10 Hydraulic Reliability Checks You Probably Aren't Making

When most people think of preventive maintenance and reliability procedures for a hydraulic system, regularly changing filters and checking the oil level are the only things considered. When the machine fails, there is often very little information about the system to refer to when troubleshooting. However, proper reliability checks should be done with the system running under normal operating conditions. These checks are vital for preventing equipment failures and downtime.

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Inspecting the Filter Bypass Valve

Most hydraulic filter assemblies have a bypass check valve to prevent damage in the event the element becomes plugged with contamination. This valve will open whenever the differential pressure across the filter reaches the spring rating of the valve (usually 25 to 90 pounds per square inch, depending on the filter design). When these valves fail, they normally fail in the open condition due to contamination or mechanical malfunction. When this occurs, the oil will bypass the element without being filtered. This will lead to premature failure of the downstream components.

In many cases, this valve can be removed from the housing and inspected for wear and contamination. Consult the filter manufacturer's documentation for the specific location of this valve and the proper removal and inspection procedures. This valve should be inspected periodically when the filter assembly is being serviced. 

Hose Condition

Leakage is one of the biggest problems in a hydraulic system. Proper hose assembly and replacement of defective hoses is one of the best ways to reduce leakage and prevent unnecessary downtime. Hoses should be inspected regularly for leakage and deterioration. Hoses that show wear on the outer jackets or leakage at the hose ends should be replaced as soon as possible. A hose that has "blisters" indicates a failure of the inner lining of the hose, allowing the oil to leak through the metal braid and collect under the outer jacket.

Hose Length

Whenever possible, hose length should not exceed 4-6 feet. Excessive hose length will increase the possibility of the hose rubbing against other hoses, a catwalk or a beam. This will lead to premature hose failure. In addition, when a pressure spike occurs in the system, the hose can absorb some of the shock. When this occurs, the hose length can change slightly. The hose should be made long enough so that it has a slight bend to absorb these shock spikes.

Hose Routing

Where possible, hoses should be routed in a way that prevents them from rubbing against each other. This will prevent premature failure of the outer jacket of the hose. In the event it is not possible to route hoses to prevent rubbing, protective sleeves should be used. Several types of sleeves are commercially available for this purpose. Sleeves can also be made by cutting an old hose to length and slitting it lengthwise. The sleeve can be placed over the hose at the rub point. Plastic cable ties should also be used to fasten the hoses together. This prevents relative movement of the hoses at the rub points.

Check for Proper Clamping

Proper hydraulic pipe clamps should be used. Conduit clamps are generally not acceptable for hydraulic lines due to the vibration and pressure spikes inherent in a hydraulic system. Clamps should be inspected regularly for loose mounting bolts. Broken clamps should be replaced. In addition, clamps should be properly spaced. A good rule of thumb is to space clamps approximately 5-8 feet apart and within 6 inches from where the pipe terminates.

Breather Cap Maintenance and Inspection

Breather caps are some of the most neglected items in a hydraulic system, but keep in mind that a breather cap is a filter. The reservoir oil level constantly changes as cylinders extend and retract, and the breather cap (filter) is the first line of defense against contamination. To prevent contamination from entering the reservoir from the outside, a proper breather filter of the appropriate micron rating should be used.

Several manufacturers offer breather filters in the 3-micron range that also remove water from the air by using desiccant materials. The desiccant will change color when saturated with moisture. These filter assemblies will pay for themselves many times over when changed on a regular basis.

Measuring Drive Motor Current Draw

The amount of power required to drive a hydraulic pump is dependent on the system pressure and flow. As the pump wears, the internal bypassing increases due to increased internal clearances. This results in decreased pump output.

With the pump delivering less flow to the system, the power required to drive the pump will decrease proportionally. Therefore, the drive motor current draw will decrease. A record should be made of the current draw when the system is relatively new to establish a baseline reference.

Temperature Checks

As system components wear, the internal clearances increase. This leads to increased bypassing. Whenever this bypassing occurs, heat is generated. This heat does no useful work in the system; therefore, energy is wasted. By using an infrared camera or some other type of heat-detecting device, this bypassing can be found.

Bear in mind that heat is generated whenever a pressure drop occurs, so localized heat will always occur in any device that meters flow, such as a flow control or proportional valve. Making regular inlet and outlet oil temperature checks on heat exchangers will give an indication of the overall heat exchanger's efficiency.

