When To Use a Bolt Tension Calibrator

Torque is a poor way to ensure joint tightness on high-load and safety-critical joints. A better method, mandated in some situations, is to use a bolt tension calibrator.

Some readers may be familiar with Skidmore testing. This blog explains when to use a Skidmore-Wilhelm bolt tension calibrator rather than tightening to a specified torque.

The Unreliability of Torque Measurement

The goal of establishing torque values is to ensure the joint is not too loose or too tight. Once verified, these values specify the tightness of a joint. Even when following torque testing best practices, there are better approaches than this when achieving a specific clamping force is essential.

The problem is that the friction between the bolt and the thread it’s going into strongly influences torque. With high friction, the specified torque may be reached long before achieving the required clamping force.

Tension: The Torque Alternative

A Skidmore-Wilhelm bolt tension calibrator measures the clamping force achieved in a connection, hence the bolt's tension. This approach accounts for friction variation and verifies that the bolting method will give the required performance. People familiar with this device often call it Skidmore testing.

When to Use a Bolt Tension Calibrator

There are three situations where you should choose Skidmore bolt testing.

1. When required for Pre-Installation Verification (PIV)
2. As part of Rotational Capacity (ROCAP) testing
3. Whenever bolt testing is needed or beneficial

Pre-Installation Verification

PIV has two objectives: to ensure the bolt, nut, and lubricant are up to the task and the assembly process is appropriate. PIV is mandated by Research Council on Structural Connections (RCSC) standards when bolts are going to be pre-tensioned, as is the case in many construction projects.

The RCSC tells us that PIV achieves five things:

1. Confirms that the joint – bolt, nut, and lubricant – meets design objectives
2. Verifies the pre-tensioning method is appropriate
3. Establishes the torque to be applied
4. Demonstrates that the torque applied will achieve the initial required
5. Verifies the correct equipment used for bolt tightening

A PIV needs a Skidmore-Wilhelm bolt tester that’s been through appropriate calibration.

Rotational Capacity Testing

Highway bridge construction projects require ROCAP testing. Its purpose is to show that a bolted assembly will produce the desired pretension load.

The American Association of State Highway & Transportation Officials (AASHTO) details specific requirements. These are also incorporated into State DOT and FHWA specifications and described in ASTM F3125.

Note that these standards require ROCAP testing in addition to PIV.

Anyone needing to carry out ROCAP testing should follow the ASTM procedure. For information only, the main steps are:

1. Install the bolt in a Skidmore
2. Tighten the assembly to 10% of the proof load.
3. Rotate the nut until achieving the rotation detailed in the specification

The rationale for ROCAP testing is straightforward: bolt manufacturers determine strength with a direct, axial pull, but this isn't what the bolt sees in service. When used on a construction project, the bolt experiences torque and tension. This rotation could fail a lower load.

Whenever Bolt Testing is Needed or Beneficial

Besides highway bridges and other construction projects, there are many situations where testing bolt tension will help to ensure the required joint strength. An obvious example is flanged pipe joints used in petrochemical and oil and gas applications. Correct bolt tension is key to satisfactory gasket performance here, especially when flanges are hard to reach.

Should other examples be needed, consider the bolted joints used in wind turbines or power generation. In these and many others, correct tension ensures safety and reliability.

Don’t overlook the use of a hydrostatic test pump for tensioning bolts.

Bolting Tools and Services From Aztec Bolting

In construction, oil and gas, petrochemical production, and power generation, it’s often essential to achieve a specified level of bolt tension. Aztec carries industry-standard Skidmore-Wilhelm testers, as required for PIV and ROCAP testing, and also offers on-site bolting services. Contact us to learn more.