RS Technologies


	Torque-Tension Testing

Overview

The amount of tension or clamping force exerted by a fastener when it is tightened is what actually holds the assembly together. Once the assembly is brought together, the fastener responds like a tension spring and the assembly acts like a compression spring. As the fastener is turned and load is applied, the fastener is stretched and the parts are compressed. This results in an elastic joint in which the fastener is the flexible member and the assembly is the rigid member.

Scope

This test procedure specifies a generic method for evaluating the effect of different surface finishes, coatings, or manufacturing processes on the torque vs. tension characteristics of threaded fasteners.

Typical Fastener Test Stand Components (as pictured)

Test Equipment

Test Bolts, Nuts, & Washers: Sufficient length to fit test fixtures, with uniform application of coating and lubrication.
Power Tool: a fixtured dc electric nutrunner capable of maintaining the required constant test speed to tighten and remove the test bolt or nut.
Torque Transducer: rotary torque sensor to measure the torque applied to the test fastener and equipped with an angle encoder to measure fastener rotation.
Tension Load Cell: a clamp force sensor that holds the test bolt and nut and measures the clamp load or fastener tension that is generated when the fastener is tightened. To determine friction coefficients, load cell should be capable of measuring thread torque.
Data Acquisition and Control Equipment: multiple-channel recorder capable of recording the torque, angle, clamp load, and thread torque if so equipped.

Click here for more information about our Basic Torque Tension Test System.

Test Procedure

Determine the torque and tension loads that are appropriate for the size of fastener being tested. These are typically specified by the required test standard or special requirements of the customer.
Select an appropriately sized nut and test washer. A new nut and washer should be used with each bolt that will be tightened.
Place the bolt being tested through the test fixture (load cell and adapters) and test washer so that the bolt extends beyond the test nut approximately six threads after bolt rundown.
Assemble the nut onto the bolt approximately two threads.
Place the nutrunner tool on the bolt and tighten to the specifed torque or tension value. Simultaneously record the torque that is applied to the fastener and the tension that is achieved.
Loosen the nut and return to the original starting position.
Repeat Steps 5 and 6 for the specified number of bolts to obtain data that can be statistically analyzed to produce torque at tension at 3 sigma limits.

Test Report

The test report for torque tension testing generally requires a multiple graphic plot of torque versus tension for the prescribed number of rundowns. A statistical graphic plot is also desirable. The testing can be further documented by a report of the numerical data to which statistics can be added.

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