David G. Lewicki and Mark A. Stevens

Glenn Research Center, Cleveland, Ohio

Testing of Two-Speed Transmission

Configurations For Use In Rotorcraft

NASA/TM—2015-218816

June 2015
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated 

to the advancement of aeronautics and space science. 

The NASA Scientific and Technical Information (STI) 

Program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Officer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI Program provides access 

to the NASA Technical Report Server—Registered 

(NTRS Reg) and NASA Technical Report Server—

Public (NTRS) thus providing one of the largest 

collections of aeronautical and space science STI in 

the world. Results are published in both non-NASA 

channels and by NASA in the NASA STI Report 

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of significant 

scientific and technical data and information 

deemed to be of continuing reference value. 

NASA counter-part of peer-reviewed formal 

professional papers, but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or of 

specialized interest, e.g., “quick-release” reports, 

working papers, and bibliographies that contain 

minimal annotation. Does not contain extensive 

analysis.

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or co-sponsored by NASA.

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 757-864-6500

• Telephone the NASA STI Information Desk at

757-864-9658

• Write to:

NASA STI Program

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681-2199
David G. Lewicki and Mark A. Stevens

Glenn Research Center, Cleveland, Ohio

Testing of Two-Speed Transmission

Configurations For Use In Rotorcraft

NASA/TM—2015-218816

June 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

NASA STI Program

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681-2199

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

703-605-6000

This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
Testing of Two-Speed Transmission

Configurations For Use In Rotorcraft

David G. Lewicki and Mark A. Stevens

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Large civil tiltrotors have been identified to replace regional airliners over medium ranges to alleviate 

next-generation air traffic. Variable rotor speed for these vehicles is required for efficient high-speed 

operation. Two-speed drive system research has been performed to support these advanced rotorcraft 

applications. Experimental tests were performed on two promising two-speed transmission 

configurations. The offset compound gear (OCG) transmission and the dual star/idler (DSI) planetary 

transmission were tested in the NASA Glenn Research Center variable-speed transmission test facility.

Both configurations were inline devices with concentric input and output shafts and designed to provide 

1:1 and 2:1 output speed reduction ratios. Both were designed for 200 hp and 15,000 rpm input speed and 

had a dry shift clutch configuration. Shift tests were performed on the transmissions at input speeds of 

5,000, 8,000, 10,000, 12,500, and 15,000 rpm. Both the OCG and DSI configurations successfully 

perform speed shifts at full rated 15,000 rpm input speed. The transient shifting behavior of the OCG and 

DSI configurations were very similar. The shift clutch had more of an effect on shifting dynamics than the 

reduction gearing configuration itself since the same shift clutch was used in both configurations. For 

both OCG and DSI configurations, low-to-high speed shifts were limited in applied torque levels in order 

to prevent overloads on the transmission due to transient torque spikes. It is believed that the relative lack 

of appreciable slippage of the dry shifting clutch at operating conditions and pressure profiles tested was a 

major cause of the transient torque spikes. For the low-to-high speed shifts, the output speed ramp-up 

time slightly decreased and the peak out torque slightly increased as the clutch pressure ramp-down rate 

increased. This was caused by slightly less clutch slippage as the clutch pressure ramp-down rate 

increased.

Introduction

As part of the Revolutionary Vertical Lift Technology Project of the NASA Advanced Air Vehicles 

Program, research is being performed for large civil tiltrotors to replace regional airliners over medium 

ranges to alleviate next-generation air traffic. Aircraft design is to carry 90 passengers for 1,000 nm with 

performance of 300 knots at 28,000-ft altitude. Aircraft sizing-design studies have identified that a twospeed rotor configuration is required (Refs. 1 and 2). Rotor speed at cruise is required to be reduced, 

possibly down to 50 percent of hover speed, to keep rotor blade advancing tip speeds reasonable. To 

support this configuration, research in developing two-speed gearboxes is being led at the Glenn Research 

Center.

One early study of a multi-speed rotorcraft drive system consisted of a high-speed traction drive 

variator and a planetary differential gear unit (Ref. 3). A unit was tested and performed as designed.

However, the mechanical efficiency was lower than that of a conventional drive system. Also, the use of 

traction drives for a main drive path is not an accepted practice for U.S. rotorcraft manufacturers.

Previous studies have been sponsored by NASA to investigate multi-speed drive systems (Ref. 4). These 

NASA/TM—2015-218816 1
studies looked at earlier tiltrotor applications where 15 percent speed reduction for cruise was required.

Dual-path configurations and compound planetary configurations looked promising. However, major 

concerns such as shifting and drive system weight were identified. An electro-mechanical, infinitelyvariable transmission, comprising a pair of planetary trains interconnected with two electric machines and 

clutches, has been proposed (Ref. 5). Again, the mechanical efficiency suffered as well as the added 

complexity of two planetary systems. A unique concept called a pericyclic continuously variable-speed 

transmission is being investigated (Ref. 6). It uses pericyclic kinematics to achieve single-speed reduction 

ratios between 1.05:1 to 50:1 with variable-speed capability in one configuration. This concept, however, 

has yet to be tested.

An early example of clutches being employed in rotorcraft was in the XV-1 Convertiplane (Ref. 7).

This aircraft was an autogyro with additional tip-jet propulsion on the rotors. When operating as an 

autogyro, a propeller was clutched-in to an internal combustion engine and de-clutched when flying in the 

tip-jet mode. In 1953, a two-speed helicopter transmission (Ref. 8) was flight tested on an Air Force H-5H 

helicopter. Here, engine speed was constant and main rotor speed was varied using a two-speed 

transmission. The flight tests showed that the two-speed transmission had great advantages in enhancing 

the rotorcraft performance. Another early example of the variable speed rotor concept was demonstrated 

with the XH-51 compound helicopter (Ref. 9). Here, rotor speed was varied by adjusting engine speed. In 

the flight tests, rotor speed was gradually decreased to reduce effects of advancing side tip speed 

compressibility effects resulting in lower vibrations, structural loads, and improved handling 

characteristics during high speed flight. Recently, a variable rotor speed approach was developed for the 

A160 Hummingbird unmanned aerial vehicle (Refs. 10 and 11). Here, a two-speed transmission is 

utilized to switch between two nominal rotor speeds. Then, rotor speed is continuously adjusted about 

either nominal speed through the engine power turbine set point.

Results from these previous references indicates that the incorporation of multi-speed concepts in 

rotorcraft application is not a trivial process and adds complexity and weight. Thus, further research and 

development in this area is required, such as described in (Refs. 12 and 13). There, studies were 

completed that investigated possible drive system arrangements that can accommodate up to 50 percent 

speed change. Several concepts where presented and evaluated. Two promising configurations were 

identified: 1) the offset compound gear transmission, and 2) the dual star/idler planetary transmission.

These two configurations were designed and fabricated as part of the NASA Revolutionary Vertical Lift 

Technology Project. Further detailed information on the design of these configurations is given in 

(Ref. 14).

The objective of the current study is to present the results of the testing on the offset compound gear 

transmission and the dual star/idler planetary transmission. Testing was performed in the NASA Glenn 

Research Center variable-speed transmission test facility. Shift tests were performed on the transmissions 

up to the design speed of 15,000 rpm. Speed and torque transients as well as transmission component 

characteristics such as lubrication system parameters, vibration, and temperatures are presented.

Apparatus

Test Facility

Testing was performed in the NASA Glenn Research Center variable-speed transmission test facility.

The variable-speed transmission test facility utilizes two identical alternating-current (AC) variable-speed 

motors which can be used as either a driver or a regenerative load. The facility implements a four 

quadrant, flux vector, high-performance drive system containing a common direct-current (DC) bus. The 

NASA/TM—2015-218816 2
common DC bus approach allows the power being absorbed from one motor to be utilized by the driving 

motor.

The test facility is shown in Figure 1. The two motors are custom-designed, 480-V, 3-phase, AC 

induction motors. The motors are water cooled using a dedicated cooling pump system. Each motor is 

supported internally by a pair of high-precision ball bearings, which are lubricated using an air-oil mist 

lubrication system. The motors also contain resistance temperature detectors (RTD's) on the motor 

windings and bearings, as well as a 1,024-tooth gear tachometer for speed control. The motors are rated 

for constant torque of 140 ft-lb from 0 to 7,500 rpm, and constant power of 200 hp from 7,500 to 

15,000 rpm.

For the current studies, the left motor is the driver motor, providing the required input speed for the 

test. The right motor is the load device, providing open-loop torque, closed-loop torque, or closed-loop 

power control. The test transmission is installed between the two motors. The test transmission is 

configured with speed sensors and torquemeters on the input and output shafts. Due to the high speeds, 

the motors and transmission were installed using a high-precision laser alignment tool. The motors and 

test transmission incorporate jacking screws to aid in the alignment. High-speed couplings connect the 

components together.

The control scheme of the test facility is shown in Figure 2. The facility is powered by 480-V, 400-A

AC service power which is isolated, filtered, converted to DC, and fed to the DC bus. The left motor is 

the driver and is configured for speed control. The left motor controller adjusts the speed of the motor 

based on the user set point using the motor tachometer for speed reference and its internal proportionalintegral-derivative controller (PID) methodology. The right motor is the power absorber. For open-loop 

control, a fixed control is sent to the motor based on the user set point. For closed-loop control, 

information from the torquemeters is sent to the load programmable logic controller (PLC), which, in 

turn, sends the appropriate signal control to the motor based on the user set point. Under closed-loop 

torque control, the system tried to maintain a constant torque. Under closed-loop power control, the 

system tried to maintain constant power (speed times torque). Again, power absorbed by the right motor 

is fed back to the DC bus for use by the driver motor. Various analog, digital, and temperature 

measurements are fed to the data pod PLC. The left and right motor controllers, data pod PLC, load PLC, 

and control and data acquisition PC are all connected through an Ethernet hub. The control PC is the 

interface to the user, providing capabilities to adjust speed and torque, turn on lube system pumps, collect

facility data, and monitor data for exceedance and shutdowns. Last, an emergency stop is available to 

immediately shutdown the facility if required.

The test transmission has a dedicated external oil lubrication system whose simplified schematic is 

shown in Figure 3. The system provides lubrication and cooling for the gears, bearings, and sprag clutch 

in the test transmission as well as providing high-pressure oil to the shift control clutch. Heated oil in the 

supply tank is pumped through a heat exchanger for precise temperature control. From there, the oil is 

split to the test transmission manifold and the high-pressure pump. The oil from the manifold is fed to jets 

that lubricate the gears and bearings in the test transmission as well as a feed to the sprag clutch in the test 

transmission. The oil from the high-pressure pump is fed to a servovalve that controls the shift clutch in 

the test transmission. Oil fed to the test transmission is returned to the return tank using a scavenge pump.

