Robert F. Handschuh

Glenn Research Center, Cleveland, Ohio

Ryan Ehinger, Eric Sinusas, and Charles Kilmain

Bell Helicopter Textron Inc., Fort Worth, Texas

Double Helical Gear Performance Results

in High Speed Gear Trains

NASA/TM—2010-216760

July 2010

AHS2009–000238
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Robert F. Handschuh

Glenn Research Center, Cleveland, Ohio

Ryan Ehinger, Eric Sinusas, and Charles Kilmain

Bell Helicopter Textron Inc., Fort Worth, Texas

Double Helical Gear Performance Results

in High Speed Gear Trains

NASA/TM—2010-216760

July 2010

AHS2009–000238

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216760 1 

Double Helical Gear Performance Results in High Speed Gear Trains 

Robert F. Handschuh 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ryan Ehinger, Eric Sinusas, and Charles Kilmain 

Bell Helicopter Textron Inc. 

Fort Worth, Texas 76101 

Abstract 

The operation of high speed gearing systems in the 

transmissions of tiltrotor aircraft has an effect on overall 

propulsion system efficiency. Recent work has focused on 

many aspects of high-speed helical gear trains as would be 

used in tiltrotor aircraft such as operational characteristics, 

comparison of analytical predictions to experimental data and 

the affect of superfinishing on transmission performance. 

Baseline tests of an aerospace quality system have been 

conducted in the NASA Glenn High-Speed Helical Gear Train 

Test Facility and have been described in earlier studies. These 

earlier tests had utilized single helical gears. The results that 

will be described in this study are those attained using double 

helical gears. This type of gear mesh can be configured in this 

facility to either pump the air-oil environment from the center 

gap between the meshing gears to the outside of tooth ends or 

in the reverse direction. Tests were conducted with both 

inward and outward air-oil pumping directions. Results are 

compared to the earlier baseline results of single helical gears. 

Introduction 

High speed, heavily loaded and lightweight gearing 

components are found in propulsion systems for rotorcraft. 

The high pitch line velocity that is part of these systems makes 

the thermal aspects of the gear system design very important. 

Transmission systems using helical gear trains in tiltrotor 

aircraft provide the proper spacing between the parallel engine 

and rotor shafts as shown in Figure 1 (Ref. 1). These gear 

trains can induce additional thermal problems as the idler 

gears in this system receive two meshing cycles (on opposites 

sides of the gear teeth) per revolution. Therefore, weight 

optimized aerospace drive system components can have 

difficulty when it is required to operate with a primary 

lubrication system failure. 

In past studies using this gear train system, testing has 

focused on basic operational characteristics (Refs. 2 and 3), 

comparison of analytical predictions to experimental results 

(Ref. 4), and the affect of superfinishing (Ref. 5). The results 

from these studies have shown and quantified the effect of 

operational conditions on the power to rotate the gear system 

at high speed and load. 

In prior testing the individual thermal/energy loss 

mechanisms were separated into gear meshing, bearing, and 

windage losses. The gear meshing and bearing losses are fairly 

well understood (Ref. 6) and some design modifications to the 

gears and bearings can provide some benefit to increase 

performance. The gear windage losses, however, are the least 

understood, but can be a source of dramatic tiltrotor 

performance improvement once this mechanism is better 

understood. 

In the work to be described in this paper, a double helical 

gear train was tested. The test facility has gears that are bolted 

to the shafting (in practice this interface is avoided to remove 

fasteners and shafting complexity). This permitted the gearing 

to be mounted from either side of the gear inner flange and 

thereby the system could be tested two ways. One way was to 

have the gears pump the air-lubricant environment (from 

lubricating and cooling jets) outward towards the axial ends of 

the teeth. The second way was to have the gear mesh pump the 

environment inward from the ends of the teeth towards the 

center gap. The double helical gears were also tested with and 

without shrouding as used in the prior studies. Temperature 

data from axial thermocouple rakes and tooth end arrays will 

also be provided. Finally, the double helical results will be 

directly compared to prior studies from the same facility 

(single helical gear results; (Refs. 2 to 7)). 

Test Facility and Test Hardware 

Test Facility 

The test facility used for this study is shown in Figure 2. 

The facility is a closed-loop, torque-regenerative testing 

system. There is a test gearbox and slave gearbox that are 

basically mirror images of each other. Each gearbox has an 

input gear, three idlers, and one bull gear. The gearboxes are 

joined together through the input gears and bull gears via 

shafting. 

