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Flared tube fitting stops hydraulic leakage
Maintains seal between beveled edge of fitting and flared tube
By Charles J. Murray -- Design News, July 20, 1998
Almont, MI--Hydraulic leakage: In most plants, it's an annoyance--a problem that draws attention but doesn't warrant immediate repair.
For those who judge the drip-drip-drip of a leaky fitting to be little more than an inconvenience, however, consider this: A fitting that leaks a drop per ten seconds loses about 40 gallons of hydraulic oil per year. A drop every second leaks 400 gallons per year. And a big assembly plant with 2,500 such fittings loses about 1 million gallons per year.
Fortunately, one engineer has devised a leak-free version of a particular type of connector: the flared tube fitting. Al Niemiec, a consulting engineer, developed a spring-loaded technique that addresses the leakage between the flared end of the tube and the beveled edge of the fitting.
Niemiec's solution solves a longstanding problem. In most such fittings, he says, the seal between the flare and the bevel leaks. The reasons for such leaks are numerous: expansion and contraction caused by thermal cycling, as well as pressure surges, line straightening, and cyclic loading. "Any of those things can create additional forces on the mating parts," Niemiec says. "And that relaxes the force engagement on the threads." Usually, he says, the relaxation of the force engagement is subtle--but it's enough to create a gap in the seal.
That's why Niemiec created the new flared tube fitting. It distinguishes itself through its use of a separate and independent piston with a beveled edge. The piston fits within the fitting but because it is independent, it doesn't necessarily move with it. As a result, when the nut backs off, the piston doesn't follow. Instead, it maintains contact with the flared tube.
To accomplish that, the piston works in conjunction with a spring. If the threaded nut backs off, the spring is allowed to expand. When it does, it forces the piston forward. As a result, the beveled edge of the piston never loses contact with the flare, and the seal is maintained.
Niemiec's patented design actually relies on two forces to maintain the seal. The user supplies the first force by pre-loading the spring during assembly of the fitting. The hydraulic system pressure supplies the second force. Since the piston is independent and free to move with respect to the rest of the fitting, it can take advantage of the hydrostatic force that bears against its back surface. As a result, Niemiec says his fitting can supply "several hundred pounds of sealing force" in a system operating at 1,000 psi.
Niemiec says that the design could help save thousands of gallons of hydraulic oil in large assembly plants, particularly those that employ injection molding machines. Such machines, he says, often use several hundreds of hydraulic fittings, many of which leak. "Once the manufacturer has started production, they are not going to shut down their machinery to take care of a small leak," he says. "Usually, they wait until a scheduled plant shutdown."
The new fitting could also help cut oil disposal costs. "The dripping oil usually gets flushed into a sump," Niemiec says. "And when the sludge from that sump has oil in it, it can be very costly to dispose of it."
Niemiec says that the new design could serve in many kinds of production machinery, and in automotive applications, such as brakes and power steering units. It is particularly well-suited for difficult-to-access applications, even in commercial plumbing, he says.
Thus far, the new design has been prototyped, but has not been produced in volume. Tests, however, have proven that the fitting applies continuous pressure between the bevel and flare. "No matter how much thermal expansion and contraction you have, the piston will always travel enough to maintain engagement of the seal," Niemiec says.
Additional details...Contact Al Niemiec, Box 551, Almont, MI 48003.
Other Applications
- Hydrostatic fan drives
- Automotive power steering
- Injection molding machines
- Commercial plumbing
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