--- title: "Pitch, bearing contact and composite gear tests" subject: "Metrology" chapter: 11 key: "semester_3/metrology:gear-testing-methods-notes" source: https://www.bctengineer.com/q/semester_3/metrology/gear-testing-methods-notes site: BCT Engineering --- # Pitch, bearing contact and composite gear tests Metrology · Chapter 11: Gear measurement - Asked in: year not given, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/gear-testing-methods-notes - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-11 ## Question Write short notes on: (a) pitch measurement of spur gears, (b) tooth bearing contact test, and (c) the composite method of gear testing. ## Short answer (a) Pitch measurement: fixed stop and dial gauge for adjacent/cumulative errors; base pitch gauge. (b) Tooth bearing test: blue coating, rolled with master; contact patch indicates alignment or profile errors. (c) Composite method: double-flank roll tester records centre distance variation as total and tooth-to-tooth composite error. ## Answer #### (a) Pitch measurement Pitch errors cause noise and uneven motion. Two types are measured: - **Single (adjacent) pitch error:** the difference between the actual and the theoretical distance between adjacent teeth. A pitch-measuring instrument with a fixed stop on one tooth and a sensitive dial gauge on the next tooth is set to zero with a master tooth; then each tooth is checked in turn. - **Cumulative pitch error:** the error in the position of any tooth with respect to the first, measured over several pitches. Also, the **base pitch** is measured by a base pitch gauge that reads the distance between parallel tangents on the base circle: $p_b = \pi m\cos\alpha$. #### (b) Tooth bearing contact test The gear is coated thinly with Prussian blue (marking compound) and rolled in contact with a master gear under light braking. The contact patch left on the teeth shows the area of bearing. Ideally the contact should be at the centre, covering about 70 to 80 % of the tooth height and length. A contact at the ends or the tip indicates misalignment, taper, wrong centre distance or profile errors. The test is simple and it is used for large gears and final assembly checks. #### (c) Composite method The gear under test is meshed with a master gear on a double-flank roll-testing machine. One gear is mounted on a fixed spindle and the other on a spring-loaded sliding carriage so that the two stay in tight mesh. As they rotate, the centre distance varies with errors of tooth thickness, profile, pitch and runout. The variation is recorded as a chart and read as: - **Total composite error:** the total variation of centre distance in one full revolution. - **Tooth-to-tooth composite error:** the variation in one pitch rotation. It is fast and checks the combined effect, so it is common in mass production, but it does not tell which error is present. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is cumulative pitch error? A. Error in position of any tooth with respect to the first, over several pitches B. Difference between actual and theoretical distance of adjacent teeth C. Variation of centre distance in one revolution D. Difference between addendum and dedendum Answer: A. Single pitch error is for adjacent teeth; cumulative error accumulates over several pitches. (Key point: Pitch errors) 2. What does a tooth contact patch at the ends or tip of the teeth indicate? A. A perfect gear with ideal contact B. Misalignment, taper, wrong centre distance or profile errors C. Only a poor surface finish D. Excess lubricant Answer: B. Ideal contact is central, covering about 70 to 80 % of height and length; end or tip contact signals alignment or profile faults. (Key point: Bearing contact test) 3. How does the composite method detect errors? A. A dial gauge is moved tooth by tooth against a fixed stop B. Prussian blue is rolled over the teeth C. The gear meshes tightly with a master on a double-flank roller tester and the variation of centre distance is recorded D. A vernier measures the chordal thickness at each tooth Answer: C. A spring-loaded carriage keeps tight mesh, so centre distance varies with tooth thickness, profile, pitch and runout errors. (Key point: Composite method) 4. What is a limitation of the composite method? A. It is slow and unsuited to mass production B. It measures only pitch error C. It needs a coating compound D. It is fast but does not show which individual error is present Answer: D. It checks the combined effect (total and tooth-to-tooth composite error) but cannot separate the causes. (Key point: Composite limitation) ## Related in Metrology - [Gear tooth vernier: chordal thickness](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Agear-tooth-vernier-chordal-thickness) — Metrology, chapter 11 ## Related in other subjects - [Tooth forces on a spur gear pinion](https://www.bctengineer.com/question.md?key=semester_6%2Ftheory_of_mechanism_and_machine_i%3Aspur-gear-tooth-forces-numerical) — Theory of Mechanism and Machine I, chapter 8 - [Spur gear terminology and standard tooth system](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Aspur-gear-terminology) — Machine Design I, chapter 9 - [Spur gear pair dimensions numerical](https://www.bctengineer.com/question.md?key=semester_6%2Ftheory_of_mechanism_and_machine_i%3Aspur-gear-pair-geometry-numerical) — Theory of Mechanism and Machine I, chapter 4 - [Lewis method module for spur gears](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Alewis-spur-gear-module-design) — Machine Design I, chapter 9