--- title: "Basic terms of limits, fits and tolerances" subject: "Metrology" chapter: 9 key: "semester_3/metrology:limits-fits-terminology" source: https://www.bctengineer.com/q/semester_3/metrology/limits-fits-terminology site: BCT Engineering --- # Basic terms of limits, fits and tolerances Metrology · Chapter 9: Limits, Fits and Tolerance - Asked in: year not given, 8 marks - Video: [LIMITS FITS AND TOLERANCES: What is limit fit & tolerance and its need? Animation](https://www.youtube.com/watch?v=joBy4BoJszo) — ADTW Study - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/limits-fits-terminology - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-9 ## Question Define the following terms with a neat sketch: basic size, actual size, limits of size, deviation (upper and lower), fundamental deviation, tolerance, allowance, hole and shaft, zero line, and clearance and interference. ## Short answer Basic size is the nominal dimension. Limits are max/min permissible sizes; tolerance is their difference. Upper/lower deviation is the algebraic difference from basic size. Clearance: hole larger than shaft; interference: shaft larger than hole. Allowance is the minimum clearance or maximum interference. Fundamental deviation (letter) positions the tolerance zone; IT grade gives its magnitude. ## Answer ```text + ------------------ upper limit | tolerance zone | (upper deviation) ------------------ lower limit 0 ============================ zero line (basic size) (lower deviation - if below the line) ``` Deviations are measured from the zero line: positive above, negative below. | Term | Meaning | |---|---| | Basic (nominal) size | The size from which limits are derived; the theoretical size on the drawing, e.g. 40 mm | | Actual size | The size found by measurement of the finished part | | Limits of size | The two extreme permissible sizes: maximum limit and minimum limit | | Deviation | Algebraic difference between a size and the basic size: upper deviation = max. limit - basic size (ES for hole, es for shaft), lower deviation = min. limit - basic size (EI, ei) | | Fundamental deviation | The deviation (upper or lower) nearest to the zero line; it fixes the position of the tolerance zone, given by a letter (A to ZC for holes, a to zc for shafts) | | Tolerance | Difference between the maximum and minimum limits = upper deviation - lower deviation; the permissible variation in size. Size of zone is given by IT grade (IT01 to IT16) | | Allowance | The intentional difference between the dimensions of the mating parts; the minimum clearance (positive) or the maximum interference (negative). It is the difference between the maximum material limits of hole and shaft | | Hole / shaft | The internal and external features of a part; a hole is not necessarily round (e.g. a keyway width), a shaft includes any external feature | | Zero (reference) line | The line representing the basic size; deviations are positive above and negative below | | Clearance | The positive difference between the hole size and the shaft size (hole larger than shaft) | | Interference | The negative difference (shaft larger than hole before assembly) | **Tolerance zone:** the zone between the upper and lower limits, defined by the grade (size) and the fundamental deviation (position), as in $40\ \text{H7}$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Tolerance is defined as: A. Maximum limit plus minimum limit B. Difference between the maximum and minimum limits (upper deviation minus lower deviation) C. Basic size minus actual size D. Difference between hole and shaft sizes Answer: B. Tolerance is the permissible variation in size, equal to the upper limit minus the lower limit. (Key point: Tolerance) 2. Upper deviation is calculated as: A. Minimum limit - basic size B. Basic size - maximum limit C. Maximum limit - basic size D. Maximum limit - minimum limit Answer: C. Upper deviation = maximum limit - basic size (ES for hole, es for shaft); lower deviation uses the minimum limit. (Key point: Deviation) 3. The fundamental deviation fixes: A. The size of the tolerance zone B. The position of the tolerance zone relative to the zero line C. The basic size D. The IT grade Answer: B. Fundamental deviation is the deviation nearest the zero line and sets the position of the zone; the IT grade sets its size. (Key point: Fundamental deviation) 4. Interference exists when: A. The hole is larger than the shaft before assembly B. The shaft is larger than the hole before assembly C. Both parts have equal size D. The tolerance is zero Answer: B. Clearance is a positive hole-minus-shaft difference; interference is the negative difference, shaft larger than hole. (Key point: Clearance and interference) 5. Allowance is: A. The permissible variation in size B. The deviation nearest to zero line C. The intentional difference between mating dimensions: minimum clearance or maximum interference D. The difference between actual and basic size Answer: C. Allowance is the difference between the maximum material limits of the hole and shaft, the minimum clearance or maximum interference. (Key point: Allowance) ## Related in Metrology - [Limits of a 40 H7/g6 hole-shaft fit](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Afit-40-h7-g6-calculation) — Metrology, chapter 9 - [Classify fits from given limits of size](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Aclassify-fits-from-limits) — Metrology, chapter 9 - [Clearance, transition and interference fits](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Atypes-of-fits-with-examples) — Metrology, chapter 9 - [Hole basis and shaft basis systems](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ahole-basis-vs-shaft-basis) — Metrology, chapter 9 ## Related in other subjects - [Limits, fits and tolerance calculation](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Alimits-fits-tolerance-numerical) — Manufacturing and Production Processes, chapter 3 - [Copy string converting lower to upper using pointer](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Apointer-array-elements-copy-string-upper) — Computer Programming, chapter 9, asked 3× - [Assembly constraints and tolerance fit calculation](https://www.bctengineer.com/question.md?key=semester_4%2Fcomputer_aided_design%3Aassembly-constraints-tolerance-fit) — Computer Aided Design, chapter 5 - [8085 ALP: add upper and lower nibbles](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aalp-8085-add-nibbles) — Microprocessor, chapter 2