Angle monitoring

Features - Assembly

Torque control alone may not achieve the target clamp load.

July 8, 2019

While most manufacturers use sophisticated equipment to control the torque when tightening threaded fasteners, angle monitoring is also essential to ensure consistent quality. Torque measurement may be misleading because every fastener, hole dimension, material strength, and lubrication condition differs, making each assembly unique. Torque control may not achieve the target clamp load, leading to potential rework or safety-critical errors.

A fastener behaves similarly to a stiff elastic band: the more it stretches, the more force reacts to return the fastener to its original position. This clamping force holds the joint components together and helps the assembly remain secure. Elastic fasteners must stretch enough to apply the proper clamp load but not so much that the joint components or fastener become damaged.

As a bolt rotates, it stretches, and the joint compresses. Angle rotation and fastener thread pitch dictate the axial distance, equivalent to the stretch and compression.

Angle monitoring can indicate proper tightening. The angle is proportional to the bolt stretch, which is directly related to clamping load. Measuring only torque may miss variability or wasted energy to overcome friction. Angle monitoring technology gives operators peace of mind that they are securing each fastener and joint with the appropriate clamp load.

Insight QC electric tool controller

Fastening control

With torque control technologies, manufacturers set torque targets based on clamping requirements for each bolting application. However, torque targets make assumptions about the range of friction that the tool encounters during fastening. Angle monitoring ensures the assumptions hold true.

The science

Only about 10% of a fastening tool’s torque productively stretches the fastener. The remaining torque helps overcome friction under the head and in the threads. Friction varies with differences in lubrication, part dimensions, surface finish, plating, and fastening speed. Too much friction causes low clamp load. Torque controllers alert operators when they meet torque requirement, even if the proper clamp load is not applied.

Tools with torque and angle monitoring measure torque – the force that binds parts together within a specific friction range, using angle monitoring to verify that range. Precision fastening tools, such as the DC and QX Series from Ingersoll Rand, offer torque and angle capabilities.

Ensure proper tightening

Angle monitoring is essential in safety-critical applications, such as aerospace assembly. In airframe assembly, for example, manufacturers use break-off fasteners to visually confirm the fastener stretches enough to apply the proper clamp load. Break-off fastener material characteristics primarily control the clamp load achieved during fastening. A piece of the fastener breaks off when the tool applies enough clamping. Even if break-off fasteners come apart as expected, tools that measure torque and angle can confirm fastener material characteristics.

Tools detect abnormal conditions that operators would not discover when measuring only torque, including:

Cross threading: A tool with angle monitoring can detect more-than-average torque at onset and stop to mitigate cross threading.

Shallow holes: With too-shallow parts, the fastener bottoms out at the end of the hole, causing a quick torque increase. If tools that only measure torque shut off, operators may meet zero clamp load. Angle monitoring identifies difficult-to-detect shallow holes and alerts operators.

Weak parts: Cracked, weak, or warped parts may slip through assembly. While operators achieve target torque on defective parts, angle monitoring catches defects.

Also, angle monitoring automates part inspection, eliminating manual angle measurements.

Ingersoll Rand

About the author: John Linehan is technical director, electrical systems engineering for Ingersoll Rand advanced assembly power tools. He can be reached at or 800.483.4981.