The Importance of Check Valves in Hydraulic Systems
Figure 1. In this system, oil flows
from the left side port, through the check valve
and out the right side port.
Check valves are the simplest form of hydraulic devices in that they permit free oil flow in one direction and block oil flow in the opposite direction. Check valves may also be used as a directional or pressure control in a hydraulic system.
In Figure 1, oil is flowing in from the left side port, through the check valve and out the right side port. If the pressure equalizes or is higher in the right side port, the check valve will close and block flow in the opposite direction.
The spring rating varies based on how the valve is used in the system. One of the most common locations for a check valve is immediately downstream of the hydraulic pump (Figure 2). Notice that no spring is shown with the check valve symbol.
When used in this application, the spring pressure rating is usually 1-5 pounds per square inch (psi) and therefore not shown with the symbol. In this case, the valve is used as a directional control in that it allows oil flow from the pump to the system but blocks flow in the reverse direction. This is commonly called a pump isolation check valve. This valve serves four purposes within the system, which are detailed below:
Figure 2. Check valves are often located
immediately downstream of the hydraulic pump.
Block Pressure Spikes
The check valve will block pressure spikes back to the pump. Depending on the pressure, oil flows from the pump to the system at a speed of 15-30 feet per second. When a directional is de-energized to block flow or a cylinder fully strokes, the oil is rapidly deadheaded. The pressure in the line can quickly increase by two to three times. The check valve should then close and block the pressure spikes to the pump.
I recall a plywood plant changing four pumps due to cracking of the pumps’ housings. This occurred over a week’s time on the debarker hydraulics. When the plant ran out of pumps, the staff finally took out the check valve and found that the piston and spring were no longer in the valve.
This $150 check valve cost the company $15,000 in replacement pumps and another $50,000 in machine downtime. That was one expensive check valve. The truth is that if one mechanic had looked at the schematic and known why the check valve was in the system, the replacement of the pumps and subsequent expenses would have been avoided.
Prevent Oil Lines from Draining
When a system is shut down, it is important to maintain oil in the lines. In many cases, the pump is mounted below the level of the system valves, cylinders and motors. The check valve downstream of the pump will prevent the lines from draining once the electric motor is turned off. If the oil in the lines drains through the pump and into the reservoir, a vacuum will occur.
Air will be pulled into the lines through the O-rings and seals of the valves and actuators. This can create issues when restarting the system, as the air will need to be bled out.
Block Oil Flow from the Accumulator
Some systems have a hydraulic accumulator installed downstream of the pump and check valve. When the system is turned off, there is pressurized fluid inside the accumulator. The check valve will block flow from the accumulator, preventing the reverse rotation of the pump.
You can observe the pump shaft or electric motor fan to verify that the check valve is good. Please note that all systems using an accumulator should have a method of bleeding the hydraulic pressure down to zero psi when the system is turned off.
Figure 3. In some systems,
one pump is used as a backup or spare,
with each having a check valve at the outlet port.
Prevent Oil Flow from the Online Pump to the Offline Pump
On many systems, one pump is used as a backup or spare (Figure 3). Each pump will have a check valve at the pump outlet port. The check valve will block flow from the online pump to the offline pump, preventing reverse rotation.
I remember being called into a papermill that kept losing one of the two pumps on its chemi-washer drives. The shaft seal of one pump continually blew out. When the mill ran out of spares, personnel had to ship their last pump by air freight to the factory in New York.
The timeline was so critical due to downtime costs that the pump was still warm when they received it back from the factory. Just prior to installing the pump, we removed the check valve in the case drain line and found it stuck in the closed position. This prevented the oil in the pump case from draining, which resulted in blowing out the seal.
Frequently, a check valve is used for pressure control. A common application is to employ it as a relief valve to protect a heat exchanger (as shown in Figure 4). In this case, the spring rating is usually 65-100 psi.
If the oil is cold, the inlet pressure to the cooler may reach the check valve’s rating. The check valve will then open and direct the pump volume around the cooler. A check valve will also provide protection for an air-type heat exchanger if the tubes become contaminated.