A 500 hp DC drive motor powers the facility and its output 

speed is increased using a 5.7:1 speed-increasing gearbox. The 

output of the speed-increasing gearbox then passes through a 

torque and speed sensor before connecting to the slave 

gearbox. 
NASA/TM—2010-216760 2 

Figure 1.—Layout view of typical tiltrotor nacelle (airplane mode). 

Figure 2.—NASA high-speed helical gear train test facility. 

Each gearbox has separate oil supply and scavenge pumps 

and reservoirs. All flow rates have been calibrated at various 

temperatures and pressures prior to installation for accurate 

flow rate measurements. Lubrication system flow rate is 

controlled using the supply pressure. Temperature is 

controlled via immersion heaters in the reservoir and heat 

exchangers that cool the lubricant returned from the 

gearboxes. Each lubrication system has very fine 3-m 

filtration. Nominal flow rate into the test or slave gearboxes at 

80 psi is approximately 15 gpm. 

The lubricant used in the tests to be described was a 

synthetic turbine engine lubricant (DoD-PRF-85734). This 

lubricant is used in gas turbine engines as well as the drive 

systems for rotorcraft. 

Test Hardware 

The test hardware used in the tests to be described is 

aerospace quality hardware. The basic gear design information 

is contained in Table 1. The input and bull gear shafts 

typically have a combination of roller bearings with ball 

bearings to contain the resultant thrust loads, whereas the idler 

gears only have roller bearings. For the double helical gear 

tests, the two thrust bearings (input shaft and bull gear shaft) 

needed in the single helical gear arrangement were removed. 

The double helical gear configuration does not produce an 

axial load from the gear forces; therefore the thrust bearings 

are not needed. A photograph of the double helical gears in the 

inward pumping gear arrangement without the shrouds is 

shown in Figure 3. A photograph of the shrouds from earlier 

tests is shown in Figure 4 (shrouding was configurable for 

either single or double helical gear trains). 

TABLE 1.—BASIC GEAR DESIGN DATA FOR 

DOUBLE HELICAL GEAR TRAIN 

Number of teeth input and 2nd idler 50 

Number of teeth 1st and 3rd idler 51 

Number of teeth bull gear 139 

Axial face width each side, mm (in.) 39.6 (1.560) 

Gap between tooth flank sides, mm (in.) 41.1 (0.555) 

Normal plane module, mm (normal plane 

diametral pitch (1/in.) 

0.254 (10) 

Normal plane pressure angle, degree 20 

Transverse plane module, mm 3.10 (8.192) 

Helix angle, degree 35 

Gear material Pyrowear 53 
NASA/TM—2010-216760 3 

Figure 3.—Photograph of test gearbox 

with shrouds removed. 

Figure 4.—Shrouding used during testing. 

Test Instrumentation and 

Data Acquisition 

Test Instrumentation 

The instrumentation used in these tests was the same as in 

prior studies except for the axial fling-off temperature rakes. 

With the double helical gear mesh tests, another set of probes 

was fabricated so that the fling-off temperatures from the 

various axial locations of the gear could be measured. The 

Figure 5.—Comparison of rake probes for single 

and double helical gear train studies. 

Figure 6.—Array probe used at the 

tooth end—meshing position. 

Figure 7.—Locations of array and rake thermocouples within 

the test gearbox. 

probe is shown in Figure 5 along with the one from prior 

single helical testing. Also in these tests, axial arrays 

(9 thermocouples) were used at the mesh exit regions (Fig. 6). 

The temperature probes were at locations in the gearbox as 

shown in Figure 7. 

Data Acquisition 

The test facility data system monitors three important 

facility parameters during operation. Speed, torque (supplied 

torque and loop torque), and temperature measurements were 

made during all the testing conducted. The test system loop 

torque is measured on the shaft connecting the bull gears from 
NASA/TM—2010-216760 4 

the test and slave gearboxes. A telemetry system was utilized 

in this location to measure loop torque. 

The data recording system used in this study has the 

capability of taking data from all parameters at a rate of one 

sample per second (tests in this study recorded data every 

2 sec). The data is displayed to the test operator in real time. 

Data is stored in a spreadsheet format and each sensor can be 

viewed at any time during a test and when post processing the 

results. 

