MRO Magazine

Ultrasonic System Listens And Measures

MRO Magazine   

Lubrication Machinery and Equipment Maintenance Test and Measurement

According to bearing manufacturers, lubrication problems account for 36% of bearing failures -- and over-lubrication is a more frequent cause of these failures than under-lubrication. As a result, sev...

According to bearing manufacturers, lubrication problems account for 36% of bearing failures — and over-lubrication is a more frequent cause of these failures than under-lubrication. As a result, several technology companies are providing simple tools to ensure that a bearing gets the right amount of lubrication at the right time.

A relatively recent innovation in bearing lubrication is an ultrasonic device that monitors what’s happening as a technician greases a bearing in order to prevent over-lubrication. According to Allan Rienstra, president of the company behind the device, SDT North America, Cobourg, ON, the development of an acoustic grease gun adaptor was the result of customer demand.

The device helps solve issues with lubrication. “In the past, most companies were working on a time-based schedule, relied on oil analysis samples, or just greased when the time suited them,” said Rienstra. “Most of these systems did not work well because there are too many variables between locations.

“Many of our customers already had digital ultrasonic data collectors and asked us if we could make an interface directly from their ultrasound sensor to any standard grease gun. We were able to provide this solution and allow our customers to continue to use the equipment they already owned and were familiar with,” he said.


The SDT equipment functions on the principle of acoustic vibration monitoring, which measures ultrasonic waves. Ultrasonic waves are high-frequency acoustic vibrations that rely on media — solids, liquids or gases — to transport themselves. These vibrations are sound pressure waves and are produced by rubbing, friction and turbulence. The equipment detects and measures them with high-frequency piezoquartz crystals. The measurement used by SDT is decibels/microvolt (dBV).

The ultrasonic system measures the level of friction produced by a bearing and stores it in a database for trending, alarming and scheduling. As lubrication conditions change, so do friction levels, which are best measured in the ultrasonic range.

“The SDT system works by both listening and digital measurement,” says Rienstra. “It tells the operator when to grease, how much grease to apply, and helps avoid over-greasing during re-greasing.

“The only way to maintain an accurate condition- based lubrication program is to measure and trend. Listen-only systems are too subjective.”

For those who already own an SDT ultrasonic system, they only need purchase an ultrawave lube tech kit consisting of an acoustic grease gun adaptor, a mechanical interface between the ultrasonic sensor and the meter. All ultra-waves come with an internal airborne sensor, so when lubricators are not using the equipment, it can still be used for air leak surveys, electrical inspections, steam trap checks, etc.

“Lubrication problems begin to make noise in the 40+ kHz range,” said Rienstra. “As the problem progresses, the lower frequency harmonics kick in. Most units that are dedicated for lubrication are what we call wide-band detectors. Some are even listening to audible sound. SDT employs a 2 kHz filter centred near 40 kHz, which blocks parasite noises that would otherwise interfere with the lubrication process. We combine a wide band sensor with a tunable filter, which provides extremely fine tuning.”

Case studies

“We’ve talked to one company that had stopped greasing motor bearings altogether. When asked why, the staff replied that lubricators had no idea how much grease the bearing needed. They would just keep adding shots without guidance.

“As a result, seals were getting blown, shields were pressurized into the rolling elements of the bearings, and grease was entering the windings of the motor. The company decided it was cheaper to let the bearings fail than to ruin the entire motor.

“In another case, at a General Mills plant in the US that makes cereal, a lot of the bearings on the cooking lines run on very close tolerances and with a lot of heat. One of the problems for grease in high-temperature environments is the separation of the oils from the carrier. Once this begins to happen, the carrier becomes waxy and hard. It forms a layer around the cage of the bearing, shielding the rolling elements from new grease. The bearings are left to run dry until they fail — a few hours to a day at best.”

By using the ultrasound equipment, lube techs at the plant were able to listen to the bearing as it was greased and measure its friction value, which identified a problem bearing with a waxy buildup on the cage. The problem could not have been discovered using conventional methods — including temperature — because the ambient temperature was already quite high.

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