Torque and Bolts for Dummies

torque on a bolt

The freelancers we nominate for our daily award fall into several broad categories, but the vast majority of them qualify on the basis of the same mistake: attempting to explain something they don't themselves understand. Lots try to give instructions for a task they've never tried, while others pretend to have knowledge they don't. We've had a lot of the former lately, but that doesn't mean we don't have links to posts where the writer is quite clearly talking through his or her hat. Here's one now: Troy Holmes with his WiseGEEK  post, "How Much Torque can a Bolt Accept?" (now at niche site AboutMechanics.com)

We'll ignore the weird verb – we might have said "withstand" or, more colloquially, "take" – and concentrate on Holmes' clumsy attempt to answer the question. But first: the amount of torque a bolt can withstand is determined by bolt size (diameter and, to a lesser extent, length) and tensile strength, which is a function of the material used to make the bolt. Torque is measured in units of pound-feet (Imperial) or Newton-meters (SI). Now, to what Troy had to say:

"...Each bolt has specific tolerance levels before failure. This tolerance level is determined by the material make-up, width, depth, and size of a bolt."

Truth be told, we think "width" refers to the diameter of a bolt, but "depth, and size"? Who knows? Of course, Holmes has more to say... and a lot of it is suspect. We're thinking rubbish like,

"The breaking pressure of a bolt is represented in a formula known as pounds per square inch (PSI)..."

Umm, no, that's not right. PSI is pressure, not torque, and it's a unit of measure, not a "formula."

"...Each bolt is designed to support a specific torque tolerance in PSI before it will fail. This is identified as the pressure of one-pound [sic] of force placed on one-inch [sic] of area of a bolt."

Again, no. A size of bolt is designed to withstand a given torque depending on the grade of steel. The bushwa about one pound on one inch of area? That's utterly ridiculous.

"A torque wrench is a special tool designed to measure the PSI placed on screws or bolts."

Not "measure" but control, and not "PSI" but lb-ft (pound-feet).  Other than that, Troy's substantially correct. Clumsy, but correct. And finally,

"[for] lug bolts. Most automobiles require a minimum of 50 foot pounds of PSI."

Most cars have studs and lug nuts instead of "lug bolts," but more to the point, "50 foot pounds of PSI" makes no friggin' sense: expanded, that would become "foot pounds of pounds per square inch"... Moron.

Welcome to the Antisocial Network, Troy (and welcome back, clueless editor W. Everett). Oh, and congratulations on your first Dumbass of the Day award. We suspect we'll see you again.

copyright © 2019-2022 scmrak

SI - FASTENERS