Tucson, AZ-When a tree falls in the forest, it can
do more than make noise. If it lands on a piece of mobile equipment, it can
rupture the hydraulic lines that link the machine's cab to its hydraulic
actuators. It can also crush the hydraulic valves, spilling gallons of oil onto
the ground and necessitating a costly clean-up. Worse, when the clean-up is
finished, maintenance personnel must figure out how to repair the equipment on
site, without easy access to all the necessary parts.
Up to now, mobile equipment users have considered such mishaps an
unfortunate--but almost unavoidable--cost of working on site. Accidents, after
all, happen.
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An expandable, two-quart rubber reservoir surrounds the pump, motor,
and gearbox. |
Maybe, though, accidents needn't be so catastrophic. At least, that's the
thinking behind a pair of revolutionary new hydraulic actuators built by Sargent
Controls' CEI Division, Oakhurst, CA. The actuators take a new approach to the
protection of hydraulic valves, fittings, reservoirs, and high pressure lines.
Instead of asking mobile equipment manufacturers to shield those components
behind hunks of sheet metal, the Controls Division engineers have moved the
components to a safer location: within the actuator itself.
That's a dramatic departure from the business-as-usual approach of hydraulic
parts manufacturers, which have traditionally built cylinders, valves,
reservoirs, fittings, and pressure lines as separate components. But by
integrating them into a single unit, CEI may have solved a multitude of problems
for mobile equipment manufacturers and users. If, for example, a tree now falls
on a backhoe, it won't inflict the kind of hydraulic damage that it once would
have. Why? Because the hydraulic lines will no longer run the 20 or 30 feet from
the machine's cab to its actuators. Instead, they'll be contained inside the
actuator's control head, along with the valve, pump, motor, reservoir and
fittings.
That means less damage for users and simplicity for manufacturers. "The
design engineer no longer has to worry about pumps, pipes, or reservoirs," notes
Don Porter, a Sargent Controls consultant and co-inventor of the system. "He
doesn't have to worry about installing those parts or protecting them. He
doesn't have to worry about space constraints. And the user doesn't have to
worry about tightening fittings or maintaining the system. It's all
self-contained."
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ORIGINAL SIZE: This is the length of the traditional valve sleeve
using O-rings, 6.237 inches long. |
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REDESIGNED SIZE: This is the length of the sleeve without the O-rings,
3.725 inches long. |
Packaging challenge. For engineers at Sargent Controls' CEI
Division, the first step in creating such a product was to incorporate the
hydraulic valves in the actuator. They included five in all--two pilot-operated
check valves, two solenoid-operated pilot valves, and a 40 gallon- per-minute
main stage control valve. All five valves, along with their controls, are packed
into a control head measuring about 5 x5 x 6 inches. The control head is rigidly
attached to the cylinder.
To accomplish all that, engineers overcame numerous packaging challenges. The
first was the obvious issue of the main stage control valve. Most such valves
would be too long to fit into such a confined area. But the Controls Division
engineers solved the problem by employing a control valve with a shrink fit
sleeve that uses no O-rings. By doing so, they eliminated as much as 2.5 inches
from the length of the valve. That, they say, was just enough to fit the valve
inside the head.
Designers of the valve say that the O-ring-less configuration has been used
in previous valve designs. But the concept wasn't popularized because users
prefer to replace the lap assembly when the valve wears out, and that's a
problem for O-ring-less designs. Sargent Controls engineers say they solved the
problem, however, through the use of special heat treatments for the lap
assembly. The heat treatment enabled them to create a lap assembly that never
needs replacement.
Equally important was the design of the hydraulic passages in the control
head itself. Because the space was so confined, engineers employed a
Pro/ENGINEER CAD system (Parametric Technology Corp., Waltham, MA) to lay out
the configuration of hydraulic passages. "We had to make sure we didn't have any
intersecting passages," Porter notes. "And the best way to accomplish that was
to do it in CAD first, rather than make mistakes in the hardware."
Initially, engineers also hoped to employ off-the-shelf pilot-operated check
valves in the control head. They soon found, however, that those, too, were
either too long or too fat. Conventional valves, they learned, were about 50%
too large in most cases. As a result, the staff used its own servovalve
expertise to shorten the fluid passages in the company's pilot-operated check
valves. Result: a smaller, easier-to-fit pilot-operated check valve.
To facilitate simplicity of manufacturing, engineers also laid out all the
intricate hydraulic passages so that all were perpendicular with a face of the
controls head. As a result, none of the passages required angled drilling.