For the tests, the oil was set to 160 °F oil inlet (to the test transmission) temperature and 80 psi oil inlet 

pressure. The lubricating fluid used was a synthetic base helicopter transmission oil conforming to the 

DOD-L-85734 specification.

Details of the shift clutch control system is shown in Figure 4. The purpose of this system is to 

provide the shift clutch piston with the proper pressure profile to achieve a required high-to-low or lowto-high speed shift. The high-pressure pump, as previously described, provides oil at 900 psi and 160 °F

NASA/TM—2015-218816 3
temperature to the shift clutch servovalve. The servovalve is a flow control throttle valve and designed for 

precise pressure control with high dynamic response requirements. The output pressure of the servovalve, 

which is fed to the test transmission shift clutch piston, is set using a dedicated closed-loop, highfrequency, PID controller. User defined set points and pressure profiles are entered into the controller, 

which in turn, supply the required pressure to the shift clutch piston. The system can provide up to 800 psi 

pressure at 1 gpm flow rate.

Test Transmissions

Two different two-speed transmission configurations were tested in the current study. The first was 

the offset compound gear (OCG) transmission configuration. The second was the dual star-idler (DSI) 

planetary transmission configuration. Both are inline devices with concentric input and output shafts and 

designed to provide 1:1 and 2:1 output speed reduction ratios. Both are designed for 200 hp and 

15,000 rpm input speed.

Transmissions used in rotorcraft applications are designed for light weight and minimized volume.

The transmissions in the current study were designed, however, in a modular fashion. This was done to 

simplify modifications required when changing the design of various components in the test transmission

such as the reduction gearing and shift clutch. In addition, a number of commercially available parts, 

many from the automotive industry, were used to reduce acquisition cost and fabrication time. Thus, the 

test transmissions in the current study were heavier and larger than actual rotorcraft counterparts, but were 

used to demonstrate prototype feasibility. The gearing, however, was designed using aerospace practices.

Offset Compound Gear (OCG) Transmission

A view of the OCG transmission is shown in Figure 5. As seen in the cross-sectional view 

(Figure 5(a)), the modular design is comprised of three regions: 1) the reduction gearing region, 2) the 

shift clutch region, and 3) the oil transfer region. The OCG employs a unique and patented arrangement in 

the reduction gearing region (Refs. 15 and 16). This is shown in a three-dimensional view in Figure 5(b).

For this arrangement, the external OCG input gear, mounted on the input shaft, drives the internal OCG 

cluster gear 1. The external OCG cluster gear 2, integral with cluster gear 1, drives the internal OCG 

output gear. Note that the centerline of the cluster shaft is offset to that of the input and output shafts, thus 

giving the name “offset compound gear”. By using this unique arrangement of external and internal 

gearing, the envelope of the gearing is reduced compared to that using all external gears. Basic 

dimensions for the gearing are given in Table I. The OCG has a 2.005:1 reduction ratio and the output is 

driven in the same direction as the input. All gears were involute spur gears made from carburized and 

ground AISI 9310 alloy steel per AMS 6265M. The gears were made to high precision AGMA class 13.

The input shaft, OCG cluster gear shaft, and output shaft were all supported by duplex ball bearing sets.

All duplex bearings were installed in machined aluminum split cap and base supports which were 

mounted to the transmission housing steel base. The transmission housing itself was a square box 

structure made from aluminum plates, primary designed to contain the oil used in the transmission as well 

as ease of access to internal parts.

The two-speed OCG transmission operates as follows. The input shaft is directly connected to the 

forward side of the shift clutch. The aft side of the clutch is connected to the output shaft as in typical 

automotive applications. Under zero piston pressure, the clutch by default is mechanically engaged

through spring force and the output shaft operates at the same speed of the input shaft. All the power 

flows directly from input shaft to output shaft at 1:1 reduction ratio. Here, the input shaft still turns the 

OCG input gear, OCG cluster, OCG output gear, and low-speed output drum (connected to the OCG 

output gear), but under no power. A sprag clutch is installed between the output shaft and low-speed 

NASA/TM—2015-218816 4
output drum and allows overrunning of the output shaft when its speed is greater than the output drum. To 

shift down in speed, pressure is applied to the shift clutch piston. When adequate pressure is applied, the 

clutch disengages and the output shaft coasts down in speed. When the output shaft speed reaches that of 

the low-speed output drum, the sprag clutch engages and the output shaft is driven at a 2.005:1 speed 

reduction through the OCG gearing. To shift back up in speed, the pressure to the shift clutch piston is 

released and the clutch is mechanically engaged, reverting back to the 1:1 ratio direct drive.

The shift clutch used in the studies was a commercially-available carbon clutch used in the 

automotive racing community. It is classified as a dry clutch and does not require oil for lubrication or 

cooling. A close-up of the clutch is shown in Figure 6. The clutch contains a pair of carbon-carbon plates.

One of the plates is rigidly connected to the drive cover and the other is splined to the clutch inner shaft.

Upon assembly, the plates are locked together as the clutch drive cover is bolted to the adapter shaft. This 

locks the input shaft (bolted to the adapter shaft) to the output shaft (bolted to the clutch inner shaft). To 

disengage the clutch, pressurized oil is applied to the piston, which pushes on the release fingers to 

separate the clutch plates. The bearings supporting the adapter shaft are sealed grease-packed deep-groove 

bearings which are axially preloaded by a Belleville washer spring pack located between the bearings. A 

grease-packed spider bearing is located between the input shaft and low-speed output drum. Dams and 

holes in the low-speed output drum were installed to keep the clutch area as dry as possible. Slight 

modifications were made from the original clutch design (described later) and the clutch capacity was 

approximately 150 ft-lb with a release load of approximately 425 lb, resulting in a required release oil 

pressure of approximately 325 psi.

As previously mentioned, oil was fed to the test transmission manifold to lubricate and cool the 

components in the transmission. Figure 7 shows details of the manifold as used for the OCG tests.

Separate taps were used for the input shaft duplex bearing, output shaft duplex bearing, OCG cluster shaft 

duplex bearing, sprag clutch, and front and rear manifolds located in the test transmission housing. All 

taps had throttle valves to regulate oil flow rate. For the OCG tests, the front manifold fed the OCG input 

gear mesh into-mesh jet and OCG output gear mesh out-of-mesh jet. The rear manifold fed the OCG input 

gear mesh out-of-mesh jet and OCG input gear mesh into-mesh jet. Per trial and error, all throttle valves 

were set wide open except for the sprag clutch, which was throttled slightly closed. The approximate oil 

flow rates to the various OCG components as indirectly measured is given in Table II.

Lastly, Figure 5(a) shows the oil transfer region toward the aft side of the transmission. This region 

contains the oil transfer mechanism to supply oil to the rotating shift and sprag clutches. This was quite a 

design challenge due to the sealing requirements at these high speeds and high pressures. The oil transfer 

mechanics along with the custom designed output shaft supplied oil to three separate passages: 1) highpressure oil supply to shift clutch piston, 2) high-pressure oil return bleed from shift clutch piston, and 

3) low-pressure oil supply to sprag clutch. The oil in the passages were sealed in the oil transfer 

mechanism through use of Vespel polyimide (DuPont) sealing rings.

Dual Star-Idler (DSI) Planetary Transmission

The second transmission tested was the DSI planetary transmission as shown in Figure 8. This 

configuration is identical to the OCG except the OCG half-speed gearing was replaced with the DSI 

planetary configuration. As seen in the front view (Figure 8(b)), the DSI planetary has an input sun gear 

which drives three sets of star and idler planet gears, which drive an internal ring gear as the output. The 

planetary carrier is fixed to ground. The idler gears are needed to permit the ring gear to rotate in the same 

direction as the sun gear. All planet gears were straddle mounted with pairs of high-precision, high-speed, 

axially-preloaded duplex ball bearings. Basic dimensions for the gearing are given in Table III. The DSI 

NASA/TM—2015-218816 5
has a 2.020:1 reduction ratio. All gears were involute spur gears made from carburized and ground AISI 

9310 alloy steel per AMS 6265M. The gears were made to high precision AGMA class 13.

The DSI configuration used the same shift clutch as the OCG tests. Thus, the operation of the shifting 

was identical to that of the OCG. Regions 2 and 3 for the DSI were identical to that of the OCG with one 

exception. A sprag clutch aft bearing was added to provide additional support to the low-speed output 

shaft drum. Lubrication to the DSI was slightly different than the OCG. The same manifold of Figure 7

was used but the OCG cluster duplex bearing tap was closed. The front manifold fed all 1 DSI planet 

bearings. The rear manifold fed all the DSI gearing using nine total jets, three per planet pairs. As with the 

OCG, all throttle valves were set wide open except for the sprag clutch, which was throttled slightly 

closed. The approximate oil flow rates to the various DSI components as indirectly measured is given in 

Table IV.

Test Instrumentation

The left and right motors and test transmission were equipped with speed, torque, temperature, and 

vibration sensors which were recorded as part of the facility data acquisition system. Lube system 

parameters such as temperature, oil pressure, and oil flow rate for the high and low pressure systems and 

clutch systems were also recorded. Facility data was collected at 5-Hz acquisition rate.

Figure 9 shows details of the facility vibration sensors. Six accelerometers were used, two mounted 

on each the left motor, test transmission, and right motor, in the vertical and horizontal directions. The 

accelerometers were broadband piezoelectric accelerometers with a nominal 11.5 pC/g charge sensitivity 

and linear frequency response from 1 to 8 kHz ± 1 dB. Overall root-mean-square (rms) vibration levels 

were measured from these accelerometers and collected as part of the facility data acquisition system.

Due to early sprag clutch development issues, proximity probes were installed on the low-speed 

output shaft drum to measure overall movement and run-out of the drum during steady state and speed 

shift conditions (Figure 10). The probes were eddy current proximity probes that provided output voltage 

directly proportional to the distance between the probe tip and the observed conductive surface (lowspeed output shaft drum) and measured both static (position) and dynamic (vibration) values. The 

voltages were converted to distance (mils) using a static calibration process. Two probes were used. One 

was installed in the vertical position and one in the horizontal position. Typical range of measurement for 

the probes were 20 to 80 mils clearance between the probe tips and drum. Initial installed clearance 

between the probes and drum ranged from 35 to 50 mils. The measured results during operation were 

collected as part of the facility data acquisition system.

Figure 11 shows locations of installed thermocouples in the test transmissions. For both the OCG and 

DSI tests, thermocouples were installed on the input and output shaft duplex bearings. For the OCG tests, 

thermocouples were installed on the cluster duplex bearing. For the DSI tests, thermocouples were 

installed on all 12 planet gears bearings. The thermocouples were type J thermocouples. All were 

installed pressed against the bearing outer diameter using a spring-bolt attachment. The measured results 

during operation were collected as part of the facility data acquisition system.