Figure 4. A check valve may also be
used as a relief valve to
protect a heat exchanger.
A few years ago while teaching a class at a sawmill, I observed the students doing their hands-on exercises on the edger. Although a check valve was shown on the schematic to protect the air cooler, the lines to the check valve were plugged off. I asked one of the mechanics about it. He said the check valve was taken off years ago and that they had changed the cooler the week before because of ruptured tubes.
When troubleshooting hydraulic systems, most everyone looks for something large to be the problem, such as a pump, valve or cylinder, but every component has a function. Be sure you understand the purpose of the check valves in your systems.
Read more on hydraulic system best practices:
10 Hydraulic Reliability Checks You Probably Aren't Making
The Seven Most Common Hydraulic Equipment Mistakes
How Do You Know if You're Using the Right Hydraulic Oil?
Top 5 Hydraulic Mistakes and Best Solutions
About the Author
Hydraulic machinery
Type of machine that uses liquid fluid power to perform work
This article is about power machinery. For civil engineering concerning water management, see Hydraulics
"Hydraulic equipment" redirects here. For exercise equipment using hydraulic cylinders for resistance, see Resistance training
open center hydraulic circuit.
A simplehydraulic circuit.
An excavator ; main hydraulics: Boom cylinders, swing drive, cooler fan, and trackdrive
Fundamental features of using hydraulics compared to mechanics for force and torque increase/decrease in a transmission.
Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are a common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine and becomes pressurized according to the resistance present. The fluid is controlled directly or automatically by control valves and distributed through hoses, tubes, or pipes.
Hydraulic systems, like pneumatic systems, are based on Pascal's law which states that any pressure applied to a fluid inside a closed system will transmit that pressure equally everywhere and in all directions. A hydraulic system uses an incompressible liquid as its fluid, rather than a compressible gas.
The popularity of hydraulic machinery is due to the very large amount of power that can be transferred through small tubes and flexible hoses, the high power density and a wide array of actuators that can make use of this power, and the huge multiplication of forces that can be achieved by applying pressures over relatively large areas. One drawback, compared to machines using gears and shafts, is that any transmission of power results in some losses due to resistance of fluid flow through the piping.
History
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Joseph Bramah patented the hydraulic press in .[1] While working at Bramah's shop, Henry Maudslay suggested a cup leather packing.[2][clarification needed] Because it produced superior results, the hydraulic press eventually displaced the steam hammer for metal forging.[3]
To supply large-scale power that was impractical for individual steam engines, central station hydraulic systems were developed. Hydraulic power was used to operate cranes and other machinery in British ports and elsewhere in Europe. The largest hydraulic system was in London. Hydraulic power was used extensively in Bessemer steel production. Hydraulic power was also used for elevators, to operate canal locks and rotating sections of bridges.[1][3] Some of these systems remained in use well into the twentieth century.
Harry Franklin Vickers was called the "Father of Industrial Hydraulics" by ASME.[why?]
Force and torque multiplication
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A fundamental feature of hydraulic systems is the ability to apply force or torque multiplication in an easy way, independent 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 cases, hydraulic ratios are combined with a mechanical force or torque ratio for optimum machine designs such as boom movements and track drives for an excavator.
Examples
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Two hydraulic cylinders interconnected
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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 lbf because C2 is a hundred times larger in area (S = πr²) 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.
