Tolerance in Engineering: Definition, Limits & Types

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  • 0:04 What Is Tolerance?
  • 0:48 Phyical Tolerances
  • 2:04 Limits of Tolerance
  • 2:36 Attribute Tolerances
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: Joseph Komor

Joseph has taught High School Engineering and is a constulatant for the Space Program.

Explore the numerous types of tolerance specifications and when to use them. Learn the definitions and why they're important in translating design to reality.

What Is Tolerance?

Have you ever gazed at a wall and admired the gears in a clock? Those gears along with the other multiple moving components in a clock are engineered to ensure they work in unison. In the world of manufacturing, nothing can be manufactured or built to perfection. It is the work of the designer or engineer to look at the parts, and determine how precise they have to be to still function.

A tolerance is a range of how far a true measurement can range from what's intended. Creating a tolerance requires in depth knowledge of the function within a system, and what would happen if it was below the specified tolerance. There are two major tolerances that are important to consider: a physical measurement (dimension) or an attribute (time, temperature, sound level).

Physical Tolerances

Physical tolerances are most common and are usually called out in engineering drawings or specifications. They specify the deviation from a specified dimension. For an example, if a clock gear needs to be 0.75'' in diameter, then a tolerance might be plus or minus .005''. This would then make the acceptable range for the clock gear diameter to be between the range of 0.745'' and 0.755''. This is a range of .010''. As designers, they must consider the function of the watch if the gear was at the largest or the smallest of this range.

While many assemblies like a clock are complex, engineers must look at what is called a tolerance stack-up, which is when they look at all the moving parts together. If every gear is at the lowest point of the tolerance, then it may not function properly. It's critical for designers to look at their application and figure out their physical tolerance allowance. For a simple standalone piece of furniture, for an example, may require only a 1/16'' tolerance. However, an engine may require a precision of .005'' or lower.

Limits of Tolerance

Limits are a type of tolerance that specifies a different lower and upper deviation. This means that if an engineer finds that the same gear diameter tolerance can be larger but not smaller, then a limit tolerance may be applied. A 0.75'' diameter gear might have the tolerance of minus .003'' and a plus .020''. This would make for a proper tolerance that ensures that the stack up of other tolerances doesn't affect the function of the clock.

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