Making spur gears

[Parksy]

Hi all

For my latest project I plan to use spur gears to run two camshafts on either side with the crankshaft run in the middle.

I haven't given any thought with regards to the distance between the crankshaft and camshaft and the gears required to rotate them.

What is the best way to work out what kind of material dimensions I need to create the pinion and driven camshaft gears? Number of teeth for each gear? Obviously the camshaft gear will have twice the number of teeth, but working out how many is where I get confused.
The distance from the crank centre to camshaft centre i worked out to be 28.286mm.


Thank you all

Andy

 

[10K Pete]

The first thing is the ratio between the cam and the crank. Most cams run half the crank speed, so that's the first thing to know: 2:1.

The only thing left is to see what sizes of 2:1 gears will span the center distance and how big they are. If the diameters are too big to fit the space, then and idler gear may be necessary. It can be any size as long as the ratio between the cam gear and crank gear is 2:1. The size of the idler does not affect the ratio. Don't think too hard about it just accept that fact. You'll get your brain to wrap around that concept in due time.

Then there's the pitch or tooth size. Too big won't work as the smaller gear won't have enough teeth. Keep the tooth count on the smaller gear as large as you can, within the ratio.

So it's really a matter of finding sizes to fit...

More specifics regarding the center distances and available space would be required for me to get further in to it for you....

Pete

 

[Jasonb]

Ideally when designing the engine you space the crank & cam ctrs to suit available gearing.

Closest with a quick look seems to be 0.9MOD 42/21 which are available off the shelf come out at 28.35ctrs if you can alter the cam position.

 

[Fluffy]

G'Day Andy,
I have not been on the forum for months & thought that I would have a look this arvo as I have some spare time.

It has been quite a while since I have done gear calcs, so here goes!!!

Gear centre distance = 28.29 mm.

To find the factor for 2:1 ratio; 28.29 divided by 3 = 9.43mm.

Reference circle diameter of crank gear 9.43 x 2 = Ø18.86 mm.
If using 1.0 module then the number of teeth = 18.
(the actual module calculation is 1.048, 1.0 should be OK)
The gear tip diameter = (18 + 2) x 1. So tip diameter = Ø20 mm.

Reference circle diameter of the cam gear = Ø37.72 mm or 9.43 x 4.
Number of teeth = 36.
The gear tip diameter = (36 + 2) x 1. So tip diameter = Ø38mm.

Whole depth of teeth = 2.25 mm.
Chordal thickness of teeth = 1.64 mm.

As I said its been a while & I think that the calcs are OK. This may give you an idea as to the profile of the spur gears that may suit your situation.

Regards,
Don.

 

[Parksy]

Thank you all, I appreciate your efforts very much so!

After posting this question I decided to have a look on eBay, and over in China I found all types of spur gears, of ranging sizes, teeth numbers, modules etc. And funnily enough, according to their dimensions, found a set that could possibly fit, being an 18t and 36t mod 1.0. The gears only cost a few dollars so it's no big loss if they don't work and if they need minor trimming then so be it. So fluffy, your maths is pretty good to me!
If I don't have any luck I'll just make a set.

Thanks again

Andy

 

[moerman]

I never did gear calculations so please enlighten me. The number of teeth is OK in the calculation but how can the diameter be 2x as big instead of 1.4x as big? This gives me an uneasy feeling.

 

[John S]

Those 1 mod gears will have a centre distance of 27mm.

If your centre distance is what you say and written in stone then the only way is to make at least one of the gears to corrected dimensions. Out at the moment but I can work it all out for you tonight.

 

[Parksy]

Those 1 mod gears will have a centre distance of 27mm.

If your centre distance is what you say and written in stone then the only way is to make at least one of the gears to corrected dimensions. Out at the moment but I can work it all out for you tonight.

I was worried so I also purchased a 19t mod 1.0 and 38t mod 1.0, just in case. If you're able to work this out, I'd appreciate this!

If neither work then I have enough gears to add an idler to it.

Hopefully I can make this work.

 

[John S]

19 and 38 are 28.5

In 1 MOD it's easy to work out.

Add 19 + 38 = 57, then divide by 2 to get half of each centres = 28.5
Then multiply this figure by the MOD which in this case is 1 so says the same.

If it was a 1.5 mod gear it would be 28.5 x 1.5 = 42.75

Don't forget if you are putting idlers in the direction changes so the cam opening order needs reversing but I'm guessing you haven't made the cam yet ?

 

[Parksy]

Brilliant! Thanks for sharing that. I haven't made the camshafts or even decided on direction of rotation yet.

 

[Parksy]

I forgot to ask John, that figure you quoted 28.5, is that taking into account that the gears are meshed with zero back lash?

 

[John S]

No that is with standard clearance which on 1 MOD won't be much.
You will not be able to run or even need to run zero backlash, at model engine speeds the noise and wear would be horrendous.

 

[Fluffy]

Andy,
Just a passing thought.
If you are going to make your own gears, canvas reinforced bakelite type F1 makes very good gears. It is easy to machine & much quieter than metal gears. Additionally, if something crews up when the engine is running the bakelite gears act as shear devices & saves other components.
I replaced some of my lathe back gears with this material & they have been running successfully for over 10 years now.
Regards,
Don.

 

[bazmak]

Myford used Bakelite gears (TUFNOL) for the tumbler reverse for the same 3 main reasons.The chinese use plastic/nylon.If you make some when you have the setup right then its easy and cheap to make lots of spares