Load limits on EQ mounts

[LaurenceT]

I know this has never been discussed here before😄 but........

I only have a dinky EQ mount now, the Star Adventurer GTi, and I'm not considering wildly overloading it but I would like to try my ancient refurb project of an Orion ED80 which just might (or maybe not) exceed the limit of 5kg.

Anyway, I've been reading quite a bit about the need to reduce the scope weight to some undefined percentage of the manufacturers stated limit because of various factors. I've had a search of Skywatchers mounts specifications and I can't see anything about reducing the limits of 

any of their EQ mounts, they just state the limit. Indeed on Astrobin a contributor anecdotally claims that a Skywatcher engineer confirmed that the load limit for my mount does not require reducing for imaging, I can't see why this should only apply to my little mount.

I'd just like to know whether there is any engineering or science based evidence ie with experimental data, to show if this load reduction is anything other than anecdotal.

[Tomatobro]

19 minutes ago, LaurenceT said:

anything other than anecdotal

I Agree. Lots of stuff gets repeated so often it takes on the status of fact.

The GTI states a max load of 5 kg. My setup is 4 kg but with its extended C of G (from the saddle) to get it to balance I had to make an extension for the CW bar so that the CW sits about 30 mm past the end of the CW bar.

It tracks well according to the ASIAIR and the subs look good.

For imaging its difficult to see how much more compact I could make it (ED72, guide scope and camera, zwo focuser, 5 position filter wheel, camera and ASIair) so even at 4 kg the CW needs to be further out (using the supplied CW).

5Kg would suggest that the designers had in mind a single scope with a DSLR which would bring the C of G closer to the saddle and keep the CW travel within the limits of the CW bar.

[LaurenceT]

My larger rig weighs in at 4.3kg but somehow it balances OK without any extension but your extension bar looks excellent.

[Elp]

Distance of your payload from the CoG/RA rotation also makes a difference, in fact it's typically an explicitly stated spec for HD mounts.

When I had my azgti my Z61 rig is around 6.5Kg all in, it worked and seemed to be okay (I was knowingly overloading it), tried my C6 for imaging which has a larger box volume but similar weight, and you can hear the mount struggle when slewing with subs also affected when imaging. So I suspect the weight being shifted away from a central axis played a part.

[Tomatobro]

one of the things I tried on a SW mount was to put the guide scope on the CW bar and use a light weight CW. It worked surprising well

[Stephenstargazer]

Mount specs should be quoted as a mass and a distance from the rotation point. Vixen do this in their manuals. You multiply the two factors to get the 'moment' the mount has to carry.

[GTom]

Discussions on strain wave mounts brought me enlightenment in this topic: MOMENT ARMS and wind cross sections are of paramount importance.

Geometrics allowing guidescope and asiair/miniPC mounted UNDER the OTA could move your center of gravity much closer to the saddle.

And true, YMMV! Seen several examples on the SA GTi with unbelievably large scopes (e.g. 130PDS!) while the 3x more expensive CEM26 struggles with an f4 150mm Newtonian that is well within spec payload.

 

Also, vibrations and flexures are much more of a problem when there are narrow links in the train, such as a skinny pier extension (stock SA GTI...), skinny wedge (AZ GTi...) or small diameter counterweight shaft.

Edited September 11 by GTom

[saac]

On 26/07/2024 at 11:23, LaurenceT said:

I know this has never been discussed here before😄 but........

I only have a dinky EQ mount now, the Star Adventurer GTi, and I'm not considering wildly overloading it but I would like to try my ancient refurb project of an Orion ED80 which just might (or maybe not) exceed the limit of 5kg.

Anyway, I've been reading quite a bit about the need to reduce the scope weight to some undefined percentage of the manufacturers stated limit because of various factors. I've had a search of Skywatchers mounts specifications and I can't see anything about reducing the limits of 

any of their EQ mounts, they just state the limit. Indeed on Astrobin a contributor anecdotally claims that a Skywatcher engineer confirmed that the load limit for my mount does not require reducing for imaging, I can't see why this should only apply to my little mount.

I'd just like to know whether there is any engineering or science based evidence ie with experimental data, to show if this load reduction is anything other than anecdotal.

