1、t 98FTMll Improving Pump Gear Geometry Through Secondarv Gear Tooth w Finishing by: S.T. Haye, Burgess-Norton Manufacturing Company 1 I TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesO Improving Pump Gear Geometry Through Secondary Gea
2、r Tooth Finishing Steven T. Haye, Burgess-Norton Manufacturing Company .The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract The direction of the hydraulic gear pump
3、industry is higher-pressure pumps that run quieter and more efficiently. In light of this, there is great importance in developing improved pump gears that will satisfy the demands of tomorrows hydraulic industry. eobjective of improved geometry for reduced noise and contact stresses fallsin step wi
4、th the entire gear industry. Following is a discussion of three gears that have been developed for their strength and quality. One of the gears is a fuel pump gear with stringent controls placed on tooth alignment variation. Another gear was developed for high fatigue strength and improved gear gecm
5、etry for high-pressure oil pumping applications. The third gear is not a pump gear but was developed for better gear geometry to reduce noise in a copier application. Copyright O 1998 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 22314 October. 1998 ISBN:
6、1-55589-729-0 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesIMPROVING PUMP GEAR GEOMETRY THROUGH SECONDARY GEAR TOOTH FINISHING Steven T. Haye, Manufacturing Engineer Burgess-Norton Mfg. Co., Geneva Illinois 601 34-21 89 ABSTRACT The direction of the
7、 hydraulic gear pump industry is higher-pressure pumps that run quieter and more efficiently. In light of this, there is great importance in developing improved pump gears that will satisfy the demands of tomorrows hydraulic industry. The objective of improved geometry for reduced noise and contact
8、stresses falls in step with the entire gear industry. Following is a discussion of three gears that have been developed for their strength and quality. One of the gears is a fuel pump gear with stringent controls placed on tooth alignment variation. Another gear was developed for high fatigue streng
9、th and improved gear geometry for high-pressure oil pumping applications. The third gear is not a pump gear but was developed for better gear geometry to reduce noise in a copier application. tolerances. Until recently, improvement efforts have been focused on the P/M tool geometry and better contro
10、l of the P/M process. Recent gear improvements have come from secondary gear tooth finishing processes, which are now in full production. Typical results are presented in this paper. PUMP GEAR NOMENCLATURE Gear pump manufacturers have been improving their pumps to obtain higher pressures and efficie
11、ncies to meet the demands of the marketplace. Because of this, there is a tangible need for better gear geometry. The five main features of the pump gear tooth are: arc tooth thickness, involute, tooth alignment variation (also known as lead error), pitch error, and roll (a functional characteristic
12、 of the previous features). Arc Tooth Thickness: INTRODUCTION In the past, P/M pump gears offered a cost effective, near net shape alternative to high cost wrought steel gears. The P/M industry was able to offer cost savings because of the limited secondary machining required to produce P/M gears. T
13、ypical secondary operations are to finish the internal bore concentric to the pitch diameter and to a specified size, the outside diameter concentric to the bore and to a specified size, and the two ends parallel to each other, perpendicular to the bore, and to a specified size. Typically, once the
14、gear was pressed, the gear teeth were left untouched. Because tooth geometry is a critical feature of the gear, there have been substantial efforts to improve gear quality without forfeiting the cost advantage of P/M. Continuous improvement activities have been applied to improve gear Arc tooth thic
15、kness (also known as chordal or circular tooth thickness) is a measure of size (thickness) of the gear tooth at the pitch diameter (see figure 1). Control of arc tooth thickness will result in better control of backlash. Backlash is the amount of play between two mating gears. Reduced backlash will
16、reduce the noise in the gear system, particularly in gear sets that operate in both directions. An Tooth Thickness Backlash Figure 1 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services, I Involute: I Involute is the actual shape of the tooth (see figure 2
17、). The involute is typically evaluated using a k-chart which defines the acceptable tolerance zone for a gear tooth (see figure 3). Gears produced to narrower tolerance limits on the k-chart demonstrate better conjugate rolling action and will perform better with mating gears. i Involute Form Figure
