Technical Memorandum

NASA/TM—2014-218396
Test Facility Simulation Results for Aerospace
Loss-of-Lubrication of Spur Gears
Robert F. Handschuh and Lucas J. Gargano
Glenn Research Center, Cleveland, Ohio
October 2014

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NASA/TM—2014-218396
Robert F. Handschuh and Lucas J. Gargano
Glenn Research Center, Cleveland, Ohio
National Aeronautics and
Space Administration
Glenn Research Center
Cleveland, Ohio 44135
October 2014

Level of Review: This material has been technically reviewed by technical management.
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Robert F. Handschuh and Lucas J. Gargano*
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Abstract
Prior to receiving airworthiness certification, extensive testing is required during the development of
rotary wing aircraft drive systems. Many of these tests are conducted to demonstrate the drive system’s
ability to operate at extreme conditions, beyond that called for in the normal to maximum power
operating range. One of the most extreme tests is referred to as the loss-of-lubrication or run dry test.
During this test, the drive system is expected to last at least 30 min without failure while the primary
lubrication system is disabled for predetermined, scripted flight conditions. Failure of this test can lead to
a partial redesign of the drive system or the addition of an emergency lubrication system. Either of these
solutions can greatly increase the aircraft drive system cost and weight and extend the schedule for
obtaining airworthiness certification.
Recent work at NASA Glenn Research Center focused on performing tests, in a relevant aerospace
environment, to simulate the behavior of spur gears under loss-of-lubrication conditions. Tests were
conducted using a test facility that was used in the past for spur gear contact fatigue testing. A loss-of-lubrication
test is initiated by shutting off the single into mesh lubricating jet. The test proceeds until the
gears fail and can no longer deliver the applied torque. The observed failures are typically plastically
deformed gear teeth, due to the high tooth temperatures, that are no longer in mesh. The effect of several
different variables to gear tooth condition during loss-of-lubrication have been tested such as gear pitch,
materials, shrouding, lubrication condition, and emergency supplied mist lubrication in earlier testing at
NASA.
Recent testing has focused on newer aerospace gear steels and imbedding thermocouples in the
shrouding to measure the air—oil temperatures flung off the gear teeth. Along with the instrumented
shrouding, an instrumented spur gear was also tested. The instrumented spur gear had five thermocouples
installed at different locations on the gear tooth and web. The data from these two types of measurements
provided important information as to the thermal environment during the loss-of-lubrication event. This
data is necessary to validate on-going modeling efforts.
Background and Introduction
The drive system used in rotorcraft applications is required to pass a 30-min loss-of-lubrication test
prior to aircraft certification. This means that the drive system must continue to operate for 30 min with
the primary lubrication system inoperative (Ref. 1). To achieve this capability, gears are extremely
susceptible to failure due to a starved lubrication condition. High load, high pitch line velocity, or the
combination of both can cause the gears, due to the meshing process, to heat to the point of failing.
Failure mechanisms of gear contacts can be summarized by Figure 1 (Ref. 2). In Figure 1, two of the
mechanisms are of most importance for loss-of-lubrication. The first is the wear region where extreme
loading at slower sliding speeds under starved lubrication conditions generates sufficient heat to expedite
wear. The other region is the scoring region where high relative sliding of the teeth influences the heat
being generated. In either case, the friction coefficient increases due to the loss of the full
elastohydrodynamic fluid film, thus increasing the heat generation rate.
*NASA Glenn Research Center, LERCIP intern.
NASA/TM—2014-218396 1

