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Wheelift — Equalizing Fluid Suspension
 The Wheelift™ technology offers major changes in the design of heavy capacity wheeled carriers. Wheelift on-center rotation axle assemblies provide all-directional travel capability specifically designed to operate on relatively thin floor slabs and severely uneven or undulating floor surfaces. The Wheelift equalizing suspension is ideally suited to very large AGVs and large machines that would be more versatile if they were able to move in any direction. Actually many huge machines can be fitted with large multiples of wheels and axles that can provide massive load carrying capability, provided there is a uniform suspension and steering capability to ensure full load sharing across every axle. Wheelift solves this dilemma by using fluid equalizing suspension and laterally articulating, on-center rotation axles. Each wheel module has a vertical lift ram as its center column. The Wheelift design allows individual on-center rotation of each axle for all-directional, all-wheel steering. The fluid lifting center column allows several inches of vertical lift for compliancy, angular alignment, and self-loading applications. Each axle independently responds to severe irregularities in the floor surfaces.
 This technology is near identical in operating functionality to the European specialized carrier, multi-axle trailers that carry loads weighing hundreds of tons in European over-the-road, and OUTDOOR transport applications. Firms like Goldhofer, Scheuerle, Kamag, Cometto, Mafi and Gussin build Self-Propelled Modular Transporters (SPMTs) that are used by all of the big rigging companies throughout the US and the resto of the world for special purpose over-the-road transport. The Wheelift™ open axle center column design, makes all of those same capabilities of load-share possible for INDOOR applications, by having heavier load carrying per axles in a much smaller and lower profile configuration. These new capabilities allow Doerfer to design and build heavier capacity AGVs than anyone else in the world.
Why Do Conventional Wheel Systems have Limited Capacity?
If all floor surfaces were perfectly flat and the floor slabs would not defect as heavy loads move across them, then no suspension would be necessary. However,since no floors are perfect and the concrete slabs WILL deflect as very heavy loads move across them, some method must be used to ensure equal load sharing between wheel sets. Extra-heavy capacity industrial platforms, heavy trailers, heavy forklifts and other heavy mobile equipment often carry loads that are too heavy for springs to be considered (and possibly too heavy for pneumatic tires as well). If the floors are not perfect and with out some means of flexture, such as the frames that are flexible enough to deflect and share the loading across all of the supporting wheels, the slightest raise in the flloor will cause the majority of the load can be on just two points, with a third point will take only some of the remaining load. When this happens, then the wheels and the floors can then be subjected to severe overloading.
In a typical heavy vehicle application with conventional suspension, single or tandem rear axles are mounted solid to the vehicle frame. To load-share, the front “steering axle assemblies” attach to a rocker beam that allows the axle with its steerable wheels to articulate laterally, thereby allowing the forward axle to tilt laterally and share their load at the front of the vehicle to form a three-point equalized loading suspension with the rigidly mounted rear axles. For the heaviest loading applications, the tires are generally solid rubber, or are steel wheels on steel rails, and their axles either normally have several wheels mounted side by side to provide greater load carrying capability; or, in the case of very large rail-mounted cranes, their wheels are in line with interconnecting load sharing boggie assemblies.
Often the problem arises that there is nothing other than the compression of the tires and the flexing of the carrier frame to assure that each of the wheels are actually bearing uniformly during travel on uneven surfaces. For that reason, heavy load applications reach a practical limit. For higher loading, steel rails (overhead cranes and transfer cars) have been one of the few viable alternatives for in-plant process applications.
More On Doerfer Company's Wheelift Suspension
There are two basic fluid suspension designs used with a typical Wheelift wheel supported mobile work platforms and transporters: The two primary systems "Three-point fluid equalizing " and "Overpressure equalizing" suspension systems.
For either system, the wheel modules are arranged in line laterally across the load carrying vehicle. A vehicle will have at least two and possibly a dozen or more "axle lines" with the number of wheel modules ranging from as few as three to ANY number of independent axles (easily as high as twenty or more).
