hydraulic system

Part 3 of 3: Hydraulic Troubleshooting Guide 101

Here’s the final part of our 3 part series on hydraulic trouble shooting 101.

STEP 5 – Relief Valve…

If the test in STEP 3 has indicated the trouble to be in the relief valve, point D, the quickest remedy is to replace the valve with one known to be good. The faulty valve may later be disassembled for inspection and cleaning.

Pilot-operated relief valves have small orifices which may be blocked with accumulations of dirt. Blow out all passages with an air hose and run a small wire through orifices.

Check also for free movement of the spool. In a relief valve with pipe thread connections in the body, the spool may bind if pipe fittings are over-tightened. If possible, test the spool for bind before unscrewing threaded connections from the body, or screw in fittings tightly during inspection of the valve.

STEP 6 – Cylinder…

If the pump will deliver full pressure when operating across the relief valve in STEP 2, both pump and relief valve can be considered good, and the trouble is further downstream. The cylinder should be tested first for worn-out or defective packing by the method described in our guide “Cylinder and Valve Testing”. Other Components…

Check other components such as bypass flow controls, hydraulic motors, etc. Solenoid 4-way valves of the pilot-operated type with tandem or open center spools may not have sufficient pilot pressure to shift the spool.

If you still have problems…

If you still have questions or problems after trying to troubleshoot your hydraulic system, feel free to give us a call and have one of our hydraulic specialists come and give you a hand.

Craig Cook

Part 2 Of Hydraulic Troubleshooting Guide 101

Here’s part 2 of our 3 part series on hydraulic trouble shooting 101.

STEP 3 – Pump or Relief Valve…

If high pressure cannot be obtained in STEP 2 by running the pump against the relief valve, further testing must be conducted to see whether the fault lies in the pump or in the relief valve. Proceed as follows: If possible, disconnect the reservoir return line from the relief valve at point H. Attach a short length of hose to the relief valve outlet. Hold the open end of this hose over the reservoir filler opening so the rate of oil flow can be observed. Start the pump and run the relief valve adjustment up and down while observing the flow through the hose.

If the pump is bad, there will probably be a full stream of oil when the relief adjustment is backed off, but this flow will diminish or stop as the adjustment is increased. If a flowmeter is available, the flow can be measured and compared with the pump catalog rating. If a flowmeter is not available, the rate of flow on small pumps can be measured by discharging the hose into a bucket while timing with a watch.

For example if a volume of 10 gallons is collected in 15 seconds, the pumping rate is 40 GPM, etc. If the gauge pressure does not rise above a low value, say 100 PSI, and if the volume of flow does not substantially decrease as teh relief valve adjustment is tightened, the relief valve is probably at fault and should be cleaned or replaced as instructed in STEP 5.

If the oil substantially decreases as the relief valve adjustment is tightened, and if only a low or moderate pressure can be developed, this indicates trouble in the pump. Proceed to STEP 4.

STEP 4 – Pump…

If a full stream of oil is not obtained in STEP 3, or if the stream diminishes as the relief valve adjustment is tightened, the pump is probably at fault. Assuming that the suction strainer has already been cleaned and the inlet plumbing has been examined for air leaks, as in STEP 1, the oil is slipping across the pumping elements inside the pump. This can mean a worn-out pump, or too high an oil temperature.

High slippage in the pump will cause the pump to run considerably hotter than the oil reservoir temperature. In normal operation, with a good pump, the pump case will probably run about 20F above the reservoir temperature.

If greater than this, excess slippage, caused by wear, may be the cause. check also for slipping belts, sheared shaft pin or key, broken shaft, broken coupling, or loosened set screw.

I’ll give you step 5 and 6 in my next post.

Craig Cook

Part 1 Of Hydraulic Troubleshooting Guide 101

Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.

Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system, other components could also be at fault, but this would require the services of an experienced technician.

By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, and then if necessary, each component in that area can be tested or replaced.

STEP 1 – Pump Suction Strainer

Probably the trouble encountered most often is cavitation of the hydraulic pump inlet caused by restriction due to a dirt build-up on the suction strainer. This can happen on a new as well as an older system. It produces the symptoms described above: increased pump noise, loss of high pressure and/or speed. If the strainer is not located in the pump suction line it will be found immersed below the oil level in the reservoir (point A).

Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump. The suction strainer should be removed for inspection and should be cleaned before re-installation. Wire mesh strainers can best be cleaned with an air hose, blowing from inside out. They can slso be washed in a solvent which is compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum base hydraulic oil. Do not use gasoline or other explosive or flammable solvents.

The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced. When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints (points B, E, G, H, J, and K). There must be no air leaks in the suction line.

Check the reservoir oil level to be sure it covers the top of the strainer by at least 3″ at minimum oil level, with all cylinders extended. If it does not cover to this depth there is danger of a vortex forming which may allow air to enter the system when the pump is running.

STEP 2 – Pump and Relief Valve

If cleaning the pump suction strainer does not correct the trouble, isolate the pump and relief valve from the rest of the circuit by disconnecting at point E so that only the pump, relief valve, and pressure gauge remain in the pump circuit. Cap or plug both ends of the plumbing which was disconnected. The pump is now deadheaded into the relief valve.

Start the pump and watch for pressure build-up on the gauge while tightening the adjustment on the relief valve. If full pressure can be developed, obviously the pump and relief valve are operating correctly, and the trouble is to be found further down the line. If full pressure cannot be developed in this test, continue with step 3.

Step 3 and 4 will be included in next week’s post. Stay tuned.

Craig Cook

Fluid Power Safety Products

Fluid Power Safety 101

Safety is a critical aspect to any fluid power system, not just from the basic level of keeping components plumbed properly, but also in overall levels of machine safeguarding. It is critical to evaluate the entire system, including the electrical portion, to minimize exposure to unnecessary risk. Systems are rated based on the weakest link in the control chain. a Several standards (including ISO 13849-1:2006, ANSI/ASSE Z244.1-2003 (R2008) and ANSI/PMMI B155.1-2011) define the control system as including input, sensing, and interlock devices as well as output devices such as pneumatic and hydraulic valves. a The function of a fluid control valve mimics that of an electrical-control relay and, therefore, is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including:

Must be functionally redundant.
Must be monitored for faults (including diminished performance faults, which may create the loss of redundancy), without depending on external machine controls or safety circuitry.
Must return to a safe position in the event of a loss of pressure or other such event.
Must be able to inhibit further operation upon detection of a fault condition until such condition is corrected.
Should have a dedicated, specific function-reset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power.
Must not automatically reset.

Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the other two elements handle the stop function (energy release). Many self-designed systems risk having hidden, potential flaws, which can lead to unsafe conditions because they are unseen, unexpected and, therefore, excluded from design and safety reviews. A good example is the spool cross-over conditions or ghost positions of a valve, which are usually not shown on schematics. a

Two general abnormal conditions can affect valve safety. The first is similar to an electrical-control fault, such as when a relay might be stuck in the open or closed position. The second is when a valve develops diminished performance, as when a valve becomes sticky or sluggish. In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.19-2010 Standard mandates a monitoring system that detects these conditions for critical applications and the ANSI/PMMI B155.1-2011Standard requires diminished performance monitoring if stopping time can be affected. An easy solution is to use a self-monitoring, Category-3 or -4 valve, designed to detect both conditions. a

The use of double valves remained relatively unheard of for many years except in a few select industries, such as stamping presses, which first initiated control reliability requirements. Double valves provide dual internal functions (redundancy) so that an abnormal function of one side of the valve does not interfere with the overall normal operation. At the same time, the double valves sense abnormal operation on either side of the valve and then inhibit further operation until the problem has been corrected and the valve deliberately reset. This sensing and inhibiting function is commonly referred to as monitoring. a

Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as does a double valve critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting. In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve actuates abnormally, the second one continues to function and redundancy is lost. The circuit doesn’t recognize lost redundancy nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up and control integrity no longer exists. a

Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue. Typical applications include E-stop, two-hand-control, light curtains, safety gates, pneumatic locking devices for safety gates, hydraulic brakes, air brakes, amusement rides, hoists, elevators, pinch-point applications, or any other application where control system integrity depends on valve operation. a

Craig Cook

How air is getting into your pressurized hydraulic system?

