Slow monitors that don’t pick up the exact end-point when a part is seated properly can hamper high-volume press-fitting operations. This process-control problem is especially widespread in automotive-powertrain assembly. A transmission, transfer case or differential can 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 widely found 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, 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.Enter high-resolution monitoring
HBM Inc., in Marlborough, Massachusetts, 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. 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. The parameter of interest is the relationship between force and displacement over time.
Fig. 1 |
Fig. 2 |
Windowing the force displacement curve
In windowing, force and displacement are monitored during the press-fit cycle (Fig. 1) and portrayed as a curve on a computer monitor (Fig. 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 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 Fig. 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 tolerant of overstressing during assembly. The very properties that make them so wear-resistant also make them susceptible to damage. Still, press fitting remains the method of choice for mounting bearings—nobody has found a better way.
Older press-fit monitoring systems use multiplexing or “sample and hold” approaches to data acquisition—very slow by today’s standards. Either approach 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 new high-resolution system, and too slow to catch the start of the rise in force at the end point.
Sampling every 50 microseconds
The new system hinges on an MGCplus precision-measurement amplifier that takes simultaneous measurements of force and displacement every 50 microseconds throughout a 3–5 second press-fit cycle. The displayed curve reflects reality for every millisecond in a fast moving process. The data couplet is precise, and the end point is sensed quickly enough to avoid damaging parts at high line rates.
High-resolution monitoring, which can cost five times as much as mainstay systems, isn’t necessary in less-demanding press-fit applications. However, it opens the door to much closer control over a difficult operation in many processes, with higher throughputs and in 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 previously not possible in press fitting.
As to versatility, the MGCplus amplifier can accommodate a variety of sensor types that older data acquisition (DAQ) amplifiers cannot, transforming 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 Southeastern United States. The system tracks 200 points along the characteristic curve in a four-second cycle to keep dimensional tolerances within 0.003 in. and forces within 3 percent of nominal, and to pick up the end point. Over the cycle, forces range from 0 to 15,000 N.
Each press-fit test station consists of an HBM Inc. WA-series quarter bridge displacement transducer mounted to the press ram, along with an HBM force transducer. Both inputs feed to an 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 amplifier provides 100-percent traceability.
Quarter bridge displacement transducers, close up
Unlike conventional half-bridge displacement transducers, the WA series quarter bridge transducer serves as one leg of a Wheatstone bridge and works on the principle of an active inductive quarter bridge. The result is a displacement transducer with a characteristic curve flat within 0.1 percent over the entire measurement range. For a variety of reasons, including compactness and higher accuracy, the WA displacement transducer is replacing half-bridge displacement transducers and LVDTs (linear variable displacement transducers) in high-end applications.
Stable in noisy plant environments
Despite the many interference sources in the plant, the inherent stability of the sensors and ruggedized amplifiers keeps the measurements virtually immune to extraneous noise. The system has proven so stable that the plant has extended the intervals between calibration checks from once a month to once a quarter.
Windowing press fits isn’t new in itself. What’’s new is the higher data density, raising the potential for higher line speeds, with much closer control over dimension and force and greater protection of the workpiece.
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