Robert F. Handschuh and Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Engagement of Metal Debris Into a Gear Mesh

NASA/TM—2010-216759

July 2010

MPT2009–17
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Robert F. Handschuh and Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Engagement of Metal Debris Into a Gear Mesh

NASA/TM—2010-216759

July 2010

MPT2009–17

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216759 1 

Engagement of Metal Debris Into a Gear Mesh 

Robert F. Handschuh and Timothy L. Krantz 

National Aeronautics and Space Administration 

Glenn Research Center

Cleveland, Ohio 44135 

Abstract 

A series of bench top experiments was conducted to 

determine the effects of metallic debris being dragged through 

meshing gear teeth. A test rig that is typically used to conduct 

contact fatigue experiments was used for these tests. Several 

sizes of drill material, shim stock, and pieces of gear teeth 

were introduced and then driven through the meshing region. 

The level of torque required to drive the “chip” through the 

gear mesh was measured. From the data gathered, chip size 

sufficient to jam the mechanism can be determined. 

Introduction 

In some space mechanisms the loading can be so high that 

there is some possibility that a gear chip might be liberated 

while in operation of the mechanism (Refs. 1 to 5). Also, due 

to the closely packed nature of some space mechanisms and 

the fact that a space grease is used for lubrication, chips that 

are released can then be introduced to other gear meshes 

within this mechanism. In this instance, it is desirable to know 

the consequences of a gear chip entering in between meshing 

gear teeth. To help provide some understanding, a series of 

bench-top experiments was conducted to engage chips of 

simulated and gear material fragments into a meshing gear 

pair. One purpose of the experiments was to determine the 

relationship of chip size to the torque required to rotate the 

gear set through the mesh cycle. The second purpose was to 

determine the condition of the gear chip material after 

engagement by the meshing gears, primarily to determine if 

the chip would break into pieces and to observe the motion of 

the chip as the engagement was completed. This document 

also presents preliminary testing done with metal “debris” 

other than chips from gears, namely steel shim stock and drill 

bits of various sizes and diameters. 

Test Equipment 

The gear testing was done using a Glenn Research Center 

Spur Gear Fatigue test rig. This rig uses two identical spur gears 

engaged with one another. The shaft mounting is an overhung 

arrangement for the test gears, and the nearby bearings are roller 

bearings. The overhung distance from the bearing supports is 

approximately 32 mm (1.25 in.). Because the test machine was 

designed for experiments of gear fatigue, the shaft and bearing 

supports are relatively large and stiff. Figure 1 shows the test 

gearbox with a pair of test gears mounted. 

Test Gears, Test “Debris” and Chips, and 

Procedure 

The gears used for this study were case-carburized and 

ground, and were manufactured from the steel alloy AISI 

9310. The gears design information is shown in Table 1. The 

gears were shot-peened after hardening and before final 

grinding. The gears were representative of gears utilized in a 

space mechanism that was being simulated. 

The gear design specifications and the actual mounted 

center distance determine the root clearance and backlash of a 

pair of gears. The backlash and root clearances were 

determined for the first pair of test gear mounted on the test 

fixture. The backlash was measured as 0.15 mm (0.006 in.) 

using a standard measurement setup (Fig. 2). The root 

clearance was measured as 0.94 mm (0.037 in.). The root 

clearance was determined by running a piece of soft material 

(solder) through the mesh and measuring the deformed 

material with a caliper. 

Three types of “debris” were placed into the meshing gear 

teeth for testing, namely steel shim stock, drill bit shanks, and 

pieces or “chips” from gear teeth. The shim stock materials 

used were steel and stainless steel. The shim thickness ranged 

from 0.13 to 1.88 mm (0.005 to 0.074 in.). The shims were cut 

to a length of approximately 2.5 to 3.8 mm (0.100 to 

0.150 in.), representing approximately half of the tooth height. 

The shim lengths were wider than the faces of the gears and 

were placed to engage the full-face width of the gear teeth 

(Fig. 3). The drill bit shanks used for these experiments had 

shank diameters ranging from 1.07 to 1.96 mm (0.042 to 

0.077 in.). 

The gear chip pieces used in this work were liberated from 

a spare test gear (case-carburized and ground AISI 9310 

steel). Ten gear chips were used for testing. Eight of the ten 

chips were created by scoring a mark on the gear tooth using a 

small rotating cutting wheel and then striking the score-line 

with a cold chisel. This created chips with irregular shapes 

and the chips were of varying sizes (mass). Two of the gear 

chips were made by cutting the gear tooth using a metals-lab 

cutting wheel. These two chips have a more regular shape and 

(as compared to chips made by striking) relatively smooth 

edges. To quantify the sizes of the chips, the mass of each 

chip was determined (Table 2). The figures that follow show 

at least one image with a scale marker for each chip used for 

testing. 
NASA/TM—2010-216759 2 

TABLE 1.—BASIC GEAR DIMENSIONS 

Number of teeth 42 

Module, (Diametral pitch); 

mm (1/in.) 