Figure 1.—Effects of speed and load on gear wear mechanisms.
Recent studies (Refs. 3 to 5) made initial attempts at evaluating the loss-of-lubrication behavior of
tested components typically used at NASA Glenn for contact fatigue studies. In prior and current NASA
testing, all tests were run in a dry sump manor, where all lubrication is jet fed and gravity drained. This
type of lubrication system results in efficient operation of the drive system and is the lubrication mode
typical in all rotorcraft main transmissions.
In these prior tests, many aspects were considered including run-dry conditions, vapor-mist
lubrication, grease lubrication, test gearbox drainage, and shrouding. These test results indicated that
testing in a relevant environment was essential for the generation of useful test results. Therefore, the
current test arrangement evolved to represent an aerospace environment. Gear shrouding was used in all
tests conducted in this paper.
The test results described in this paper document the on-going research assessment of aerospace
quality components subjected to loss-of-lubrication conditions. Static instrumentation located in the gear
shrouding was used on all tests. Also, one test gear was instrumented with thermocouples to provide
temperature data on the gear. This gear was subjected to normal and loss-of-lubrication conditions.
Results from these tests are described in this report.
Test Facility
The test facility used for conducting all loss-of-lubrication simulation tests is the NASA Glenn
Contact Fatigue Test Facility (Refs. 6 to 8). The facility is shown in the sketch illustrated in Figure 2. The
facility is a torque regenerative test rig that locks torque in the loop via a rotating torque applier. The test
gears have a 1:1 ratio. Facility speed and torque can be varied as needed during a test. The basic gear
design information of the tested gears is listed in Table 1.
As previously mentioned, the facility went through an evolutionary period (Ref. 5) where shrouding,
visual access, gearbox lubricant removal, and instrumentation were added to the test section. A
photograph of the current test gearbox arrangement is shown in Figure 3.
NASA/TM—2014-218396 2

Figure 2.—Cross-sectional sketch of the test gearbox
used for loss of lubrication testing.
TABLE 1.—BASIC GEAR DESIGN INFORMATION
Shrouds
NASA/TM—2014-218396 3
28 tooth gear
Module, mm [diametral pitch (1/in.)] 3.175 (8)
Pressure angle (deg.) 20
Pitch diameter, mm (in.) 88.9 (3.5)
Addendum, mm (in.) 3.175 (0.125)
Whole depth, mm (in.) 7.14 (0.281)
Chordal tooth thickness, mm (in.) 4.85(0.191)
Face width, mm (in.) 6.35 (0.25)
Figure 3.—Current test gearbox arrangement utilizing shrouds (bottom exit shown).

Figure 4.—Example of test gear arrangement with outer gearbox cover removed, top exit shrouds shown.
Static shroud thermocouples were utilized during all tests. High temperature glass was also utilized as
the outside shroud to encase the gears. Ballistic plastic and another layer of high temperature glass
provided visual (and video) access to the gears during operation. Lubricant for normal operation was fed
at the into-mesh location through part of the shroud (Fig. 4). The normal operational flow rate was
approximately 0.42 l/min (0.11 gpm) at 207 kPa (30 psig) jet pressure with the lubricant inlet temperature
of ~110 C (230 F). Instrumentation and normal lubricant jet locations are shown in Figure 4. A turbine
engine—drive system lubricant, MIL-L-85734, was used during the testing to be described in this report.
Testing Methodology
Prior to loss-of-lubrication testing discussed in this paper, the gears were broken-in to allow normal
run-in wear to occur. The gears were operated for at least 1 hr at ~50 percent of maximum torque and at
full facility speed (10000 rpm). After this period, the load was then increased to the maximum load and
run for at least several more hours prior to conducting a loss-of-lubrication test. Most tests were run until
the teeth failed to continue to carry torque (plastically deformed) or stopped just prior to this condition.
During all tests, the static instrumentation and live video were carefully monitored. Data from all
sensors were collected at 1 Hz and stored for post-processing.
Testing and Discussion of Results
The test results to be described in this report were from the same lot of gears manufactured to the
basic gear design information as shown in Table 1. The gear material was an aerospace gear steel (Ref. 9),
that was carburized and final ground. The surface roughness was 0.41 micro-meters (16 micro-inch) or
better. The gears have some tip relief starting at the highest point of single tooth contact and a small
amount of crowning across the face width that is symmetric.
NASA/TM—2014-218396 4

