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Fluid power safety in the workplace, part 6

Fluid power safety in the workplace, part 6

This is part 6 in a series on the importance of following good safety protocol in fluid power system maintenance and design. It highlights real-life examples of the dangers and injuries that can occur and provides advice on preventing them. Find part 1 here; part 2 here; part 3 here; part 4 here; and part 5 here.

Fluid power components are misapplied for many reasons. In some cases, it is human error. In others, it is poor design. However, in the majority of cases, it is simply a lack of training and education.

Although some components are forgiving if misapplied, others are not so forgiving. The not-so-forgiving components are capable of causing far-reaching devastation — accidents that can result in severe injury, death, substantial property damage, or all of these. It is not uncommon to hear stories about misapplied components that literally “exploded like a bomb,” sending shards of metal scattering in all directions.

Correct component selection, application, and installation is serious business. Therefore, engineers, designers, assembly-line personnel, maintenance personnel, and safety personnel must be appropriately trained. Following are a few examples of the cause and effect of component misapplication.

Figure 1. Arrangement of two pilot-operated check valves on this cylinder do not allow for thermal expansion of oil — which can result in failure of rod gland’s retaining ring.

Example 1: pilot-operated check valve
A company that designs and builds drill rigs gave a recently graduated engineer the task of designing the mast assembly for a prototype drilling rig. The design included a pair of cylinders to lift the heavy mast from the horizontal (travel) position, into the vertical (work) position.

The young engineer had received no formal training in fluid power, so he asked for help from the company’s local fluid power distributor’s sales engineer. Together they designed the valve circuitry and sized the components needed to do the job. Upon completion, the machine was moved into the yard for testing. The test went according to plan, and everything appeared to operate normally. The mast was left in the raised position, and the prototype team left for lunch.

Upon their return, they found the twisted remains of the mast lying on the ground. During an investigation they discovered that the gland had blown out of one of the mast support cylinders, which is why the mast collapsed.

They focused their investigation of the accident on the cylinder rod gland and found that the rod-gland retaining ring had sheared, but they couldn’t determine why. They enlisted the help of the cylinder manufacturer’s applications engineer to help them with root-cause analysis. In the end, it was determined that the misapplication of a pilot-operated check valve caused the failure.

When the mast was in the raised position, the cylinder rod was fully extended. The oil in the cap end of the cylinder was trapped by the pilot-operated check valve, A in Figure 1. As heat from the sun baked the cylinders, the oil heated up and expanded. (A rule-of-thumb for the temperature/pressure relationship holds that for each 1° F the oil is heated, the pressure increases by about 50 to 60 psi.) However, with nowhere for the oil to go, the expansion increased pressure in the cylinder to create a force high enough to shear the retaining ring. The misapplication of a pilot-operated check valve, in this case, could have resulted in severe injury and/or multiple deaths, had the prototype personnel not left for lunch.

An investigation into the cause of the failure determined that the following conditions contributed to or caused the accident:

• neither the engineer, the salesperson, nor the technician were properly trained to design or work on a hydraulic system,
• the incorrect valve was installed on the machine, and
• although the rig was equipped with mast locks, the locks were not used by the prototype team.

The correct component in this case would have been either a pilot-operated check valve with integral thermal expansion capability, or a counterbalance valve.

Figure 2. Top, incorrect spool blocks all ports in neutral position, preventing the counterbalance valve from operating properly. Bottom, float-center configuration allows counterbalance valve to tame pressure spikes.

Example 2: counterbalance valve and directional control valve
The maintenance and engineering personnel at a sawmill, along with the machine manufacturer, were confounded at the fact that one particular hose on a certain machine had a history of bursting unexpectedly. The hose supplier determined that the problem was caused by a combination of extreme pressure spikes and violent hose whipping. While this problem caused undue production losses, the plant personnel were particularly concerned for the safety of the people who worked on and around the machine. A burst hose could leave them vulnerable to severe burn or skin-penetration injuries.

The saw mill finally asked for help from a fluid power design consultant to help solve the problem. After asking a number of questions, the consultant physically inspected the machine. He also reviewed the design of the system and spent some time comparing the components on the machine with the components on the circuit schematic.

He discovered that the problem was caused by the misapplication of a directional control valve that operated the cylinder to which the problem hose was connected.

The dynamics of the load, cylinder operation, and inertia were considered when the machine was originally designed. To tame the anticipated pressure spikes, a counterbalance valve was installed in series with the transmission line between the cylinder and the directional control valve. The counterbalance valve, if working properly, prevented excessively high pressure spikes by permitting controlled deceleration of a load.

However, the selection of the neutral (center) configuration of the directional control valve plays a critical role in the ability of a counterbalance valve to do its job. Thus, the neutral position of the directional control valve must provide an unrestricted flow path from the discharge port of the counterbalance valve to tank. It must also be capable of connecting the discharge port of the counterbalance valve to the opposite end of the cylinder to prevent cavitation.