Pump and motor
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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 one-tenth of the torque then available at the motor shaft, but the shaft speed (rev/min) for the motor is also only one-tenth of the pump shaft speed. This combination is actually the same type of force multiplication as the cylinder example, 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
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A hydraulic circuit is a system comprising an interconnected set of discrete components that transport liquid. The purpose of this system may be to control where fluid flows (as in a network of tubes of coolant in a thermodynamic system) or to control fluid pressure (as in hydraulic amplifiers). For example, hydraulic machinery uses hydraulic circuits (in which hydraulic fluid is pushed, under pressure, through hydraulic pumps, pipes, tubes, hoses, hydraulic motors, hydraulic cylinders, and so on) to move heavy loads. The approach of describing a fluid system in terms of discrete components is inspired by the success of electrical circuit theory. Just as electric circuit theory works when elements are discrete and linear, hydraulic circuit theory works best when the elements (passive components such as pipes or transmission lines or active components such as power packs or pumps) are discrete and linear. This usually means that hydraulic circuit analysis works best for long, thin tubes with discrete pumps, as found in chemical process flow systems or microscale devices.[4][5][6]
The circuit comprises the following components:
- Active components
- Transmission lines
- Passive components
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 that 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 the tank and the fluid is not pumped to 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 the 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.
Open loop and closed loop circuits
Open loop circuits
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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". Open center circuits use pumps which supply a continuous flow. The flow is returned to the tank through the control valve's open center; that is, when the control valve is centered, it provides an open return path to the 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 the 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 loop circuits
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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 gear pump) 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 losses in the motor housing from rotating effects and losses in the ball bearings can be considerable as motor speeds will reach - rev/min or even more at maximum vehicle speed. 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 200-250 bar is recommended.
Closed loop systems in mobile equipment are generally used for the transmission as an alternative to mechanical and hydrodynamic (converter) transmissions. The advantage is a stepless gear ratio (continuously variable speed/torque) and a more flexible control of the gear ratio depending on the load and operating conditions. The hydrostatic transmission is generally limited to around 200 kW maximum power, as the total cost gets too high at higher power compared to a hydrodynamic transmission. 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
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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 systems
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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), 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, 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.
Load-sensing systems
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Load-sensing systems (LS) generate less power losses as the pump can reduce both flow and pressure to match the load requirements, but require 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 :
For more information, please visit hydraulic check valves.
P o w e r l o s s = Δ p L S ⋅ Q t o t {\displaystyle Powerloss=\Delta p_{LS}\cdot Q_{tot}}
The average Δ p L S {\displaystyle \Delta p_{LS}} is around 2 MPa (290 psi). 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. load pressure + ΔpLS) x sum of flow.
Five basic types of load sensing systems
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- Load sensing without compensators in the directional valves. Hydraulically controlled LS pump.
- Load sensing with up-stream compensator for each connected directional valve. Hydraulically controlled LS pump.
- Load sensing with down-stream compensator for each connected directional valve. Hydraulically controlled LS pump.
- Load sensing with a combination of up-stream and down-stream compensators. Hydraulically controlled LS pump.
- Load sensing with synchronized, both electric controlled pump displacement and electric controlled valve flow area for faster response, increased stability and fewer system losses. This is a new type of LS-system, not yet fully developed.
Technically the down-stream mounted compensator in a valve block 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 two 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 the type (4) system, the functions with up-stream compensators have priority, for example the 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.
Components
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Hydraulic pump
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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 to hydraulic machinery applications are:
- Gear pump: cheap, durable (especially in g-rotor form), simple. Less efficient, because they are constant (fixed) displacement, and mainly suitable for pressures below 20 MPa ( psi).
- Vane pump: cheap and simple, reliable. 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 swashplate causes the pistons to reciprocate a greater or lesser distance per rotation, allowing output flow rate and pressure to be varied (greater displacement angle causes higher flow rate, lower pressure, and vice versa).
- Radial piston pump: normally used for very high pressure at small flows.
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.
Control valves
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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.
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 (25 µm). 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 regulators reduce the supply pressure of hydraulic fluids as needed for various circuits.
- Sequence valves control the sequence of hydraulic circuits; to ensure that one hydraulic cylinder is fully extended before another starts its stroke, for example. Hydraulic circuits can perform a sequence of operations automatically, such as trip-and-reclose three times, then lockout, of an oil-interrupting recloser.[7]
- 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.
- Pilot controlled check valves are one-way valve that can be opened (for both directions) by a foreign pressure signal. For instance if the load should not be held by the check valve anymore. Often the foreign pressure comes from the other pipe that is connected to the motor or cylinder.
