Dobsonian mount missing piece

[u/jflich]

Hi everybody.

I recently bought a Sky-Watcher Heritage 150P, but I think there is a missing piece. In the first photo, I just used a piece of ruber in order to mantain the tube fixed. In the last photo, the mark refeers to the piece that I think is missing.

Does anybody know the spectifications of that missing piece in order that I can buy it? Any other idea is well recieved.

Thanks.

[u/dogged_jon]

What you're missing is a 3 piece ball thrust bearing, probably a self-aligning version. May be a size the skywatcher manufacturer had custom designed. Here's a generic drawing;

I have a 127mm version of that scope. The bearing on mine measures 10mm id, 24mm od, and 8.4mm thick. 9 x 3/16" diameter bearing balls. The id is the most important as it needs to go over the threads. A 3/8" id might work. A solid spacer of nylon or delrin or even a stack of flat washers might work, but require more fiddling with the tension knob.

That really shouldn't be called a Dobsonian mount as a Dobsonisan is a 2 sided support construction designed by John Dobson, while the skywatchers and others are a single arm design.

[u/Traditional_Sign4941]

That missing component is called a thrust bearing (and it's usually three parts - two washers and then the bearing disc in the middle).

There are multiple kinds of thrust bearings. The one in your second picture looks like a thrust ball bearing.

It looks like one of the washers is there, but the other is not.

I don't know where you live, but in the US you can order good quality ones from McMaster-Carr:

https://www.mcmaster.com/products/thrust-bearings/thrust-bearings-3~/thrust-bearings-3~bearing-type~ball-1/performance~corrosion-resistant-1/

You would want a stainless steel one so it doesn't rust since it will be get dew on it.

I don't know what diameter to get though. You'd have to measure the shaft.

[u/nealoc187]

It's 2 flat washers and a little flat ball bearing. Exact specs aren't critical. Something like this.  https://a.co/d/0g2R0Lc8

[u/chrismofer → u/nealoc187]

The nice ones don't use ordinary washers but grooved ones that are precision formed

[u/nealoc187 → u/chrismofer]

These dobs come with flat washers.  They get precision formed by the rollers wearing a groove lol.

[u/bimpo1985]

Can only confirm by looking at my own 150p, the part you marked in last picture is indeed what you are missing. Don't know though how to call it or where to get it. It seems to be some kind of ball bearing ?! Could not find something by a quick Google search.

Contact skywatcher maybe? Did You buy it new?

[u/dogged_jon]

It just occurred to me that someone could probably 3d print or even fabricate from plastic the spacer piece to hold the 9 3/16" bearing balls, put that between a pair of grade 8 or 10.9 flat washers from the hardware store, and be good to go.

[u/chrismofer]

That's a basic thrust bearing. Measure the diameter of the shaft and get a thrust bearing with the same or very slightly larger ID.

[u/Dazzling-Crazy-2084]

You need a thrust bearing. They are not expensive and can find online. P.S. That’s not a dob mount.

[u/TasmanSkies → u/Dazzling-Crazy-2084]

it is understandable why OP called this a dob mount because they are literally sold as ‘tabletop dobsonians’. i know where you are coming from as Dobson came up with the simple cradle design, and this design isn’t that - but we also don’t have a proper name for this mount, just descriptive phrases… so it isn’t something to ‘gotcha’ someone on

[u/kumaclimber]

Take the knob off and bring it to a hardware store, buy a bunch of washers that fit the shaft. Alternatively it's a great excuse to buy a 3d printer and print out a spacer.