Five test results will be described in this report. The tests in the loss-of-lubrication condition were
conducted at one of two load levels as documented in Table 2. The gear basic design information along
with the load level produced maximum level of bending and contact stress found via the analysis
technique of Reference 10.
An example of each of the two load levels will now be described. At the lower level of the two loads,
two tests were conducted with the loss-of-lubrication time of 40.8 and 43.1 min in length. The data from
the static thermocouples imbedded in the shrouds along with other facility temperatures is shown in
Figure 5.
The gears in the post-test condition are shown in Figure 6. Note that the test was conducted until the
gear tooth meshing heat generation and resultant temperature was high enough to plastically deform the
gear teeth. This type of post-test condition occurred whenever the test was permitted to continue until
total loss of torque delivery was achieved. A typical loss of lubrication failure at this condition is shown
in Figure 7.
Two other tests were conducted at an elevated level of load (~83.6 N*m (740 in*lb)). These two tests,
in the loss-of-lubrication mode, produced failures in 7.9 and 9.2 min. The test temperature data from the
7.9 min loss-of-lubrication test is shown in Figure 8. As can be seen from this data, the temperature did
not reach an increased steady-state condition prior to increasing to failure as it did in the test shown in
Figure 5. After the primary lubrication system was disconnected the temperature just continued to rise.
The post-test condition of the test gears is shown in Figure 9 with the gearbox cover removed.
TABLE 2.—TEST CONDITION STRESS ANALYSIS RESULTS (REF. 10)
Torque Maximum contact stress Maximum bending stress
N*m (in*lb) GPa (ksi) GPa (ksi)
59.3 (525) 1.67 (242) 0.214 (30.9)
83.6 (740) 1.88 (272) 0.296 (43)
Figure 5.—LOL data for 1.72 kPa (250 psi) load pressure ~59.3 N*m (540 in*lb)
torque tested at 10000 rpm.
NASA/TM—2014-218396 5

Figure 6.—Post-test condition of the gears (gearbox
cover and outer high temperature glass removed).
Figure 7.—Example of loss-of-lubrication test during final seconds of operation.
NASA/TM—2014-218396 6

Figure 8.—Loss-of-lubrication data from higher load test 83.6 N*m (740 in*lb).
Figure 9.—Post-test photograph of increased load test gears with
outer shroud high temperature glass fractured.
NASA/TM—2014-218396 7

Instrumented Gear Test
The most desired test for loss-of-lubrication behavior is to have on-component information. While it
would be of great benefit to have instrumentation at the gear-meshing surface, this has been shown to be
very difficult to accomplish. Even with full elastohydrodynamic film the lubricant viscosity of turbine
engine lubricants, as used in rotorcraft drive systems, is insufficient to develop films thick enough to keep
the on-surface instrumentation from wearing away. Therefore, thermocouples were installed at locations
in areas of the gear where contact does not occur were used.
The gear used in these tests is shown in Figure 10. It was the gear installed in the left side of the
gearbox. The left side gear acts as the driving gear of the test section of the facility. A total of five
thermocouples were attached to the gear at the tooth tip mid-face width, root mid-face width, and on the
side of the gear at the pitch radius, root radius and mid-web locations. As shown in Figure 10, the
thermocouple wires were covered by a thin metal foil that was spot welded to the side of the gear to
protect the instrumentation during operation. The rest of the required hardware is shown in Figure 11
prior to installation in the test rig. The assembly in the test rig is shown in Figure 12.
Figure 10.—Instrumented LOL test specimen before testing.
NASA/TM—2014-218396 8

Figure 11.—Loss-of-lubrication test specimen and related components.
Figure 12.—Test set up for instrumented test gear.
NASA/TM—2014-218396 9

The initial data for warm-up and operation of the facility in the normal to loss-of-lubrication mode is
shown in Figure 13. During this same test the lubricant was shut off and the test run until failure. The data
from loss-of-lubrication initiation to failure for the shroud (static) and other facility thermocouples is
shown in Figure 14.
The data in Figures 13 and 14 indicate that the bulk temperatures of the gear were exceeding the
temperature that was found from the static shroud thermocouples. In the normal lubrication condition this
value was 20 to 40 F depending on the location. In the loss-of-lubrication mode, this amount was as high
as 500 F higher on the gear than that of the static shroud thermocouples. In Figure 13, the data beyond
8200 sec is believed to be invalid due to post-test inspection of the thermocouple wiring that was melted
together at the common locations through the shaft. The thermocouple wiring coating was exposed to
bulk temperatures in the gear beyond the melting point of the wire coating.
Figure 13.—On-gear and out-of-mesh thermocouple data from star-up to
instrumentation failure during loss-of-lubrication.
Figure 14.—Data from shroud and facility (static) thermocouples.
NASA/TM—2014-218396 10

