Energy conservation and other advantages in Mobile Equipment Through CLOSED CIRCUIT HYDROSTATIC TRANSMISSION C. Ramakantha Murthy Technical Consultant
Various features/advantages of HST Hydrostatic transmissions offer many important operating features. They are : i) remain stalled and undamaged under full load at low power loss ii) hold a pre-set speed accurately against driving or braking loads ii) operate efficiently over a wide range of torque/speed ratios iii) operate in reverse at controlled speed within design limits unaffected by output loads iv) transmit high power per unit volume displacement with low inertia.
Various features/advantages of HST contd.. v) does not creep at zero speed. But fine inching may be possible with a little complexity in hydraulic circuit vi) provide faster response than any other type of transmission vii) provide dynamic braking viii) provide long life with a little careful maintenance (mainly contamination control) ix) provide flexible transmission lines.
HYDRAULIC TRANSMISSION SYSTEM - TYPES OPEN CIRCUIT TRANSMISSION CLOSED CIRCUIT TRANSMISSION (HST)
Open Circuit Transmission Closed Circuit Transmission
Open Circuit Hydrostatic Transmission : One in which the power flow is not continuous: Fluid drawn From a reservoir by the pump is delivered to the motor inlet Port and returned from the motor to the reservoir. In this system, outflow from the actuator returns to tank normally through a Directional Control Valve Change of direction is achieved by shifting spool of DC valve Tank is sized based on the main pump flow capacity which in turn results in bigger tank requirement
CLOSED CIRCUIT TRANSMISSION Hydrostatic Transmission (HST)
Closed Circuit Hydrostatic Transmission : A Transmission in which the flow path in the power circuit is a continuous, un-interrupted path from the pump discharge port to the motor inlet port and back to the pump inlet port.
Closed Circuit System Outflow from an actuator, normally a motor, returns to the pump input port. Thereby, oil would be continuously in circulation inside circuit (Closed Loop) However, to compensate internal losses and also to provide transfer medium for heat dissipation, auxiliary pump called charge pump is used. This also provides fluid for servo control. Charge pump draws fluid from the tank. Tank is sized based on the charge pump capacity. Since charge pump is much smaller compared to main pump, tank can be much smaller in size.
Closed Circuit System contd. In a closed system, change of direction of actuator is achieved by operating swash plate in opposite direction. Speed variation is gradual & smooth. This is achieved by gradually varying swash plate angle. Flow output from pump is always in proportion to actual demand of actuator. Hence, power losses are less and so the heat generation which would result in smaller cooler requirement. http://hydraulicspneumatics.com/200/techzone/hydraulicpumpsm/art icle/false/6450/techzone-hydraulicpumpsm
Progression of constant-power HST circuits - from a bare pump and motor to an assembly with basic accessories.
Principle of Operation: Closed Circuit - Servo
Closed Circuit Pump Controls Manual Displacement Control Hydraulic Displacement Control Electrical Displacement Control
Displacement types - Motors Fixed Displacement Variable Displacement Dual displacement
Different Circuit Configurations Closed Circuit HST
Functional hydrostatic transmissions summarized according to types of pumps and motors involved: Fig. A shows HST with fixed-displacement pump and motor; Fig. B has fixed motor and variable-displacement pump; Fig. C has fixed pump and variable-displacement motor, and Fig. D has a variable-displacement pump and motor.
CONSTANT POWER, TORQUE AND SPEED SYSTEM Although this transmission is inexpensive, its applications are limited, primarily because alternative forms of power transmission are much more energy efficient. Because pump displacement is fixed, the pump must be sized to drive the motor at a fixed speed under full load. When full speed is not required, fluid from the pump outlet passes over the relief valve. This wastes energy in the form of heat
CONSTANT POWER, VARIABLE TORQUE AND VARIABLE SPEED SYSTEM Using a variable-displacement pump instead of one with a fixed displacement creates a constant torque transmission. Torque output is constant at any speed because torque depends only on fluid pressure and motor displacement. Increasing or decreasing pump displacement increases or decreases motor speed, respectively, while torque remains fairly constant. Power, therefore, increases with pump displacement.
VARIABLE POWER, CONSTANT TORQUE AND VARIABLE SPEED SYSTEM The most versatile HST configuration teams a variable-displacement pump with a variable-displacement motor. Theoretically, this arrangement provides infinite ratios of torque and speed to power. With the motor at maximum displacement, varying pump output directly varies speed and power output while torque remains constant. Decreasing motor displacement at full pump displacement increases motor speed to its maximum; torque varies inversely with speed, and horsepower remains constant.
VARIABLE POWER, VARIABLE TORQUE AND VARIABLE SPEED SYSTEM The curves in this Figure, illustrate two ranges of adjustment. In Range 1, motor displacement is fixed at maximum; pump displacement is increased from zero to maximum. Torque remains constant as pump displacement increases, but power and speed increase. Range 2 begins when the pump reaches maximum displacement, which is maintained while the motor's displacement decreases. Throughout this range, torque decreases as speed increases, but power remains constant. (Theoretically, motor speed could be increased infinitely, but from a practical standpoint, it is limited by dynamics.)
