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Sep 30, 2011 | Wheelift

A New Concept to Improve Heavy Transporter and AGV Capacity Performance

Doerfer Company is announcing a new technology that is revolutionizing in-plant heavy load carrying capabilities. Doerfer's Wheelift Systems group is introduces this 140-Ton capacity, all-directional travel, self-loading transporter that extends far beyond any present forms of in-plant handling. These new heavier capacity designs are ideally suited to heavy Automatic Guided Vehicle (AGV) applications as well.

Many companies that build or assemble large products and heavy fabrications, often have to turn down opportunities because of capacity because they have reached their maximum practical size their facilities will handle. People with those problems will be especially interested in this technology. "In the right application, this technology is going to be an incredible advantage to some companies," said Mel Terry, director of Wheelift Systems. "Anyone that has ever moved anything heavy, has experienced many of the inherent problems associated with conventional handling means and the limitations of their current systems as loads get heavier." Terry also projects that more and more firms will begin to request these heavier capacities as they realize this technology allows for for virtually unlimited expansion of their capabilities in existing facilities.

More importantly, the transporter shown in this article demonstrates new freedom for engineers who need to design much larger systems. The Uniload wheel modules bring capabilities that have been never-before-attainable on a practical operating basis. This system uses large numbers of small, heavy capacity axle assemblies ("wheel modules") that have independent suspension and that have fully omnidirectional travel by being able to be power rotated about their vertical lifting center column. Every axle is essentially independent and is steered in precise relation to all other wheel modules in the system, regardless of their location geometry -- thereby allowing all-directional travel over uneven floor surfaces.

The Transporter Description

The transporter in this article is specifically designed for in-plant, heavy process handling applications. The OEM 140T is a basic carrier platform that was built for to a customer's specification as a carrier platform on which that firm could mount its newly designed dual mold changer platform. The transporters is powered by a 140 horsepower propane conversion gas engine that is encased in a sound dampening enclosure. While Doerfer transporters are normally designed to have very low load decks, this customer specified that the load deck was to be , the vehicle is 48 inches in height by 8.2 feet wide, with a load deck length of 14.5 feet, and an overall length of 20 feet. While this particular vehicle travels 70-feet per minute, speed selections may vary depending on the intended plant conditions and whether the control is from a walkinbg or riding operator, or is to have a driverless automatic guided travel control.

Also by specification, this particular transporter has the capability of five inches of vertical lift, but lift heights for similar applications can be up to 15 inches. As required in many machine interface applications, Uniload wheelsets are designed to provide lower deck heights than have ever before been possible with other than fixed travel rollers on steel rails.

The Uniload Wheel Modules

The load carrying wheel assemblies in this application consist of eight independent, fluid suspension axle assemblies, each rated at 40,000 pounds and having two 18 inch diameter by 9-inch face width solid urethane wheels that mount on an open-center axle. The on-center-rotation center support column is a custom-designed, heavily constructed, hydraulic ram who's base connects to the axle with a pair of trunnion pins. These connections allow the axles to articulate laterally plus/minus four degrees. In addition to providing fluid exchange equalizing lift between interconnect groups of axle assemblies. The vertical lifting center column hydraulic ram has a squared upper shaft that transfers steering torque through the top mount slewing gear and the wheel module's hydraulic steering motor.

Each of the eight Wheelift axle assemblies are independently steerable. This transporter has two driver wheelsets mounted diagonally opposed at the front and rear corners of the vehicle; both drivers have two in-hub mounted hydraulic motors. The diagonally opposed mounting allows and equally divided tractive effort, whether the transporter is driving forward, sideways, or rotating about its center. The vehicle has six idler wheelsets that proportionately steer in the exact geometry needed for any turning requirement, which provides versatile capabilities for omni-directional travel in the most demanding of transporter applications.

