Maintenance troubleshooting best practices for fluid power

Fluid power insights

• Fluid power describes pneumatic and hydraulic systems, which consist of seals, bushings, bearings and other components built into them.
• Maintenance is crucial for pneumatic and hydraulic systems because they are often integrated into many critical aspects of a manufacturing system.

Fluid power is used to describe pneumatic and hydraulic systems. As with power transmission components, there are shafts, seals, bushings and bearings built into pneumatic or hydraulic actuators.

Actuators are used to move or hold products, or to exert a force against it, such as in a press. Pneumatic and hydraulic actuators are typically cylindrical in shape, and so are called cylinders. Cylinders operate by means of the fluid (air or hydraulic) exerting force against a piston attached to a rod. Figure 1 shows a typical cylinder with two ports; when air enters the cap-end port, the cylinder extends, and when it enters the other port and exhausts from the back or cap-end port it retracts. Actuators such as parallel grippers (Figure 2) may also have internal gearing or other mechanisms built in.

Figure 1: Example of a pneumatic cylinder. Courtesy: Automation LLC

The speed and pressure at which the air enters and exits the ports controls the way the cylinder operates. The cross-sectional area and volume of the cylinder also helps determine the force and speed of the rod. The formula for the force exerted by a piston is F = P x A, where P is the air or fluid pressure exposed to the rod, and A is the area of the cylinder face. There are many possible factors that affect this force calculation, consult a good pneumatics reference if exact calculations are needed.

Here is an example of a force calculation for a 1.5” bore air cylinder at 60 lbs/in²:

To find the area, first calculate the radius, 1.5”/2 = 0.75”.

Area = πr² = 3.14159 (0.75 x 0.75) = 1.767

60 psi x 1.767 = 106.0287 lbf (pounds-force)

This can also be converted to Newtons (1 lbf = 4.44822N) or kilograms force (kgf) (1 lbf = 0.453592), so 106.0287 lbf = 471.639N = 48.0938 kgf

Flow controls restrict the air into or out of the ports, this changes the speed of the rod in either the extending or returning direction. Flow controls act differently depending on whether air is being metered in (on the cap end port to extend the cylinder) or metered out (restricting the flow on the rod end while extending the cylinder). It is generally considered a better control technique to meter out, since this is the best way to slow an unloaded cylinder. Flow controls are usually placed on both ports. Just like tightening a screw, turning the flow control clockwise restricts flow more, slowing the actuator.

Figure 2: Example of a parallel gripper mechanism. Courtesy: Automation LLC

Valves are used to turn on and off the airflow to the actuator. There are many different valve configurations available, some have two solenoids to control air to each of the A and B ports and exhaust the other. With a single solenoid valve, the A port is actuated when it is energized, and the B port exhausts. When the solenoid is de-activated a spring returns the valve position pressurizing the B side and exhausting the A side. Some single acting valves are also configured to exhaust both sides when turned off, using a spring on the actuator itself to return the cylinder.

Valves are often arranged in groups on a manifold. This allows multiple devices to be controlled with one centralized air supply and control connection. The control may use a multiconductor cable to carry signals to (and from) the unit, or use a communication protocol to a control unit.

Many valves require a minimum pressure (usually at least 15-20 psi) to shift the position of the valve. Low pressure applications may require more care in valve selection. Piloted valves use a separate higher pressure to shift the valve if the pneumatic circuit’s air is not sufficient.

Fittings and plastic air tubing are used to connect the valves to the actuators. Fittings often thread onto the ports on the cylinder and manifold, and are sometimes built into the valve. Fitting threads are usually wrapped with Teflon tape for a proper seal.

Figure 3: Example of a valve manifold. Courtesy: Automation LLC

Fittings and hose are available in both standard and metric sizes, and with different threads. Hose diameters such as 6mm and 1⁄4” are very close in size and are easily confused. This can make connections looser or tighter than they would be if using the proper size. Adapters are sometimes used to interface from one size of hose or fitting to another. It is important to identify the sizes and threads used in a pneumatic system and not try and force something to fit where it is not designed to.

Fitting threads, hose connections and the hoses themselves can leak air, even when sized correctly. This can usually best be detected by listening for a “hissing” sound. Since all ports are not pressurized at all times, users will have to activate valves to check all of the connections.