As a mechanical engineer I’m pretty confident that these so called safety margins are all anecdotal if well intended. The main design limit is dictated by the holding torque provided by the drive and gearing and is not a structural limit as such. The proof is as always easy to ascertain, load your setup, image or view and look for any degradation in performance. There is no better evidence than gained by practical investigation.

Jim

Edited September 11 by saac

[PhilB61]

I used my AZ-Gti exclusively in EQ mode and it developed significant play in the RA axis after being loaded with a 200-500 Nikon zoom lens and DSLR camera, plus guide scope etc (required approx 2.7kg counterweight) for a winter season of imaging. This was easy enough to adjust out after accessing the locking ring and does not seem to have re-occured so may have just been just bedding in or poorly setup from factory although I didn't notice it originally.

[TiffsAndAstro]

23 hours ago, saac said:

As a mechanical engineer I’m pretty confident that these so called safety margins are all anecdotal if well intended. The main design limit is dictated by the holding torque provided by the drive and gearing and is not a structural limit as such. The proof is as always easy to ascertain, load your setup, image or view and look for any degradation in performance. There is no better evidence than gained by practical investigation.

Jim

Is there any harm I could do testing say a 5.3kg scope on my sw SA GTi who's payload limit is 5kg?

[Tomatobro]

By harm I guess you mean permanent damage to the mount. As a GTI owner I would think not.

If you can balance it within the limits of the counterweight bar and supplied counterweight then its tracking will be reasonable but if you have to add additional counterweight or lengthen the bar then tracking will suffer.

My setup is less than the 5 kg limit but I had to add a small extension to the counterweight bar to get it to balance.

[saac]

1 hour ago, TiffsAndAstro said:

Is there any harm I could do testing say a 5.3kg scope on my sw SA GTi who's payload limit is 5kg?

I doubt you would do any harm as in structural failure. You may find battery drain is a little quicker and pending on the drive the slew rate may be slower. As per @Tomatobro comment I'd pay attention to balancing. Longer term you may find that gear teeth and bearing wear a little quicker than had been lifed by the design spec. That said, these mounts are not rotating at high rpm nor do they accumulate high "mileage".  So, on balance I think any additional wear on a 0.3 kg excess would be marginal.  Give it a go, see what happens  

Jim  

Edited September 12 by saac

[TiffsAndAstro]

40 minutes ago, saac said:

I doubt you would do any harm as in structural failure. You may find battery drain is a little quicker and pending on the drive the slew rate may be slower. As per @Tomatobro comment I'd pay attention to balancing. Longer term you may find that gear teeth and bearing wear a little quicker than had been lifed by the design spec. That said, these mounts are not rotating at high rpm nor do they accumulate high "mileage".  So, on balance I think any additional wear on a 0.3 kg excess would be marginal.  Give it a go, see what happens  

Jim  

This was my thinking too, but I was somewhat weary.

It already sounds like a coffee grinder. If I get the scope I'll give it a try at least Ty

[TiffsAndAstro]

1 hour ago, Tomatobro said:

By harm I guess you mean permanent damage to the mount. As a GTI owner I would think not.

If you can balance it within the limits of the counterweight bar and supplied counterweight then its tracking will be reasonable but if you have to add additional counterweight or lengthen the bar then tracking will suffer.

My setup is less than the 5 kg limit but I had to add a small extension to the counterweight bar to get it to balance.

That's really interesting Ty. My CW were very close to bottom of CW shaft and in balance. But I've moved them further up and it's still in balance.... I think both axes on sw gti are not very sensitive/free flowing with clutches loosened.

I won't be adding extra CW so this seems a reasonably low risk test. 

Will post what happens if I get such a scope

[GTom]

On 26/07/2024 at 13:52, Tomatobro said:

one of the things I tried on a SW mount was to put the guide scope on the CW bar and use a light weight CW. It worked surprising well

I am thinking about this for my Lihgtrack setup, but it certainly adds to the cabling nightmare. At least the guidecam I am using is USB2, and USB2 cables aren't as stiff as the v3 ones... Beyond 500mm FL I'd start worrying about differential flexure.

Edited September 19 by GTom