How to Rebuild an Astro Motor
By Kirk Massey (New Creations R/C)
First, be aware that ASTRO prefers that motors needing rebuilding be sent back to the factory.
They're not just trying to get the business, but are trying to ensure that their products
perform as they should. With this in mind I'm writing for those of you who are facing a
deadline for a meet, or just want to do it yourself. Although these motors are complicated,
there are some helpful precautions and guidelines to follow.
Start with a clean work place, making sure there is no grinding dust or metal shavings
around. Place clean paper towels on the work area. This helps keep parts clean and
makes it easier to spot and keep up with small shims and washers. First be sure to mark
the positive brush on the end bell if you are working with an older motor that lacks this
mark. A felt tip pen or permanent marker works fine.
Now locate the two scribe marks on the black motor case. They are usually by the negative
brush holder which should also have a scribe mark in front of it on the end bell. Scribe a
line on the black motor case that lines up with the mark on the end bell. This will mark
the timing location. Remove the brushes first and then the screws that hold the motor
Holding the black motor case, front end bell and armature, remove the brush end bell
being careful not to loosen the "wavy" copper washer that is normally on the rear of
the armature. If it is not there, look in the brush end bell as it sometimes sticks to
the rear bearing. If that's the case, tap the end bell open side down on your work bench
and the washer will fall off. Hold the black case and front end bell firmly while grasping
the armature by the commutator. Carefully pull the armature out of the motor case not
allowing the armature to be pulled into the magnets. (Note: Cobalt magnets are very brittle,
and the edges are easily chipped if struck by the armature.)
Inspect the motor case for loose magnets or any signs that the armature has been rubbing
the magnets. Also look for any foreign matter sticking to the magnets or in between them.
Inspect the commutator for burned segments. If one or more is recessed deeply or cocked
slightly, replace the armature. ASTRO recommends that the armature be trued if the
commutator copper has turned a reddish color from overheating, or if you replace the brushes.
Do NOT try to do this on a lathe by chucking up on the laminates of the armature because the
shaft is not in the exact center of the armature laminates. To do so will cause the
commutator to be cam shaped and will wear out the brushes very rapidly by oscillating them
in and out on every rotation and greatly increase arcing of the brushes. The correct way
is to chuck the armature shaft near the commutator and provide a counter sunk live center
on the other end of the armature shaft. This assumes that the armature shaft is true and
has never been bent. If the shaft is not true then your best option is replacing the
Check the bearings for roughness or dryness. Then remove them from the end bells by pressing
them out on a drill press or arbor press. Don't drive bearings in or out with a punch as
this will damage the bearings. After disassembly clean the end bells, the black body case,
and armature, but not the bearings, with motor cleaner, and blow dry with compressed air.
Press the bearings back in the end bells and install the front bell back on the black motor
case at the opposite end from the timing scribes. Install the same shims on the front of the
armature that you removed earlier. If you are installing a new armature start with one thin
shim. Carefully install the armature in the body case by holding the front end bell and
black body firmly and guiding the armature into the front bearing.
Now install the rear shims as they came off. On a new armature start with one thick washer
and then the "wavy" copper washer, and then the brush end bell. Holding the end bells firmly
in place pull the armature shaft in and out. There should be no loose free play. If there
is, add shims to the rear until there is no loose free play, yet with slight pressure on the
front shaft the "wavy" washer should compress about 1/64". This can be observed by watching
the armature shaft move in and out of the rear bearing.( NOTE: on older motors when installing
new armatures, you must pay close attention to the clearance between the commutator tangs and
the front edge of the brass part of the brush holder. The older motors are shorter, and you
must file the front lower edge of the brass brush holder to clear the tangs.) If the armature
won't turn when you first assemble the motor, or it rubs internally, don't force it as the
commutator will be damaged. Remove the end bells and place all the shims in the back and file
the brush holder until you have at least 1/64 " clearance. Be sure to keep the filings
out of the bearing by covering it with masking tape. Remove the filings before reassembly.
If at all possible keep the armature laminates centered on the magnets for performance. Line
up the timing marks and install the screws that hold the motor together. Then install the
brushes, capacitors, and leads. Connect the motor to a battery or power supply and check
for proper rotation; from the front of the motor this is counter clockwise on direct drive,
and clockwise on geared drive. The next step according to Bob Boucher of ASTRO is, more
than any other, where more people ruin their motors. So read carefully.
To Break in the Motor and Seat New Brushes:
ASTRO recommends running the motor at lower voltage with a smaller prop so that the motor
only draws 10 amps. To do this try installing a prop two inches smaller in diameter than
what is recommended for flight. For example, a Cobalt .05 on direct drive could use a 6x4
instead of the 8x4 recommended for flying. Reduce cell count if necessary to reduce the
Break in time varies greatly depending on the type of brushes in your motor. The FAI Drag
brushes #5044 are very soft and have low resistance. They can seat in a few minutes whereas
the #5045 and #5046 brushes take up to an hour to seat. After a few minutes of running,
check motor temperature. If it's uncomfortably warm, let it cool off before more running.
Also check the brushes. Brushes are fully seated when the wear pattern shows fully across
the brush face. The motor is now ready for retiming.
Remove the prop and loosen the retaining screws to adjust the timing. Apply power and find
the neutral timing point. (Rotate center case both directions to find least amps drawn)
Then, advance the timing by turning the black motor case in the same direction as the motor
shaft is turning, until the current draw goes up by 2 amps for sport wind motors, and 4 amps
for FAI motors. For example, on a standard .05 motor the neutral draw might be 3 amps.
Advance until the draw is 5 amps. Then re-tighten the screws holding the motor together.
NOTE: there are two sets of tapped holes in the brush end bell on newer motors to receive
the screws holding the motor together. The left hand holes are for direct drive use and the
right hand holes are for geared drive. Now you are ready to test your refurbished motor in
your latest creation. Enjoy!
You will need an accurate ammeter to set the timing. Some better multimeters read to 10 amps.
Astro and AI/ROBOTICS are two places with amp/voltmeters for motor testing. For those
short on cash a shunt allowing amps to be read on your voltmeter is available from Steve
Kowalsky, 32 Hollo Dr. Hollbrook, NY. 11741 for $10.00 pp. It comes with Sermos connectors,
Astro connectors available at extra cost.
This article lifted (stolen) from the NOTES newsletter of the DEAF club, Frank Korman editor.
January 1994 issue. A phone call to Kirk confirmed OK to share with all.