“Oil hydraulics” is a term for power-converting or power-transmitting systems and devices that actuate Basic structure of oil hydraulic device and JIS symbols. Oils, Lubricants, Grease, Jelly .. HFA fluids – oil in water emulsions, according to ISO 12 Biodegradable hydraulic fluids according to ISO 15 A hydraulic element needs to be machined to a high degree of precision. 2. Leakage of hydraulic oil poses a problem to hydraulic operators. 3. Special treatment.
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Classification of hydraulic fluids based on ISO viscosity grade. Selection of hydraulic fluid for the system. .. one is mineral oil based hydraulic fluid. The main. Hydraulic oil is a fluid lubricant used in hydraulic systems for transmitting power. The following characteristics and properties are important for hydraulic oils. include storage tank, filter, hydraulic pump, pressure regulator, control valve, hydraulic pump takes hydraulic fluid (mostly some oil) from the storage tank and.
Closed loop circuits[ edit ] 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. Advantages: No directional valve and better response, the circuit can work with higher pressure.
The pump swivel angle covers both positive and negative flow direction. Disadvantages: The pump cannot be utilized for any other hydraulic function in an easy way and cooling can be a problem due to limited exchange of oil flow.
High power closed loop systems generally must have a 'flush-valve' assembled in the circuit in order to exchange much more flow than the basic leakage flow from the pump and the motor, for increased cooling and filtering. The flush valve is normally integrated in the motor housing to get a cooling effect for the oil that is rotating in the motor housing itself. The leakage flow as well as the extra flush flow must be supplied by the charge pump. A large charge pump is thus very important if the transmission is designed for high pressures and high motor speeds.
High oil temperature is usually a major problem when using hydrostatic transmissions at high vehicle speeds for longer periods, for instance when transporting the machine from one work place to the other. High oil temperatures for long periods will drastically reduce the lifetime of the transmission.
To keep down the oil temperature, the system pressure during transport must be lowered, meaning that the minimum displacement for the motor must be limited to a reasonable value.
Circuit pressure during transport around bar is recommended. Closed loop systems in mobile equipment are generally used for the transmission as an alternative to mechanical and hydrodynamic converter transmissions.
Large wheel loaders for instance and heavy machines are therefore usually equipped with converter transmissions. Recent technical achievements for the converter transmissions have improved the efficiency and developments in the software have also improved the characteristics, for example selectable gear shifting programs during operation and more gear steps, giving them characteristics close to the hydrostatic transmission. Constant pressure and load-sensing systems[ edit ] Hydrostatic transmissions for earth moving machines, such as for track loaders, are often equipped with a separate ' inch pedal ' that is used to temporarily increase the diesel engine rpm while reducing the vehicle speed in order to increase the available hydraulic power output for the working hydraulics at low speeds and increase the tractive effort.
The function is similar to stalling a converter gearbox at high engine rpm. The inch function affects the preset characteristics for the 'hydrostatic' gear ratio versus diesel engine rpm. Constant pressure CP systems[ edit ] 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, and 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 lifetime is prolonged. Large wheel loaders for instance and heavy machines are therefore usually equipped with converter transmissions. Recent technical achievements for the converter transmissions have improved the efficiency and developments in the software have also improved the characteristics, for example selectable gear shifting programs during operation and more gear steps, giving them characteristics close to the hydrostatic transmission.
Hydrostatic transmissions for earth moving machines, such as for track loaders, are often equipped with a separate ' inch pedal ' that is used to temporarily increase the diesel engine rpm while reducing the vehicle speed in order to increase the available hydraulic power output for the working hydraulics at low speeds and increase the tractive effort.
The function is similar to stalling a converter gearbox at high engine rpm. The inch function affects the preset characteristics for the 'hydrostatic' gear ratio versus diesel engine rpm.
The closed center circuits exist in two basic configurations, normally related to the regulator for the variable pump that supplies the oil:. 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 generates a constant power loss related to the regulating pressure drop for the pump regulator:.
If the pump flow is high the extra loss can be considerable. The power loss also increases if the load pressures vary a lot. The cylinder areas, motor displacements and mechanical torque arms must be designed to match 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.
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 reaches the maximum swivel angle.
This feature is important for machines that often run with the pump at maximum swivel angle and with several activated functions that must be synchronized in speed, such as with excavators. With type 4 system, the functions with up-stream compensators have priority.
Steering-function for a wheel loader. No official standardized name for this type of system has been established but Flowsharing is a common name for it.
Hydraulic pumps supply fluid to the components in the system. Pressure in the system develops in reaction to the load. Hence, a pump rated for 5, psi is capable of maintaining flow against a load of 5, psi. Pumps have a power density about ten times greater than an electric motor by volume.
They are powered by an electric motor or an engine, connected through gears, belts, or a flexible elastomeric coupling to reduce vibration. Piston pumps are more expensive than gear or vane pumps, but provide longer life operating at higher pressure, with difficult fluids and longer continuous duty cycles.
Piston pumps make up one half of a hydrostatic transmission. Directional control valves route the fluid to the desired actuator. They usually consist of a spool inside a cast iron or steel housing.
