1、14FTM17 AGMA Technical Paper The Impact of Surface Condition and Lubricant on Gear Tooth Friction By S. Rao and A. Isaacson, Gear Research Institute and G. Sroka and L. Winkelmann, REM Surface Engineering2 14FTM17 The Impact of Surface Condition and Lubricant on Gear Tooth Friction Suren Rao and Aar
2、on Isaacson, Gear Research Institute and Gary Sroka and Lane Winkelmann, REM Surface Engineering 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 Frictional losse
3、s in gear boxes are of significant interest to gear box designers as these losses transform into heat. The direct result is a reduction in the fuel efficiency of the vehicle involved. Further, in many instances, this heat has to be absorbed and dissipated so that lubricant properties and gear box pe
4、rformance are not significantly compromised. This effort is to measure and document the comparative friction losses in a gear mesh due to gear tooth surface condition and lubricant. Three distinct surface conditions are considered. They are ground, isotropic superfinished (REM ISF) and tungsten inco
5、rporated diamond-like carbon coating (W-DLC) which is a wear resistant coating. Two lubricants, MIL-PRF-23699 (ISO VG 22) and Mobil SHC 626 (ISO VG 68) are considered. The experimental effort is conducted on a high speed, power re-circulating (PC), gear test rig, which had been specially instrumente
6、d with a precision torque transducer to measure input torque to the four-square loop. The torque required to drive the loop is measured under various speeds and tooth loads within the torque loop, with test gears with different surface conditions and with different lubricants. Two operating torque l
7、evels within the four-square loop at speeds ranging from 4,000 rpm (pitch-line velocity of 19 m/sec) to 10,000 rpm (pitch-line velocity of 47 m/sec) are evaluated. Input torque measurements, as measured by the precision torque transducer, on ground test gears operating in MIL-23699 lubricant are use
8、d as a base line. The increase or decrease in the input torque to the four-square loop is a measure of the change in friction losses at the test gear mesh due to changing surface condition, tooth load and or lubricant. Based on the collected data, a qualitative analysis of the effect of gear tooth s
9、urface condition on frictional losses is presented. Further, the surface characteristics of the tooth flanks of the ground, superfinished and coated gears are also described. Plans for future work, to obtain a quantitative measure of the effective coefficient of friction at the tooth surface, are al
10、so proposed. Copyright 2014 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 October 2014 ISBN: 978-1-61481-109-1 3 14FTM17 The Impact of Surface Condition and Lubricant on Gear Tooth Friction Suren Rao and Aaron Isaacson, Gear Research Institute a
11、nd Gary Sroka and Lane Winkelmann, REM Surface Engineering Introduction The impact of gear tooth surface quality and treatments on frictional losses in a gear mesh is of significant interest to the aerospace gear community as these losses are converted to heat that has to be dealt with. Further, the
12、 impact of lubricant on frictional tooth mesh losses is also of interest. The most exhaustive experimental study quantifying gear tooth friction is by Yoshizaki 1, in which spur gears with various geometries were operated in a power re-circulating test rig and frictional losses were measured. Variou
13、s lubricants and additives were also evaluated and tooth surface finishes (Rmax) ranging from 0.5 to 4 m were considered. In Britton 2, another experimental study, that specifically evaluated the effect of superfinishing on gear tooth friction on a power re-circulating gear test rig is described. A
14、30% reduction in frictional losses is measured and documented. In another experimental study on gear tooth friction, Petry-Johnson 3 measured frictional losses in a power re-circulating test rig operating ground and chemically polished gears with two different tooth sizes in three different lubrican
15、ts. This data was further utilized to define guide lines for the design of gear meshes and transmissions. Martins 4 experimentally measured the friction coefficient in FZG (ground) gears utilizing two lubricants. Several attempts to model and predict the friction losses 5, 6, 7 are also evident in l
16、iterature, where the experimental effort is utilized to correlate to analytical results. Based on the available literature, a comprehensive experimental study to compare gear mesh friction losses with different tooth surface conditions, different lubricants and under various operating conditions was
17、 considered a worthwhile effort. In this study the special variables being evaluated include superfinishing and a W-DLC coating compared to a ground base line. Two lubricants are also evaluated. Experimental set-up A high-speed, power re-circulating (four-square) gear test rig was utilized for the p
