Many incidents that occur when working on mobile hydraulic machinery are careless accidents, such as slips and falls. "Fortunately, you don't hear a lot about the hydraulic injuries," says Scott Kane, global platform manager - mobile, Parker Hannifin Corp. "But when they do occur, they are very severe."
Hydraulic injuries occur mainly due to the failure to relieve pressure prior to working on a hydraulic system. "It is guys disconnecting a hose or cracking a seal... while there is still pressure in the system. Then they end up getting sprayed," says Kane.
A lot of potential energy is stored in the hydraulic system. According to Dennis Kemper, account application engineer, Gates Corp., hydrostatic systems can produce pressures as high as 8,000 psi and temperatures up to 250° F. Work groups can generate pressures up to 4,000 psi and temperatures up to 212° F. Drain or return line hoses don't have much pressure, but still have temperatures up to 275° F.
A ruptured hose releases this energy. "Unless you have ever seen a hydraulic hose rupture under 5,000 psi, you have no idea what kind of potential energy is there," says Kane. "People take it for granted."
There is also the possibility for serious burns. "Allow the system to cool to ambient temperature before performing any work on it," advises Raymond Wilkins, chief engineer, fluid conveyance, Eaton's Hydraulics Operations.
Because of the potential risks, technicians need to be qualified. "Repair personnel should be trained in safety procedures specific to hydraulics before working on hydraulic systems," says Wilkins.
According to Wilkins, the most generic safety procedures include ensuring cylinder-supported machine components have been blocked and/or properly restrained, then pressure relieved from the system and all cylinders, in accordance with applicable safety standards and practices. "Exactly how these steps are accomplished depends on the details of the particular system being repaired, and should be spelled out in detail in the OEM repair manual and written procedures for the equipment in question," he adds.
It's critical to avoid shortcuts. For example, Kane notes that most manufacturers of cylinders for mobile applications incorporate a counterbalance valve. The function of the valve is to prevent the cylinder from dropping if the hose breaks. "That counter balance locks everything out," he states.
But the valve is not intended to be relied on as a lockout device. "I have seen guys who have the cylinder in a raised position, and when they do maintenance, they will assume the counterbalance valve will hold the cylinder up," Kane comments. "I am not going to stick my head in there and rely on a counterbalance valve. If you have to have something extended, block it, chain it, pin it up."
Watch for clues
Hydraulic hoses often give you visual clues of impending failure. "Cover abrasion, a cracked or missing rubber cover, bent or kinked hose, oily wet hose cover and blistered cover are all signs of hose damage that may lead to failure," says Wilkins.
Of course, it isn't possible to spot all potential failures. "The obvious problems ? wear or slight leaks ? are easy to identify, but some of the failures are from within," says Kemper. "Pinholes or bursts occur because of wear of the wire or due to a bend too close to the stem. You will never see them until the hose fails."
This places an emphasis on preventive maintenance. "Hose assemblies can last a long time but not indefinitely, so a regular PM program is needed," says Kemper. "We have seen people try to use hoses for 15 years or more. While they got away with it, others were not so lucky, and that is when serious injuries occur."
"Hose is an elastomeric product," Kane adds. "It is like a belt on your car. It is going to wear out."
During a routine inspection of the hoses, Kemper recommends looking for high abrasion or wear; tight bends that exceed the minimum bend radius recommended by the hose assembly manufacturer; and areas where the hose is bent quickly from the stem. This could cause pinholes and premature failures.