Hydraulic machinery

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

In this type of machine, hydraulic fluid is pumped to a high pressure and transmitted throughout the machine to various actuators. The hydraulic pumps are powered by engines or electric motors. Pressurized fluid is controlled by the operator with 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.

Contents

[edit] Hydraulic power

The science of Fluid pressure provides some of the theory of hydraulics.

  1. A force acting on a small area can create a much larger force by acting on a larger area by virtue of hydrostatic pressure.
  2. A large amount of energy can be carried by a small flow of highly pressurized fluid.

Hydraulic machinery offers a very large amount of power and force with relatively small components. A typical hydraulic cylinder with a 75 mm (3 inch) bore, for example, can supply 89 000 N (20,000 lbf). The power transmission in a hydraulic system is easily controlled with valves.

Some parts of a hydraulic system will operate at about 2000 kPa (300 psi) (pilot controls, vehicle brakes). The main hydraulic actuators (for example, cylinders or fluid motors) will typically operate in the range of 7000 - 42000 kPa (1000 - 6000 psi). With advances in materials and design, there is a trend toward higher pressure, with some systems operating to 100 000 kPa (15,000 psi). Some exotic systems with titanium hardware will operate at over 350 000 kPa (50,000 psi).

[edit] Hydraulic circuits

For hydraulic fluid to do work, that fluid must flow to the actuator, 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 then to 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.

A simple open center hydraulic circuit.
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A simple open center hydraulic circuit.
The equivalent circuit schematic.
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The equivalent circuit schematic.




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.

Load-sensing circuits pumps can reduce both flow and pressure to match the load requirements. This results in a much more efficient system when partially loaded.

[edit] Pumps

A magnified view of an external gear pump.
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A magnified view of an external gear pump.

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 5,000 psi is capable of maintaining flow against a load of 5,000 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.

Common types of hydraulic pumps for hydraulic machinery applications are;

  • gear pump: cheap, durable, simple. Less efficient, because they are constant displacement, and suitable for, largely, pressures below 200 Bar (3000 psi).
  • vane pump: cheap and simple, reliable (especially in g-rotor form). Good for higher-flow low-pressure output.
  • Axial piston pump: many designed with a variable displacement mechanism, to vary output flow for automatic control of pressure. There are various axial piston pump designs, including swashplate (sometimes referred to as a valveplate pump) and checkball (sometimes referred to as a wobble plate pump). The most common is the swashplate pump. A variable-angle swash plate causes the pistons to reciprocate. 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


[edit] Control valves

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, 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.

Tolerances are very tight in order to handle the high pressure and avoid leaking, spools typically have a clearance with the housing of less than a thousandth of an inch. 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.

Pressure relief valves are used in several places in hydraulic machinery; on the return circuit to maintain a small amount of pressure for brakes, pilot lines, etc... On hydraulic cylinders, to prevent overloading and hydraulic line/seal rupture. On the hydraulic reservoir, to maintain a small positive pressure which excludes moisture and contamination.

Pressure reducing valves reduce the supply pressure as needed for various circuits.

Counterbalance valves supply resistance to flow under certain conditions, to help a forklift maintain position of a load, for example.

Sequence valves control the sequence of hydraulic circuits; to insure that one hydraulic cylinder is fully extended before another starts its stroke, for example.

Shuttle valves provide a logical or function.

Check valves are one way valves, allowing an accumulator to charge and maintain its pressure after the machine is turned off, for example.

Cartridge valves are off the shelf components with a standardized envelope, making them easy to populate a proprietary valve block. They are available in many configurations; on/off, proportional, pressure relief, etc. They generally screw into a valve block and are electrically controlled to provide logic and automated functions.

Hydraulic fuses are in-line safety devices designed to automatically seal off a hydraulic line if pressure becomes too low, or safely vent fluid if pressure becomes too high.