18、 2 Outside r-7- Diameter (+I - I - (-1 - Deviation - K-Chart Figure 3 It should be noted that the involute trace is affected by runout Runout of the pitch diameter to the bore will give the appearance of excessive involute error (see figure 4 with excessive runout). Because of this, involute should
19、only be measured on gears with minimal runout and involute traces such as those seen in figure 4 should be viewed with caution. Involute Traces with Minimal Runout Involute Traces with Excessive Runout Figure 4 Tooth Alignment Variation: Tooth alignment variation is the relationship of the tooth fla
20、nk traced along the pitch line to the axial centerline of the gear (see figure 5). The pitch line is a line that travels along the tooth at the pitch diameter. The centerline of the gear is a theoretical line that travels through the center of the gear. In the case of a spur gear, it is desirable fo
21、r the pitch line to run parallel to the centerline. Control of tooth alignment variation results in better tooth contact patterns and in the case of pump gears, higher efficiencies due to the reduction of oil leak paths. Two mating pump gears with perfect tooth alignment variation will have a line o
22、f contact throughout the entire gear creating a seal that will prevent oil from leaking between the gear mesh. In some cases, perfect tooth alignment is undesirable. If an alignment error between two mating gears exists (the centerlines of the two gears are not parallel to each other), the contact p
23、attern will be shifted towards one end of the gear set. If there is no refinement to the lead of the gear, one gear will tend to end load the other COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesgear (see figure 6). This could result in wear and galli
24、ng of the gear set. To offset this, a lead crown is specified. A crowned tooth is convex along the pitch line (see figure 7). Conventional P/M typically creates a reverse crown, or hollow tooth, which may be undesirable in certain applications. Pitch Line Line Figure 5 Figure 6 Figure 7 Pitch Error:
25、 Pitch error or tooth spacing error is the error between the flank of one tooth and the flank of the following tooth (see figure 8). Excessive pitch error will cause high noise and undue stress on the gear teeth primarily because of an inconsistency in the mesh from one tooth to the next. In a perfe
26、ct gear set, two teeth will unmesh smoothly since the adjacent teeth are spaced properly. If there is significant error in tooth spacing, however, this transition will not be smooth. Roll: .The above features are considered attribute characteristics. They are individual measure- ments of the quality
27、 of the gear tooth geometry. One of the most common methods of measuring functionality of a gear is a roll measurement. Figure 8 This measurement is usually performed with a certified master gear and the work gear. The roll test produces three components of output: runout, tooth to tooth error, and
28、total composite error (see figure 9). The runout of a gear is the variation in center distance caused by eccentricity of the pitch diameter to the bore. The runout is depicted on the roll chart as the average high and low points on a sinusoidal curve. Runout causes inefficiencies and noise in a gear
29、 set because the gear set is at one moment in a tighter mesh than at the next moment due to changes in tooth mesh attributed to changes in center distance. The tooth to tooth error is the error between subsequent teeth due to pitch error, spacing error, involute error, and tooth damage such as nicks
30、 or dings on the teeth. Tooth to tooth error Total I Figure 9 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesis depicted on the roll chart as the maximum distance from peak to valley of two teeth. Tooth to tooth error will result in higher noise in th
31、e gear system as weil as excessive wear. Total composite error is a combination of runout . and tooth to tooth error. It is the variation between the highest peak and the lowest valley on one revolution of the test gear held in tight mesh with a rolling master gear. In typical gear cutting methods,
32、the root is generated with the same tool that generates the tooth profile. This method leaves an undercut between the root and the start of the tooth profile (see figure 10). One advantage of PIM gearing is the ability to design a root fillet that is optimum for the application. A full root fillet r
33、adius can be designed to both strengthen the gear and in the case of pump gears, to minimize the amount of trapped volume making the gear set more efficient (see figure 10). PM Full Root Figure 10 PIM GEARS VS. WROUGHT STEEL GEARS One disadvantage of conventional P/M gearing is that tooth profile is
34、 generated in the first step of the process and is susceptible to distortion in subsequent operations, particularly sintering, coining, and heat treating. P/M is prone to taper of the gear tooth as well as a hollow tooth (see figure 11). Processes to improve tooth geometry following these operations
