Precision system parameters
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This document discusses factors that influence gear reliability and methods for measuring gear accuracy. It describes three quality classifications for gears from commercial to ultraprecision and standards for gear tolerances. Methods for measuring individual geometric features like profile and lead as well as functional measurements using composite errors are presented. Tolerances, variations, and required inspection information are also covered.
Precision system parameters
- 1. Today’s development of modern products is confronted with rising functional requirements, higher complexity, integration of hardware, software and sensor technology and with reduced product and development costs. These, along with other influential factors on the reliability, are shown in Figure 1 Figure 1-Factors which influence reliability Reliability methods in the product life cycle Reliability Phenomenon of Precision Design Securing of system reliability 1
- 2. 2 It is important when designing a gear system to thoughtfully consider the application. Will the environment introduce conditions that negate the precision inherent to the gears you chose? The key points to remember when selecting the quality grade of the gears for your design are: • • The precision does not guarantee accuracy, and • • Tolerances are designed to mitigate deviations in accuracy.
- 3. Quality Classification • Commercial Gears : This is the least precise gear group. Applications are broad and varied, largely in competitive consumer products industrial equipment. These gears are produced by methods favoring high Production and low cost. Sizes range from small to very large and encompass both coarse and fine pitches. • Precision Gears : Closer control of gear function and performance is provided by this gearing. Application and demand is more limited than for the commercial class. The instrument field is largest user of these gears, but many precision power gears are components in machine tools, aircraft engine drives, and turbines. Fabrication requires quality machines, good procedures, and sometimes secondary refining processes such as grinding or shaving. • Ultraprecision Gears : This classification is an extension of the precision class and represents gears which have the best quality. Applications are limited to high-quality instruments, special control systems and computers, and military navigation and fire control systems. 3
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- 5. These standards cover tolerances and measuring methods. These are the current new standards that replaced the older standards ANSI/AGMA 2000- A88 (for spur and helical gears), and AGMA 390.3a (for bevel and worm gears). Other important international standards are also widely used. These include the German standards DIN 3962 and 3963 for spur and helical gears and DIN 3965 for bevel gears ; Japanese standards JIS B 1702 for spur and helical gears; and JIS 1704 for bevel gears ; International Standards Organization ISO 1328 , British standards BS 436. Quality is the characteristic properties of a gear distinguishing the nature of its manufacturing tolerances. Variation is the measured plus or minus change from the specified value, see below figure. Gear quality For a description of the application of gear tooth quality, 5
- 6. 6 Tolerance is the amount by which a specific dimensions is permitted to vary. The tolerance is the difference between the maximum and minimum limits and is an absolute value without sign, see below figure. Allowable variation is the permissible plus or minus deviation from the specified value, see side figure.
