Full Thread versus Partial Thread Screws: Choosing What You Really Need

December 10, 2018

This is a tricky one, for full thread screws feature attributes that their counterparts just don't use. Choosing between the two fastener types, an inexperienced light construction worker might shrug his shoulders. After all, what's the difference? The same shank size and head shapes are available, so why can't the construction guy just flip a coin and get on with the job?

Outlining the Fastener Forms

The differences between them are subtle. The length and diameter and even the thread type fool the eye at first. Then you see the blank area below the head of a partially threaded fastener. Flicking back to the full thread variant, the thread starts below the fastener head, and it continues all the way to the terminating end. So, what's the big idea? Why is this "grip length" zone featureless on partially profiled screws and bolts? Well, think how much stronger this area must be, just by keeping its full cylindrical outlines.

What Is A Fastener's Grip Length?

Right below the fastener head, massive amounts of torsion stress and load weight become concentrated. If your fastener uses threads here, it'll grip and anchor thin surfaces. If, however, this area is fully occupied by the length of a thick drill hole, those threads do nothing. They serve no purpose. In this case, partial thread screws use their uncut shank zones, their grip length areas, to deliver more shearing strength. Without slotted threads, zones that can shear, this cylindrically featureless area gains much needed mechanical resilience. Therefore, when a construction project demands extra holding power, select partially threaded fasteners. For thinner surfaces, choose full thread screws or bolts.

Delivering Extra Fastening Features

Back with the partially profiled shanks, they're not susceptible to load-induced bending or shearing. Furthermore, they make short work of difficult alignment issues. With thin sheet metal fastening, this is a common full thread application. The automotive industry uses such fastener shanks, for they distribute smaller loads, deliver greater gripping features, and they're designed to satisfy grip strength challenges. For thicker metal sheets and dense surface elements, you turn instead to the partially threaded fasteners described above. It's in such load-centric applications that shear strength issues are solved.

There's a number of technically themed labels that are used to describe the threads of a fastener. On the shank or rod, helically shaped crest and flank edges slot their way around the fastener. The third profile shaping feature is the helical root, the groove that mates with a nut or surface hole. Imagine shearing forces as they impact a screw root. On partial thread screws, there are no root grooves to weaken a fastener's connection.

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