- Counterbalance valves are in fact a special type of pilot controlled check valve. Whereas the check valve is open or closed, the counterbalance valve acts a bit like a pilot controlled flow control.
- Cartridge valves are in fact the inner part of a check valve; they 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 in complex hydraulic systems may 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.
Actuators
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- Hydraulic cylinder
- Hydraulic motor (a pump plumbed in reverse); hydraulic motors with axial configuration use swashplates for highly accurate control and also in 'no stop' continuous (360°) precision positioning mechanisms. These are frequently driven by several hydraulic pistons acting in sequence.
- Hydrostatic transmission
- Brakes
Reservoir
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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 and also work as a heat accumulator to cover losses in the system when peak power is used. 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
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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.
Hydraulic fluid
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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
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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. Filters are used from 7 micron to 15 micron depends upon the viscosity grade of hydraulic oil.
Tubes, pipes and hoses
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Hydraulic tubes are seamless steel precision pipes, specially manufactured for hydraulics. The tubes have standard sizes for different pressure ranges, with standard diameters up to 100 mm. The tubes are supplied by manufacturers in lengths of 6 m, cleaned, oiled and plugged. The tubes are interconnected by different types of flanges (especially for the larger sizes and pressures), welding cones/nipples (with o-ring seal), several types of flare connection and by cut-rings. 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 n 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.
Seals, fittings and connections
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Components of a hydraulic system [sources (e.g. pumps), controls (e.g. valves) and actuators (e.g. cylinders)] need connections that will contain and direct the hydraulic fluid without leaking or losing the pressure that makes them work. In some cases, the components can be made to bolt together with fluid paths built-in. In more cases, though, rigid tubing or flexible hoses are used to direct the flow from one component to the next. Each component has entry and exit points for the fluid involved (called ports) sized according to how much fluid is expected to pass through it.
There are a number of standardized methods in use to attach the hose or tube to the component. Some are intended for ease of use and service, others are better for higher system pressures or control of leakage. The most common method, in general, is to provide in each component a female-threaded port, on each hose or tube a female-threaded captive nut, and use a separate adapter fitting with matching male threads to connect the two. This is functional, economical to manufacture, and easy to service.
Fittings serve several purposes;
- To join components with ports of different sizes.
- To bridge different standards; O-ring boss to JIC, or pipe threads to face seal, for example.
- 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.
- To incorporate bulkhead hardware to pass the fluid through an obstructing wall.
- A quick disconnect fitting may be added to a machine without modification of hoses or valves
A typical piece of machinery or heavy equipment may have thousands of sealed connection points and several different 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 fittings, are metal to metal compression seals deformed with a cone nut and pressed into a flare mating.
- Face seal, metal flanges with a groove and o-ring seal are fastened together.
- Beam seals are costly metal to metal seals used primarily in aircraft.
- Swaged seals, tubes are connected with fittings that are swaged permanently in place. 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 more than 6,000 psi (41 MPa) of fluid pressure.
See also
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References and notes
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- Hydraulic Power System Analysis, A. Akers, M. Gassman, & R. Smith, Taylor & Francis, New York, , ISBN 0---9
Pros and Cons of Counterbalance Valves
Posted: August 26th, Author: David Bickford
Principal Engineer, Innovation and Performance Excellence
York Precision Machining & Hydraulics LLC
A guide to optimal safety and productivity
Table of Contents
When load-holding equipment fails, the damage can ruin lives and businesses. Persons injured or killed, property lost, fines, lawsuits and months of terrible publicity, with business lost and brands discredited.
In this article, we're going to discuss load-holding equipment. We're starting here with counterbalance valves (CBV). CBVs help hold a load in place and are also referred to as load-holding valves (LHV). These valves are common safety components of load-carrying equipment. When they operate as expected, they improve the safety of machinery. However, many equipment operators don't want to rest their safety on only the counterbalance valves — and for good reason. Understanding the pros and cons of these valves and knowing other supplemental options to combine with LHVs for safety will give you the ability to safely and productively use equipment that meets your load-holding requirements. In this guide, we'll discuss what load-holding valves are, how they work, their advantages and disadvantages and more.
