1. Introduction.
Design Unit has carried out surface fatigue durability tests on six pairs of MEGA-GEARS using a ‘double-ended’ back-to-back test rig with a centre distance of 160mm. In order to achieve direct comparison with test data for conventional involute helical gears, the MEGA-GEARS were finish ground after carburising.
The MEGA-GEARS were supplied by PEM Ltd in the hobbed condition and the gears were carburised and tempered using the process parameters agreed for previous gear fatigue strength research. The hardened gears were bored, faced and finish ground to ISO Grade 5 accuracy and a surface finish (Ra) of 0.3 – 0.5µm.
Gear metrology (involute, lead and pitch) and surface roughness measurements were carried out as agreed with the British Gear Association for test gears used in surface fatigue testing as well as a full metallurgical inspection including micro-hardness, Barkhausen Noise, residual stress and retained austenite measurements.
2. Experimental Method.
2.1. Test Rig and Test Gears.
The micro-pitting tests were carried out on a ‘double-ended’ back-to-back gear rig, with a centre-distance of 160mm, capable of testing at torques of up to 6000Nm.
The test gears used were 1 :1 ratio helical MEGA-GEARS (carburised SAE8822) with a facewidth of 38mm. Two pairs of gears were finish ground with a chamfered tip relief of 30µm over a 2.5mm roll length. The remaining gears were ground without tip relief. Gear details are given in Annex 3.
2.2. Test Conditions and Procedure.
The helical MEGA-GEARS supplied by PEM Ltd were tested under the following conditions:
| • Pinion speed | : | 3000rpm | |
| • Lubricant | : | 100 cSt Mineral Oil + 4% Anglamol | |
| • Lubricant temperature | : | 70°C | |
| • Lubricant flow rate | : | 34 l/min | |
| • Lubricant tank volume | : | 70 l | |
| • Lubricant filtration | : | 20μm full flow |
The gears were run for 50×106 load cycles (280 hours) with visual inspections carried out after each 1Ox106 load cycles. The test was terminated if the failure limit was exceeded at any stage.
2.3. Definition of Failure.
The gears were deemed to have failed when the area of macro-pitting was greater than or equal to 4% of one tooth flank or 1 % of all flanks. The macro-pitting failure torque can be determined to approximately ±6% (±2.5% Hertzian contact stress).
2.4. Materials Analysis.
The base material condition was analysed to confirm that the heat treatment specification had been met and the material was in the condition expected. This involved checks on hardness, microstructure, residual stress and retained austenite. Barkhausen Noise and hardness checks were also carried out after finish grinding to inspect for grinding burn.
One tooth from a MEGA-GEAR in the carburised and tempered condition was cut, mounted and metallographically polished. Micro- hardness profiles were then measured in three positions on one flank. The polished section was then etched using a 2% nitric acid solution until the microstructure was revealed. Microstructural examination was carried out using a video camera attached to an optical microscope.
Two further teeth were cut and residual stress and retained austenite profiles were measured on the tooth flank. Both measurements were carried out using X-ray diffraction on an XSTRESS 3000 residual stress analyser. Residual stress was measured parallel (0°) and perpendicular (90°) to the direction of grinding. Chemical etching was used as a strain-free method of material removal in order to measure residual stresses and retained austenite levels at depths of 10, 20, 40, 80, 150, 300 and 500µm from the surface.
3. Test Results and Discussion.
Back-to-back testing of the gears was carried out at three torque levels. The nominal contact stress at the pitch diameter, as calculated by BS-ISO 6336 (Annex 2) is summarised in Table 1. The torque and contact stress for the standard 20° pressure angle gears is summarised in Table 2.
3.1. MEGA-GEARS (100cSt mineral oil+ 4% Anglamol, 70°C, 3000rpm).
The results of the surface fatigue strength tests carried out on the MEGA-GEARS supplied by PEM Ltd are summarised in Table 3. The maximum depth of micro-pitting measured on the gears run without tip relief at a Hertzian contact stress of 1280MPa was 18µm after 50×106 load cycles. Figures 1 and 2 show the condition of pinion and wheel flanks on gears run without tip relief at a Hertzian contact stress of 1280MPa. The appearance of the pinion and wheel flanks is very similar with a narrow band of micro-pitting extending across the full facewidth in the dedendum although the area of micro-pitting is slightly greater on the wheel flanks (i.e. the driven gear flank).
The maximum depth of micro-pitting measured on gears run with tip relief at a Hertzian contact stress of 1470MPa was 18µm after 22×106 load cycles. Figures 5 and 6 show the condition of pinion and wheel flanks on gears with tip relief run at a Hertzian contact stress of 1470MPa. Again, the appearance of the pinion and wheel flanks is very similar with a narrow band of micro-pitting extending across the full facewidth in the dedendum and the area of micro-pitting is slightly greater on the wheel flanks. However, a case spalling failure (initiated sub-surface at the case-core junction) occurred on the wheel across approximately 30% of one tooth flank. The test was terminated after 22×106 load cycles because of the case spalling failure (Figure 6).
