1. Technical Field
The present invention is directed toward a testing apparatus for a drive unit, and more particularly toward an electronically controlled apparatus for testing a hydrostatic drive unit. The testing apparatus is operable to test a drive unit over a range of motor speeds and system pressures to simulate in a laboratory environment the operating conditions which the drive unit would experience in the actual use.
2. Background Art
Present hydrostatic drive units include a variable displacement hydraulic pump driven by an input shaft. A hydraulic fluid is pumped from the hydraulic pump to a fixed or variable displacement hydraulic motors for driving an output shaft. In these devices, there is no mechanical linkage between the input shaft which drives the hydraulic pump and the output shaft which is driven by the hydraulic motor.
Many hydraulic pumps or motors of the variable displacement type have a rotating cylinder block with pistons axially movable therein. The displacement of the hydraulic pump is proportional to the stroke of the pistons within the cylinder block. The pistons engage a tiltable swashplate to vary the stroke of the pistons and prescribe the displacement of the pump. Operation of the drive unit, as characterized by the rotational speed of the output shaft of the motor, can thus be effected by positioning of the tiltable swashplate.
Hydrostatic drive units may be further characterized by the system fluid pressure within the two fluid lines which connect the variable displacement pump and the hydraulic motor and form a closed-loop fluid circuit. As the hydraulic pump directs fluid at a flow rate commanded by the position of the pump swashplate toward the motor, motor pistons are displaced and the output shaft is forced to rotate. Any rotational resistance encountered by the shaft results in the build up of pressure within the fluid lines connecting the pump and motor.
In order to test the operability, durability, and performance of newly designed hydraulic drive unit components, test stands are used. The test stands primarily incorporate a drive means for driving either the hydraulic pumps or motors under simulated working conditions for extended periods of time. Under these simulated conditions, the displacement of the hydraulic pumps and motors are varied throughout the safe operating range for which the components were designed.
The simplest type or prior art stands incorporate a drive means for driving either a hydraulic pump or motor and a meter for reading the variations of output from the hydraulic device. More elaborate test stands incorporate the combination of hydraulic pumps and motors together with cooling and metering systems in order to assure that optimum efficiency is obtained. It is not possible with the prior art devices, however, to automatically control the operation of the test specimen according to a predetermined profile of motor speeds and fluid system pressures. In order to modify the operation conditions, the meters must be manually read and the test stand must be appropriately adjusted. Further, it is not possible with earlier devices to run more than one type of test on a given test stand, i.e. switch from performance testing to endurance testing without moving to another test stand.
The prior art also fails to provide systems which can test a hydrostatic drive unit at a zero delta system pressure drop between the two fluid lines comprising the closed loop. This is a particularly important feature when assessing the performance characteristics of the drive unit when going from a driving or uphill load to an absorbing or downhill load.
Additionally, energization of the prime mover for driving the test specimen can be costly over extened test schedules. While some costly stands currently provide a regenerative feature wherein a major portion of the input power can be recovered and reused in testing additional hydraulic devices to minimize operating expense, none provide a power regenerative feature in which prime mover energy can be regenerated at variable hydrostatic motor speeds.
The present invention is directed toward overcoming one or more of the problems set forth above.