When “tight” is too tight

As power tool manufacturers pack more power output into cordless tools, that extra torque which helps workers bully through tough fastening jobs may simply be too much power for smaller fastening tasks.

Some of the newest cordless drill/drivers on the market today can put out more than 500 in.-lbs. of torque, and impact drivers, their more powerful and smaller cousins, can produce nearly 1,700 in.-lbs. of torque. That amount of force can literally turn the head off a screw or strip the threads right off a fastener.

Or worse, the worker’s zeal to achieve a tight fit takes the fastener just past its maximum torque level, leaving it looking like it’s ready to hold for the life of the project, only to have it fail once vibration or sheer forces are exerted onto it.

Fastener physics

“There is a minimum tightness for any joint,” says Richard Wright, mechanical engineer and chairman of Wright Tool Company. “Below this point, the joint may come apart in service either by working loose or by bolt breakage.”

Experts at Rockford International, a fastener supplier, report that preload is the critical factor in creating a safe, effective fastener joint. Preload is the tightening of a fastener to its proper clamp load, which is the amount of tension in the bolt that is equal to or greater than the maximum external load to which the joint will be exposed.

Proper preload keeps the bolt or cap screw from loosening, protects it from fatigue and keeps the joint from slipping or separating.

Acceptable preload falls between the clamp load and the proof load for the fastener. Proof load is the load just below yield strength that can be applied to a bolt without causing permanent deformation beyond 0.0005". It is approximately 75 percent of its tensile strength.

Clamp load can vary depending upon the application, but it is usually achieved at about 55 percent of tensile strength. Therefore, proper preload should fall approximately at 55 percent to 75 percent of the tensile strength of the bolt.

Preload is worker-defined

The person running the wrench largely determines proper preload. Any technician can cause any fastener to fail simply by over tightening or under tightening it.

Rockford International experts recommend that the best way to install a fastener is with a torque wrench, but even that does not guarantee right preload. Torque values needed to properly tighten a fastener vary greatly with the condition of the fastener’s threads.

An unplated fastener with clean, dry threads requires about one-third more torque than a plated fastener to achieve the same preload. The same unplated fastener, when lubricated, needs only 60 percent of the torque required for dry threads. Manufacturers produce torque specification tables for installing fasteners, but due to the condition of the fastener at installation, they can only be considered a guideline.

Workers hold the power

The best way to address the over tightening dilemma with today’s powerful tools is to attack the tightening task with minimum, instead of maximum power, says Peter McCourt, vice president of sales of electrical fixing and fastening products at Erico. “As long as the tool has an adjustable torque level, a professional user can tighten fasteners without over tightening them. The key is to start with the lowest setting and drive fasteners, working up to higher torque levels to the point where the fastener gets fully driven into the material. Many tool users start at the highest power level, then work back. The more delicate the base material, the more careful you’ll want to be,” he concludes.

Published in the May 2005 issue of Contractor Tools and Supplies magazine.

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