WHAT DO TORQUE WRENCHES
HAVE TO DO WITH TIGHTENING FASTENERS?

By Richard B. Wright, President of Wright Tool Company

What do torque wrenches have to do with tightening fasteners? Not as much as you might think, unless you select them with care, treat them properly, and, most important of all, understand their limitations. To understand the problems that are blamed on torque wrenches-inaccurate settings or readings and improperly tightened fasteners-we need to examine the fundamentals of why we tighten fasteners. We know that if a bolted joint is not tightened enough it may come apart, or it can break from being too tight. It can even break from being too loose. Torque wrenches help overcome some lack of knowledge and experience but only if they are properly used.

Shear Joints
Bolts that keep two parts from sliding apart are said to be in shear. For this type of joint, the fastener only needs to be tight enough or otherwise secured so that it does not come apart. This type of joint fails only when the bolt falls out of the joint. Tightening the fastener is one way to prevent that. However, there are some joints that are so stiff or subjected to so much vibration that no amount of tightening is sufficient to keep the joint together. If bolt tension is enough to keep the joint together a torque wrench is a good tool to see that enough tension is applied. If tension is not adequate to do the job then other means must be used, such as lock washers, jam nuts, lock nuts, or, in extreme cases, wire. When in doubt, there is no harm in treating a joint as a tension joint.

Tension Joints
The shear joint does not rely on tension in the bolt to hold parts together. The tension joint, however, relies on the tension of the bolt to hold two joined parts together. The greater the tension on the bolt, the more force holding them together. Steel and most metals have a property known as fatigue, which means that they lose strength upon repeated loads. For example, a bolt with an ultimate strength of 1,000 lbs. will carry a load of 1,000 lbs. once. It will carry a load of 500 lbs. millions of times. It will carry a load between 500 lbs. and 1,000 lbs. for a limited number of times between 1 and 1 million, depending on whether the load is closer to the 1,000-lbs.-value or the 500-lbs.-value. It doesn't matter how long the heavy load is carried; it can carry 1,000 lbs. forever, as long as it is only one cycle. Most joints are subject to variations in load that either adds or decreases the tension in the bolt.

How can we get all of the strength we pay for, or do we assume the bolt strength is only 500 lbs. and use twice as many bolts in the joint? If the joint has been properly designed, the clamping surfaces around the bolt will take almost all of the variation if the bolt is properly tightened. In this scenario, a load of close to 1,000 lbs. can be used because the load will not fluctuate very much. But if the joint is not properly tightened, the joined parts will separate and the full increase on the load will be on the fastener, which will overload it. Or, the load could go to zero, which means it is in a fatigue condition and the bolt will fail because of the repeated application of a load that it could have easily carried except for the fatigue factor. Therefore, a bolt in a tension joint will fail from fatigue if it is not tightened enough, and it will fail from fatigue and overload, if it is tightened beyond its ultimate strength.

In our example the bolt's ultimate strength is 1,000 lbs.; in actual practice there is a safety factor that extends the range beyond the 1,000 lbs., before the bolt actually breaks from simple overload. Therefore, it is extremely important that we achieve the proper tension on the bolt-not too much and not too little.

The ability to get the correct tension is why we use torque wrenches. Torque wrench sales are growing every year, which is a reflection that mechanics are performing better and more careful work. However, there is a catch to all of this. Torque wrenches do not measure tension-they never did and never will. So why buy a torque wrench?

What is torque?
There is no simple and reliable way of measuring tightening tension in a bolt. Quality torque wrenches are reasonably accurate and reliable if properly cared for and recalibrated regularly, but they measure torque and not tension.

Torque is equal to the force multiplied by the length of the lever that is being used to tighten the fastener. The K-factor varies greatly because of the many factors that can influence it, such as fastener coatings and lubricants. The total range is between 0.1 and 0.33. For specific fastener finishes and lubricants the amount of variation may be as low as 20% under carefully controlled conditions. Therefore, it is very important that the torque value be appropriate to the conditions, and that the conditions, such as cleanliness and the amount and quality of the lubricant, be carefully maintained.