Mufflers or silencers reduce noise and prevent foreign matter from entering the exhaust ports. They come in various sizes and threads. They are usually placed on the exhaust ports of valves and are assumed to be present even if not shown.

Note that not all the valves are externally actuated. Needle valves restrict flow in both directions and check valves allow flow in one direction only. The valves themselves may also have diagrams printed on them showing what type of valve it is. Learning to read these symbols is an important part of pneumatic troubleshooting.

The numbers expressed in the designation are the number of ports and the number of positions for the valve. The blocks show the condition of the air flow when each solenoid is energized and when it is in the off position.

The purpose of a 5/3 center blocked valve that has an additional exhaust solenoid on it, sometimes called an “air dump,” is to exhaust residual stored pressure when the rest of the system loses air.

The middle block shows the condition of the flow when the two solenoids are off. The pressure port is labeled 1, and when turned off, the two sides keep the pressure on each side of the piston holding it in position. If the air dump is turned on, the air will escape through the check valves, freeing the cylinder. This is a requirement in some cases for safety, to release the stored energy in the circuit with an emergency stop. The numbering and port designation can differ depending on the brand of valve being used. Consult the manufacturer’s documentation for the product.

Soft starts are 3/2 valves that gradually increase air to downstream components when energized. They are commonly used in industrial applications. This exhausts the air when turned off, also often used with emergency stops. They also prevent systems from “banging” when pressure is applied.

Air preparation is a term that describes the components needed to prevent contamination from solids, water and oil. Plant air from a compressor flows through multiple devices, pipes and fittings that can add particulates, oil and moisture. Even though air dryers, filters, water separators and regulators are often present at the compressor, the air is should also be treated at the machine to prevent damage.

Manual shutoff valves can be used to remove incoming air from the system. They can also be locked for safety purposes.

Filters are used to remove particulates and water from the incoming supply. They have a clear glass or plastic bowl so that the liquid can be seen. Mist Separators are used to remove aerosol state oil mist from the exhaust of process air. They can look similar to the filter shown below, but it may also be large self-contained systems. Regulators reduce and control the incoming pressure for machinery. They use a spring with a diaphragm to allow excess pressure to escape and usually have a gauge attached.

Lubricators were important at one time to lubricate rubber parts like seals inside pneumatic components. Seal materials have evolved over time however and lubricators are usually only required for pneumatic tools and some air motors or air clutches. Most modern machines do not use lubricators. FRLs/FLRs, or filter-regulator-lubricators, are unitary self-contained units that accomplish filtering, pressure regulation and lubrication.

Figure 4: Example of FRL assembly. Courtesy: Automation LLC

Filter regulators include filtering and pressure regulation without lubrication. They can be combined into a single unit.

Five pneumatic adjustments and maintenance tips

Consider these five things that may require attention in a pneumatic system:

1. Supply – Pneumatic systems require clean dry air to operate properly. Ensuring that the plant air is as uncontaminated and dry as possible is an important part of ensuring machines have as long a life expectancy as possible.

2. Air preparation – Set pressure regulators at the manufacturer’s recommended value to ensure proper machine operation. Check filter bowls and drain/clean as necessary. If lubricators are required, ensure the oil reservoirs are filled with the proper lubricant.

3. Actuators and control – Periodically adjust flow controls to ensure proper movement of cylinders and actuators. Faster is not always better, ensure that actuators and products move in a smooth controlled way. There may be flow controls on both ports of air cylinders, remember that metering out (the port used to exhaust the cylinder) is the best way to prevent the jerking movement of a cylinder with no load.

4. Connections – Check fittings and hose connections for leaking air. Rewrap fitting threads with teflon tape and replace hoses or fittings as necessary. Listen for air leaks with solenoids activated in each direction. Solenoids usually have manual activators or buttons to energize them with. Use the manual functions on operator interfaces to test the valves also.

5. Sensing – Set sensors on cylinders and tooling with air applied, centering the sensor over the internal cylinder magnet or tooling. More will be covered on sensors later.

Hydraulics

Hydraulic power is another form of fluid power. Unlike pneumatics however, hydraulic systems require a contained fluid throughout the system. Whereas a pneumatic system can be connected to a plantwide system of compressed air, hydraulic systems require their own pumps and fluid supply.