Also shown in Figure 11 are three high frequency accelerometers that were installed inside the test 

transmissions. One was mounted on top of the input shaft duplex bearing housing and one on the output 

shaft duplex bearing housing. For the OCG tests, an accelerometer was mounted on top of the cluster 

shaft duplex bearing housing. For the DSI tests, an accelerometer was mounted on top of the planetary 

carrier. All were installed to measure vibration in vertical direction. The accelerometers were broadband 

high-frequency piezoelectric accelerometers with a nominal sensitivity of 10 mV/g and linear frequency 

response from 1 to 30 kHz ± 3dB. The outputs of the accelerometers along with tachometer pulses on the 

input and output shafts were routed to anti-aliasing filters with a 50 kHz cut-off frequency, then to a 

NASA/TM—2015-218816 6
separate PC-based data acquisition system. The data were acquired at 125 kHz sampling rate in 20-sec

durations. Raw data were collected before, during, and after shifts for later post-processing.

Test Procedure

For the OCG test, the test transmissions as pictured in Figure 5 was built up on a bench then installed 

in the Glenn variable-speed transmission test facility. Upon installation, the lubrication system was 

initially checked. Oil pressures were adjusted and flow measurements were taken as shown in Table II.

Green runs were initially performed on the test transmissions in which speeds and torques were gradually 

increased in order to break-in the parts and check operation of the test facility. The OCG was the first 

hardware to be tested and a number of facility issues were addressed during the start of testing (described 

later).

Once completed, the shift tests were performed to demonstrate feasibility of the OCG concept. The 

operation procedure for these tests was as follows. First, the lubrication system was started and the oil 

was heated to 160 °F. Upon reaching steady state, the transmission was run at the appropriate input speed 

of the test per speed control of the left motor. The right motor was set to power control at the appropriate 

level for the test. Upon reaching semi steady-state conditions, the clutch release profile was activated 

which linearly increased the clutch pressure from 0 to 400 psi at a rate of 20 psi/sec. At approximately 

325 psi, the clutch disengaged and the transmission performed a high-to-low output speed shift. The low 

output speed condition (2:1 reduction ratio) was run for a short period of time, then the output load was 

reduced to zero. A clutch engagement profile was activated which linearly decreased the clutch pressure 

from 400 to 150 psi at a rate of 20 psi/sec. The clutch engaged at approximately 220 psi and the 

transmission performed a low-to-high output speed shift. These tests were run at 5,000, 8,000, 10,000, 

12,500, and 15,000 rpm input speed. For these tests, facility data was continuously saved at 5 kHz. Highfrequency vibration data was saved in 20-sec bursts.

After completion, the OCG was removed, disassembled, and inspected. The DSI configuration as 

pictured in Figure 8 was then installed. The lubrication system was checked and flow measurements were 

taken (Table IV). Green runs were performed and the same shifts tests as performed for the OCG were

then repeated for the DSI configuration. After completion, additional tests were performed on the DSI

configuration. The effects of clutch pressure ramp rate and right motor control mode on shifting 

characteristics were studied. Clutch pressure ramp rates of 20, 4, and 1 psi/sec were each performed for 

right motor control modes of power, torque, and manual control.

Results and Discussion

OCG Test Results

Shift tests were performed on the OCG transmission for input speeds of 5,000, 8,000, 10,000, 12,500, 

and 15,000 rpm. Transmission output speeds and corresponding shift clutch pressures for the OCG tests 

to characterize the shifting dynamics are given in Figure 12. Plotted are the output speeds (red lines, right 

vertical axis scale) and clutch pressures (blue lines, left vertical axis scale) versus run time for the shifts at 

5,000, 8,000, 10,000, 12,500, and 15,000 rpm input speed. In order to align the plots for comparison, the 

horizontal-axis run times were defined as follows. For the high-to-low speed shift segments, the output 

speed started ramp-down at 20 sec. For the low-to-high speed shift segments, the output speed started 

ramp-up at 100 sec.

For each high-to-low speed shift test, the clutch disengagement profile started at approximately 5 sec

on the plots. By 12 to 15 sec, the clutch pressures were linearly increasing. At 25 sec, the clutch reached 

NASA/TM—2015-218816 7
400 psi. Between 21 and 22 sec, the clutch exhibited momentary drops in pressure. These were caused by 

the full disengagement of the shift clutch where the applied oil fluid pressure overcame the spring 

engagement pressure in the clutch. Before the pressure drop, the clutch appeared to slip and the output 

speed started to decrease. After the drop, the clutch was fully disengaged and the output speed coasted 

down to half speed until engaged by the sprag. It is interesting to note that the time of the pressure drops 

did not occur the same for the different tests. It appears less slipping occurred at higher input speeds.

For each low-to-high speed shift tests, the clutch engagement profile started at approximately 90 sec on 

the plots. At 100 sec, the clutch exhibited momentary increases in pressure. Here, the spring engagement 

force overcame the oil fluid pressure and the output speed started to increase until reaching full input 

speed which is when the clutch was fully engaged.

Table V summarizes the ramp-down and ramp-up time characteristics. Shown are ramp times (sec) 

and rates (rpm/sec). Ramp-down rates for shift tests from 5,000 to 12,500 rpm input speed steadily 

increased with input speed, then suddenly dropped at 15,000 rpm input speed. Ramp-up rates were similar 

for input speeds of 5,000 to 10,000 rpm and slightly higher for speeds of 12,500 and 15,000 rpm.

Figure 13 shows the transient input and output torques for the OCG shift tests. As with Figure 12, the 

horizontal axis is defined for start of the high-to-low output speed drop at 20 sec and start of the low-tohigh speed increase at 100 sec. The left column is input torque for the complete shifts and the right 

column is the output torque for the complete shifts. As previously stated, the OCG transmission coasts 

down in speed after clutch disengagement during the high-to-low speed shift. Figure 13 shows the 

corresponding drops in torque starting at 20 sec during these coast downs. As the sprag is engaged the 

input torques increases back to their original values and the output torques increase to approximately 

twice their original values due to the system being set in power control. Table VI depicts the average 

measured output torques during the high-to-low speed shift tests, before and after shifts, as well as the 

peaks. It was originally desired to perform all the shift tests at the same torque level. Shifts at 5,000, 

8,000, and 10,000 rpm were all run at the same approximate input torque level (Figure 13(a) to (f)). It was 

found, however, that transient overloads occurred at higher speeds during sprag engagement. Because of 

this, the output power set point was decreased with increased input speed in order to prevent test 

transmission overload (facility shutdown limits were set at 70 ft-lb torque). This is shown in the reduced 

power set points and resulting output torques at higher speeds in Table VI. One other point to mention is 

that the peak values listed in Table VI are from unfiltered 5-Hz facility data, where the data plotted in 

Figure 13 are filtered values, thus not reflecting the true peaks.

From very early shift attempts, it was discovered that a significant transient torque spike occurred at 

the start of clutch engagement for the low-to-high speed shift tests. This was due to the system trying to 

quickly speed up the output shaft during engagement. Because of this, the applied torque was set to zero

for these tests in order to prevent transmission overload. Table VII depicts the average measured torques

for the low-to-high speed shifts, before and after shifts, as well as the peaks. Even under no applied load, 

significant transient torque spikes occurred during the transitions. As with Table VI, peak values listed are 

from unfiltered data. Figure 12 and Figure 13 and Table V to Table VII characterized the transient speeds 

and torques of the OCG shifting dynamics. The transmission successfully shifted at its full design input 

speed of 15,000 rpm. But due to transient torque spikes, shifts were limited in applied torque levels in 

order to prevent overloads on the transmission. It is believed that the relative lack of appreciable slippage 

of the dry shifting clutch at these operating conditions and clutch pressure profiles (20 psi/sec ramp rate) 

was a major cause of the transient torque spikes. It should also be noted that the transmission was not 

initially designed to shift under much applied torque.

To further characterize the results of the OCG testing, additional transmission parameters are 

presented in Figure 14 to Figure 17. In these plots the data from all input speeds test are plotted 

NASA/TM—2015-218816 8
continuously on the horizontal axes. The input speeds for each shift are shown in the first graph of the 

plots as a reference. The parameters are color coded to indicate reduction ratio. Data at 1:1 output speed 

(high-speed) are shown in blue. Data at 2:1 output speed reduction (low-speed) are shown in red. The 

main purpose of the plots are to show the effect of reduction ratio on these parameters. It is not intended 

to show shift transients as was done with Figure 12 and Figure 13.

Transmission lubrication oil flow rates and inlet oil pressures are shown in Figure 14 for the OCG 

shift tests. Occasional drops in oil flow rate (Figure 14(b)) and corresponding spikes in oil inlet pressure 

(Figure 14(c)) occurred during the low-speed output regime. The cause is believed to be due to the 

occasional blocking of flow in the sprag clutch. The sprag clutch had four oil feed holes in its inner race, 

four oil exit holes in its outer race, and 16 sprag elements. Since the number of holes was an even divisor 

of the number of elements, it was possible for the elements to align with the holes and temporarily block 

oil flow. The mean oil flow rate at the low-speed output conditions did not appear to be affected by input 

speed, whereas the mean flow rate at the high-speed output decreased with input speed. This was also 

probably related to sprag clutch oil blockage.

Figure 15 shows transmission temperatures for the OCG shift tests. The forward and aft OCG cluster 

gear duplex bearing temperatures are shown in Figure 15(b). The transmission was not run long enough at 

each condition to achieve fully steady state results. One can see, however, an increase in temperature with 

input speed. Also, a very slight decrease in temperature occurred at the start of each low-speed output 

shift. The forward and aft bearings were very similar in response, with the forward bearing running 

slightly hotter. Figure 15(c) shows temperatures of the input and output shaft duplex bearings. As with the 

cluster, the bearing temperatures increased with increased input speed. The output shaft bearing showed a 

significant decrease in temperature at the low-speed output conditions due to the reduced shaft speed, and 

thus, reduced speed of the bearing.

Figure 16 shows the vibration from the facility accelerometers (from Figure 9) for the OCG shift 

tests. For the most part, the motor vibrations were fairly constant throughout the tests and not affected by 

test transmission reduction ratio (Figure 16(b), (e), and (f)). The motor vibrations slightly increased with 

speed and were largest at the highest tested speed condition (15,000 rpm). The OCG transmission 

vibration significantly increased at 8,000 rpm input speed (Figure 16(c) and (d)). The cause of this is 

unknown. Other than that, the OCG vibrations were fairly constant for the tests with a slight increase with 

input speed and a slight increase at the low-speed output regime.

Figure 17 shows the low-speed output shaft drum movement for the OCG shift tests as measured by 

the proximity probes. Shown is the measured clearance between the horizontal probe tip and shaft drum 

(Figure 17(b)) and the measured clearance between the vertical probe tip and shaft drum (Figure 17(c)).

