What is in this article?:
- Fluid power safety in the workplace, part 6
- Example 3: mobile directional control valve
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.
Example 3: mobile directional control valve
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.
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:
• 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!
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.