20, 1998 Design News
DESIGN APPLICATIONS From the regional editors
Flared tube fitting stops hydraulic
Maintains seal between beveled
edge of fitting and flared tube
Charles J. Murray, Senior Regional
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.
Conventional flared tube fittings employ a
solid body. When the fitting engagement loses
its pre-load, the seal between the flare and
bevel can leak. The new design employs a small
piston that moves independently from the rest
of the fitting. If the fitting?s pre-load
relaxes, spring force pushes the piston?s
bevel against the flared tube, maintaining the
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
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,"
Additional details?Contact Al Niemiec,
Box 551, Almont, MI 48003.