Catching shaft torque data on the fly

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

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 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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Induction-powered systems

For long-term data collection, induction- powered systems are another viable solution. A transmitter and coil in a collar are mounted on the shaft. A stationary coil surrounds the collar. The coil pair acts as a transformer to induce power across an air gap between the two coils. In this way, the unit is continuously powered by a noncontact means.

The data signal output from the sensor is induced back across the coil pair to a demodulator. Data input and power output are transmitted through the same cable. System simplicity and ease of installation combined with long term data collection makes this method suitable as a feedback loop for monitoring and controlling plant processes.

Advantages of induction powered systems are:
• Suitable for long term installations.
• Maintenance free.
• Installs easily on shaft.
• Induced power.
• Multiple channels available.

Disadvantages:
• More expensive.

Telemetry

Telemetry is a reliable method for diagnostic testing, especially where data collection is needed for a short-term. Here, a strain gage and a small transmitter are mounted to the rotating shaft. Methods of mounting vary from glass strapping tape (which requires no machine disassembly) to custom-designed collars for high rotation speeds. The transmitter is powered by a battery that also excites the sensor.

The transmitter broadcasts the strain gage output as an FM frequency, which is picked up by a receiver and converted to a voltage. This voltage signal from the stationary receiver can be read and recorded by an oscillographic recorder or sent to a computer.

Some transmitters are reduced in size to accommodate space limitations and minimize adverse effects of centrifugal force. These transmitters use the 88 to108 MHz band. The FCC restricts broadcasting power in this band, which limits the transmission distance to 1 to 2-in. Thus, the receiving antenna surrounding the shaft must be within 2 in. for optimum reception.

For larger diameter shafts, a transmitter using the 49 MHz band can be used. Higher power in this band provides a longer transmission distance, up to 200 ft. These transmitters are now being miniaturized to offer signal strength in a smaller package.

Advantages of telemetry include:
• Minimal machine downtime.
• Adaptable to various shaft sizes.
• Requires minimum shaft area.
• Portable.
• Least expensive.
• Low maintenance.

Disadvantages are:
• Transmitter is battery powered.
• Short term limitations.
• Possible RF interference.
• High rpm’s require secured mounting.
• Possible interference with multiple channel applications.

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Selection

All three of these methods — slip ring, induction powered, and telemetry — are capable of providing information on shaft torque, axial loads, temperature, or pressure, based on data from a rotating strain gage or other type sensor. And, they all offer comparable data accuracy.

Determining which of these (or other) methods are best suited for a particular application depends on parameters such as test duration, transmission distance, signal level required, speed of rotation, number of data channels required, space available around the shaft, and cost.

For help in matching a measurement method to a specific application, consult the manufacturers of the systems being considered.

Overcoming the technology hurdle

Engineers are increasingly turning to shaft torque measurement to obtain accurate information on machine performance. Often, these engineers are not familiar with the necessary instrumentationand testing procedures. But, the value of data gatheredfar outweighs the difficulty in learning these procedures. For those who don’t want to (or can’t) perform the machine monitoring tests, the alternative is to hire a testing consultant to obtain the data.

Rotating shaft sensors at work

Here are a few examples of using sensors to measure data on rotating shafts:

Icebreakers. Ship owners needed to measure propulsion shaft torque and thrust on icebreakers and tug boats during sea trials in the frigid waters of Antarctica. This data helps to determine if the vessels meet performance requirements and improve future designs. Torque measurements, for example, indicate how well a vessel will perform when ice forms on its propeller and shaft. These measurements are one of the most difficult diagnostic tasks in marine research because of the small axial strains that must be measured.

As test contractor, Fleet Technology Ltd., Kanata, Ontario, chose telemetry because of its ease of installation and shaft adaptability. This method eliminated the burdensome and expensive process of replacing a section of the large-diameter shaft with a spool type load cell.

A custom transmitter was designed and built to provide high amplification of the low strain gage signals. The sea trial tests were successful. And the telemetry system provided high quality, high signal-to-noise ratio signals for both shaft torque and thrust readings according to Bruce Cowper, mechanical systems engineer at Fleet Technology.

Fiber manufacturing machinery.Fiber manufacturers in North Carolina required precise temperature control on heated rolls used to manufacture high-density fiber yarn. The yarn is produced by drawing extruded polyester over a series of induction- heated rotating godet rolls. Temperature control provides consistent fiber characteristics, thus helping to reduce rejects, sampling, sorting, and stockpiling.

A custom-built induction-powered system, designed to operate in the hot environment, was mounted on the roll motors. An RTD sensor, imbedded in each rotating roll, was connected to a rotating coil, also on the roll. With the addition of a stationary induction coil, this system transmits temperature data via the two coils to the temperature controls. Over 300 sensing units were subsequently installed.

Food processing. In Hebron, Maryland, a customer of the R&G Tool Co. needed to monitor stresses on a 500-ftlong chain that drives 40 different machines in a chicken processing plant. A portable system was required that could be carried along the chain line to measure the tension at each machine. Finding high-stress locations would enable the company to make adjustments and reduce the chances of chain breakage, thereby avoiding machine downtime and idling of 200 employees.

R&G developed a custom-designed telemetry system to solve the problem. A special load cell was built and installed on the chain. Telemetry carried the signal to the portable receiver and a recorder to gather the needed data.

Susan J. Woodward is the vice president of Binsfeld Engineering Inc., Maple City, Mich.

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