As with most mobile equipment, Caterpillar's IT38G tool carrier uses hydraulics for multiple functions of its operation. However, it also incorporates a variable-speed, loadsensing hydraulic fan drive, which helps conserve fuel and reduce engine emissions.
John Baus, lead engineer for medium wheel loaders at Caterpillar, Peoria, Ill., explains that the IT38G has articulated steering, which makes it easy to maneuver, especially in tight spots. Baus says steering is accomplished with a two-piece chassis that pivots under the operator cabin. When the operator turns the steering wheel, say, to the right, a cylinder on the left side of the chassis extends, while an identical cylinder on the right side retracts.
Turning the steering wheel drives a gerotor-type pump, which directs hydraulic fluid in a pilot circuit to a proportional directional valve. This valve, in combination with other components, routes the correct flow of hydraulic fluid to and from the appropriate cylinders. Load sensing designed into the steering circuit directs power through the steering system only when needed. When steering is not needed, more power from the engine is available for implements.
Hydraulic fan drive
A trend in mobile equipment — especially large equipment — is the use of hydraulic fan drives to conserve energy and reduce emissions. Performance of diesel engines is very sensitive to operating temperature. An engine exhibits it highest power output per unit of fuel consumption within a relatively narrow range of temperatures. Likewise, a diesel engine produces the lowest amount of emissions per unit of power also within a relatively narrow temperature range. These two temperature ranges are not identical, but they generally overlap. Therefore, operating the engine within this overlapping temperature range produces maximum fuel economy and minimum emissions.
To keep an engine operating within this range, the cooling system maintains a relatively constant engine temperature even though engine speed, load, ambient temperature, and other factors are changing. The IT38G uses a variable-speed hydraulic fan drive to do just that.
The fan drive is powered by a variable-displacement axial-piston pump with load sensing. The load sensing feature minimizes power consumption of the fan drive when cooling demands are low — for example, when the equipment is operating in cold weather. From the pump, pressurized fluid flows through control valves to a fixeddisplacement gear motor, which drives the fan.
The large fan in this machine acts as a flywheel, so if inertia of the fan should cause it to become an over-running load, cavitation could occur in the motor or pump. To prevent cavitation, Baus says the hydraulic fan circuit contains an anti-cavitation valve — essentially a pilot-operated check valve controlled by backpressure in the circuit.
Additional features
Another trend in mobile hydraulics incorporated in the IT38G is to provide numerous oil sampling valves for fluid assessment. Making the oil sampling process easier speeds maintenance and increases the likelihood that machines will be well maintained.
To address the trend toward more environmentally friendly operation, Cat's Ecology Drains are used place of drain plugs. Ecology Drains incorporate a check valve that opens when a mating fitting is connected to the drain. This eliminates the potential for fluid spillage that can occur with drain plugs.
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