What Are Counterbalance (Load-Holding) Valves?
A counterbalance, or load-holding, valve is a mechanism typically located near the actuator that uses hydraulic pressure to keep a load from moving. Generally, these valves have three related functions: support, control and safety.
When required for load support, these valves prevent a hydraulic actuator under load from drifting in position. When lowering loads, the counterbalance controls the rate or speed of motion of the load on the actuator.
What Is the Purpose of a Counterbalance Valve?
A hydraulic counterbalance valve controls the actuator in hydraulic systems for overriding or suspended loads. CBVs achieve this goal by creating back pressure at the return line to the actuator. The valve manages the pressure from impacts and loads to:
- Promote load stability.
- Prevent equipment damage.
- Manage hose failure.
- Stop runaway loads.
- Handle induced pressure.
- Manage variable and high back pressure.
- Compensate high system instability.
- Protect the system from burst hoses.
With all of these responsibilities, counterbalance valves are critical for safety. They manage pressure, flows and elements of extreme environments to stabilize loads. Low instability encourages safety and productivity in the hydraulic system.
Counterbalance valves are used in systems that have uncontrolled movements from overrunning loads. Since the pilot-operated check valve cannot control loads, counterbalance valves perform this function to prevent the load from dropping.
How Do Counterbalance Valves Work?
Counterbalance valves are hydraulic devices that function using this basic principle: fluid can freely flow through a check valve into the actuator, and reverse flow will be blocked using a relief valve until a pre-set pressure is reached that is set based on the system pressure and load capability. This pressure is higher than the system pressure when the load is applied and allows the fluid to flow in the opposite direction and the actuator to function. When pressure is removed, the valve goes below this set value, closes, and the load holds its place.
The preset pressure to the pilot port will determine the direction the load can move. To lift a load, the valve allows free flow through the check valve, so the cylinder can extend. When fluid flows to the rod end of the cylinder, this pressure will pilot open the valve, so you can lower the load. This pressure will decrease if the load starts to run away, and the counterbalance valve will adjust to match the cylinder speed to the pump flow.
[Load Holding Valve]: In this illustration, the lines are connected to the hydraulic cylinder and feed the hydraulic fluid to drive the cylinder in extension or retraction. Fluid supplied to the lower end of the cylinder provides force to drive the piston to extend the rod and position the crane boom. Fluid supplied above the piston to the cylinder rod end retracts the piston and rod and lowers the crane boom. The hydraulic system raises and lowers the crane boom to position the load over the location the load is to be lifted or lowered. If the hydraulic system has a failure, the boom will descend and the load would land on whatever it is elevated over, causing injury and damage to people and property.
What Types of Equipment Use Counterbalance Valves?
Many forms of equipment use load-holding valves in their overhead lift hydraulics systems, such as:
Advantages of Counterbalance Valves
Load-holding valves offer some advantages for systems that use them. These benefits of counterbalance valves have made them a popular component of hydraulic systems across numerous sectors. Everything from the military to amusement parks use load-holding valves to provide function and safety to their equipment. Their main advantages are:
1. Safety Through Load Control
The most well-known attribute of load-holding valves is the measure of safety they offer to devices equipped with them. By automatically regulating the descent of loads and holding equipment steady while lifted, these valves are meant to protect workers in the area.
2. Simplified Design
The simple design of load-holding valves allows for multiple variations to accommodate individual needs. For systems with numerous hydraulic lifts, installing two-stage or restricting load-holding valves gives greater control over the entire system, while allowing for machinery that does more things.
Disadvantages of Counterbalance Valves
Load-holding valves also come with some serious shortcomings. You may want to replace or supplement your valve with solutions to compensate for these disadvantages, which include:
1. Not Fail-Safe, Limited Safety
The main disadvantage of counterbalance valves is that they are not fail-safe. In other words, they will protect your workers and equipment until they don't. If a valve opens too rapidly, you can experience instability. In the worst cases, your equipment could fail entirely, and your load could come crashing down. This typically occurs when the valve gets stuck in the open position. Also, CBVs are dependent on other hydraulic circuit components. Using a CBV as a primary safety means can mean that a failure in most points in the circuit, could cause a load to come down, regardless of the integrity of the CBVs. So their safety is dependent on the system, not simply their stand alone presence.