- 7. Structure of the system of Accuracy ISO system of accuracy (ISO 1328) comprises 13 accuracy grades of which grade 0 is the highest and grade 12 is the lowest degree of accuracy. The DIN (3961) gear tooth tolerance system contains 12 gear tooth qualities. The finer qualitities are intended for master gears and special requirement. The ANSI/AGMA 2015 classification system is an alpha numeric code which contains two items, accuracy grade and prefix. The AGMA classification number shall consist of a prefix letter ‘‘A’’ identifying the tolerance source, and an accuracy grade identifying the specific tolerances. Quality requirements for various gear applications in terms of DIN &AGMA standards 7
- 9. 9 it can be assumed to be unity. Equation 36 therefore, can be modified to: Precision Gearbox
- 11. The manufacturer or the purchaser may wish to measure one or more of the geometric features of a gear to verify its accuracy grade. A gear which is specified to an AGMA accuracy grade must meet all the individual tolerance requirements applicable to the particular accuracy grade and size as noted in tables. 11
- 12. Individual Working Quality Parameters In the working test gear teeth are mated with counter gear teeth and the combined effects of their individual geometrical deviations (individual errors) on the working action are determined as composite and cumulative errors. These can be assigned to one of the gears (the test gear) if the gear used as the mating gear is a master gear with deviations which are negligibIy small compared with the deviations of the test gear. It is common to use master gears with a quality at least three grades higher than the specified quality of the gear under test. If the deviations of the matine gear are not negligibly small (e.g. when the working test is performed with two transmission gears) the composite and cumulative errorş can only be assigned jointly to the gear pair. Composite and Cumulative Errors Centre distance a set to a fixed value. Single-flank engagement of the right flanks left flanks through torque constraint, Tangential composite errors = relative deviations of angle of rotation compared with the corresponding zero-deviation angular settings brought about by a reference measuring system. The centre distance is established through the action of a load acting coaxially with the line of centres and varies with the rotation of gear and mating gear. Double-flank engagement. Radial composite errors = center distance alterations. 12
- 13. 13 • Certain necessary information should be provided to the operator(s) of the measuring equipment. The information required will vary depending on the type of measurement(s) required. • Most measurement processes require basic gear and blank data, number of teeth, pitch, pressure angle, helix angle, tooth size, outside diameter, root diameter, facewidth, design profile, design helix, etc. • Certain measuring tasks require additional information. For exampleı to measure profile, the profile control diameter and start of tip break must be provided. • With mechanical measuring equipment, additional information may be required: base circle diameter (radius), base helix angle, sine bar setting, etc, • The design engineer or engineering department should be responsible for supplying this minimum required inspection information to those performing the measurements. Required Inspection Information 1. Analytical inspection or measurement 1.1 Macro geometry Measurement 1.2 Micro geometry Measurement 1.2.1 Profile and Lead Measurement 1.2.2 Pitch and Runout Measurement 2. Functional inspection or measurement 2.1 Double-Flank Inspection 2.2 Single-Flank Inspection MEASUREMENT OF GEAR ACCURACY Essentially, there are two major classes of gear accuracy measurement
- 14. Span measurement using a disc micrometer 1.1 Macro geometry Measurement Four different techniques are commonly used to inspect the tooth thickness. These are measurement of the chordal thickness; over balls, pins or wires measurement; span measurement; Measuring blocks . 14
- 15. 1.2 Micro geometry Measurement There are mainly two major categories of instruments used for analytical inspection of errors in microgeometry of gears. The first is traditional Mechanical generative instruments, and the second is computer numerical control (CNC) instruments or coordinate measuring machines (CMM). Measuring objects in gear metrology and CNC-measuring device Universal gear measuring instruments (GMI) Gears are measured using CMMs or special mechanically or CNC-controlled gear measuring instruments (GMI). The devices differ with respect to the methods of measurement applied, the measurement strategy and the software used for the evaluation of the measured data. (A) CNC gear tester (WENZEL GearTec GmbH-Germany, Gear Metrology Machine at IIT Indore-India) and (B) accuracy inspection of external helical micro gear. 15
- 16. 1.2.1 Profile and Lead Measurement In practice, the gears are mounted on a shaft or fixtures and are mounted vertically between centers or accommodated in a rotating chuck affixed to the measurement table. A single measuring probe is used for both profile and lead measurement. The position of probe during measurement of microgeometry parameters (A) profile measurement, (B) lead measurement, and (C) pitch and runout measurement The machine compares the actual gear profile to the reference profile created against the gear specifications and may record the deviation graphically on a chart. 16