As seen from the figure, a significant difference in clearance occurred when comparing tests at 1:1 and 

2:1 output speed. At 1:1 ratio, the low-speed output shaft drum is unloaded. At 2:1 ratio, torque flows 

through the drum. Thus, the position of the drum was affected by reduction ratio and torque flow path.

This is further evident in the orbit plots of Figure 18. Here, the vertical and horizontal clearances in 

Figure 17 are converted to vertical and horizontal positions and plotted against each other in orbit plots.

Data from the five different input speed conditions are divided into separate plots. Again, the plots clearly 

show a shift in shaft drum position when comparing 1:1 to 2:1 ratio test results. Also evident is reduced 

drum position oscillation at the higher tested input speeds of 12,500 and 15,000 rpm.

Measurement results of the high-frequency vibration of the OCG cluster gear bearing housing (from 

accelerometer 2 in Figure 11) are shown in Figure 19. Shown are raw time trace plots in 0.10-sec 

snapshots of the high-to-low speed OCG shift tests for each of the five input speeds tested. As stated 

before, data were sampled at 125 kHz. The left column are results before the shift (1:1 ratio output speed) 

and the right column are results after the shift (2:1 ratio output speed). Since the accelerometers were 

NASA/TM—2015-218816 9
mounted directly on the duplex bearing housing inside the transmission, the vibration levels were 

relatively high. Also, the accelerometer digitizing range was mistakenly set too low and signal clipping 

occurred at ± 125 G's. Before the shift (left column), the OCG cluster was unloaded. Impulse spikes at the 

OCG cluster shaft and input shaft periods can be seen. After the shift (right column), the torque was 

transmitted through the OCG cluster and the vibration levels significantly increased. Significant impulse 

spikes at the cluster period occurred at 12,500 rpm input speed and somewhat at 15,000 rpm.

Fast Fourier transforms of the time traces are shown in Figure 20. The spectra were based on 5 sec of 

vibration data using a Hanning window and 32,768 points (3.8 Hz bandwidth) per spectrum resulting in 

19 frequency averages per plot. Again, the left column are results before the shift (1:1 ratio output speed) 

and the right column are results after the shift (2:1 ratio output speed). Table VIII gives shaft and gear 

mesh frequencies for the OCG configuration at the various input speeds tested. Comparing this with 

Figure 20, the major vibration energy occurred at the gear mesh fundamental (and somewhat two times

the gear mesh fundamental) frequencies. Also, as with Figure 18, vibration levels significantly increased 

after shift, which was when the OCG was loaded.

DSI Test Results

As with the OCG, shift tests were performed on the DSI planetary transmission for input speeds of 

5,000, 8,000, 10,000, 12,500, and 15,000 rpm. Transmission output speeds and corresponding shift clutch 

pressures for the DSI tests to characterize the shifting dynamics are given in Figure 21. As with Figure 12

and Figure 13 from the OCG tests, the horizontal axis is defined for start of the high-to-low output speed 

drop at 20 sec and start of the low-to-high speed increase at 100 sec. The results of speed, clutch pressure, 

and clutch pressure drops, are very similar to those from the OCG tests. Table IX summarizes the rampdown and ramp-up time characteristics. The overall results are also very similar to those from the OCG 

tests.

Figure 22 shows the transient input and output torques for the DSI shift tests. As with speed, the DSI 

torque transients were very similar to that from the OCG tests. Table X depicts the average measured 

output torques during the high-to-low speed shift tests. Table XI depicts the average measured torques for 

the low-to-high speed shifts. The peaks during the shift transients were a little erratic, but in general, 

matched those from the OCG tests. All test set points were identical for both the OCG and DSI tests with 

one exception. The set point power for the DSI tests at 12,500 rpm input speed was 22 hp, where the set 

point power for the OCG tests at 12,500 rpm input speed was 38 hp. The lower set point was used due to 

torque limit exceedance experienced at attempts at 38 hp during DSI testing. In summary, the DSI 

planetary transmission successfully shifted at its full design input speed of 15,000 rpm. As with the OCG, 

shifts were limited in applied torque levels in order to prevent overloads on the transmission. Since the 

transient behavior of the OCG and DSI tests were very similar, it is deduced that the shift clutch (same 

dry carbon clutch used with both configurations) had more of an effect on dynamics than the reduction 

gearing configuration itself.

Transmission lubrication oil flow rates and inlet oil pressures are shown in Figure 23 for the DSI shift 

tests. As seen from the initial oil flow measurements (Table II and Table IV), the overall oil flow rate of 

the DSI was greater than that for the OCG due to the increased number of jets to the planetary gears and 

bearings. As with the OCG tests, drops in oil flow rate (Figure 23(b)) and corresponding spikes in oil inlet 

pressure (Figure 23(c)) occurred during the low-speed output regime. The drops appeared more frequently 

compared to the OCG tests, and the frequency increased with input speed. The cause of the difference in 

flow drops between the DSI and OCG is not completely known, but may be related to the additional 

backup bearing used in the DSI sprag assembly, causing an additional restriction to the oil flow. As with 

NASA/TM—2015-218816 10
the OCG tests, the mean oil flow rate at the low-speed output conditions was not affected by input speed, 

whereas the mean flow rate at the high-speed output decreased with input speed.

Figure 24 shows transmission temperatures for the DSI shift tests. Temperature of all 12 DSI planet 

bearings were recorded. The hottest bearing of the 12 was the idler #3 aft bearing. The coolest was the 

star #3 forward bearing. Both of these temperatures are shown in Figure 24(b). All 12 bearings exhibited 

similar trends and were banded within the two depicted in Figure 24(b). One concern related to the DSI 

design was the extremely high speed of the planet gears and bearings. Table XII gives the speeds of the 

planetary components. As seen the table, the star planets run up to 39,474 rpm at full input speed. Even 

though the transmission was not run completely to steady state, the measured planet bearing temperatures 

were reasonable considering the speed at which they operated. The DSI planet bearings were significantly 

hotter than the OCG cluster bearings due to their increased operating speed and possible insufficient oil 

drainage provisions.

Figure 24(c) shows temperatures of the input and output shaft duplex bearings. As with the planets, 

the bearing temperatures increased with increased input speed. The output shaft bearing showed a 

significant decrease in temperature at the low-speed output conditions due to the reduced shaft speed, and 

thus, reduced speed of the bearing. The duplex bearings, in general, behaved similar to those for the OCG 

tests.

Figure 25 shows the vibration from the facility accelerometers (from Figure 9) for the DSI shift tests.

The motor vibrations were fairly constant throughout the tests and not affected by test transmission 

reduction ratio (Figure 25(b), (e), and (f)). The DSI transmission vibration slightly increased with input 

speed (Figure 25(c) and (d)). The DSI transmission vibration was higher at 2:1 output compared to 1:1 for 

5,000, 8,000, and 10,000 rpm input speeds, but were lower at 2:1 for speed of 12,500 and 15,000 rpm.

Figure 26 shows the low-speed output shaft drum movement for the DSI shift tests as measured by 

the proximity probes. Figure 27 shows the corresponding orbit plots. As seen from the figures, the 

position of the drum did not show a shift in shaft drum position when comparing 1:1 to 2:1 ratios test 

results. Thus, the sprag backup bearing was successful in locating the shaft drum. Also, the overall 

movement of the drum was lower with the backup bearing installed when comparing results from 

Figure 27 and Figure 18.

Measurement results of the high-frequency vibration of the DSI planetary carrier (from accelerometer 

2 in Figure 11) are shown in Figure 28. Shown are raw time trace plots in 0.10-sec snapshots taken at 

125 kHz sample rate of the high-to-low speed DSI shift tests for each of the five input speeds tested. The 

vibration levels were relatively high but lower than those from the OCG cluster gear housing. The 

accelerometer digitizing range was mistakenly set too low and signal clipping occurred at ± 125 G's for 

tests at input speeds 5,000 to 12,500 rpm. This was corrected and no clipping occurred for the test at 

15,000 rpm. The vibration levels for the loaded planetary (after shift, right column) were significantly 

higher than those for the unloaded planetary (before shift, left column). Significant impulse spikes at the 

output shaft period occurred for tests at 10,000 and 12,500 rpm input speed.

Fast Fourier transforms of the time traces are shown in Figure 29. As with the OCG analysis, the 

spectra were based on 5 sec of vibration data using a Hanning window and 32,768 points (3.8 Hz 

bandwidth) per spectrum resulting in 19 frequency averages per plot. Again, the left column are results 

before the shift (1:1 ratio output speed) and the right column are results after the shift (2:1 ratio output 

speed). Table XII gives shaft and gear mesh frequencies for the DSI configuration at the various input 

speeds tested. Comparing this with Figure 29, the major vibration energy occurred at the planetary gear 

mesh frequencies. Also, as with Figure 28, vibration levels significantly increased after shift which was 

when the DSI planetary was loaded.

NASA/TM—2015-218816 11
Additional shift tests were performed on the DSI transmission to determine the effect of clutch 

pressure ramp rate and right motor control mode on the shifting characteristics. Clutch pressure ramp 

rates of 20, 4, and 1 psi/sec were each performed for right motor control modes of power, torque, and 

manual control. Figure 30 gives the output speed and clutch pressure and Figure 31 gives the input and 

output torques for shifts using power control mode. Figure 32 gives the output speed and clutch pressure 

and Figure 33 gives the input and output torques for shifts using torque control mode. Figure 34 gives the 

output speed and clutch pressure and Figure 35 gives the input and output torques for shifts using manual 

control mode. Table XIII summarized the output speed ramp times and rates as well as output torques for 

the high-to-low shifts of all these tests. Table XIV gives the output speed ramp times and rates as well as 

output torques for the low-to-high shifts of all these tests. Figure 36 depicts the speed ramp times, rates, 

and peak output torques. Note that all tests were performed at 5,000 rpm input speed. For the high-to-low 

shifts, the set points for all tests were adjusted to produce the approximate same amount of applied output

torque before shift. For all low-to-high speed shifts, the applied torque was set to zero in order to prevent 

transient overloads (right motor, manual mode, 0 percent).

From Table XIII and Table XIV and Figure 36, low-to-high shift output speed ramp-up times were 

less than high-to-low shift output speed ramp-down times for the same control and clutch pressure ramp 

rate. Low-to-high speed ramp-up times were governed by the relatively quick engagement of the dry 

clutch. High-to-low ramp-down times were basically a coast down of the output shaft, and thus, had 

longer ramp-down times compared to ramp-up times. For the low-to-high speed shifts, the output speed 

ramp-up time slightly decreased as the clutch pressure ramp-down rate increased. This was caused by 

slightly less clutch slippage as the clutch pressure ramp-down rate increased. For the high-to-low speed 

shifts, open-loop manual control mode produced larger speed ramp-down times compared to closed-loop 

power and torque control modes, while power and torque control mode ramp-down times were similar to 

each other. This was caused by the following. At the start of the high-to-low speed shifts, the output shaft 

coasted down in speed which resulted in a reduction in torque (Figure 31, Figure 33, and Figure 35).