The test procedure for collecting the data to be presented 

was the following. For a given set of conditions (speed, 

torque, lubricant pressure and lubricant oil inlet temperature) 

the facility was operated for at least 5 min or until the 

temperatures of interest had stabilized (~2 F). 

Experimental Results 

In this part of the paper, just the results attained with the 

double helical gears will be discussed. Comparison to the 

single helical gear tests will be discussed in the next section. 

The gears were operated at various speed, torque, lubricant 

flow rate and lubricant inlet temperature conditions. For all the 

results shown in this section, the lubricant inlet temperature 

for the test gearbox was held at 200 F. Also, the slave 

gearbox conditions (lubricant temperature and flow rate) 

remained unchanged for all of the test conditions of the data to 

be presented. For the experimental results to be described in 

this paper the nomenclature shown in Table 2 will be used. 

The first comparison is made between temperatures across 

the gear face width and the direction of gears pumping of the 

air—lubricant mixture, as shown in Figure 8. Four different 

sets of data are shown in the figure for the 2nd and 3rd idler 

gear location. Thermocouple probes 1 and 6 were near the 

ends of the teeth and thermocouple probes 3 and 4 are near the 

center gap of the double helical gears. The outward pumping 

arrangement had the lowest temperature at the gap locations. 

The inward pumping configuration had an increase in 

temperature towards the tooth gap. Also, for the inward 

pumping configuration, the inward pumping had a much 

higher temperature at the gap region when the shrouding was 

in place. 

The next comparison to be made is shown in Figure 9. In 

this figure the maximum thermocouple rake and array 

temperatures are plotted for the two speed conditions (12500 

and 15000 rpm input shaft speed), three bull gear shaft torque 

levels (33, 67, and 100 percent of maximum), outward or 

inward pumping configuration, and with or without shrouds. 

All data shown in Figure 9 was taken with the standard inlet 

conditions mentioned above. In the figure two bands of data 

are shown due to the two speed conditions. The rake or array 

maximum temperatures increased linearly with applied torque. 

At either input shaft speed (band), the inward pumping gear 

arrangement produced the highest (shrouded, rake probe) and 

TABLE 2.—FIGURE NOMENCLATURE 

OP Outward Pumping 

IP Inward Pumping 

S Shrouds 

NS No Shrouds 

SH Single Helical 

DH Double Helical 

ISF Superfinished Gear Flanks 

GRND Ground Gear Flanks 

Figure 8.—Rake temperature as a function of location across 

the face width, 200 F lubricant inlet temperature, 

15000 rpm, 5000 hp (2nd to 3rd idler location). 

lowest (shrouded or unshrouded, array probe) temperatures. 

For the inward pumping arrangement, a limited amount of 

lubricant from the meshing gears would be assumed to strike 

the array probe in comparison to that flung from the gear mesh 

radially and pumped axially towards the center of the face 

width of the gear. 

The last comparison with respect to just the double helical 

gear tests is shown in Figure 10. In Figure 10(a) and (b) the 

standard conditions (200 F lubricant inlet temperature, and 

80 psi lubricant jet pressure) were provided for two speeds and 

three load cases as described earlier. In Figure 10(a) drive 

motor power (amount of power to rotate the entire test rig) and 

in Figure 10(b) lubricant temperature rise (outlet minus inlet 

temperatures) is plotted versus bull gear shaft torque. From 

Figure 10(a) or (b) it can be seen that either the amount of 

power or the temperature rise was minimized using outward 

pumping configuration with shrouds. Outward pumping also 

required the most power or had the highest temperature rise if 

the shrouds were removed. When the gears were run with 

them pumping towards the gap, there was less of an effect 

with or without the shrouds. 
NASA/TM—2010-216760 5 

Figure 9.—Maximum rake and array temperatures as a function of torque, speed, and shrouding 

(inward and outward pumping). 

Figure 10.—Effect of shrouding on drive motor power (a) and lubricant temperature rise (b). 
NASA/TM—2010-216760 6 

Comparison to Prior Testing 

In this part of the paper the results from double helical gear 

testing will be compared to prior experimental results from 

single helical gears. As was shown earlier in this paper, two 

shaft speeds of interest will be shown with varying amounts of 

torque applied. The two speed levels pertain to the two 

tiltrotor flight operating conditions of hover (high shaft speed) 

and forward flight (lower shaft speed). Comparisons will be 

made for the amount of power to rotate the gear train in the 

test facility, amount of power absorbed by the lubricant 

(thermal efficiency), inlet lubricant jet pressure (lubricant 

flow), lubricant temperature increase across the gearbox, and 

thermocouple information from the radial and axial probes. 