Figure 5: Example of a filter regulator. Courtesy: Automation LLC

Because pneumatic systems use air which is compressible, there is a delay in actuator movement. With hydraulic systems there is no delay in movement, and the available force is much higher for a similar sized actuator. Where most pneumatic systems operate in the 60-100 psi range, hydraulic systems can provide 1,000 to 5,000 psi or even more.

Many of the components of a hydraulic system are similar to those of a pneumatic system, but there are a few differences. A reservoir of hydraulic fluid (oil) is required. Compressed air can be pulled from the atmosphere, whereas fluid has to be supplied.

The filter is made of different materials than typical air filters. They remove both water and solid contaminants from the system. More hydraulic failures are a result of contamination than any other cause.

A pump is required to provide pressure for the system. This is usually driven by an electric motor, but there are other types, even engines are used for some large systems. A relief valve is used to ensure pressure does not exceed the requirement. Fluid is returned to the reservoir.

An accumulator stores energy to maintain pressure, dampen vibrations and pulsing, and improves the efficiency of the system. They are pre-charged with an inert gas, typically nitrogen. A moveable barrier, usually a piston or rubber bladder separates the oil and gas. The gas is usually pressurized to 80-90% of the working pressure of the system.

Directional valves are often used for moving the actuator. These are usually 4/2 or 4/3 configuration. Because of the force required to shift the spool inside the valve body, these may be pilot-actuated, using a pressurized fluid (often about 50 psig) to move the spool.

Proportional valves are sometimes used to control speeds of actuators. These can be controlled from 0-100% by an analog signal from a programmable logic controller (PLC) or another type of controller.

Pressurizing fluids creates heat, which can break down the hydraulic oil and reduce its life. A hydraulic oil cooler is a heat exchanger that removes heat to the outside air. These may be air cooled or water cooled.

Figure 6: Example of a hydraulic system. Courtesy: Automation LLC

Five hydraulic adjustment and maintenance tips

Many of the maintenance tasks for pneumatic systems apply to hydraulics also. Because the system runs on hydraulic fluid however there are various areas with additional concerns.

1. Prevent the system from overheating – Hydraulic fluid gets hot as it is pushed through pumps, tubing and relief valves. If the temperature is too low, water can condense in the reservoir, if it is not properly removed by the filter it can cause pump cavitation. If the temperature is too high, oxidation causes varnish and sludge deposits that can clog the filters. Typical plant hydraulic system fluids run in the 110 to 150 °F range. Perform regular checks of the oil cooler and monitor the temperature in the reservoir.

2. Keep the system clean – Keep the reservoir covered and ensure the area around drain lines and breather fill openings are kept free of debris. Dirt, water and metal dust or shavings can make their way through these openings. Clean the cap before replacing it on the reservoir. Store hydraulic fluid in a clean environment.

3. Keep fluid clean and test for contaminants – Check and change filters on a regular basis. Filter oil added to the system through portable filters. Add hydraulic fluid of the same brand and viscosity grade as needed. Sample the fluid for color, visible signs of contamination and odor.

4. Six visual and other checks:

   1. Inspect hydraulic hoses, tubing and fittings for leaks and frays.

   2. Inspect the inside of the hydraulic reservoir for signs of aeration. Use a flashlight and into the fill hose for signs of foaming or small whirlpools, these may be a sign of a leak in the suction line or faulty shaft seals.

   3. Check return, pressure and hydraulic filter indicators and pressure gauges against manufacturer’s documentation.

   4. Check system temperature independently using an infrared thermometer, also look for “hot spots” on the motor or proportional valves. High temperatures on the valves can be caused by the valve sticking.

   5. Listen to the pump for unusual noise. Cavitation the formation of bubbles or “cavities” of air in the pump. It is caused by areas of lower pressure around an impeller. It will damage the pump, decrease flow and cause vibration if not treated.

   6. Check breather caps, fill screens and all filters regularly.

5. Hydraulic safety: Tubes and hoses can develop pinholes that could cause a high- pressure stream of fluid. As mentioned previously, the fluid could also be hot. Engineers should never run their hands over a hydraulic line to check for leaks. Use a sheet of paper or cloth with gloves.

– This has been edited from the “Maintenance and Troubleshooting in Industrial Automation” book by Frank Lamb, the founder and owner of Automation Consulting LLC and a member of the Control Engineering editorial advisory board.

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