Air typically enters the hydraulic system through the the pump inlet and, under certain conditions, past the rod seal of a double-acting cylinder.
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But air can also invade the system through joints in pressurized plumbing.
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When fluid travels through a pipe or hose at relatively high velocity – in a pressure line for example, and has to change direction through a tee or elbow, a venturi effect can be created.
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Because the sealing arrangement of the hydraulic connector is designed to withstand positive pressure – but not negative pressure, air can be drawn into the system – even when the plumbing has no apparent leaks.
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If you made a glass model of a pipe elbow and connected a measuring point in the middle of the angle, you would see a negative pressure when fluid passed through the elbow at high velocity.
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And if you looked carefully, you’d likely see air bubbles entering the system through the seal of the measuring connection.
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What it comes down to is use as few sharp angles – tee-pieces, elbows, etc in hydraulic plumbing as possible. a

Craig Cook

Cost of Air Leaks

Air leaks are much easier to ignore than oil leaks because they don’t draw attention to themselves in the same way.

You don’t need to worry yourself with clean-up and disposal costs. Contaminant ingression is possible, but is generally not a major concern.

And unless the leak is significant, safety is not usually a big issue either. So that leaves make-up fluid (air).

While air is free – clean, dry compressed air is NOT.

In considering the cost of make-up air for a pneumatic system the following need to be considered:

Depreciation (wear and tear) of the compressor; Conditioning costs – filtration, drying and lubrication; and Energy cost of compression.

The ideal leakage rate is of course zero, but when calculating the free-air delivery (FAD) required by a pneumatic system a rule of thumb is to allow for leakage of 10% of total flow rate.

Consider a 10 cubic meter/minute system leaking 10% or 1 cubic meter/minute. The power required to compress 1 cubic meter (35.3 cubic feet) of air per minute to a pressure of 6 bar (90 PSI) is approximately 5.2 kilowatts. At an electricity cost of $0.10 per kilowatt-hour this leakage is costing over 50 cents per hour in electricity consumption alone.

In a 24/7/365 operation that amounts to $4500 per year!

While a leakage rate of 10% of flow rate may sound high and would be unsustainable in a hydraulic system, air leakage rates as high as 25% are not unheard of even in apparently well maintained pneumatic systems!

As demonstrated by the above example, the annual cost of air leaks in pneumatic systems can be significant in power consumption alone.

Craig Cook

Monitoring Filter Condition

Let’s change gears and discuss the benefits of monitoring the condition of your hydraulic filter elements.

Warning of filter-bypass is typically afforded by visual or electric clogging-indicators. These devices indicate when pressure drop (delta P) across the element is approaching the opening pressure of the bypass valve (where fitted).

In the case of a return filter for example, if the bypass opens at a delta P of 3 Bar, the clogging indicator will typically switch at 2 Bar.

Replacing standard clogging-indicators with differential pressure gauges or transducers enables continuous monitoring of the filter element.

Think of it in terms of the engine coolant temperature on your car or truck. Would you prefer to have a gauge on the dashboard that allows you to monitor coolant temperature continuously or a light which only glow when the engine has already overheated?

Similarly, continuous monitoring of the filter elements in a hydraulic system can provide valuable clues to the performance of the filter and the condition of the system.

This allows trending of fluid cleanliness against filter pressure-drop, which may be used to optimize oil sample and filter change intervals. For example, the optimal change for a return filter in a particular system could be higher or lower than switching pressure of the clogging indicator.

Continuous monitoring may also provide early warning of component failures and element rupture. For example, if the delta P across a pressure filter suddenly increased from 1 to 3 Bar (all other things equal), this could be an indication of an imminent failure of a component upstream.

Similarly, a sudden decrease in delta P could indicate a rupture in the element – something a clogging indicator will never alert you to.

Craig Cook

Simple Explanation of Seals

There are numerous sealing products, technologies, and techniques to keep oil in and contaminants out of the hydraulic system, each providing unique benefits. Further, in some harsh applications, the features of individual seals do not fulfill the demanding requirements for the equipment and a sealing system is required.

