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Equipment Discussions >> ATM, Optics and DIY Forum

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Tom and Beth
Post Laureate


Reged: 01/08/07

Loc: Tucson, AZ
Re: Fixing old equatorial mount new [Re: idp]
      #5383277 - 08/23/12 06:01 PM

Hello Ivano,

The 1.5 inch shaft GEM like yours was indeed very common during the 50s through 70s, and other manufactures were Cave, Atlas and Meade. I'm not sure if this is still a valid Email but try Donald Rothman [newyearskid@webtv.net] for Counterweights and the Motor. If you don't have the 6 inch setting circles for the mount, they are easy to make out of wood or plastic. Matter of fact, there's a link down below in MOUNTs for making the dials.


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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Fixing old equatorial mount new [Re: Geo.]
      #5383564 - 08/23/12 09:26 PM

Quote:

The motor has to be 1/2 revolution PER HOUR or 12 per day. 1/2 RPM is 720 revolutions per day (1440 minutes = 24 hours). So the worm wheel would have to have 720 teeth and it looks a lot less than that. 1/2 RPH would only require 120 teeth on the worm wheel (1440/ 12 R/day = 120), which is what it looks like.




Hi George,

The motor does not drive directly the worm gear; indeed, there is another reduction gear in between that is barely visible in the pictures I posted (I was interested in showing the other side). The reduction rate seems to be about 1/4, I have not counted the teeth yet as my problem is the motor; gears are now cleaned and greased and seem to fare pretty well!

Ivano


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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Fixing old equatorial mount new [Re: Tom and Beth]
      #5383567 - 08/23/12 09:29 PM

Quote:

Hello Ivano,

The 1.5 inch shaft GEM like yours was indeed very common during the 50s through 70s, and other manufactures were Cave, Atlas and Meade. I'm not sure if this is still a valid Email but try Donald Rothman [newyearskid@webtv.net] for Counterweights and the Motor. If you don't have the 6 inch setting circles for the mount, they are easy to make out of wood or plastic. Matter of fact, there's a link down below in MOUNTs for making the dials.




Hello Tom and Beth,

Thanks for the good tip! As for the setting circles, yes I am going to make them and there are good templates on a thread in this forum; however, posters mainly have to do with dobsonians so no one has posted a good template for an AR circle.

Ivano


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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Fixing old equatorial mount new [Re: idp]
      #5383580 - 08/23/12 09:40 PM

By the way: thanks to all for invaluable advice. I think I will still try to fix the original motor before I give up.
As for the counterweights, I still hope I can find some scrap ones around. Otherwise, I could make by filling a tin can with concrete even though concrete is not nearly as heavy as iron or steel.

Yesterday I (almost) completed the secondary mirror cell and spider, the tube is painted and I light-proofed it as good as I could. I still need a working motor, counterweights and lightshields, and then I'm basically ready to go!

Ivano


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Datapanic
Carpal Tunnel
*****

Reged: 10/17/09

Loc: Tucson, Arizona
Re: Fixing old equatorial mount new [Re: idp]
      #5384383 - 08/24/12 11:26 AM

Quote:

Bad news.

Back from Stellafane, I decided to finally reconnect all the wires and see if the electric motor still works (I worked on other parts of the telescope last week). While I was stripping one end of the white wire from the motor, the other one... slipped out of the motor. I pulled out the cover, two wires are accessible, the one that slipped out (lover hole in the case; it came out from the same hole as the black wire) is not.
Game over?

Ivano




The 3-wire Hurst motors are reversible so you may not be out of business provided that the red and black wires when powered will turn the motor in the right direction. I'm not sure if you still need the capacitor between the red and white wires if you use the motor for single direction.

It may be possible to find a replacement on Hurst Motors Website that will work - it just needs to rotate at the same speed and direction as the old motor and fit its mounting. These motors are expensive! Sometimes they will show up on surplus sites as well. I found a pair of 2-rpm CA's that way for $80 or so each.




Edited by Datapanic (08/24/12 12:59 PM)


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DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Fixing old equatorial mount new [Re: Datapanic]
      #5384640 - 08/24/12 01:51 PM

The Hurst reversible motors use two coils and the capacitor to make a phase shift between them. Depending on which coil is leading in phase determines what direction it turns, so you need both coils, ie all the wires to make the motor run.
As for the counter weight, many sporting goods stores sell single bar bell weights made of cast iron.

