Vibration and Operational Characteristics 

of a Composite-Steel (Hybrid) Gear

NASA/TM—2014-216646

June 2014

Robert F. Handschuh

Glenn Research Center, Cleveland, Ohio

Kelsen E. LaBerge

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Samuel DeLuca and Ryan Pelagalli

Glenn Research Center, Cleveland, Ohio

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Vibration and Operational Characteristics 

of a Composite-Steel (Hybrid) Gear

NASA/TM—2014-216646

June 2014

Robert F. Handschuh

Glenn Research Center, Cleveland, Ohio

Kelsen E. LaBerge

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Samuel DeLuca and Ryan Pelagalli

Glenn Research Center, Cleveland, Ohio

ARL–TR–6973


	



	


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NASA/TM—2014-216646 1 

Vibration and Operational Characteristics of a 

Composite- Steel (Hybrid) Gear 

Robert F. Handschuh 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Kelsen E. LaBerge 

U.S. Army Research Laboratory 

Glenn Research Center 

Cleveland, Ohio 44135 

Samuel DeLuca1

 and Ryan Pelagalli1

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

Hybrid gears have been tested consisting of metallic gear teeth and shafting connected by composite 

web. Both free vibration and dynamic operation tests were completed at the NASA Glenn Spur Gear 

Fatigue Test Facility, comparing these hybrid gears to their steel counterparts. The free vibration tests 

indicated that the natural frequency of the hybrid gear was approximately 800 Hz lower than the steel test 

gear. The dynamic vibration tests were conducted at five different rotational speeds and three levels of 

torque in a four square test configuration. The hybrid gears were tested both as fabricated (machined, 

composite layup, then composite cure) and after regrinding the gear teeth to the required aerospace 

tolerance. The dynamic vibration tests indicated that the level of vibration for either type of gearing was 

sensitive to the level of load and rotational speed. 

Introduction 

Drive systems of the future will be required to have increased power-to-weight ratios in addition to 

reduced maintenance, noise, and cost. These requirements have been under constant evolution, resulting 

in improvements over the last several decades. Materials, manufacturing, processing, lubricants, and 

advanced drive system analysis have led to drive system designers pushing the technology and steadily 

making improvements (Refs. 1 to 5). Typically these technology improvements are implemented when 

drive system upgrades occur and when new vehicles are designed. The implementation is timed in this 

manner to reduce costs, as qualification of a drive system for rotorcraft is an expensive process. 

Power-to-weight ratio is one of the most important drive system attributes. Parametrically the 

resultant gearbox and lubrication system weight, wt, for rotorcraft of all types, sizes, number of engines, 

and number of main rotors is shown in Figure 1 (Ref. 6). Here, the parametric value is a function of the 

input and output speeds vin and vout, respectively (both in revolutions per minute), of the drive system and 

the transmitted power P (in horsepower). Once this calculation is made, the anticipated weight of the 

gearbox and lubrication system, wt, can be found. Using advanced technologies, results in drive systems 

that are lighter weight for a given gear ratio and power level such that they fall below the curve shown in 

Figure 1. This improved capability offers the rotorcraft extended range and/or payload. 

Materials play an important role in improving the power-to-weight ratio. Currently rotorcraft drive 

systems utilize lightweight structure materials (aluminum and magnesium) for the housing and minimize 

 

1

 NASA Intern working in Lewis Educational and Research Collaborative Internship Project. 
NASA/TM—2014-216646 2 

gear weight via careful analysis and machining. Generally, rotorcraft gears have only enough mass for 

load carrying ability. Minimizing the mass of a gear leads to a lack of heat storage capability. This 

attribute can cause problems during a primary lubrication system failure in which gears would operate 

under starved or dry conditions. Therefore all operation scenarios need to be carefully considered. 

Composite materials have been considered for some rotorcraft drive system housings, such as the work 

contained in Reference 7. Many issues were worked out to attain project success. However, cost and some 

other technical issues yet to be resolved have been a roadblock to incorporating this technology into 

production. 

