Should I choose fine or coarse threaded bolts?

Bolted joints are many times ignored when it comes to ensuring long-term reliability of equipment. With most fastened joints, maintenance practices can be sloppy when the original design is good with little impact to overall equipment reliability. This can lead to complacency when executing details of joint assembly and maintenance. While the immediate reliability impact may not be noticed, the insurance that a properly assembled bolted joint can provide should not be overlooked. Bolted joints are an integral part of equipment and component reliability as they connect a majority of our process equipment by coupling shafts together, holding down equipment or holding together the critical components.

Just like any reliable system, there are many details to get correct, and like any failure there are many details to overlook when it comes to maintaining a reliable bolted joint. While there are elaborate solutions to some joint issues, most reliable joints come down to getting the basics right. As with most equipment and component reliability, there are several key areas to focus on such as design, installation, maintenance and operation. While design is a foundation for equipment reliability, the scope of this article will concentrate on installation and maintenance of the bolted joint for best reliability.

Installation and assembly for a reliable bolted joint has some fundamental elements. Here are 7 key installation areas to pay attention to assure a reliable bolted joint.

1. Check that the two mating surfaces are flat. This will result in even loading on the joint with consistent compression and ensure that the bolt load is perpendicular to the joint. In addition, some joints may also be subject to leaks if the surfaces are not flat.

2. Check bolts, nuts and washers to make sure they are correct for the assembly. Are the bolts the correct grade (SAE 2, 5, 8, metric 8.8, 10.9, etc.)? Are the bolts the correct metallurgy such as carbon steel, stainless steel or B7 for high temperature? Is the bolt length correct and threaded length correct? Some joints are designed to keep threaded portions out of shear plane of joint. Before tightening, verify that any shank portion of a screw does not bottom out in the hole as in Figure 11 and Figure 1.2. This will not only lead to damaged threads but will lead to a loose joint as no preload or clamp force will go to the joint even though you will see plenty of torque during assembly. Neighboring bolts will also now shoulder that joint stress and increase the probability of failure.

3. Ensure proper thread engagement at assembly. Since a majority of the bolt stress is taken on the first few threads, inspect the condition of the first several threads being engaged. For full engagement, make sure the bolt sticks a couple threads through the nut to fully engage bolt and nut to avoid short bolting condition. The lead threads are not full so the nut should not be flush with the end of the bolt but stuck through a few threads.

4. Don&#;t adjust assembly while it is tight as this will put uneven loads on the bolt and can eventually lead to bolts loosening up during machine operation. Loosen bolts if any machine or flange movements need to be made such as for alignment while the machine is down and locked out.

5. Choose correct torque tools and methods. Different methods have different associated errors. Torque methods include by feel, torque wrench (dial, clicker, digital, hydraulic), turn of nut (used many times on very large bolted connections) and ultrasonic bolt elongation. Don&#;t use an impact wrench to preload bolts. Overload and large variations of bolt preload typically result. Also use smooth motions when using a torque wrench. Jerking the wrench can also cause large preload errors.

6. Choose the correct bolt torque amplitude to achieve proper bolt preload. Bolt torque can be calculated by the equation below once the friction factor, K, and target bolt preload, F, are determined.

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Torque(in-lbs)= K * d* F

Where

K = Nut Friction Factor (dimensionless)

D = Nominal Bolt Diameter (inches)

F = Bolt Preload (lbs); use 75% proof load for joints that will be disassembled; use 90% proof load for joints that are permanent like structural members.

The calculated torque is only as good as the variables used in the equation. Know your nut friction factor which is influenced by installation condition (dry, lubricated, corroded, etc.). Use lubrication on bolts if possible at assembly (oil, anti-seize, Loctite, etc.). Most will lubricate the threads of a bolt but miss lubricating under the head of the bolt. Typically over 60% of bolt torque is lost by friction under the bolt head. Using lube will reduce bolt preload scatter, protect threads and make disassembly easier later. If dry torque is used then make sure the correct magnitude is used. Bolt coatings can also greatly affect friction factor. Use lubricated torque to increase the life of the nut particularly when the bolt is threaded into the machine-screw application.

7. Follow bolt torque sequence using a crisscross pattern. Also use torque bolts with ramped torque amplitudes of 30%, 60% and 100% torque to assure even joint preload. While executing this is preferred on all bolted joints, it can be more critical on some large joints where the assembly can get cocked or easily have large preload scatter such as on tapered hub type designs.

Once a good joint is designed and assembled, proper maintenance of the bolted joint will become critical to keep it together without failure. Retighten bolts after machine warm up especially on critical applications. Thermal growth and other factors can move the joint which can loosen. For extra insurance other methods may be employed to ensure long-term bolt preload. These may include Loctite, special lock washers or nuts, tie wire bolt heads or tack weld bolt heads so backing out will not occur. A torque wrench should be checked annually to verify that it is still producing consistent accurate results. A torque wrench should also be stored at the lowest possible setting. Use new fasteners as much as possible. This is cheap insurance for most joints. Declining and less consistent, repeatable bolt preload results with each reuse of bolts and nuts.

When key design, assembly or maintenance elements are not carefully attended, bolt failure can result. Some common bolt failure modes are fatigue, overload, stripped threads, shear or corrosion. Fatigue and overload are two of the more common failure modes. Many bolt failures occur at the first engaged thread of the bolt as that is the point of the highest stress. Failures under the head or first thread are also common areas of bolt failure as shown in Figure 2. As long as a bolt is not over torqued on installation then the most likely source for overload is an extreme external load on the joint.

Fatigue can be a more subtle and challenging failure mechanism to keep in check but can be improved with the right precautions. Here are three key actions to improve the fatigue strength of the bolted joint.

1. For the bolts, use bolts that were made with rolled threads instead of cut threads as fatigue strength is greater with roller threads. Most standard bolts are made with rolled threads, but special machine design applications that use special-made bolts may not be. Using a higher grade material will also increase fatigue strength as endurance limit is increased.

. In the assembly, assure that the head angularity is less than 1°. Fatigue increases sharply beyond 1° head angularity. Counter bore to true bolt head angularity is a common fix for some applications such as motor feet.

3. During assembly ensure that bolt preload is sufficient as previously discussed. This will minimize the cyclic load that the bolt will see which will increase the resistance to bolt fatigue. As seen on the joint diagram on Figure 3, once the external force on the joint exceeds a low-bolt preload, the joint compression can quickly be compromised and then all the external force may be seen directly on the bolt. Consequently a bolt with higher preload will result in the joint absorbing more of the external cyclic load and insulate the bolt from fatigue.

While most bolted joints have a significant enough design that errors in details of installation and maintenance may be overlooked without noticeable impact to overall reliability, there are many challenging applications where precision assembly must be followed to avoid equipment failures. For the others, following bolted joint best practices just builds in extra insurance for the unknown things that can happen to our equipment. Complex applications with rotating bending stresses, reversing loads, thermal stresses and screw (blind drilled) machine designs can offer real challenges to achieve a reliable bolted joint. Designs for these applications need to be especially robust to counter these external factors. Bolted joints are simple on the surface, but it takes doing all the little things right to ensure a reliable bolted joint of any design. If you have a bolted joint that is all of a sudden having failures, review the fundamentals for joint assembly to improve bolted joint reliability.

Randy Riddell is reliability manager at SCA. Edited by Joy Chang, digital project manager at CFE Media, .

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