Portable telemetry system includes transmitters and a receiver.

What does an icebreaker, a fiber manufacturer, and a chicken processing plant have in common? All are under pressure to improve their machinery operations and, in order to do so, they must often obtain operating data from locations where access is difficult at best.

Too often when machines break down, users fix them but don’t address the cause. Inevitably, the breakdowns continue, incurring more downtime and loss of production. “Fixing” becomes a costly, temporary solution for underlying problems.

In today’s economy, companies are taking a hard look at how their equipment operation affects bottom-line profits. One way is to measure machine operational data and use it to improve the machine performance and eliminate failures. Such data can help to:
• Increase production and reduce downtime.
• Qualify machine performance.
• Verify manufacturer’s specifications.
• Detect and evaluate overloads.
• Aid in troubleshooting by isolating the cause of failure.
• Resolve repetitive maintenance problems.

Measuring techniques

During sea trials of ships such as this icebreaker, Nathaniel B. Palmer, engineers measure torque and thrust on propeller shafts to evaluate propulsion system performance. Photo courtesy of Edison Chovest Offshore.

Data from rotating shafts affords engineers with information needed to identify and eliminate the cause of repeated machine failure: overloads, excessive torque, side loading (from chain or belt drives), axial tension, or compression loads.

Because such data must be obtained from a moving component, it can be difficult to measure. Methods available in the past were cumbersome and costly, often discouraging manufacturers from obtaining the necessary data. But improved technology has increased the accessibility of sensor data from rotating equipment.

Three commonly used methods for measuring data on rotating shafts are slip rings, induction-powered systems, and telemetry.

Slip rings

A slip ring system uses rotating rings and stationary brushes to transmit data from a shaft-mounted strain gage to monitoring or control devices.
Select figure to enlarge.

A common method for measuring rotating- shaft parameters, slip rings rely on rotating rings riding on stationary brushes. The rotating rings are usually made of coin silver, gold, or other noble metals. Brushes are made from silver graphite, gold, or platinum. The slip rings are mounted on the shaft and connected to a strain gage (or other) sensor, also on the shaft. The silver graphite brushes, rubbing on the coin silver rings, provide an electrical path for power input and data output signals.

Slip ring advantages include:
• Suitable for long term.
• Externally powered.
• Multiple channels available.
• Familiar technology.
• Less expensive.

Disadvantages of this method are:
• Requires shaft modifications.
• Wearing parts require maintenance.
• Signal tends to erode over time.
• Requires additional sensor amplifier.

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