Parametric hob sprocket for ladder chain

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mcdanlj
Posts: 68
Joined: Sun Dec 01, 2019 12:10 am

Parametric hob sprocket for ladder chain

Post by mcdanlj »

I saw this thread on twitter about using FreeCAD to design a custom 3D printable hob sprocket for ladder chain, and I wanted to poke at the design.

The complete sketch supporting the hob sprocket shape looked like a lot of work, so I created something similar as a parametric design that lets you choose the number of teeth, thickness of the sprocket, and the size of a key for a keyway (which you can disable in the design tree if you don't need one). The keyway is synchronized to a tooth, which can be important for timing and also may make it stronger. Choose a clearance that matches your 3D printer, or if you are cutting with a CNC router maybe you can set it to 0. Grab your calipers and measure the thickness of the chain wire, across two rungs (chord pitch will be calculated by subtracting a rung wire thickness from this measurement), and the inside width that determines the width of the disk that forms the sprocket.

If you are making an idler and don't want a keyway, just disable the Keyway from the design key. Otherwise you want to know the width of the key and the height of the key above the curve of the shaft.


Screenshot from 2022-03-02 21-44-12.png
Screenshot from 2022-03-02 21-44-12.png (66.7 KiB) Viewed 2470 times


Unlike a typical sprocket as you would for example find on a bicycle, this one is full disk width to the tip, which is intended to support ladder chain. Incidentally, it's easier to 3D print, and easier to CNC route from stock.


Screenshot from 2022-03-02 21-43-15.png
Screenshot from 2022-03-02 21-43-15.png (160.12 KiB) Viewed 2470 times


If you want something looser and longer-lasting, you probably want an ansi profile, and I'll let someone else make an example. But if you've found yourself with some ladder chain and want to 3D print or CNC route custom sprockets, this is meant to allow you to measure a few things, make a few choices, and print/route out sprockets.


LadderChainHobSprocket.FCStd
(111.76 KiB) Downloaded 74 times

i don't know if I'll ever use this, but I had fun making it!
random
Posts: 20
Joined: Sun Mar 02, 2014 5:28 pm

Re: Parametric hob sprocket for ladder chain

Post by random »

Well, I'll for sure use it. I like that you rounded the tips of the teeth. My initial version has the tangent sections of the circles extend all the way out to points.
I needed one for each end of the ladder chain and only had the one printed, so this will be a huge help in designing the other.
kisolre
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Joined: Wed Nov 21, 2018 1:13 pm

Re: Parametric hob sprocket for ladder chain

Post by kisolre »

Just curious is there something wrong in using the sprocket profile generator in PD and padding to desired thickness? Is this profile superior to the already existing one?
mcdanlj
Posts: 68
Joined: Sun Dec 01, 2019 12:10 am

Re: Parametric hob sprocket for ladder chain

Post by mcdanlj »

kisolre wrote: Thu Mar 03, 2022 7:56 am Just curious is there something wrong in using the sprocket profile generator in PD and padding to desired thickness? Is this profile superior to the already existing one?
Well, I hadn't yet discovered that particular feature of Part Design, so that's the first reason. :D

The second, better, reason is that I don't know whether any of those profiles is a hob sprocket, but none of them look to me like a hob sprocket. My (limited) understanding from the few minutes of search I did after seeing the twitter thread, and going down the rabbit hole trying to learn, is that the hob sprocket has higher wear but lower backlash, and the originating use case turns out to care about backlash, because it's being used for control rather than power transmission.

At least the ANSI profiles in the sprocket profile generator, and I think all the profiles, are (as is normal for chain) optimized for power transmission and reduced wear.

Using chain for control isn't typically the best engineering choice. I understand the selection of ladder chain to be an aesthetic choice in this context, and the use of a hob sprocket is a concession as a result of that choice. (I would have used closed-loop timing belt for this purpose.)

Using chain for control can be useful where the environment can be too hot for timing belt, in which case a hob sprocket profile might be useful for engineering reasons.
kisolre
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Joined: Wed Nov 21, 2018 1:13 pm

Re: Parametric hob sprocket for ladder chain

Post by kisolre »

mcdanlj wrote: Thu Mar 03, 2022 1:16 pm Well, I hadn't yet discovered that particular feature of Part Design, so that's the first reason.
So now that you know about it, care to compare them? I am genuinely curious.
mcdanlj
Posts: 68
Joined: Sun Dec 01, 2019 12:10 am

Re: Parametric hob sprocket for ladder chain

Post by mcdanlj »

kisolre wrote: Thu Mar 03, 2022 2:34 pm So now that you know about it, care to compare them? I am genuinely curious.
I... think I just did? It doesn't look like any of those profiles are hob sprockets, which have different geometry.

But here my expertise is exceeded. This was just me seeing a sketch that looked like it could be done easier, simpler, and better utilizing a polar pattern, and providing an example of how that would work, including parameterization. I'm a developer not a mechanical engineer, and my knowledge of sprockets is limited to infrequent bicycle chain maintenance and what I read on the web yesterday when I went down a rabbit hole opened up on twitter. :lol:

It seems obvious to me that the Part Design approach is much better generally and I'm glad to have learned about it. FreeCAD is full of things I haven't even noticed.
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