Tech Stuff

Torque values and threaded fastener clamping loads: Part I: From clamping loads to tightening steps

Text and photos by Mike Mavrigian

In a manner of speaking, bolts can be compared to rubber bands. Once the underside of the bolt head makes contact with the parent surface, the head can’t enter the threaded hole, so additional rotation of the bolt head causes the bolt shank of a high-tension bolt to begin to stretch. The objective is to reach the ideal point where this stretch provides the needed clamping force to properly secure the component being installed.

When tightened properly (to specification), the bolt has stretched within its designed elastic range.

When the bolt is loosened, the “metal memory” elasticity allows the bolt to return to its free, uninstalled length. That’s why we can think of a bolt or stud as a rubber band. If the bolt is under tightened, and does not enter its elastic range, it won’t provide its designed clamping force. If overtightened beyond its elastic range, the bolt can enter a yield point, and can permanently weaken. If there’s no elasticity, the bolt can’t do its job in terms of providing clamping force. It’s just sitting there, filling a hole. Taken a step further, if overtightened beyond its elastic range, it can shear. We’ll delve into this in greater detail, but hopefully you get the drift.

Bolt or stud diameters are based on the load required for component clamping performance. That’s why 1/4-inch or 6 mm bolts may be used in one location, and 3/8-inch bolts or 10 mm bolts in another, etc. A smaller-diameter bolt requires less torque value to achieve ideal clamping load, and a larger-diameter bolt requires more torque value to achieve ideal clamping load.

Although not a perfect analogy, you can somewhat view threaded fasteners as “fuses.” The diameter is based on the requirement for the specific job, just as the amp rating of a fuse is based on the requirement for a particular circuit. Taking advantage of a threaded fastener’s clamping load potential isn’t a matter of guesswork. Especially for critical fasteners, such as any involved in the brake system, steering system, suspension, engine, transmission, differential and wheels, all threaded fasteners must be tightened to their specific-application torque value. If you don’t pay attention to torque values, it’s like buying a set of pistons and sticking them into cylinder bores without measuring oil clearance, or like building a front suspension without measuring any wheel angles. It’s just not a good idea.

How do you tighten a bolt or a nut? While many folks hand-tighten by “feel,” the more astute rely on a monitoring device such as a torque wrench. Unfortunately, when most folks use a micrometer-type (click type) torque wrench, they simply adjust the wrench handle to the desired force (if the wrench is the micrometer type), slap a socket onto the wrench, stick it over the bolt head and tighten until they hear a click, without regard to the variables that may enter the picture.

Always tighten main cap bolts in the specified sequence for even-load distribution.
<p>Always tighten main cap bolts in the specified sequence for even-load distribution.</p>

First of all, is the torque wrench accurate? If it’s a cheap one, or if it’s been banged around in the shop for years, it’s probably out of calibration. Secondly, is the fastener being tightened lubricated properly? Third, is the clamping force being created suitable for the diameter and type of metal?

As I just noted, when a bolt is tightened far enough to enter its elastic range, it begins to stretch (it’s this stretch that creates the needed clamping force). What many folks don’t realize is that quite often as much as 90% of the torque applied during tightening is used, not to create clamping force, but to overcome friction. Friction occurs between mating threads, and between the underside of the bolt head (or nut) and the parent material of the object being installed.

Excess friction can occur if galling or “thread seizing” takes place. This is especially common with threaded fasteners made of alloys such as aluminum, stainless steel and titanium. If galling occurs (at any level of severity), this will make your torque readings inaccurate, since the galling effect will add significant friction at the thread mating area. This will result in a severely under-tightened fastener since your torque reading or indicator click will take place well before your desired clamping load is reached.

Several factors can affect fastener tension, including type of material, material hardness, lubrication (or lack thereof), fastener hardness, surface finish/plating, thread fit and tightening speed. So, the next time you tighten a bolt, consider the many factors at play. It’s not as simple as tightening until you hear a click.

In order to give the threaded connection the best chance of performing properly, pay attention to the following:

1. Make sure the threads (both male and female) are clean.

2. Make sure the threads are in good condition, and free of deformation or burrs.

3. When necessary, apply the required lubricant to the threads before assembly (this may involve engine oil, molybdenum disulphide, an anti-seize compound, or an anaerobic thread locking compound, depending on the situation.

4. Keep your torque wrenches clean and calibrated. Depending on their amount of use, consider sending your torque wrenches out for recalibration once per year. Also, store your torque wrenches in a safe place. Don’t toss them around the shop. They’re delicate instruments that deserve proper care.

5. When tightening, whether using a common hand wrench or a torque wrench, slow down! Fast tightening creates friction and heat at the thread area, which can lead to thread galling. Fast tightening can also lead to inaccurate tightening, as the torque wrench must overcome the increased friction.

6. When you reach the torque limit (your desired torque value), approach this slowly and watch the needle or feel for the click. If you tighten too fast, you may pull the wrench past the pre-set limit (unknowingly adding a few more foot-pounds of torque).

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