Many high-volume press-fitting operations are hampered by slow monitors that don’t pick up that exact end point when the part is seated properly. This process control problem is especially widespread in automotive powertrain assembly. A transmission, transfer case or differential may contain four to six bearings that are press-fit into place on an assembly line turning out 3,000 transmissions a day. Press-fits are also found widely in assembly or rebuilding of turbomachinery, aircraft, industrial machinery, motion control systems and precision medical devices. 

 

Whatever the industry or application, the dilemma is the same: 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. This can lead to excessive scrap or rework, early failure in service, warranty problems or poor performance. And, slowing down the process to ensure proper seating impairs throughput and competitiveness.   

 

Enter high-resolution monitoring

HBM Inc. of Marlborough, Mass., has developed a high-resolution press-fit monitoring system that picks up the end point of the process to within 50 microseconds. 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 United States and Europe, where part tolerances are in the single-micron range and force tolerances are less than 0.5 percent of scale.

 

“It’s basically a refinement of the ‘windowing’ technique used for years in press-fit monitoring,” says Steve Webb, engineering manager 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 a dynamic test, explains Webb.

 

“The parameter of interest is the relationship between force and displacement over time,” he says.

 

Figure 1

 

Windowing the force displacement curve

In windowing, force and displacement are monitored during the press-fit cycle (Figure 1) and portrayed as a curve on a PC screen (Figure 2).  

 

Figure 2

 

For the part to be approved, that curve must pass through a series of prescribed force/displacement windows on the screen. Equally important, 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 end point is indicated by an exponential rise in force at the end of the cycle, as seen in the area marked “end window” in the Figure 2. That rise in force 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 can create data drift or lags between the force and displacement readings, rendering a curve that doesn’t quite portray reality in such a fast moving process. Monitoring resolution may be in the eight- to 16-bit range. That is exponentially slower than the 24-bit rate used in the HBM new high-resolution system, and too slow to catch the start of the end-point rise in force.  

 


Photo of the MGCplus

 

Sampling every 50 microseconds

The new system hinges on an MGCplus precision measurement amplifier that takes true simultaneous measurements of force and displacement every 50 microseconds throughout a three- to five-second press-fit cycle.

 

“The displayed curve reflects reality for every millisecond in a fast moving process,” says Webb, “The data couplet is precise, and the end point is sensed quickly enough to avoid damaging the parts at higher line rates.”     

 

High-resolution monitoring, which can cost five times as much as mainstay systems, isn’t necessary in less-demanding press-fit applications, says Webb. However, it opens the door to much closer control over a “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 percent of scale. These are levels of accuracy simply not possible before in press-fitting,” he says.

 

As to versatility, he 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 inches, forces within 3 percent 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 percent traceability.

 

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This article was provided by HBM Inc. For more information, call 800-578-4260, e-mail info@usa.hbm.com or visit www.hbm.com.