Bolt Tightening Put To The Test

It’s important to get the basics right first when deploying your reliability/maintenance processes.

Nothing gets more fundamental than analyzing how nuts and bolts interact. Why would we spend time and energy experimenting with bolts and bolted joints? With as many years as they’ve been used, haven’t we learned all there is to know?

In reality, proper fastener practices and procedures have continually advanced and evolved. The 1980 Suzuki GS1000 service manual specification for head bolt torque has 25 words and one simple drawing. The 2015 Yamaha WR450F service manual has 173 words, 14 steps, and three drawings. Significant changes indeed. The three-person team that conducted this research included Pat Akins, Reliability Director for the Weyerhaeuser Wood Products facilities, CMRP; Ella Vormwald, Univ. of Tennessee, Knoxville, Senior majoring in Electrical Engineering; and Steve Mundy, Nuclear Engineer, consultant and instructor with the Univ. of Tennessee’s RMC.

Torquing a bolt establishes preload. Preload is intended to manage tension and shear loads for the two items bolted together. This is accomplished by forcing the bolt to stretch. The idea is that the bolt recoil will maintain a clamping force on the joint that exceeds the load it is expected to encounter. Additionally, they bolt and nut should be capable of handling shock loads, thermal stresses, and vibration.

Tools and Methods

For the study, a Skidmore-Wilhelm (model P) fastener strain gauge was used to test bolt clamp force. Other components used in the research included torque wrenches with verified recent calibrations (one of which provided degrees-of-rotation), lubricants, bolts, and the FTRM (Fastenal Technical Reference Guide). While many references have different values, FTRM Rev. 18 was the source for values used in calculating the torque wrench setting and providing accepted data to compare to experimental data.

The torque wrench, degree-of-rotation, and turn-of-the-nut methods for tightening bolts were tested. Bolts were finger-tightened before setting torque. The initial trial for the first two is the same: tighten with a torque wrench until it “clicks” and record the clamp force. Subsequent trials for the torque wrench method include loosening the bolt, tightening again until it “clicks,” and recording the clamp force. For the degree-of-rotation method, the user recorded the bolt’s amount of rotation from the initial trial, then tightened to the same degree of rotation each time, regardless of the torque required. For the turn-of-the-nut method, the fastener was rotated a predetermined amount, based on the ratio of its diameter to the clamp length. Ten trials per test were performed to obtain consistency.

Trials and Results

One set of trials was to repeatedly set the torque of dry fasteners first by tightening the nut and then repeat by tightening the bolt. Trials used zinc-plated 3/8-in.-16, Gr 5 and Gr 8 bolts.

It made little difference whether the nut or the bolt head was tightened. Another observation was that repeated torquing of dry fasteners resulted in significant drop in clamping force. After seven uses, Gr 8 settled to no higher clamping force than Gr 5. This was attributed to the higher torque creating additional damage to the sliding surfaces and thus more resistance to the applied torque.

Another series of tests was designed to determine how lubricants affect the clamping force with a repeated torque applied. In this case, GR 8 bolts were used and the same torque applied (37 ft.-lb.), regardless of lubricant. (Most torquing procedures reduce the recommended torque when fasteners are lubricated.) Lubricant was applied to both sides of the washer and to the threads of the nut.

The observation was that most lubricants effectively reduced friction such that much higher and more consistent clamping force was obtained than with dry fasteners. Thread-locking compound did not provide as much lubricating effect.

Another test compared the resulting clamping force achieved by the degrees-of-rotation method to that of a torque wrench. In this scenario, the bolt was first torqued to 37 ft.-lb., the degrees-of-rotation noted, then the bolt was loosened and returned to the same degrees nine times. All tightening was performed with dry 3/8-in.-16 zinc-plated Gr 8 fasteners.

Observations were that the amount of force required to return the bolt to the initial degrees increased significantly with each cycle, almost to the point of breaking. Returning the bolt to the original degrees provided repeatable clamping force, but at the risk of breakage.

Best Practices

Plants should reassess their bolting and torquing requirements to ensure they are consistent with today’s best practices. Bolts should be clean and free of corrosion, burs, or damage. A bolt that does not rebound to its original length should not be reused. When in doubt, throw it out.  

Lubricating the bolt provides much greater clamping force than dry bolts and tends to have more repeatable results. While the turn-of-the-nut and degrees-of-rotation methods provide more repeatable clamping force, using this method for repeated torque cycles of dry fasteners presents opportunity for failure. The turn-of-the-nut method is a common practice when using new bolts on structural configurations, but the bolts are not to be reused. For this reason, the turn-of-the-nut method is not appropriate for industrial equipment. EP

Based in Knoxville, Klaus M. Blache is director of the Reliability & Maintainability Center at the Univ. of Tennessee, and a research professor in the College of Engineering. Contact him at kblache@utk.edu.