35、 result in high precision P/M gears that are still very cost competitive. Even with the additional operation(s) required to requalify the gear teeth, the cost advantages of P/M are still realized due to the highly accurate near net shape of a P/M gear. This is due to minimal stock that not only O re
36、duces the material waste, but also reduces cycle times on expensive machining operations. Requalifying tooth geometry on P/M gears provides the qualities that are seen with wrought steel, gears while maintaining the advantage of the optimized root fillet radius. Figure 11 demonstrates the difference
37、s between con- ventional P/M gears and gears that are req ualified. u Typical PM Machinad Straight Lead Error Chert Lead Error CW Maaiinad Crownad Typical PIM Lead Em Chart Machining Stcck Figure 11 CASE STUDIES Following are three case studies, which explore the use of these technologies in specifi
38、c but common applications. Each case required the requalification of the gear teeth for different reasons, but the common goal of gearing is high accuracy, low cost gears. CASE ONE - REQUALIFICATION. OF THE GEAR TOOTH TO REDUCE TOOTH ALIGN- MENT VARIATION: In hydraulic gear pumps, oil is carried fro
39、m a reservoir (low-pressure side), along the OD of the gears and is compressed in the gear mesh. The resistance to this compression or charge generates the flow of oil. Valves restrict this flow creating pressure (see figure 12). COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Inf
40、ormation Handling ServicesBecause of the difference in pressures, the gears must be produced as tightly toleranced as possible to avoid leakage from the high-pressure side back to the low-pressure side. This is true for any gear pump, whether the pump is for oil, water, fuel, or other fluid that is
41、pumped using gears. When pumping lower viscosity fluids, such as fuel, the system becomes more sensitive to the gear accuracies due to the fact that the thinner fluids will easily squeeze through any imperfections in the mating gear set. The leak path is more prevalent in the gear mesh because of th
42、e higher pressures that are applied there making it increasingly important to have a tight seal between the two gears in mesh. This is accomplished by controlling the amount of tooth alignment variation of each gear. O n Low Pressure Side 7 Drive Gear Idler Gear Compression Zone J High Pressure Side
43、 Figure 12 In this case study, reduction in tooth alignment variation was required to improve efficiencies in the gear pump at low RPMs (cold cranking efficiencies). By requalifying the gear teeth, the tooth alignment variation was reduced by more than fifty percent (see figure 13). Although governi
44、ng tooth alignment variation was the primary goal for this gear set, several other gains were realized: a very precise involute was generated allowing better rolling properties; the runout was reduced to well below typical P/M standards; and the MOW variation was reduced to twenty percent of the ori
45、ginal P/M specification. All of these advances worked towara a gear set that runs much more efficiently and quietly with little extra cost to the original P/M design. The AGMA quality of this gear increased from a Class 6 to a Class 9 or better. . - Tooth Alignment Vanation Before Secondary Machinin
46、g Operations Tooth Alignment Variation After Secondary Machining Operations Figure 13 CASE TWO - REQUALIFICATION OF THE GEAR TOOTH TO IMPROVE INVOLUTE AND STRENG TH: Occasionally it is desirable to develop a gear that has an elongated, thin tooth. The purpose for this design is to increase tooth con
47、tact ratio (see figure 14). The gear is designed this way to reduce noise in the pump. More teeth in con- Figure 14 tact at one time reduces the amount of noise generated when one set of teeth leave contact with each other and another set of teeth begin meshing. Because of the elongated, thinned too
48、th, there are more demanding requirements on the arc tooth thickness and involute tolerances. There is a minimal amount of backlash available between gear sets (this also aids in noise reduction). Small deviations in the involute or arc tooth COPYRIGHT American Gear Manufacturers Association, Inc.Li
49、censed by Information Handling Servicesthickness can produce undesirable results. Positive involute can cause the gear set to fail during operation. Another consideration with this type of gear design is the bending fatigue strength. The gear tooth may be weaker than other designs and close attention must be paid in the development and manufacturing of the gear. Higher densities and materials that improve bending complement this gear geometry. Another challenge associated with this type of gear is the P/M compaction and coining tooling. P/M tooling consists of a die cavity and two p
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