Sound Checks

Regular sound checks should be made, especially with hydraulic pumps. Cavitation occurs whenever the pump cannot get the total amount of oil it is calling for at the suction port. This will produce a steady, high-pitched whining sound. If not corrected, the pump will deliver a reduced output until it destroys itself.

The most common cause of cavitation is a plugged suction strainer. It may also be caused by excessive oil viscosity (low temperature) or excessive drive motor revolutions per minute (RPM). Aeration occurs whenever outside air enters the suction port of the pump. It will produce a more erratic sound. Causes of aeration include an air leak in the suction line, low fluid level or a bad shaft seal on a fixed displacement pump.

Pressure Checks

Pressure checks should be made regularly. This will give an indication of the condition of several components in the system, such as accumulators and various pressure control valves. If the pressure drops more than 200 pounds per square inch (PSI) while the actuators are moving, this may indicate a problem. A record should be made of these pressures when the system is operating normally to establish a baseline reference. 

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About the Author

We all know distributors are a valuable resource for end-users of fluid power products, often providing local, on-site, customer support. Closer to end-users than manufacturers, their customers should be able to rely on that local support.

To help provide that support, knowledge about the products they sell helps to meet the support needs of their customers/end-users. Fluid power distributors can take what they learn about customers&#; specific pumps, motors, and valves, and put that knowledge to use for their clients. If you are a distributor and your end-users can benefit from knowing more about the products you offer, this article is for you.

As pumps and motors are responsible for supplying power and energy within a hydraulic system, valves are the mechanical devices used to control that power by regulating the flow of fluid.

They can be used to redirect pressurized fluid, control the level of flow to a certain area, or completely close a line.

Although there are dozens of varieties that all serve different purposes, in this article, we will focus on these three major types of hydraulic valves:

• Pressure Control Valves
• Flow Control Valves
• Directional Control Valves

Each type has its own functions within a fluid power system and applications in which they provide the ideal result. Each hydraulics brand provides various configuration choices based on valve type. Acquainting yourself with these differences can be useful when offering clients support on valve-related issues.

If you are looking to provide guidance when your customer has a question or a problem, or if they&#;re asking you to consult on purchase decisions more confidently, we recommend starting with basic knowledge of how pressure control valves, flow control valves, and directional flow valves operate.

Pressure Control Valves

Hydraulic pressure control valves are used to regulate the fluid pressure that is passing through hydraulic systems and prevent rises in pressure that may result in system problems. They are also used to maintain a set pressure in a part of a hydraulic circuit.

Different types of pressure control valves include:

Each type has its own functions within a fluid power system and applications in which they provide the ideal result. Each hydraulics brand provides various configuration choices based on valve type. Acquainting yourself with these differences can be useful when offering clients support on valve-related issues.If you are looking to provide guidance when your customer has a question or a problem, or if they&#;re asking you to consult on purchase decisions more confidently, we recommend starting with basic knowledge of how pressure control valves, flow control valves, and directional flow valves operate.Hydraulic pressure control valves are used to regulate the fluid pressure that is passing through hydraulic systems and prevent rises in pressure that may result in system problems. They are also used to maintain a set pressure in a part of a hydraulic circuit.Different types of pressure control valves include:

• Pressure relief valves
• Pressure Reducing valves
• Sequence valves
• Counterbalance valves

Hydraulic relief valves limit system pressure to protect components. The valves also limit the hydraulic system&#;s maximum output force and control the amount of heat generated by balancing the hydraulic force with an adjustable spring force.

Pressure reducing valves are used to balance hydraulic circuit pressure with pressure in the main circuit. When the outlet pressure reaches a preset level, the valve opens, a piston moves to release pressure, and it keeps the outlet pressure constant.

Sequence valves are used to control the sequential operation of actuators. If the inlet pressure exceeds a preset level, they add pressure to the outlet side. A check valve allows for this directional control.

Counterbalance valves are used to maintain hydraulic pressure or load backpressure on a cylinder by releasing flow levels to keep pressure constant. Like sequence valves, they use a check valve to provide a counterbalance of flow, lifting the cylinder up to freely pass.

Flow Control Valves

Hydraulic flow control valves do what they say: They control flow rates in hydraulic circuits, with an end goal of controlling speed elsewhere in a fluid power system.

In hydraulic systems, these valves are used to regulate flow rates to actuators, such as motors and cylinders, and with it the speed of those components.