Energy Conservation in Closed Circuit HST In constant pressure hydraulic systems gas loaded accumulators can be used for energy storage. During braking and lowering of loads, the kinetic energy of the load can be stored in the accumulator and then the energy will be reused for acceleration and lifting of the load. Secondary controlled motors and hydrostatic transformers are components that can be used in conjunction with accumulators to improve the overall efficiency of hydrostatic systems.
The ICE is an essential drive train unit in mobile machinery. Comparison of the efficiency of the different drive train components (engine, pump, hydraulic motor) shows that the engines are the least efficient. Therefore it has also a major impact on the overall drive train efficiency. Operating the engine as long as possible in its minimum specific consumption zone, will drastically improve the overall efficiency. The developed optimization routine is maximizing the total efficiency by selecting the optimal combination of engine speed and displacement of the pump and motor to meet a certain load and speed demand on the output axis.
Use of Accumulators in Closed Circuit Transmission An accumulator is rarely necessary in a closed loop circuit. However, the two functions that an accumulator could serve in a hydrostatic system are: to absorb shock in the high pressure loop or to provide instantaneous low loop make-up flow. This instantaneous make-up flow requirement is associated with fluid compressibility problems, sometimes called the bulk modulus effect. Systems with long lines or applications which experience load induced pressure spikes are vulnerable to fluid compressibility and normally require supplemental low loop make-up fluid for a short time duration. However, the accumulator volume adds to the total volume of the system which is subject to the fluid compressibility during a load induced pressure spike.
Gang mounting multiple pumps Gang mounting multiple pumps provides a single, compact assembly that supplies two or more independent circuits from the rear drive pad of a gasoline or diesel engine. In this example, two variabledisplacement axial-piston pumps are visible at left; a fixeddisplacement vane pump, at right, serves as a charge pump
Packaged HST Packaged HST encloses pump, motor, controls, conducting system, and all auxiliary components into a single housing. The unit shown accepts input power from a V-belt drive and transmits power to the load through its output shaft. Packaged HSTs are available in a variety of configurations, many of which bolt directly to an engine.
MAJOR ADVANTAGES OF CLOSED CIRCUIT : Higher overall efficiency resulting in better energy management Full control of drive under all conditions Power feed back to prime mover when going down a slope SHORT-COMINGS OF CLOSED CIRCUIT : Only one function possible in one closed circuit Possible only for motors. In a very restricted way for cylinders
Over-running Loads & Dynamic Braking in Open Circuits The condition of Load reversal from resistive to over-running can be critical in open circuit drives. In a closed circuit, some dynamic braking is generally inherent in the system. Load reversal tends to over-ride the hydraulic motor, functionally converting it into a pump. Pump can absorb energy and transmit it to the prime mover which now functions as an energy absorber under these conditions.
High inertia vehicles, such as railroad maintenance machines, need control of deceleration forces. The pump has a capability to be driven to a neutral swash-plate condition before the vehicle reaches a zero speed. Without some method to prevent this from occurring, the internal pressure limiter will drive the pump towards maximum displacement during dynamic braking. The pump, now acting as a motor, will feed a large amount of torque into the engine due to the pump s high pressure and displacement. This may be more torque than what the engine can handle in a braking mode. A circuit utilizing the RVPL, while sensing travel direction, can be used to limit system pressure while decelerating. By setting the RVPL at a pressure level less than the internal pressure limiter, torque can be absorbed by the prime mover at a safe level. The system is designed such that the internal pressure limiter controls the acceleration torque while the RVPL controls the deceleration torque.
Recommendations-Closed Circuit Hydraulic Reservoir Function : To remove air and to provide make-up fluid for volume changes associated with fluid expansion or contraction, possible cylinder flow and minor leakage Reservoir fluid capacity should be between 0.5 to 1.5 minutes maximum pump flow (in GPM/LPM) Large reservoirs may contribute to difficulties with fluid contamination control. Reservoirs should have baffles between inlet and outlet ports to ensure adequate dwell time and to revent surging and aeration of fluid.
CHARGE PRESSURE FILTRATION
Closed circuit or closed loop? There are four basic HST configurations; two open-circuit and two closed-circuit configurations. Both refer to how the hydraulic lines in the system are connected. In an open circuit, fluid is drawn into the pump through a reservoir, is routed to the motor, then re-enters the reservoir after passing through the hydraulic motor. In a closed circuit, the flow path is uninterrupted - fluid flows in a continuous path from the pump discharge port to the fluid motor inlet port, out the motor discharge port and back into the pump inlet.
The two types of open and closed circuit systems are open loop and closed loop, which refer to the control arrangement of the HST. An open-loop system has no means of feedback for speed, pressure, flow, or torque regulation. Any variable control settings are accomplished manually by the operator. Closed-loop control, however, incorporates feedback devices that provide communication between the pump and motor, so the HST automatically adjusts to variations in operating conditions of the load, engine, or both
Application Open Circuit Transmission COPYRIGHT C (1998) VICKERS, INCORPORATED 18-8
Application Closed Circuit Transmission Parking Brake X Propel Speed Control Reverse Forward Steering Cylinders Left Propel Right Propel C1 C2 Power Steering Motor To lift, tilt, auxiliary System Traction Control Valve COPYRIGHT C (1998) VICKERS, INCORPORATED 18-9
Configuration of the Swing Drive for the Excavator