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.
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.
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.
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.