18、urpose of this experimental study. This rig consists of a test gear box connected to a reversing gear box, as shown in Figure 1. An electrohydraulic torque applicator establishes and measures the torque within the four-square loop and consequently the load on the gear teeth. The motor driving the fo
19、ur-square kinematic loop is only supplying the power to overcome the frictional losses in the test gear box mesh and the reversing gear box mesh. This input torque, outside the four-square loop, was measured with a precision, bearing-less, digital torque-meter, under different experimental condition
20、s to establish a comparative measure of the frictional losses in the test gear mesh under those experimental conditions. Figure 1. Four-square gear test rig schematic 4 14FTM17 As stated above the four-square gear test rig consists of a test gear box and a reversing gear box. The reversing gear box
21、consists of very high accuracy helical gears with a face width of 100 mm. The gears in the test gear box are 28 teeth, 3.175 module, 20 degree pressure angle, 6.25 mm face width, spur gears fabricated from AMS 6308 steel, carburized and hardened to 60-64 on the Rockwell C scale. Due to the significa
22、nt difference in face widths between the gears in the test gear box and the reversing gear box, gear failure in fatigue testing is restricted to the test gear box only. Figure 2 illustrates the test gears mounted in the test gear box with the direction of rotation illustrated by the arrow. Oil jet l
23、ubrication was employed in the tests and the “oil into the mesh” nozzle is at the bottom and the “oil out of the mesh” nozzle is at the top in Figure 2. As the test gear box and the reversing gear box are dissimilar, the total frictional losses cannot be precisely assigned to either of the two gear
24、boxes. However, a comparative estimate of changes in gear tooth frictional losses due to surface condition or lubricant change can be assessed. Further, an arbitrary assignment of frictional losses attributable to the two gear boxes allows an approximate assessment of the changes of frictional losse
25、s due to the variables of surface and lubricant. Test effort The initial effort focused on characterization of the surface of the test gears. Negatives of the tooth surface were first fabricated using surface replication epoxy (accuracy experimentally verified to be better than 0.1 micron). These re
26、plicas were analyzed utilizing optical interferometry to obtain surface characteristics of ground, superfinished and coated gears. The results of the surface characterizations are summarized in Table 1 and are considered to be consistent with what is normally obtained in industry. A typical data out
27、put from one test run is illustrated in Figure 3. The blue line represents the measured loop torque within the four-square test rig and the orange line represents the input torque as measured by the torque transducer on the power input shaft. This particular figure shows the input and loop torques w
28、hile operating with superfinished gears at 8,000 rpm in SHC 626 lubricant. Depending on the test conditions and the thermal inertia of this test rig, the set up requires up to an hour of operation before the input torque stabilizes for measurement purposes. The repeatability of the measurements was
29、evaluated. Figure 4 illustrates torque recordings of three different repetitions of superfinished gears operating at 8,000 rpm in SHC 626 lubricant. The range of the stabilized input torque in the three repetitions was 0.09 N-m. This computes to less than +/-1% of the measured torque of 6.95 N-m and
30、 was considered acceptable. The tests conducted are detailed in Table 2 with their respective pitch line velocities. Figure 2. Test gears in gear box with oil inlet nozzles 5 14FTM17 Table 1. Surface roughness data of test gears Test pair S/N Surface Ra average Ra standard developmentRz average Rz s
31、tandard development1 006 As ground 0.241 0.028 1.26 0.11 1 008 As ground 0.257 0.028 1.33 0.222 064 (R) REM ISF 0.084 0.010 0.56 0.16 2 057 REM ISF 0.081 0.015 0.53 0.113 054 W-DLC coating 0.084 0.015 0.62 0.12 3 064 (L) REM ISF 0.084 0.010 0.56 0.16 NOTES: 1. Surface roughness measurements are repo
32、rted in micrometers. 2. Measurements reported here are directional, taken orthogonal to grinding direction. 3. Averages are computed based upon 4 measurements at 6 similar locations for each sample. Figure 3. Record of measured torques Figure 4. Three repetitive torque measurements 6 14FTM17 Table 2
33、. Tests details1)MIL-PRF-23699 4000 rpm (18.62 m/s) 8000 rpm (37.24 m/s) 10,000 rpm (37.24 m/s) 96 N-m 1, 2, 3 1, 2, 3 1, 2, 3 192 N-m 1, 2, 3 1, 2, 3 1, 2, 3 Mobil SHC 626 4000 rpm (18.62 m/s) 8000 rpm (37.24 m/s) 10,000 rpm (37.24 m/s) 96 N-m 1, 2, 3 1, 2, 3 2, 32)192 N-m 1, 2, 3 1, 2, 3 2, 32)NOT
