Torque and Power Measurement; A Brief Overview

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Throughout history mankind has relied on rotating shafts and shaft torque to power its society.

From an ancient Roman flour mill in Barbegal, France to the modern Dukovany Nuclear Power Station,

the common denominator is the rotation of a shaft generating torque and power. Naturally, measuring

the torque and power produced through a shaft-driving system is critical to understanding and

optimizing the overall machine or process. This article provides an overview of various torque and

power measurement tools including the benefits and limitations of each system.

Illustration: Overshot waterwheels are conjectured to have driven the 16 flour mills at Barbegal, France (drawing from

Scientific American).1

The Dukovany nuclear power station2

Torque is defined as a twisting force that tends to cause rotation. The word originated from the

Latin word, torquere, meaning to twist. Torque can also be quantified by the equation:

Torque = Force applied x lever arm*

*The lever arm is the perpendicular distance from the axis of rotation to the line of action of the

force.

1 http://www.waterhistory.org/histories/barbegal/,

A. Trevor Hodge, "A Roman Factory," Scientific American (November, 1990), pp. 106-111 2 http://www.cez.cz/en/power-plants-and-environment/nuclear-power-plants/dukovany.html

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Torque is measured in foot- pounds or Newton-meters. Archimedes, the famous Greek mathematician

and inventor, was so excited about force and levers he was credited with the statement, “Give me a

lever long enough and a place to stand and I will move the world.”

Power, measured in the unit horsepower or Watts, was invented by the Scottish Engineer James

Watt in the 18th century3. Through experimentation involving actual horses, Watt developed the

equations relating torque to horsepower. One of the most recognizable equations, commonly used by

mechanical engineers, is:

P

P=Power, units of horsepower

T= Torque, ft lbs force

N= Revolutions per minute

5252 = Constant

Watt’s work was instrumental in proving that mechanical systems, steam and later gasoline engines,

could effectively replace actual horse power.

Quantifying torque and power, helped engineers to develop and improve various designs but

proper tools were still needed for proper and accurate measurement. A breakthrough concept was

developed by Samuel Christie and later, improved and popularized by Charles Wheatstone, became

known as the Wheatstone bridge4. A circuit diagram is shown below.

Wheatstone bridge diagram5

The significance of the Wheatstone bridge cannot be underestimated. This concept laid the

foundation for invention of the strain gage, invented in 1938 by Edward Simmons and Arthur Ruge.

Ruge and Simmons, working independently of each other, both discovered that small diameter wires

made of electrical resistance alloys could be bonded to a structure to measure surface strain. This type

3 Cleveland, C. (2007). Horsepower. Retrieved from http://www.eoearth.org/view/article/153556 4 http://www.engineersedge.com/instrumentation/wheat_stone_bridge.htm 5 http://en.wikipedia.org/wiki/Wheatstone_bridge#/media/File:Wheatstonebridge.svg

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of gauge had the advantage of responding well to static strains. It was due to the demands of this

rapidly growing industry that the important advance into foil strain gauges was made.6

Vishay full bridge general purpose strain gage – shear/torque pattern7

Combining the concepts of the Wheatstone bridge and strain gage, engineers and inventors

were able to develop various types of equipment to accurately measure torque and power. Each has

its own advantages and disadvantages but, all rely on established concepts and common goals of

monitoring mechanical forces and being instrumental in system improvements.

Dynamometer

The dynamometer is the most common tool used in the evaluation of torque and power.

Dynamometers have the advantages of being very accurate, allowing many types of testing. From a

single system to complete vehicle testing, there are many suitable dynamometers for each requirement

available.

Heavy duty engine dynamometer, UC Riverside8

For all of their advantages, dynamometers, however, are limited by their cost, size and

capabilities. Their size limits their capability for tests onboard marine vessels and in tight spaces. In

addition, some systems are too large for any type of dynamometer testing.

Load Cell and Moment Arm

A simple, yet cost effective torque testing apparatus involves a load cell with a torque arm

arrangement mounted to a specific load. As a force is applied to the load, the armature will compress

the load cell. From the load cell data and moment arm length, torque information can be calculated.

6 http://www.sensorland.com/HistPage003.html 7 http://www.vishaypg.com/micro-measurements/stress-analysis-strain-gages/all-shear-torque-rosettes/ 8 http://cichlid.cert.ucr.edu/about/hddl.asp

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Armature and load cell to measure torque, Wright State University.9

The disadvantages of such a system relate to the limits of the load cell, accuracy and data

collection. The system does not measure actual shaft torque but, rather, forces on a moment arm. The

system is also limited by space, flexibility and rarely used on modern marine vessels or large equipment.

Rotary Torque Sensor

A more modern approach to torque and power data collection is the in-line rotary torque

sensor. Rotary torque sensors offer a range of torque ranges, a high degree of accuracy, digital output

capability and various types of data collection features, i.e. RPM, shaft direction, in addition to torque

and horsepower. Some rotary torque models also have the advantage of withstanding very high RPM

(up to 50,000 RPM) and G-Force level.

Honeywell model 2110-2116 in-line torque sensor10

However, inline sensors are costly, do not offer versatility and have torque limitations. They

also require shaft modifications before installation. This can be very costly with large diameter shafts.

Radio Torque Telemetry System

A very effective and more versatile option for measuring torque and power is the radio torque

telemetry system. Typical units are not limited to any torque level and can be used on, or specified for,

9 http://cecs.wright.edu/people/faculty/sthomas/3.jpg 10 https://measurementsensors.honeywell.com/ProductDocuments/Torque/Model_2110_Datasheet.pdf

Torque and Power Measurement; A Brief Overview

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nearly any shaft diameter. They also offer the advantages of reasonable pricing, easy calibration,

portability and convenient installation (no shaft modifications required).

Binsfeld Engineering Inc. TorqueTrak 10K radio torque telemetry system11

There are limitations with radio torque telemetry systems that must be considered. Shaft

rotational speed is critical information to have due to the mechanical limits of rotating components, i.e.

battery, adhesive tape, etc. Occasionally, radio interference from adjacent devices can pose torque

system operation challenges. Proper strain gage application, required for successful system operation,

can be tedious. Improperly installed strain gages can result in accuracy and data errors.

Binsfeld Engineering Inc. produces a selection of non-contact torque and power measurement systems

for both temporary and permanent applications. Binsfeld TorqueTrak radio telemetry systems offer all

the benefits described here including convenience, versatility and reasonable pricing as well as user-

friendly operation. BEI also provides friendly and immediate technical support for any system issue.

Summary

Understanding theories of torque, power and test equipment can benefit in the search for a

torque telemetry system. As there is no perfect solution, understanding the limitations of each system

provides insight to choosing the best and most cost effective option.

11 http://www.binsfeld.com/torquetrak/torquetrak-10k/

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Author’s Note:

Brian Carr (brian@binsfeld.com) is the U.S. Sales Manager for torque telemetry systems at Binsfeld

Engineering Inc. Brian earned a BSME at Michigan Technological University and an MSBA from Boston

University and has been selling torque measurement systems for over 3 years. In his leisure time, he

enjoys scuba diving, biking, skiing and kayaking.