March 17, 1997 Bogus Torque

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by	Stephen T. Harms

About the Author ... Stephen T. Harms is a lecturer with the Department of Aviation, San Jose State University. He holds FAA mechanic and flight certificates and has over 2,500 hours of flight time. His research interests include aviation safety, human factors, and computer aided instruction.

When an assembly is engineered, loads to be encountered by the bolted joint are calculated and a safety factor is applied to allow for unexpected loads. The engineer then sizes a fastener to maintain the proper pre-load which will withstand the tensile, bending and shear loads imposed upon the assembly.

Developing Pre-Loads

When a fastener (bolt) is tightened, both a tension and torsional stress is established within the bolt. This establishes a pre-load (clamping force) that is developed and applied to the assembly. When we torque a bolt to develop a clamping force of 8,000 pounds, this is the force the bolt is applying to the assembly, Figure 1.

Figure 1. A clamping load of 8,000 pounds is being applied to two plates.

If a bolt developing 8,000 pounds of force were used to hold two plates together, this force would be the equivalent of placing a 4-ton weight on top of the two plates, Figure 2. You can see in Figure 2 that any force applied to the plates that is less than 8,000 pounds would not be enough to separate the plates.

Figure 2. An 8,000 pound clamping load is developing the same force that a 4-ton weight would develop if placed upon two plates.

The reason for developing a high pre-load (clamping force) using a bolt is to make the bolted joint as strong, if not stronger, than the original material used for the assembly. Obviously, the greater the pre-load the greater the joint's resistance to the working loads which act to pull, bend, or shear the bolted joint apart. Increasing the torque over the manufacturer's specifications will not only increase the pre-load, but can actually cause the bolt and/or assembly to fail.

In order to ensure that the proper clamping force is obtained on every application, we must follow some rather simple procedures regarding matched fasteners. When the proper bolt, flat washer, lock washer and nut are selected we have a matched fastener that will perform to the designing engineer's specifications. When the assembly is properly torqued, this matched fastener system will ensure the fastener will withstand all design-specified loads placed upon the bolted joint.

Washers placed on the jointed material between the head of the bolt and the nut provide a larger load-bearing surface for attaining higher clamping loads. In addition, flat washers provide a consistent surface for head, or nut rotation during installation and reduce galling and burnishing by providing a smooth flat bearing surface. Flat washers should always be used under the bolt, or nut when a calibrated torque wrench is used. Where a high load distribution is required, e.g., when used with soft, thin and/or composite materials a flat washer is always used.

In addition, the washer must be correctly sized to fit the shank of the bolt to prevent damage to the bolt's shank and be concave (dish) resistant. If the washer is too soft, the washer will concave and fail to provide a uniformed load transfer and smooth bearing surface. When exposed to pounding loads a washer that is too hard could fracture and fall out of position, releasing the bolt's pre-load.

The function of the nut is to have its threads deflected slightly to evenly distribute the load through the nut and its successive threads. When an inferior nut is used with a superior strength bolt, it is very likely the threads in the nut will fail before sufficient torque can be applied to the nut. In cases where the improperly matched nut does not fail during the application of torque, a high load placed upon the bolt and/or nut can cause the nut's threads to fail while in service.
As a nut and bolt are tightened, the bolt and nut will elongate in the gripping area causing some thread deformation. Nuts generally fail when the threads of the nut strip (tear out). In other cases the nut will not fail, but a tensile failure of the bolt occurs. To help assure the bolt fails first and not the nut, the nut should have a higher proof load then the bolt.

Whenever a mismatched bolt, flat washer, lock washer, or nut assembly is used, we have unintentionally created a bogus fastener. While this mismatch can be easily prevented, a matched fastener's strength can easily be compromised when a job must be completed and we find ourselves reaching for any available hardware that will allow the job to be completed.

Bolt Stress/Strain Diagram

By over tightening we can easily damage a fastener assembly. Let's explore this further with a review of the mechanical properties of a bolt, figure 3.

Figure 3. Bolt stress/strain diagram.

Every bolt behaves like a spring. If we apply a tensile (a metal's resistance to being pulled apart) pre-load to the bolt it will elongate (stretch). Also present within the bolt is a force waiting to return the bolt to its original length. This force, known as elasticity, is a property that allows a piece of metal to resume its original shape after a deforming force is removed. It is this property of the bolt trying to return to its original length that gives the fastener its holding ability. As long as the bolt's pre-load is within the elastic range of the stress/strain diagram, permanent deformation of the bolt will not result. Once the pre-load has been removed, the bolt, just like a spring will return to its original length.

When an excessive pre-load is placed on a bolt beyond its proof load, the bolt will stretch. This limiting value of stress at which the bolt's elasticity ceases is known as the elastic limit represented as the proof load in figure 3. Beyond the proof load a bolt will not return to its original length. The bolt has now entered the plastic range where permanent deformation occurs. Therefore, when a pre-load is placed on a bolt that exceeds the bolt's proof load, the bolt will be unable to return to its original length.

A bolt can be placed in a device that will allow the bolt's clamping force to be observed. The clamping force the bolt creates can then be read on a conventional pounds per square inch scale. When this test is performed the following can be observed:

• A new matched fastener (bolt, washer and nut) when torqued to specifications, will easily reach its designed clamping load.

