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Hurst 1/2 RPM Motor Needed

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#1 JohnH

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Posted 17 July 2008 - 11:00 AM

I need to replace this motor as the one on my mount no longers works due my vehicle being involved in a traffic accident.

Either the gearbox or the motor itself are cooked, I don't know. The motor isn't shorted and each pair of wires has similar resistance, but it just makes a faint buzzing noise and heats up.

The exact model number in CA and the part is a 990341.

I know some motors can be rebuilt but who to send this to?

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#2 Skywatchr

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Posted 17 July 2008 - 12:26 PM

Try here.. Hurst Motors

#3 JohnH

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Posted 17 July 2008 - 02:16 PM

I did try Beckwith, but they said it would be 6-7weeks right now, and I need my mount working for Table Mountain in 2 1/2 weeks.

I couldn't find any other who have it in stock.

#4 Skywatchr

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Posted 17 July 2008 - 06:39 PM

:foreheadslap: That figures! All I have is a Model DA and it's a 1 RPM.

Jeff

#5 Tom and Beth

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Posted 17 July 2008 - 08:19 PM

Hi John,

Did you try removing the motor to fault isolate it (run it with no load). You COULD have jammed the worm or other portion of the drive.

EDIT: This place CLAIMS to have them, call for price
http://codemicro.com...?partid=1171728

#6 Skywatchr

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Posted 17 July 2008 - 09:10 PM

When a company hides their prices, and make you contact them for a price, it usually means it's way overpriced. They want you to talk to a salesperson so they can throw a pitch at you. At least that has been my experience every time I've inquired that way. YMMV

Jeff

#7 Tom and Beth

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Posted 17 July 2008 - 10:21 PM

Roger that, Jeff.

And yet if the other guy absolutely, positively HAS to have a new motor....

#8 Skywatchr

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Posted 17 July 2008 - 10:33 PM

Yep, sometimes you get stuck between a rock, and a hard place. :help:

Jeff

#9 JohnH

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Posted 18 July 2008 - 02:34 PM

The worm turns fine with the motor and attached gear removed. I can easily move it with little resistance with just one fingertip even with 30 lbs of scope, counterweight, spotter and a 41mm Panoptic.


I tested the leaded for the inputs to the motor and they have a high resistance, but not out of line for something that is only rated for 5 watts.

After removing it, I tested the various parts of the external circuit (switch, bulb, capacitor and solder connections) and found they have continuity.

I plugged it back in, still removed so no load on the gear, and it did not turn, it just got hot to the touch.

PS You have to admire whoever machined this mount. The little block of aluminum that he machined out to hold the wiring took at least 18 separate steps on a milling machine.

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#10 Tom and Beth

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Posted 18 July 2008 - 04:27 PM

You probably already surmised this, but your motor is dead. That is GOOD news, because an issue with the worm/drive is not as easily rectified.

BTW, don't throw any of the motor stuff away. You'll need it for when you get the replacement.

Good job on the Fault Isolation, John!

#11 jwheel

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Posted 18 July 2008 - 04:52 PM

You might also try contacting the Hurst Manufacturing Company

http://www.hurstmfg....stmfg/index.jsp

I needed a replacement motor for my mount about a year ago and was able to get one from them.

Joe Wheelock

#12 astrokido

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Posted 18 July 2008 - 04:53 PM

Let's see: the motor isn't jammed and you're getting current through both coils, both healthy signs. Have you tried replacing the capacitor?

#13 gnabgib

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Posted 18 July 2008 - 05:30 PM

John;
Perhaps the lid on the end got jammed on to far during the accident and is preventing the armature from turning. The lid is just a light press fit so pushing it off a little bit should be fairly easy. Or just remove it to see if the armature is spinning freely. A stuck armature would cause the motor to heat up rather quickly!
My .02 worth
Kevin

#14 DAVIDG

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Posted 18 July 2008 - 05:47 PM

Before you declare the motor dead, you need to test the capacitor with a cap checker. You have a reversible motor and if the cap is bad it will do just like you describe, hum and get hot.
I have the same motor and just checked the resistance on the windings. Both Red/Black and White/Black have a resistance of 815 ohms. There is no continuity between the windings. One sign that the cap is bad is continuity thru it when it is electrically isolated. The cap needs to be 0.5 ufm at 250 volts or higher and non polarized. I would also double check the plug and power cord to be sure your getting power to both black leads of the winds. I would check continuity from the prongs on the plug to the each of the windings. You should have low resistance from the hot side of the plug to the both black leads and also low resistance from neutral prong on the plug to either the white or the red lead. If you have low resistance to the red lead you should have very high resistance to the white lead and the condition should reserve when you flip the switch.
www.herbach.com has your motor but it's $170 !

- Dave

#15 JohnH

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Posted 18 July 2008 - 09:57 PM

OK I'll run through the check once more:

Power from plug makes it inside.

Capacitor tests at 504.5 nF which is .5045 microFarads and is rated to 500V.

Resistor tests at 74.5 kOhms. Somewhat irrelevant, as this is only useful for the weird little neon blub that serves as a 'power on' indicator.

Red/Black and White/black leads test at 753 and 760 ohms respectively.

The aluminum cap comes off easily, so that just leaves the gearbox being physically jammed.

