Abstract:
A surface supported vehicle lift includes at least one independent elevating runway member which accepts loads of varying weights. The runway member accepts uniform or non-uniform loads imposed from a first end to a second end. The runway member includes at least two lift leg assemblies. The lift leg assemblies include hydraulic cylinders and a pump having an electric motor to drive the hydraulic cylinders. Each lift leg assembly also includes pulsed or proportional solenoid valves and position sensors affixed thereto. Synchronizer pumps are provided to equalize flow to and from each lift leg assembly for the runway member, and each lift leg assembly includes pulsed or proportional flow divider bias thereby maintaining lateral and longitudinal synchronization between each lift leg assembly. The surface mounted vehicle lift need not be fixedly attached to a supporting surface, and in addition, may be in moveable contact with the supporting surface. As desired, a guiding means for provision of a specified elevation path may be provided.

Description:
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 when two or more runway members are used. However, the present invention is not a parallelogram machine in that no structural base element is employed. This fundamental kinematic difference results in a machine that is different in kind, as opposed to degree, to a parallelogram (four bar mechanism) machine. 
     An object of the present invention is to provide a surface supported vehicle lift which includes load elevation and lowering in a desirably improved and more adaptable manner while maintaining predictable capacity to specified non-uniform and non-symmetrical loading. 
     Another object of the present invention is to provide a surface supported vehicle lift including at least one runway member which is fitted with lateral platform axis pivoted lift leg assemblies. 
     A further object of the present invention is to provide a surface supported vehicle lift including two lateral runway members, each lateral runway member having an axis pivoted lift leg assembly selectively power controlled to position the runway member in a preselected manner. 
     Even another object of the present invention is to provide a surface supported vehicle lift including at least two lateral runway members, each runway member having a pivoted lift leg assembly instrumented to permit determination of relative or absolute position of the runway member lateral pivot axis in relation to fixed or variable selected reference data. 
     Also, a further object of the present invention is to provide a surface supported vehicle lift wherein the opposite end of a lift leg assembly pivots from a runway member and bears on a supporting surface. 
     Even a further object of the present invention is to provide a surface supported vehicle lift including a surface bearing leg end being alternatively configured to: 
     (a) rock on a supporting surface in a manner determined by the effective radii of a contacting leg end and an extending position of the lift leg assembly; or, 
     (b) rotate about an axis established by a substrate supported bearing; 
     (c) roll on a substrate; or, 
     (d) any or all of the above on a pad provided to control the unit load on a supporting substrate. 
     More particularly, a surface mounted vehicle lift includes a plurality of runway members which may be selectively elevated and lowered relative to the supporting surface by lift leg assemblies through suitable controls on an operating console. 
     The supporting surface includes a ramp portion disposed at one end of a runway member, or, alternatively, at both ends for a drive-through arrangement. Each runway member has an interior cavity adapted to receive lift leg assemblies for elevating the runway member relative to the supporting surface. The lift leg assemblies are pivotably received in interior cavities of the runway members and operate in tandem. 
     The upper end of each lift leg assembly comprises an upper transverse shaft which is pivotally connected at one end to the runway member. The other, lower end of each lift leg assembly rests and rocks upon the supporting surface. Preferably, each lift leg assembly is defined by first, second and third parallel members that are maintained in a spaced relation by an upper and lower transverse shaft. A rod end is secured to a transverse cylinder pivotal shaft of a fluid cylinder fitted to the lift leg assembly. The head end of the fluid cylinder is opposite the rod end, and is additionally pivotably secured to the runway member through a transverse head end pivotal shaft. Selective extension of the fluid cylinder rod moves the lift leg assembly outwardly from the platform member. Selective retraction of the fluid cylinder rod moves the lift leg assembly inwardly until disposed within the runway cavity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like numerals refer to like parts throughout the several views and wherein: 
     FIG. 1 is a perspective view of a surface mounted vehicle lift of the present invention in a first lifting position. 
     FIG. 2 a  is a side view of the present invention in a raised position. 
     FIG. 2 b  is a side view of the present invention in a lowered position. 
     FIG. 3 is an enlarged side view of area  3  of FIG. 2 a  with selected portions cut away to show relevant details. 
     FIG. 4 is an enlarged side view of area  4  of FIG. 2 b  with selected portions cut away to show relevant details. 
