You’ve heard the story. In 1999, a navigation error caused the Mars Climate Orbiter to enter the planet’s atmosphere incorrectly, where it burned and shook itself down to microcomponents and molecules. Why? Because information transferred between the spacecraft’s manufacturers and the navigation team was never converted from English units to metric.
While it was incorrect course information which doomed the Mars Climate Orbiter, torque values for metric and standard bolts are no less important. A bolt or screw torqued to the wrong value can fall out or deform and shear under stress. So, when you’re building spacecraft to explore our solar system—or working on more terrestrial products, like aircraft, automotive, electronics, or medical devices—understanding torque values for metric bolts is an indispensable component of mission success.
Understanding Torque Values for Metric Bolts
Torque describes radial force. Every different bolt or screw, in each type of fastening substrate, under varying temperature and atmospheric conditions, can handle a different amount of stress before failure. Each one must be fastened with a minimum amount of radial force to hold it in place. Knowing the range of torque a fastener requires allows manufacturers to ensure their bolts and screws neither fall out nor fail.
So, how is torque measured? Well, by multiplying two factors: force applied and distance from the application. Torque values are expressed as a number followed by two units, force and distance, such as 17 ozf.in, or 17 inch-ounces. But there’s more than one way to describe force and distance. In fact, there are three:
- Imperial: Most American tools use the English, or imperial, system of measurement, including ounces/pounds to describe force and inches/feet to describe distance.
- Metric: Most of the rest of the world uses the metric system as a standard, describing force with grams/kilograms and distance with centimeters/meters.
- International Standard: Some scientific applications use the International Standard system for measuring torque as well, which uses newtons to describe force and meters to describe distance.
Converting Torque Values for Metric Bolts
As these units all describe the same phenomena, simply using different scales, values can be easily converted back and forth between the imperial, metric, and International Standard systems. For instance, if you want to know how many ounces a 2-gram weight is, simply multiply by the conversion rate, 0.035274. To find out how many feet there are in, say, 42 meters, just multiply 42 by 0.3048.
If you—like most of us—don’t have the beautiful mind needed to calculate conversion rates efficiently in your head, the next best approach is to turn to a conversion chart or calculator. These references will help you rapidly and correctly convert units between measurement systems.
For any mission-sensitive values, it’s best to have redundant layers of quality assurance. This way, you can double and triple-check that conversion calculations are accurate. After all, a misplaced decimal point can cause a satellite to fall.
Using Torque Values for Metric Bolts
It’s not enough to know the correct torque values for your fasteners; you have to apply the correct amount of torque as well. Using tools which can deliver limited, designated amounts of torque is the best way to do this.
Tools are built and calibrated to deliver torque at continuous or stepped intervals along a defined scale. Continuous range tools, such as DC control electric screwdrivers, can be used to apply torque in any system since they can deliver any amount of torque within their design limits. Stepped interval tools, such as adjustable or preset torque screwdrivers, can only deliver torque in predesignated amounts.
To deliver torque measured in a different system using continuous tools, simply convert the torque value to the system the tool uses and proceed. To deliver appropriate torque with stepped interval tools, it’s best to use a tool designed to deliver torque in the appropriate system. For example, use a tool with metric intervals to deliver torque measured in metric units. While conversion calculators may get you close to your interval, the lack of precision can have costly repercussions.
Understanding the different measurement systems and how to convert values between them is essential for manufacturers operating in our interconnected world. Delivering appropriate torque takes the right tools, calibrated to the scale your fasteners are measured on. It may not seem like much, but small mistakes can have big costs.