Flow rate also determines rate of energy transfer at any given pressure. A common physics concept is used to determine this transfer:

Actuator force x distance traveled = work done on load

This equation shows that the actuator force, multiplied by the distance through which it moves (stroke), equals the work done on the load. The energy transferred must also equal the work done, and speed determines the energy transferred.

In other words, regulating speed is the primary function of flow control valves because:

Energy transfer must equal work done, and is determined by multiplying force times stroke
Speed determines the amount of energy transferred
Flow rates determine the speed of an actuator

There are a variety of flow control valve designs, each intended for specific applications. The most common types include ball, diaphragm, needle, and butterfly.

Directional Control Valves

Directional control valves have three main functions: to stop fluid flow, to allow fluid flow, or to change direction of fluid flow between hydraulic cylinders and motors.

They typically consist of a sliding spool inside a cylinder, which restricts or permits fluid flow to control the fluid path. They can also consist as a poppet to control flow, which works in the same way.

Directional control valves can stop the flow suddenly and are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves for the sound they make when operating.

There are two other types of directional valves that can control fluid flow in more complicated hydraulics systems:

1. Proportional valves -Allow infinite positioning of spools and flow volumes for more complex circuits
2. Servo valves - Operate with high accuracy, repeatability, low hysteresis, and high frequency response for sophisticated loop systems (and are often more expensive than proportional valves)

More Product Knowledge to Steer Satisfied Customers

In summary, hydraulic valves provide the control needed to keep hydraulic systems working. They act as the steering wheel to guide the moving parts of a functional hydraulics system.

Hydraulic relief valves limit system pressure to protect components. The valves also limit the hydraulic system&#;s maximum output force and control the amount of heat generated by balancing the hydraulic force with an adjustable spring force.Pressure reducing valves are used to balance hydraulic circuit pressure with pressure in the main circuit. When the outlet pressure reaches a preset level, the valve opens, a piston moves to release pressure, and it keeps the outlet pressure constant.Sequence valves are used to control the sequential operation of actuators. If the inlet pressure exceeds a preset level, they add pressure to the outlet side. A check valve allows for this directional control.Counterbalance valves are used to maintain hydraulic pressure or load backpressure on a cylinder by releasing flow levels to keep pressure constant. Like sequence valves, they use a check valve to provide a counterbalance of flow, lifting the cylinder up to freely pass.Hydraulic flow control valves do what they say: They control flow rates in hydraulic circuits, with an end goal of controlling speed elsewhere in a fluid power system.In hydraulic systems, these valves are used to regulate flow rates to actuators, such as motors and cylinders, and with it the speed of those components.Flow rate also determines rate of energy transfer at any given pressure. A common physics concept is used to determine this transfer:This equation shows that the actuator force, multiplied by the distance through which it moves (stroke), equals the work done on the load. The energy transferred must also equal the work done, and speed determines the energy transferred.In other words, regulating speed is the primary function of flow control valves because:Energy transfer must equal work done, and is determined by multiplying force times strokeSpeed determines the amount of energy transferredFlow rates determine the speed of an actuatorThere are a variety of flow control valve designs, each intended for specific applications. The most common types include ball, diaphragm, needle, and butterfly.Directional control valves have three main functions: to stop fluid flow, to allow fluid flow, or to change direction of fluid flow between hydraulic cylinders and motors.They typically consist of a sliding spool inside a cylinder, which restricts or permits fluid flow to control the fluid path. They can also consist as a poppet to control flow, which works in the same way.Directional control valves can stop the flow suddenly and are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves for the sound they make when operating.There are two other types of directional valves that can control fluid flow in more complicated hydraulics systems:In summary, hydraulic valves provide the control needed to keep hydraulic systems working. They act as the steering wheel to guide the moving parts of a functional hydraulics system.

And whether they are used to control flow by redirecting pressurized fluid (pressure control valves), controlling the level of flow and speed (flow control valves), or completely stopping the directional flow (directional control valves), knowing about different types of valves and their intended function can be a great tool to help you support your customer&#;s individual needs.

Knowing the basics of different components and how they work is the key to offering this guidance, and to provide a valuable resource to your customers / end users.

Knowing the basics of different components and how they work is the key to offering this guidance, and to provide a valuable resource to your customers / end users.

Need more help? Call the HPS Customer Service team at (888) 477- for assistance. We&#;re here to help with your fluid power questions 8 &#; 6 ET, Monday through Friday.