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Hurst Synchronous Motor Malfunction?

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#1 Rustler46

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Posted 28 October 2020 - 03:38 AM

Hello,

 

Recently I've been working on returning to service my home made 8-inch, f/4.8 reflector. The mirror was finished in 1967. A friend made the fiberglass tube around that time. The mount went through several iterations before attaining its present form. This is how it looks in 2020.

 

8%22 RFT-EQ-02375.jpg

 

The mount was finished in its present form around 1985, for use in photoelectric photometry of variable stars. But this mount and telescope have seen limited use in recent years. A while back when I assembled the mount, the Hurst synchronous motor appeared to have died, because I didn't hear its normal sound. So the motor was removed. Now I have discovered that the motor still functions, but with perhaps a gummed up gear train. 

 

When I first got it going again, I applied some torque to the output shaft, and it began rotating, albeit with some difficulty. So I applied some silicon spray lubricant to the output shaft and let is rest over night. The next day I brought it inside the house to warm up, even placing it in a sunlit window. It seemed to run OK. So I left it running for a while. Later it seemed quite warm to the touch. So I pulled the plug, and set it out of the sun. Later in the evening I ran it for a few hours. And it again became quite warm to the touch - not hot enough to burn, but too warm to continue holding for a long time.

 

So this leads to my question:

 

Is it normal for such a motor to run so hot? 

 

If not perhaps it is being loaded excessively by the gear-train. The output shaft is not loaded. Here are some photos of the motor assembly:

 

EQ Mount-02377.jpg

I believe the motor is the upper assembly with the internal gear train below.

 

EQ Mount-02378.jpg

This motor was supplied with a Mathis 6-inch worm gear set.

 

EQ Mount-02381.jpg

This shows where the chain drive from the Hurst motor attaches to the worm.

 

Any help you can provide would be appreciated.

 

Best Regards,
Russ

 

 

 

 



#2 BobT

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Posted 28 October 2020 - 06:42 AM

I think your first suspicion, gummed up gearbox, is correct.  Being in storage that long, the grease has probably solidified.  I have some old Hearst geared steppers but I don't remember if the gearboxes can be disassembled or not.  Hopefully you can de-gunk them.  If not, and you can't find a replacement, I suggest you take a look at converting to the DIY OnStep system.  I've built two versions of OnStep, the first driving a converted 1960's StarLiner mount and the other currently driving my Losmandy G11.  Excellent project!.

 

BobT

 

Edit:  Nice mount!


Edited by BobT, 28 October 2020 - 06:43 AM.

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#3 Simcal

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Posted 28 October 2020 - 06:43 AM

Found this: http://www.hurst-mot...om/ca_dd_g.html. Do the model numbers match up ( in the hurst table )?

 

Perhaps call them to determine if it's serviceable or if there is a replacement?

 

Hurst Manufacturing
1551 East Broadway Street
Princeton, IN 47670
(833)266-4332 Toll Free
(812)386-7504 Fax

 

Problem, and thus heat, could be any of:

 

- reduction gears are gummed / locked up with old goo (that's the housing attached at the drive end of the motor )

- capacitor is shot and needs replacement

- windings are shorting due to deterioration of the insulation

- drive axle is gummed up with old lube goo


Edited by Simcal, 28 October 2020 - 06:45 AM.

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#4 Couder

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Posted 28 October 2020 - 07:32 AM

If the motor was that warm to the touch, I would think at least some of the grease would have melted. I had a clock shop for over 30 years, and generally once motors like that acted up, they were not reliable. Of course there are exceptions but I said generally not always.


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#5 Rustler46

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Posted 28 October 2020 - 03:05 PM

Found this: http://www.hurst-mot...om/ca_dd_g.html. Do the model numbers match up ( in the hurst table )?

 

Perhaps call them to determine if it's serviceable or if there is a replacement?

