Fixing old equatorial mount
Posted 24 August 2012 - 12:51 PM
As for the counter weight, many sporting goods stores sell single bar bell weights made of cast iron.
Posted 25 August 2012 - 05:53 PM
Posted 26 August 2012 - 12:44 PM
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.
Posted 29 August 2012 - 03:03 PM
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
Posted 10 December 2012 - 12:57 AM
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.