The Direct Tension Indicator

Production facilities that utilize moving webs have to exert some form of tension control. However, this is not the only case in which having control over tension is imperative. Those who build structures need to know if the fasteners they use in slip-critical or other situations will hold up. They need a device that will notify them about whether the tension is correct. One of the pieces of equipment that accomplishes this successfully is a direct tension indicator.

Tension Indicators (DTIs): Description and Function

DTIs became a part of American manufacturing in the early 1970s. These single-use mechanical load cells resemble hardened washers but feature raised protrusions on one face measure the tension. They measure the bolt tension that occurs during the tightening process. The operator affixes the director tension indicator (DTI) beneath the nut or bolt head. The protrusions on the DTI produce a gap. When pressure, through the rotation of the nut along the bolt thread, occurs, the action crushes the protrusions. These become flat, reducing the existing gap.

The very design of the DTI specifies the gap at which this action occurs. This allows engineers to inspect the bolts, checking to see if they comply with the specified tension requirements. This system effectively provides a consistent tightening. Moreover, it occurs regardless of three significant factors:
1. Friction coefficient
2. Contamination
3. Thread-fit or coating

The ease of use and the simplicity of inspection explain why many companies opt for direct tension indicators. Today, they find applications in structural fastening assemblies. They indicate whether the tension is proper in fastening assemblies for steel bridgework

The Direct Tension Indicator

DTIs are simple but effective. They control tension using a device resembling a washer. Yet, the very simplicity of a direct tension indicator works in its favor. It is a favourite of many state DOTs when it comes to choosing the most effective structural fasteners for bridgework and other crucial public works.

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