Measuring clamp force is the actual goal in joint assembly with threaded fasteners. Clamp force is the force holding the materials together. "If we could measure clamp force, we wouldn't need torque," says Ray Reynertson, president and COO of Sturtevant Richmont.

However, there is no quick and convenient way to measure clamp force in an assembly environment. In the lab, assemblers can measure clamp force by using load cells and load washers. "These go around the bolt, in between the two pieces that are being put together. A wire extends into a display unit. The unit then tells you the force," says Doug Hall, CEO of AIMCO. "You would use that in the lab when you're trying to figure out what clamp force you're getting. But that is not practical to use on an assembly line. You can't do that on every single fastener. So measuring torque is the only way."

But what is torque? "Basically, distance times force equals torque," says Reynertson. In more layman terms, torque measures the turning or twisting force applied to an object. By precisely applying torque, the objective is to clamp parts together with a tension greater than any external force trying to separate them. The part then remains under constant stress and is immune to fatigue.

Unlike clamp load, torque is one of the most easily measured variables to determine how well parts are assembled. "Traditionally, it has been assumed that if the torque is sufficient enough, it should clamp the parts together. So you should have a good assembly," says Dave Miller, business development manager at PCB Load & Torque, a division of PCB Piezoelectronics.

How Is Torque Determined?

When determining correct torque specifications, engineers must take several variables into consideration. The first consideration is the maximum load that may be placed on the fastener. The second consideration is the strength of the material that is being joined, and the third consideration is whether the joint is hard or soft. "Because a hard joint has very stiff materials clamped together, you're not going to have to turn the fastener much to get the needed clamp force. A soft joint has softer material, so the fastener will have to be turned a little farther before you get the right amount of clamping force," says Miller.

A variety of tables and standards are available to provide recommended torque values, depending on the joint type and the size, design and material of the fastener. However, with the influx of plastics and composites in assembly, these traditional torque tables have been turned on their head. "If you have a fairly complex stack-up, where you have maybe a couple of steels, brackets of a softer, stamped material, and a plastic, it could take quite a bit of time to determine exactly what the assembly torque should be," says Miller. "If the joint is critical enough, you may have to use some advanced assembly methods, such as a torque-to-turn method, where you apply so much torque to bring the parts into contact, and then you turn the fastener a given number of degrees to reach that amount of clamp force."

Sometimes, engineers simply work by trial and error using the established standards and tables. "They start with a specific torque, and if over the course of a few months, no real quality issues arise... then it probably works. There is no great need to change it or to be that much more accurate," says Hall.

Measurement Methods

According to Reynertson, assemblers should measure torque in anything that resists rotation. If it turns, torque measurements should be taken.

But when and how should torque be measured? There are three places that torque can be measured: before assembly, during assembly and after assembly. Measurements taken during each phase will answer different questions. In some cases, all three measurements must be taken to ensure quality.

Measuring torque before assembly answers the question: "Can my tool do the job?" Assemblers simply run their tool several times to check its torquing accuracy.

The best way to measure torque during assembly is to rely on electric and pneumatic tools with built-in reaction torque sensors. These sensors measure the amount of torque being applied to the fastener. If the tool doesn't have a built-in sensor, a rotary torque sensor can also be placed between the tool and the workpiece. "Often, [a rotary torque sensor] can be a little more accurate, because it is not going through the gearing of the tool," says Miller. "You can measure torque dynamically that way and get a pretty good idea of what the tool is producing and how much energy it is transferring into the fastener."

Generally, power tools are used in conjunction with a computer-based data collection system. Automatic storage and retrieval of tightening data can be an important tool in statistical process control.

Measuring torque after assembly requires a torque audit. A torque audit answers the question: "Is my product good before it goes out the door?" After assembly, inspectors check that all the fasteners are in and measure how much torque is left over in the fasteners.

Torque audits are typically performed with a click wrench or dial wrench. A click wrench gives off a clicking sound when a certain torque is reached. A dial wrench displays the torque value on a dial with a needle. An inspector would put the click wrench or dial wrench on the bolt and then start to put pressure on the fastener. At a certain point during the pulling, the fastener will break loose as it is being turned in the tightening direction. When a slight movement is attained, the quality inspector stops tightening. If the fastener is at the correct torque to begin with, the wrench should click before the fastener starts to move. That means the fastener has the minimum amount of torque in it. On a dial wrench, the needle will point to the measurement.

Typically, 100 percent of critical joints will be inspected, either by recording the data at the time the fastener was tightened or during post-assembly. For less critical fasteners, a more random audit might occur to ensure the assembly has been completed correctly.

The torque reading should be checked as soon after the tightening operation as possible and before any subsequent process. If the fasteners are left too long, or subjected to different environmental conditions before checking, friction-and consequently the torque values-can vary. Variation can also be caused by embedding of the threads and nut face, and joint surface. This results in tension reduction and affects torque.

And most importantly, assemblers should ensure that the individual conducting the audit is correctly trained. "Anytime you have a measurement system that involves a human being, [that person] has to know how to do it right... That is independent of whatever tools you happen to be using," says Frank Skog, who recently retired as product marketing manager at ASI DataMyte Inc.

Factors That Affect Torque

Friction in the threads of the fastener and under the head of the fastener can affect the amount of torque that is applied to that fastener. "Most of the energy going into clamp force is lost to friction. Only about 10 percent of the energy is left over for clamp load. So only 10 percent of the energy is holding the pieces together; 90 percent of the energy dissipated into heat. So what that tells us is that if you change something about that joint that changes that friction, you can make drastic changes in clamp force even if the torque stays the same," says Hall. "If you change your friction 5 percent from 90 to 95, you're changing the clamp force by half. You've cut it from 10 percent to 5 percent." So even if the tool is providing the exact amount of torque each time, the assembly could still come loose. Many times, assemblers mistakenly assume that the tool is malfunctioning, but most commonly, something simply changed in the joint that changed the friction. The same amount of torque is being used, but because the friction increased, the clamp load decreased.

Something as simple as adding or subtracting lubricant to the fastener can produce great swings in the clamp force. For example, if a certain amount of lubrication is expected in a joint, the torque specification may be set at a certain measurement. If that fastener is not lubricated sufficiently, there will be greater friction in the joint. Even though the tool may have reached the required specification, it may not have been enough to tighten the fastener, because of the excess friction that is present.

"If you change things midstream, it can have a big effect. You then have to re-evaluate what torque you're going to use," says Hall.