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Old Jan 08, 2013, 08:08 AM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
18,096 Posts
There is a chance that you have not actually bent anything, you might have just pulled the shaft forward in the inner races and now the bearings are pulled together too tightly.

Look at these two sequences to see what I mean:

The right way to mount a prop:

end of shaft---> nut + washer + prop + washer + nut <---magnet housing top{

The wrong way to mount a prop:

end of shaft---> nut + washer + prop + washer <---magnet housing top{

The only difference in those two methods is that the nut that is closest to the magnet housing has been left off. That nut would normally be bottomed out where the threads end and the shaft becomes unthreaded.

In the wrong method prop is resting on the housing and as the nut is tightened it can pull the shaft forward through the inner bearing races. The pulling forces on the shaft is restrained at the back of the motor by a Circlip and that pulls the bearings towards each other and that is probably creating the drag you feel.

You may be able to relieve the pressure on the bearing by removing everything from the shaft, loosening the grub screw at locks the magnet housing to the shaft. That grub screw, incidentally, has probably slipped downwards on the shaft a short distance as the over-tightening happened.

Use a close fitting straight shank (not! ball ended) hex wrench to loosen the grub screw, if you use a ball ended hex wrench or loose fitting wrench it will probably strip the hex socket in the grub screw.

Just loosening the grub screw may provide some relief on the drag. If it does not, put the motor back in the mount, rest the mount on a block of wood that has a hole in it that is large enough to clear the Circlip. A hole that is about 5/16"/8mm or so is about right.

That will put the shaft point upward and the motor restrained on the block of wood. Put two nuts on the very end of the shaft and lock them together to protect the threads on the end of the shaft. Now use a small brass hammer and give the nuts a gentle tap. If that relieves the drag you are done. In a normal motor there will be a slight amount of play that will let the magnet housing move up and down, like one or two thousandths of an inch 0.025-0.050mm) or so.

Now you can replace the grub screw and you should put blue (not red!) Loctite on it when you do that. It is not a bad idea to upgrade the headless (1.5mm socket) grub screw with a button head screw that has a 2mm hex socket and the same 3-0.5mm thread. You can see one of those here:

And you can find them at the LHS in the Traxxas car replacement parts assortment or on eBay.

I never mount props on the DTA-750 motors by clamping them to the shaft with nuts. It is just a poor choice for getting adequate clamping force on the prop hub. I use wobbly prop savers and Thera-Band prop- saver bands as seen in the attached photos. And the full story on fiitting and using prop savers and making the bands is in this thread:

Prop Savers - Fitting, Using, Testing, & Making Thera-Band Bands -

I have hundreds of hours of flying time with prop mounted with those bands (using two or even three bands) and have not once ever lost a prop in flight. I hear the silliest stories from multi-rotor guys about how prop savers cannot be used because they cause vibrations, are not reliable or strong enough, and other silly reasons. Those are simply lame excuses.

If you fit a prop saver as shown at that link, use two or three Thera-Band bands, and have a balanced prop, the chances of losing a prop in flight are very remote. The bands are cheap (a fraction of a cent to a few pennies at the most) and can be replace occasionally for further insurance.

You have my sympathy on the lack of success on the search engine here. If you use it in the Advanced Search mode and make sure to set the date range for the search to Any Date... it will work a little better. But for most searches you will get better search results from google by using a search like " dt700".

If the above does not fix your problem, you may have actually damaged the magnet housing by deforming the bell. That will be harder to deal with. If you need help on getting the motor apart this thread will help:

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Last edited by jackerbes; Jan 08, 2013 at 03:37 PM.
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