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CFD Increases Reliability for Caterpillar
Posted Mon July 08, 2002 @10:23AM
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Application Hydraulics is playing an increasingly important role in the performance of heavy equipment and as such, the reliability and effectiveness of hydraulic systems is becoming paramount. Caterpillar was the first company to investigate and use hydraulic systems within the engines of its larger vehicles. Most recently, the company has been using computational fluid dynamics (CFD) to make vital adjustments to some of its hydraulic systems.

Caterpillar aims to design and manufacture superior hydraulic components, supported by high quality system maintenance and service, and its vehicles are renowned for their performance and reliability. So when it became clear that there was a problem with some of its larger vehicles, Caterpillar pulled out all the stops to find a solution.

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Priyatosh Barman, Six Sigma black belt at Caterpillar explains: "Some dealers had informed us that certain hydraulic pumps on the larger vehicles were failing before their expected shelf life. Usually, the larger the machine, the more analytical sophistication is required to ensure reliability so we turned to CFD (computational fluid dynamics) to try to isolate the problem and prevent its recurring."

When the engineers investigated, they found that there was too much air in the flow through the hydraulic pump, causing it to fail more frequently and preventing the vehicle from functioning to maximum capacity. The over-aerated flow was coming in from another part of the hydraulic system, a partially filled tank which receives the return flow of the fluid from the implement pump. The flow arrives into the tank at high speed and had previously been slowed using perforated rectangular baffles. It was found that although these baffles were to a certain extent calming the flow and weakening its circulation, a high level of aeration was still occurring, leading to the pumpís failure.

The over-aerated flow was being carried, via suction lines, to both the implement and fan pumps, increasing the chance of cavitation in both pumps. Once this had been identified as the source of the problem STAR-CD, CFD simulation software from The CD adapco Group was used to find ways to calm the strong fluid circulation inside the tank; reducing aeration and further problems.

Hydraulic simulation is not traditionally assisted by CFD, dealing as it does with fluid pressure rather than flow, but the Caterpillar design engineers could see that in this case, CFD might offer them a more detailed look at the problem, as well as help to design the solution.

"We started by using STAR-CD to look at exactly what the fluid was doing inside the tanks. The software allowed us to predict the fluid circulation within the tank and to find the fluid pressure and speed distribution for standard flow rates," continues Mr Barman. Using this information to create a CFD model, Caterpillar was able to examine all the conditions within the tank, including the details of the perforated baffles, and identify how each affected the flow around the system.

Mr Barman continues: "As well as modelling the way in which the established solution was working, we were also able to model three alternative designs in order to find the optimum solution to this problem." The options included a variety of baffle designs. Perforations were added or removed on the CFD model and the results recorded. "STAR-CD was able to predict significant differences of flow circulation and other flow parameters, in each case giving us an accurate picture of the most appropriate design," says Mr Barman. "The various models allowed us to look at what wasnít working properly and how we could improve the design of the tank and baffles in order to avoid any further problems with the hydraulic system."

The size of the flow circulation was found to have an impact on the level of aeration in the tank and comparisons made using the STAR-CD software demonstrated the optimum elements of each design. The software led the designers to a solution that would not necessarily have been found without its help. To solve the problem, the baffles were surrounded with a porous circular tube. This design was found to distribute the fluid jets from the baffle holes in a more uniform manner, decreasing the flow circulation within the hydraulic tank.

A further improvement was discovered during the simulation process: as well as making the flow more uniform, the porosity of the baffles was found to play a major role in controlling the pressure within the tank.

"Using the STAR-CD software gave us three major benefits which were invaluable in finding the best solution to this problem," concludes Mr Barman. "The software allowed us to find and address the problem quickly, to spend very little time on physical prototypes and, in the end, less money was needed to resolve the problem and improve the overall design of the tank."

Baseline velocity contours
Velocity contours of the baseline model.

Preferred velocity contours
Velocity contours of prefered model.

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