In addition to structural components, there have also been recent efforts to incorporate lower density 

composites in dynamic components, such as shafts and gears. This report focuses on the potential application 

of composite material in rotorcraft drive system gears. The web of the test gear was replaced with composite 

material. The material properties of the composite material used in this study are compared with those of a 

typical aerospace gear material in Table I. One property that is of real importance is the density. The density of 

the composite material used in this study is approximately 25 percent of that of typical gear steel. Also, there 

was an anticipated benefit expected that the material change should help with mesh-generated vibration and 

noise that is transmitted from the gears to the shafts and bearings. 

The objective of this study is to describe how composite webbed gears (referred to here as “hybrid” gears) 

were fabricated and the resultant effect on the gear natural frequency, transmitted vibration, and noise. 

TABLE I.—COMPOSITE AND AEROSPACE STEEL MATERIAL PROPERTIES 

Property Composite material AISI 9310 gear steel 

Modulus of elasticity, Pa (psi) Tension: 44109

 (6.4106) 

Compression: 42109

 (6.1106) 200109 (29106

)

Poisson’s ratio 0.3 0.29 

Density, kg/m3 (lb/ft3) 1800 (112) 7861 (491)

Thermal conductivity, W/mC (Btu/hftF) T700 fiber-axial: 9.4 (5.43) 55 (32) 

Useful maximum temperature as gear material, C (F) 150 (302) 175 (347) 

Coefficient of thermal expansion, 10-6 m/mK–1 (10-6 in/inF–1) In plane: 2 (1.1) 13 (7.3) 

Failure strain, percent Tension: 1.9

Compression: 0.94 

Elongation, percent 15 
NASA/TM—2014-216646 3 

Hybrid Gear Manufacture 

The hybrid gears manufactured in this study followed the process as described in Reference 8. A brief 

description will now be provided. 

The test gear design used for this study has the design shown in Table II. Figure 2 provides a pictorial 

explanation of the hybrid gear assembly process. A hexagonal region was machined out of a steel gear 

leaving two steel gear components: a gear rim with the teeth and a hub region for attachment to the 

facility shafting. The braided prepreg composite material was built up in a fixture around the steel hub 

and rim as shown in steps 3 to 8 of Figure 2. A total of 36 layers of composite material were built up then 

cured in the fixture (step 9) at a final temperature of 177 °C (350 °F). The fixture for fabrication and 

curing used the inner diameter of the hub and the gear measurement over pins in an attempt to keep the 

gear teeth aligned with the axis of rotation. An example of the cured gear is shown in Figure 3. 

TABLE II.—GEAR DESIGN FOR THIS STUDY 

Number of teeth ....................................................................... 42 

Module, mm (Diametral pitch 1/in.) ............................... 2.12 (12) 

Circular pitch, mm (in.)......................................... 6.650 (0.2618) 

Whole depth, mm (in.) .............................................. 4.98 (0.196) 

Addendum, mm (in.) ................................................. 2.11 (0.083) 

Chordal tooth thickness, mm (in.) ......................... 3.249 (0.1279) 

Pressure angle, deg ................................................................... 25 

Pitch diameter, mm (in.) ................................................ 88.9 (3.5) 

Outside diameter, mm (in.)...................................... 93.14 (3.667) 

Measurement over pins, mm (in.).......................... 93.87 (3.6956) 

Pin diameter, mm (in.)............................................... 3.66 (0.144) 

Backlash ref., mm (in.) .............................................. 0.15 (0.006) 

Tip relief, mm (in.) ........................ 0.013–0.018 (0.0005–0.0007) 

All-steel gear weight, kg (lbf) ............................. 0.3799 (0.8375) 

Hybrid gear weight, kg (lbf) ................................ 0.3242 (0.7147)
NASA/TM—2014-216646 4 

A total of four gears (two pairs) were manufactured and used in this study. The first gear pair (A) was 

used for fatigue testing, to be described later, while the second pair (B) was used for both static and 

dynamic vibration tests. Because of material differences (i.e., coefficient of thermal expansion), the initial 

gear geometry was not preserved after the curing process, causing significant pitch variation, and other 

anomalies compared with the original steel aerospace gear. Figure 4 provides example pitch variation 

measurements for two of the four hybrid gears after curing. These pitch variations were greater in gear set 

A than in set B. This resulted in a reduction in backlash, normally around 0.15 mm (0.006 in.), of 0 to 

0.038 mm (0 to 0.0015 in.). Even with the reduced backlash, gear set A withstood fatigue testing without 

damage to the contact surfaces. 