     FIG. 5 is a schematic view of a control and operating system for use with a surface mounted vehicle lift of the present invention. 
     FIG. 6 is a perspective view of the lift of FIG. 1 in a second lifting position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the Figs. is illustrated a surface mounted vehicle lift  10  which is adapted to support a load such as a large vehicle  200 . The surface mounted vehicle lift  10  includes at least one runway member  20 , and preferably two runway members  20  which may be selectively elevated and lowered relative to the supporting surface  30  by lift leg assemblies  40  through suitable controls on an operating console  100 . 
     FIGS. 1 and 2 a - 4  show that the supporting surface  30  includes ramp portions  32  disposed at one end of a runway member  20  or, alternatively, at both ends for a drive-through arrangement. Each runway member  20  has an interior cavity adapted to receive lift leg assemblies  40  for elevating the runway member  20  relative to the supporting surface  30 . 
     More particularly, the lift leg assemblies  40  are pivotably received in interior cavities of the runway members  20 . And, the lift leg assemblies  40  operate in tandem. For ease of illustration and understanding, the second lift leg assembly  40 ′ and runway member  20 ′ are usually identified with a primed suffix (′) to indicate that they are substantially identical to the first lift leg assembly  40  and the first runway member  20 . 
     Referring to FIGS. 1 and 3, the upper end of each lift leg assembly  40  comprises an upper transverse shaft  42  which is pivotally connected at one end to each runway member  20  and  20 ′. The other, lower, end of each lift leg assembly  40  rests and rocks upon the supporting surface  30 . Preferably, each lift leg assembly  40  is defined by first, second and third parallel members,  46 ,  48 , and  50 , respectively, that are maintained in a spaced relation by an upper and lower transverse shaft,  42  and  44 , respectively. 
     With particular reference to FIG. 3, a fluid cylinder rod  62  is secured to a transverse cylinder pivotal shaft  64  of a fluid cylinder  83 ,  85 ,  83 ′,  85 ′ fitted to the lift leg assembly  40 . The head end  66  of the fluid cylinders  83 ,  85 ,  83 ′,  85 ′, is opposite the rod end  62 , and is additionally pivotably secured to the runway members  20  and  20 ′ through a transverse head end pivotal shaft  68 . Selective extension of the fluid cylinder rod  62  moves the lift leg assembly  40  outwardly from the platform members  20  and  20 ′. Selective retraction of the fluid cylinder rod  62  moves the lift leg assembly  40  inwardly until disposed within the runway cavity. 
     Again, FIG. 3 shows the lift leg assembly  40  lower end transverse shaft  44  as it bears tangentially upon the supporting substrate surface  30 . 
     Referring back to FIG.  1  and to FIG. 6, the opposite end of a lift leg assembly  40  pivoting from the runway members  20  and  20 ′ bears on a supporting surface  30 . The surface  30  bearing end of the lift leg assembly  40  may be alternatively configured to: 
     (a) rock on a supporting surface  30  in a manner determined by the effective radii of the contacting leg end and the extending angular position of the lift leg assembly  40 ; 
     (b) roll on the supporting surface  30  by means of a freely rotating lower end transverse shaft  44  mounted roller  45  of the lift leg assembly  40  that is not dependent on the position of the lift leg assembly  40 ; or, 
     (c) rotate about an axis established by a substrate supported bearing (not shown). 
     If rolling contact by rollers  45  upon the supporting surface  30 , is employed for each lift leg assembly  40 , then longitudinal restraining means for the resulting relatively low and predictable force magnitudes may be applied to the runway members  20  and  20 ′ with elevation path guidance  47 , thereby achieving runway member  20  and  20 ′ with elevation and descent in motion that is purely vertical, or guided as otherwise desired. If lift leg assembly  40  with roller  45 ,  45 ′ substrate  30  contact is employed and elevation path guidance  47 ,  47 ′ is applied to the runway members  20  and  20 ′, then the lift leg assemblies  40 ,  40 ′ can be electively and independently disposed in the direction of articulation rotation as shown in FIG.  6 . 
     If rolling bearing contact upon the supporting surface  30  is employed, the rotational direction of a given lift leg assembly  40  deployment from a runway member  20  and  20 ′ may be the same as, or be in opposition to, any other given lift leg assembly  40  deployment rotational direction. 
