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  #1  
Old Thu 01 November 2007, 10:52
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Dust-proofing stepper motors and consequences of dust ingress

To Gerald:

Maybe I'm crazy but it seems like I recall you instructing someone to ensure that their stepper motors were oriented so that the wires leading into the body were on the bottom to reduce dust contamination and bearing failure. I searched and searched for the post but couldn't find it. If you did indeed made that post, then the drawings for the geared motor mounts will need to be changed. My motors can only be mounted with the wires on the side so I'm planning to put a small dab of silicon sealant on the hole.

If you never made that post then ignore this one.
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  #2  
Old Thu 01 November 2007, 11:12
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
As I look at my 7.2 Gearsteppers on the bench, your correct about orientation, but due to the alignment and swing angle to the rack, we are subject to only that orientation. The non-geared folks can rotate their motors in 90degree clocks and maintain centerline. down is good, sideways not so bad, up..pretty bad. I will silicone my entry point as well! Sean
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  #3  
Old Thu 01 November 2007, 12:05
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Doug & Sean, it is something like the wire entry orientation that should be on an assembly drawing. Might sound like something small, but we did seize up a motor because of that, and there were no spare motors on this continent. (Against all advice, the motor was opened, cleaned out and bearings replaced).

I didn't think that the geared motors don't have the options on wire orientation. But then I seem to recall that the lone geared motor I have seen had a reasonable seal and clamp for the wires? (Our un-geared motors are fairly open)
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  #4  
Old Thu 01 November 2007, 13:39
Alan_c
Just call me: Alan (#11)
 
Cape Town (Western Cape)
South Africa
Send a message via Skype™ to Alan_c
Is it not possible to seperate the motor from the gearbox and swing it 90 degrees and rebolt it - that should not expose the inside of the motor.

Maybe a dumb question never having handled one, but its seems simple enough?
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  #5  
Old Thu 01 November 2007, 13:49
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Alan, people are going to tell you the sky will fall if you do that!

But, I was curious and peeked inside our one geared motor - the motor shaft has teeth cut directly into the shaft and this is the input pinion gear. The backlash may be affected if undertaken without wearing those white Japanese gloves . . . . .

Ours had lots of loctite on the screws - the motor didn't come off willingly. But it didn't seem to come to harm after re-assembly (This was over a year ago)
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  #6  
Old Thu 01 November 2007, 20:30
driller
Just call me:
 
Quote:
Originally Posted by Gerald D View Post
Alan, people are going to tell you the sky will fall if you do that!

But, I was curious and peeked inside our one geared motor - the motor shaft has teeth cut directly into the shaft and this is the input pinion gear. The backlash may be affected if undertaken without wearing those white Japanese gloves . . . . .

Ours had lots of loctite on the screws - the motor didn't come off willingly. But it didn't seem to come to harm after re-assembly (This was over a year ago)
NEVER-EVER-EVER take apart a stepper motor.

But the gear head is a separate part. you should be able to unbolt the screws and without separating them, just rotate the motor, and then bolt it back together.

If you find one screw is longer than the others, make sure it goes back in the same gear box hole. not the same motor hole.

A word of minor caution. I had separated a gear head from a servo motor and the unit was sealed with a gasket. if you find such a gasket, you can get a replacement at the auto-parts store.

Oh, and did I mention to NEVER-EVER-EVER remove the end of the stepper or remove the stepper armature ?

Dave
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  #7  
Old Thu 01 November 2007, 21:07
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
Of course now you have to tell us why to NEVER EVER do that.
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  #8  
Old Thu 01 November 2007, 21:13
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Gerald,

I looked at one of my geared motors tonight and I could see something that looked like windings through the hole where the wires entered. What do you think about dabbing on a little bit of silicon? Of course, I would be careful to make sure it didn't enter the motor.
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  #9  
Old Fri 02 November 2007, 00:08
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Dave, trust me, you can pull the armature out of an Oriental Motor PK299 and live to tell the tale. Got the t-shirt. Sky didn't fall.