Since the torque levels decreased, a corresponding increase in load motor current was produced under 

closed-loop control, which produced more drag in the load motor and a decrease in ramp-down time 

compared to that using open-loop manual control.

As previously stated, all low-to-high shift tests were performed unloaded. For these tests, the peak 

output torque increased as the clutch pressure ramp-down rate increased. Again, this was due to slightly 

less clutch slippage with increased clutch pressure ramp-down rate. Even at low clutch pressure rampdown rates, a relatively small amount of clutch slippage occurred for the dry clutch configuration. This 

produced transient torque spikes during low-to-high speed shifts, and thus, prevented operation of low-tohigh speed shifts under any appreciable applied torque. For the high-to-low shift tests, the peak output 

torque was the same for all clutch pressure ramp-up rates for power control mode. Other than that, there 

was no other trend in peak output torque with clutch pressure ramp-up rates or control modes for the highto-low shifts.

Lessons Learned

The testing described in this current study was performed in the newly developed NASA Glenn

Research Center variable-speed transmission test facility. A number of facility development issues were 

encountered during initial testing of the first transmission tested, the OCG configuration. Details of 

various lessons learned are described in (Ref. 14). A summary of these results are as follows.

The first was failure of the output shaft coupling (Figure 37). The facility uses commercially available 

high-speed, metal bellows couplings between the test transmission and left (driver) and right (loader) 

motors. The couplings employ taper lock hubs which are tightened together to lock connecting shafts 

NASA/TM—2015-218816 12
together. Due to requirements of rather small shaft diameters in the test transmission, intermediate sleeves 

were needed in between the shaft and coupling assembly. It is believed these sleeves lowered the torque 

capacity of the coupling from that stated in the manufacturers data sheets. During initial testing of the 

OCG configuration, unexpected transient overloads were encountered. These transient torque spikes 

contributed to coupling slippage, which reduced the torque through the output shaft. The facility was in 

closed-loop torque control mode at the time, and thus, issued a reversing rotation to the right motor to 

increase torque. Excessive slippage, spinning, and eventual scoring of the shaft surfaces were 

encountered. The issue was resolved by limiting the allowed torque to 70 ft-lb through use of facility 

shutdowns and eventually incorporating keys in the connections.

The second issue encountered was failure of a sprag clutch (Figure 38). The sprag shown in this 

figure was a high capacity clutch with 34 sprag elements. It is believed that insufficient lubricant flow 

was used in this condition, as the clutch failed during overrunning (1:1 ratio output speed) operation. It is 

believed that only 0.2 gpm oil flow rate was used in this test. The issue was resolved by increasing the 

flow rate to 0.9 gpm and using a sprag with 16 (instead of 34) elements. In addition, a backup sprag 

support bearing was added in later testing to further control the position of the low-speed output shaft 

drum. The aft sprag bearing and spider bearing (located radial between low-speed shaft and input shaft, 

Figure 6) form a duplex-bearing support, which properly supports both the low-speed shaft and sprag, and 

is also axially preloaded. With this, all shafts have duplex bearing support.

The original dry carbon-carbon clutch was a commercially available clutch used in the automotive 

racing industry. It had excessive torque capacity, but required a high release force to disengage the clutch.

The original design included two diaphragm (conical) springs with integral machined radial fingers 

mounted in series for disengagement. The two springs provided the rated capacity but required an oil 

pressure of approximately 700 psi applied to the piston to release the fingers and disengage the clutch.

Sealing issues were encountered with the above when trying to provide the excessively high oil pressure.

The clutch was then modified by removing one diaphragm spring and installing correction shims. This 

modification reduced the clamping rated capability yet provided adequate clamping force and required 

less force (oil pressure) to release the clutch. The resulting design is shown in Figure 39.

As previously mentioned, both transmission configurations successfully shifted at their full design 

input speed of 15,000 rpm, but due to transient torque spikes, were limited in applied torque levels in 

order to prevent overloads on the transmission. It is believed that the relative lack of appreciable slippage 

of the dry shifting clutch at operating conditions and pressure profiles tested was a major cause of the 

transient torque spikes. A second configuration, a wet clutch, has been designed and fabricated and is 

shown in Figure 40. The wet clutch contains sets of steel-friction plates as used in automotive 

applications. The steel plates are attached to the input shaft through their outer diameters. The friction 

plates are attached to the inner shaft through their inner diameters. Upon assembly, the plates are locked 

together through use of drive springs, thus locking the input shaft to the clutch inner shaft. To disengage 

the clutch, pressurized oil is applied to the piston, which pushes on the springs to separate the clutch 

plates. The separated clutch plates are kept cool and lubricated through use of the oil system used to 

lubricate the gears and bearings in the test transmission, which is independent of the above clutch control 

circuit. The required pressure to release the wet clutch is significantly lower than that of the dry clutch 

due to the increased cross-sectional area of the wet clutch piston. Initial bench testing of the wet clutch 

resulted in a required release pressure of approximately 140 psi, compared to 325 for the dry clutch. It is 

believed that the reduced required release pressure along with the wet clutch characteristics will allow 

better control of slippage to address the transient torque spikes. Future shifts tests with the wet clutch 

configuration are planned.

NASA/TM—2015-218816 13
Lastly, as mentioned, issues where encountered in the oil transfer region to maintain adequate oil 

pressure to the shift clutch. The initial design used cast iron oil transfer seal rings, as used in commercial 

automotive transmissions. These seal rings exhibited excessive wear during high speed and high pressure 

operation (Figure 41). The rings were replaced with DuPont Vespel polyimide material and exhibited 

much improved wear rates.

Conclusions

Experimental tests were performed on two promising two-speed transmission configurations designed 

for use in advanced rotorcraft application: 1) the offset compound gear (OCG) transmission, and 2) the 

dual star/idler (DSI) planetary transmission. Testing was performed in the NASA Glenn Research Center 

variable-speed transmission test facility. Shift tests were performed on the transmissions at input speeds

of 5,000, 8,000, 10,000, 12,500, and 15,000 rpm. Speed and torque transients as well as transmission 

component characteristics were collected. The following results were obtained:

1) Both the OCG and DSI configurations successfully perform speed shifts at full rated 15,000 rpm 

input speed.

2) The transient shifting behavior of the OCG and DSI configurations were very similar. Since the 

same shift clutch was used in both configurations, it was deduced that the shift clutch had more of an 

effect on shifting dynamics than the reduction gearing configuration itself.

3) For both OCG and DSI configurations, low-to-high speed shifts were limited in applied torque 

levels in order to prevent overloads on the transmission due to transient torque spikes. It is believed that 

the relative lack of appreciable slippage of the dry shifting clutch at operating conditions and pressure 

profiles tested was a major cause of the transient torque spikes.

4) For low-to-high speed shifts with the DSI configuration, the output speed ramp-up time slightly 

decreased and the peak out torque slightly increased as the clutch pressure ramp-down rate increased.

This was caused by slightly less clutch slippage as the clutch pressure ramp-down rate increased.

5) The planet bearings in the DSI configuration successfully operated at high speed during test.

References

1. Johnson, W., Yamauchi, G.K., and Watts, M.E., “NASA Heavy Lift Rotorcraft Systems 

Investigation,” NASA/TP—2005-213467, 2005.

2. Acree, C.W., Yeo, H., and Sinsay, J.D., “Performance Optimization of the NASA Large Civil 

Tiltrotor,” International Powered Lift Conference, London, UK, Jul. 22-24, 2008.

3. Goi, T., Kawakami, K., Yamakawa, E., and Tanaka, H., “Variable Rotor Speed Transmission with 

High Speed Traction Drive,” American Helicopter Society 55th Annual Forum, Montreal, Quebec, 

Canada, May 25-27, 1999.

4. Handschuh, R.F., and Zakrajsek, J.J., “Current Research Activities in Drive System Technology in 

Support of the NASA Rotorcraft Program,” American Helicopter Society Vertical Lift Aircraft 

Design Conference, San Francisco, CA, Jan. 18-20, 2006.

5. Ai, X., Mohr, T., and Anderson, S., “An Electro-Mechanical Infinitely Variable Speed 

Transmission,” SAE 2004 World Congress, Detroit, MI, Mar. 8-11, 2004.

6. Saribay, Z., Smith, E., Lemanski, A., Bill, R., Wang, K.W., and Rao, S., “Compact Pericyclic 

Continuously Variable Speed Transmission Systems: Design Features and High-Reduction Variable 

Speed Case Studies,” American Helicopter Society 63nd Annual Forum, Virginia Beach, VA, May 1-

3, 2007.

NASA/TM—2015-218816 14
7. Harris, F. D., “An Overview of Autogyros and the McDonnell XV-1 Convertiplane,” NASA/CR—

2003-212799, 2003.

8. Munter, P. L., “Design and fight test of a two-speed helicopter transmission,” American Helicopter 

Society 10th Annual Forum, Washington, D.C., May 24-26, 1954.

9. Lentine, F. P., G. W. P. and Oglesby, T. H., “Research in maneuverability of the XH-51A compound 

helicopter,” USAAVLABSTR Technical Report 68-23, U.S. Army Aviation Materiel Laboratories, 

Fort Eustis, VA, Jun. 1968.

10. Karem, A., “Hummingbird A160 and optimum speed rotor (OSR),” AHS International Specialists’

Meeting - Unmanned Rotorcraft: Design, Control and Testing, Proceedings, Phoenix, AZ, Jan. 18-20, 

2005.

11. Brown, F., Robuck, M., Kaiser, E., Ohlerking, W., Schellhase, E., Mychalowicz, E., and Paul, D., 

“Design and Development of a Dual Ratio Transmission for the A160T UAV Rotorcraft,” American 

Helicopter Society 66th Annual Forum, Phoenix, AZ, May 11-13, 2010.

12. Stevens, M.A., Handschuh, R.F., and Lewicki, D.G., “Concepts for Variable/Multi-Speed Rotorcraft 

Drive System,” American Helicopter Society 64th International Forum, Montreal, Canada, Apr. 29 -

May 1, 2008.

13. Stevens, M.A., Handschuh, R.F., and Lewicki, D.G., “Variable/Multispeed Rotorcraft Drive System 

Concepts,” NASA/TM—2009-215456, Army Research Laboratory Report ARL-TR-4758, Mar.

2009.

14. Stevens, M.A., Lewicki, D.G., and Handschuh, R.F., “Concepts for Multi-Speed Rotorcraft Drive 

System - Status of Design and Testing at NASA GRC,” American Helicopter Society 71st Annual 

Forum and Technology Display, Virginia Beach, VA, May 5-7, 2015.

