Read ISO 6336 VS AGMA 2001 Gear Rating Comparison for Industrial Gear Applications text version

ISO 6336 VS AGMA 2001 GEAR RATING COMPARISON for INDUSTRIAL GEAR APPLICATIONS

Glen Cahala

Rexnord Industries, LLC Milwaukee, Wisconsin

Abstract- In 1997 the International Standards Organization (ISO) released the long awaited 6336 ISO Calculation of Load Capacity of Spur and Helical Gears. Through the combined efforts of gear technical associations representing countries around the globe, the ISO 6336 gear rating standard has emerged to lead international Gear Manufactures into the new millennium. The load capacity ratings calculated with the ISO 6336 standard are often significantly different than those calculated with the American Gear Manufactures Association (AGMA) standard 2001. In today's global market Gear Users and OEM's must, at a minimum, have the knowledge of how each standard compares to the other. Ideally a translation code from one standard to the other would best meet the needs of the Gear User and OEMs for comparing standards. This paper provides the Gear User and OEMs with a technique to make such a comparison. Rating comparisons are performed on actual manufactured industrial gearing utilizing both ISO and AGMA standards, with a side-by-side analysis. I. INTRODUCTION With numerous differences between the International Standards Organization (ISO) and American Gear Manufactures Association (AGMA) gear rating standards, how can the gear user evaluate the ISO gear rating standard, and quickly and confidently make a comparison to the more familiar AGMA? The applicable AGMA and ISO gear rating standards of interest are 2001 and 6336, respectively. While AGMA utilizes many standards to rate various types and applications of gearing, the ISO 6336 standard is most comparable to the AGMA 2001. This paper presents the gear user with valuable insight on the significant differences in ratings which these two standards often produce. Rating comparisons are performed on actual manufactured gearing with a sideby-side analysis of both standards. A brief comparison of carburized materials grades, and quality levels is also included. The paper is written for the end users, in laymen terms for those less familiar with gearing terminology and rating practices. It is not the intent of this paper to explain why the AGMA and ISO standards are different, but only to compare the differences, and offer a translation technique of the ISO rating to AGMA. II. BACKGROUND The AGMA gear rating standard is predominantly an empirical calculation method used to predict the suitability of a gear set for a given application. The AGMA standards have been developed by industry, through years of successful operation of manufactured gearing. The AGMA standards are periodically revised and edited to reflect the ever changing dynamics in which gears operate, how they are designed, and manufacturing technologies. The ISO 6336 gear rating standard is also used to predict the suitability of a gear set for a given application. However, the ISO standard was primarily developed by academics, and utilizes a theoretical approach. Through the use of formulas and equations the ISO standard was produced from the theories and principles of mechanics. One might say that the comparison of AGMA 2001 to ISO 6336 is the empirical approach vs. theoretical approach. While AGMA 2001 offers only one method of calculating the gear set rating, ISO 6336 offers 5 methods which are identified by the letters A to E. Method A is considered the most comprehensive and accurate method with the precision of the other methods descending with the alphabetical notation. Method A requires precise measurements of actual test gearing, and usually requires prototyping. Method B allows for theoretical calculations and is used to rate the gearing in this paper.

III. TEST DATA

The comparison begins with the selection of a population sample, or sample test data. What are the gear meshes which should be evaluated? How many meshes can be considered representative of all combinations? With an infinite number of possible combinations the scope of this paper will certainly need to be limited. While double, triple, and even quadruple reduction ratios are commonly used in the cement industry, the complexity of the rating comparison is significantly increased as the number of reductions increase. Therefore, a single reduction ratio has been chosen to demonstrate the general behavior of both standards which can be considered adequately representative of the multiple stage ratios. A complete product line of single reduction, parallel shaft speed reducers, consisting of 6 speed reducer sizes has been evaluated at two nominal ratios of 5.9:1 and 4.1:1. All of the subject gearing is single helical, carburized and symmetrically mounted between two bearings. The high speed shaft of the speed reducer is evaluated at 990 rpm for all six reducer sizes and both nominal ratios. Table I contains gear center distance, face width, and exact ratios corresponding to the two nominal ratios.

