Improper bolting leads to decreased machine reliability and increased life cycle cost

Sept. 15, 2002
Improper bolting has the biggest impact on machine reliability and life cycle cost.

I would like to discuss the most ignored maintenance practice, improper bolting.

Proper bolting is a fundamental requirement of equipment reliability, but is often ignored. To prove this point, take a walk through your plant. Using only your fingers, see how many critical bolts,foundation, machine housing or piping flanges, you can loosen. If your plant is typical, you will find a substantial percentage of them loose enough to removed without the need of a wrench.

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There is nothing magic about proper bolting techniques. It's simply a matter of proper fastener selection, installation and periodic inspection. Visual inspection alone will uncover widespread abuse of acceptable bolting practices. For example, you'll find the use of standard SAE Grade 1 bolts instead of coupling bolts; the use of multiple flat washers to make a connection; the use of helical spring lock washers; and many others. In one plant, I found that maintenance technicians had ground the shoulder of a 1/2-in. Grade 5 bolt to 3/8 inches to permit lateral movement and to facilitate alignment of a 500-hp motor. Yet, the technicians wondered why these bolts failed continuously.

Selecting the proper fastener

Contrary to popular opinion, all fasteners are not the same. Each application has specific requirements, such as torque, weight, vibration, etc., that determine the type or grade of fastener required. Follow these requirements without exception. Failure to do so reduces machine reliability and increases maintenance costs.

One primary consideration for bolt selection is the torque and impact loading of the machine. These factors determine the grade of bolt needed. While they are difficult to visualize, the torque and recurring impact generated by production equipment are substantial. In many applications, torque and recurring impact exceed 100,000 psi. The fastener must have enough tensile strength to absorb these forces and prevent damage caused by mechanical looseness. Bolt tensile strength ranges from 64,000 psi for a SAE grade 1 bolt to 150,000 psi for a grade 8 bolt. If a grade 1 bolt is used in a 100,000-psi application, it will gradually stretch and eventually fracture. The resultant looseness also damages the machine. Mechanical looseness causes excessive movement within the machine or misalignment between machine-train components. Regardless of where it occurs, looseness accelerates wear and reduces operating life.

Using washers properly


Another prevalent problem is the misuse of washers. This is especially true of split-ring or helical spring lock washers. Most applications only require flat washers, which are designed to distribute bolting pressure and machine load uniformly. However, the washer must be selected and installed properly. At a minimum, match the washer to the SAE Grade of the bolt and nut. In addition, the washer's inside diameter should be just large enough to fit around the bolt. Too large an inside diameter causes more damage that not using a washer.

Avoid using split-ring or helical spring lock washers as much as possible. If one must be used, exercise extreme caution to ensure that it's torqued properly. Too little torque means that normal machine vibration or impacting will fatigue the lock washer and cause it to fail. If this occurs, the fastener is no longer functional and machine damage will result. Too much torque crushes the lock washer. As a result, its anti-rotation characteristic is lost, and the broken washer eventually works its way out of the fastener.

Torque wrenches make life easier

A wonderful, new invention should always be used for fastening machine components. It's called a torque wrench. Don't laugh,too many plants don't use, or even have, one. Instead, technicians use the biggest cheater bar or the strongest millwright to lock fasteners in place. This simply doesn't work. Every application has a specific torque requirement. Too little torque doesn't provide enough mechanical stiffness and permits the fastener to loosen further as the machine operates. Too much torque stretches the bolt, which reduces tensile strength and leads to premature failure.

Follow proper bolting practices universally. The costs generated from poor practices can be substantial. Proper fastening procedures require that the proper fasteners, i.e., bolt, washer, and nut, are used in each application; fasteners are properly assembled; and proper torque is applied. None of this is difficult. Most operation and maintenance manuals provided by original equipment manufacturers include information on proper fastener selection, assembly and installation. To ensure best practices, list this information on work orders. The only remaining issue is to get rid of the pipes and other levers used to tighten fasteners, and to mandate the use of calibrated torque wrenches. While this final task may be the most difficult, with proper supervision, it's not insurmountable.

A simple change in bolting procedures will generate more positive change in reliability and life cycle cost than any other single maintenance practice.

Contributing Editor Keith Mobley can be reached via email at [email protected].

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