I did find the entry for the motor at the site you gave me. However, they say it will be 6 to 7 weeks to get one. I do have a call in to several other distributors, but so far I haven't heard a thing.

I was digging around in a volume of Sky and Telescope for the year 1977, and found a nearly identical setup, with even the same motor. The only difference is it used a three position switch for those frequent trips to the southern hemisphere.

#16 DAVIDG

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Posted 19 July 2008 - 01:28 PM

Is the switch a double pole, double throw, center off, type or just an On/Off ? The motor is reseversible so if the gear box is jammed you might get lucky and by changing the direction get it to move and free it up. If the switch allows this great, or just flip the position of the Red and white leads and that will change the direction. Good luck.

- Dave

#17 JohnH

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Posted 19 July 2008 - 03:35 PM

The switch has three electrical connections but only two positions.

I tried reversing the poles like you suggested but there is no change. That 6-7 week wait is looking better and better all the time..........

#18 Skywatchr

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Posted 19 July 2008 - 06:54 PM

Boy, it's too bad you couldn't be able to find a decent DC servo motor to replace the AC motor. There has gotta be some way to get it back up and running in less time.

Jeff

#19 DAVIDG

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Posted 19 July 2008 - 08:46 PM

Just to be totally on the safe side I would replace the cap. In a couple of very rare ocassions I have seen a cap check OK using a tester but be bad when high voltage is applied.
I just pulled the back cover off of my motor and right below the top coil is the armature. I can stick a small screwdriver in the gap and move the armature. You might want to see if you can do this on your motor. Maybe just moving it will free something and get you back in business.

- Dave

#20 JohnH

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Posted 20 July 2008 - 10:16 AM

The motor is a pretty tight affair, without too much room.

I can see a plastic spacer of sorts underneath the two plastic discs that cap the top winding and some metal under all of that.

I'll go get a thin, sharp punch to pick at that and see if that moves at all.

Nope, it doesn't move. It either is jammed or isn't meant to move. Hmmmmmmmmmmmm.

"These kinds of puzzles, do not fit together."

#21 Eddgie

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Posted 20 July 2008 - 11:00 AM

Well, if impact jammed it up, my bet is that impact will un-jam it.

I would try tapping it onto the head of a hammer from different orientations.

I mean at this point, it sounds like you don't have anything to loose.

These are very small gears and the torqe of the motor is greatly reduced trough them so by the time you get to the output, the torque is so reduced that even a tiny mis-alignment might bind them up.

I wouldn't pound it, but I sure would try doing some "Taps" against an anvil in different orientations and try the motor after each run.

I might also simly try getting something snug around the output gear and forcing it forward and back. Maybe protect the teeth between a couple of peices of wood and use a vice grip. Just torque it back and forth a bit to try to transfer the energy back through the gearbox to unbind it.

In fact, I might try this first....

Good luck.

#22 JohnH

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Posted 20 July 2008 - 12:22 PM

Your last message gave me the clue I needed (and the encouragement) and I used a spade bit (bad use of a not very accurate tool) and I pried up the three tabs that appeared to hold the top winding and the soft iron the other part is made of (it holds the magnetic fields needed for the motor to operate) and actually look directly. I reasoned that the motor was toast, and I had little to lose by playing around.

There is indeed a rotating flywheel underneath, which spins very freely with little resistance. I held the motor up by just the gear, and tried spinning this with a fingertip to see if the torque gets transfered through the gearbox.

The drive gear did turn, so the gears work and the trouble must lie with the part I removed.

The upper coil was held by six 'petals', three above and three below, that are folded down to be press-fit inside the aluminum shell of the rest of the housing. Three of these are just short ones that have holes for the tabs that are folded in from the cap to cover the insides, and the other three extend down further. They all extend around the rotor and have a fairly close tolerance, around 1/32".

This part had been compressed when the mount slid into the seat back of my van. I made sure these legs were straight and put the coil back.

The disc still didn't rotate freely, so I checked to see if there was still clearance around it. There was, so I pried it up a bit and this time, the disc would spin freely. I pulled the winding again, and all the legs had no protruding bits. I looked at the part and noticed that the legs were squashed down slightly so I used the drill bit again to raise them, and I made the legs of it right angles.

Now it would spin freely with the winding placed. The motor was binding on the top.

I connected it to the circuits of the mount and it works properly now. I don't know if it tracks at the right speed, but I'll find out tonight.

Thanks for all the help on this people. See you at Table Mountain.

PS. Now if I can just fix the van as easily. ;)

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#23 Skywatchr

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Posted 21 July 2008 - 01:07 PM

Ah, heck, a little duck tape and the van will be good as new! :roflmao:

#24 JohnH

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Posted 21 July 2008 - 01:52 PM

Regretably, the van is a company vehicle and has to be presentable at all times. Summers are slower, so I have time to get it fixed up.

My insurance rates are another matter however........

#25 JohnH

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Posted 23 July 2008 - 10:02 AM

Followup: The Codemicro people finally got back to me awith a price for the motor in question and they do have one in stock for $170.93, about $23 cheaper than any other place I looked. I'm actually glad they took so long, as I time to tinker and ask questions here and I actually learned that this mount isn't actually a scary thing deep down, just simple, accurate and elegant.

I might just buy one to have a replacement on hand for an emergency. I still have to test the mount the, but I haven't had a clear night since I passed on the evening when I fixed it.






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