Proper Torque Wrench Techniques

By Walt Moore, Senior Editor | September 28, 2010

What's a torque wrench? It's a referee, says Greg Katsis, marketing manager for Sturtevant Richmont, a manufacturer of torque-measurement systems in Franklin Park, Ill. It makes sure that big, burly mechanics and those of slighter builds agree on when a bolt is "tight."

For components to work as the engineer intended, says Katsis, mating pieces must be held together with a precise amount of clamping force, which is determined by the amount of "tension" created in bolts as they're tightened. Tension is a force trying to stretch the bolt, and clamping force is proportional to tension, but working in the opposite direction. Using enough properly sized bolts yields the needed clamping force without stretching bolts to their elastic limit — the point beyond which they can't return to their original length.

To make sure mating parts are securely fastened, you need to determine clamping force. One way to do that is to measure bolt tension.

For example, "direct tension indicators" (DTIs), installed under the bolt head or nut, are basically washers with little stamped dimples that flatten as tension increases. The amount of deformation (determined by measuring the gap between the washer surface and the fastener) correlates to tension. Other tension indicators include systems that measure bolt stretch ultrasonically, as well as bolts with a substance in the head that changes color as tension increases (like the eye of a maintenance-free battery).

Perhaps a simpler way to determine clamping force, however, is to measure bolt tension indirectly, that is, by measuring the force (torque) applied to threaded fasteners. Engineers know how to fairly accurately correlate torque values to bolt tension and, thus, to clamping force. Although proponents of measuring bolt tension directly would say that their method is more accurate (mostly because torque measurements don't account for changes in friction, such as thread corrosion), measuring torque remains the common method for determining clamping force.

And the most widely used instrument for measuring torque applied to threaded fasteners is the torque wrench.

Torque basics, Hooke's law

Torque is force around a point or axis — a twisting force — and is calculated by multiplying an applied force by the distance through which the force works. In English units, the force is measured in pounds or ounces, and the distance in feet or inches. In the International System of Units (SI), force is measured in newtons and distance in meters. To illustrate, 10 pounds of force exerted at the end of a 3-foot breaker bar yield 30 pound-feet of torque at the socket on the end of the bar.

(The people who know about such things tell us that torque is most accurately expressed as "pound-feet" or "ounce-inches." The expressions, "foot-pounds" and "inch-ounces," they say, are technically measures of work, not torque. Common usage, however, has made the latter completely acceptable, and you might find torque-wrench scales identified either way.)

Everyone agrees, though, that Hooke's Law is at the heart of torque-wrench operation. Robert Hooke (1635–1703) was a brilliant Englishman — philosopher, inventor, architect and scientist — who is credited with discovering the law of elasticity. In layman's terms, Hooke's Law says that the amount an elastic (flexible) material bends (deforms) is in direct proportion to the force acting upon it. In more scientific terms, "strain" (deformation) is proportional to "stress" (force applied). And as long as the material's elastic limit is not exceeded, it will return to its original shape.

So, if you set a thin, flat bar of steel on edge and anchor it at one end, the bar will flex in direct proportion to how hard you push against its flat side. Release the bar and it springs back. Now, if you can correlate the flex of the bar to a scale, you'll have an indication of the force (torque) being exerted at the anchored end. This, in principle, is how torque wrenches work, but different types of wrenches use different mechanisms for applying Hooke's Law.

Beams, dials, clicks and digits

The flat-beam is the oldest, simplest and, according to some, the most accurate of all torque-wrench designs. It's still made in a wide variety of capacities, from small ounce-inch models to those with a top capacity of perhaps 300 pound-feet or more.

The flat-beam torque wrench has lost ground to other types of wrenches, however, because it has what some consider inherent disadvantages. To wit, you always need to see the scale when using it, and you have to interpolate readings that fall between scale markings. The flat-beam does not click, buzz, flash or vibrate when it reaches the torque setting you want, but it takes more than its share of abuse and rarely needs calibrating.

The dial-type torque wrench also employs Hooke's Law, often by deflecting a torsion bar at the "square drive" (the socket-attachment point). The torsion bar's movement may then be transferred via a "floating beam" to a clock-like mechanism with a dial scale. The standard dial-type, however, has the same disadvantages of the flat-beam — a line-of-sight scale and between-the-lines readings.

But, that said, Ray Pagels, director of sales for Precision Instruments, a long-time torque-wrench manufacturer in Des Plaines, Ill., makes the point that new dial-type wrenches can be adjusted to a desired torque, then can beep or flash when that torque is attained. Dial-type wrenches are available in a wide range of capacities and, according to Pagels, can be designed to deliver accuracies of 1 percent.

Perhaps the most widely used torque wrench in today's equipment shops is the micrometer-adjustable "clicker." Specific designs vary, but typically, as the tool's adjustable barrel is turned to select a specific torque value, a large spring is compressed against an interface mechanism (toggle, cam or low-friction roller) that engages a deflecting beam. When the deflection of the tool is sufficient to overcome the compressive forces at the interface, the beam disengages and the wrench "clicks."

Among the most recent of torque-wrench designs is the digital. The digital torque wrench typically reads to a greater degree of precision than other types, sometimes to plus/minus 1 percent from 20 to 100 percent of capacity. Our understanding of these wrenches is that they employ Hooke's Law by attaching a strain gauge (which passes varying amounts of electrical current as its shape changes) to a torsion bar. As the user tightens fasteners and deflects the torsion bar, the strain gauge interprets the deflection and reports it as a numerical value on the wrench's display.