Auxiliary valves. Complex hydraulic systems will usually have auxiliary valve blocks to handle various duties unseen to the operator, such as accumulator charging, cooling fan operation, air conditioning power, etc... They are usually custom valves designed for the particular machine, and may consist of a metal block with ports and channels drilled. Cartridge valves are threaded into the ports and may be electrically controlled by switches or a microprocessor to route fluid power as needed.

[edit] Actuators

[edit] Reservoir

The hydraulic fluid reservoir holds excess hydraulic fluid to accommodate volume changes from: cylinder extension and contraction, temperature driven expansion and contraction, and leaks. The reservoir is also designed to aid in separation of air from the fluid. Design engineers are always pressured to reduce the size of hydraulic reservoirs, while equipment operators always appreciate larger reservoirs.

Some designs include dynamic flow channels on the fluid's return path that allow for a smaller reservoir.

[edit] Accumulators

Accumulators are a common part of hydraulic machinery, they store energy by using pressurized gas. One type is a tube with a floating piston. On one side of the piston is a charge of pressurized gas, on the other side is the fluid. Bladders are used in other designs.

Examples of accumulator uses are backup power for steering or brakes, or to act as a shock absorber for the hydraulic circuit.

[edit] Hydraulic fluid

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 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.

[edit] Filters

Filters are a very important part of hydraulic machinery. Metal filings are continually produced by mechanical components and need to be removed, along with other contamination.

Filters may be positioned in a variety of 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 after the pump, and before 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 any contaminants that may enter the reservoir (from external sources) are not filtered until they pass through the system at least once.

[edit] Hose, tubes and pipes

Hydraulic hose is graded by pressure, temperature, and fluid compatibility. 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.

Hydraulic pipe is thick enough to have threads cut into it for connections. It is rarely used for high pressure systems, which prefer tubes or hoses. Pipe lends itself to weldments and can be used to fabricate manifolds. Steel suppliers carry black pipe, which is non-galvanized and suitable for welding.

Hydraulic tubes are preferred over hoses whenever possible, since they are more durable. They are also preferred to pipe since they weigh less. Hydraulic tubes usually have flared ends and captive nuts to make connections. They may also be steel weldments with floating nuts and face seal fittings on the ends. Tubes can be fabricated to a high degree of complexity.

Tubes and pipes for hydraulic applications traditionally have not been plated or painted, since the oil and temperature they operate under drive away moisture and reduce rusting. Anti corrosion coatings, such as zinc chromate, are becoming more popular.

[edit] Seals, fittings and connections

In general, valves, cylinders and pumps have female threaded bosses for the fluid connection, and hoses have female ends with captive nuts. A male-male fitting is chosen to connect the two. Many standardized systems are in use.

Fittings serve several purposes;

  1. To bridge different standards; O-ring boss to JIC (hydraulic), or pipe threads to face seal, for example.
  2. To allow proper orientation of components, a 90°, 45°, straight, or swivel fitting is chosen as needed. They are designed to be positioned in the correct orientation and then tightened.
  3. To incorporate bulkhead hardware.
  4. A quick disconnect fitting may be added to a machine without modification of hoses or valves

A typical piece of heavy equipment may have thousands of sealed connection points and several different types of seals, below are some of the most common types;

  • Pipe fittings, the fitting is screwed in until tight, difficult to orient an angled fitting correctly without over or under tightening.
  • O-ring boss, the fitting is screwed into a boss and orientated as needed, an additional nut tightens the fitting, washer and o-ring in place.
  • Flare seal, a metal to metal compression seal with a cone and flare mating.
  • Face seal, metal flanges with a groove and o-ring are fastened together.
  • Beam seal, an expensive metal to metal seal used mostly for aircraft.
  • Swaged seals, tubes are connected with fittings that are swaged in place (non-serviceable). Primarily used in aircraft.

Elastomeric seals (O-ring boss and face seal) are the most common types of seals in heavy equipment and are capable of reliably sealing 6000+ psi (41368+ kPa) of fluid pressure.

[edit] See also

[edit] External links

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