On the gears run without tip relief at a Hertzian contact stress of 1470MPa, the maximum depth of micro-pitting was 22µm after 27×106 load cycles. Figures 7 and 8 show the condition of pinion and wheel flanks on gears without tip relief run at a Hertzian contact stress of 1470MPa. As for previous tests, the appearance of the pinion and wheel flanks is very similar with a narrow band of micro-pitting extending across the full facewidth in the dedendum and the area of micro-pitting is slightly greater on the wheel flanks. The test was terminated as a result of a case spalling failure that occurred on the wheel after 27×106 load cycles.
The gears run without tip relief at a Hertzian contact stress of 1630MPa showed a maximum depth of micro-pitting of 18µm after only 1×106 load cycles. Figures 12 – 14 show the condition of pinion and wheel flanks on gears without tip relief run at a Hertzian contact stress of 1630MPa. The appearance of the pinion and wheel flanks is very similar with a narrow band of micro-pitting extending across the full facewidth in the dedendum. A case spalling failure occurred after only 1×106 load cycles across approximately 20% of one tooth flank and therefore, the test was terminated (Figure 13).
Case spalling that occurred on the MEGA-GEARS at 1470MPa and 1630MPa was caused by inadequate case thickness of the finish ground gears and would not be expected in production gears of adequate case thickness.
3.2. Conventional involute helical gears.
The results of previous surface fatigue strength research carried out using conventional involute helical gears with tip relief (40µm over a 4mm roll length) are summarised in Table 2. The gears were manufactured using 17CrNiMo6 and carburised using the process parameters agreed for previous gear fatigue strength research.
Conventional involute helical gears (with tip relief) run at a pitchline Hertzian contact stress of 1480MPa showed a maximum depth of micro-pitting of 8µm after 50×106 load cycles. Figures 3 and 4 show the condition of pinion and wheel flanks on gears (with tip relief) run at a Hertzian contact stress of 1480MPa. The appearance of the pinion and wheel flanks is similar with micro-pitting in the dedendum and at the transition to tip relief, although the area of micro-pitting is greater on the wheel flanks.
The area of micro-pitting is visibly lower on the MEGA-GEARS run without tip relief under the same test conditions (100cSt mineral oil + 4% Anglamol, 70°C, 3000rpm) and the micro-pitting is confined to a narrow band extending across the full facewidth in the dedendum (Figures 1 and 2). However, the maximum depth of micro-pitting is significantly higher (18µm) after 50×106 load cycles (Tables 1 and 2).
The maximum depth of micro-pitting measured on conventional involute helical gears (with tip relief) run at a Hertzian contact stress of 1480MPa was 24µm after 20×106 load cycles. Figures 9 and 10 show the condition of pinion and wheel flanks on gears (with tip relief) run at a Hertzian contact stress of 1480MPa. Micro-pitting is present in the dedendum and at the transition to tip relief although, as seen previously, the area of micro-pitting is greater on the wheel flanks. Macro-pitting failures also occurred on both pinion and wheel flanks after 10 – 20×106 load cycles (Figure 13).
Again, the area of micro-pitting is visibly lower on the mega-gears run with and without tip relief under the same test conditions (100cSt mineral oil + 4% Anglamol, 70°C, 3000rpm) and the micro-pitting is confined to a narrow band extending across the full facewidth in the dedendum (Figures 5 – 8). The maximum depth of micro-pitting is also very slightly lower on the mega-gears (18µm with tip relief, 22µm without tip relief) after a comparable number of load cycles (Tables 1 and 2). Furthermore, the conventional involute helical gears failed in macro-pitting (initiated from micro-pitting in the dedendum) after 10 – 20×106 load cycles whereas the MEGA-GEARS failed in case spalling after 22 – 27×106 load cycles. With adequate case depth, case spalling would not be expected to occur.
No reference data is available for conventional involute helical gears in carburised 17CrNiMo6 tested at 5000Nm torque. However, it would be expected that micro-pitting would occur very rapidly.
3.3. Materials Analysis.
The results of the materials analysis work carried out on the MEGA-GEARS are summarised in the form of a data sheet in Annex 1. The micro-hardness profiles measured after carburising and tempering show a case hardness in the range 690 – 735Hv and a case depth in the range 0.69 – 0.93mm (after finish grinding). The case hardness is in the specified range of 650 – 750Hv and the case depth measured is slightly lower than would be specified for a conventional gear of this size – a case depth in the range 1.25 – 1.75mm (target 1.50mm) was specified for 6.0 – 8.0mm module gears testes as part of previous gear fatigue strength research.
