Hydraulic Jack
A power-driven pump operates a rotary motor
Basic hydraulic transmission scheme
6.3 Operating principle of hydraulic transmission
6.3.1 Reciprocating motion
Hydraulic transmission with reciprocating motion
Left Figure:
1.Pump; 2. Discharge valve; 3.Suction valve; 4.reservor; 5.directional control valve; 6.actuator (hydraulic cylinder)
Right figure:
1. Pump; 2.safety valve; 3.directional control valve; 4.actuator (hydraulic cylinder)
6.3.2 Rotary motion
6.4 Hydraulic circuit types
6.4.1 Close circuits:
In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank. Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation and pressurizes the loop. The charge pump is normally installed on the back of the transmission pump and has an output of at least 20% of the transmission pump's output.
Advantages
- Supplement oil
- Increases suction pressure and so, increasing the system pressure, therefore this system can provides high power
- Reverse easily direction of actuator when it has high load
Disadvantages
- High temperature of circulating oil
- High ability of oil leakage
- High cost because of auxiliary components
6.4.2 Open circuits
Advantages
- Supplement oil easier
- Oil is cooled in reservoir
Disadvantages
- Supply smaller power
6.4.3 Differential circuits
6.5. Main components in static hydraulic transmission
6.5.1 Hydraulic pump
The pump is driven by a prime mover which is usually an electric motor or a petrol or diesel engine. The energy input from the prime mover to the pump is converted into high-pressure energy in the fluid which is transmitted through pipes and in turn is converted into rotational energy by a motor or translational energy by a cylinder.
When a hydraulic pump operates, it performs two functions. First, its mechanical action (the mechanical energy of the prime mover- an internal combustion engine or electric motor is transmitted to the pump) creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system. A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load.
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