- Dave


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Startraffic
professor emeritus
*****

Reged: 02/12/06

Loc: Lat. 39.143345, Long. -77.1748...
Re: Fixing old equatorial mount new [Re: DAVIDG]
      #5385874 - 08/25/12 06:45 AM

Ivano,
Give these guys a try. Note: they don't have a parts listing on line, but they may have what you need. http://www.baynesvilleelectronics.com/index2.ivnu HTH

Clear Dark Skies
Startraffic
39.138274 -77.168898


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gfeulner
super member
*****

Reged: 11/23/07

Loc: Bergen county, New Jersey
Re: Fixing old equatorial mount new [Re: idp]
      #5386880 - 08/25/12 06:53 PM

Ivano- I just ordered a 1RPM motor from Digi-Key for my Cave mount. I'm pretty sure they will have what you are looking for. Gerry

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Geo.
Carpal Tunnel
*****

Reged: 10/01/08

Loc: Upstate NY
Re: Fixing old equatorial mount new [Re: gfeulner]
      #5387986 - 08/26/12 01:44 PM Attachment (20 downloads)

These drives aren't designed to be rebuilt. From your photos it looks like the field coil is retained by barb clips formed by indentations in the case. Sort of a one way process. These can be over come by getting some flat shims between the barb and the retainer. Most likely the wire has broken at the end of the coil. With luck there will be enough of a tail to reconnect it. Coils can be rewound. Google - Making Old Synchronous Clocks Safe to Use.

The other course is to find a replacement timer motor. A WAG is that you'll need 100-150 oz. inches torque.

The speed of a synchronous motor is determined by the power mains cycle rate (60Hz in No. America). The speed that is stamped on the case is AFTER reduction. Yes my maths were off.

To Calculate the speed of a induction motor, apply this formula:

Srpm = 120 x F
..........-------
...............P

Srpm = synchronous revolutions per minute.
120 = constant
F = supply frequency (in cycles/sec)
P = number of motor winding poles

So

Srpm = 120 x 60.........7200
...........---------- = --------
................P................P

2 to 24 poles are common in synchronous clock drives this would give is a rotor speed of 300-1200 RPM.

Easier to work the other way. Most Meade forks have 180 tooth worm wheels. The worm is driven one revolution every eight minutes. Thus the drive output is 1/8 RPM or 0.125 RPM. (0.125 RPM * 60' = 7.5 RPH.or 1440/8 = 180)

Assuming 300RPM at the rotor and wanting 0.125 RPM at the gearhead output we need a reduction of 2400:1. You need high reductions to have enough torque to to the work required. The worm gears have a further reduction of 180:1. So the total reduction is 432,000:1. There are 86,400 seconds in a day. Thus our motor operates at 5 revolutions per arc second.

So lets make a couple of assumptions: your worm wheel has 180 teeth, thus a 180:1 ratio, the worm axle reduction is 4:1. Our gear reduction is 720:1 before the motorís gearhead. Our motorís gearhead output is 1/2 RPM. Our worm is rotating at 0.25 the gearheadís output, or 0.125 RPM which gets us one worm wheel per 24 hours.. If the Hurst is doing 300 RPM then the gearhead produces a 600:1 reduction and fairly high torque output. The later Hurst I have seems to have 12 poles so it would operate at 2X your drives speed (assumed) but require 2X the reduction to get to .050 RPM.

Due to those external reductions gears you have some flex ability. Driect drive with 1/8 RPM, or reduction gears to get a 1, 1/2 or 1/4 RPM to that 1/8 final output. Also an idler gear will reverse the motor's final rotation.


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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Fixing old equatorial mount new [Re: Geo.]
      #5393527 - 08/29/12 04:03 PM

Hi Geo,

Very interesting and very informative post, thank you! eventually decided that I've messed up enough already, so a friend who actually knows quite a bit about telescope mechanics is now taking care of the mount. He also suggested that I should get a new motor, which I will. Dommage. The optical tube is now basically finished (the secondary mirror cell kept me busy for a while); I tried it a couple of nights ago on the Moon on a dobson mount and, well, they told me the primary mirror should be good


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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Fixing old equatorial mount new [Re: idp]
      #5563426 - 12/10/12 12:57 AM

Hi to all, work has kept me far from my project for the last months but I am still looking for a new motor. I may now be able to buy a Hurst CA 990338, which looks very similar to the (defunct) one that drove my mount but its output speed is 1 RPM while I need 1/2 RPM.

However, someone on this thread wrote that the difference in these motors is the reduction gear, otherwise they are the same and it could be possible to swap them (it does not look easy though). Can anyone confirm that this is actually the case? It comes with the capacitor and it can be wired for CW or CCW.

Thanks,

Ivano


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