Expensive gears

[GrahamRounce]

I realise that precision hardened industrial steel gear wheels are going to be expensive, but I wonder why (apparently) there are no cheapo hobbyist-level ones available?

I expected to find thousands from "£5 for 10" upwards when I looked for say 3mm thick 20-50mm dia mod 3 's, ideally with grub fixing screw, but no.

Considering they can surely just press them out these days...

Or maybe there's some kind of junk scrap machinery that's a good source??

 

[Agent_zed]

a lot of the smaller gears I've seen in tools etc have been plastic. How accurate do you need the gears to be as old school mechano has a lot of gears

 

[GrahamRounce]

I'm thinking of slow-moving with potentially lots of strain, so could nylon cope?

I did look (on ebay and in my memory!) at Meccano ones, but the teeth would be a bit small and the larger ones too thin....

It's for a robot knee joint, in case anyone's wondering. Mk's I to VI were with this

kind of setup, with motor > universal coupling > threaded rod > "tendon", but it was too faffy, even before trying to put in limit switches and a linear potentiometer for feedback.
So I thought I'd simplify it back down again, with a big gear at the joint, and a train of lesser ones. Or perhaps sprocket & chain...

Starting to get fed up with it, anyway...

 

[ChaiLatte]

If you want 3 MOD and 20mm dia., how many teeth will it have?

Mod3 15 teeth is 51mm OD.

https://www.ebay.co.uk/itm/354668461540

 

[J-G]

I realise that precision hardened industrial steel gear wheels are going to be expensive, but I wonder why (apparently) there are no cheapo hobbyist-level ones available?

I expected to find thousands from "£5 for 10" upwards when I looked for say 3mm thick 20-50mm dia mod 3 's, ideally with grub fixing screw, but no.

Considering they can surely just press them out these days...

Or maybe there's some kind of junk scrap machinery that's a good source??

You seem to have little understanding about the size of gears !

MOD 3 gears are large - I use mostly MOD 0.5 but on occasion MOD 0.3 and 0.7 - You talk of "20-50mm dia." when in reality you ought to be talking "Nº of Teeth". At MOD 3, a 5 tooth gear is 21mm dia. with a root of 6.6mm - essentially impractical, though not impossible - a 9T would be 33mm OD - 18.6mm Root - so may well support a 12mm bore. 15T is 51mm OD, 36.6mm Root.

It may well be possible to use injection molding to bash out more conventional size gears but "press them out" of 3mm thick Brass or Alum is not sensible - 0.5mm thick yes but you want a collar that takes a grub screw as well!

I suggest that you re-evaluate what you actually need as far as Nº of Teeth and Ratios, then determine what 'MOD' size is viable.

 

[Just4Fun]

Anything here any good?

 

[J-G]

https://www.ebay.co.uk/itm/354668461540

I wouldn't consider anything from that listing to be viable! - there is no mention of the thickness and they state specifically that the 'bore' is rough size!

 

[GrahamRounce]

@J-G: Ok, yes. Scratch "Mod 3" and insert "3mm tooth spacing". I'm sure there is a logic to the mod system but I haven't yet got my head to an intuitive understanding.

 

[J-G]

Ok, yes. Scratch "Mod 3" and insert "3mm tooth spacing". I'm sure there is a logic to the mod system but I haven't yet got my head to an intuitive understanding.

That's still putting the cart before the horse! - You are now talking 3mm CP (Circular Pitch) and that gives you 19.486mm OD for a 28T gear and 49.363mm OD for a 74T gear.

As I've said - work from Nº of Teeth and Ratios, then match that data to MOD (or CP) to determine the diameter - though, to be fair, all factors should be considered at the same time to arrive at a sensible outcome.

I did consider offering to send you my spread-sheet but I don't think it would help until you have a better grasp of the concept of gearing.

 

[Spectric]

For this robot knee joint are gears the best option, if you look at plant and machinery, lets say an excavator then the digger arm has the same movement as you are looking for but uses hydraulic rams. You could use pneumatic rams if not wanting the hydraulic oil or if wanting a toothed system then a simple lead screw rotated by a motor, this gives the advantage that by counting turns the control system will have feedback on position.

The reason some gears are expensive is that they do require a lot of precision machining, but injection moulded plastic is a lot cheaper.

 

[ChaiLatte]

I wouldn't consider anything from that listing to be viable!

The listing has a table for each module size giving tooth count and dimensions. In addition, when a person looks at a particular listing on eBay, it shows various other similar options in both size and material. I would respectfully suggest it has greater potential to assist the OP than an exclamation-point heavy diatribe on his lack of understanding.

In the interest of actually helping the OP to gain "a better grasp of the concept of gearing", might I suggest Ivan Law's 'Gears and gear cutting', pdf copies of which can be found online.

 

[mwinfrance]

"they can surely just press them out these days" made me smile.

 

[J-G]

The listing has a table for each module size giving tooth count and dimensions. In addition, when a person looks at a particular listing on eBay, it shows various other similar options in both size and material. I would respectfully suggest it has greater potential to assist the OP than an exclamation-point heavy diatribe on his lack of understanding.

In the interest of actually helping the OP to gain "a better grasp of the concept of gearing", might I suggest Ivan Law's 'Gears and gear cutting', pdf copies of which can be found online.

I won't honour that silly response with a reply except to agree that Ivan Law's book is worth a read and that I should have looked more deeply into the posting which does have tables of sizes.

 

[Thingybob]

Have you looked in the Hobbies catalogue they have quite a good range