Hydrostatic transmission
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1. The document discusses different types of hydrostatic transmissions, including open-circuit, closed-circuit, and reversible systems. 2. Key components of hydrostatic transmissions are described, including the charge pump, relief valves, motors, and pumps. The selection process for pumps and motors is also outlined. 3. Various pressure control valves used in hydrostatic systems are explained, such as relief valves, counterbalance valves, sequence valves, and pressure reducing valves. Shuttle valves are also introduced.
Hydrostatic transmission
- 1. Department of Agricultural and Food Engineering Indian Institute of Technology, Kharagpur Presented by Rajeev Kumar (11AG61R14)
- 2. Wheel Engine Hydrostatic transmission Planetary gear box with 4:1, 1:1 reduction Differential with 3.4545 reduction Final drive 3.143 reduction Final drive 3.143 reduction Wheel Fig. Block diagram of proposed drive train of hydrostatic transmission for tractor
- 6. 1. Open circuit transmission • It is Fixed speed, non-reversible open circuit transmission. • An anti cavitations check valve is supply the fluid to motor if pump is stopped and hence prevents the motor cavitations. If FDP change to VDP The curves for this transmission with variable displacement pump and fixed déplacement Motors (know as constant torque transmission). Type of Hydrostatic transmission (HST)
- 7. If FDM change to VDM • It is called constant power transmission • The motor output power is independent of the motor displacement and constant pressure drop across the motor. • But motor torque is decrease with increase of Dm • Motor speed is decreases increase of Dm
- 8. Reversible open-loop transmission • In this HST, direction of rotation of the hydraulic motor is determined by the directional control valve. • A cross line relief valve network is built into this circuit to deal with the pressure surge which will occur if the DCV is operated rapidly to reverse the motor drive. • In any FDP or FDM, a FCV is used. If FCV is used then system become hot . As excess fluid will have to discharge across the PRV. • If A and B are blocked than fluid will go through open centre DCV.
- 9. Closed-loop transmissions • For closed loop HST system, a make up pump must be used. • Check valve Network is responsible for isolating the fluid from low pressure to high pressure line. • In non-reversing, closed-loop transmissions it may be possible to use an elevated reservoir to supply make-up oil so eliminating the necessity for a make-up pump and associated circuitry. • A check valve situated after the filter prevents any possibility of the motor causing reverse flow through the filter. • A brake valve is shown to give controlled retardation of the driven load and an anti-cavitation check valve is placed across the motor
- 10. Closed-loop reversible HST with scavenging circuit • For hydrostatic braking, the cooling and filtering of the fluid can be by a separate conditioning. • In this circuit, RV1 = 1.5, RV2 = 1 bar. • When the hydraulic motor is being driven the shuttle valve .then Rv2 sets the boost pressure at the pump and bleeds off excess oil from the circuit through the cooler. This is known as 'scavenging' or flushing. • RV3 is a motion control valve which functions both as a cross-line relief valve and a brake valve.
- 11. Closed-loop transmission characteristics • The characteristics of reversible drives are mirror images of the non-reversible drives. • 1. VDP and FDM. • Dp × Np = ηvs × Dm × Nm Nm =(Np/ ηvs × Dm) × Dp Nm = constant × Dp • Tm × 2π = Dm × Pm ηt • Tm × Nm = Dm × Nm × Pm × ηo Tm × Nm = Dp × Np × Pm × ηo
- 12. Closed-loop transmission characteristics 2. FDP and VDM Dp × Np = ηvs × Dm × Nm , Nm =(Np × Dp/ ηvs ) × (1/Dm) Tm × 2π = Dm × Pm ηt Tm × Nm = Dm × Nm × Pm × ηo Tm × Nm = Dp × Np × Pm × ηo
- 13. Fixed-speed, non-reversible, open-circuit transmission
- 15. Closed-loop hydrostatic transmission with make-up pump
- 16. Closed-loop HST with gravity feed make-up.
- 17. Closed-loop reversible HST with scavenging circuit.
- 18. The charge pump is critical component of the hydrostatic transmission. It is the heart of the hydrostatic transmission, without the charge flow and charge pressure, the transmission will cease to function. The charge pump provides several functions to hydraulic circuit 1. The primary function of charge pump is to replenish the fluid lost through leakages. 2. Provide flow, under pressure for maintaining back pressure on pump, motor. Charge pump sizing
- 19. 3. Provide fluid for the servo piston valve on the system having servo – controlled transmission. 4. Provides cooled, cleaned fluid for temperature control and flushing The pressure relief valves have a simple system which allow or avoid the flow rate depending on the pressure differences between both sides and the crack pressure. • In the hydrostatic transmissions, the static pressure is much bigger than the dynamic one because the speed of the liquid is low. as small as the transmission as fluid flow speed must be higher.