In fact, have replaced bearings on 3 of our original steppers now already - the first one I pulled the armature before I read about the dire consequences, so for the next two I managed to change the bearings without pulling the rotor out. All the motors are still as good as the 4th one. (It would seem that older magnet technology was very fragile - modern magnets don't suddenly go off).
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  #10  
Old Fri 02 November 2007, 00:11
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The story is here:
http://www.talkshopbot.com/forum/messages/26/7470.html
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  #11  
Old Fri 02 November 2007, 06:06
driller
Just call me:
 
Two reasons.

first, the magnets inside are charged and lose charge when removed. the coil and rotor are in a harmony of magnetic balance. if you separate them, you let the cat out of the bag.

I go back to a test Mariss did, I think he found about a 20% loss in motor power after disassembly and re-assembly.

Second, unless you are set up to control the armature, it will smack hard into the case when you try to pull it out.

that will damage the unit and may generate magnetic splinters. you can get them off while the parts are separated, but you have to grind the parts together for re-assembly.

If you have an old stepper, one you are willing to throw away, try it..

And, if you want to play with your new steppers, try this. This is really neat !
connect two motors, wire for wire. nothing else.

now, rotate one motor and watch the other. it is one-for-one. the power generated by the magnetic field on the one you turn will power the other motor. has to to with induction coils and magnetics and such. Also shows how as a magnetic filed is generated and collapses, it releases electricity. And that goes to why you cannot use full motor amps from the power supply. some current comes from the power generated by the coil not being powered as the magnetic field collapses.

It is totally a harmless test, and in fact, early steppers were used as remote control units by wiring them up like that.

Another neat trick it so connect two wires of any coil together to lock the motor. take a new motor, spin it, you will feel the cogging. now, connect any two wires that are on the same coil together and presto ! the motor is locked.

I am easily amused.

Dave

Last edited by driller; Fri 02 November 2007 at 06:11..
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  #12  
Old Fri 02 November 2007, 06:24
driller
Just call me:
 
Ah, I don't know if Mariss ever did the test on the newer motors.

here is his post on the older motors.

http://groups.yahoo.com/group/CAD_CA.../message/56536


Hi,

When I don't know for sure, I run an experiment:-)

1) I took two identical motors, (Rapidsyn 23D6209, 4.7A @ 1.7VDC)
connected in full-winding mode and set the current to 2.4A per phase
on a G201. The power supply was 25VDC.

2) I removed the rotor from one motor, waited 5 seconds, then
replaced it. The other motor was unmolested and would serve as the
baseline unit.

3) Both motors were then run on a dyno and data was taken at 5 revs
per second (1,000 full steps per second).

4) The baseline motor delivered 18.25W to the dyno at that speed for
a torque of 82.3 in-oz.

5) The molested motor delivered 11.75W for a torque of 52.9 in-oz.

6) The calculated torque (and power) output for the disassembled
motor was 64% of the intact motor.

7) I removed the rotor again, leaving it out overnight before
reassembling it for the second time and ran the above tests again.
There was no additional deterioration in performance.

Conclusions:

1) Demagnetization occurs immediately on disassembly.

2) The motor loses about 1/3 of its holding torque and and power as a
consequence.

This experiment applies to standard magnet motors. I have not tried
it on the newer, rare-earth magnet motors. I like the ones I have too
much to find out. Any volunteers?

Mariss


[follow-up post]


Re: A little experiment on demagnetization

Hi,

An addendum to the previous post. Something bothered me about the
data. The motors I used for the test came from a very nice 4" X,Y
table I got from Tim Goldstein made by New England Associates
Technology.

Both motors had been modified by NEAT so that the wires came out the
front of the motors rather than the rear as is usual. That is the
reason I didn't want the motors after I re-motored the table with
some really nice Vexta PK268-03A "square" motors (sorry Tim).

The thing that troubled me was 82 in-oz is low for that motor.

I looked thru my vast motor harem until I found another 23D6209
motor, this time with conventional leads.

I ran the same setup on this motor and got 23.5W at 5 revs per second
for a torque of 106 in-oz. That is more in line with what I would
expect for that motor.

