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.
Force and torque multiplication
A fundamental feature of hydraulic systems is the ability to apply force or torque multiplication in an easy way, independant of the distance between the input and output, without the need for mechanical gears or levers, either by altering the effective areas in two connected cylinders or the effective displacement (cc/rev) between a pump and motor. In normal case hydraulic ratios is combined with a mechanical force or torque ratio for optimum machine designs, like in boom movements and trackdrives for an excavator.
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 = π r ²) 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
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. 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 pump regulator. This setting must cover the maximum required load pressure. Pump delivers flow according to required sum of flow to the consumers. The CP-system generates large power losses if the machine works with large variations in load pressure and the average system pressure is much lower than the pressure setting for the pump regulator. CP is simple in design. Works like a pneumatic system. New hydraulic functions can easily be added and the system is quick in response.
Constant pressure systems (CP-system), unloaded . Same basic configuration as 'standard' CP-system but the pump is unloaded to a low stand-by pressure when all valves are in neutral position. Not so fast response as standard CP but pump life time is prolonged.
Load-sensing systems (LS-system) generates less power losses as the pump can reduce both flow and pressure to match the load requirements, but requires more tuning than the CP-system with respect to system stability. The LS-system also requires additional logical valves and compensator valves in the directional valves, thus it is technically more complex and more expensive than the CP-system. The LS-system system generates a constant power loss related to the regulating pressure drop for the pump regulator:
Power loss =
The average Δ p LS is around 2 MPa (290 psi). If the pump flow is high the extra loss can be considerable. The power loss also increase if the load pressures varies a lot. The cylinder areas, motor displacements and mechanical torque arms must be designed to match in load pressure in order to bring down the power losses. Pump pressure always equals the maximum load pressure when several functions are run simultaneously and the power input to the pump equals the (max. load pressure + Δ p LS ) x sum of flow.Five basic types of load-sensing systems
(1) Load sensing without compensators in the directional valves. Hydraulically controlled LS-pump.
(2) Load sensing with up-stream compensator for each connected directional valve. Hydraulically controlled LS-pump.
(3) Load sensing with down-stream compensator for each connected directional valve. Hydraulically controlled LS-pump.
(4) Load sensing with a combination of up-stream and down-stream compensators . Hydraulically controlled LS-pump.
(5) Load sensing with synchronized, both electric controlled pumpdisplacement and electric controlled valve flow area for faster response, increased stability and less system losses. This is a new type of LS-system, not yet fully developed.
Technically the down-stream mounted compensator in a valveblock can physically be mounted "up-stream", but work as a down-stream compensator.
System type (3) gives the advantage that activated functions are synchronized independent of pump flow capacity. The flow relation between 2 or more activated functions remains independent of load pressures even if the pump reach the maximum swivel angle. This feature is important for machines that often run with the pump at maximum swivel angel and with several activated functions that must be syncronized in speed, such as with excavators. With type (4) system, the functions with up-stream compensators have priority. Example: Steering-function for a wheel loader. The system type with down-stream compensators usually have a unique trademark depending on the manufacturer of the valves, for example "LSC" (Linde Hydraulics), "LUDV" (Bosch-Rexroth Hydraulics) and "Flowsharing" (Parker Hydraulics) etc. No official standardized name for this type of system has been established but Flowsharing is a common name for it.
Open and closed circuits
Open-loop: Pump-inlet and motor-return (via the directional valve) are connected to the hydraulic tank.The term loop applies to feedback; the more correct term is open versus closed "circuit".
Closed-loop: Motor-return is connected directly to the pump-inlet. To keep up pressure on the low pressure side, the circuits have a charge pump (a small gearpump) that supplies cooled and filtered oil to the low pressure side. Closed-loop circuits are generally used for hydrostatic transmissions in mobile applications. A
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