• Remove the pre-load and retighten the bolt and nut. The bolt will not reach the same clamping load as the previous test. Now, remove the nut and flop it over. Retorque the bolt. The clamping load will again increase to just below its original value obtained in the first test.What caused this to happen? The small amount of burnishing that took place on the contact threads increased the turning friction between the threads, thus changing the pre-load to an unknown value. Rotate (flop) the nut and the unused surface of threads (from the first torque application) are now making contact with the bolt's threads reducing the turning friction. Add some oil to the threads and the clamping load will again change by decreasing the thread's turning friction.

• Next, the bolt is tightened without a torque wrench. The clamping force continues to increase and then levels off even though the bolt and nut is still being tightened. The bolt has now entered the plastic range. Clamping force will no longer increase due to the bolt's elongating (stretching). The revealing part of this demonstration is when the bolt is removed in many cases there are no glaring signs that the bolt has been damaged. If however, the bolt has gone well into the plastic range damage will show up as a necking (narrowing) in the threaded area of the bolt.

• Retorque the bolt to the specified torque value and in most cases the bolt and nut will take the torque value as set on the torque wrench, but the bolt will not be able to obtain its designed clamping force. Thus, the technician will have applied the correct torque, but the fastener may not be applying the correct clamping force.

Once the bolt enters the plastic range and yields, the thread pitch increases (elongates) in a small section of the bolt. By placing a nut on the bolt and tightening it by hand, this damage can, in some cases, be detected, Figure 4.

Figure 4. Maximum stress in a bolt occurs within the first 1- 2 threads from the shank and within the first several threads of the bolt closest to the contact surface as indicated by the vectors.

Where the elongation damage has occurred, the nut generally becomes tight on the threads. The nut can in most cases still be turned by hand with a slight increase in turning resistance when passing over the threads in this damaged (elongated) area.

This area of excessive tightness between the nut and bolt may be your only clue that this bolt has been damaged beyond serviceable limits. Checking the bolt in this manner is a fast and easy check when no other non-destructive inspection (NDI) method is available. Upon closer inspection the bolt will show slight signs of damage in the form of galling or burnishing. In any case the bolt and nut should be discarded if any doubt exists.

The Need to Pre-Load a Bolt

By applying a specified torque to a bolt, we are pre-loading, or stretching the bolt so the applied pre-load exceeds the stress level the bolt is expected to encounter during normal service. This pre-loading of the bolt allows the bolt to exist in a level stress condition. When in this state the bolt should never exceed its elastic limit and will not be subjected to fatigue stress, Figure 5.

Figure 5. Applied loads must remain below the pre-load established in the bolt to prevent fatigue failure of the bolt/fastener assembly.

Therefore, when the bolt has the proper pre-load, also known as a pre-stressed condition, the bolt will not be affected by the cyclic, or changing loads occurring during normal flight operations.

Furthermore, to assure that the proper pre-load value has been obtained, a calibrated torque wrench must be used. Using torque tables will ensure the proper pre-load is obtained and reference to these tables is one way the technician has of assuring the bolt is pre-stressed correctly.

Tribology — the study of friction — plays an important role in obtaining proper pre-loads. Any lubricant, burrs or irregularities on the threads of the bolt, nut or washer will affect the torque value and the amount of pre-stress applied to the fastener. Torque tables are very specific and will state if the torque applied is obtained using dry, or oiled threads.

Oil when present on threads will reduce the turning friction. This reduced friction will cause the bolt to be over stressed by placing a greater pre-load on the bolt if oiled threads were not intended. With too much pre-load on the bolt, the bolt may encounter a tensile failure.

Alternatively, burrs or some other irregularity can cause excessive turning friction. In this case the bolt may never reach the intended pre-load (an under stressed) condition. Paint or corrosion under the flat washer can become displaced during usage and this will allow the pre-load to diminish. The bolt will now be subjected to all the destructive cyclic forces of fatigue acting upon it. Fatigue failure will eventually occur.

Extremely critical bolts (such as connecting rod bolts on piston engines) are pre-stressed based upon the actual stretch of the bolt, rather than by a torque value. Measuring the actual elongation of the bolt is the most accurate method of ensuring the correct pre-load has been applied. This method is not affected by the friction variances caused by dirt, burrs, paint, worn threads and inaccurate torque wrenches.

Summary

A technician must be aware that in the everyday removal and reinstallation of bolts, many variables are at work to undermine quality work. Every technician must be alert to anything that might affect the pre-loading of the bolt. Some good practices to follow are:

• Bolt and nut threads must always be cleaned and in good condition before their reuse.

• All mating surfaces should be flat, clean and free of paint so the washer can distribute its load properly.

• Holes should be free of nicks and corrosion before installing the bolt.

• Torque wrenches must be properly calibrated and used! Elbow torque is never a proper substitute for a torque wrench.

Every technician must be made aware of the needs to properly pre-load a bolt and the factors affecting a bolt's clamping force. By following proper procedures in selecting matched fasteners and applying proper torque, every technician may rest assured an important link in the safety chain remains strong.