"Straight holes are easier to drill," Porter says. "And none of this is any good
if the plant can't build it."
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Sargent controls engineers used a direct-acting design to reduce the
size of the pilot-operated check valve. |
Step two. The entire system, known as the SMART Cylinder, is
the first step in the creation of a nearly-autonomous hydraulic actuator.
The next step, known as the CHAMP cylinder, packs even more functionality
into the package. In the CHAMP, engineers have incorporated the valves, lines,
reservoir, rotary piston pump, brushless dc motor and planetary gearbox.
In essence, the CHAMP solves a problem that even the SMART cylinder cannot
solve. If a tree, or some other heavy object, severs the supply or return lines
that lead to the cylinder that's still a problem for the SMART Cylinder because
the cylinder is hydraulically linked to the machine's hydraulic pump.
In the CHAMP, however, all hydraulic linkages to the machine's cab are
eliminated. Instead of hydraulic links, the CHAMP Cylinder is connected to the
joysticks in the cab by electrical wiring. That's possible because each actuator
is an autonomous hydraulic system, so it needs only an electrical signal from
the cab to activate it.
The CHAMP Cylinder also differs from the SMART Cylinder in size. The
cylindrically shaped unit measures about 5 inches in diameter and about 9 inches
long, making it considerably larger than its predecessor.
That extra volume is consumed by the rotary piston pump and the system that
powers it. A brushless dc motor from BEI/Kimco Magnetics Div., San Marcos, CA,
supplies the power for the pump. It operates through a planetary gearbox, which
provides 4:1 speed reduction.
Moving the entire hydraulic system inboard also meant that engineers needed
to deal with another problem: what to do with the hydraulic oil. As with any
system, hydraulic oil from the cylinder must be held in a reservoir when the
cylinder's rod retracts. In these cylinders, however, there was little room for
a reservoir.
Still, the Controls Division engineers squeezed a two-quart reservoir into
the head. The molded rubber reservoir, which is expandable, surrounds the motor,
gearbox, and pump. Engineers say that the two-quart volume was sufficiently
large because, unlike conventional system reservoirs, it didn't need to hold
extra oil in long stretches of tubing.
Mobile system solution. The firm's engineers envision the
SMART Cylinder and CHAMP Cylinder being used in a wide variety of applications,
including backhoes, wheel loaders, cranes, forklifts, dump trucks, refuse trucks
and agricultural machines. In all those applications, they say, the new
cylinders offer advantages. "Even if you crush it, there's very little oil
loss," notes engineer Wayne Shapiro, Sargent Controls business development
manager and a co-inventor of the system. "The only oil you lose is in the
reservoir and the cylinder itself. There are no long lines and no remote
reservoirs." That's particularly important, they say, because oil clean-ups can
be fantastically expensive.
What's more, the new cylinders help address fluid contamination, an issue of
growing importance to manufacturers and users alike. Because the system is
completely closed, engineers say, there's less potential for adding water and
dirt to the oil. "Most reservoirs have a cap for adding oil," Porter says. "But
that's also an entranceway for contamination. Here, the water and dirt can't get
in, because it's sealed." When oil needs to be replenished, Porter says, users
can do it through a tiny charging valve.
The biggest advantages of all, however, may accrue to equipment
manufacturers. With the CHAMP Cylinder, OEM engineers no longer need to worry
about laying out hundreds of feet of hydraulic tubing, or adding other hydraulic
components. "When you first look at it, you might think this cylinder takes up
more space," Porter says. "But when you consider that it replaces the entire
hydraulic system, you realize that it's smaller, lighter, and uses less oil than
conventional designs because you don't have the long lines of tubing."
DESIGN CHALLENGES
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Traditional main-stage control valves too long. Solution: Shrink-fit sleeve
with no O-rings.
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Potential for intersecting hydraulic passages because space in the control
head so small. Solution: Design passages with CAD software.
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Traditional check valves too long or too fat. Solution: Engineers used
servovalve expertise to shorten fluid passages.
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With hydraulic system moving inboard, what to do with the hydraulic oil.
Solution: Molded-rubber reservoir surrounding motor, gearbox, and pump to hold
two quarts of oil.
OTHER APPLICATIONS
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Logging equipment
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Backhoes
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Wheel loaders
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Cranes
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Forklifts
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Garbage trucks
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Dump trucks
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Waste gates
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Flood gates
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