For a 3-point suspension arrangement, the total numbers of wheel modules are divided in the three groups. Fluid lines interconnect between each of the axle center columns in each group. In this configuration, regardless of the number of Wheelift wheel modules being used, there are three basic support points that carry the load. Due to the ability of the axles to articulate, and the free movement of fluid between axle cylinders, and the free movement of fluid between suspension cylinders, every tire is free to react independently to changing floor conditions within each of the three separate groups. The system assures that no wheel can ever be overloaded and, more importantly, the floors are not being damaged from wheel point loading.
There are instances however, where loads with high centers-of-gravity (CG) or an irregular load distribution dictate that three-point suspension is not adequate. For those applications an "Overpressure" load distribution circuit design is employed, wherein lift, lower, and leveling are controlled by separate valving in four "lift zones" and each axle or zone has its own set of pressure relief valves that control the maximum load that wheel modules are being allowed to carry. The operator or AGV controller raises the vehicle or work platform to a desired travel height. There is a preset maximum load pressure for the independent loading of the wheel modules in each group. As the vehicle is traveling and one or more wheel modules encounter a raise or bump in the operating surface, the load (cylinder pressure) will rise in the affected wheel module. Once that pressure reaches the preset level, the relief valve open slightly and the cylinder compresses as fluid bleeds off to maintain the desired pressure setting. Wheels in this system will experience different load pressures, but no wheel can be overloaded — the floors can only be subjected to the wheel’s preset allowable loading. This system is more complex, but desirable where severe off-center or high CG loads are likely to be encountered.
The Wheelift equalizing suspension provides a number of valuable functions not found in conventional equipment. One capability is LOAD LEVELING. With either of the foregoing circuit designs the load platform can rise to any level within the lift stroke of the lifting rams. By controlling the lift zones separately, the platform can be leveled or tilted as desired.
The primary functions of the lift cylinders are that of COMPLIANCE to the operating surface during travel. If the floors are relatively uniform, only a small portion of the systems lift stroke is needed for assuring equalized loading. If the floor undulations are severe or the transporter will be negotiating ramps, the full stroke of the lift cylinders may be needed. In the case of equalizing suspension, the transporter would be raised to one half of the average lift stroke of all the cylinders. The cylinders are then free to travel plus or minus one half of their stroke. In the case of an 18" (450mm) nominal diameter wheel modules, the lift stroke is five inches (130mm); therefore, the transporter would negotiate floor undulations of up to five inches (130mm) in height over the length of the transporter wheelbase.
The inherent ability to control the platforms lift height can become one of the most valuable assets. The ability to travel in both the lowered and raised positions lends itself to SELF-LOADING applications. An operator can lower a transporter to near the bottom of its lift stroke and drive under a load that is supported on raised rails or stands. The transporter drives under, and then lifts to engage the load's base. Further lift brings the entire assembly well clear of the floor to be driven to a new location. The transporter can precisely align the load at whatever process station it is being delivered to, then lower the load onto supports and drive out from under. This capability allows one transporter to be used to carry a wide variety of different load platforms. The only commonality of the loads need be the height and width of the "tunnel space" that the transporter drives into. Steel mills have used this capability for many years.
For instance, as shown to the right, a SELF-LOADING application might be similar to specialty steel mill processes shown here. The SPMT transporters shown were built by CETEC circa 1972 and the photo depicts the same transporter that is carrying a pallet structure with eight 25-ton steel coils between process points, may on the return, pick-up and carry a pallet structure holding 150-ton hot metal ladle that must travel to or from furnace locations. A few moments later, the same transporter may haul of pallet load of hot billets to an annealing oven. The only commonality between the loads is the tunnel space under the pallet. With the SELF-LOADING feature, transporters can be designed for multi-tasking applications in other industries as well.
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