2.12 (12) 

Circular pitch, mm (in.) 6.65 (0.2618) 

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

Addendum, mm (in.) 2.11 (0.083) 

Chordal tooth thickness, mm (in.) 3.25 (0.1279) 

Helix angle, (deg) 0 

Pressure angle, (deg) 25 

Pitch diameter, mm (in.) 88.9 (3.50) 

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

Root fillet, mm (in.) 1.02 (0.04) 

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

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

Backlash, mm (in.) 0.15 (0.006) 

Tip relief, mm (in.) 0.015 (0.0006) 

TABLE 2.—MASS OF THE TEST GEAR CHIPS 

Chip identification number Mass, 

(grams) 

1 0.090 

2 0.100 

3 0.106 

4 0.116 

5 0.139 

6 0.252 

7 0.342 

8 0.045 

9 0.096 

10 0.032 

Testing using the shims and drill bits were done with no 

lubrication on the gear teeth. For tests done with gear chips, a 

generous amount of a Teflon-based, space- qualified grease 

was applied with a brush to the teeth prior to testing. During 

testing, one of the gear shafts (the driven gear) was free to 

rotate. That is, there was no resistive torque applied. The 

driving gear was rotated with a torque wrench by hand in a 

very deliberate manner. The peak torque reading recorded by 

the torque wrench was recorded as the engaged debris was 

rolled through the mesh. 

Test Results 

The results for the shim stock and drill bit testing will be 

discussed first. To relate the peak torque required to roll the 

object through the gear mesh to the size of the object, the 

volume of the object was calculated assuming the full-face 

width of the gear was engaged and using a nominal height of 

3.18 mm (0.125 in.) for the shim objects. The resulting 

relationship of peak torque to the object volume is provided in 

Figure 4. For the smallest shims tested, only a very small 

nominal drag torque of the rig setup was required to rotate the 

shim through mesh. The drill bit shanks required a 

significantly greater peak torque for an equivalent volume of 
NASA/TM—2010-216759 3 

shim stock material. This can be explained by the higher 

hardness of the drill bit shank relative to shim stock, thereby 

requiring larger peak torque before any plastic deformation (or 

shaft deflection) of the bit shank or gear teeth takes place. 

For the drill bit test data a straight line can be passed 

through the data. In the data for the much softer shim stock, 

there initially appears to be a linear relation up to a certain 

volume. Then a region of this data is nearly non-varying and 

only increasing torque slightly as the volume was increased. 

Finally for this data, a rather large increase in torque was 

recorded as the shim volume was in excess of 35 mm3

. 

A plausible explanation for this can be stated as follows. 

Initially the shim stock plastically compresses and an increase 

in volume causes a proportional change in torque. As the 

volume is increased, enough of the tooth tip—root clearance is 

still available to plastically compress the shim material 

resulting in little increase in the torque needed. Finally, as the 

volume of the tooth tip to root clearance is used up, the torque 

required to “extrude” the shim increase substantially. 

For the case of the shim stock material, the main effect was 

elastic deformation of the support structure (shaft and 

bearings) and plastic deformation of the shim stock. 

Essentially the shim stock was extruded. The shim stock took 

the shape of the root-fillet region of the driven gear with 

intrusion by the tip of the driving gear (Fig. 5). The peak 

torque was roughly a linear function of the engaged shim 

volume. By visual examination, it was judged that the gear 

teeth were not damaged by engagement of the shim stock. For 

the case of the drill bit shanks, the main effect was elastic 

deformation of the supporting structure and some plastic 

deformation of the teeth. The drill bit shanks permanently 

deformed and damaged the tooth tip that made contact with 

the bit shank during the meshing process (Fig. 6). 

Next, the results of testing with the gear chips will be 

discussed. The peak torque required to rotate the gears was, to 

a good approximation, a linear function of the mass of the 

engaged chip (Fig. 7). For chip #2 of Table 2, the peak torque 

exceed the measuring capacity of the torque wrench, and since 

the peak torque was not know precisely the data point was not 

included in the plot of Figure 7. A larger capacity torque 

wrench was used for subsequent testing. 

