Roll a million miles

DEVICE UNDER TEST

Reduce time to simulate 1 million miles of testing. Vary fatigue life by applying pressure from 500 lbs to 5000 lbs and oil ranging in temperature from 70°F to 280°F while spinning the roller at speeds from 500 rpm to 5000 rpm. Count roller revolutions and monitor vibration, looking for signs of failure.

• Bosch Rexroth: servo motor and variable-frequency servo drive. www.boschrexroth.com.
• National Instruments: 24-input, 24-output isolated PCI digital I/O card; 24-bit PCI analog-input card; multifunction PCI data-acquisition card; graphical programming language. www.ni.com.
• Omega Engineering: proportional-integral-derivative (PID) controller for oil temperature control. www.omega.com.
• PCB Piezoelectronics: accelerometer. www.pcb.com.

Engineers at GT Technologies (Westland, MI, www.gttechnologies.com) needed to perform life tests on cam follower rollers. Because these rollers go into truck diesel engines, they need to last 1 million miles or more. But a million-mile test could take as long as two years to complete. To cut test time, Erik Maassen, manager of testing, turned to VI Engineering (www.viengineering.com) to design and build a roller fatigue tester.

A roller under test spins in the center of this fixture while three rollers apply a load. A PC controls speed and load while monitoring roller performance. Courtesy of VI Engineering.

During a test, an accelerometer connected to the 24-bit analog-input card monitors the spinning roller. The PC performs a fast Fourier transform (FFT) of the accelerometer signal to produce a frequency plot, and a data-acquisition card uses signals from the motor controller to count revolutions.

The tester software operates in manual and automatic modes. In manual mode, a technician spends 10 to 15 min adjusting the roller speed and load. Using the results of the FFT and the geometry of the rollers, the technician selects five frequency bands from within a 100-Hz-to-10-kHz frequency range for the system to monitor during the automated test (a frequency band might be 100 Hz to 120 Hz). The technician sets amplitude limits for each frequency band before starting the automated test. "When the amplitude of a frequency band exceeds a limit," said Maassen, "it indicates failure, usually caused by cracks or material loss. At that point, the system aborts the test."

VI Engineering designed the system with a pneumatic oil pump as opposed to an electric pump to spray oil on the roller. The pneumatic oil pump was less expensive, but it needed a pressure gauge with a regulator to sense if the air pressure was too high. It was also louder than the electric pump. "The electric pump would have been better based on total cost and performance," said systems engineer Raj Balsamy.

Initially, when the system sprayed oil on a roller spinning above 4000 rpm, the oil splashed outside the test fixture because of open spaces in the cover. Engineers added guards to keep the oil in place, but it took many tries to find the best solution.

Balsamy also found that even with shielded cables, noise can interfere with wanted signals. He noticed that a 0–5 V signal indicating number of revolutions from the motor picked up noise from a nearby AC power cord. The solution: Use twisted-pair shielded cable, not just shielded cable.