I would conclude both motors had been partially disassembled (and
demagnetized) to make the wiring modifications. The actual torque
loss is more like 50% rather than 33% when compared to a know good
motor (106 in-oz vs. 53 in-oz).

Mariss
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  #13  
Old Fri 02 November 2007, 06:25
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
Dave,

Now I know. Thanks.

Marc
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  #14  
Old Fri 02 November 2007, 11:52
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
For those wondering how they assemble the motors if they are so fragile, the answer is that they are magnetized after assembly. De-magnitized motors can be re-magnetized with special coil and a couple of thousand amps for a very short period. I can't remember how many thousands of amps, but it is a huge number.

Another fun test for a stepper: turn shaft by hand and feel how easy it is. Now twist all the lead-wires together and try again . . . . .
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  #15  
Old Fri 02 November 2007, 12:18
zetacnc
Just call me: Fabio
 
Atibaia
Brazil
Hello all,

I don't know some of you tried to dismantle a stepper motor, but when you are about to extract the rotor, put a steel collar a little wider than the rotor diameter and almost the same rotor heigh just in front of the openned motor, and slip the rotor to inside collar...

This way you do not break magnetic flux - looks like you maintain a kind of "magnetic short circuit", like that cheap "horseshoe magnetics" we buy for your sons...

Do not try to extract rotor from steel collar, or the magnetic flux will break...

Now you can even rewind some burned pole, or change a ruined ball bearing...

To close again the motor, just position the collar with the rotor inside in the top of the openned motor, and slip the rotor do it's original position and close it...
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  #16  
Old Wed 07 November 2007, 02:15
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
While visiting this thread to check for drawing updates, the wire entry thing struck me in a new place......do NOT seal the hole airtight. These motors cycle hot/cold and thus they need to breathe as the air inside expands/contracts. Seal this hole and you force it to breathe somewhere else - possibly via the bearing shields and the lightly greased balls.

My theory is that we were fond of using high pressure air to blow the dust off everything. With the wires originally pointing up, a puddle of dust lay there and it was tempting to "blow the dust out of the motors" - we just blew it in. Orientating the entry down removes the urge and the puddle.

The other factor is that I am hesitant to say use silicon on an expensive motor that is under warranty. But the OEM couldn't object if I said to orientate the entry away from dust and blowguns. (Some silicon RTV's are acetic - they cause corrosion. I "potted" something in this acid silicon once before and it was a disaster - the tracks lifted off the pc board)
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  #17  
Old Wed 07 November 2007, 03:08
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
The oriental geared steppers came with a nice bundle of wire in their harness with no unified outer jacket. I will just slide the shrink wrap tube up close - maybe a little in- the motor before I shrink it all clean and closed. Thanks for the advice.
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  #18  
Old Wed 07 November 2007, 10:30
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Thanks Gerald. I rarely make a move without approval from you or Mike Richards so I haven't plugged the holes yet. I'll try the shrink wrap too and will be careful with my compressed air. I'm finally getting a weekend off so maybe I can get some pictures posted even though I don't have the logos yet.
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  #19  
Old Wed 07 November 2007, 22:31
tpworks
Just call me: Tom
 
Atlanta, GA
United States of America
couldn't you just remove the screws to the gearhead turn the motor and replace the screws?
Tom
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  #20  
Old Thu 08 November 2007, 00:15
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Sorry Tom, I posted in the wrong thread and you didn't see the preceding discussion there. Over here in this thread you can see we discussed that rotation before.

I have a horrible feeling this discussion jinxed it for Daya in Sri Lanka - see his woes in this thread today.




Quote:
Originally Posted by Gerald_D View Post
Were your stepper motors open on the back? I strongly suggest you put a label/tape over those holes because the dust gets into the bearings. That hole traps dust and your operator will try to blow it out with compressed air - he will force the trapped dust into the bearing. (I had to replace bearings on my first stepper motors because of dust).
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  #21  
Old Thu 08 November 2007, 06:19
driller
Just call me:
 
Mounting the motors on the gear head so the wire is down is a good idea.

putting tape on the bearing to prevent dust is another good idea.


