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In many plants, the maintenance instructions provided with work orders can be terribly vague. Details on the proper tightening of threaded fasteners, however, is essential in avoiding looseness-related vibration in rotating equipment and leakage in piping flanges.

Assuming work-order instructions specify the proper size and quality of necessary threaded fasteners, rarely do they provide details on the appropriate torque value, fastener-lubrication requirements, and sequence in which to tighten such fasteners. All three of these elements are required to achieve controlled tightening. Let’s address each of them here, one at a time.

TORQUE VALUE
Torque relates to the force applied to the threaded fastener. The imperial unit is pound-feet (lb-f) or pound-inches (lb-in). The SI unit is the Newton meter (Nm). One lb-f = about 1.36 Nm and one lb-f = 12 lb-in. If you apply on pound of force one inch from the rotational center of the threaded fastener, you increase the force by a factor of 12 if you increase the length of your wrench to 12 inches, or one foot.

Remember: the longer the wrench, the greater the force. That’s why a pipe placed on the end of the wrench is sometimes called a “cheater bar.” The force can also be increased with a geared torque multiplier.

The reason torque is so important is because it’s necessary to achieve enough tension on the threads of a fastener to secure it and, thus prevent it from loosening—particularly in the presence of vibration.

As I recently pointed out in my article/newsletter column titled “Regarding Your Threaded Fasteners” (Nov. 4, 2019), the threads of a fastener behave like a spring: They’re elastic. When it comes to tightening procedures, we want to achieve enough force to secure the fastener, but not so much that we exceed the elastic limit of the fastener.

For bolts that will be reused, we typically aim to tension to about 75% of the fastener’s proof load, or elastic limit. For single use fasteners (torque-to-yield bolts) such as engine head bolts, we typically torque to 90-95% of the proof load. The size of the fastener, the grade of the fastener, and the thread type (fine or coarse) determine the appropriate torque value. These values must be included in the fit, tolerance, quantity, and quality details for all maintenance instructions.

FASTENER LUBRICATION
While we apply torque and measure the force with a torque wrench, the goal is to achieve tension on the threads—and torque doesn’t always equate to tension.

The formula for calculating required torque includes the “K” factor to account for friction. Most torque charts include values for lubricated and non-lubricated fasteners. (Forget the non-lubricated value. It’s guesswork, at best.) As much as 90% of the torque applied is lost to friction.

Frictional losses starve the threads of the tension they need to create a securely fastened joint. About half of the frictional force is lost in the threads and the other half is lost on the nut face. (NOTE: it’s best to keep a bolt static and turn the nut when tightening a fastener.)

In light of the frictional-force loss in the threads and on the nut face, work instructions must direct the maintainer to apply lubricant to the threads AND nut face. Machine oil is acceptable. Extreme-pressure (EP) gear oil is better. A good molybdenum disulfide thread lubricant or nickel anti-seize, though, is best for controlling tension and creating a strong, tight fastened joint. (NOTE: It’s a good practice to clean and de-bur the threads prior to applying the lubricant.)

FASTENING SEQUENCE
Most fastened joints involve multiple fasteners. It’s very important for maintenance-work instructions specify the sequence in which such fasteners are tightened to prevent possible soft- soft-foot issues in equipment and leaks in pipe flanges.

First, we must specify the geometrical sequence. Tighten the fasteners using a geometrical crisscross pattern to ensure that the force is applied evenly.

Then, we must specify the torque sequence, which should include 1) wrench/spanner tight, following the geometrical sequence; 2) one-third torque value, following the geometrical sequence; 3) two-thirds torque, following the geometrical sequence; 4) final torque, following the geometrical sequence; and 5) final pass, following the geometrical sequence.

THE ROI FROM PRECISION
Incorporating details on torque values, fastener-lubrication requirements, and, required-fastening sequence in all corrective- and preventive-maintenance instructions will help ensure that personnel are building precision into your site’s fastened joints. Once that takes hold, sit back, relax, and watch as vibration and leak problems begin to evaporate.