Controlling the Vehicle

All of the vehicle's onboard functions are controlled through the SynchroSteer computerized steering controller and from the operator's handheld pendant and programmable input panel. The handheld controller provides "mode selections" for the vehicle’s five various omni-directional steering patterns, which include:

  • "Normal" - For proportioned "random path" all-wheel steering in the normal forward and reverse travel directions, the leading and trailing wheel modules will rotate up to 30 degrees to accommodate tight turning radiuses
  • "Lateral" - For sideways travel, all wheelsets rotate 90º to realign for nnow perpendicular "random path" travel. Once all wheelsets reach the lateral travel mode the vehicle has the same fully steerable (+/- 30º) travel while driving sideways.
  • "Oblique" - The operator might choose oblique or "crab" travel when nearing an "unload" or "docking" position. This mode is used for slight course changes while holding parallel to a particular line travel.
  • "Rotate" - When the operator selects rotate, the vehicle will stop and the axles will rotateon-center until each axle is in line with the vehicle center point. The vehicle then turns about that exact centerline.
  • "RotatePLUS" - In this mode, there are four additional programmable center points that may be used for special purpose turning requirements. A typical example is for very narrow aisle maneuvering, where two aisles intersect. The operator may move the transporter very close to one side of the travel aisle and stop when the middle of the transporter reaches the intersecting aisle's corner. The operator will select a center point that is only a few inches to one side of the vehicle (rather than the center of the vehicle). The transporter will now rotate using the actual intersecting aisle corner as the center of rotation. This maneuver reduces the aisle width needed for turning by roughly 17 percent.
In addition to controlling steering geometry and synchronizing drive motor speeds, SynchroSteer controls the engine and all of the vehicle's hydraulic functions.


Equalizing suspension is achieved because interconnecting fluid lines join the Uniload wheel modules into three suspension groups. In this case two wheel modules on each side of the forward end form two groups, while the remaining four wheelsets on the rear or engine-end form the third group, who's center is the suspension point of that load zone.

In certain applications, nitrogen or other gases can be used to provide compliant suspension, rather than hydraulic fluid. Too, in a system where a large number of wheel modules are being used and it is desirable to keep the individual loading points of the three-point liquid suspension as far to the outside as possible, the center modules can be on a gas suspension to allow each center axle to react independantly while the load is still accurately balanced on the outer three points. In such a case, the transporter will have two or more welder-size nitrogen bottles mounted in a special rack. The center-mounted wheel modules are connected to a nitrogen charged circuit that provides the precision sensitive adjustment of an air spring suspension system. With manual or electronic control, the pressurized nitrogen circuit allows the lift cylinders to equally share the load with the fluid suspension wheelsets without influencing the center of gravity of the load being carried. The automatic load sharing is accomplished with the simple physics of gas charging. And, other than for major changes in the weight of the load being carried, does not require operator or computer input or monitoring.

Travel Height and Elevation Control

Using distance sensing devices mounted vertically in each lifting zone, the vehicle is capable of measuring the distance above the floor and controlling lift for the normal travel requirements. For transporters that are a carrying platform for loads that must roll on or off, the transporter must lower onto solid landing pads or have corner-mounted hydraulic outriggers that can lift the entire transporter and its load to the desired transfer elevation. The transporters three-point suspension system cannot be used to support loads rolling on and off the transporter.

While several wheels may be on crowns or depressions, the load height will remain the same and at virtually the same relative pressure will be bearing against the floor for every wheelset. With the equalized loading of the Uniload fluid suspension system and its articulating axles, the individual tires are assured of not being overloaded. Even at a standstill, as heavily loaded wheelsets rotate to new directions, opposing wheels roll on true radius arcs, virtually eliminating lateral scuffing, overloads, and other conditions that could otherwise result in severe damage to the floor and wear on the wheel treads.

Market Focus

The primary industry focus of this customers application was in the quick-change capability of very large injection molds for the automotive side of the plastics industry. Similar transporters and AGVs would be employeed for die-change vehicles for automotive metal stamping, and heavy coil handling applications where they might carry three to eight 50-ton coils at a time. "SynchroSteer multi-axis steering controls and all-directional drives work ideally under the navigational commands of any AGV system," said Terry.

There are many inherent problems associated with conventional handling means and the limitation of vehicles as loads get heavier. Unfortunately this technology, while making new types of movements possible, is inhearently more expensive complex hydraulic and computerized control systems that make this unique capability possible. Generally speaking, 30-tons seems to be the beginning threshold. In actuality, if the requirement can be met with any type of conventional technology, this system probably can not be cost-justified. However, the future holds good prospects for lighter loads. As Terry explained, "Eventually we will be building smaller wheelsets in enough quantity that pricing on lighter systems may be practical."