The spool slides to different positions in the housing, and intersecting grooves and channels route the fluid based on the spool's position. The spool has a central neutral position maintained with springs; in this position the supply fluid is blocked, or returned to tank. Sliding the spool to one side routes the hydraulic fluid to an actuator and provides a return path from the actuator to tank. When the spool is moved to the opposite direction the supply and return paths are switched.
When the spool is allowed to return to neutral center position the actuator fluid paths are blocked, locking it in position. Directional control valves are usually designed to be stackable, with one valve for each hydraulic cylinder, and one fluid input supplying all the valves in the stack. The valve block will be mounted to the machine's frame with a three point pattern to avoid distorting the valve block and jamming the valve's sensitive components. The spool position may be actuated by mechanical levers, hydraulic pilot pressure, or solenoids which push the spool left or right.
A seal allows part of the spool to protrude outside the housing, where it is accessible to the actuator. The main valve block is usually a stack of off the shelf directional control valves chosen by flow capacity and performance.
Some valves are designed to be proportional flow rate proportional to valve position , while others may be simply on-off. The control valve is one of the most expensive and sensitive parts of a hydraulic circuit. The hydraulic fluid reservoir holds excess hydraulic fluid to accommodate volume changes from: The reservoir is also designed to aid in separation of air from the fluid and also work as a heat accumulator to cover losses in the system when peak power is used.
Design engineers are always pressured to reduce the size of hydraulic reservoirs, while equipment operators always appreciate larger reservoirs. Reservoirs can also help separate dirt and other particulate from the oil, as the particulate will generally settle to the bottom of the tank.
Some designs include dynamic flow channels on the fluid's return path that allow for a smaller reservoir. Accumulators are a common part of hydraulic machinery. Their function is to store energy by using pressurized gas.
One type is a tube with a floating piston.
On the one side of the piston there is a charge of pressurized gas, and on the other side is the fluid. Bladders are used in other designs.
Reservoirs store a system's fluid.
Examples of accumulator uses are backup power for steering or brakes, or to act as a shock absorber for the hydraulic circuit. Also known as tractor fluid , hydraulic fluid is the life of the hydraulic circuit. It is usually petroleum oil with various additives. Some hydraulic machines require fire resistant fluids, depending on their applications. In some factories where food is prepared, either an edible oil or water is used as a working fluid for health and safety reasons.
In addition to transferring energy, hydraulic fluid needs to lubricate components, suspend contaminants and metal filings for transport to the filter, and to function well to several hundred degrees Fahrenheit or Celsius.
Filters are an important part of hydraulic systems which removes the unwanted particles from fluid.
Metal particles are continually produced by mechanical components and need to be removed along with other contaminants. Filters may be positioned in many locations. The filter may be located between the reservoir and the pump intake.
Blockage of the filter will cause cavitation and possibly failure of the pump. Sometimes the filter is located between the pump and the control valves.
This arrangement is more expensive, since the filter housing is pressurized, but eliminates cavitation problems and protects the control valve from pump failures. The third common filter location is just before the return line enters the reservoir. This location is relatively insensitive to blockage and does not require a pressurized housing, but contaminants that enter the reservoir from external sources are not filtered until passing through the system at least once.
Hydraulic tubes are seamless steel precision pipes, specially manufactured for hydraulics. The tubes are supplied by manufacturers in lengths of 6 m, cleaned, oiled and plugged. In larger sizes, hydraulic pipes are used. Direct joining of tubes by welding is not acceptable since the interior cannot be inspected. Hydraulic pipe is used in case standard hydraulic tubes are not available.
Generally these are used for low pressure. They can be connected by threaded connections, but usually by welds. Because of the larger diameters the pipe can usually be inspected internally after welding. Black pipe is non-galvanized and suitable for welding. Hydraulic hose is graded by pressure, temperature, and fluid compatibility. Hoses are used when pipes or tubes can not be used, usually to provide flexibility for machine operation or maintenance.
The hose is built up with rubber and steel layers. A rubber interior is surrounded by multiple layers of woven wire and rubber. The exterior is designed for abrasion resistance. The bend radius of hydraulic hose is carefully designed into the machine, since hose failures can be deadly, and violating the hose's minimum bend radius will cause failure.
Hydraulic hoses generally have steel fittings swaged on the ends. The weakest part of the high pressure hose is the connection of the hose to the fitting. Another disadvantage of hoses is the shorter life of rubber which requires periodic replacement, usually at five to seven year intervals. Tubes and pipes for hydraulic applications are internally oiled before the system is commissioned. Usually steel piping is painted outside.
Where flare and other couplings are used, the paint is removed under the nut, and is a location where corrosion can begin. For this reason, in marine applications most piping is stainless steel.
Components of a hydraulic system [sources e.This means that strict cleanliness is required when assembling units. System type 3 gives the advantage that activated functions are synchronized independent of pump flow capacity. CP is simple in design, and works like a pneumatic system. New hydraulic functions can easily be added and the system is quick in response.
Today most hydraulic fluids are based on mineral oil base stocks.
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