Odd-Shaped Seal Stops Random Leaks

Canal Winchester, Ohio, expect near-perfect reliability from their Piggyback construction forklifts. Therefore, it was perplexing — and disturbing — when occasionally and without warning, a Piggyback would blow the drive hose to the front-wheel-drive motor. Hydraulic leaks are construction-equipment pariahs because, among other reasons, oil can contaminate the ground and shut down a work site. “It was frustrating,” said Bill Pohl, general manager at Princeton. The vehicles used premium hose and fittings assembled to exacting specifications, yet the problem occurred about once a month, he explains. A brand new, correctly manufactured and thoroughly checked Piggyback would leave the plant, enter service and, within days, develop a leak that shuts down the machine. “The lost time, repair, and cleanup all cost money and upset the customer,” says Pohl. “In our business, reliability is everything.” Initial analysis of the breakdowns eliminated the hose design, hydraulic circuit, and machine construction as culprits. Each unit, which has about 200 hydraulic connections, appeared to be in excellent condition — dry and tight — when it left the plant. Yet after only about 30 hr of operation, the 0.75-in. hose in the 5,000-psi hydrostatic drive circuit would occasionally fail. Eventually, technicians pinpointed missing or pinched O-rings in the suspect fitting — a situation likely caused by the difficult-to-reach assembly area of the tightly aligned hose connections. When Princeton confirmed that random seal loss or failure was the cause, engineers at Parker Hannifin’s Tube Fittings Div., Columbus, Ohio, recommended a different O-ring that would virtually eliminate the possibility of seals misaligned or popping out during assembly. Parker’s Trap-Seal, a trapezoidalshaped O-ring, exerts a positive retaining force in the O-ring face seal (ORFS) fitting groove that prevents it from falling out or getting pinched in the connection. Compared with conventional O-rings, Trap-Seal has a larger overlap between the