DN Staff

May 13, 2010

5 Min Read
Hydro-Mechanical Torque Control

Today's diesels used in mid-size, off-road andconstruction equipment are clean, efficient power sources, but they are farfrom indestructible. Stalling a turbocharged diesel is to be avoided at allcosts and carries with it a high probability of cooking the turboA-chargerlubricant. It may also cause the unit to self-destruct when the engine isre-started, a reason why engine builders include a mandatory cool-down periodin the normal shutdown sequence.Since most functions on these types of machinesare hydraulically powered, OEMs have turned to hydraulic suppliA-ers forsolutions. One approach that has been successful on large equipment is the useof a pump equipped with an autoA-matic hydro-mechanical torque control system tolimit the amount of torque extracted from the engine.

"The idea here is to limit the torque at thepump shaft across a wide range of engine speeds," says Todd McIntyre, productmanager for Eaton Corp. "It's the mirror image of a power control sysA-tem thatlimits the amount of power at a fixed speed. The OEM essentially has to choosewhich engine parameter they want to optimize, and if it's torque then this isthe answer."

"For compact and mid-size equipment it isalmost always torque that needs to be controlled," McIntyre adds. "The problemwas that, until now, such a sysA-tem was not readily available on pumps in thissize range."

An example of the pump's impact is in atelehandler application with engine specifications of 50 hp, 2,650 rpm and1,194 inch-lb of torque. Designed withA-out torque control, in order to liftwith 3,000 psi, the design requires a 2.50 cid (41 cc) 420 pump and flow islimited to 29 gpm - even for light loads.

By using torque control, the engineer is able to specify a 3.80cid pump, and the larger displacement allows greater flow (faster operation) ofthe bucket when empty or not lifting with 3,000 psi. Engaging torque controland de-stroking the pump to 2.50 cid preA-vents engine stalls with heavy loads.

The new Eaton 420 Series pump with hydro-mechanical torquecontrol is essentially an enhanced version of a stanA-dard pressure-compensatedpump using a mechanical feedback loop to control swash plate position. Insimple terms, the pressure compensator setting is varied according to swashplate position rather than acting at a single set point. Since torque is afunction of pressure and disA-placement, torque is now held constant. Themechanical feedback determines the amount of flow the pump can deliver at agiven pressure, because the pressure determines the swash plate angle.

"In practical terms, that means the 420 pump will not extractmore than a fixed amount of torque from the engine," McIntyre says. "An addedbenefit is faster function speeds when the work load is low because the flowrate changes."

He cites the example of how raising a backhoe bucket filled withconcrete might happen slowly due to the torque control setting protectingagainst engine stall. But if the operator empties the bucket, it will movefaster because the flow rate will be higher. "Curling a bucket at 20 gpm canrequire 10 to 15 more engine horsepower than curling the same bucket at 17 gpmon a small machine," he says.

The same amount of engine torque is available in both casesbecause the pump automatically limits the torque used no matter what thedemand. It will not stall the engine, regardless of what the operaA-tion mayrequire.

Based on customer input, McIntyre says Eaton engineers chose toutilize a mechanical feedback system for the new 420 pump. "The mechanicalsystem provides the functionality required by today's equipment at a lower costthan a comparable electronic solution," McIntyre notes. He says the nextgeneration of machines will almost certainly need electronic controls. But bythen, new emissions requirements will be in place and the engines will alreadyhave extensive electronic management systems.

"It makes good sense to tie the pump control into the samesystem," he adds. "It's yet another example of the emerA-gence ofelectro-hydraulic solutions as the wave of the future."

For the present, though, Eaton's hyA-dro-mechanical solutionoffers a number of advantages to OEMs in addition to controlling torque. Insome cases, it can help them keep their equipment from migrating into operatingareas covered by tighter emission regulations.

"At the moment, a piece of equipA-ment with a 68 hp engine fallsunder a different set of emission requirements than the same piece of equipmentwould with a 75 hp engine," McIntyre says. "If the OEM can provide thenecessary machine functionality with the smaller engine by using a 420 pump tocontrol torque, they potentially can reduce their exposure to additional costsassociated with more stringent standards."

This factor will likely become increasA-ingly important as newregulatory requireA-ments impact the power density of diesels. For example,being able to stay with the smaller engine could mean avoiding the cost ofre-designing engine mounts, coolA-ing arrangements and engine compartA-mentgeometry.

"Beyond that, the pumps themselves are more durable becausebearing loads are constant across the whole operating range. That translatesinto longer bearing life which directly impacts both operatA-ing and maintenancecosts," McIntyre says. "They also are more efficient because it is relativelyeasy to achieve an optimum corner horsepower where both pressure and flow aremaximized simultaA-neously for maximum output."

While the concept of hydro-mechaniA-cal torque control is notnew, the ability to package the necessary functions in a pump small enough tomeet the needs of compact and mid-size equipment required a good deal ofengineering. In addition to hydro-mechanical torque control, the new "B" design420 Series pump also offers a more robust casting and the option of a low-noisecompensator.

Later this year, Eaton plans to introduce a new 620 pump (98cc,280 bar continuA-ous) for moderate flow, high-pressure-applications. It isreported 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 andprovides improved reliability, 28 percent fewer parts and a proven comA-pensator.Target markets include wheel loaders, motor graders, rail and highwaymaintenance and excavators.

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