Torque Wrench Measurement

Understanding the numbers and scales of your torque wrench is key to applying the correct torque to your bolts and nuts.

As someone who has delved into the world of mechanical tools and machines, I understand that the concept of torque wrench measurement can be a bit intimidating for beginners. So, I decided to create this guide that simplifies the fundamentals of torque measuring units, and explains how to read the graduations on the scale and set the torque wrench.

Unit of Torque Wrench

The measuring units used in torque wrenches are Nm, lb-ft, or lb-inch. The most common units used for measuring torque are Pound-foot (lb-ft) and Newton meters (Nm).

Torque is a value calculated by multiplying the force applied to a lever by the distance from which the force is applied to the pivot point. That’s why you will see that torque values have a force and a distance unit.
You can find more information and a detailed explanation here.

What is Nm on a Torque Wrench?

Nm or Newton-meter is how torque is measured in the metric system (SI). The N stands for Newtons, which is a force unit.
For example, 20Nm is the resulting force from applying a 20 Newtons force to the tip of a lever with a length of 1 meter.

Pound Foot (lb-ft)

Lb-ft is an imperial system unit where lb stands for the force applied in pounds and ft for the distance measured in feet.
For example, 10 lb-ft is the resulting force from applying a force of 10 pounds at a one-foot distance from the fastener’s head.

Note: The unit of torque is pound-foot(lb-ft) and not foot-pound(ft-lb); the latter is the unit of energy. On some wrenches, it might be marked incorrectly, like the ones I own.

Nm to lb-ft Conversion

Converting from Newton-Meter (Nm) to Pound-foot (lb-ft) or vice-versa is easy.

One Pound-foot (lb-ft) is equal to 1.356 Nm.

This means multiplying the torque in lb-ft by 1.356 will give you the equivalent Nm value. When you want to convert from Nm to lb-ft, divide the torque value in Nm by 1.356
Example: 90Nm is approximately equal to 70 lb-ft. (90/1.356=70.059)

How to Read the Torque Measurement on a Torque Wrench?

Depending on the torque wrench type, torque values are shown in different ways.

Let’s see how to read torque measurements on the most common types of torque wrenches:

Click and Micrometer Torque Wrenches

These torque wrenches have the torque values printed or engraved on the bottom part of the handle. They usually have two columns of numbers; one is in Pound-foot (lb-ft), and the other in Newton meters (NM). Between the numbers of both columns, you will see a vertical line perpendicular to the numbers.

The bottom part of the handle has several numbers and vertical lines. That part of the handle, called the dial, is the part you twist to choose a torque value. Turning the dial clockwise will put more tension in the internal spring, increasing the desired torque, and turning it counterclockwise will reduce the spring’s tension, reducing the torque value.

On a micrometer-type torque wrench, as you turn the handle, the numbers on the micrometer handle will also rotate. Notice that turning the handle affects both the scale and micrometer numbers simultaneously.

To set the wrench to a desired value, you will have to line up the vertical line of the scale with the vertical line above each number with the number marked on the dial, which is also marked with numbers that go from 0 to 9.

For example, if your want to dial in a value of 100 lb-ft, you will have to find that number on the scale and rotate the handle until the top of the dial is aligned with the horizontal line, which is connected to the 100 lb-ft value. Besides, you have to align the vertical line above the 0 on your dial with the vertical line of the scale.

Split Beam Torque Wrench

Reading torque in a split beam torque wrench is simpler. You just have to turn a small screw until the gauge reaches the desired torque value.

Digital Torque Wrench

Reading torque on these torque wrenches is even simpler. You just have to choose the desired unit system (metric NM or imperial fl-lbs), and you will be able to read and adjust torque in a matter of seconds.

Common Torque Requirements

• Tightening Wheel Nuts and Lugs
  Wheel nuts and logs require torque values between 70 to 90f lb-ft or 82 to 122 Nm.
  This value varies depending on the wheel lug nuts size and the vehicle specs.
• Cylinder Heads
  Cylinder heads’ studs are usually torqued using the torque + angle method.
  Just to give you an idea of what size of torque wrench you would need for torquing them, cylinder heads torque specs are between 12 to 15 lb-ft or about 16 to 20 Nm.
• Wood Deck Lag Bolt
  The torque specs for lag bolts used for building wood decks and framing vary depending on the type of wood used and the size of the bolt. For example, if you are fastening a deck made of softwood, and you are using ½” bolts, you should torque them no more than 13 lb⋅ft. If you are using ⅝” bolts, you should adjust it to 15 lb⋅ft.

Always check manufacturers’ user manuals and references like repair or building instruction manuals to find the torque settings of the parts you will tighten.

Torque Wrench Accuracy

Clicker type torque wrenches are accurate enough to prevent over-torquing or under-tightening for most general-purpose jobs. But for more accurate tasks, I recommend getting a digital torque wrench.

It’s important to note that torque wrenches like click torque wrenches that allow you to keep applying force when the preset torque has been reached, can lead to inaccurate torque settings if you keep adjusting the fastener after you hear the first click.

Another element that contributes to better accuracy is the scale where you set the torque value. The more detailed the scale, the more accurate the final torque will be.

How Accurate are Torque Wrenches?

Torque wrenches are pretty accurate tools; they have an average precision of about +/-3 or 4%. Micrometer, split beam, digital, and gauge torque wrenches are the most accurate types, with a precision of between 1 to 3 percent.

These wrenches are excellent for high-precision jobs where applying the right torque can mean the difference between proper functioning or catastrophic failure.

Importance of Calibration

Most torque wrenches lose calibration over time. It’s important to keep them calibrated to ensure you always apply the correct torque, preventing damage to the fasteners, joints, and other problems, which can result in a headache and ruin a great job.

Calibrating a torque wrench is as important as using it to tighten fasteners properly. Using a non-calibrated tool is as bad as not using it, with the addition that you will think you have properly tightened something when you haven’t.

Conclusion

It’s important to know how your tool works to achieve better and faster results. Measuring torque with a torque wrench can seem a little tricky in the beginning, but after a few uses, and once you get used to your tool, you will find it as easy as using any other tool. Remember to check your torque wrench’s calibration at least once a year or after the number of uses stated in your tool’s user manual.

Using a torque wrench, in many cases, can sound like an unnecessary extra step for your job, but it’s the best way to ensure safety and give your work a high-quality finish.