15. Stevens, M.A., Handschuh, R.F., and Lewicki, D.G., “Offset Compound Gear Inline Two-Speed 

Drive,” United Stated Patient No. 8,091,445 B1, Jan. 10, 2012.

16. Stevens, M.A., Handschuh, R.F., and Lewicki, D.G., “Offset Compound Gear Inline Two-Speed 

Drive,” United Stated Patient No. 8,668,613 B1, Mar. 11, 2014.

NASA/TM—2015-218816 15
TABLE I.—BASIC GEAR DIMENSIONS FOR OCG TRANSMISSION CONFIGURATION

Input gear Cluster gear 

1

Cluster gear 

2

Output gear

Type External Internal External Internal

Number of teeth 25 37 31 42

Diametral pitch, teeth/in. 8.7273 8.7273 8.0000 8.0000

Pitch diameter, in. 2.8646 4.2396 3.8750 5.2500

OD (ext), ID (int), in. 3.0938 4.0850 4.1250 5.0000

Pressure angle, deg 20.0 20.0 20.0 20.0

Face width, in. 0.375 0.375 0.375 0.375

TABLE II.—OIL FLOW RATES AT 80 psi SUPPLY

PRESSURE, 160 °F OIL INLET TEMPERATURE,

FOR OCG TEST CONFIGURATION

Component Flow rate, 

gpm

OCG duplex bearing 0.1

OCG gears 0.3

Output shaft duplex bearing 0.1

Input shaft duplex bearing 0.1

Sprag clutch 0.9

TABLE III.—BASIC GEAR DIMENSIONS FOR DSI TRANSMISSION CONFIGURATION

Sun gear Planet star 

gear

Planet idler 

gear

Ring gear

Type External External External Internal

Number of teeth 50 19 20 101

Diametral pitch, teeth/in. 12.0000 12.0000 12.0000 12.0000

Pitch diameter, in. 4.1667 1.5833 1.6667 8.4167

OD (ext), ID (int), in. 4.3101 1.7367 1.8101 8.2633

Pressure angle, deg. 20.0 20.0 20.0 20.0

Face width, in. 0.600 0.600 0.600 0.600

TABLE IV.—OIL FLOW RATES AT 80 psi SUPPLY

PRESSURE, 160 °F OIL INLET TEMPERATURE,

FOR DSI TEST CONFIGURATION

Component Flow rate, 

gpm

DSI bearings 0.2

DSI gears 0.8

Output shaft duplex bearing 0.1

Input shaft duplex bearing 0.1

Sprag clutch 0.9

NASA/TM—2015-218816 16
TABLE V.—OCG SHIFT TEST CHARACTERISTICS,

RAMP-DOWN AND RAMP-UP TIMES AND RATES

Shift 

no.

Input speed, 

rpm

Ramp down 

time, 

sec

Ramp down 

rate, 

rpm/sec

Ramp up 

time, 

sec

Ramp up 

rate, 

rpm/sec

1 5,000 3.4 742 2.0 1,280

2 8,000 4.6 869 3.2 1,244

3 10,000 5.6 903 3.8 1,304

4 12,500 6.0 1,044 4.2 1,511

5 15,000 10.0 747 4.8 1,557

TABLE VI.—OCG SHIFT TEST CHARACTERISTICS,

OUTPUT TORQUES FOR HIGH-TO-LOW SPEED SHIFTS

Shift 

no.

Set points Output torque, 

ft-lb

Input speed, 

rpm

Output power, 

hp

Before shift After shift Peak

1 5,000 18 18.9 37.8 37.8

2 8,000 30 19.6 39.4 39.4

3 10,000 38 19.9 40.1 57.8

4 12,500 38 15.9 32.5 50.0

5 15,000 20 6.9 14.1 31.0

TABLE VII.—OCG SHIFT TEST CHARACTERISTICS,

OUTPUT TORQUES FOR LOW-TO-HIGH SPEED SHIFTS

Shift 

no.

Input speed, 

rpm

Output torque, 

ft-lb

Before shift After shift Peak

1 5,000 0.7 1.1 37.1

2 8,000 1.1 1.6 33.6

3 10,000 1.3 2.0 43.4

4 12,500 1.1 2.0 53.7

5 15,000 1.1 2.3 51.5

TABLE VIII.—OCG CONFIGURATION SHAFT AND GEAR MESHING FREQUENCIES

Speed, 

rpm

Shaft frequencies, 

Hz

Input mesh frequencies, 

Hz

Output mesh frequencies, 

Hz

Input

shaft

Cluster

shaft

Output

shaft

Input

shaft

Cluster

shaft

Output

shaft 1X 2X 3X 1X 2X 3X

5,000 3,378 2,494 83 56 42 2,083 4,167 6,250 1,745 3,491 5,236

8,000 5,405 3,990 133 90 66 3,333 6,667 10,000 2,793 5,586 8,378

10,000 6,757 4,987 167 113 83 4,167 8,333 12,500 3,491 6,982 10,473

12,500 8,446 6,234 208 141 104 5,208 10,417 15,625 4,364 8,728 13,091

15,000 10,135 7,481 250 169 125 6,250 12,500 18,750 5,236 10,473 15,709

NASA/TM—2015-218816 17
TABLE IX.—DSI SHIFT TEST CHARACTERISTICS,

RAMP-DOWN AND RAMP-UP TIMES AND RATES

Shift 

no.

Input speed, 

rpm

Ramp down 

time, 

sec

Ramp down 

rate, 

rpm/sec

Ramp up 

time, 

sec

Ramp up 

rate, 

rpm/sec

1 5,000 3.4 752 2.0 1,269

2 8,000 4.8 842 3.0 1,330

3 10,000 5.2 973 3.6 1,410

4 12,500 7.8 808 4.2 1,445

5 15,000 10.4 726 5.6 1,333

TABLE X.—DSI SHIFT TEST CHARACTERISTICS, OUTPUT

TORQUES FOR HIGH-TO-LOW SPEED SHIFTS

Shift 

no.

Set points Output torque, ft-lb

Input speed, 

rpm

Output power, 

hp

Before shift After shift Peak

1 5,000 18 19.1 38.2 38.2

2 8,000 30 19.7 39.8 46.7

3 10,000 38 20.0 40.3 50.2

4 12,500 22 9.2 18.7 32.6

5 15,000 20 7.0 14.3 36.6

TABLE XI.—DSI SHIFT TEST CHARACTERISTICS, OUTPUT

TORQUES FOR LOW-TO-HIGH SPEED SHIFTS.

Shift 

no.

Input speed, 

rpm

Output torque,

ft-lb

Before shift After shift Peak

1 5,000 1.0 1.1 36.5

2 8,000 1.1 1.7 34.7

3 10,000 1.1 1.9 53.4

4 12,500 1.1 2.2 51.9

5 15,000 1.0 3.0 44.7

TABLE XII.—DSI CONFIGURATION SHAFT AND GEAR MESHING FREQUENCIES

Speed, 

rpm

Frequencies, 

Hz

Gear mesh frequencies, 

Hz

Input 

shaft

Planet 

star

Planet 

idler

Output 

shaft

Input

shaft

Output

shaft

1X 2X 3X

5,000 13,158 12,500 2,475 83 41 4,167 8,333 12,500

8,000 21,053 20,000 3,960 133 66 6,667 13,333 20,000

10,000 26,316 25,000 4,950 167 83 8,333 16,667 25,000

12,500 32,895 31,250 6,188 208 103 10,417 20,833 31,250

15,000 39,474 37,500 7,426 250 124 12,500 25,000 37,500

NASA/TM—2015-218816 18
TABLE XIII.—EFFECT OF CONTROL MODE AND CLUTCH PRESSURE RAMP RATE ON

SHIFT CHARACTERISTICS FOR DSI CONFIGURATION HIGH-TO-LOW SPEED SHIFTS

Shift 

no.

Control 

mode

Set point Clutch 

pressure rate, 

psi/sec

Ramp down 

time, 

sec

Ramp down 

rate, 

rpm/sec

Output torque, 

ft-lb

Before 

shift

After shift Peak

6 Power 18 hp 20 4.2 606 18.9 38.1 38.1

7 Power 18 hp 4 7.4 342 18.9 38.1 38.1

8 Power 18 hp 1 11.4 222 18.9 38.3 38.3

9 Torque 20 ft-lb 20 3.8 675 20.0 20.2 42.5

10 Torque 20 ft-lb 4 9.0 281 20.0 20.1 29.1

11 Torque 20 ft-lb 1 11.2 226 20.0 19.8 36.2

12 Manual 11 % 20 5.2 488 19.3 19.0 19.0

13 Manual 11 % 4 9.4 269 19.3 18.9 27.4

14 Manual 11 % 1 12.8 197 19.2 18.9 26.5

TABLE XIV.—EFFECT OF CONTROL MODE AND CLUTCH PRESSURE RAMP RATE ON SHIFT

CHARACTERISTICS FOR DSI CONFIGURATION LOW-TO-HIGH SPEED SHIFTS

Shift 

no.

Control 

mode

Set point,

%

Clutch 

pressure rate, 

psi/sec

Ramp up 

time, 

sec

Ramp up 

rate, 

rpm/sec

Output torque, 

ft-lb

Before 

shift

After 

shift

Peak

6 Manual 0 20 2.0 1,266 1.1 1.2 39.6

7 Manual 0 4 2.6 982 0.7 0.9 29.9

8 Manual 0 1 3.2 762 0.8 1.1 24.2

9 Manual 0 20 2.0 977 0.9 1.1 35.1

10 Manual 0 4 2.6 946 0.9 1.1 30.5

11 Manual 0 1 3.2 767 0.6 0.9 26.7

12 Manual 0 20 2.0 1,246 0.7 0.9 35.5

13 Manual 0 4 2.4 1,052 0.9 1.2 32.5

14 Manual 0 1 3.0 831 0.9 1.2 28.1

NASA/TM—2015-218816 19
Figure 1.—NASA Glenn Research Center variable-speed transmission test facility.

Left motor

Test transmission

Right motor

(a) Artist sketch view.

Left motor

Speed sensor

Torquemeter

Coupling

Test transmission Right motor Coupling

Torquemeter

Speed sensor

(b) Photo view.

NASA/TM—2015-218816 20
Figure 2.—Variable-speed transmission test facility control system.

Left

motor

(driver)

Right

motor

(power

absorber)

Test

transmission

Speed

sensor

Torque

sensor

Torque

sensor

Speed

sensor

Emergency

Stop

DC bus

Left

motor

controller

Right

motor

controller

Ethernet

hub

Load

PLC

Data pod

PLC

Torque

Speed

DC power in

Motor tach Motor tach

Analog,

digital,

thermocouple

inputs

Control & data

acquisition PC

NASA/TM—2015-218816 21
Figure 3.—Test transmission oil lubrication system.