If you're trusting a counterbalance valve to keep a heavy load from damaging equipment and products or from hurting employees, you should plan for the worst and supplement it--with a locking device that is actually fail-safe.
2. Requires Checks and Adjustments
Check Valves require adjusting and checking the preset position. If the system loading changes it may necessitate the CBV to be adjusted. External pilot-operated load-holding valves don't need constant adjustments whenever you change loads, but other types of counterbalance valves do. This added step ensures control of the load every time and helps to maintain the valve's integrity.
Unlimited Safety: Why It Pays To Use A Fail-Safe Locking Device
Because load-holding valves or other components in a hydraulic circuit can fail, you need a supplementary safety accessory to prevent injuries or deaths on load-holding equipment. A mechanical locking mechanism, such as the Bear-Loc®, can be a valuable replacement or addition to any piece of load-bearing equipment. Bear-Loc® offers some major benefits, including:
· Reliability:The Bear-Loc® is a positive locking device that locks instantly and automatically when hydraulic pressure is lost accidentally or on purpose. For the person, property or equipment that's situated under a high or heavy load, that's a priceless, life-saving assurance.
· Durability: The key to Bear-Loc’s effectiveness is interference fit: it is composed of a rod and liners enclosed in a sleeve which forms an interference fit with the outside diameter of the rod.Because the Bear-Loc® can move freely when hydraulic pressure is applied, components do not experience wear as other locking devices do.
· Versatility: Bear-Loc® also features infinite position locking, zero backlash and high system stiffness when required. The Bear-Loc® works on large and small equipment, with customization options to lock up to four million pounds, accommodate sleeves from one inch to seven feet, and fit rods from one inch to 27 inches across.
Given all these features, and because the Bear-Loc® does not depend on valves, moving parts or other components to obtain its positive mechanical lock, it is the most reliable, positive, fail-safe rod locking device in the industry. The Bear-Loc® can work solo or in conjunction with existing equipment to keep people, equipment and projects safe and to optimize productivity.
How Do You Select a Counterbalance Valve?
The market has many options for counterbalance valves. When looking for a CBV, it is important to understand your options and choose a valve designed for your application.
Follow these steps to choose a hydraulic counterbalance valve for your application:
- Consider a basic CBV: Many applications are compatible with basic counterbalance valves. They are a cost-effective solution for static and dynamic loads. For example, a cherry picker is compatible with a basic CBV.
- Select a pilot ratio: Another factor to consider is the pilot ratio. High pilot ratios are best for consistent and stable loads, and low pilot ratios work for unstable and variable loads.
- Choose a relief function: CBVs have two types of relief functions — direct acting and differential area. Direct acting valves are stable, whereas differential area valves are designed for high-flow applications.
- Determine if you have a closed center directional valve: Some machines, like the wheel loader, have a closed center directional valve and counterbalanced valves. This application is not ideal for basic CBVs as they would close when the pressure builds up.
- Consider the back pressure: Some equipment, like cranes, creates constantly changing back pressure. Systems like these require a counterbalance valve that can adapt to the system conditions as they change.
- Look for hose burst protection: While all counterbalance valves protect from burst hoses, some CBVs are designed for additional protection. Special CBVs for hose burst work with the directional valve to control motion. Excavators commonly have this type of counterbalance valve to control the flow and pressure in the cylinders.
Stepping Up Safety for Your Hydraulic System
Below is a side by side comparison of a crane boom hydraulic system. The red outline in each indicates the scale of the number of components necessary to properly function for safety dependency. You can see that the Bear-Loc® can isolate safety down to dependency on itself and removes many other components that have potential to degrade or fail out of the safety equation.
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