Fluid motor control system providing speed change by combination

Motors: expansible chamber type – Fluid supply through diverse paths to single expansible chamber

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Details

60421, 60426, 60486, 60484, 91510, F01B 2502

Patent

active

043544207

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
This invention relates to control systems for fluid pressure driven motors and more particularly to the control of motors in which displacement may be changed to vary motor speed or torque.
In one specific form the invention relates to control systems for plural fluid motors which propel a vehicle and in which an automatic reduction in vehicle speed is desirable under a specific operating condition such as, for example, when the vehicle is undergoing a turn.
2. Background Art
Control of the speed of a fluid motor may be accomplished by varying the rate of flow of working fluid through the motor or by varying the displacement of the motor. Some fluid pressure driven systems use both speed control techniques. In some hydrostatic drive vehicles, for example, motor displacement is shifted in steps to provide for plural speed ranges while flow regulation enables continuous selective variation of speed within each such range.
The extent of speed change that can be realized by adjusting the displacement of fluid motors has in many cases been undesirably limited and subject to imprecision. While this may result in part from practical limitations on the size and complexity of the fluid motor that restrict the range of displacement change that can be designed into the motor, the problem is often aggravated by an effect which tends to reduce the magnitude of the speed change which is realized by a given change of displacement and to do so in a variable and somewhat unpredictable manner. In particular, certain characteristics of the pressurized fluid sources which supply working fluid to the motor tend to counteract the effect of a given displacement change in the motor.
The pressurized fluid sources which are commonly used to supply working fluid to fluid motors are often arranged to maintain a constant output pressure by varying output flow in accordance with changes of demand for fluid by the motor. One common form of pressurized fluid source includes one or more variable displacement pumps each of which is controlled by a pressure compensator that automatically adjusts the pump displacement as necessary to maintain a constant output pressure during periods of variable demand. If motor displacement is increased to reduce motor speed and if the pressure compensated pump is not at that time delivering the maximum possible flow, then the pressure compensator senses an incipient output pressure drop and reacts by increasing pump displacement and therefore the output flow from the pump. The increased flow of working fluid which is then available to the motor at least partially offsets the speed reducing effect of the motor displacement increase. An opposite but otherwise similar effect occurs if motor displacement is reduced with the intention of increasing motor speed. The pressure compensator at the pump reacts by reducing pump output flow. Consequently, the increase of motor speed is less than would otherwise be expected.
This reduction of the speed changing effect of motor displacement adjustments is not confined to systems using pressure compensated pumps. Another common form of pressurized fluid source has one or more fixed displacement pumps. To limit the output pressure to a predetermined maximum, one or more relief valves discharge a portion or all of the output flow from the pumps back to tank except at times when demand by the motor or other fluid powered devices supplied by the source exceeds the maximum output flow of the pump. If motor displacement is increased to reduce speed, less fluid discharges through the relief valve and more becomes available to the motor with the result that the resulting speed reduction is again less than would otherwise be the case. If motor displacement is reduced, an increased proportion of the output flow of the pump may discharge through the relief valve with the result that the increase of motor speed is again less than would be realized under constant flow conditions.
These effects undesirably limit the range of motor speed change which can

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