The residual stress profile shows that a slight tensile residual stress (approximately 20 – 40MPa) is present on the surface of the tooth flanks in the carburised and tempered condition and is most likely to be due to decarburisation during heat treatment. However, the effect would be removed by finish grinding. The sub-surface residual stress is compressive in the range 130 – 290MPa and is typical for carburised steel.
The retained austenite level near the surface as measured by X-ray diffraction is slightly higher than specified after carburising and tempering with a maximum of 48% at a depth of 40µm from the surface. However, the finish grinding operation will have removed the near surface peak shown on the profile to give a surface retained austenite level of approximately 40% (as per specification).
The near surface microstructure in the hardened case consists of tempered martensite and retained austenite and is typical for carburised steel.
The results of the Barkhausen Noise measurements taken on the gear tooth flanks after finish grinding are in the range 15 – 25 and are typical for a carburised and ground surface-with no grinding burn present. Hardness measurements were also taken on the gear tooth flanks after finish grinding and the flank hardness is in the range 700 – 760Hv.
4. Conclusions.
| • | At the conditions used in these tests, the area of micro-pitting generated on the MEGA-GEARS (with and without tip relief) was significantly lower than that generated on conventional involute helical gears (with tip relief). |
| • | The depth of micro-pitting generated on the MEGA-GEARS was similar to or greater than that generated on conventional involute helical gears under the same test conditions. |
| • | The MEGA-GEARS failed at Hertzian contact stresses greater than 1280MPa as a result of case spalling initiated below the surface at the case-core junction. |
| • | ISO6336 stress calculations show that the contact stress at the pitchline is very similar to that for a conventional 20° pressure angle gear of the same size and helix angle. |
| Table 1: | Nominal contact stress at pitch diameter for PEM MEGA-GEARs as calculated by BS-ISO 6336. |
| Test Torque (Nm) | Contact Stress of MEGA-GEAR to BS-ISO 6336 | ||
| 3000 | 1280 | ||
| 4000 | 1470 | ||
| 5000 | 1630 |
| Table 2: | Nominal contact stress at pitch diameter for 20° pressure angle, 30° helix angle gears as calculated by BS-ISO 6336. |
| Test Torque (Nm) | Contact Stress of 20° pressure angle gear, 30° helix gears to BS-ISO 6336 | ||
| 3000 | 1290 | ||
| 4000 | 1480 |
| Table 3: | Surface fatigue strength data for helical MEGA-GEARS (Hertzian contact stress calculated using BS-ISO 6336). |
| Applied Torque (Nm) | Hertzian Contact Stress (MPa) | Tip Relief (Yes/No) | Number of Load Cycles (x106) | Failure Mode | Maximum Depth of Micro-pitting (μm) |
| 3000 | 1280 | No | 50 | No failure | 18 |
| 4000 | 1470 | Yes | 22 | Case spalling | 18 |
| No | 27 | Case spalling | 22 | ||
| 5000 | 1630 | No | 1 | Case spalling | 18 |
| Table 4: | Surface fatigue strength data for conventional involute helical gears (Hertzian contact stress calculated using BS-ISO 6336). |
| Applied Torque (Nm) | Hertzian Contact Stress (MPa) | Tip Relief (Yes/No) | Number of Load Cycles (x106) | Failure Mode | Maximum Depth of Micro-pitting (μm) |
| 3000 | 1290 | Yes | 50 | No failure | 8 |
| 4000 | 1480 | Yes | 20 | Macro-pitting | 24 |
ANNEX 1
MATERIALS DATA SHEET
ANNEX 2
CALCULATIONS TO BS-ISO 6336
Calculations to 1S06336
This appendix shows the stress analysis for the PEM MEGA-GEARS and the standard helical gears for the test torques. It is noted that at the pitchline, the contact stress according to 1S06336 is very similar for the two designs. While the MEGA-GEAR has the greater radius of curvature, this advantage is balanced out by the reduction in transverse contact ratio and the resultant reduction in total length of line of contact. However, at the start of active profile, the effective radius of curvature is more than twice as great in the MEGA-GEAR as in the conventional gear. At this phase of mesh, the MEGA-GEAR will operate with lower contact stress provided optimum, blended tip relief can be generated.
ANNEX 3
MEGA-GEAR GEAR SPECIFICATION
MEGA-GEAR Specification
The MEGA-GEAR supplied by PEM was not fully specified. From measurement, the gear details are as shown in the table below. For gear measurement, the equivalent geometry can be used. For gear grinding, the maximum grinding wheel pressure angle that can be dressed is αw=26.5°. Therefore, the gear geometry also had to be expressed in terms suitable for grinding.