If a fastener has been used before and tightened to a high value, it may stretch slightly and its pitch will not match exactly, forcing the bolt into a mismatched condition. This is one reason why high-strength bolts should not be reused in critical applications. Another reason is the act of removing and re-installing a fastener counts as a fatigue cycle. The second use will not have as long a service life as the first.

Plated bolts are different than bare steel bolts, and cadnium-plated bolts are different than zinc-plated bolts. A proposed change from cadmium to zinc plating would require much higher torque to get the same bolt tension. The "K" is doubled.

Factory installed fasteners are usually clean and well lubricated. If they are reinstalled in the field, they should be cleaned and relubricated. Because these factors vary so much, and because different joints are designed differently, the value from a service manual is more reliable than values from a general purpose handbook, although handbook values are substantially better than guessing.

Although occasional problems may occur with achieving accurate torque readings, a torque wrench should still be used. Proper tightening is very important, and torque wrenches continue to be the most accurate device, and substantially more accurate than the judgment of even the most experienced mechanic. A good mechanic should have a sense of how a joint should feel when it is being tightened, and should use that information in conjunction with a torque wrench.

Both joints with crossed threads and joints that don't feel right (indicating possible burrs or other defects on the threads that will produce an abnormal amount of resistance) should not be tightened. In situations such as these, the tension obtained with a torque wrench will not be the correct one.

Accuracy
Because of the inaccuracies between torque and tension, many users conclude that the solution is to buy a more accurate torque wrench. The significant difference is really not between quality torque wrenches but between the torque/tension relationships, which has at least a 20% range of variation. A good torque wrench might have an accuracy range of 4% or, perhaps, 8%. Because the factors producing differences between the torque and the tension are completely unrelated to factors producing errors in the torque wrench, the combination of the errors is not the sum of the two. For the example cited, it would not be 24% and 28%, but rather 20.4% and 21.5%. The difference of 4% on the torque wrench is only 1.1% in the overall accuracy.

The most reliable and rugged torque wrench should be purchased. A torque wrench should never be used for loosening because more torque is often needed. For this reason, it is unnecessary to have a wrench that measures in both directions.

If a wrench is out of calibration, the setting or readings have shifted from what the actual torque is. For example, if a torque wrench is in error by 10 ft. lbs. (which could easily happen if the spring element is stretched too far), and the user tightens a fastener to the specified torque (which might be 100 ft. lbs.), the actual torque applied would be 110 ft. lbs. This adds directly to the errors described above, which can easily produce a 10% variation resulting in a bolt tension that could be 20% too high. In some cases this would be enough to cause bolt breakage-maybe not immediately, but eventually after a period of use due to fatigue.

Care of Torque Wrenches
Be careful to handle a torque wrench in the same way you would a pair of micrometers. Keep it in its box, protect it from shock and regularly check the calibration against a tester. Do not overload it, and do not use it as a plain long handle, which will destroy calibration. Most torque wrenches use some kind of spring to measure load. Springs drift and shift from use, making recalibration very important. Overloading should also be avoided, as it will stretch the spring. Finally, release the load on click-type wrenches when they are not in use.

A Word About Torque Multipliers

Because the torque required to remove a fastener is more than is required to install it, a torque multiplier can easily be overloaded and damaged by the additional torque. Depending on the load, a long flex handle, a slugging wrench or an impact gun should be used. A torque multiplier should not be used for removing fasteners.

As with all tools, proper use is critical to obtaining the desired performance. Used correctly, the torque wrench can be a simple solution to common bolting challenges.

About the Author:

Richard B. Wright is president of Wright Tool Company, a leading manufacturer of professional-quality sockets and wrenches. Located in Barberton, Wright Tool manufactures more than 3,000 tools for the industrial, contractor, and MRO markets.

Mr. Wright holds a degree in Mechanical Engineering from the California Institute of Technology and an M.B.A. degree from the University of Pennsylvania Wharton School. A licensed engineer, Mr. Wright holds several patents in the field of hand tools and electrical instrumentation.