- 20. 1) Pump leakage Pump Leakage = actual capacity– Theoretical capacity Pump Leakage = actual capacity– ηv × actual capacity Pump Leakage = actual capacity (1- ηvp) 2. Motor leakage Similarly, Motor Leakage = actual capacity (1- ηvm) 3. Servo control requirement For most of the application with stroke time 1-3 second
- 21. 4. Loop flushing The amount of loop flushing will normally vary from 7 to 15 lpm depending on the charge pump displacement, input speed, and relative setting between the pump and motor charge relief valves. So take loop flushing is 13 lpm. 5. Total charge Flow required = Pump leakages + Motor leakages + Servo control requirement + Loop flushing. 6. From Charge pump performance curve Find the Pressure across charge pump. Again from Charge pump performance curve Find Power requirement for charge Pump. Power = discharge × Pressure
- 22. Fig. Charge pump performance curve
- 23. 2. Selection of Motor: Motor torque = (Required Thrust × Radius of wheel)/ (Gear reduction from motor to wheel) Required Thrust = (Gross traction coefficient × Dynamic Weight of Rear wheel) Gross traction coefficient is found from brixius equation. 2.1. Maximum speed of motor
- 24. 2.2. Minimum speed of motor (at maximum torque requirement) 2.3. Minimum torque 2.4. Displacement of motor. • From motor chart characteristics select the motor.
- 25. 3. Selection of Pump For HST, Main Pump should be bi-directional VDM. 3.1 Displacement of Pump Dp × Np × ηvp × ηvm= Dm × Nm From pump chart characteristic select the pump, and Pressure across pump.
- 26. CONCLUSIONS The hydrostatic transmission system gives infinite no. of gear and therefore hydrostatic transmission has the ability to adjust engine speed and transmission gear ratio together to operate at the point of maximum fuel efficiency for given travel speed and power requirement The hydrostatic transmission was found to be lesser in weight as well as compact in size as compared to the existing gear box.
- 27. The hydrostatic transmission for agricultural tractor offers gear reduction in two stages i.e. hydrostatic low and hydrostatic high. The hydrostatic transmission for agricultural tractor offers velocity in two stages i.e. hydrostatic low and hydrostatic high.
- 28. References Adarsh, K.,1986.Design & development of hydrostatic steering and hydrostatic transmission system for agricultural tractor .Unpublished M. Tech. Thesis, Agricultural and Food engineering Department, IIT, Kharagpur, India. Pinches,M .and Ashby,J.1989. Power hydraulics, pp 163- 179.
- 29. THANK YOU
- 30. PRESSURE-CONTROL VALVES • Relief valve: The function of a relief valve is to set the maximum pressure in a hydraulic system . • Counterbalance valve: is to set a back pressure , the usual pressure setting is 1.3 times the load induce pressure. Disadvantage of this valve is to reduces the available force. • Sequence valve : is to pass a signal when a certain pressure has been reached . • The principal feature of most pressure controls valve is that the hydraulic forces are resisted by a spring.
- 31. ln the poppet type relief valve pressure at port P acts on the exposed surface of the poppet to apply a force which is resisted by the spring force. Relief valves of the ball or poppet type have a rapid response to pressure surges. but the pressure flow characteristic is not constant. But it damage the seat so leakage is there. • Guided piston relief valve is of much quieter operation but is best suited for low pressure application(up to 100 bar) under flow condition. • The response time is still fast although slightly slower than the direct poppet-type relief valve.
- 33. • .
- 34. • The pressure override is the difference between the cracking pressure or opening pressure and the pressure drop across the valve when it is passing the maximum rated flow at the same valve setting. • The differential piston /poppet type relief valve is suitable for pressures up to 350 bar. • The pilot-operated relief valve is a two-stage valve which gives good regulation of pressure over a wide range of flow
- 35. Differential poppet relief valve: force to overcome spring = pressure x (a - b). Pilot-operated relief valve
- 36. Over-center valve (pilot- operated counterbalan ce valve or brake valve).
- 37. Pressure-reducing valves • These are used to limit the pressure in part of the circuit to a value lower than that required in the rest of the circuit • The pressure-reducing valve is a normally open valve which throttles or closes to maintain constant pressure in the regulated line • Direct-acting pressure-reducing valves are available for low flow rates up to about 45 1/m and pressures up to 210 bar; they can be supplied with or without a reverse-flow check valve. • Pressure-reducing valves may be: 1. Non-relieving, i.e. they do not limit any pressure increase downstream of the Valve set up by an external force 2. Relieving type. This limits the pressure downstream of the valve even when it is increased by an external force.
- 38. Shuttle valves • The shuttle valve is a single-ball check valve with two inputs A and B, and one output C. • It is used for load-sensing and will accept a signal from the higher of two pressure inputs