Hydraulic machinery

I want to introduct something about Low Noise Type Pilot Operated Relief Valves.

Structure: Control Terms of Payment: T/T Pilot operated relief valves here have been particularly develop-ed as low-noise types. Able to protect pumps and control valves against excessive pressures, they are used to control the pressure in the hydraulic system to a constant level. Remote control and unloading are permitted by using vent circuits.




Excavator. Main hydraulics: Boom cylinders, swing, cooler fan and trackdrive.



Princples for hydraulic force and torque multiplication.

Hydraulic machinery are machines and tools which use fluid power to do work. Heavy equipment is a common example.

In this type of machine, high-pressure liquid called hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders. The fluid is controlled directly or automatically by control valves and distributed through hoses and tubes.

The popularity of hydraulic machinery is due to the very large amount of power that can be transferred through small tubes and flexible hoses, and the high power density and wide array of actuators that can make use of this power.

Hydraulic machinery is operated by the use of hydraulics, where a liquid is the powering medium. Pneumatics, on the other side, is based on the use of a gas as the medium for power transmission, generation and control.

Contents

1 Force and torque multiplication

2 Hydraulic circuits

3 Constant pressure and load-sensing systems

3.1 Five basic types of load-sensing systems

4 Open and closed circuits

5 Hydraulic pump

6 Control valves

7 Actuators

8 Reservoir

9 Accumulators

10 Hydraulic fluid

11 Filters

12 Tubes, Pipes and Hoses

13 Seals, fittings and connections

14 Basic calculations

15 See also

16 References and notes

17 External links


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Force and torque multiplication

A fundamental feature of hydraulic systems is the ability to apply force or torque multiplication in an easy way without the need of mechanical gears or levers, either by altering the effective areas in two connected cylinders or the effective displacement between a pump and motor.

Examples

(1) Two hydraulic cylinders interconnected:

Cylinder C1 is one inch in radius, and cylinder C2 is ten inches in radius. If the force exerted on C1 is 10 lbf, the force exerted by C2 is 1000 lbf because C2 is a hundred times larger in area (S= ?r2) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch. The most common use for this is the classical hydraulic jack where a pumping cylinder with a small diameter is connected to the lifting cylinder with a large diameter.

(2) Pump and motor:

If a hydraulic rotary pump with the displacement 10 cc/rev is connected to a hydraulic rotary motor with 100 cc/rev, the shaft torque required to drive the pump is 10 times less than the torque available at the motor shaft, but the shaft speed (rev/min) for the motor is 10 times less than the pump shaft speed. This combination is actually the same type of force multiplication as the cylinder example (1) just that the linear force in this case is a rotary force, defined as torque.

Both these examples are usually referred to as a hydraulic transmission or hydrostatic transmission involving a certain hydraulic "gear ratio".

Hydraulic circuits



A simple open center hydraulic circuit.



The equivalent circuit schematic.

For the hydraulic fluid to do work, it must flow to the actuator and or motors, then return to a reservoir. The fluid is then filtered and re-pumped. The path taken by hydraulic fluid is called a hydraulic circuit of which there are several types. Open center circuits use pumps which supply a continuous flow. The flow is returned to tank through the control valve's open center; that is, when the control valve is centered, it provides an open return path to tank and the fluid is not pumped to a high pressure. Otherwise, if the control valve is actuated it routes fluid to and from an actuator and tank. The fluid's pressure will rise to meet any resistance, since the pump has a constant output. If the pressure rises too high, fluid returns to tank through a pressure relief valve. Multiple control valves may be stacked in series[1]. This type of circuit can use inexpensive, constant displacement pumps.

Closed center circuits supply full pressure to the control valves, whether any valves are actuated or not. The pumps vary their flow rate, pumping very little hydraulic fluid until the operator actuates a valve. The valve's spool therefore doesn't need an open center return path to tank. Multiple valves can be connected in a parallel arrangement and system pressure is equal for all valves.

Constant pressure and load-sensing systems

The closed center circuits exist in two basic configurations, normally related to the regulator for the variable pump that supplies the oil:

Constant pressure systems (CP-system), standard. Pump pressure always equals the pressure setting for the pumpregulator. This setting must cover the maximum required load pressure. Pump delivers flow according to required sum of...(and so on)
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