The post-test condition of the gears used in this test is shown in Figure 15. What can be noted is that a
bending failure occurred before the loss-of-lubrication test, as the failed tooth has no evidence of running
in this post-test condition. A hypothesis is that the failed tooth was initiated by a spot weld in the root-fillet
region from the thin metal protection strap that was used to cover the thermocouple wiring.
In Figure 16, the tooth that failed in bending shows no apparent damage due to loss-of-lubrication
since it occurred before this lubrication condition was initiated. Upon closer examination, the thin metal
strapping used to overcoat the wiring and spot-welded to the tooth, was found to be the initiation site of
the bending failure. Future test hardware for this purpose will not have this as a hold-down feature for the
tooth root-fillet area.
Figure 15.—Post-test condition after loss-of-lubrication test was completed.
Gearbox housing, slip ring and outer high temperature glass removed.
Figure 16.—Instrumented spur gear post-loss-of-lubrication test condition.
NASA/TM—2014-218396 11

Conclusions
A series of five loss-of-lubrication tests were conducted in an aerospace simulated environment using
consistent sets of test hardware. The following is a summary of the test results:
1. Applied torque can have a drastic effect on loss-of-lubrication time. An increase of torque by
40 percent, 59.3 to 83.6 N*m (525 to 740 in.-lbs) resulted in a decrease in loss-of-lubrication
operation time by 75 percent (42 to 8 min).
2. Operation in loss-of-lubrication mode at lower torque produced an elevated steady state temperature
condition. The higher torque level did not have this operating time at an elevated steady state
temperature condition. During the higher torque tests, the temperature continued to increase until
failure of the teeth.
3. On-component thermocouple data revealed that the gear under normal conditions have bulk
temperatures that are 20 to 40 F higher than the fling-off temperatures measured by the static shroud
thermocouples.
4. On-component thermocouple data indicated that during loss-of-lubrication, conditions bulk
temperatures on the gear are from 150 to 500 F higher at certain times during this test mode when
compared to the static shroud temperatures.
References
1. ADS-50-PRF, Aeronautical Design Standard, Rotorcraft Propulsion Performance and Qualification
Requirements and Guidelines, U. S. Army Aviation Troop Command, April, 1996.
2. Handschuh, R.: Thermal Behavior of Spiral Bevel Gears, NASA–TM-106518, ARL-TR-403,
January 1995.
3. Morales, W. and Handschuh, R.: A Preliminary Study on the Vapor/Mist Phase Lubrication of a
Spur Gearbox NASA TM-1999-208833, February 1999.
4. Morales, W., Handschuh, R., and Krantz, T.: Feasibility Study of Vapor-Mist Phase Reaction
Lubrication Using a Thioether Liquid, NASA/TM—2007-215035, December 2007.
5. Handschuh, R., Polly, J., and Morales, W.: Gear Mesh Loss-of-Lubrication Experiments and
Analytical Simulation, NASA/TM—2011-217106, November 2011.
6. Oswald, F.: Mechanical Components Branch Test Facilities and Capabilities, NASA/TM—2004-
212722, January 2004.
7. Krantz, T. and Kahraman, A.: An Experimental Investigation of the Influence of the Lubricant
Viscosity and Additives on Gear Wear, NASA/TM—2005-213956, ARL-TR-3126, October 2005.
8. Krantz, T.: The Influence of Roughness on Gear Surface Fatigue, NASA/TM—2005-213958,
October, 2005.
9. Pyrowear Alloy 53, Carpenter Technology Corporation, AMS 6308B.
10. Vijayakar, S. and Abad, S.: Helical 3-D User’s Manual, Advanced Numerical Solutions, Hilliard,
OH, February 2005.
NASA/TM—2014-218396 12

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