TABLE I SAMPLE POPULATION

TEST SAMPLE UNIT 1 2 3 4 5 6 43 39 35 31 27 23 17 16 15 14 13 12 CENTERS (in) FACE (in) EXACT RATIO 4.1:1 Nom. 4.048 4.038 3.958 4.083 4.048 3.952 EXACT RATIO 5.9:1 Nom. 5.933 5.842 5.882 5.833 6.000 5.933

these gear tooth attributes can be found in AGMA standard 390.03, Gear Classification, Materials and Measuring Methods for Unassembled Gears. The sample gearing is manufactured to an AGMA quality level Q-10. The calculated corresponding ISO quality level for pitch and profile is ISO quality level 7 with lead tolerances equivalent to an ISO quality 8 or 9. The ISO quality level used for the 6336 ratings in the population sample is Q-7, because it represents the highest quality level for the gear set. It is important to note that the AGMA tolerance gets tighter with larger quality levels i.e. a quality level 12 is more stringent than a quality level 8. For ISO just the opposite is true, an ISO quality level 7 is more stringent than a quality level 9. A general rule of thumb for comparing quality level is that the sum of the AGMA and ISO quality numbers describing the same gear is approximately 17. This is known as "The rule of 17". V. Materials Comparison The AGMA and ISO standards also allow for varying materials quality. AGMA Grade 2 carburizing steel is approximately equivalent to ISO material quality MQ. The material quality level is utilized to establish allowable contact stress (Sac) and allowable bending stress values (Sat). Table II compares these two materials. The AGMA and ISO allowable values are within 3.6% for contact stress and 3.3% for bending stress.

TABLE II

CARBURIZING GRADE MATERIALS COMPARISON

AGMA Grade 2 70,000 225,000 ISO MQ 72,518 217,556

Sac (psi) Sat (psi)

IV. Quality Level Comparison Before gear ratings can be calculated a gear quality level comparison is required to determine what AGMA quality level corresponds to the equivalent ISO quality level. Quality level establishes allowable manufacturing limits of gear tooth profile, pitch, and lead. A brief definition of each gear tooth attribute follows: Profile, is the curve forming the side of the tooth section. Pitch, is the distance from one point on a tooth to the same point on the adjacent tooth. Lead, is the axial advance of the helix for one revolution. Detailed definitions of

VI. Rating Comparison The stage is now set to rate the gearing of the sample population to both the AGMA and ISO standards. Gear manufacturers utilize computer programs to perform these calculations, which can be extremely complicated and nearly impossible to perform by hand. AGMA 2001 and ISO 6336 calculate two rating attributes for gearing, durability (pitting resistance) and strength (bending). Figures 1 and 2 are

the strength and pitting resistance ratings for the entire population sample plotted with reference to the reducer center distance. The reducer center distance is representative of the size of the unit, typically the larger the center distance, the larger the speed reducer. As the physical size of the unit increases so does the cost. There is more material, more machining, and thus more cost.

STRENGTH RATING vs. CENTER DISTANCE

80,000

A note of clarification is required for the 43 inch center distance data point for both standards and ratios of Figure 1. The explanation for the spike is that the gear tooth form used for the 43 inch center distance is of a different fundamental design than the others. The ISO pitting resistance ratings are also consistently higher than the AGMA pitting resistance ratings for both ratios, as shown in Figure 2. The rating increase for the ISO standard ranged from 66% to 7% for the nominal 5.9:1 ratio and 98% to 63% for the 4.1:1 ratio. Increases of this magnitude must be tempered by applying the proper application factor. It would appear that higher service or application factors must be used for gearing rated to ISO, compared to gearing rated to AGMA. A higher service or application factor will ensure adequate gear life for reducers rated to the ISO standard.

PITTING RESISTANCE RATING vs CENTER DISTANCE

ISO 6336 NOMINAL RATIO 4.02 : 1

70,000

ISO 6336 NOMINAL RATIO 5.89 : 1

60,000 RATING (HP) 50,000 40,000 30,000 20,000 10,000 0 21 26 31 36 41 REDUCER CENTER DISTANCE (IN)

AGMA 2001 NOMINAL RATIO 4.02 : 1 AGMA 2001 NOMINAL RATIO 5.89 : 1

90,000 80,000

ISO 6336 NOMINAL RATIO 4.02 : 1 AGMA 2001 NOMINAL RATIO 4.02 : 1 ISO 6336 NOMINAL RATIO 5.89 : 1 AGMA 2001 NOMINAL RATIO 5.89 : 1

Fig. 1 AGMA and ISO Gear strength ratings for sample population.

