Today's diesels used in mid-size, off-road and
construction equipment are clean, efficient power sources, but they are far
from indestructible. Stalling a turbocharged diesel is to be avoided at all
costs and carries with it a high probability of cooking the turboA-charger
lubricant. It may also cause the unit to self-destruct when the engine is
re-started, a reason why engine builders include a mandatory cool-down period
in the normal shutdown sequence.
Since most functions on these types of machines
are hydraulically powered, OEMs have turned to hydraulic suppliA-ers for
solutions. One approach that has been successful on large equipment is the use
of a pump equipped with an autoA-matic hydro-mechanical torque control system to
limit the amount of torque extracted from the engine.
"The idea here is to limit the torque at the
pump shaft across a wide range of engine speeds," says Todd McIntyre, product
manager for Eaton Corp. "It's the mirror image of a power control sysA-tem that
limits the amount of power at a fixed speed. The OEM essentially has to choose
which engine parameter they want to optimize, and if it's torque then this is
the answer."
"For compact and mid-size equipment it is
almost always torque that needs to be controlled," McIntyre adds. "The problem
was that, until now, such a sysA-tem was not readily available on pumps in this
size range."
An example of the pump's impact is in a
telehandler application with engine specifications of 50 hp, 2,650 rpm and
1,194 inch-lb of torque. Designed withA-out torque control, in order to lift
with 3,000 psi, the design requires a 2.50 cid (41 cc) 420 pump and flow is
limited to 29 gpm - even for light loads.
By using torque control, the engineer is able to specify a 3.80
cid pump, and the larger displacement allows greater flow (faster operation) of
the bucket when empty or not lifting with 3,000 psi. Engaging torque control
and de-stroking the pump to 2.50 cid preA-vents engine stalls with heavy loads.
The new Eaton 420 Series pump with hydro-mechanical torque
control is essentially an enhanced version of a stanA-dard pressure-compensated
pump using a mechanical feedback loop to control swash plate position. In
simple terms, the pressure compensator setting is varied according to swash
plate position rather than acting at a single set point. Since torque is a
function of pressure and disA-placement, torque is now held constant. The
mechanical feedback determines the amount of flow the pump can deliver at a
given pressure, because the pressure determines the swash plate angle.
"In practical terms, that means the 420 pump will not extract
more than a fixed amount of torque from the engine," McIntyre says. "An added
benefit is faster function speeds when the work load is low because the flow
rate changes."
He cites the example of how raising a backhoe bucket filled with
concrete might happen slowly due to the torque control setting protecting
against engine stall. But if the operator empties the bucket, it will move
faster because the flow rate will be higher. "Curling a bucket at 20 gpm can
require 10 to 15 more engine horsepower than curling the same bucket at 17 gpm
on a small machine," he says.
The same amount of engine torque is available in both cases
because the pump automatically limits the torque used no matter what the
demand. It will not stall the engine, regardless of what the operaA-tion may
require.
Based on customer input, McIntyre says Eaton engineers chose to
utilize a mechanical feedback system for the new 420 pump. "The mechanical
system provides the functionality required by today's equipment at a lower cost
than a comparable electronic solution," McIntyre notes. He says the next
generation of machines will almost certainly need electronic controls. But by
then, new emissions requirements will be in place and the engines will already
have extensive electronic management systems.
"It makes good sense to tie the pump control into the same
system," he adds. "It's yet another example of the emerA-gence of
electro-hydraulic solutions as the wave of the future."
For the present, though, Eaton's hyA-dro-mechanical solution
offers a number of advantages to OEMs in addition to controlling torque. In
some cases, it can help them keep their equipment from migrating into operating
areas covered by tighter emission regulations.
"At the moment, a piece of equipA-ment with a 68 hp engine falls
under a different set of emission requirements than the same piece of equipment
would with a 75 hp engine," McIntyre says. "If the OEM can provide the
necessary machine functionality with the smaller engine by using a 420 pump to
control torque, they potentially can reduce their exposure to additional costs
associated with more stringent standards."
This factor will likely become increasA-ingly important as new
regulatory requireA-ments impact the power density of diesels. For example,
being able to stay with the smaller engine could mean avoiding the cost of
re-designing engine mounts, coolA-ing arrangements and engine compartA-ment
geometry.
"Beyond that, the pumps themselves are more durable because
bearing loads are constant across the whole operating range. That translates
into longer bearing life which directly impacts both operatA-ing and maintenance
costs," McIntyre says. "They also are more efficient because it is relatively
easy to achieve an optimum corner horsepower where both pressure and flow are
maximized simultaA-neously for maximum output."
While the concept of hydro-mechaniA-cal torque control is not
new, the ability to package the necessary functions in a pump small enough to
meet the needs of compact and mid-size equipment required a good deal of
engineering. In addition to hydro-mechanical torque control, the new "B" design
420 Series pump also offers a more robust casting and the option of a low-noise
compensator.
Later this year, Eaton plans to introduce a new 620 pump (98cc,
280 bar continuA-ous) for moderate flow, high-pressure-applications. It is
reported that this pump will provide excellent power density and up to 135 hp
(at 2,200 rpm). The unit is 12-percent shorter than Eaton's PVH pumps and
provides improved reliability, 28 percent fewer parts and a proven comA-pensator.
Target markets include wheel loaders, motor graders, rail and highway
maintenance and excavators.
For more information:
Eaton's piston pumps:
http://designnews.hotims.com/27744-508
Hydro-Mechanical Torque Control
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