The thing I don't like in the pictures is the hanging wires. they should be in a cable guide like the Igus. the wires will last LOTS longer in a cable tray.

For the loose steppe wires, I had stripped some cable TV wire, pulled out the soft center and used the outer plastic on my stepper wires. keeps them together nicely.
Thread drift.

I too have a gap at the motor. I am thinking about a rubber boot of some kind. Something flexible, but not air tight. seems some of my tools have something like that, I am thinking I could take a boot, use a cable tie and keep the dust out.

Dave
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  #22  
Old Thu 08 November 2007, 08:20
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by driller View Post
Mounting the motors on the gear head so the wire is down is a good idea.
But we can't disassemble the motor gearbox while it is under warranty . . . .
The current motor plates for the X & Y axes can take either geared or ungeared motors. For the new drawing set, I added as an option a plate purely for the geared motor. I will look at revising that plate to solve the "problem".

Quote:
Originally Posted by driller View Post
putting tape on the bearing to prevent dust is another good idea.
The pic of Daya's motor is a mystery (almost). Oriental supplies all their motors with a sticky label covering the hole. In Daya's case, someone removed the label. I think that all the other owners of the geared motor will confirm that the hole is closed.

If you look closely at the pic, there is a recess just behind the cover where the dust gets and lies in. (This recess is the OD size of the bearing). Imagine putting a blowgun in there to "blow the motor out" . . . . that accumulated dust is then forced into the bearing shield at high pressure.
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  #23  
Old Thu 08 November 2007, 11:47
soulvoid
Just call me: Håvard
 
Stavanger
Norway
Quote:
Originally Posted by Gerald D View Post
The pic of Daya's motor is a mystery (almost). Oriental supplies all their motors with a sticky label covering the hole. In Daya's case, someone removed the label. I think that all the other owners of the geared motor will confirm that the hole is closed.
Hi Gerald,

My motors from Motion king also have the open hole at the back. Really bad idea removing the label there if there was one there. Easy to cover that hole though.

I'm more worried about the front myself. Someone suggested sealing the front with an oil seal or by using silicone or acrylic mastic, wait till it hardens and then spin the axle by hand to loosen it. It will seal some for sure, but I don't know about longevity. Also I'd worry about the silicone or acrylic getting into the bearing at the front. Been thinking about a thin rubber gasket between the motor and the mount with an undersized hole for the axle would do the trick (for some time atleast).
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  #24  
Old Thu 08 November 2007, 12:10
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
V-Rings are the answer:

In Europe, Forsheda is the well-known brand:
http://www.allsealsinc.com/forsheda.html

I seem to remember that the correct size for the motor's half inch shaft was still too small to seal the hole in the casing. We put a heat shrink sleeve or two on the motor shaft and used a bigger V-ring to go over the heatshrink.

The geared motors apparently don't have the same dust issues - the backs are sealed with the label and the pinion end has a good seal?
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  #25  
Old Thu 08 November 2007, 13:19
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Oriental pk296A2A-SG7.2
Shaft tolerance really close, shrink tube or v-seal should be fine.
Aft end of assembly is almost completely sealed, except for wire entry point.
Attached Images
File Type: jpg gear_frt.jpg (31.5 KB, 409 views)
File Type: jpg gear_back.jpg (52.9 KB, 412 views)
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  #26  
Old Thu 08 November 2007, 19:09
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Useful pics Sean, thanks.

The direct-drive Oriental Motor PK299 series:
Dust.jpg
....gap about 2.5mm [0.1"]

Let's get some perspective here . . . . . . .

Daya's case in Sri Lanka is only a suspicion of dust, nothing confirmed. Suspicion based on large and blowgun tempting hole in motor.

Our case of dusty bearings appears to be absolutely unique - have not seen any other report of this. Could be because we sealed the whole table in a dust room where everything got dust-laden and then at the end of every day blew it all clean with 10bar [150psi] blowgun. Ours were direct drive motors with large openings around the shaft.

So, the chances of dust ruining the bearings is rather small. This thread is about reducing the 1% chance down to 0.1%. V-Rings are cheap. They work rather well - even on garbage trucks
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