Engineering parts are never made to one exact size, so drawings specify a range of acceptable sizes called a tolerance. Tolerances and fits control how a shaft and hole assemble, move, locate, and carry load. A small change in diameter can decide whether a part slides freely, presses tightly, or fails to assemble.
Understanding fits helps engineers design parts that are manufacturable, reliable, and cost effective.
A fit is determined by comparing the largest and smallest possible sizes of the hole and shaft. Clearance fits always leave space between parts, interference fits always create overlap, and transition fits may do either depending on actual manufactured sizes. In the hole-basis system, the hole is kept at a standard lower limit and different shaft tolerance zones are chosen to create the desired fit.
These choices affect assembly force, bearing performance, alignment, wear, and the need for heating, cooling, or pressing during installation.
Key Facts
- Tolerance = upper limit size - lower limit size.
- Clearance = hole diameter - shaft diameter when the hole is larger than the shaft.
- Maximum clearance = largest hole - smallest shaft.
- Minimum clearance = smallest hole - largest shaft.
- Interference = shaft diameter - hole diameter when the shaft is larger than the hole.
- Hole-basis system: the hole lower deviation is usually 0, and shaft deviations are selected to create the fit.
Vocabulary
- Tolerance
- Tolerance is the allowed variation between the maximum and minimum acceptable size of a manufactured feature.
- Clearance fit
- A clearance fit is a fit where the shaft is always smaller than the hole, so the parts can assemble without force.
- Interference fit
- An interference fit is a fit where the shaft is always larger than the hole, so the parts must deform slightly or be pressed together.
- Transition fit
- A transition fit is a fit where the actual parts may have either small clearance or small interference depending on their manufactured sizes.
- Hole-basis system
- The hole-basis system is a standard fit system where the hole size range is fixed and different shaft size ranges are selected to obtain the needed fit.
Common Mistakes to Avoid
- Using nominal size as the only size is wrong because real manufactured parts can be anywhere within the tolerance limits.
- Calculating clearance with the wrong limits is wrong because maximum clearance uses the largest hole and smallest shaft, while minimum clearance uses the smallest hole and largest shaft.
- Calling any tight assembly an interference fit is wrong because a transition fit can sometimes be tight and sometimes loose depending on actual part sizes.
- Choosing very tight tolerances by default is wrong because tighter tolerances usually increase machining time, inspection cost, and scrap rate.
Practice Questions
- 1 A hole is specified as 25.000 mm to 25.030 mm and a shaft is specified as 24.970 mm to 24.990 mm. Find the minimum clearance, maximum clearance, and type of fit.
- 2 A hole is specified as 50.000 mm to 50.020 mm and a shaft is specified as 50.010 mm to 50.035 mm. Find the maximum clearance or minimum interference values and identify whether the fit is clearance, transition, or interference.
- 3 A pulley must be permanently mounted on a motor shaft without a key, while a bearing inner race must be removable for maintenance. Explain which type of fit is more suitable for each case and why.