The first comparison, Figure 11, will be between the 

various configurations with respect to the amount of power to 

rotate the system at two rotational speeds with varying 

amounts of bull gear shaft torque. The single helical (SH) gear 

results are shown for superfinished (ISF) and conventionally 

ground (GRND) gear surfaces. All data is for shrouded (S) 

gear conditions. (In subsequent figures NS—means that the 

shrouds were removed for the test conditions.) For the 

conditions tested at the two different rotational speeds and 

three levels of bull gear torque, the double helical gears 

configured in the outward pumping, shrouded arrangement 

provided the lowest power loss at all conditions. 

The next comparison is for power loss to the lubricant, 

Figure 12, between the various configurations shown. The 

level of losses (hp) were determined via the flow rate, 

lubricant properties, and the temperature differential from 

lubricant inlet to exit. In this figure, once again the double 

helical gear train had the lowest power loss for both speed and 

all loading conditions. This data is also for all configurations 

using shrouds. 

Next the effect of lubricant system jet pressure (volumetric 

flow rate) is presented in Figure 13. The tests were all run at 

33 percent of maximum torque and a 200 F lubricant inlet 

temperature. As before, the data is contained in two “speed 

bands”. Lower jet pressure reduces the amount of lubricant 

available to keep the gear surfaces fully flooded and provide 

adequate cooling. In this facility the jets feed lubricant into 

and out of mesh. The lubrication system operates in a dry 

sump mode—all lubricant is jet fed and scavenged away by 

lubrication pumps. There is also concern that by lowering the 

jet pressure, the jets cannot penetrate deep enough to the roots 

of the teeth so the level of loading was kept to 33 percent of 

maximum. The trends were that higher pressure (more cooling 

flow) reduced the temperature change from inlet to exit of the 

gearbox. Also the double helical outward pumping 

arrangement produced the lowest temperature increase for all 

conditions tested. 

In Figure 14 the next comparison is made between the 

temperature change as a function of two speeds and three 

torque levels for six different gearing configurations. Once 

again the double helical, outward pumping configuration with 

shrouds had the lowest temperature rise for the lubricant 

between inlet and exit of the gearbox. Nearly all the data 

showed a linear dependence on the torque applied. 

The last two Figures 15 and 16 will show the temperature 

probe data for the single and double helical gear 

configurations. The thermocouple array maximum 

temperature differential is shown for two speeds and three 

torque levels, all at 200 F lubricant inlet temperature. The 

temperature difference was found from the maximum 

thermocouple array measured minus the lubricant inlet 

temperature (~200 F). For this data, one can see three bands 

of data. The single helical gears have a much higher 

temperature differential from the array probe thermocouples 

than the double helical gears. The double helical, inward or 

outward pumping gears, had a similar result for temperature 

difference at 15000 rpm as the single helical at 12500 rpm. 

Also the double helical gear configurations at 12500 rpm were 

the lowest. An explanation for this is fairly straightforward. 

Since a single helical gear has the entire face width to pump 

the entrapped lubricant—air environment, it would be 

expected to have a much higher exit (array) temperature as 

opposed to one only moving across approximately half the 

face width. 

The last comparison to make will be with the rake probes 

that measured the air- lubricant being flung radially from the 

gear teeth. In Figure 16 the temperature differences between 

the maximum rake temperature and lubricant inlet temperature 

is plotted versus applied torque for the two speed conditions 

(12500 and 15000 rpm). All test data shown was taken with 

shrouds in place. At the 12500 rpm input shaft speed, all data 

was within ~7 F of each other. At the higher speed 

conditions, both double helical gear configurations were better 

with the outward pumping double helical configuration being 

the best at all torque conditions. 
NASA/TM—2010-216760 7 

Figure 11.—Drive motor power versus torque, two speeds at 200 F oil in temperature. 

Figure 12.—Power losses versus torque, two speeds at 200 F oil in temperature. 
NASA/TM—2010-216760 8 

Figure 13.—Temperature change of the lubricant versus pressure at 200 F, two speeds, 19000 in*lb 

torque at bull gear shaft. 

Figure 14.—Temperature change of lubricant versus torque, two speeds at 200 F. 
NASA/TM—2010-216760 9 

Figure 15.—Temperature of array thermocouple maximum difference from inlet temperature versus 

torque at two speeds, 200 F lubricant inlet temperature. 