     The prime mover of every lift leg assembly  40  member  46 ,  48 , and  50  of a lift assembly is sized to elevate the most adverse lift leg assembly  40  member  46 ,  48 , and  50  load for a selected nonuniform runway member  20  and  20 ′ load distribution or set. 
     A given lift leg assembly  40  may have a multiplicity of capacity ratings which correspond to every degree of nonuniform loading selected and may include uniform loading as one such selection. 
     The motion of elevation of a given runway member  20  and  20 ′ may be any arbitrary path within a runway member  20  and  20 ′ horizontal-vertical plane for which path control is provided, including pure vertical translation. 
     In the operation of the lift  10 , a closed loop control system  300  is employed wherein a logic algorithm compares all lifting system platform leg elevations to a desired position, and modulates power to the prime movers to correct runway members  20  and  20 ′ elevation. FIG. 5 shows, for example, a basic schematic diagram of a control system  300  for operation of a preferred embodiment of the present invention. 
     For an initial operational condition of system power applied and the runway members  20  and  20 ′ stationary in a lowered position, the nomenclature and state of the control system components is hydraulic fluid reservoir  70  full, pump suction filter  71  filled, electric motor  73  off, main pump  72  stationary, isolation check valve  75  closed, pressure relief valve  74  closed, descend valve  77  closed, descend flow controller  76  inactive, synchronizer pumps  78  stationary, solenoid valves  80 ,  82 ,  80 ′,  82 ′ closed, trim flow controllers  79 ,  81 ,  79 ′,  81 ′ inactive, lift leg assembly hydraulic actuator piston rods  62  of fluid cylinders  83 ,  85 ,  83 ′,  85 ′ retracted, runway member position sensors  84 ,  86 ,  84 ′,  86 ′ retracted and transmitting equal lowered runway members  20  and  20 ′ position signals, programmable logic controller, abbreviated “PLC”,  87  operating and comparing the signals of position sensors  86 ,  84 ′, and  86 ′ to  84 . The signals are processed and determined equal by the programmable logic controller (PLC)  87  and the outputs of the PLC  87  to the solenoid valves  80 ,  82 ,  80 ′,  82 ′ are off, and thus the solenoid valves  80 ,  82 ,  80 ′,  82 ′ are in the closed position. 
     Elevation of the runway members  20  and  20 ′ is begun by application of electrical power to the electric motor  73  and the resulting main pump  72  rotation will provide flow to the hydraulic system. For rated runway members  20  and  20 ′ loads, the hydraulic pressure developed is below the set point of the pressure relief valve  74  which remains closed, and check valve  75  opens. The descend valve  76  is closed and flow is provided to the hydraulically parallel ports of the synchronizer pumps  78 . The synchronizer pumps  78  are essentially four equal positive displacement gear pumps connected by an internal common rotating shaft. The hydraulic fluid flow from the main pump  72  through the synchronizer pumps  78  is therefore divided equally in volume and independently in pressure, and metered to the four hydraulically independent ports of the synchronizer  78 . The solenoid valves  80 ,  82 ,  80 ′,  82 ′ are closed and thus equal hydraulic fluid flow volume is supplied to each of the lift leg assembly hydraulic cylinders  83 ,  85 ,  83 ′,  85 ′. The hydraulic fluid pressure in each cylinder  83 ,  85 ,  83 ′,  85 ′ increases until the hydraulic actuator piston rod  62  force developed by the cylinder slightly exceeds the load resistance acting on that cylinder, and runway members  20  and  20 ′ elevation results. The initial runway members  20  and  20 ′ elevating rate is determined by the volumetric output of the main pump  72 , and this initial rate will then slowly decrease with increasing runway members  20  and  20 ′ elevation as a result of the change in lift leg assembly  40 ,  40 ′ and cylinders  83 ,  85 ,  83 ′,  85 ′ geometry due to the relative rotations of the lifting lift leg assemblies  40 ,  40 ′ and cylinders  83 ,  85 ,  83 ′,  85 ′. 
     The volumetric fluid flow to each cylinder  83 ,  85 ,  83 ′,  85 ′ is maintained essentially equal over the short term by a division of the main pump  72  output into four equal volume flows by the synchronizer pumps  78 . 
