Adapted from an article that appeared in Sound & Vibration, June 2005

Chemical processing plants that produce olefin and polyolefin materials for the plastics industry employ mixers of gigantic proportions. It’s not uncommon to see 2,000 HP electric motors driving speed-reducing gearboxes of similar size for mixing a batch of ethylene, propylene, or polyethylene — materials that are eventually converted to pellets for plastic injection and blow-molding machines. The heavy loads placed on the gearboxes under such conditions are reason enough to have teams of engineers and technicians forever vigilant of the sounds and vibrations these giants continually generate. Their mission is to prevent premature gearbox failures.

A reliability engineer for a large chemical company has the responsibility to monitor and analyze these gearboxes, and develop specifications for their repair and overhaul. He is a member of the machinery support group that analyzes gearbox vibrations, spent oil, and reciprocating compressors where he monitors pressures and volumes. Several large gearboxes on site occasionally need to be line-bored and completely stripped down for overhaul. But because of the high cost of the repair, it’s imperative that the gearbox not be overhauled either too soon or too late. This means that reliability and accurate data must be logged continuously regarding mechanical vibrations from the gear teeth and bearings to establish a signature for both acceptable and marginal components.

Potential Solution. The machinery support group currently uses walk-around data collectors for most of their vibration monitoring requirements. The equipment is capable of collecting long time records, but to do this, the high-frequency data must be filtered out. Unfortunately, the higher frequencies contain the information needed to completely analyze the most critical element, the gear teeth. The machinery group also has reel-to-reel tape recorders, FFT analyzers, and ordinary oscilloscopes. But the equipment is severely limited to measuring only two channels simultaneously, gathering hardly enough data to sufficiently characterize a gearbox noise or vibration issue. Also, virtually no post processing of the most critical data is possible.

Typical mixers for olefin and polyolefin are designed to operate 24 hours per day for as long as ten years. But engineers at one leading chemical company are realizing longer lives because they monitor their gearboxes with an IOtech ZonicBook containing eZ-Analyst software and study vibration signatures that warn of looming failures. The data help the engineers improve gearbox designs and set up a repair schedule that prevents major failures for what they predict will be close to 30 years.

IOtech’s Solution. Because the cost of missing an opportunity to prevent a gearbox failure far exceeds the cost of new equipment that could predict failures, the reliability engineer looked at other data acquisition systems. He needed a monitoring and analyzing system that could provide plots of vibration data recorded simultaneously on several gears and bearings. And as a result of his search, he purchased a ZonicBook™ vibration analyzer with eZ-Analyst™ software. The eight channel IOtech instrument has a built-in RPM/frequency port, piezoelectric accelerometer input, and anti-aliasing filters that let the machinery group monitor and analyze the output of several accelerometers attached to the gearbox case and bearing hubs. They also monitor gear and shaft speeds with tachometer signals derived from reflective tape sensors.

The test setup monitors a tachometer speed signal from each bearing and vibration in two or three directions with accelerometers. Proximity probes are placed in the X and Y-axes at each bearing location, along with a pair of thrust probes and a single key phasor that provides a once-per-revolution timing signal. The gearbox contains driver and driven elements, so four bearing and eight vibration probes are needed for radial vibration.

The biggest advantage, however, is the fact that with eZ-Analyst software, the reliability engineer can stack up the plots, one above the other synchronized in time, to see anomalies as they relate to each other.

As a result of numerous recordings with the ZonicBook, the machinery group now can gather enough information to go further than just recommending a replacement with the same kind of gearbox; they can provide design-improvement data. The reliability engineer is now working closely with the gear manufacturer, and between them, they recently pulled off three or four fantastic overhauls. They dropped the noise level substantially, and expect to get 30 years out of a gearbox before it needs an overhaul.

The reliability engineer said that he found the ZonicBook easy to configure, compared to the other analyzers he has that that are much more confusing to set up. He claims that software-based analyzers, like the ZonicBook, are much easier to set up and use because he can just pick and choose options from the menu. The other analyzers were purchased in the early 1980s. They lacked the ability to use a mouse, a standard keyboard, and set up an efficient configuration.

Also, the reliability engineer finds the IOtech equipment very durable and portable. He even had it in areas that were extremely dusty and humid, and never experienced a failure or problem. The new IOtech tools are a big plus over those that he had on his shelf before.

Conclusion. A leading manufacturer and user of olefins and polyolefins, employs an IOtech ZonicBook with eZ-Analyst software for analyzing gearbox vibrations. The system lets engineers analyze the data collected, establish vibration signatures for the gearbox components, and develop a realistic repair schedule. The comprehensive maintenance program prevents surprise failures and considerably lowers repair costs by ensuring that the gearboxes are serviced at optimal times.