To reduce the pitch variations and return the tooth geometry to its original form, the teeth on gear set 

B were reground after performing initial vibration tests. The geometry of one of the reground gears is 

shown in Figure 5. Although the tooth geometry was restored, the regrind process resulted in excess 

backlash of 0.58 to 0.64 mm (0.023 to 0.025 in.), which is nearly 4 times the normal steel gear backlash. 

Some modifications would be needed in adapting this process to production, such as waiting until after 

composite curing to perform the final tooth grind to minimize repeat effort. 
NASA/TM—2014-216646 5 

Free Vibration Modes 

In an effort to understand how adding a composite web to a spur gear effects the vibration and noise 

generated and/or transmitted, first free-free vibration tests were performed in the form of impact tests. 

Impact tests were performed on both the composite hybrid gear and its steel counterpart. The vibration 

response was collected with an accelerometer connected to the gear’s inner bore via a small aluminum 

bracket affixed (glued) to the bore to provide a rigid mount that was easily removed. The gear was 

suspended by an elastic band. Using an instrumented modal hammer, the gear was impacted radially in 

line with the accelerometer. This is slightly different from the setup previously reported, that placed the 

accelerometer in the axial direction adjacent to the bore (Ref. 8). Figure 6 shows the test setup and 

accelerometer bracket. Data acquisition was triggered by the instrumented hammer and was collected at a 

sample rate of 50 kHz. Figure 7 shows representative vibration plots of both a hybrid and a steel gear. 

Although the majority of the vibration has diminished in the composite gear after 5 ms, the steel gear 

vibrates out past 10 ms. 

The accelerance for the steel gear impact tests are shown in Figure 8. The first natural frequency for 

the steel gear is around 7000 Hz. This is consistent with a previous finite element analysis (FEA) that 

provided estimated natural frequencies at 7187 and 7270 Hz (Ref. 8). Similarly, the natural frequencies 

seen at 12.1 and 12.7 kHz are also close to FEA approximations. Those seen at 15.2, 16.5, and 17.2 kHz 

were not seen in previously published axial impact tests. This is likely due to the fact that the mode 
NASA/TM—2014-216646 6 

shapes around 15 and 16 kHz involve only radial motion as was seen in the FEA-estimated mode shapes. 

Since damping for any particular mode is related to the width of the peak in the frequency spectrum, the 

sharp spikes at 12.1, 12.7, 16.5, and 17.2 kHz represent the modes with limited damping, whereas the 

modes at 7 and 15.2 kHz exhibit a higher level of damping. 

Hybrid gear impact tests displayed in Figure 9, show the first natural frequency around 6400 Hz. 

This is consistent with previous testing (Ref. 8). These results also show a strong natural frequency at 

10.7 kHz. It is difficult to separate the other resonant frequencies, as they are highly damped and overlap. 

Neither of these natural frequencies is consistent with those identified by FEA that predicted the first 

frequency to be higher than that of the steel gear at 7780 Hz (Ref. 8). Further modeling efforts are needed 

to obtain useful hybrid gear FEA predictions, focusing on ply arrangement effects and a better model of 

the interface between the steel and composite material. 
NASA/TM—2014-216646 7 

Test Facility 

The dynamic tests reported herein were conducted in the NASA Glenn Spur Gear Fatigue Rig. The 

facility, shown in Figure 10, is a closed-loop test rig that operates at speeds up to 10 000 rpm and 57.9 Nm 

(513 inlb) torque. Speed and load are independently adjustable. Load is adjusted by varying the supply 

pressure to the rotating torque actuator located in one of the slave gears. From FEA, the torque mentioned 
NASA/TM—2014-216646 8 

above induces a bending stress of 0.212 GPa (30.8 ksi) and a contact stress of 1.17 GPa (170 ksi). These 

stresses were calculated using the full three-dimensional FEA method described in Reference 9. 

Vibration data were attained using accelerometers with a bandwidth of 10 kHz with ±5 percent 

accuracy (18 kHz at ±10 percent). The accelerometers were mounted as shown in Figure 11 such that one 

was parallel to the line of action (LOA) of the gears and the other perpendicular (off-LOA). Data were 

taken via laboratory analog-to-digital converters and read into a computer at a rate of 50 kHz. 
NASA/TM—2014-216646 9 

Experimental Results 

The experimental results of the three types of tests performed are described in the following sections. 

