assemblies used for elevating and lowering of parallel runway members wh

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