Vixen GP motors

[andyhicks]

Hi all,

Ive been messing with my Vixen GP mount and I can't seem to get the motors to do their jobs.

Both motors are working at all the various speeds. When I attach the the slo mo controls and unlock the clutch ring they work and I can move the scope about on both axis. but when I use the motors there is no movement on either, not even tiny movements! the gear wheels are moving and I've tightened the clutch ring so the gear shafts are moving too..??

I took it apart and gave it a bit of grease and a clean and to be honest everything seems solid. It works its just the motors wont drive it?

any ideas?? this is my first motorised mount.

Andy

[Merlin66]

If the gears and shafts are turning.....the worm wheel and final drive must be working.... unless you're way out of balance with the scope.

Loosen the clutches and re-balance the scope :

a) with the tube horizontal - slide it in the rings to find the balance point

across the RA shaft - move the counterweights so it will sit without moving

Try the drives again.... at normal tracking rate the RA movement is obviously very very slow....

[andyhicks]

Thanks for the reply! Im fretting a bit here..

I got a excited then thinking you'd cracked it because Ive had the scope and the counterweight off a lot and no I havent balanced it after that!

Just looked at it again and realised Ive been trying to get it working without the scope or the weight on so its just the bare mount! bummer, that means it not a balancing problem!

Im a bit worried now and I cant see why it wont work?! everythings moving and slo mo controls work but no drive from the motors...on both axis!

e2a: I understand the movement will be small but i cant measure any.

[MartinB]

My experience of these motors is that they don't have much torque and sometimes struggle at much above sidereal. However, it would be unusual for them not to be able to drive the gears at all. Have you tried tracking a star? As Ken says the RA movement is very slow and probably not visible.

Are the slow mo controls fairly stiff? If the worm is set too tight to the gear it will bind and stop movement. It is a fine line with these mounts between having a load of backlash andgearing that binds.

[haitch]

Put a finger on the motor shaft whilst pushing the buttons and see if you can feel the pulses of the stepper motor. I usually can with mine (which has DK-3 motors being direct clones of the Vixens) even when it is just driving the RA forwards at 1x.

[andyhicks]

I think ive made it worse when I took it apart!

Normally when I turn it on I hear the motor pulsing and even on the slowest setting I can watch the teeth of the gears and see movement, very slow but its there, . On the highest speed 32x it just ticks along and when I hold the button the gear shaft turn faster. It will probably do a full turn in about 3 seconds and I can notice a lot of movement with a full turn with the slo mo contol knobs on.

I think maybe I over tightened something when I put it back together because now when I press the buttons it still moves as before but just letting it run there is no sound anymore and no sign of the teeth moving.

aw man what have I done!

[haitch]

Just to check (as I've done similar before)...you've not connected the RA cable to Dec thenlooking for a slow movement on the RA Have you?

I spent a whole evening once wondering how my polar alignment could be so badly off before realising I'd got the motor cables the wrong way round.

[andyhicks]

Haha thanks Haitch, I wish it was as simple as that! No there in the right place.

The RA seems better now after some tinkering, still need to look at the dec but im doing that now.

Im going to leave it running for a bit and see what happens

thanks

[andyhicks]

Think Ive got it back to at least how it was before! the standard pulsing is back and the gears will pull my finger in so they're working, the gear shaft is moving so now just have to wait and see if the mount will move!

[MartinB]

Don't worry Andy, these are very sturdy mounts and you won't have damaged it. I don't know if you have sussed how to adjust the backlash but have a look here

SPa1

[andyhicks]

Brilliant link Martin thanks!

I realised the tension on the gears was affecting them and there are a lot of screws involved. Ive managed to get it working now, the motors are driving the mount! the DEC is slightly stiffer than RA so I think ill follow those steps and start again.

thanks again for your help

e2a: can you explain backlash? Im not sure if my eyes are playing tricks but I think the mount will move, stop and start again.. is that backlash?

[MartinB]

Yes, if you move the scope one way and then reverse the direction it seems to take a while for the scope to respond, that is the backlash being taken up. You get it more in dec than Ra since the RA is constantly moving and taking up any backlash that develops.

Backlash is an essential feature of worm gears. The mesh has to be a little loose otherwise it will bind. When you disengage the motors and use the slow mo you can actually feel the slop in the gears. The trick is to tighten the gears until this slop is reduced to a minimum but hasn't gone completely. It is a lot of work to fine tune. Don't get too obsessed by it though, it's more important that the gears move freely than eliminating all possible backlash.