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FULLERKREG® Direct Tension Indicators (DTI's) are the simplest, easiest to implement, and most cost-effective method for ensuring quality in bolt tightening operations. Compliant with ASTM F959 / F959M, RCSC, AASHTO, and FHWA specifications, FULLERKREG® DTI's use a patented orange silicone compound to provide instant visual confirmation of correct bolt preload — no feeler gage required.
Thinking that reaching the torque value means the bolt is properly tightened. Torque is a rotational force — the force required to rotate the nut along the bolt threads. You can apply a very high torque value to tighten a bolt without actually achieving proper tightness. On construction sites, many factors cause additional torque increase, such as dirt, dust, high temperatures, rust, rain, moisture, damaged threads, poor thread precision, excessive surface coating, improper storage, insufficient lubrication, and even issues with the torque tool itself.
Tension is the linear force generated when a bolt is stretched. The bolt's elongation clamps two components together firmly. It is the true force that creates a solid joint. Tension is the actual goal of bolt tightening operations.
Standard Direct Tension Indicators (DTIs) are specially designed, stamped, heat-treated, coated, and tested washers featuring several semicircular raised protrusions (bumps). When used with high-strength bolts in steel structural connections, the tension gradually increases during tightening. The raised protrusions on the washer are compressed and gradually flattened by the pressure from above. When the blade of a feeler gage can no longer enter the gap and touches the bolt shank, it indicates that the deformation of the washer protrusions has reached the required amount (designed for a compression load equivalent to 70% of the bolt's minimum tensile strength or minimum preload).
The Direct Tension Indicator is a displacement control method that directly responds to tension, free from the influence of thread conditions and other resistances that affect torque. Standard DTIs were developed in North America as early as the 1970s. However, because deformation verification and inspection still required the use of a feeler gage, they were not particularly convenient to use and therefore did not become widespread in steel structural bolting operations.
In 1996, FULLERKREG developed the FULLERKREG® Direct Tension Indicators (FULLERKREG® DTI's). This innovation fills the grooves on the backside of the protrusions with a soft orange silicone compound and incorporates discharge channels. When the washer protrusions are gradually flattened under load, the orange silicone compound is extruded through the channels and appears around the perimeter of the tension indicator. This allows the deformation of the tension indicator to be confirmed and inspected visually.
This innovative invention fully realizes the functionality and practicality of Direct Tension Indicators. FULLERKREG® Direct Tension Indicators make bolt tightening a simple and rapid operation: start turning the nut (bolt head)... tighten... when the appropriate amount of orange silicone compound (pre-calibrated to the required preload) extrudes around the indicator, stop turning the nut (bolt head)... correct bolt tensioning is complete.
Currently, there are four methods for applying preload to high-strength bolts: Calibrated Wrench, Turn-of-Nut, Torque Control Bolts, and Direct Tension Indicators. When used correctly, all four methods can produce the correct preload. However, except for FULLERKREG® Direct Tension Indicators, the others are all torque-based methods for bolt tightening, each with varying degrees of construction complexity and effectiveness differences. Steel construction professionals unanimously agree that FULLERKREG® Direct Tension Indicators are the most reliable and simplest of these methods for achieving correct preload.
Easier & Superior to Turn-of-Nut and Calibrated Wrench Methods
No need to mark or remember to stop tightening at 1/3, 1/2, or 2/3 turns. No need for daily or per-batch bolt condition and torque wrench verification or recalibration.
Superior to Torque Control Bolts
Direct tension control, not torque control. No special proprietary tightening tools required. No broken-off ends that become potential jobsite safety hazards and sources of bolt corrosion. No issues with the tail end breaking off due to torque before the steel components are joined.
Suitable for High-Strength Bolts of Any Length; Bolt Quality Inspection
Even for extremely short or long bolts, FULLERKREG® Direct Tension Indicators activate accurately when the tension value is reached. Derived inspection function — to examine bolt performance when pre-tension locking quality cannot be reached repeatedly or when there are doubts about bolt sourcing.
Prevents Excessive Torque, Accurately Controls Bolt Tightening
The installer knows exactly when to stop tightening. Some applications require bolts to be slightly tighter without over-tightening. FULLERKREG® Direct Tension Indicators achieve effective, accurate control that was previously impossible.
Achieves Optimal Snug-Tight Conditions
The orange silicone compound appears in trace amounts, small amounts, or full amounts (design preload), allowing bolt tightening status to be known and controlled. This facilitates completing correct tension on every bolt in sequence, achieving the most uniform and optimal bolt snug-tight condition and consistency across all bolt groups for the best steel assembly quality. All participating parties highly recognize the locking quality of FULLERKREG® Direct Tension Indicators.
Simple Visual Direct Tension Indicator; Saves Construction Time
The orange silicone compound is easily identified visually. No other inspection tools are needed during installation; both novice and experienced installers can easily master it. Visual confirmation of locking tension shortens identification time and increases tool relocation speed, saving installation labor hours.
Safer for Inspectors
Inspectors (quality control checkers) can easily see the amount of orange silicone compound extruded from a distance to identify bolt tightening status. They do not need to carry inspection tools and climb up and down steel structures to personally inspect bolts at every joint, nor rely solely on initial feeler gage sampling of a few bolts. In fact, the adoption of FULLERKREG® Direct Tension Indicators allows every single bolt to be inspected.
| Application Requirement | Recommended Method |
|---|---|
| Visual confirmation required, no special tools | FULLERKREG® DTI (Squirter Type) |
| Standard structural steel, torque wrench available | Calibrated Wrench or Turn-of-Nut |
| High-volume, speed-critical assembly | FULLERKREG® DTI (fastest inspection) |
| Wind turbine tower flange M36–M48 | FULLERKREG® DTI (GL Certified) |
| Bridge structural steel (FHWA / DOT projects) | FULLERKREG® DTI (FHWA compliant) |
| Bolt quality verification / sourcing audit | FULLERKREG® DTI (derived inspection function) |
FULLERKREG® ASTM F959 DTI Squirter Washers — product range overview:
| [ Photo 1 ] | [ Photo 2 ] | [ Photo 3 ] | [ Photo 4 ] |
FULLERKREG Fasteners — Precision ASTM and Structural Fastener Solutions, Direct from Manufacturer.
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