Here are the basics of seal designs:

Wipers:

Provide aggressive wiping force to prevent mud, water, dirt and other contamination
Allow lubricating oil film to return to system on inward stroke
Protect main sealing elements, thus increasing life of seals
Are often made from Polyurethane, which offers high abrasion resistance
Are often used as a linkage pin grease seal

Rod seals:

Contain system fluid from escaping to atmosphere
Must provide sealing function at low and high pressure
Require excellent extrusion and wear resistance
Should provide good pump-back capability for lubricating oil film
Often must withstand up to 6000 psi

Buffer seals:

Must withstand high pressure exposure
Protect the rod seal against pressure spikes
Pressure relieving capability prevents pressure build-up between seals
Increase rod seal life
Allow for wider extrusion gaps
Require high wear resistance

Wear bands:

Prevent contact between metal parts in the cylinder
Center rod and piston from housing elements
Increase seal life

Craig Cook

Stopping the “Water Hammer” in Your Hydraulic System

In last week’s email, we talked about decompression and how to control it.

Let’s wrap up the topic of noise by talking about a phenomenon known as ‘water hammer’.

Water hammer is the term used to describe the effect that occurs when the velocity of the fluid moving through a pipe suddenly changes.

Sudden change in fluid velocity causes a pressure wave to propagate within the pipe. Under certain conditions, this pressure wave can create a banging noise, similar to that you would expect to hear when beating a pipe with a hammer. Hence the phrase.

Not surprisingly, common symptoms of this problem are high noise levels, vibration and broken pipes.

When a moving column of fluid hits a solid boundary – when a directional control valve closes suddenly for example, its velocity drops to zero and the fluid column deforms, within the rigid cross-sectional area of the pipe, to absorb the energy associated with its motion – similar to a car hitting a concrete wall.

However unlike a car, the fluid is almost incompressible so the deformation is small and a store of energy accumulates in the fluid – similar to compression of a spring.

The magnitude of the pressure rise that results from the subsequent release of this stored energy can be expressed mathematically as follows:

Pr = P + u p c

Where P is initial pressure, u and p are initial fluid velocity and density respectively and c is the speed of sound through the fluid.

Accumulators and other damping devices are sometimes installed in an effort to deal with this problem. However, the significance of the pressure rise equation shown above is that fluid velocity is the only variable that can be altered to address the root cause.

Put simply, reducing the velocity of the fluid column that hits the solid boundary, reduces the magnitude of the subsequent pressure rise.

Returning to the traffic crash analogy – the slower the car is traveling when it hits the wall, the less damage is caused.

In hydraulics, the easiest way to do this – on paper at least, is to increase the diameter of the pipe. This reduces fluid velocity for a given flow rate.

The other alternative is to control deceleration of the fluid column by choking valve switching time to the point where the pump’s pressure compensator and/or system relief valve reacts fast enough to reduce flow rate through the pipe and therefore velocity of the fluid.

Craig Cook

Another Reason For Noise In Your System That Can Give You Problems

There is another intermittent and problematic source of noise in hydraulic systems – decompression.

This problem arises because hydraulic oil is NOT incompressible. The ratio of a fluid’s decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity.

The bulk modulus for hydrocarbon-based hydraulic fluids is approximately 250,000 PSI, (17,240 bar) which results in a volume change of around 0.4% per 1,000 PSI (70 bar).

When the change in volume exceeds 10 cubic inches (160 cubic centimeters) decompression must be controlled.

The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work.

If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path.

Uncontrolled decompression stresses hydraulic hose, pipe and fittings. It creates noise and can cause pressure transients that can damage hydraulic components.

Decompression is an inherent problem in hydraulic presses for example, due to the large volume cylinders operating at high pressures.

Although hydrocarbon-based hydraulic fluids compress 0.4% – 0.5% by volume per 1,000 PSI, in actual application it is wise to calculate compression at 1% per 1,000 PSI. This compensates for the elasticity of the cylinder and conductors and a possible increase in the volume of air entrained in the fluid.

For example, if the combined captive volume of the hydraulic cylinder and conductors on a press was 10 gallons and operating pressure was 5,000 PSI, the volume of compressed fluid would be 0.5 gallons (10 x 0.01 x 5).

This equates to potential energy of around 33,000 watt-seconds. If the release of this amount of energy is not controlled, you can expect to hear a bang!

Decompression is controlled by converting the potential energy of the compressed fluid into heat. This is achieved by metering the compressed volume of fluid across an orifice.

Craig Cook