Figure 4.—Shift clutch pressure control system.

Oil

supply

tank

Oil

return

tank

Manifold

Flow meter /

Pressure

transducer

Heat

exchanger

Supply

pump

Heaters

High-pressure

pump

Shift-clutch

servovalve

To shift clutch

To gears,

bearings,

sprag clutch Test

transmission

Scavenge

pump

User defined

set points &

pressure profiles

High-pressure

oil supply

Pressure

PID

controller

Shift clutch

servovalve

Shift

clutch

piston

Pressure

transducer

Clutch

bleed

Test transmission

NASA/TM—2015-218816 22
Figure 5.—Offset compound gear (OCG) transmission configuration. 

Input

shaft

Input

shaft

duplex

bearing

OCG

cluster

gear 1

OCG

input

gear

OCG

cluster

duplex

bearing

OCG

output

gear

Low-speed

output

drum Sprag

clutch

Output

shaft

Oil

transfer

mechanism

Shift

clutch

Shift

clutch

piston Output

shaft

duplex

bearing

(a) Cross-sectional view.

Region 1,

half-speed gearing

Region 2,

shift clutch

Region 3,

oil transfer

Forward

side

Aft

side

OCG

cluster

gear 2

(b) 3D view of OCG gearing (region 1).

Input shaft

OCG cluster gear 1

OCG input gear

OCG output gear

OCG cluster gear 2

NASA/TM—2015-218816 23
Figure 6.—Dry shift clutch.

Figure 7.—Test transmission oil supply manifold.

 

Carbon

plates

Drive

cover

Adapter

shaft

Release

fingers

Piston

Input

shaft

Output

shaft

Clutch

inner

shaft

Low-speed output drum

Preload

spring pack

Spider bearing

Front

manifold

Rear

manifold

Output

shaft

duplex

bearing

OCG

cluster

duplex

bearing

Input

shaft

duplex

bearing

Sprag

TC clutch

Oil

supply

(80 psi)

Pressure

gage

 

Throttle

valves

NASA/TM—2015-218816 24
Figure 8.—Dual star-idler (DSI) planetary transmission configuration. 

(a) Cross-sectional view.

(b) Front view of DSI planetary.

DSI idler #3 planet gear

DSI sun gear

DSI ring gear

DSI star #3 planet gear

DSI star #2 planet gear

DSI idler #2 planet gear

DSI star #1 planet gear DSI idler #1 planet gear

Input

shaft

Input

shaft

duplex

bearing

DSI

planet

gear

DSI sun

gear

DSI

planet

bearing

DSI

ring

gear

Low-speed

output

drum Sprag

clutch

Output

shaft

Oil

transfer

mechanism

Shift

clutch

Shift

clutch

piston Output

shaft

duplex

bearing

Sprag

clutch

aft

bearing

DSI

carrier

DSI carrier

Region 1,

half-speed gearing

Region 2,

shift clutch

Region 3,

oil transfer

Forward

side

Aft

side

NASA/TM—2015-218816 25
Figure 9.—Facility accelerometers.

Figure 10.—Low-speed output drum proximity probes.

5

3

1

2

4

6

No Description

1 Left motor, vertical

2 Left motor, horizontal

3 Test transmission, vertical

4 Test transmission, horizontal

5 Right motor, vertical

6 Right motor, horizontal

NASA/TM—2015-218816 26

Vertical probe

Horizontal probe

Low-speed

output drum
Figure 11.—Thermocouple and high-frequency accelerometer instrumentation.

1

1

2

3

2

3 4

5 through 16

TC No Description

1 Input shaft duplex bearing, forward

2 Input shaft duplex bearing, aft

3 Output shaft duplex bearing, forward

4 Output shaft duplex bearing, aft

OCG:

5 OCG cluster gear duplex bearing, forward

6 OCG cluster gear duplex bearing, aft

DSI:

5 DSI, star #1 forward bearing

6 DSI, star #2 forward bearing

7 DSI, star #3 forward bearing

8 DSI, idler #1 forward bearing

9 DSI, idler #2 forward bearing

10 DSI, idler #3 forward bearing

11 DSI, star #1 aft bearing

12 DSI, star #2 aft bearing

13 DSI, star #3 aft bearing

14 DSI, idler #1 aft bearing

15 DSI, idler #2 aft bearing

16 DSI, idler #3 aft bearing

Acc No Description

1 Input shaft duplex bearing housing

OCG:

2 OCG cluster gear duplex bearing housing

DSI:

2 DSI planetary carrier

3 Output shaft duplex bearing housing

 

Accelerometer

Forward Thermocouple

side

Aft

side

NASA/TM—2015-218816 27
Figure 12.—Transmission output speed and clutch pressure for OCG shift tests. (a) 5,000 rpm 

input speed. (b) 8,000 rpm input speed. (c) 10,000 rpm input speed. (d) 12,500 rpm input 

speed. (e) 15,000 rpm input speed.

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

3000

4000

5000

6000

7000

8000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

4000

6000

8000

10000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

4000

6000

8000

10000

12000

14000

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Clutch pressure, psi

0

100

200

300

400

6000

8000

10000

12000

14000

16000

 

(a)

(b)

(c)

(d)

(e)

Clutch

pressure Output

speed

NASA/TM—2015-218816 28

Output speed, rpm
Figure 13.—Transmission input and output torques for OCG shift tests. (a) and (b) 5,000 rpm input speed. 

(c) and (d) 8,000 rpm input speed. e), f) 10,000 rpm input speed. (g) and (h) 12,500 rpm input speed. 

(i) and (j) 15,000 rpm input speed.

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Output torque, ft-lb

0

10

20

30

40

50

 

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

High-to-low shift

Low-to-high shift High-to-low shift

Low-to-high shift

NASA/TM—2015-218816 29
Figure 14.—Transmission lubrication characteristics for OCG shift tests.

Figure 15.—Transmission temperatures for OCG shift tests.

Input speed, rpm

0

5000

10000

15000 Oil flow rate, gpm

1.0

1.5

2.0

Time, sec

0 240 480 720 960 1200

Oil inlet pressure, psi

75

80

85

90

 

(a)

(b)

(c)

Blue: high-speed output

Red: low-speed output

Input speed, rpm

0

5000

10000

15000

OCG cluster gear

duplex bearing

temperatures,

deg F

140

160

180

200

220

Time, sec

0 240 480 720 960 1200

Duplex bearing

temperatures,

deg F

140

160

180

200

 

Blue: high-speed output

Red: low-speed output Forward

Aft

Output shaft, forward

Input shaft, aft

(a)

(b)

(c)

NASA/TM—2015-218816 30
Figure 16.—Facility vibrations for OCG shift tests.

Input speed, rpm

0

5000

10000

15000

Left motor

vibration, G

0

2

4

6

8

10

OCG transmisssion

vibration, G

0

5

10

15

20

OCG transmission

vibration, G

0

5

10

15

20

Right motor

vibration, G

0

2

4

6

Time, sec

0 240 480 720 960 1200

Right motor

vibration, G

0

2

4

6

 

1, Vertical

2, Horizontal

(a)

(b)

(c)

(d)

Blue: high-speed output

Red: low-speed output

(e)

(f)

3, Vertical

4, Horizontal

5, Vertical

6, Horizontal

NASA/TM—2015-218816 31
Figure 17.—Low-speed output shaft drum clearance from proximity probes 

for OCG shift tests.

Figure 18.—Low-speed output shaft drum orbit from proximity probes for 

OCG shift tests.

Input speed, rpm

0

5000

10000

15000

Horizontal

clearance, mils

34

36

38

40

42

Time, sec

0 240 480 720 960 1200

Vertical

clearance, mils

32

34

36

38

40

 

(a)

(b)

(c)

Blue: high-speed output

Red: low-speed output

 

1 mil

1 mil

(a) 5,000 rpm (b) 8,000 rpm

(c) 10000 rpm (d) 12,500 rpm (e) 15,000 rpm

Blue: high-speed output

Red: low-speed output

NASA/TM—2015-218816 32
Figure 19.—OCG cluster gear duplex bearing housing (accelerometer 2) vibration time traces for OCG high-to-low 

speed shift tests. (a) 5,000 rpm input speed, 18.9 ft-lb output torque, before shift. (b) 5,000 rpm input speed, 

37.8 ft-lb output torque, after shift. (c) 8,000 rpm input speed, 19.6 ft-lb output torque, before shift. (d) 8,000 rpm 

input speed, 39.4 ft-lb output torque, after shift. (e) 10,000 rpm input speed, 19.9 ft-lb output torque, before shift. 

(f) 10,000 rpm input speed, 40.1 ft-lb output torque, after shift. (g) 12,500 rpm input speed, 15.9 ft-lb output torque, 

before shift. (h) 12,500 rpm input speed, 32.5 ft-lb output torque, after shift. (i) 15,000 rpm input speed, 6.9 ft-lb 

output torque, before shift. (j) 15,000 rpm input speed, 14.1 ft-lb output torque, after shift. Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Time, sec

0.00 0.02 0.04 0.06 0.08 0.10

Vibration, G

-150

-75

0

75

150

Time, sec

0.00 0.02 0.04 0.06 0.08 0.10

Vibration, G

-150

-75

0

75

150

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

Cluster shaft period

Cluster Input shaft period

Cluster

Cluster

Cluster

Cluster

Cluster

NASA/TM—2015-218816 33
Figure 20.—OCG cluster gear duplex bearing housing (accelerometer 2) vibration spectra for OCG high-to-low speed 

shift tests. (a) 5,000 rpm input speed, 18.9 ft-lb output torque, before shift. (b) 5,000 rpm input speed, 37.8 ft-lb 

output torque, after shift. (c) 8,000 rpm input speed, 19.6 ft-lb output torque, before shift. (d) 8,000 rpm input speed, 

39.4 ft-lb output torque, after shift. (e) 10,000 rpm input speed, 19.9 ft-lb output torque, before shift. (f) 10,000 rpm 

input speed, 40.1 ft-lb output torque, after shift. (g) 12,500 rpm input speed, 15.9 ft-lb output torque, before shift. 

(h) 12,500 rpm input speed, 32.5 ft-lb output torque, after shift. (i) 15,000 rpm input speed, 6.9 ft-lb output torque, 

before shift. (j) 15,000 rpm input speed, 14.1 ft-lb output torque, after shift. Vibration, G

0.0

0.1

0.2

0.3

0.4

0.5

Vibration, G

0.0

0.5

1.0

1.5

Vibration, G

0

1

2

3

Vibration, G

0

5

10

15

Vibration, G

0

1

2

3

Vibration, G

0

1

2

3

Vibration, G

0

1

2

3

4

5

Vibration, G

0

2

4

6

8

10

Frequency, kHz

0 5 10 15 20 25 30

Vibration, G

0

1

2

3

4

5

Frequency kHz

0 5 10 15 20 25 30

Vibration, G

0

5

10

15

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

NASA/TM—2015-218816 34
Figure 21.—Transmission output speed and clutch pressure for DSI shift tests. 