It was noted that immediately after testing the disengaged 

chips were hot, indicative of the tremendous friction forces 

and plastic deforming work being done during the meshing 

process. Some of the smaller chips tended to remain in the 

root of the gear after the engagement. Larger chips were 

forced out of the mesh and were found on the table top just 

below the gear mesh. The gear on the left had been rotated 

clockwise with the torque wrench (the input torque direction), 

so the motion tended to throw and/or allow the chip to drop to 
NASA/TM—2010-216759 4 

the tabletop. For the two largest chips (chips #6 and #7 of 

Table 2), the engaged gear teeth were deformed and damaged 

significantly enough that the gears could no longer be rotated 

with hand torque past the damaged tooth. An image of such 

damage is provided in Figure 8.

Images of the chips both before and after engagement 

testing are provided in Figures 9 to 18. The images show that 

the main effects on the chips were deformation of the chips to 

conform to the available tooth root spaces. In general, the 

chips remained intact. For the case of chips #5 and #8, small 

pieces were liberated from the main piece as can be seen in 

the figures. Chip #10 especially shows that the 

case-carburized material, even though generally described as 

brittle, can be significantly deformed and exhibit some 

toughness. Although the chip experienced some “tearing”, the 

chip did not “shatter” or otherwise exhibit extreme brittleness. 
NASA/TM—2010-216759 5 
NASA/TM—2010-216759 6 

Finally all the data generated is shown in Figure 19. The 

drill bit and shim stock mass data was found using the data 

from Figure 4. The drill bit data was very comparable to the 

tooth chip data following the same trend. The shim stock data 

had a lower torque level over all the data taken. This must be 

an artifact of the nominal material hardness. The scatter in the 

tooth chip data is possibly due to non-symmetric shapes in 

comparison to the symmetry of the drill bit data. 

Summary and Conclusions 

A series of bench-top experiments was conducted to 

provide an understanding of the engagement of metal debris 

into a gear mesh. The gears used for testing were 

case-carburized 12-pitch spur gears made from the steel alloy 

AISI 9310. The metal debris that was engaged into the pair of 

meshing test gears was shim stock, drill bit shanks, and chips 

of gears liberated from a test gear. 

It was found that the peak torque required to rotate the 

gears with the object engaged was proportional to the size 

(mass) of the engaged object. Engaging objects of higher 

hardness required a significantly greater peak torque relative 

to an equivalent sized object of lesser hardness. 

For the largest chip sizes tested, sufficient deformation 

occurred to the gear teeth to prevent smooth motions of the 

gear when the damaged tooth is engaged. 

During this study, no new chips and no obvious tooth 

fracture occurred. 

Even though case-carburized steel hardened in the manner 

of the aerospace gears is generally described as “brittle”, the 

chips exhibited significant deformation and exhibited some 

toughness. Chips did not “shatter” or otherwise exhibit 

extreme brittle behavior. 

Although large chips caused significant damage to the test 

gear teeth the data generated in this study could be used for 

minimum torque needed to drive a gear chip of a certain mass 

for the tested system. Therefore the torque required to drive a 

gear chip through mesh would be expected to be system 

dependent. 

References 

1. Krantz, T. and Handschuh, R.: A Study of Spur Gears 

Lubricated With Grease-Observations From Seven 

Experiments, NASA/TM—2005-213957, ARL–TR–3159, 

September 2005. 

2. Proctor, M., Oswald, F., and Krantz, T.: Shuttle 

Rudder/Speed Brake Power Drive Unit (PDU) Gear 

Scuffing Tests With Flight Gears, NASA/TM—2005- 

214092, December 2005. 

3. Handschuh, R. Krantz, T., L. Lerch, B. and Burke, C.: 

Investigation of Low-Cycle Bending Fatigue of AISI 9310 

Steel Spur Gears, NASA/TM—2007-214914; 

ARL–TR–4100, July, 2007. 

4. Krantz, T. and Tufts, B.: Pitting and Bending Fatigue 

Evaluations of a New Case-Carburized Gear Steel, 

NASA/TM—2007-215009; ARL–TR–4123, December, 

2007. 

5. Krantz, T., Oswald, F., and Handschuh, R.: Wear of Spur 

Gears Having a Dithering Motion and Lubricated With a 

Perfluorinated Polyether Grease, NASA/TM—2007 

-215008; ARL–TR–4124, December, 2007. 
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14. ABSTRACT

A series of bench top experiments was conducted to determine the effects of metallic debris being dragged through meshing gear teeth. A 

test rig that is typically used to conduct contact fatigue experiments was used for these tests. Several sizes of drill material, shim stock, and 

pieces of gear teeth were introduced and then driven through the meshing region. The level of torque required to drive the “chip” through the 

gear mesh was measured. From the data gathered, chip size sufficient to jam the mechanism can be determined. 

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