Blind bolt achieves non-blind strength

DN Staff

September 25, 1995

4 Min Read
Blind bolt achieves non-blind strength

Irvine, CA--Weaker and less reliable than conventional nuts and bolts, blind bolts have long been a construction-industry stepchild. This viewpoint is especially true in the assembly of tubular-steel sections used in bridge and building construction. Reluctant or unable to specify blind fasteners for these applications, engineers weld the beams, pre-attach nut plates to the interior, or cut access holes and use standard bolts.

Each method has its disadvantages. Welds demand skilled labor, alter the material properties, often require a thicker gauge, and take considerable time. Nut plates lack flexibility, and access holes weaken the structure. In Japan, where tubular steel has been popular for more than fifty years, none of these methods satisfied a range of projects that involved attaching reinforcement plates to existing bridges.

To address the problem, engineers at Nippon Steel approached Huck International and asked the company to develop a high-strength blind fastener. The result is ULTRA-TWIST(TM), the first blind bolt to met ASTM's A-325 standard.

At first, Huck engineers suggested the logical and simple approach: use a larger, standard blind fastener. But their Japanese counterparts, wanting to avoid the potential confusion and hassle of drilling two slightly different hole sizes, said no. The blind and non-blind fasteners had to be the same diameter.

Searching for additional performance, engineers attacked every limitation in existing blind twist-off bolts. First on the list came tensile strength. Blind bolts are weaker than standard bolts because they are saddled with the need to form a head on the inaccessible side of the work. They usually incorporate a sleeve that buckles and expands as the user tightens the nut. But the sleeve takes away from the diameter of the core pin, reducing the fastener's strength.

The A-325 standard permits bolts to range in hardness from about RC25 to RC35. Harder pins possess higher tensile strengths. For safety, however, designs must account for the lower number. By narrowing the hardness range of the core pin and concentrating that range to the upper end of the scale, engineers increased the pin's functional strength to that of a standard-sized bolt. "This is a very tough manufacturing requirement for us," says Shahriar Sadri, Huck's vice president of research and development.

Threading lowers a fastener's tensile limit as well, by reducing the pin's cross section. To eke out every last bit of trength, engineers created and patented a new thread design for ULTRA-TWIST(TM). Subtly hallower and with more rounded form, the thread design maximizes the pin's cross-sectional area. The result: "We are getting the same performance from a down-sized fastener as from a full-size nut and bolt," says Sadri.

Huck presented the new design to their Japanese partners, and the design performed well--except for clamp-up. "We lost about 30% of our clamping force by the action/reaction between the two sleeves," Sadri explains. As with other blind-bolt designs, this first iteration used the nut to press a drive-sleeve against a bulb-sleeve. Even after the bulb sleeve fully formed the blind-side head, some of the clamping load still carried through the sleeves.

Back at their CAD stations, Huck engineers designed an innovative way to stop the sleeves from sharing the load by means of a shear washer. Sandwiched between a bearing washer and the drive sleeve, the shear washer transmits the nut's compressive force to the bulb sleeve to form the head. Once the head is fully formed, continued force shears the washer and allows all the clamping force to react through the bearing washer and into the joint. "With the shear washer we recover 100% of all the tightening load through the joint," says Sadri. As an added benefit, by separating the head-forming and clamping actions, each size ULTRA-TWIST(TM) bolt accommodates a 0.25-inch grip range.

After extensive field testing by Nippon Steel, Huck's new high-strength blind bolt is available in the United States. Users install the bolts with a standard electric shear wrench in 35 to 50 seconds, simplifying the assembly of most any blind joint. Based on studies in Japan, the company claims the fastener cuts labor costs by 50% while precluding welding.

While the bolts are deceptively simple looking, Huck claims the development of ULTRA-TWIST(TM) was one of the company's bigger challenges. "I don't think anyone ever had to optimize a fastener this much before," Sadri says.

Additional details...Contact Mark Brenner, Huck International, Inc., Dept. C, 6 Thomas St., Irvine, CA 93718, (714) 855-9000.

Other Applications

  • Building construction

  • Reinforcement for mining tunnels

  • Any inaccessible steel structure requiring high strength

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