--- title: "Clearance, transition and interference fits" subject: "Metrology" chapter: 9 key: "semester_3/metrology:types-of-fits-with-examples" source: https://www.bctengineer.com/q/semester_3/metrology/types-of-fits-with-examples site: BCT Engineering --- # Clearance, transition and interference fits Metrology · Chapter 9: Limits, Fits and Tolerance - Asked in: year not given, 8 marks - Video: [Fit and Types of Fits - Clearance Fit / Interference Fit & Transition Fit (Animation)](https://www.youtube.com/watch?v=0QuVB012loc) — Engineering Orukalai - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/types-of-fits-with-examples - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-9 ## Question Explain clearance, transition and interference fits with sketches and give one application of each. Write the hole basis and shaft basis fits in the ISO system with an example for each type. ## Short answer Clearance fit: shaft always smaller (e.g., H7/g6 bearing). Interference fit: shaft always larger, fixed by press (e.g., H7/p6 gear). Transition fit: tolerance zones overlap, small clearance or interference (e.g., H7/k6 keyed pulley). Hole basis: hole H, fits H7/g6, H7/k6, H7/p6. Shaft basis: shaft h, fits G7/h6, K7/h6, P7/h6. ## Answer A **fit** is the relationship between a hole and a shaft that are assembled, resulting from the difference of their sizes before assembly. #### Types ```text Clearance Transition Interference |hole| |hole| |shaft| | | | | | | |shaft| |hole| |shaft| | | (hole zone (zones (shaft zone above shaft overlap) above hole) ``` 1. **Clearance fit:** the shaft is always smaller than the hole, so there is always a gap (minimum clearance $\ge 0$). The parts can slide or rotate. Example: shaft in a plain bearing (H7/g6, H8/f7), sliding fit of a piston in a cylinder (running fits). 2. **Interference (force/press) fit:** the shaft is always larger than the hole, so the assembly needs force (pressing, or heating or cooling the parts) and they are fixed together by friction. Example: bush in a housing, gear on a shaft, wheel on an axle (H7/p6, H7/s6). 3. **Transition fit:** the sizes overlap, so, depending on the actual dimensions, there may be a small clearance or a small interference. It is used for accurate location without much load: pulley on shaft with a key, spigot location, coupling hub (H7/k6, H7/n6). #### Hole basis and shaft basis - **Hole basis system:** the hole has zero lower deviation (H); the fits are obtained by changing the shaft's fundamental deviation. Examples: H7/g6 (clearance), H7/k6 (transition), H7/p6 (interference). - **Shaft basis system:** the shaft has zero upper deviation (h); the fits are obtained by changing the hole's deviation. Examples: G7/h6 (clearance), K7/h6 (transition), P7/h6 (interference). The **minimum clearance** is the difference between the lower limit of hole and the upper limit of shaft; the **maximum clearance** is the difference between the upper limit of hole and the lower limit of shaft. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines a clearance fit? A. The shaft is always smaller than the hole, so there is always a gap B. The shaft is always larger than the hole C. The tolerance zones of hole and shaft overlap D. The hole has zero lower deviation Answer: A. In a clearance fit the hole zone lies above the shaft zone, so minimum clearance is at least zero and parts can slide or rotate. (Key point: Clearance fit) 2. How is an interference fit assembled and held? A. By sliding them by hand; a key holds them B. By pressing or heating/cooling the parts; friction holds them fixed C. By a small gap filled with oil D. By overlapping zones giving a random clearance Answer: B. The shaft is always larger than the hole, so force or thermal change is needed and friction fixes the parts. (Key point: Interference fit) 3. Why does a transition fit give either a small clearance or a small interference? A. The shaft zone is wholly above the hole zone B. The hole zone is wholly above the shaft zone C. The hole and shaft tolerance zones overlap D. The hole size is fixed at the basic size Answer: C. Because the zones overlap, the actual sizes decide whether a clearance or interference results. (Key point: Transition fit) 4. In the hole basis system, which set correctly lists clearance, transition and interference fits? A. G7/h6, K7/h6, P7/h6 B. H7/p6, H7/k6, H7/g6 C. h7/G6, h7/K6, h7/P6 D. H7/g6, H7/k6, H7/p6 Answer: D. Hole basis keeps H (zero lower deviation) and changes the shaft letter: g6 clearance, k6 transition, p6 interference. 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