- 17. 1.2.2 Pitch and Runout Measurement • For measurement of pitch and runout, the probe is initially brought into contact with any tooth flank on the reference circle diameter point at mid face. • This initial flank is considered as the datum tooth flank. • The probe is then retracted from the tooth space, and the gear is indexed by an angle as appropriate for one tooth or pitch. • The probe then moves back into the reference circle diameter of the next tooth flank, and its location is recorded. • This process is repeated for a full rotation. The same procedure is conducted simultaneously for the opposite tooth flanks. Gear runout may also be inferred from these measurements. Radial runout test by means of CMM or GMI; a) Testing with rotating table, b) Testing without rotating table Pitch inspection by direct angular measurement 17
- 18. 2. Functional inspection or measurement Functional or composite gear inspection is a qualitative method of evaluating gear accuracy where the main objective is to compare a gear to the required specifications as provided by a reference gear. Essentially, the results of functional gear inspection reveals if a gear will work as intended. This method involves rolling two gears of the same specification together (where one is a master or reference gear and other is a work gear whose quality is to be evaluated) and measuring the resultant motion to determine composite error, tooth-to-tooth error, transmission error, etc. The gears can also be tested in pairs instead of using a master gear. Functional inspection may be subdivided into two basic types: A double flank tester in tight mesh Schematic representation the basic operational principle of a single-flank inspection machine. 18
- 19. The major differences between these two methods are as follows : • Single-flank inspection implies that only one flank is in contact during gear rolling, whereas in double-flank inspection, rolling occurs such that both flanks (right and left flanks) are in contact. • Double-flank inspection allows for center distance variation (therefore, also referred to as the “variable center distance method”), whereas in single-flank inspection, the center distance remains fixed (therefore also known as “fixed center distance method”). • Single-flank inspection evaluates transmission errors whereas double flank inspection cannot detect angular tooth position defects and thus cannot evaluate transmission errors. • Double-flank gear roll testers are usually manually operated and thus relatively inexpensive. 19
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- 25. Interpretation of Runout and Adjacent pitch error Runout and pitch deviations of an eccentric gear
- 26. The new edition of ISO 1328 introduces changes in the range of calculating of the tolerance values. Moreover, it gives many significant details on considerations for elemental measurements such as datum axis, direction of measurement, direction of tolerance, measurement diameter, data filtering, data density, required measuring practices The new edition introduces guidelines on minimum set of parameters to be measured Table. Among the parameters there are two not considered in the standard, i.e. s –tooth thickness and cp –contact pattern. Parameters to be measured 26
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- 28. Gear blank dimensional deviations and gear housing dimensional deviations can have a strong effect on the contact conditions and operation of the gear pair. Since it is usually more economical to manufacture blanks and housings to tight tolerances than to manufacture gear teeth to high accuracy, consideration should be given to holding gear blank and housing tolerances to minimum values, consistent with the manufacturing facilities available. This practice allows the gears to be made to less exact tolerances and usually produces the most economical overall design. 28
- 29. Center Distance Allowances The center distance tolerance is the allowable deviation specified by the designer. The nominal center distance is determined by considerations of minimum backlash and interference between the tips of the teeth of each gear member with the non--involute profile at the root of its mate. In the case where the gears carry load in only one direction, with infrequent reversals, the control of maximum backlash is not a critical consideration and the allowance in center distance can be governed by consideration of contact ratio. When backlash must be closely controlled, as in motion control gears, or when the load on the teeth reverses, the tolerance for center distance must be carefully studied, taking into account the effect of: – deflections of shafts, housings and bearings; – misalignment of gear axes due to housing deviations and bearing clearances; – skew of gear axes due to housing deviations and bearing clearances; – mounting errors; – bearing runouts; – temperature effects (a function of temperature difference between housing and gear elements, center distance and material difference); – centrifugal growth of rotating elements; – other factors, such as allowance for contamination of lubricant and swelling of non-metallic gear materials. 29
- 30. Evaluation of Sound Pressure Level of Gearbox • Masuda, T., Abe, T., and Hattori, K. (January 1, 1986). "Prediction Method of Gear Noise Considering the Influence of the Tooth Flank Finishing Method." ASME. J. Vib., Acoust., Stress, and Reliab. January 1986; 108(1): 95– 100. https://doi.org/10.1115/1.3269309 30