Stud and Nut Tension

To examine stud and nut tension, we’ll conduct an experiment that tests the tension achieved in a fastener, given specific amounts of torque. This will demonstrate how friction affects the tension achieved in the fastener, and the dramatic consequences of increasing that friction.

With the help of a Skidmore-Wilhelm torque-tension tester, we will monitor the tension achieved in the fastener with varying amounts of torque. To ensure accurate torque is being applied, we will use the RAD-1800 pneumatic torque wrench.

Using a new stud and a new nut:

At this point, we want to see what the tension will be if we continue to use the same stud and nut, but decrease the amount of torque applied ­- back to the original 500 ft/lbs. If the result shows a lower tension than was originally achieved, we will then see how much more torque is required to achieve 28,000 ft/lbs (as shown above, this was originally arrived at with 600 ft/lbs of torque).

Results

With each successive application of the fastener, the amount of tension decreases. To achieve the same amount of tension that we reached in the third attempt, we needed to apply 33% more torque. We made two more applications, at 500 ft/lbs of pressure, and the tension continued to decrease (15,000 and 14,000).

Observations

The nut had galled somewhat where it contacted the object surface; however, the nut and the stud were both still in pretty good condition. In reference to tightening a fastener, John Bickford notes in his book, ‘An Introduction to the Design and Behavior of Bolted Joints’, that, “Typically, about 10% of this input work ends up as potential energy stored in the joint and bolt “springs.” The rest is lost in a variety of ways. Most is lost as heat, thanks to friction restraints between the nut and joint surface and between male and female threads.”

Standard torque specifications

The following torque specification chart applies to standard materials for bolts, nuts, and studs, based on S.A.E. bolt steel classifications.

Recommended maximum torque values + or – 5 %

NOTE: Where materials are other than S.A.E. Standards, refer to the following conversion chart:

The above torque values are offered as a guide only. Torque specifications, especially for critical joints, should be determined under actual assembly conditions, due to many variables involved which are difficult to predict and affect the torque/tension relationship.