NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Gearbox Reliability 

Collaborative Bearing 

Calibration 

J. van Dam

Technical Report

NREL/TP-5000-47852

October 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Gearbox Reliability 

Collaborative Bearing 

Calibration 

J. van Dam

Prepared under Task No. WE10.1131

Technical Report

NREL/TP-5000-47852

October 2011
NOTICE

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Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste. 
iii

Table of Contents

List of Figures .................................................................................................................... iv

List of Tables ..................................................................................................................... iv

Introduction ......................................................................................................................... 1

Bearings .............................................................................................................................. 2

Calibration fixture ............................................................................................................... 6

Data acquisition system .................................................................................................... 10

Calibration procedures ...................................................................................................... 12

Assembly....................................................................................................................... 12

Load cells zero .............................................................................................................. 12

Load cell leveling .......................................................................................................... 13

Static calibration ........................................................................................................... 13

Crane (Dynamic) calibration......................................................................................... 14

Strain gage zero ............................................................................................................. 14

Analysis............................................................................................................................. 15

Static calibration ........................................................................................................... 15

Crane (dynamic) calibration ......................................................................................... 16

Results ............................................................................................................................... 18

Issues with calibration ................................................................................................... 23

Comparison with FEA model ........................................................................................... 24

Acknowledgements / Team............................................................................................... 24

Annex A Transformation from load cell readings to bearing loads .................................. 25

Background: .................................................................................................................. 25

Assumptions:................................................................................................................. 25
iv

List of Figures

Figure 1. Groove locations on planet A .............................................................................. 2

Figure 2. Groove locations bearing BR .............................................................................. 3

Figure 3. Groove locations bearing CR .............................................................................. 3

Figure 4. Close up of one of the strain gages on the bearings. . ......................................... 4

Figure 5. Spec sheet for strain gages used on the planet bearings ...................................... 5

Figure 6. Calibration stand in the 100 kip load frame in the 251 highbay. . ...................... 7

Figure 7. Drawing of calibration fixture ............................................................................. 8

Figure 8. Two A bearings assembled in the tapered sleeve with spacer in between. ......... 9

Figure 9. Calibration fixture in the load frame. . .............................................................. 10

Figure 10. Example of time series of strain during static calibration. ............................. 13

Figure 11. Crane calibration method set up. . ................................................................... 14

Figure 12. Example of static calibration. ......................................................................... 15

Figure 13. Example of fitted slope in static calibration with CR8_1A and CR8_1B at 

TDC. ............................................................................................................... 16

Figure 14. Example of a time series of crane calibration with identified peaks and 

valleys ............................................................................................................. 17

Figure 15. Crane calibration results for Bearing 1 groove 1 ............................................ 17

Figure 16. Results of the static calibration. . .................................................................... 19

Figure 17. Results of crane calibrations ............................................................................ 20

Figure 18. Comparison of slope of static calibration and positive slope of crane 

calibration ......................................................................................................................... 21

Figure 19. Positive slope of crane calibration versus the slope from the static 

calibration ....................................................................................................... 21

Figure 20. Found slopes separated for A and B locations ................................................ 22

Figure A.1. Drawing showing basic load path .................................................................. 25

Figure A.2. Base plate showing distance between loadcells ............................................ 26

Figure A.3. Dimension of yoke......................................................................................... 26

Figure A.4. Drawing indicated thickness of yoke end plate. ............................................ 27

List of Tables

Table 1. Load cell details .................................................................................................. 10

Table 2. Strain gage wire colors ....................................................................................... 11

Table 3. Correlation between bearing calibration names and GRC DAS names; Set 1. .. 11

Table 4. Correlation between bearing calibration names and GRC DAS names; Set 2 ... 12

Table 5. Static calibration files ......................................................................................... 18

Table 6. Crane calibration files ......................................................................................... 19

Table 7. Calibration results ............................................................................................... 22

Table 8. Calibration results separated for A and B location gages ................................... 22

Table 9. Calibration results for EtherCAT ........................................................................ 23
1 

Introduction

NREL has initiated the Gearbox Reliability Collaborative (GRC) to investigate the root 

cause of low wind turbine gearbox reliability. 

The GRC follows a multi-pronged approach based on a collaborative of manufacturers, 

owners, researchers and consultants. The project combines analysis, field testing, 

dynamometer testing, condition monitoring, and the development and population of a 

gearbox failure database. 

At the core of the project are two 750kW gearboxes that have been redesigned and rebuilt 

so that they are representative of the multi-megawatt gearbox topology currently used in 

the industry. These gearboxes are heavily instrumented and are tested in the field and on 

the dynamometer. 

One of the measurements the collaborative is targeting is the load distribution on the 

planet bearings. For this purpose, the planet bearings were instrumented with strain 

gages. The strain gages measure the roller load as the roller moves over an arch ground 

into the inner race of the planet bearings. The objective of the calibration was to measure 

the correlation between the load applied to the bearing and the strain gage response. This 

report documents the set-up, the calibration procedures, the issues, and the results. 

The calibrations took place between 16 July and 9 December 2008 in the 100 kip load 

frame at the National Wind Technology Center. 
2 

Bearings 

Two sets of planets bearings were used, one set for each of the two GRC gearboxes. Each 

set consists of six bearings for three planets (two bearings per planet). The bearings were 

numbered 1 through 12. The planets are labeled A, B and C. Each bearing has three 

grooves in the inner race. One of the grooves is in top dead center (TDC) on all bearings. 

The other two grooves are different for each planet. Note that each groove arc has a 2mm 

flat spot as indicated at the top right of Figures 1 through 3. 

Figure 1 through Figure 3 show the groove locations for three bearings. (Note that these 

drawings are included only to provide a visual representation of groove locations. High 

resolution drawings are available upon request.) Since the groove pattern is not 

symmetric on planets B and C, an additional designation was added to the bearing label. 

BL is the downwind bearing on planet B. BR is the rotor side or upwind bearing for 

planet B. The configuration is similar for planet C.

Figure 1. Groove locations on planet A
3 

Figure 2. Groove locations bearing BR

Figure 3. Groove locations bearing CR

Each groove contains two strain gages. The one closest to the rim of the inner race is 

designated A, while the other one is B. The gages are Poisson gages, type CEA-06-

062UT-350 shown in Figure 5. Figure 4 shows a gage in a groove. The gage factor for all 

the gages is 2.095.
4 

Figure 4. Close up of one of the strain gages on the bearings. The edge of the bearing is 

on the right. PIX #19679. 
5 

Figure 5. Spec sheet for strain gages used on the planet bearings
6 

Calibration fixture

A calibration fixture, shown in Figure 6, was designed by Powertrain Engineers (Figure 

7). The idea behind the test stand is that it allows for independent rotation of the inner 

race and outer race of the bearings. This allows the grooves to be put in any orientation 

while rolling the roller elements over them when they are under load. 

The bearings were installed on a shaft that was equivalent to the actual planet pins to

show the effect of shaft bending, if present.

The bearings were inserted into the tapered sleeve (item 7 in Figure 7). Between the 

bearings, a spacer was keyed to the shaft (Figure 8). The bearings were keyed to the 

spacer through a slot in the inner race. 

The tapered rollers (item 9 in Figure 7) can be pulled onto the sleeve by tightening the 

M20’s (item 19 in Figure 7) and by applying hydraulic pressure to the ports. This process 

contracts the tapered sleeve to reduce the radial clearance in the bearings to match the 

installed clearance in the gearbox. 

Assembly of the bearings into the calibration assembly took place inside a “clean room”. 

The clean room was constructed using plastic over a wooden frame. Air was blown into 

the clean room by an external air handler equipped with a HEPA filter.

Pictures and drawings of the calibration fixture are shown in Figure 6 and Figure 9. 
7 

Figure 6. Calibration stand in the 100 kip load frame in the 251 highbay. PIX #19676.
8 

Figure 7. Drawing of calibration fixture
9 

Figure 8. Two A bearings assembled in the tapered sleeve with spacer in between. 

PIX #19680.
10 

Figure 9. Calibration fixture in the load frame. PIX #19678.

Data acquisition system

The data acquisition system used for the calibration is the Low Speed Shaft Data 

Acquisition System (LLS DAS). This DAS also was used for the GRC field test. It 

consists of one enclosure containing two cDAQ backplanes. The USB output of the 

chassis was converted to fiber optic and sent to the control room. In the control room, the 

signal was converted back to USB and plugged into the DAS computer. 

Four 25 kip load cells were used on the bottom of the calibration fixture. The load cells 

were configured in the National Instruments’ mode, “Full Bridge I,” with an excitation 

voltage of 2.5V. Load buttons were inserted into the bottom of the load cells. The load 

frame had a 100 kip load cell for control of the applied load. Load cell details are 

provided in Table 1 

Table 1. Load cell details

Channel Make model Serial number Slope

LC1 MTS 661.20E-03 13570 20451800

LC2 MTS 661.20E-03 78893 20226400

LC3 MTS 661.20E-03 78892 20082000

LC4 MTS 661.20E-03 109696 20013400

Load frame control load cell MTS 661.23E-01 390276 NA
11 

The strain gages in the bearing grooves were configured as Half Bridge Type I with a kfactor of 2. Table 2 shows how the gages were connected to the DAS. The naming 

convention for the channel names assigned to the strain gages was:

Bearing name_number_groove number_gage location in groove. 

Thus, BL2_1A is the gage closest to the rim in groove 1 of bearing number 2, which is 

the downwind bearing on planet B. Table 3 and Table 4 provide the correlation between 

the channel names used in the bearing calibration and the channel names used in the 

dynamometer and field testing. Set 1 was used in gearbox 1, which went to the field test. 

Set 2 was used in gearbox 2, which was used for the dynamometer testing.

Table 2. Strain gage wire colors

Gage wire color cDAQ wire color Signal

Red Purple Excitation - 

White Grey Signal +

Black Blue Excitation +

Table 3. Correlation between bearing calibration names and GRC DAS names; Set 1.

Type Bearing Groove

from TDC 

CW=+ Position DAQ Module Channel

Bearing 

calibration 

name GRC DAS name

A 1 1 -86 deg A Master 1 0 A1_1A AD_274_25

B Master 1 A1_1B AD_274_75

2 0 deg A Master 2 A1_2A AD_00_25

B Master 3 A1_2B AD_00_75

3 86 deg A Master 2 0 A1_3A AD_86_25

B Master 1 A1_3B AD_86_75

A 4 1 86 deg A Master 2 A4_1A AU_86_25

B Master 3 A4_1B AU_86_75

2 0 deg A Master 3 0 A4_2A AU_00_25

B Master 1 A4_2B AU_00_75

3 -86 deg A Master 2 A4_3A AU_274_25

B Master 3 A4_3B AU_274_75

BL 6 1 0 deg A Master 4 0 BL6_1A BU_756_25

B Master 1 BL6_1B BU_756_75

2 52 deg A Master 2 BL6_2A BU_308_25

B Master 3 BL6_2B BU_308_75

3 104 deg A Master 6 0 BL6_3A BU_00_25

B Master 1 BL6_3B BU_00_75

BR 9 1 -104 deg A Master 2 BR9_1A BD_00_25

B Master 3 BR9_1B BD_00_75

2 -52 deg A Master 7 0 BR9_2A BD_308_25

B Master 1 BR9_2B BD_308_75

3 0 deg A Master 2 BR9_3A BD_756_25

B Master 3 BR9_3B BD_756_75

CL 3 1 -70 deg A Master 8 0 CL3_1A CU_290_25

B Master 1 CL3_1B CU_290_75

2 -26 deg A Master 2 CL3_2A CU_334_25

B Master 3 CL3_2B CU_334_75

3 0 deg A Slave 1 0 CL3_3A CU_00_25

B Slave 1 CL3_3B CU_00_75

CR 8 1 0 deg A Slave 2 CR8_1A CD_00_25

B Slave 3 CR8_1B CD_00_75

2 26 deg A Slave 2 0 CR8_2A CD_334_25

B Slave 1 CR8_2B CD_334_75

3 70 deg A Slave 2 CR8_3A CD_290_25

B Slave 3 CR8_3B CD_290_75
12 

Table 4. Correlation between bearing calibration names and GRC DAS names; Set 2

Type Bearing Groove

from TDC 

CW=+ Position DAQ Module Channel

Bearing 

calibration 

name GRC DAS name

A 7 1 -86 deg A Master 1 0 A7_1A AD_274_25

B Master 1 A7_1B AD_274_75

2 0 deg A Master 2 A7_2A AD_00_25

B Master 3 A7_2B AD_00_75

3 86 deg A Master 2 0 A7_3A AD_86_25

B Master 1 A7_3B AD_86_75

A 12 1 86 deg A Master 2 A12_1A AU_86_25

B Master 3 A12_1B AU_86_75

2 0 deg A Master 3 0 A12_2A AU_00_25

B Master 1 A12_2B AU_00_75

3 -86 deg A Master 2 A12_3A AU_274_25

B Master 3 A12_3B AU_274_75

BL 2 1 -104 deg A Master 4 0 BL2_1A BU_756_25

B Master 1 BL2_1B BU_756_75

2 -52 deg A Master 2 BL2_2A BU_308_25

B Master 3 BL2_2B BU_308_75

3 0 deg A Master 6 0 BL2_3A BU_00_25

B Master 1 BL2_3B BU_00_75

BR 5 1 0 deg A Master 2 BR5_1A BD_00_25

B Master 3 BR5_1B BD_00_75

2 52 deg A Master 7 0 BR5_2A BD_308_25

B Master 1 BR5_2B BD_308_75

3 104 deg A Master 2 BR5_3A BD_756_25

B Master 3 BR5_3B BD_756_75

CL 11 1 -70 deg A Master 8 0 CL11_1A CU_290_25

B Master 1 CL11_1B CU_290_75

2 -26 deg A Master 2 CL11_2A CU_334_25

B Master 3 CL11_2B CU_334_75

3 0 deg A Slave 1 0 CL11_3A CU_00_25

B Slave 1 CL11_3B CU_00_75

CR 10 1 0 deg A Slave 2 CR10_1A CD_00_25

B Slave 3 CR10_1B CD_00_75

2 26 deg A Slave 2 0 CR10_2A CD_334_25

B Slave 1 CR10_2B CD_334_75

3 70 deg A Slave 2 CR10_3A CD_290_25

B Slave 3 CR10_3B CD_290_75

Calibration procedures

Assembly

The bearings were inserted into the tapered rollers. The M20’s were torqued (typical 

values 325-350 ft-lb) until the bearings' diametrical clearance was less than 0.0005, but 

still positive. The step between the tapered rollers and the inner sleeve was measured 

around the perimeter throughout this tightening process ensuring the straight fit of the

roller into the sleeve. A maximum of 0.005” was allowed between maximum and 

minimum measurements. If needed, hydraulic pressure could be applied to the hydraulic 

port on the rollers with a maximum pressure of 2000psi. 

Load cells zero

The load cells were all zeroed while hanging from the base plate. An Enerpac was used to 

jack up the base plate. 
13 

Load cell leveling

To ensure all load cells had about the same contact with the load frame base plate, the 

load cells could be turned on their thread to be extended or retracted. This was an 

iterative process. First, two load cells on the diagonal (for example load cells 1 and 3) 

would be extended. They were adjusted until they read about the same under load. Then, 

the load cells on the other diagonal were extended slowly until the readings of all four 

load cells were equal (within 3%). 

Static calibration

For the static calibration, one of the grooves was put on top dead centre (TDC). The 

spanner wrench was used to move the outer races until the strain gages on one of the 

bearings reached a maximum. It was assumed that this meant a rolling element was on 

the centre of the groove. A new data file was started and the load was increased from

10,000 lbs to 80,000 lbs, and then decreased to 10,000 lbs in 10,000 lb steps. This was

repeated at least three times. Figure 10 shows an example of the bearing response during 

this type of calibration where gages, CR8_1A and CR8_1B were at TDC. Plots of the 

responses of the other four gages are shown here for reference purposes only. Rolling 

elements were not positioned over these grooves and, in some cases, this loading 

configuration produced negative (compressive) strain. 

-150

-100

-50

0

50

100

150

200

250

30 40 50 60 70 80 90 100

Time [sec]

strain [ue]

 CR8_1A

 CR8_1B

 CR8_2A

 CR8_2B

 CR8_3A

 CR8_3B

Figure 10. Example of time series of strain during static calibration.

Data file: 2008_12_09 10_30_18 100Hz
14 

Crane (Dynamic) calibration

Dynamic effects were investigated using an overhead crane. For the crane calibration, a 

groove was put in the TDC position and a wire rope was wound around the rollers 

(Figure 11). The crane was used to unwind the wire rope and thus rotate the outer races of 

the bearings. The outer races of the bearings were allowed three full rotations, thus 

assuring each roller went past the groove at least once. This test was done for five load 

levels: 20, 35, 50, 65 and 80 kip (total load on the fixture). 

Figure 11. Crane calibration method set up. PIX #19677.

Strain gage zero

To obtain a zero on the strain gage, the groove was rotated to dead bottom. The load 

frame was shut off, leading to an applied load of about 50lbs. Since there was positive 
15 

clearance in the bearing, it was assumed that the rolling element on the bottom would not 

be touching the inner race. A short data file was collected.

Analysis 

Static calibration

For the static calibration, the strain was plotted against load. An example is given in 

Figure 12. There is hysteresis present; the strain for the increasing load was consistently 

lower than for the decreasing load. As in Figure 10, CR8_1A and CR8_1B are the only 

gages at TDC. Then, the data was filtered for increasing load only. A linear regression 

was applied to the data. The entire load range was used for the fit (Figure 13). 

-150

-100

-50

0

50

100

150

200

250

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

L oad on bearing [lbf]

s train [ue]

 C R 8_1A

 C R 8_1B

 C R 8_2A

 C R 8_2B

 C R 8_3A

 C R 8_3B

Figure 12. Example of static calibration. Data file: 2008_12_09 10_30_18 100Hz
16 

y = -0.0033697x + 22.5598119

y = -0.0022x + 7.2073

y = 0.0003x + 6.116

y = 0.0048x - 12.453

y = -5E -05x + 8.3836

y = 0.0037x - 13.14

-150

-100

-50

0

50

100

150

200

250

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

L oad on bearing [lbf]

s train [ue]

 C R 8_1A

 C R 8_1B

 C R 8_2A

 C R 8_2B

 C R 8_3A

 C R 8_3B

L inear ( C R 8_2A)

L inear ( C R 8_2B )

L inear ( C R 8_3B )

L inear ( C R 8_1A)

L inear ( C R 8_3A)

L inear ( C R 8_1B )

Figure 13. Example of fitted slope in static calibration with CR8_1A and CR8_1B at TDC. 

Data file: 2008_12_09 10_30_18 100Hz 

Crane (dynamic) calibration

Time series were plotted and the peaks and valleys were identified (Figure 14). The strain 

value of each peak and valley is plotted against the load for that peak or valley. Data for 

the different load steps is then combined into one plot (Figure 15). A linear regression 

was fit through the peaks and the valleys separately. 
17 

-2500

-2000

-1500

-1000

-500

0

500

10 20 30 40 50 60 70 80

Time [sec]

strain [ue]

 A4_2A

 A4_2B

4_2A max

4_2Amin

4_2B max

4_2Bmin

 A1_2A

 A1_2B

1_2A max

1_2Amin

1_2B max

1_2Bmin

Figure 14. Example of a time series of crane calibration with identified peaks and valleys

y = 0.004045x + 822.528507

y = -0.001740x + 783.601404

y = -0.001069x - 813.795120

y = 0.003573x - 760.687262

-1000

-500

0

500

1000

1500

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000

Bearing load [lbf]

Strain [ue]

1_1A max

1_1Amin

1_1B max

1_1Bmin

Linear (1_1A max)

Linear (1_1A max)

Linear (1_1Amin)

Linear (1_1Bmin)

Linear (1_1B max)

Figure 15. Crane calibration results for Bearing 1 groove 1 (A1_1A and A1_1B)
18 

Results 

Table 5 and Table 6 provide the key for the filename and calibration. The data file name 

format is “yyyy_mm_dd hh_mm_ss”. 

Table 5. Static calibration files

Bearing Groove Data file

1 2008_11_06 10_14_54

1 2 2008_11_06 09_22_29

3 2008_11_06 10_33_48

1 2008_11_06 15_44_11

2 2 2008_11_06 14_58_42

3 2008_11_06 14_51_18

1 2008_12_08 15_37_56

3 2 2008_12_08 14_50_11

3 2008_12_08 14_44_11

1 2008_11_06 10_31_27

4 2 2008_11_06 09_24_48

3 2008_11_06 10_23_36

1 2008_11_06 14_53_32

5 2 2008_11_06 15_00_48

3 2008_11_06 15_46_16

1 2008_10_09 14_58_57

6 2 2008_10_09 13_40_01

3 2008_10_07 15_27_01

1 2008_10_06 10_12_05

7 2 2008_10_06 10_36_37

3 2008_10_06 10_44_27

1 2008_12_09 10_30_18

8 2 2008_12_08 14_52_15

3 2008_12_08 15_40_04

1 2008_10_07 15_30_26

9 2 2008_10_09 13_40_01

3 2008_10_09 15_03_08

1 2008_11_05 12_51_12

10 2 2008_11_05 13_00_03

3 2008_11_05 13_51_09

1 No good data

11 2 2008_11_05 13_02_31

3 2008_11_05 12_54_36

1 2008_10_06 10_49_05

12 2 2008_10_06 10_22_48

3 2008_10_06 10_08_05
19 

Table 6. Crane calibration files

Load level [lbf]

Grooves: 20 35 50 65 80

3_3 & 8_1 2008_12_08 14_18_44 2008_12_08 14_23_46 2008_12_08 14_27_36 2008_12_08 14_30_59 2008_12_08 14_34_12

3_2 & 8_2 2008_12_08 14_55_48 2008_12_08 15_00_03 2008_12_08 15_03_27 2008_12_08 15_06_53 2008_12_08 15_10_08

3_1 & 8_3 2008_12_08 15_16_17 2008_12_08 15_19_29 2008_12_08 15_22_56 2008_12_08 15_26_32 2008_12_08 15_32_55

1_2 & 4_2 2008_11_06 09_32_20 2008_11_06 09_37_03 2008_11_06 09_44_08 2008_11_06 09_48_58 2008_11_06 09_52_18

1_1 & 4_3 2008_11_06 09_51_15 2008_11_06 10_00_24 2008_11_06 10_03_37 2008_11_06 10_07_02 2008_11_06 10_10_53

1_3 & 4_1 2008_11_06 10_40_26 2008_11_06 10_51_13 2008_11_06 10_54_32 2008_11_06 10_58_41 2008_11_06 11_02_10

2_3 & 5_1 2008_11_06 14_07_08 2008_11_06 14_11_53 2008_11_06 14_16_31 2008_11_06 14_23_49 2008_11_06 14_46_53

2_2 & 5_2 2008_11_06 15_06_12 2008_11_06 15_10_13 2008_11_06 15_13_55 2008_11_06 15_17_43 2008_11_06 15_20_58

2_1 & 5_3 2008_11_06 15_25_51 2008_11_06 15_29_28 2008_11_06 15_32_42 2008_11_06 15_36_17 2008_11_06 15_39_56

11_1 & 10_1 2008_11_05 12_32_19 2008_11_05 12_35_51 2008_11_05 12_39_05 2008_11_05 12_42_40 2008_11_05 12_46_30

11_2 & 10_2 2008_11_05 13_06_17 2008_11_05 13_09_40 2008_11_05 13_14_26 2008_11_05 13_19_40 2008_11_05 13_23_49

11_1 & 10_3 2008_11_05 13_28_07 2008_11_05 13_31_56 2008_11_05 13_35_39 2008_11_05 13_38_51 2008_11_05 13_43_11

6_3 & 9_1 2008_10_07 15_36_40 2008_10_07 15_40_08 2008_10_07 15_44_01 2008_10_07 15_47_47 2008_10_07 15_51_13

6_2 & 9_2 2008_10_09 13_45_54 2008_10_09 13_49_56 2008_10_09 13_53_37 2008_10_09 13_57_07 2008_10_09 14_00_39

6_1 & 9_3 2008_10_09 14_22_42 2008_10_09 14_25_51 2008_10_09 14_29_12 2008_10_09 14_33_42 2008_10_09 14_37_24

7_2 & 12_2 2008_10_06 14_29_53 2008_10_06 14_34_03 2008_10_06 14_37_42 2008_10_06 14_41_12 2008_10_06 14_44_37

7_3 & 12_1 2008_10_06 15_04_50 2008_10_06 15_08_08 2008_10_06 15_11_46 2008_10_06 15_15_36 2008_10_06 15_18_47

7_1 & 12_3 2008_10_06 15_37_38 2008_10_06 15_41_21 2008_10_06 15_55_55 2008_10_06 15_59_18 2008_10_06 16_02_31

The results of the static calibration are found in Figure 16. The results of the crane 

calibration are found in Figure 17. 

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

1_1A1_2A1_3A2_1A2_2A2_3A3_1A3_2A3_3A6_1A6_2A6_3A7_1A7_2A7_3A

10_1A10_2A10_3A

4_1B4_2B4_3B5_1B5_2B5_3B8_1B8_2B8_3B9_1B9_2B9_3B

11_1B11_2B11_3B12_1B12_2B12_3B

[ue/lbf]

Figure 16. Results of the static calibration. Note that not all gages are labeled in this 

figure. Gage 4_3B appears to be miswired.
20 

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

0.012

1_1A1_2A1_3A2_1A

2_2A

2_3A

3_1A

3_2A

3_3A

6_1A6_2A6_3A

7_1A

7_2A

7_3A

10_1A10_2A10_3A

4_1B4_2B

4_3B

5_1B5_2B

5_3B8_1B

8_2B

8_3B

9_1B9_2B9_3B

11_1B11_2B11_3B12_1B12_2B12_3B

[ue/lbf]

slope through peaks

slope through valleys

sum of slopes

Figure 17. Results of crane calibrations

One of the strain gages (4_3B) was wired or hooked up backward to the DAS. It is 

recommended that the negative slope be implemented because the comparison with the 

other gages during the first rotations of the gearbox will quickly show, if this should be 

reversed. 

There is excessive scatter in the resulting, calculated slopes. The slopes calculated using

the two methods do not seem to show much correlation (Figure 18 and Figure 19). 

Therefore, we recommend using the population average for all gages except 4_3B, where 

the minus of the average slope should be applied initially. 
21 

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

1_1A1_2A1_3A2_1A2_2A2_3A3_1A3_2A3_3A6_1A6_2A6_3A7_1A7_2A7_3A

10_1A10_2A10_3A

4_1B4_2B4_3B5_1B5_2B5_3B8_1B8_2B8_3B9_1B9_2B9_3B

11_1B11_2B11_3B12_1B12_2B12_3B

[ue/lbf]

Static

Crane

Figure 18. Comparison of slope of static calibration and positive slope of crane calibration

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0

0.0010.0020.0030.0040.0050.0060.007

Static slope

postive crane slope

Figure 19. Positive slope of crane calibration versus the slope from the static calibration
22 

Table 7 shows the average slopes found from the two methods and the standard deviation 

of the mean. 

Table 7. Calibration results

Methods Mean StDev

[uε/lbf] [lbf/uε] [uε/lbf]

Crane (peaks) 0.004267 234.36 0.001036

Crane (valleys) -0.00169 -591.72 0.000661

Crane (range) 0.005961 167.76 0.001316

Static 0.004241 235.79 0.001244

In Table 8 and Figure 20, the slopes are separated for the A and B locations. The static 

calibration results indicate that perhaps separate mean values should be applied for these 

two groups. However, the crane calibration results do not support this conclusion (Figure 

20). 

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

Slope from static calibration [ue/lbf]

Positive slope from crane calibration [ue/lbf]

A gages

B gages

Figure 20. Found slopes separated for A and B locations

Table 8. Calibration results separated for A and B location gages

Methods Mean [uε/lbf]

A B 

Crane (peaks) 0.004193 0.004341

Crane (valleys) -0.002079 -0.001308

Crane (range) 0.006273 0.005649

Static 0.004716 0.003766
23 

After the calibrations were completed, the GRC data acquisition system was changed 

from the cDAQ platform that was used during the calibrations to an EtherCAT based 

system. The EtherCAT system no longer uses the strain gage bridge configuration to 

calculate microstrain, but instead puts out V/V. Table 9 gives the average slopes that 

would need to be used for the EtherCAT-based data acquisition system. 

Table 9. Calibration results for EtherCAT 

Method slope Slope

[lbf/V/V] [N/V/V]

Crane (peaks) 360.45 1603.26

Crane (valleys) -910.07 -4047.97

Crane (range) 258.01 1147.65

Static 362.65 1613.05

Issues with calibration

The following issues were found with the calibration set up, which likely resulted in the 

scatter seen in the calibration results: 

• Four point contact on bottom: There were four load cells on the bottom, an over 

constraint for a plane. It was decided to put the load cells on a sandwich of hardened 

plates, with grease in between, to add some compliance. 

• Four point contact on the top: The yoke had four rollers that contacted the dummy 

planet gear. This was over-constraint for a plane. It was decided to loosen the yoke a 

half turn to give it compliance. 

• Movement of the yoke rollers on their shaft: The rollers moved side-to-side on their 

shafts. Also, the bearings were not located in the center of those rollers. The rollers 

were not all mounted in the same orientation during the original assembly (some had 

the bearing offset to the outside and some had it offset to the inside). 

• Main shaft not hardened: The main shaft was supported on needle bearings. The main 

shaft acts as the inner race for those bearings. At high loads (80-90kip), the needles 

dug into the main shaft causing the whole setup to hang up (difficult to impossible to 

rotate). 

• Yoke shafts not hardened: The yoke rollers have needle bearings inside. The yoke 

shaft acts as the inner race for the needles. At high loads (80-90kip), the needles dug 

into the shaft making it impossible to rotate the outer race of the planet bearings. 

• Bearings were not axially constrained on main shaft. Due to the four load cells, it was 

nearly impossible for the load cells to attain an even reading while keeping the stand 

level. Small deviations in level caused the bearings to “walk off” the shaft during the 

crane calibrations and this caused a shift in the load from one side to the other. 
24 

Comparison with FEA model

Romax prepared a Finite Element Analysis (FEA) of the bearing grooves. The results are 

described in a separate report: “Max Qiao, Analysis of Static Calibration Test for Planet 

Bearing for NREL by Romax Technology Limited, 12/01/2009.” 

The strain sensitivity reported in that report was 0.006325uε/lbs. This number differs

from the number NREL found (0.004267 uε/lbs) for several reasons:

• Wrong groove geometry provided: The groove geometry provided by Romax did not 

include the flat spot in the center of the groove. Anecdotal information from some 

testing done on different groove geometries predicts a reduction of 20-25% in strain if 

the flat spot is present. 

• Gage factor: For the purpose of the calibration, NREL did not address the actual gage 

factor because the calibration was from the load to microstrain and back. For 

comparison with the FEA, NREL would apply the actual gage factor, which is 2.10 

instead of 2.0. Thus, the strain levels measured by NREL would need to be increased 

by 5%. 

• Strain averaging: The values reported by Romax were the strains at the actual strain 

gage centers. The strain gradient in the groove is quite large. By interpolating the 

strain, and averaging it over the actual location of the active part of the strain gage, it 

was found that a 5% lower strain would be measured by the gages. 

Accounting for the above deviations, the Romax slope is 0.004819 uε/lbs. The NREL 

slope was 0.004480uε/lbs. These values are about 7% apart.

Acknowledgements / Team

The following people contributed to the physical bearing calibration: Ed Overly, Troy 

Boro, Bill Gage, Braden Kappius, Hal Link, Mike Jenks, Scott Hughes, Jerry Hur, and 

Jim Adams. 
25 

Annex A Transformation from load cell readings to bearing 

loads

Background:

Shown in Figures A.1 through A.4, the bearing calibration jig is supported by four load 

cells located on the corners of the stand. These load cells are numbered 1 through 4 in a 

clockwise direction. Load cells 1 and 2 were on the west, load cells 3 and 4 were on the 

east. Load cells 1 and 4 were on the North.

Assumptions:

The loads on the bearing act through the center of the banana rollers. This is indicated in 

Figure A.1 

Figure A.1. Drawing showing basic load path

LC1_2 LC3_4

F_west F_east

s1

s2
26 

Two basic calculations:

• Moment around west bearing: LC1_2*(s1-s2)+F_east*(2*s2)-LC3_4*(s1+s2) = 0

• Sum of vertical forces: LC1_2 + LC3_4 = F_west +F_east

The load cells are located in the holes indicated near the corners of the base plate. To be 

able to install the load cells, the dimension of the hole was changed from the drawing 

below (figure A.2). 

Figure A.2. Base plate showing distance between loadcells

s1 = 524/2 = 262

Figure A.3. Dimension of yoke
27 

Figure A.4. Drawing indicated thickness of yoke end plate.

The bearing load acts at (d1+d2)/2 from the center of the jig

d1= 276.80/2 – 103.45 = 34.95

d2 = 276.80/2- 25.4 = 113

s2 = 73.975

Thus: 

LC1_2*(s1-s2)+F_east*(2*s2)-LC3_4*(s1+s2) = 0

F_east=[LC3_4*(s1+s2)-LC1_2*(s1-s2)]/(2*s2)=[LC3_4*335.975-

LC1_2*188.025]/147.95 

F_west = LC1_2 + LC3_4 – F_east

d1 

d2 

s2