So, the next time you pick up a torque wrench, remember, that even though it can do some hard work, it's first of all a precision measuring device. Treat it so, and it might last for decades.

Precision Instruments' Split-Beam Click Wrench
Precision Instruments' Split-Beam Click Wrench relies on measuring the deflection of a set of beams. The wrench is adjustable, and "clicks" when the desired torque is attained. It's available in capacities to 600 pound-feet with a flex (12-degree) ratcheting head.
Precision Instruments
Precision Instruments engineer Larry Cook tells us that the dial-type wrench is the only type not affected by the user's hand position. This phenomenon has to do with the relation of the wrench's torsion bar and floating beam.
Precision Instruments' micrometer-adjustable torque
This Precision Instruments' micrometer-adjustable torque wrench uses a spring to push two members — a plunger and beam — into compression at a roller-bearing interface. The wrench "clicks" when the desired torque is attained and the two members disengage. Always return a micrometer-adjustable wrench to its lowest scale setting to relieve pressure on the spring and to prolong its calibration.

Quick Tip

 

How often should you have a torque wrench calibrated? Most manufacturers suggest that calibration be based on cycles, that is, how often the wrench is used. Your supplier can suggest calibration intervals based on your operation. If you have a number of torque wrenches, you may be able to perform your own calibration, using an analog tester such as this Precision Instruments' model.


Avoid Capacity Extremes

When manufacturers state the accuracy of their torque wrenches, you may read qualifying statements, such as "Accuracy of plus/minus 3 percent Indicated Value (from 20 to 100 percent of capacity) exceeds ASME B107.14M and ISO 6789."

In this instance, the manufacturer is cautioning you against using the bottom 20 percent of the wrench's capacity. Assuming that the scale of a 150 pound-feet wrench starts at zero, you'd want to avoid taking readings below 30 pound-feet, because these readings may not meet stated accuracies. Most will tell you, too, not to continually use the wrench at extreme upper capacities. Size the wrench to the job.


Ancestor of Them All

The flat-beam torque wrench illustrates Hooke's Law right out in the open. A measuring beam and an indicator beam are attached to a cylindrical housing that remains stationary during use. As the measuring beam (with its attached scale) bends, the torque developed is read directly under the indicator beam's pointer. To check calibration, hold the wrench vertically by the square drive (socket-attachment point). If the pointer reads "0", it's ready to use.

Graphic: Sturtevant Richmont


Clockwise and Counterclockwise

If your application calls for using a torque wrench in both the clockwise and counterclockwise direction, make sure the selected wrench is designed to work in both directions. Some don't, and some that do have lesser accuracies in the counterclockwise direction.


The Latest and Greatest

The digital torque wrench is among the most recent developments in torque-measuring systems. The LCD display on this wrench rotates 60 degrees to facilitate viewing in confined areas. Some digital torque wrenches also may provide audio/visual/tactile indicators and download information to computers for record keeping.

Graphic: Sturtevant Richmont


Interchangeable Heads

Some torque wrenches allow interchanging the heads, from, say, the standard square drive to a ratcheting open-end wrench or a tube wrench. As long as the head has the same centerline as the square drive, the tool's torque capacities remain the same.

Graphic: Sturtevant Richmont


Using Torque Wrenches
  • Do apply force smoothly at 90 degrees to the wrench handle.
  • Do grip the handle in the correct position to assure accurate readings.
  • Do push down on the wrench in vertical applications, bending your knees, not your waist.
  • Do release the tension on micrometer-adjustable wrenches after use.
  • Do use good sockets.
  • Don't jerk the wrench, or torque values could be distorted.
  • Don't continue to pull the wrench after it "clicks."
  • Don't pull the wrench toward you in vertical applications.
  • Don't use "cheater bars," or torque values will be distorted.

Accuracy: Full-Scale Versus Indicated-Value

According to Greg Katsis, marketing manager for Sturtevant Richmont, industry standards are established for the accuracy of torque-wrench types. In plus/minus values, the flat-beam must be accurate to 2 percent, the dial to 3 percent and clickers to 4 percent. Manufacturers, of course, may choose to exceed these values if customer requirements dictate.

Katsis notes, however, that manufacturers may base the percent accuracy on either "full-scale" or "indicated-value" measurements. For example, a wrench with a maximum capacity of 100 pound-feet and rated at plus/minus 4 percent "full scale," he says, has a potential variation of 4 pound-feet on either side of any reading. If the dial says 20 pound-feet, for instance, the actual torque value could range from 16 to 24 pound-feet.

On the other hand, a similar wrench with the same 4-percent accuracy, but rated on "indicated value," has a much narrower (and better) range of accuracy. At a reading of 20 pound-feet, for example, this wrench would have a potential accuracy spread of only 19.2 to 20.8 pound-feet, that is, plus/minus 4 percent of the indicated value.


 

 

 


Acknowledgements
Construction Equipment thanks Greg Katsis, marketing manager for Sturtevant Richmont; and Ray Pagels, director of sales, and Larry Cook, engineer, Precision Instruments, for assistance in assembling this report.

 

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