     At this point, consideration must be given to the fact that a number of effects tend to desynchronize over the long term the position of the hydraulic actuator piston rods  62 . Accordingly, the heights of the runway members  20  and  20 ′ are desynchronized over the long term at the lift leg assemblies  40 ,  40 ′ upper transverse shafts  42 . Four common contributors to this desynchronization are: (1) dimensional variations (tolerance and wear) in the kinematic linkage; (2) differing internal leakage of the separate pumps within the synchronizer pumps  78  for various operating pressures; (3) the hydraulic fluid transport of small but differing volumes of air to the cylinders  83 ,  85 ,  83 ′,  85 ′; and, (4) lateral and longitudinal nonuniform runway member  20  and  20 ′ loadings resulting in some degree of deflection to the elastic structures. 
     The problem of desynchronization is solved by the present invention in that dynamic measurement and correction of relative runway members  20  and  20 ′ heights is accomplished by a runway synchronization trim subsystem. The runway synchronization trim subsystem is illustrated in FIG. 5 as consisting of the runway position sensors  84 ,  86 ,  84 ′,  86 ′, the PLC  87 , the solenoid valves  80 ,  82 ,  80 ′,  82 ′, and the trim flow controllers  79 ,  81 ,  79 ′,  81 ′. Two strategies of runway members  20  and  20 ′ height control are valid, the use of an absolute master lift leg assembly or the use of a relative master lift leg assembly, one being identified by the numeral  40  and the other by numeral  40 ′. For an absolute master control, a specific runway single lift leg assembly  40  or  40 ′ is defined as the master and all others as slaves. 
     The optimum selection of the control strategy as absolute or relative is dependent on the detail design parameters of the system. Key factors include: runway members  20  or  20 ′ length; number of lift leg assemblies  40 ,  40 ′ per runway member  20  or  20 ′; location of the lift leg assembly  40 ,  40 ′ on the runway member  20  or  20 ′; and ratio of maximum rated non-uniform load to maximum rated uniform load, which is selected to result in minimizing the bending stress in the lift leg assembly  40 ,  40 ′, or the runway members  20  and  20 ′, or both. For either control strategy selected, absolute or relative, the operational result is: The output of a master lift leg assembly  40 ,  40 ′ runway member  20  or  20 ′ position transducer  84 ,  84 ′,  86 ,  86 ′ is compared to the output of a slave lift leg assembly  40  runway member  20  or  20 ′ position transducer  84 ,  84 ′,  86 ,  86 ′ and for any runway member  20  or  20 ′ elevation difference greater than a small selected maximum (dead-band) found, the PLC  87  applies and maintains power to the output channel in the PLC  87  for the more elevated runway member  20  and  20 ′ lift leg assembly  40 ,  40 ′. 
     This repetitive comparison process results in the opening of one or more of the solenoid valves  80 ,  82 ,  80 ′,  82 ′ for each detected error. The trim flow controllers  79 ,  81 ,  79 ′,  81 ′ are each preset to a flow regulation rate of about 20% of the design flow to any given lift leg assembly  40  hydraulic cylinder  83 ,  85 ,  83 ′,  85 ′ from the flow division of the synchronizer pumps  78 . The opening of the solenoid valve  80 ,  82 ,  80 ′,  82 ′ of any more elevated lift leg assembly  40 ,  40 ′ thus results in a reduction of the flow rate to the hydraulic cylinder of that lift leg assembly  40 ,  40 ′ and the rate of elevation of that lift leg assembly  40  is reduced by about 20%, permitting the lower elevation lift leg assembly  40  to synchronize with the higher lift leg assembly  40  and thus the elevation error to be corrected, at which time the open solenoid valve  80 ,  82 ,  80 ′, or  82 ′ will be closed by PLC  87  removal of power from that lift leg assembly  40 &#39;s solenoid valve  80 ,  82 ,  80 ′, or  82 ′ and the resultant closing of the open solenoid valve  80 ,  82 ,  80 ′, or  82 ′. 
