Hydraulics has given a tremendous boost to the performance and versatility of mobile equipment. After centuries of human and animal powered equipment, machine builders harnessed the power of steam. This advancement expanded the power of machines to enable them to move heavier loads faster. To this day, you still sometimes hear excavators referred to as steam shovels. But the greatest leap forward in mobile equipment performance came with the advent of modern hydraulics.

These advancements continue today with ever-increasing power capacities, higher reliability through improved filtration and diagnostic techniques, higher energy efficiency by implementing load-sensing circuitry, and enhanced operation through electronic control. Perhaps nothing better illustrates the strides achieved in higher power capabilities for mobile equipment than the photo at right. It shows one of the first all-hydraulic excavators dwarfed by the largest built to date, the RH400, designed and built by Orenstein and Koppel AG, of Berlin, Germany. The hydrostatic drives, implements, and other systems are powered by a pair of Cummins 2700-hp QSK60 diesel engines. The hydraulic systems are supplied by 18 variable-displacement, axial-piston pumps, eight of which are Model A7VSL1000HDs from Mannesmann Rexroth.

This earthmoving giant was delivered to Syncrude Canada Ltd., Fort McMurray, Alberta for its new North Mine project. A total of four RH-400s are used for mining oil sand, which is trucked to crushing stations and eventually mixed with hot water to form an oil sand slurry that is pumped through a pipeline to an extraction station. The process is expected eventually to produce 82 million barrels of oil per year. When the project reaches full capacity, in 2006, the four RH400s will deliver oil sand to as many as 30 trucks, each with a capacity of 320 tons. It takes only four passes of an RH400 to completely fill one of the trucks. The high reliability and efficiency of the RH400's hydraulic system, combined with other advancements, will enable production to increase while decreasing operating costs. In fact, officials project that the North Mine project will use 15% less energy than had been consumed by its predecessor, the East Mine.

Hydraulics essential to mobile forklift

Unless you work in the material handling or shipping industry, you probably think of a forklift as 4-wheeled vehicle that zips through warehouses unloading pallet-mounted cargo from trucks, loading cargo onto trucks, or moving freight around a warehouse to make room for more freight. Although this is an accurate perception of the traditional forklift truck, more and more lift trucks depart from this convention.

Two applications have driven this diversity in forklift truck design. First, outdoor warehousing has become commonplace, so the traditional 4-wheel truck with small tires and rear drive and steering proves unsuitable in many situations. Instead, forklift trucks that can tackle uneven terrain were developed to serve this need. Second, many companies need to load or unload cargo at facilities that have no forklift truck. For these cases, the lift truck is loaded onto the truck bed and transported with the freight to the destination. Once there, the lift truck is unloaded and used to move the freight from the truck's cargo bed to a specific area at the destination.

The TrailerMate, manufactured by Eagle-Picher Industries, Inc., Lubbock, Texas, combines both of these features and more - all made possible by the innovative use of hydraulics. The TrailerMate features a two-wheel hydrostatic drive (three-wheel optional) that not only provides propulsion, but speed control and braking as well. Steering is accomplished through a rear-mounted wheel that is positioned by a rotary actuator. As with most forklifts, the mast assembly is raised and lowered using hydraulics. But, in addition, the TrailerMate incorporates a Pantograph Reach mechanism that is, essentially, a horizontally mounted double scissors jack. A pair of hydraulic cylinders actuates this mechanism to permit the fork assembly to reach loads positioned up to 4-ft in front of the TrailerMate.

HST provides propulsion

The hydraulic system of the TrailerMate TM50 is powered by a 46-hp (@2700 rpm) diesel engine driving a Sauer-Sundstrand Series 40 pump for the hydrostatic transmission (HST). A tandem-mounted gear pump provides flow for steering, the mast assembly, and stabilizers. In the HST circuit, the variable-displacement, axial-piston pump delivers up to 46 cc/rev (2.8 in.3/rev) and has a maximum pressure rating of 350 bar (5000 psi). Output flow from the pump is routed to a parallel circuit feeding a pair of 35 cc/rev (2.14 in.3/rev) fixed-displacement, axial-piston motors (which also are part of the Series 40). Each motor is connected through a splined shaft to a Torque-Hub planetary wheel drive from Fairfield Mfg. to drive the front wheels.

The HST also incorporates a traction control system that can be engaged when the TrailerMate is operated under conditions that prevent the tires from getting good traction.The operator activates the traction control system through a switch, which routes hydraulic fluid through a flow divider/combiner. With the system activated, the divider/combiner valve routes hydraulic fluid equally to both wheels. This prevents the wheel with lower traction from spinning.

Using the HST simplifies the design providing not only propulsion, but braking as well. This eliminates the need for the many braking components that would be necessary if the TrailerMate used a mechanical drive. A spring-applied/pressure-released brake holds the TrailerMate in place when it is parked. A switch on the parking brake is wired to the engine's electrical circuit to keep the engine from starting unless the brake is applied.

Rotary actuator simplifies steering

Steering is accomplished though a third wheel mounted in the center-rear of the vehicle. (As an option, this wheel incorporates an axial-piston motor to provide 3-wheel drive.) Supplied by Helac Corp., this rotary actuator uses a piston-and-helix configuration to generate high torque through a full 180° rotation. The rotary actuator offers several advantages over conventional steering cylinder and linkage setup. The rotary actuator:
eliminates mechanical linkages and the maintenance associated with them
exhibits constant steering effort throughout the full range of steering
needs no periodic adjustment for wear
achieves longer life
is more compact, and
provides more precise steering control.

In addition to these benefits of using rotary actuators in general, the TrailerMate takes advantage of what Helac calls a pivot-style mounting. This actuator design incorporates integral bearings and a heavy-duty shaft and flange assembly, making it a self-contained steering actuator and wheel mount. Installation simply involves connecting the wheel assembly at the bottom, bolting the top flange of the actuator to the underside of the vehicle frame, and connecting the hydraulic lines.

Rusty Shinn, lead engineer at Eagle-Picher, pointed out precise steering control is important, because designers wanted the TrailerMate to be easy to use. This is one reason why they decided against using a skid-steer drive arrangement. With skid-steer, steering is accomplished by varying the speed of the wheels on the left or right side of the vehicle. In fact, rotating wheels forward one side of the vehicle and in reverse on the other side allows the vehicle to turn on its own center. But because most operators would be accustomed to operating a conventional forklift truck, designers wanted the TrailerMate to have the steering feel of a conventional forklift. With the rear wheel positioned perpendicular to the front wheels, the Trailer Mate exhibits a turning radius of only 114 in, or 93 in. for the optional 3-wheel drive model.