Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Fixing old equatorial mount

  • Please log in to reply
30 replies to this topic



    Fly Me to the Moon

  • *****
  • Posts: 5832
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 24 August 2012 - 12:51 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

#27 Startraffic


    Viking 1

  • *****
  • Posts: 887
  • Joined: 12 Feb 2006
  • Loc: Lat. 39.143345, Long. -77.174802

Posted 25 August 2012 - 05:45 AM

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

Clear Dark Skies
39.138274 -77.168898

#28 gfeulner


    Viking 1

  • *****
  • Posts: 680
  • Joined: 23 Nov 2007
  • Loc: Bergen county, New Jersey

Posted 25 August 2012 - 05:53 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

#29 Geo.



  • *****
  • Posts: 3707
  • Joined: 01 Oct 2008
  • Loc: Upstate NY

Posted 26 August 2012 - 12:44 PM

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

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


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

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 motor’s gearhead. Our motor’s gearhead output is 1/2 RPM. Our worm is rotating at 0.25 the gearhead’s 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.

Attached Files

#30 idp


    Viking 1

  • -----
  • topic starter
  • Posts: 506
  • Joined: 21 Aug 2009
  • Loc: New Haven, CT

Posted 29 August 2012 - 03:03 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 :smirk:

#31 idp


    Viking 1

  • -----
  • topic starter
  • Posts: 506
  • Joined: 21 Aug 2009
  • Loc: New Haven, CT

Posted 10 December 2012 - 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.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics