Balanced bridge circuit, supplied with low-voltage direct current and connected to a readout device, measures remaining brush material, rate of brush wear, armature runout, and brush signature

Farrell, W. T.

Peek, C. R.

English

NASA Technical Reports Server

May 1970	 Brief 70-10266 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued-to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division. NASA. Code UT, Washington, D.C. 20546. 
Motor Brush Wear Measured with Strain Gages 
.
A strain gage transducer has been applied to mon-
itor brush wear in sealed electric motors which use a 
leaf-type brush support spring. Formerly it was 
necessary to disassemble the motors and measure 
the brushes mechanically. 
In this application, two strain gages are installed, 
one on each side of the spring. The gages are con-
nected to form two adjacent arms of a balanced 
bridge circuit. Two temperature-compensated preci-
sion resistors mounted on the motor case form the 
other two arms. When the bridge is supplied with 
low-voltage dc and connected to a suitable readout 
device, such as a microvoltmeter or oscilloscope, 
measurement of the amount of brush material re-
maining, the rate of brush wear, the armature runout 
and the brush signature is possible. 
The following steps are taken to prevent the calibra-
tion of the gages from changing during operation. 
The gages are bonded to the spring with high-tem-
perature cement, which is then cured for 24 hours at 
250°F and 20 psig. Following the cure, the tempera-
ture is cycled between room temperature and a level
25% above that encountered by the brush during 
normal operation to eliminate cement creep, the most 
common source of calibration shift. Cycling is con-
tinued until the installation shows no change in out-
put over the entire temperature range. The output 
voltage is then calibrated with respect to brush dis-
placement, and the unit is ready for use. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Code 207.1 
Greenbelt, Maryland 20771 
Reference: B70-10266 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. T. Farrell and C. R. Peek of 
Radio Corporation of America 
under contract to 
Goddard Space Flight Center 
(GSC- 10886) 
Category 01 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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Motor brush wear measured with strain gages

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