Fastening Threads: Torque Control vs. Angle Control

December 23, 2009
/ Print / Reprints /
/ Text Size+
Angle control is a more fundamentally sound method of measuring the tension applied to a bolted joint.

In previous columns, I’ve explained why tightening fasteners to a particular torque does not mean the resulting bolt tension will be either accurate or repeatable. That’s one of the biggest challenges of using bolted joints that depend on clamp load. However, another means of tightening that does not rely on the torque-tension relationship is available to manufacturing engineers.

This method is often called “angle control” in assembly and “turn-of-the-nut” in construction applications. It is a more fundamentally sound method of tension control. There is always a direct correlation between bolt elongation and bolt tension until the bolt yields, which is not the case with the torque-tension relationship.

Differences between torque and angle control are more easily understood visually. Figure 1a could be the result of a bench test used to calculate friction coefficient or nut factor, where the resulting tension can vary for a given torque input. The amount of variation depends on geometry, surface finish and coatings, but the extent in Figure 1 is not exaggerated for some conditions.

Unfortunately, electroplated zinc, the most common fastener coating, has some of the worst torque-tension variation. Assuming the trace occurs in the elastic range, angle can be substituted for tension on the X axis (Figure 1b) to show how this condition impacts traditional torque-controlled tightening.

Because the friction effect of each bolt varies, the angle the bolt rotates before the installation torque is reached is not constant. Higher friction will cause torque to rise more quickly. Because the angle of rotation is smaller, the bolt tension generated is also proportionally less.

This is the theory behind angle control. Since torque only indirectly creates tension while the bolt elongation that results from rotation is directly proportional to it, let’s rotate to a specified angle rather than a specified torque and we’ll always get the desired tension. Of course, it’s not that simple in actual use.

First, we need to determine the angle that will achieve our desired tension. If we were elongating the bolt as we would a steel rod in a tensile tester, Hooke’s Law would make it easy to estimate the elongation required for a given tension. However, for every unit of bolt elongation in a joint, there is a corresponding (but unequal) amount of compression in the clamped components.

For simple hard joints, such as nongasketed steel plates, this compression can be calculated by the same method used for bolt elongation. For other less ideal joints, testing is required. In fact, testing is needed to utilize angle control for nonideal joints for another reason.

Joint stiffness, represented by the slope of a tension-angle trace, is not constant while the joint is being drawn together into complete contact. This is represented in Figure 2, which also illustrates a common solution to the problem.

Testing determines the torque at which the transition phase ends and the linear portion of the trace begins. Allowing for part-to-part variation, this “snug” torque is determined. The specified angle of rotation, which must be measured with sufficient accuracy, starts from this point. As bolt length-to-diameter ratios get smaller, more tension is generated per degree of rotation, so greater accuracy is required.

Less common, but more troublesome, is when the tension-angle trace is not linear, due to joint stiffness variation. While testing is required to establish angle control on all but the simplest joints, this is also true of torque control where in-joint tests or verified friction factors are needed.

So, the bottom line comparison between angle control and torque control is that joints with consistent stiffness and variable friction favor angle control, while consistent friction and variable stiffness favors torque control. Joints with neither consistent stiffness nor friction require a return to the drawing board.

Since the first joint type is probably the most common, and torque control is definitely more common in assembly applications, there are opportunities for many manufacturers to successfully employ angle control, particularly those using tightening tools with angle encoders.

Did you enjoy this article? Click here to subscribe to Assembly Magazine.

Recent Articles by David Archer

You must login or register in order to post a comment.



Image Galleries

Behind the Scenes at Ford's Michigan Assembly Plant

People are the heart and soul of the 2012 Assembly Plant of the Year. This slideshow shows some of the men and women who build three different types of electrified vehicles alongside traditional gas-powered cars on the auto industry’s most flexible assembly line—Ford’s Michigan Assembly Plant in Wayne, MI. Photos courtesy Ford Motor Co.


 Live from The ASSEMBLY Show, Bob Wood, president of ECI Spinnomatic, talks about his company’s newest product: a fully automated riveting cell equipped with a rotary indexing table, a six-axis robot, an orbital forming unit, and laser sensors. 

More Podcasts

Assembly Magazine

assembly january 2015 cover

2015 January

The 2015 January Assembly includes our report on Leak Testing Aluminum Castings plus much more. Check it out today!

Table Of Contents Subscribe

Plant Production

How much will your assembly plant produce next year?
View Results Poll Archive


Welding: Principles & Practices

This text introduces students to a solid background in the basic principles and practices of welding.

More Products

Clear Seas Research

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications,Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.


facebook_40px twitter_40px  youtube_40pxlinkedin_40pxgoogle plus  

Assembly Showrooms

ASSEMBLY Showrooms