34、ES: 1)1 Both gears are as ground; 2 Both gears are REM/ISF; 3 Specimen gear is W-DLC coated, mate gear is REM/ISF. 2)Tests 2 and 3 were not conducted as excessive vibration and scoring damage occurred during ground gear testing at prior 10K rpm test. Results and discussions In order to provide an ad
35、equate tribological basis for the collected data it was decided to compute and document the range of specific film thickness for the experimental effort 8. The bulk temperature of the gear tooth in mesh was interpolated from an earlier experimental effort 9 in order to obtain the lubricant parameter
36、 that is required for the computation of the ratio. Based on an oil inlet temperature of 40.5C, the range of computed ratios for the MIL-PRF 23699 lubricant ranged from 0.31 to 2.5. For the SHC 626 lubricant the computed ratios ranged from 0.50 to 4.3. The lowest ratios are associated with ground ge
37、ars at high torques and low speeds while the highest ratios are associated with superfinished gears at low torques and high speeds. The ratios for the coated gears could not be determined due to lack of experimental data on tooth bulk temperature and lack of coefficient of friction data to compute t
38、he same. Tests were conducted with the various gear pairs in the test gear box and under load, speed and lubricants as defined in Table 2. One typical set of results is shown in Figure 5. As can be seen from Figure 5, the ground gears had the highest measured input torque at all speeds at 192 N-m wi
39、th MIL-PRF-23699 lubricant. Figure 5. Typical input torque measurements 7 14FTM17 The results of all the tests conducted are summarized in Table 3. To examine these results analytically, the measured input torque for the ground gear pair was subtracted from the measured input torques for the superfi
40、nished and coated gear pairs, under the same load, speed and with the same lubricant. These changes in input torques can be entirely attributed to the change in the surface condition of the gear pair under test and the change in frictional losses at the tooth flank, as all test conditions are otherw
41、ise identical. The frictional loss changes range from -0.72 N-m (ID No. 6) to +0.08 N-m (ID No. 2), as shown in Table 3. The superfinished and coated gears generally required lower input torques (compared to ground) except for one instance where the coated gear had a higher input torque (ID No. 2) t
42、han the ground gear set. As this experiment was the first test conducted with the coated gear, some “breaking in” of the coating may have influenced the measurement. If time and budget allowed, the test would be repeated with new gears for better characterization of the break in process or to confir
43、m an anomaly in the data. The ground and superfinished gears were also new at the start of testing. No data was observed that would indicate a similar break in characteristic. A comparison between superfinished and coated gears was inconsistent. In some instances the superfinished gears had a lower
44、or the same input torque as the coated gear. In some instances the coated gear performed better with a lower loss measurement. The more viscous SHC 626 oil appears to play a greater role in reducing frictional losses at lower speeds and higher loads at the same speeds. The MIL-PRF-23699 appears to m
45、ore effective at reducing losses at higher speeds and lower loads. As all other conditions are maintained the same, this difference in input torques at each speed, each loop torque and utilizing the same lubricant is entirely due to the changes in frictional losses in the meshing gear teeth mesh. Ta
46、ble 3. Test data summary showing changes in input torque ID No. Surface condition Loop torque, N-m Speed, rpm Lubricant Input torque change, N-m, relative to as ground at same condition Reduction compared to as ground (50% model) Reduction compared to as ground (67% model) 1 REM/ISF 96 4000 MIL-PRF-
47、23699 -0.03 98.8% 98.2% 2 W-DLC 96 4000 MIL-PRF-23699 0.08 102.7% 104.1% 3 REM/ISF 96 8000 MIL-PRF-23699 -0.19 94.3% 91.4% 4 W-DLC 96 8000 MIL-PRF-23699 -0.32 90.7% 85.9% 5 REM/ISF 96 10,000 MIL-PRF-23699 -0.67 82.9% 74.1% 6 W-DLC 96 10,000 MIL-PRF-23699 -0.72 81.4% 71.9% 7 REM/ISF 192 4000 MIL-PRF-
48、23699 -0.11 96.8% 95.1% 8 W-DLC 192 4000 MIL-PRF-23699 -0.36 89.7% 84.4% 9 REM/ISF 192 8000 MIL-PRF-23699 -0.53 86.4% 79.4% 10 W-DLC 192 8000 MIL-PRF-23699 -0.46 88.2% 82.1% 11 REM/ISF 192 10,000 MIL-PRF-23699 -0.13 96.8% 95.2% 12 W-DLC 192 10,000 MIL-PRF-23699 -0.15 97.1% 95.6% 13 REM/ISF 96 4000 M
49、obil SHC 626 -0.18 94.1% 91.1% 14 W-DLC 96 4000 Mobil SHC 626 -0.14 95.6% 93.3% 15 REM/ISF 96 8000 Mobil SHC 626 -0.27 92.5% 88.6% 16 W-DLC 96 8000 Mobil SHC 626 -0.31 91.6% 87.2% 17 REM/ISF 192 4000 Mobil SHC 626 -0.28 92.4% 88.5% 18 W-DLC 192 4000 Mobil SHC 626 -0.29 92.1% 88.0% 19 REM/ISF 192 8000 Mobil SHC 626 -0.38 90.6% 85.7% 20 W-DLC 192 8000 Mobil SHC 626 -0.37 91.0% 86.4% 8 14FTM17 As the input torque measurement includes the losses in the reversing gear box, which is very dissimilar to the test gear box, an assumption on the amoun