     For an initial operational condition of system power applied and the runway members  20  and  20 ′ stationary in an elevated position, the state of the components are: Hydraulic fluid reservoir  70  level reduced below full by the fluid displaced to the cylinder extensions, pump suction filter  71  full, electric motor  73  off, main pump  72  stationary, pressure relief valve  74  closed, isolation check valve  75  closed, descend valve  77  closed, descend flow controller  76  inactive, synchronizer pumps  78  stationary, solenoid valves  80 ,  82 ,  80 ′,  82 ′ closed, trim flow controllers  79 ,  81 ,  79 ′,  81 ′ inactive, lift leg assembly  40  hydraulic cylinders  83 ,  85 ,  83 ′,  85 ′ extended, runway position sensors  84 ,  86 ,  84 ′,  86 ′ extended and transmitting equal elevation runway position signals, PLC  87  operating and comparing the signals of the position sensors. The signals are processed and determined equal by the programmable logic controller (PLC)  87  and the PLC  87  outputs to the solenoid valves  80 ,  82 ,  80 ′,  82 ′ are therefore off, and the solenoid valves  80 ,  82 ,  80 ′,  82 ′ are in the closed position. 
     Descent of the runway members  20  and  20 ′ is begun by application of electric power to open the descend valve  77 . The weight of the runway members  20  and  20 ′ and load, if any, acting on the cylinder push rods  62  pressurizes the hydraulic fluid in the cylinders  83 ,  85 ,  83 ′,  85 ′, generally at different pressure levels. The resultant flow from each cylinder  83 ,  85 ,  83 ′,  85 ′ is to the independent ports of the synchronizer pumps  78 . The flow from each cylinder  83 ,  85 ,  83 ′,  85 ′ is metered essentially equal by the synchronizer pumps  78  and combined into a uniform pressure flow which maintains the isolation check valve  75  closed. The combined flow from the ports of the synchronizer pumps  78  passes through the preset descend flow controller  76 , the open descend valve  77 , and into the reservoir  70 . The descend flow controller  76  is preset to about 80% of the flow rate of the main pump  72  output to produce descent rates somewhat less than the elevation rates. The trim control strategy selected, absolute or relative master runway elevation sensor output comparison as discussed in the elevation control section, remains in effect. The more elevated runway member  20  and  20 ′ positions are detected by comparison of master and slave runway position sensors  84 ,  86 ,  84 ′,  86 ′ by the PLC  87  using a preselected algorithm, and power is applied to those corresponding PLC  87  output channels. The resultant solenoid valve  80 ,  82 ,  80 ′, or  82 ′ opening of the more elevated lift leg assembly  40  cylinder  83 ,  85 ,  83 ′, or  85 ′ provides an additional regulated flow discharge path for the more extended cylinder(s)  83 ,  85 ,  83 ′, or  85 ′. The trim flow controllers  79 ,  81 ,  79 ′,  81 ′ are set (as discussed in the elevation section) to about 20% of the flow provided to each cylinder  83 ,  85 ,  83 ′,  85 ′ by the independent port metered flow of the synchronizer pumps  78  during elevation, thus the descending rate of a cylinder  83 ,  85 ,  83 ′, or  85 ′ with an open solenoid valve  80 ,  82 ,  80 ′, or  82 ′ will be about 20% of the elevation rate due to the corresponding trim flow controller  79 ,  81 ,  79 ′, or  81 ′ and an additional 80% of elevation rate due to the descend flow controller  76 . The decent rate, therefore, is essentially the same as the elevation rate but may be adjusted if desirable to rates either greater or less than the elevation rate. As in elevating runway member  20  and  20 ′ synchronization control, the descending height error correcting solenoid valve  80 ,  82 ,  80 ′, or  82 ′ remains open until the runway error is corrected to the specified dead band, then is closed by PLC  87  detection of the equalized runway position sensor  84 ,  86 ,  84 ′,  86 ′ input and resultant closing of that solenoid valve  80 ,  82 ,  80 ′, or  82 ′. 
     In operation, a vehicle  200  is moved in place upon a vehicle lift  10  which is in a lowered position. The vehicle  200  is then chocked or otherwise held in place on the runway member(s)  20  and  20 ′ of the lift. When lifting is desired, an operator engages a circuit which causes the lift leg assemblies  40  begin to raise the runway member  20  and  20 ′ by force applied from the hydraulic cylinders  83 ,  85 ,  83 ′,  85 ′. As the runway member  20  and  20 ′ elevates the control system ensures that the runway member  20  and  20 ′ remains consistently level and typically horizontal to the supporting surface  30  both from front to rear (longitudinally) and from side to side (laterally). This is accomplished by synchronizing, as described above, the elevation of each lift leg assembly. 
     The detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom for modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit of the invention and scope of the appended claims.