In this case, the designer chose a directional control valve with an incorrect spool configuration. He or she chose a spool configuration that simply blocked all the ports in the neutral position, thus preventing the counterbalance valve from doing the job it is designed to do, Figure 2.

The consultant recommended that they replace the closed-center configured directional control valve with a float-center configured valve, and this corrected the problem.

Example 3: mobile directional control valve

Figure 3. The two directional valves are connected in series.

An engineer, along with a maintenance mechanic, were given the task of adding an additional implement to an existing machine. To get the implement to work, they had to install a new cylinder and a new directional control valve. They made their first error when they connected the two valves (the existing directional control valve and the new valve) in parallel.

When they started the machine, neither system operated because both valves were open center. They resorted to connecting the two directional control valves in series, Figure 3. This simply meant that they disconnected the tank return (discharge) transmission line from the first valve, and connected it directly to the inlet port of the new valve.

They then connected the existing return line to the tank port of the new valve. When testing the machine, everything appeared to work well.

A few weeks later, the mechanic was summoned to the machine because it had unexpectedly broken down. He arrived on the scene to find that two of the three tie-rod bolts (the bolts that hold the directional control valve sections together) on the first (original) valve were broken. He assumed that the only dynamic that could cause this type of failure was an immense pressure spike. Thus, he wrote it off as an operational problem.

He repaired the valve, and a few days later the same problem occurred. Only this time the operator wasn’t so lucky — he suffered burns to his face and hands from the hot oil as it sprayed out of the valve when the tie-rods broke.

Figure 4. The pressure settings of valves compound when they are connected in series.

The technician contacted the equipment dealer’s representative. It did not take him long to figure out the cause of the problem — the valves were connected in-series without power-beyond. Power-beyond essentially splits a valve’s internal passages into two: one passage connects the pressure relief valve to the tank port, and the other extends the pump flow to the second valve.

It is common practice to place mobile directional control valves in series. However, it is critical that every valve but the last one has power-beyond capability. Whenever pressure relief valves are connected in series, the pressure settings of the valves compound, Figure 4. The reason for the interval between installing the valve and the respective failure: the operator had to stall both circuits simultaneously for the failure to occur.

Example 4: directional control valve
A component manufacturer was experiencing problems with an automatic production machine. While the hydraulic system was idle (pump running) the piston rod (horizontal mount) on a transfer unit cylinder was creeping out and disrupting the cycle. As a result, the operator had to constantly select the manual override position and reset the machine. This created an unacceptable loss of production time. Maintenance personnel tried everything they could think of to correct the situation, including replacing the cylinder and the directional control valve.

They asked their local fluid power distributor for any suggestions. The distributor’s sales technician reviewed the problem and advised them to add a pilot-operated check valve at the cylinder’s rod-end port. This suggestion did not solve the problem, and the cylinder continued to creep.

They finally asked for assistance from one of the company’s engineers. It turned out that the directional control valve was misapplied. When a pressure-compensated pump is used in conjunction with a single-rod, double-acting cylinder, there are a few items that need to be taken into consideration:

Figure 5. Cylinder creep can be the result of the differing areas on either side of the piston.

• rod-to-bore ratio,
• whether or not the cylinder rod is under constant load,
• compensation pressure setting, and
• duration of cylinder idle time.

When the pressure at the inlet port of a directional control valve is constant, the oil will leak across the spool, and the pressure will equalize on both sides of the cylinder piston. However, because the areas on both sides of the piston are unequal (due to the rod), the force tending to extend the piston rod is greater that tending to retract it. The net result: the cylinder creeps, Figure 5.

The original directional control valve had a closed center spool configuration — this was incorrect. The problem was rectified by installing a valve with a float-center configuration, along with a pilot-operated check valve.

Although the misapplication of hydraulic components is a frequent and serious problem, there are many engineers who are excellent fluid power system designers who seldom, if ever, make errors when selecting fluid power components. When designing and engineering fluid power systems and machinery, you must avoid experimentation and trial-and error methods. The consequences of component misapplication can lead to severe injury, death, substantial property damage, or all of these.

Misapplication also contributes to the loss of tens of thousands of production and manpower hours and millions of gallons of oil that are wasted each year.

Choose components wisely. If you are not sure, ask. Component manufacturers generally have excellent applications engineers — use them!

Rory McLaren is president, Fluid Power Training Institute, Salt Lake City. For more information, call (801) 908-5456, email [email protected], or visit

Caution: Rory McLaren and the Fluid Power Training Institute do everything possible to ensure that the information and drawings contained in these reports are accurate and that the suggested procedures are deemed safe and reliable. However, these are general recommendations only and might not be applicable to all situations. You must have your engineering and service departments read these recommendations and make the necessary changes for your specific conditions.

The Fluid Power Training Institute is not responsible for actions taken by untrained or unauthorized persons. All hydraulic system service, repair, and troubleshooting should be conducted only by trained, authorized personnel.

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