High-resolution monitoring speeds bearing press-fitting

A transmission, transfer case or differential may contain four to six bearings that are press fit into place on assembly lines turning out as many as 3,000 transmissions a day, so the problem of automatically finding the exact end point when the part is seated properly is especially widespread in automotive powertrain assembly.

06/01/2007


A transmission, transfer case or differential may contain four to six bearings that are press fit into place on assembly lines turning out as many as 3,000 transmissions a day, so the problem of automatically finding the exact end point when the part is seated properly is especially widespread in automotive powertrain assembly. Press fits are also found widely in assembly or rebuilding of turbomachinery, aircraft, industrial machinery, motion control systems, and precision medical devices. Many of these high-volume press fitting operations are hampered by slow monitors that don't pick up the exact end point. Pressing the part a few microns too far can ruin it. Stop too soon, on the other hand, and the part doesn't seat properly, leading to excessive scrap or rework, early failure in service, warranty problems or poor performance. Slowing down the process to ensure proper seating impairs throughput and competitiveness.

Discrete Control, Machine Control & Discrete Sensors


The press-fitting machine includes a hydraulic cylinder to provide pressing force, whose position is sensed by a displacement transducer. The cylinder presses on a force transducer, which, drives the bearing or bushing into a hole in a housing through an adapter, called the press fit tool.

HBM, Inc. of Marlborough, MA has developed a high-resolution press-fit monitoring system that picks up the end point of the process to within 50 s. That's quick enough to permit speedup of automated press-fit operations without putting the parts at risk. The system is up and running on several assembly lines in the US and Europe, where part tolerances are in the single micron range and force tolerances are less than 0.5% of scale.

“It's basically a refinement of the 'windowing' technique used for years in press-fit monitoring,” says Steve Webb, senior applications engineer at HBM, which has more than 10,000 installations around the globe. “The difference is much faster data acquisition and truly simultaneous measurement of force and displacement, which picks up the end point more precisely.”

Unlike most in-line tests, monitoring a press fit is dynamic—the parameter of interest is the relationship between force and displacement over time. In windowing, force and displacement are monitored during the press-fit cycle and portrayed as a curve on a PC screen. That curve must pass through a series of prescribed force/displacement windows on the screen: an initial contact window, a fitting window, and end window. (see “Windowing technique for force monitoring” illustration.)

The initial contact window shows a force spike as the insert to be pressed first makes contact with the edge of the hole in the housing it will be pressed into. The force rises as both the press mechanism and the parts align themselves to allow the part to slide into the housing, then drops off as the part begins sliding into the hole.

Fitting contact window shows the more-or-less steady force required to push the part into the hole. Since it is typically an interference fit, both housing and insert need to deform slightly as the part slides in. Friction is less important because the sliding surfaces are typically lubricated.

The end window shows the sudden exponential force rise as the part reaches the limit of its travel.

The system must stop the press at the exact split second the bushing or bearing is pressed home, or the part can be crushed. The exponential rise in force at the end of the cycle takes place in less than a millisecond.

“Precision bearings may last a lifetime when they're handled properly, but they aren't very tolerant of overstressing during assembly,” says Webb. “The very properties that make them so wear resistant also make them susceptible to damage.” Yet press fitting remains the method of choice for mounting bearings because nobody has found a better way.

Older press-fit monitoring systems use multiplexing or “sample and hold” approaches to data acquisition, which are very slow by today's standards. Either system can create data drift or lags between the force and displacement readings, rendering a curve that doesn't portray reality in such a fast moving process.

The new system hinges on an MGCplus precision measurement amplifier that takes true simultaneous measurements of force and displacement every 50 s throughout a 3 to 5 second press-fit cycle. “The displayed curve reflects reality for every millisecond in a fast moving process,” says Webb, “the data is precise. And the end point is sensed quickly enough to avoid damaging the parts at higher line rates,”

Discrete Control, Machine Control & Discrete Sensors

The windowing screen highlights the three critical phases of the pressfitting operation: initial contact, fitting, and end-stop.

High resolution monitoring at the 24-bit level can cost five times as much as 8 to 16 bit range mainstay systems, opens the door to much closer control over any “problem” operation in many processes, higher throughputs, and more versatile press-fit machines. “Many users need to hold micron-level tolerances on dimension and force tolerances better than 0.5% of scale. These are levels of accuracy simply not possible before in press fitting,” says Webb.

As to versatility, Webb says that the MGCplus amplifier can accommodate a variety of sensor types that older DAQ amplifiers cannot. This can transform a dedicated press to a general purpose machine.

Case in point

In one of its first U.S. applications, a high resolution press-fit monitoring system runs through 6,400 cycles a day in a leading edge powertrain plant in the Southeast. The system tracks 200 points along the characteristic curve in a four-second cycle to keep dimensional tolerances within 0.003 in., forces within 3% of nominal—and to pick up the all-important end point. Over the cycle, forces range from 0 to 15,000 N.

Each press-fit test station consists of a highly advanced HBM type WA quarter-bridge displacement transducer mounted to the press ram, along with an HBM force transducer. Both inputs feed to an HBM MGCplus test amplifier specially designed for press-fit monitoring. The amplifier screen displays the actual force-displacement curve and the tolerance “windows” through which it is supposed to pass. Software in the HBM amplifier provides for 100% traceability.

Now in its third generation, the MGCplus DAQ system features 24-bit A/D converters for each channel plus synchronous and simultaneous measurement of all channels. Included are auto-check, auto calibration and enhanced confidence in data (ECID) for complete traceability. It works as a stand-alone device or mates readily with a PC and is compatible with all catman, LabView, Diadem and BEAM software. The standard software package includes MGCplus Assistant, which includes wizards to facilitate startup and configuring.

Despite the many interference sources in the plant, the inherent stability of the sensors and ruggedized amplifiers keep the measurements virtually immune from extraneous noise, according to Webb. The system has proven so stable that the plant has extended the intervals between calibration checks from once a month to once a quarter, he adds.

“Windowing” press fits certainly isn't new in itself” says Webb. HBM has more than 10,000 installations around the world dating back decades. “What is new here is higher data density raising the potential for higher line speeds with much closer control over dimension and force and greater protection of the workpiece.”



Author Information

C.G. Masi is senior editor at Control Engineering . Reach him at charlie.masi@reedbusiness.com .




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