Gear Metrology Report

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Transcript of Gear Metrology Report

Gear Metrology

By

Shailendra Jain Syed Samsul Amin Supervisor: Dr. Jay Raja Summer 2004

Department of Mechanical Engineering and Engineering Science The University of North Carolina at Charlotte 9201 University City Blvd. Charlotte, NC 28223-0001 USA

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Title: Metrology of Gear

Contents: I. Introduction II. Gear Classification III. Gear Terminology i) ii) Definitions Surface Metrological Features

IV. Gear Standards V. Measurement of Gear Accuracy (Measurement Techniques and Equipments) i) Functional Gear Checking ii) Analytical Gear Checking a) Runout b) Pitch Variation c) Profile d) Tooth Alignment e) Tooth Thickness VI. Surface Measurement of Gears VII. Measurement of the surface roughness of gear tooth flanks VIII. Conclusion IX. Reference X. Appendix

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I.

INTRODUCTION Gears are a means of power transmission and changing the rate of rotation of a machinery shaft. They can also change the direction of the axis of rotation and can change rotary motion to linear motion. Unfortunately, mechanical engineers sometimes shy away from the use of gears and rely on the advent of electronic controls and the availability of toothed belts, since robust gears for high-speed and/or highpower machinery are often very complex to design. However, for dedicated, high-speed machinery such as an automobile transmission, gears are the optimal medium for low energy loss, high accuracy and low play. The intricacies of a gears terrain offer challenges to even the most experienced quality control engineer. As gear specifications tighten, tolerances often drop to the submicron realm. Hobbing, shaving, and grinding machines that offer already high accuracies can lag behind the quality demands of their finished product. Culprits include uneven or incorrectly mounted cutting tools, the results of which manifest themselves in profile errors, flankline deviation, variation in tooth thickness, pitch error, and deviations in flank shape. A gear that deviates from the ideal will make itself heard and seen. Substandard gears are noisy during operation, wear down quickly, and fail prematurely. Here we present a broad and comprehensive report on Gear Metrology explaining Gear classification and terminology, Metrological aspects of Gears (Cylindrical parallel axis involute gears), Standards of Gear measurements, Measurement techniques and Measuring equipment. This report is intended to formalize the procedures used for measuring lead, profile and pitch errors in involute gears using dedicated gear measuring machines and CMMs with gear measurement software. It should be used when gear tolerances are specified in accordance with existing gear standards (e.g. ISO 1328, AGMA 390.2, BS 436) and assumes that basic background knowledge of involute geometry and the measurement techniques are familiar to the reader. ISO Technical Report TR 10064-1 1992: 1 background information is recommended for more detailed study.

II.

GEAR CLASSIFICATION Gears are of several categories, and can be combined in a multitude of ways, some of which are illustrated in the following figures: SPUR GEAR: Spur gears are the most common type of gear having radial teeth parallel to the axle. They have straight teeth, and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth HELICAL GEAR: A gear wheel meshed with another so that their shafts are at an angle less than 180 degrees. The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement. This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions. Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load. One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees. WORM GEAR: A short rotating screw that meshes with the teeth of another gear. As a worm gear is an inclined plane, it will be the driving gear in most cases. Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater. Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when

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the gear tries to spin it, the friction between the gear and the worm holds the worm in place. This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. One other very interesting usage of worm gears is in the Torsen differential, which is used on some high performance cars and trucks. BEVEL GEAR: Bevel gears are used to connect shafts, which intersect usually but not necessarily at 90 degrees. The teeth on a bevel gear are subjected to much the same action as spur gear teeth. Bevel gears are not interchangeable and in consequence are designed in pairs (except in the case of mitre bevel gears). DIFFERENTIAL GEAR: A certain arrangement of gears connecting two axles in the same line and dividing the driving force between them, but allowing one axle to turn faster than the other. It is used in the rear axles of automobiles to permit a difference in axle speeds while turning. RACK GEAR: A toothed bar into which a pinion, (worm gear spur etc.) meshes. Rack and pinion gears are used to convert rotation into linear motion. A perfect example of this is the steering system on many cars. The steering wheel rotates a gear, which engages the rack. As the gear turns, it slides the rack either to the right or left, depending on which way you turn the wheel PINION: A small cogwheel, the teeth of which fit into those of a larger gearwheel or those of a rack. COGWHEEL: A wheel with a rim notched into teeth, which meshes with those of another wheel or a rack to transmit or receive motion. III. GEAR TERMINOLOGY

Fig 1 Gear Specification

DEFINITIONS: 1. Addendum: The distance a tooth projects above, or outside of, the pitch line or circle. 2. Base circle: The base circle is a circle from which involute tooth profiles are derived. 3. Base cylinder: The base cylinder corresponds to the base circle and is the cylinder from which involute tooth surfaces, either straight or helical are derived. 4. Backlash: The amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the operating pitch circles. Backlash is the gap between gear teeth where they mesh. This leads to play in the gears. 5. Bottom Land: The root diameter.

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6. Chordal Addendum: The distance from the outer diameter to the pitch line. 7. Chordal Thickness: The tooth thickness at the pitch line. 8. Circular Pitch: The distance from the center of one tooth to the center of the next tooth measured round the circumference of the pitch circle. 9. Clearance: The amount by which the Dedendum of a gear tooth exceeds the addendum of a mating gear. 10. Center distance: The distance from the center of the gear shaft to the center of the pinion shaft. 11. Circular tooth thickness: The length of arc between the two sides of the same gear tooth, on a specified circle. (Refer figure 1). 12. Datum circle: The datum circle is a circle on which measurements are made. 13. Composite action test: A method of gear inspection in which the work gear is rolled in tight, doubleflank contact with a master gear or a specified gear to determine composite variations. 14. Composite tolerance, tooth-to-tooth (double-flank): The permissible amount of tooth-to-tooth composite variation. 15. Composite tolerance, total (double-flank): The permissible amount of total composite variation. 16. Composite variation: Variation in center distance when a gear is inspected by a composite-action test. 17. Composite variation, tooth to tooth (double-flank): The greatest change in center distance while the gear being tested is rotated through any angle of 360/N during a double flank composite test. 18. Total Composite variation (double-flank): The total change in center distance while the gear being tested is rotated one complete revolution during double-flank composite-action test. 19. Datum of axis rotation: The axis of the gear used as the basis for measurements. 20. Datum tooth: The designated tooth used as the starting point for measuring other teeth. 21. Diameter, profile control: The specified diameter of the circle beyond which the tooth profile must conform to the specified involute curve. 22. Dedendum: The depth of a tooth space below, or inside of, the pitch circle. 23. Eccentricity: The distance between the center of a datum circle and a datum axis of rotation. 24. Face width: The length of the gear teeth in an axial plane. 25. Functional face width: The portion of the face width less the edge round at each end. 26. Index variation: The displacement of any tooth from its theoretical position, relative to a datum tooth. Measurements are usually linear, near the middle of the functional tooth profile. If the measurements are made normal to the tooth surface, they should be corrected to the transverse plane. 27. Total Index variation: The maximum algebric difference between the extreme values of index variation for a given gear. Total index var