Fits and tolerances in mechanical engineering define how two mating parts, such as a shaft and hole, will assemble and operate. Choosing the correct fit affects assembly force, movement, wear, alignment and service life.
What is a tolerance?
A tolerance is the permitted variation in a dimension. Instead of requiring every manufactured part to have exactly the nominal size, engineering drawings define an acceptable range.
Three basic types of fit
- Clearance fit: the hole is always larger than the shaft, allowing free movement.
- Transition fit: depending on actual sizes, the assembly may have small clearance or interference.
- Interference fit: the shaft is larger than the hole over the specified limits and requires pressing or thermal assembly.
How to choose a fit
Consider whether the parts must rotate, slide, locate, transmit torque or remain permanently assembled. Also consider material, temperature, lubrication, surface finish and manufacturing capability.
Practical example
A rotating bearing seat needs a different fit strategy from a removable cover. A press-fit dowel may require controlled interference, while a sliding guide may require predictable clearance.
Tolerance stack-up
Do not select a mating tolerance in isolation. Review the complete assembly stack-up. Manufacturing variation in multiple parts can accumulate and affect alignment or assembly.
Drawing tips
- Use recognized fit systems where applicable.
- Show nominal dimensions clearly.
- Do not specify unnecessarily tight tolerances.
- Check the effect of temperature and material expansion.
- Confirm that inspection equipment can measure the requirement.
Understanding fits and tolerances in mechanical engineering helps convert a CAD model into a reliable, manufacturable assembly.