 

Hurst Manufacturing
1551 East Broadway Street
Princeton, IN 47670
(833)266-4332 Toll Free
(812)386-7504 Fax

 

Problem, and thus heat, could be any of:

 

- reduction gears are gummed / locked up with old goo (that's the housing attached at the drive end of the motor )

- capacitor is shot and needs replacement

- windings are shorting due to deterioration of the insulation

- drive axle is gummed up with old lube goo

Thanks for your comment, Simon. 

 

I don't know if the little gearbox has any grease at all in it. But if so, the gearbox isn't made to be easily opened. But after some exercising the motor some more, the amount of heat being produced is reduced. So if grease is present it's being massaged into a better consistency.

 

As for the capacitor, it was in used condition when attached to the motor 35 years ago. It looks to be an electrolytic. It measures 0.65 μfd @ 100VDC. So perhaps being exposed to 120 VAC it is not the best for the purpose. The motor indicates 0.5 μfd should be used. So I'm looking to find a replacement in my parts bin. I'll see if that helps with the heat. Seems like a ceramic, 250-volt replacement would last longer. Comments from electronics persons?

 

Also some more silicone spray lube might be of help on the shaft that is accessible. The motor is 1/2 RPM, so I'll time the rotation to see if it meets specs.

 

Edit:
I just ordered a replacement capacitor - a ceramic, 0.47 μfd, 250 volts.  While the actual cost to manufacture that component is much lower, 3 of them plus handling/shipping cost me $2.65 each. I got three for spares, and given the ± 10% tolerance, one of the three might be closer to the designed 0.5 μfd. But I'm pleased to have them on the way to arrive by this weekend. I could have gone to the closest Radio Shack (perhaps the only one left in the USA). But with the 50-mile round trip, well the online convenience was my preferred method. Also I didn't need to mask up and risk health (mine or that of others). A new capacitor should ensure that part of the situation is as designed.

 

So progress is being made, thanks to suggestions on the forum such as yours.

 

Kind Regards,

Russ


Edited by Rustler46, 28 October 2020 - 09:30 PM.


#6 Rustler46

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Posted 02 November 2020 - 12:15 AM

Thanks for your comment, Simon. 

 

I don't know if the little gearbox has any grease at all in it. But if so, the gearbox isn't made to be easily opened. But after some exercising the motor some more, the amount of heat being produced is reduced. So if grease is present it's being massaged into a better consistency.

 

As for the capacitor, it was in used condition when attached to the motor 35 years ago. It looks to be an electrolytic. It measures 0.65 μfd @ 100VDC. So perhaps being exposed to 120 VAC it is not the best for the purpose. The motor indicates 0.5 μfd should be used. So I'm looking to find a replacement in my parts bin. I'll see if that helps with the heat. Seems like a ceramic, 250-volt replacement would last longer. Comments from electronics persons?

 

Also some more silicone spray lube might be of help on the shaft that is accessible. The motor is 1/2 RPM, so I'll time the rotation to see if it meets specs.

 

Edit:
I just ordered a replacement capacitor - a ceramic, 0.47 μfd, 250 volts.  While the actual cost to manufacture that component is much lower, 3 of them plus handling/shipping cost me $2.65 each. I got three for spares, and given the ± 10% tolerance, one of the three might be closer to the designed 0.5 μfd. But I'm pleased to have them on the way to arrive by this weekend. I could have gone to the closest Radio Shack (perhaps the only one left in the USA). But with the 50-mile round trip, well the online convenience was my preferred method. Also I didn't need to mask up and risk health (mine or that of others). A new capacitor should ensure that part of the situation is as designed.

 

So progress is being made, thanks to suggestions on the forum such as yours.

The new replacement capacitor arrived yesterday. It replaces the failed one that was the size of a AA battery. The new one is about the size of a single split pea. Times have changed in 35 years. The new capacitor has been soldered into place. That was indeed the problem. It runs much quieter, and with very little heat buildup - just slightly warm to the touch. I'm glad it didn't destroy itself from being too hot with the defective capacitor.