(a) 5,000 rpm input speed. (b) 8,000 rpm input speed. (c) 10,000 rpm input speed.

(d) 12,500 rpm input speed. (e) 15,000 rpm input speed.

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

3000

4000

5000

6000

7000

8000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

4000

6000

8000

10000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

4000

6000

8000

10000

12000

14000

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

6000

8000

10000

12000

14000

16000

(a)

(b)

(c)

(d)

(e)

Clutch

pressure Output

speed

 

NASA/TM—2015-218816 35
Figure 22.—Transmission input and output torque for DSI shift tests. (a) and (b) 5,000 rpm input speed. (c) and 

(d) 8,000 rpm input speed. (e) and (f) 10,000 rpm input speed. (g) and (h) 12,500 rpm input speed. (i) and 

(j) 15,000 rpm input speed.

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Output torque, ft-lb

0

10

20

30

40

50

 

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

High-to-low shift

Low-to-high shift High-to-low shift

Low-to-high shift

NASA/TM—2015-218816 36
Figure 23.—Transmission lubrication characteristics for DSI shift tests.

Figure 24.—Transmission temperatures for DSI shift tests.

Input speed, rpm

0

5000

10000

15000 Oil flow rate, gpm

1.5

2.0

2.5

Time, sec

0 475 950 1425 1900

Oil inlet pressure, psi

75

80

85

90

 

(a)

(b)

(c)

Blue: high-speed output

Red: low-speed output

Input speed, rpm

0

5000

10000

15000

DSI bearing

temperatures,

deg F

160

180

200

220

240

Time, sec

0 475 950 1425 1900

Duplex bearing

temperatures,

deg F

140

160

180

200

 

Idler #3 aft bearing

Star #3 forward bearing

Output shaft

Input shaft

(a)

(b)

(c)

Blue: high-speed output

Red: low-speed output

NASA/TM—2015-218816 37
Figure 25.—Facility vibrations for DSI shift tests.

Input speed, rpm

0

5000

10000

15000

Left motor

vibration, G

0

1

2

3

4

5

DSI transmisssion

vibration, G

0

1

2

3

4

5

DSI transmission

vibration, G

0

1

2

3

4

5

Right motor

vibration, G

0

1

2

3

4

5

Time, sec

0 475 950 1425 1900

Right motor

vibration, G

0

1

2

3

4

5

 

1, Vertical

2, Horizontal

(a)

(b)

(c)

(d)

Blue: high-speed output

Red: low-speed output

(e)

(f)

3, Vertical

4, Horizontal

5, Vertical

6, Horizontal

NASA/TM—2015-218816 38
Figure 26.—Low-speed output shaft drum clearance from proximity probes 

for DSI shift tests.

Figure 27.—Low-speed output shaft drum orbit from proximity probes for 

DSI shift tests.

Input speed, rpm

0

5000

10000

15000

Horizontal

clearance, mils

44

46

48

50

Time, sec

0 475 950 1425 1900

Vertical

clearance, mils

32

34

36

38

 

(a)

(b)

(c)

Blue: high-speed output

Red: low-speed output

(a) 5,000 rpm (b) 8,000 rpm

(c) 10,000 rpm (d) 12,500 rpm (e) 15,000 rpm

1 mil

1 mil

Blue: high-speed output

Red: low-speed output

NASA/TM—2015-218816 39
Figure 28.—DSI planetary carrier housing (accelerometer 2) vibration time traces for DSI high-to-low speed shift 

tests. (a) 5,000 rpm input speed, 19.1 ft-lb output torque, before shift. (b) 5,000 rpm input speed, 38.2 ft-lb output 

torque, after shift. (c) 8,000 rpm input speed, 19.7 ft-lb output torque, before shift. (d) 8,000 rpm input speed, 

39.8 ft-lb output torque, after shift. (e) 10,000 rpm input speed, 20.0 ft-lb output torque, before shift. (f) 10,000 rpm 

input speed, 40.3 ft-lb output torque, after shift. (g) 12,500 rpm input speed, 9.2 ft-lb output torque, before shift. 

(h) 12,500 rpm input speed, 18.7 ft-lb output torque, after shift. (i) 15,000 rpm input speed, 7.0 ft-lb output torque, 

before shift. (j) 15,000 rpm input speed, 14.3 ft-lb output torque, after shift. Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Vibration, G

-100

-50

0

50

100

Vibration, G

-150

-75

0

75

150

Time, sec

0.00 0.02 0.04 0.06 0.08 0.10

Vibration, G

-100

-50

0

50

100

Time, sec

0.00 0.02 0.04 0.06 0.08 0.10

Vibration, G

-150

-75

0

75

150

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

Output shaft period

Output

Output

Output

NASA/TM—2015-218816 40
Figure 29.—DSI planetary carrier housing (accelerometer 2) vibration spectra for DSI high-to-low speed shift tests. 

(a) 5,000 rpm input speed, 19.1 ft-lb output torque, before shift. (b) 5,000 rpm input speed, 38.2 ft-lb output torque, 

after shift. (c) 8,000 rpm input speed, 19.7 ft-lb output torque, before shift. (d) 8,000 rpm input speed, 39.8 ft-lb 

output torque, after shift. (e) 10,000 rpm input speed, 20.0 ft-lb output torque, before shift. (f) 10,000 rpm input 

speed, 40.3 ft-lb output torque, after shift. (g) 12,500 rpm input speed, 9.2 ft-lb output torque, before shift. 

(h) 12,500 rpm input speed, 18.7 ft-lb output torque, after shift. (i) 15,000 rpm input speed, 7.0 ft-lb output torque, 

before shift. (j) 15,000 rpm input speed, 14.3 ft-lb output torque, after shift. Vibration, G

0.00

0.05

0.10

0.15

0.20

Vibration, G

0

1

2

3

Vibration, G

0.0

0.1

0.2

0.3

0.4

0.5

Vibration, G

0

1

2

3

Vibration, G

0.0

0.1

0.2

0.3

0.4

0.5

Vibration, G

0

1

2

3

4

5

Vibration, G

0.0

0.1

0.2

0.3

0.4

0.5

Vibration, G

0

1

2

3

4

5

Frequency, kHz

0 5 10 15 20 25 30

Vibration, G

0.0

0.5

1.0

1.5

Frequency, kHz

0 5 10 15 20 25 30

Vibration, G

0.0

0.2

0.4

0.6

0.8

1.0

 

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

NASA/TM—2015-218816 41
Figure 30.—Transmission output speed and clutch pressure for DSI power control shift tests. 

(a) 20 psi/sec clutch pressure ramp rate. (b) 4 psi/sec clutch pressure ramp rate. (c) 1 

psi/sec clutch pressure ramp rate.

Figure 31.—Transmission input and output torque for DSI power control shift tests. (a) and (b) 20 psi/sec 

clutch pressure ramp rate. (c) and (d) 4 psi/sec clutch pressure ramp rate. (e) and (f) 1 psi/sec clutch 

pressure ramp rate. Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

(a)

(b)

(c)

Clutch

pressure Output

speed

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Input torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Output torque, ft-lb

0

10

20

30

40

50

 

(a)

(c)

(e)

(b)

(d)

(f)

High-to-low shift

Low-to-high shift High-to-low shift

Low-to-high shift

NASA/TM—2015-218816 42
Figure 32.—Transmission output speed and clutch pressure for DSI torque control 

shift tests. (a) 20 psi/sec clutch pressure ramp rate. (b) 4 psi/sec clutch pressure 

ramp rate. (c) 1 psi/sec clutch pressure ramp rate.

Figure 33.—Transmission input and output torque for DSI torque control shift tests. (a) and (b) 20 psi/sec 

clutch pressure ramp rate. (c) and (d) 4 psi/sec clutch pressure ramp rate. (e) and (f) 1 psi/sec clutch 

pressure ramp rate. Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

(a)

(b)

(c)

Clutch

pressure Output

speed

 

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Input torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Output torque, ft-lb

0

10

20

30

40

50

 

(a)

(c)

(e)

(b)

(d)

(f)

High-to-low shift

Low-to-high shift

High-to-low shift

Low-to-high shift

NASA/TM—2015-218816 43
Figure 34.—Transmission output speed and clutch pressure for DSI manual control 

shift tests. (a) 20 psi/sec clutch pressure ramp rate. (b) 4 psi/sec clutch pressure 

ramp rate. (c) 1 psi/sec clutch pressure ramp rate.

Figure 35.—Transmission input and output torque for DSI manual control shift tests. (a) and (b) 20 psi/sec 

clutch pressure ramp rate. (c) and (d) 4 psi/sec clutch pressure ramp rate. (e) and (f) 1 psi/sec clutch 

pressure ramp rate. Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Clutch pressure, psi

0

100

200

300

400

Output speed, rpm

2000

3000

4000

5000

(a)

(b)

(c)

Clutch

pressure Output

speed

 

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Input torque, ft-lb

0

10

20

30

40

50

Output torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Input torque, ft-lb

0

10

20

30

40

50

Time, sec

0 10 20 30 40 50 70 80 90 100 110 120

Output torque, ft-lb

0

10

20

30

40

50

(a)

(c)

(e)

(b)

(d)

(f)

High-to-low shift

Low-to-high shift

High-to-low shift

Low-to-high shift

NASA/TM—2015-218816 44
Figure 36.—Shift ramp times, rates, and peak output torque for DSI shift tests.

Figure 37.—Output shaft coupling failure.

Ramp times, sec

0

5

10

15

20

Power control

Torque control

Manual control

Clutch pressure ramp rate, psi/sec

0 5 10 15 20 25

Peak output

torques, ft-lb

0

10

20

30

40

50

60

H-L shift

L-H shift

Ramp rates, rpm/s

0

200

400

600

800

1000

1200

1400

(a)

(b)

(c)

 

Output

shaft

Sleeve

Coupling

NASA/TM—2015-218816 45
Figure 38.—Sprag clutch failure.

Figure 39.—Modified shift clutch.

 

Inner race

Sprag

Outer race

Removed

one set of

fingers.

Assembled

modified

shift clutch.

 

NASA/TM—2015-218816 46
Figure 40.—Wet shift clutch.

Figure 41.—Oil transfer seal rings.

 

Drive

plates Piston

Drive

springs

Input

shaft Output

shaft

Clutch

inner

shaft

Low-speed output drum

Worn cast iron

oil transfer seal

ring

Vespel polyimide

oil transfer seal rings

 

NASA/TM—2015-218816 47