RATING (HP)

70,000 60,000

The strength ratings using AGMA 2001 are substantially less compared to the ISO 6336 strength ratings. An increase of not less than 42% is achieved for the entire sample population using the ISO rating standard, while several of the reducers have a rating increase over 80% and as high as 100%. The higher strength gear ratings of ISO 6336 can potentially lead to a significant increase in bending failures. In addition, the ISO strength rating will reduce the temporary overload capacity of the gearing potentially resulting in permanent plastic deformation, and eventually tooth breakage. The ISO strength ratings are aggressive when compared to AGMA. Inversely, it could be said that the AGMA ratings are too conservative. A direct correlation between AGMA and ISO strength ratings is not yet evident from the data collected. An important observation from Figure 1 is that the ISO ratings are consistently more aggressive than AGMA. In addition, Figure 1 also demonstrates the consistencies in the ratings calculated with the AGMA standard, as the data series plots for both ratios are relatively parallel. The ISO data series plots appear relatively independent for both ratios, suggesting possible inconsistencies in the standard.

50,000 40,000 30,000 20,000 10,000 0 22 27 32 37 42 REDUCER CENTER DISTANCE (IN)

Fig. 2 AGMA and ISO Gear pitting resistance ratings for sample

population. The relatively smooth trend lines of pitting resistance ratings for both standards and ratio splits, suggest that a correlation may exist. In an effort to develop the conversion factor, the data series was normalized by a dimensionless parameter to extract the connection between the two standards. The parameter used is a ratio of face width (b) to center distance (a), or the (b/a) ratio. Table III shows the (b/a) ratio for the sample population.

TABLE III SAMPLE POPULATION (b/a) RATIO

Sample Population 1 2 3 4 5 6

Centers (a) Face Width (b) 43 39 35 31 27 23 17 16 15 14 13 12

b/a Ratio 0.40 0.41 0.43 0.45 0.48 0.52

Note, that reducers which have different design criteria will not demonstrate similar increases in rating as shown in Figure 3. However, similar graphs can be developed for additional gear set families.

VII. CONCLUSION AGMA 2001 and ISO 6336 can produce significantly different gear ratings for both strength and pitting resistance. For the sample of reducers analyzed strength ratings have been demonstrated to be consistently higher with the ISO standard. The pitting resistance ratings have been shown to be ratio dependent. A relationship has been identified for pitting resistance rating conversion from one standard to the other given the ratio of gear face width to center distance as shown in Figure 3. Even the quality levels of the two standards are different, although we have identified a translation for pitch and profile with the "rule of 17". It is clear that when comparing these two standards, one should exercise caution when interpreting the results. Most importantly beware that significant differences do exist between the two standards. A reputable gear manufacture will be capable of providing a comparison when requested. What does all this mean to the selection of a speed reducer for my application? Many factors other than gearing can potentially rate a gear box, depending on the application. These include bearings, shaft stresses, thermal, over hung loads, etc. or a combination of several of these factors. However, if the gearing is the rating entity of the reducer, this analysis suggests a general way to compare the resulting ratings of the two standards.

The (b/a) ratio is plotted vs. the percent increase in pitting resistance rating from AGMA to ISO in Figure 3. One of the most noticeable features of Figure 3 is the effect the ratio has on the percent increase in rating. A considerable amount of additional rating increase over AGMA is achieved at the lower nominal ratio of 4.1:1. In addition, the plot shows a linear line, which has been superimposed to represent the best fit curve for each data series. The best fit curve is the translation needed for this family of reducers to compare the AGMA pitting resistance rating to the ISO pitting resistance rating, with an accuracy of 5%. For ratios which fall in between the two data series, linear estimations can be applied. For ratios which fall outside the two data series, no sample data has been analyzed, and caution should be used if an extrapolation is attempted. In addition, Figure 3 has been evaluated at varying pinion speeds which has shown to have little, to no effect on the percent increase in rating from AGMA to ISO.

RATINGS % CHANGE VS. FACE WIDTH CENTER DISTANCE (RATIO) PITTING RESISTANCE

100 RATING % CHANGE AGMA to ISO 90 80 70 60 50 40 30 20 10 0 0.39

5.89:1 NOMINAL RATIO 4.02:1 NOMINAL RATIO

0.41 0.43 0.45 0.47 0.49 0.51 0.53

(b/a) RATIIO

Fig. 3 Ratio of gear face width and centers plotted with the increase in rating from AGMA 2001 to ISO 6336.

The translation shown is only an approximation for comparison purposes and applies to this particular family of reducers which have similar design criteria.

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ISO 6336 VS AGMA 2001 Gear Rating Comparison for Industrial Gear Applications

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