 

While waiting for the replacement cap I kept lubing the output shaft with multiple applications of silicone and WD-40. Things are running smoothly now. I timed its rate of rotation. With its designed 1/2 RPM it took almost exactly 2 minutes for I rotation - 1 minute 59.6 seconds. It can't be much better than that.

 

I did a rewiring of the junction box where the 120 VAC input cord meets the motor wires and capacitor. Previously the plug had 3 prongs. But the cord did not have a ground conductor - my bad. That has been replaced with the correct 3 conductor wire. This ground conductor will be electrically connected to the aluminum parts of the mount, which will also ground the motor itself. At my observing site in the garden adjacent to my home, I have an outside, weather-proof electrical outlet on the pillar of my G11/G-1 mount. This is fed by a GFI circuit, giving additional protection against injury or death. Life is too precious not to do things right.

 

Next on the agenda for this old telescope and mount is to get the proper sprockets for the chain drive feeding the worm gear set. As mentioned earlier the machinist (who did much of the work on the mount) didn't realize that the exact drive ratio is important. It is not hugely inaccurate in tracking, just an irritant in needing occasional manual correction. This isn't difficult, since manual movement in RA is very easy. The 6-inch Mathis worm-gear has a nice, large diameter adjustable clutch.

 

Kind Regards,
Russ



#7 Simcal

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Posted 02 November 2020 - 07:52 AM

Excellent news Russ. There's a satisfaction in bringing life back to old machines smile.gif

 

Would it be worth swapping the chain drive for a pulley/belt drive you think?  I always kind of thought the sprocket/chain system has more backlash and the tolerances weren't as good for timing. Great for reliability though.  

 

Lots of sprocket options (and belt/pulley) in the robotics tinkering world.  Kids these days get all the cool stuff.  Here's a link if you need: https://www.robotmar.../sprockets.html

 

Your motor error could likely now be due to your utility's 60 Hz frequency error.  I've heard allowable frequency error to be around +/- 0.5% 

 

Looking at the drive construction further, I see alot of great thought went into the problem. It's quite elegant really.  The use of a u-channel for the RA housing was brilliant.

 

Thanks for sharing your project Russ.  


Edited by Simcal, 02 November 2020 - 08:07 AM.

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#8 Rustler46

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Posted 02 November 2020 - 11:58 AM

Excellent news Russ. There's a satisfaction in bringing life back to old machines smile.gif

 

Would it be worth swapping the chain drive for a pulley/belt drive you think?  I always kind of thought the sprocket/chain system has more backlash and the tolerances weren't as good for timing. Great for reliability though.  

 

Lots of sprocket options (and belt/pulley) in the robotics tinkering world.  Kids these days get all the cool stuff.  Here's a link if you need: https://www.robotmar.../sprockets.html

 

Your motor error could likely now be due to your utility's 60 Hz frequency error.  I've heard allowable frequency error to be around +/- 0.5% 

 

Looking at the drive construction further, I see alot of great thought went into the problem. It's quite elegant really.  The use of a u-channel for the RA housing was brilliant.

 

Thanks for sharing your project Russ.  

Thanks for your comments, Simcal. No doubt the chain drive will add another component to periodic error. But for visual-only use that is not a concern. But since I'll be looking to correct the slight error in drive rate, your suggestion about a drive belt sounds inviting. I would guess it would have much less periodic error than chain drive. A toothed-belt would be in continuous contact with around 1/2 half of the circumference of the toothed wheels. I have a spur gear set made to go with the worm set. I'll count teeth and get the exact ratio needed. Perhaps a belt system would give more options as to ratio.

 

The spur gear set is not useable because the motor's radius is greater than the spur gears' centerline distance. This would require the motor to be on the opposite side of the gear set as is the worm. There isn't enough room inside the aluminum C-channel for that. Thus the need for chain or belt drive.


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