Types of expansion bolts

Updated April 17, 2017

An expansion bolt is a bolt, nut and lead sleeve assembly that expands on installation to provide secure fastening. These bolts secure items into materials, such as brick and concrete, for which threads are unreliable. Expansion bolt fasteners fill the gaps caused by cracks and crevices. As a result, they are popular choices for building in high-vibration environments. Expansion bolts support heavy-duty applications, such as stadium seat installation.

Lead sleeve

Although all components of an expansion bolt are important, the lead sleeve is crucial. Because lead is malleable, it fills the surrounding cracks as well as the drilled spaces. The resulting multiple points of attachment minimise the risk of the anchoring material cracking.

Bolt configurations

The bolt/lead sleeve/tapered nut expansion bolt configuration can increase productivity in large operations. Construction workers can drill the holes in advance and then insert complete bolt assemblies into the holes. When the nut is added and tightened, the tapered nut compresses the lead sleeve, resulting in its expansion. With the bolt/lead sleeve/metal cone configuration, bolt expansion results from driving the metal cone into the sleeve during nut tightening. With the tapered bolt head/lead sleeve/metal cone configuration, bolt expansion results from the tapered bolt head driving the expansion of the lead sleeve.

Expansion bolt benefits

Expansion bolts are strong, reliable and flexible. A very small expansion bolt can secure items subject to relatively strong forces. Expansion bolts can continue to do the job even in poor concrete because they employ multiple points of contact. A single point of contact would be subject to far greater stress, which could result in the crumbling of concrete. Expansion bolts are flexible enough to allow safe installation even when holes drilled in advance are slightly misaligned.

Corrosion resistance

The steel alloys of the component bolts, nuts and cones provide corrosion resistance. Bolts with cadmium and zinc plating have increased corrosion resistance. They are a good choice for applications where the bolts may face continual exposure to water.

Installation tips

After drilling, clean the hole with a blower to ensure proper fit of the expansion bolt assembly. If new dust enters from the walls of the hole, the concrete surface may be of low quality. If the concrete surface is poor, use longer anchors (and deeper holes). Use a setting tool over the expansion bolt as a target for the hammer to make the task of expansion bolt installation easier. Use a torque wrench for proper tightening of the nut. Too little tightening results in a larger-than-necessary force on the anchor. Too much tightening risks breaking the bolt. If the bolt moves more than 6.5mm (1/4 inch) after tightening, the hole is too large. Use a larger bolt.

Standards organisations

The British Standards Institute specifies the standard and metric sizes as well as the drive types of machined parts, including expansion bolts. This also determines the standards and specifications for plating on expansion bolts and components.

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About the Author

Based in Washington, DC, award-winning editor Barbara Conn has been writing about science, technology, small business, and general interest topics since 1984. Her articles have appeared in the Capital PC User Group “Monitor.” She earned a Bachelor of Science degree in chemistry from Bucknell University.