Figure 16.—Temperature change of oil inlet to maximum rake probe data versus torque, two speeds, 200 F 

lubricant inlet temperature. 
NASA/TM—2010-216760 10 

Conclusions 

Based on the results found in this study and those 

conducted previously the following observations can be 

concluded 

1. Double helical gear trains that outwardly pump the air—

lubricant mixture axially produced the highest 

performance when compared to other tested conditions 

that included inward pumping double helical gears and 

single helical gear trains (ground and superfinished). 

2. Double helical gear trains that have an inward pumping 

arrangement produced similar performance results with or 

without shrouds. The results from these tests were in 

between the outward pumping arrangement being better 

than outward pumping without shrouds and not as good as 

outward pumping with shrouds. This was found in both 

the drive motor power needed to rotate the entire test 

facility as well as the temperature change of the lubricant 

across the gearbox (difference between inlet and outlet 

lubricant temperature). 

3. The double helical, outward pumping shrouded 

arrangement was also the best when lubricant pressure 

was reduced, providing the lowest temperature increase at 

nearly all conditions. 

4. Temperature probes were used to measure the radial and 

axial air—lubricant environment as the gears were tested. 

The array probe temperatures were much higher for the 

single helical (ground or superfinshed) in comparison to 

either inward or outwardly pumped configurations. In the 

case of a single helical gear the array probe will measure 

the air—lubricant that is pumped some distance across the 

face width in comparison to the double helical gear 

system that might only be half the distance for the case of 

outward pumping. The data from the rake probes showed 

that the most significant difference was at the 15000 rpm 

condition with the double helical gear, outward pumping 

arrangement being the one with the lowest temperature 

increase between the oil inlet temperature and the 

maximum rake probe temperature. 

References 

1. Kilmain, C., Murray, R., and Huffman, C.: V-22 Drive 

System Description and Design Technologies, American 

Helicopter Society 51st Annual Forum, May 1995. 

2. Handschuh, R. and Kilmain, C: Preliminary Investigation 

of the Thermal Behavior of High-Speed Helical Gear 

Trains, NASA/TM—2002-211336, ARL–TR–2661, 

March 2002. 

3. Handschuh, R. and Kilmain, C.: Efficiency of High-Speed 

Helical Gear Trains, NASA/TM—2003-212222, ARL–

TR–2968, April, 2003. 

4. Handschuh, R. and Kilmain, C.: Preliminary Comparison 

of Experimental and Analytical Efficiency Results of 

High-Speed Helical Gear Trains, ASME 2003 Design 

Engineering Technical Conference, September 2003, 

Chicago, IL. 

5. Handschuh, R. and Kilmain, C.: Experimental Study of 

the Influence of Speed and Load on Thermal Behavior of 

High-Speed Helical Gear Trains, NASA/TM—2005-

213632, ARL–TR–3488, July 2005. 

6. Handschuh, R. and Kilmain, C: Operational Influence on 

Thermal Behavior of High-Speed Helical Gear Trains, 

NASA/TM—2006-214344, ARL–TR–3969, November 

2006. 

7. Handschuh, R.; Kilmain, C.; Ehinger, R.: Operational 

Condition and Superfinishing Effect on High-Speed 

Helical Gearing System Performance, NASA/TM—2007-

214696; ARL–TR–4099, June, 2007. 
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Double Helical Gear Performance Results in High Speed Gear Trains 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The operation of high speed gearing systems in the transmissions of tiltrotor aircraft has an effect on overall propulsion system efficiency. 

Recent work has focused on many aspects of high-speed helical gear trains as would be used in tiltrotor aircraft such as operational 

characteristics, comparison of analytical predictions to experimental data and the affect of superfinishing on transmission performance. 

Baseline tests of an aerospace quality system have been conducted in the NASA Glenn High-Speed Helical Gear Train Test Facility and 

have been described in earlier studies. These earlier tests had utilized single helical gears. The results that will be described in this study are 

those attained using double helical gears. This type of gear mesh can be configured in this facility to either pump the air-oil environment 

from the center gap between the meshing gears to the outside of tooth ends or in the reverse direction. Tests were conducted with both 

inward and outward air-oil pumping directions. Results are compared to the earlier baseline results of single helical gears.

15. SUBJECT TERMS

High speed gearing; Propulsion; Helical gears 

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