Vibration 

Dynamic vibration data was taken with test gears installed in four different configurations: steel 

driving steel, steel driving hybrid, hybrid driving steel, and hybrid driving hybrid. It is important to note 

that the hybrid gears used for the dynamic tests were set B that were reground to aerospace tolerances, as 

discussed previously. The tests were performed at five different shaft speeds and at three levels of torque 

(load pressure). The rotational speeds were 2500, 5000, 7500, 8750, and 10 000 rpm and the torque levels 

were at 20.5, 39.2, and 57.9 Nm (182, 347, and 513 inlb, respectively). From Figure 11, the driving gear 

is the one on the left and the driven gear is on the right. All vibration data were taken from the right-side 

seal housing that is in direct contact with the shaft support bearing. 

Root-mean-square (RMS) vibration levels for each test are shown in Figure 12 for both the LOA and 

off-LOA accelerometers. Results show that the highest vibration was experienced when the hybrid gear 

was driving the steel gear. When comparing the hybrid driving steel results with the steel driving hybrid 
NASA/TM—2014-216646 10 

results, there is a significant reduction in vibration. With the driving gear mounted on the left and the 

driven gear mounted on the right, adjacent to the accelerometers, this shows that lower vibration levels 

are experienced when the hybrid gear is mounted next to the sensors. The hybrid gear pair shows some 

improvement over the steel pair at the higher rotational speeds and loads for the LOA accelerometer and 

shows a small overall improvement when comparing to off-LOA results. 

The raw data were also averaged over 20 shaft rotations to obtain the time synchronous average. This 

process is performed to remove nonsynchronous noise from the data. The RMS of the average is shown in 

Figure 13 and provides similar results as shown in Figure 12. 

Dynamic tests were performed both before and after gear set B was reground, providing a comparison 

between the two geometries. These vibration results are shown in Figure 14. The regrind process is shown 

to reduce the vibration by as much as half at lower speeds. The results presented could be improved by 

optimizing the composite curing process to minimize the distortion that occurs. The need for regrinding 

after the composite cure process may not be eliminated, but the amount of material removal required to 

bring the geometry back to aerospace quality could be reduced. 
NASA/TM—2014-216646 11 

Noise 

While the vibration data was being taken, a sound pressure level measurement was made. The 

measurement was taken using a single, hand-held acoustic probe that uses the A-weighted scale. The 

probe was held at a distance of 0.4 m (~16 in.) at the height of the centerline of the meshing gears, 

pointed directly at the test gear cover. The RMS-averaged data are shown in Figure 15. The noise data 

produced similar trends as did the vibration data discussed previously. 

Fatigue 

Hybrid gear set A was used for endurance testing as a proof of concept. The gear set used in this test 

was put into the facility after composite curing without any regrinding of the gear tooth profiles. A single 

long-term test was conducted to examine the durability of the composite-steel bond. The test was run for 

one billion cycles at 10 000 rpm and 48.6 Nm (430 inlb) at 49 °C (120 °F) oil inlet temperature. This 

corresponds to a calculated contact stress of approximately 1.35 GPa (195 ksi) and a bending stress of 

0.20 GPa (29.2 ksi). The gears survived without any visual signs of debonding or degradation of the 

composite material. 
NASA/TM—2014-216646 12 
NASA/TM—2014-216646 13 

Conclusions 

Based on the results attained in this study the following conclusions can be made: 

1. A hybrid gear mesh has shown promise for operation in a simulated aerospace environment with 

the possibility of reducing component weight. 

2. Hybrid gears need to be further processed to maintain aerospace gear quality. 

3. The hybrid gears were fatigue tested to one billion stress cycles at representative loads and speeds 

with no resulting degradation. 

4. The hybrid gears exhibited a lower natural frequency than the standard steel gears of the same size 

and dimension. 

5. Dynamic vibration results indicated the strong dependence of operational speed on the measured 

results. The hybrid driving hybrid configuration exhibited the lowest vibrations, but only at the higher 

speeds and loads. 

6. Vibration tests performed with the hybrid gear driving the steel gear typically exhibited the highest 

RMS vibration levels, particularly at the higher rotational speeds. 

7. The A-Weighted, RMS sound-level measurements trended similarly to those of the vibration 

results.

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