Multiple gears

[steamer]

Great !      Now..

What specification of gear are you choosing?     Diametral Pitch?  or Module?

From that we can get tooth counts for each gear.    With that!    You may need to tweek your dimensions accordingly   ( gears come in whole numbers of teeth..so you might have to tweek you positions of the gears slightly to accommodate ./N

For instance.   If we choose Mod 1...because that makes the math easy....  MOD = PCD/N   PCD is the pitch circle diameter expressed in mm, and N is the number of teeth.

So   If you were to make you idler 30mm    if you choose MOD 1, it will have a PCD of 30 mm and 30 teeth.   So if you were to adjust your linear positions such that a 30 mm idler worked...it's easy   ( You could go up to 31 tooth...which ever works,)   but it does need to be a whole number of teeth.

MOD 0.5 = PCD/N...for a 30mm PCD would have 60 teeth,  but all the gears would also need to be MOD 0.5 for them to mesh.

Dave

[steamer]

Now there's one more step...

What is the load your trying to drive?    RPM?  Torque?

That information will drive the size of the gear teeth..The bigger the load....generally speaking...with spur gears....the bigger the teeth of the gears.

Mod 1 gear teeth are much larger than mod 0.5. ect

For a model engine....if you want to be really safe....I'd go MOD 1     I've personally had gear trains with Nylon and Steel MOD 1 gears take 3 to 4 HP at 30000 rpm in very dirty environments, and although they won't last for very long there,   In a model engine, kept clean and lovingly lubricated,  they'll probably outlast you.

Dave

[Jasonb]

It's a lot better but the idler will need to be a whole number assuming MOD1 as at the moment you will end up with an odd number of teeth, is it possible to move No4 slightly to the left so the idler can be changed to 30PCD or further to the right so it can be 31PCD, this seems the easiest to move being just the distributor

[Jo]

We need the measurement from the centre of the crankshaft to the centre of the camshaft.

38.537mm 

As you can tell lots of the members want to help 

So lets look at the first option of the crankshaft driving the cam directly: As Jason has said 30DP seems to be close if you chose a 60 and 30 tooth gear (50.8 PCD and 25.4 PCD). This funny measurement term PCD (Pitch Circle Diameter) is not the outside diameter of the gear but the diameter at which the gears mesh.

To find the distance between the two gears centres you need to add up the PCD values for each and divide it by 2 (the distance between is the two radius added together not the diameters) so for the 60/30 tooth 30DP pairing it would be (50.8+25.4) /2 = 38.1mm between centres which is too big by increasing the DP value you reduce the distance between the gears. A pair of 31 DP gears of the same teeth would work out at (49.1+24.58)/2 = 36.8mm but this is not a common gear size.

By adding an idler gear you are not forced to use the correct gear meshing distance between the pair. When you add this extra gear it is best to use an odd number of teeth so that it wears evenly. Now you could go for a 40 tooth and 20 tooth 30DP gear pair with say a 35 tooth idler set slightly off to the side.

There are lots of different ways to do it. When calculating Gears I use a freely available tool called Metal shop calculator.

Jo

[Zephyrin]

introducing an idler gear doesn't change the end gears ratio but change the direction of rotation of the camshaft, a point to be aware when designing the cams.

[Jasonb]

As Jason has said 30DP seems to be close if you chose a 60 and 30 tooth gear (50.8 PCD and 25.4 PCD). This funny measurement term PCD (Pitch Circle Diameter) is not the outside diameter of the gear but the diameter at which the gears mesh.

Jason has not mentioned DP, Jason has only talked about PCD which from the drawings look like it is shown in metric so why complicate things with imperial DP gears?

31DP would be a right pain to make and not avaliable to buy. As such an advocate of Metric I thought you would jump at the cance to use MOD?

[Jo]

OK so Jason was not discussing Diametric Pitch first time round but continuing the confusion from the drawing.

As such an advocate of Metric I thought you would jump at the cance to use MOD?

As the owner of a very extensive set of legacy DP gear cutters and only an odd MoD cutter I tend to use what I have already got and as the existing model designs are spaced for DP gears it makes life easy.

Edit: A set of MOD 0.85 gears of 60/30 teeth gives a spacing of 38.2mm between shafts. I am not sure if these are common metric gear sizes.

Jo

[Jasonb]

Edit: A set of MOD 0.85 gears of 60/30 teeth gives a spacing of 38.2mm between shafts. I am not sure if these are common metric gear sizes.

Jo

They are not which is the whole reason for using an idler in the first place as it allows 12/24T Mod 1 gears for crank & cam and can have it's own ctr position placed anywhere and also avoids having a massive 60T cam gear sticking out the side of the engine.

[Jasonb]

This is how it could be done with standard tooth profiles and whole numbers of teeth.

The Crank and cam stay at the given 38.537ctrs & 16.2 offset and the 12:24 gives the 2:1 ratio. Idler is 30 and if the 24 distributor could be moved upwards that keeps the given 26.662horizontal position.

However this puts the distributor up very high, we are not given the vertical position of this but if the idler were reduced to 26 then that would position it closer to the OP's drawn position

[Jo]

Edit: A set of MOD 0.85 gears of 60/30 teeth gives a spacing of 38.2mm between shafts. I am not sure if these are common metric gear sizes.

Jo

They are not which is the whole reason for using an idler in the first place as it allows 12/24T Mod 1 gears for crank & cam and can have it's own ctr position placed anywhere and also avoids having a massive 60T cam gear sticking out the side of the engine.

Its all yours Jason I have better things to do with my time than have yet another argument with you

Jo

[Alyn Foundry]

Whilst on the subject of " idler wheels " there was a motorcycle manufacturer, who's name escapes me that used an odd number of teeth on their idler wheel.

This " hunting tooth " arrangement ensured an even wear on the overall train. Something any big Lister enthusiasts here will understand. My " P " type that had done half a century of work pumping water from a well for the local water company has a distinct rattle as each valve is operated.

Just another consideration, cheers Graham.

[bernienufc]

Hi All
Thank you so much for your time and efforts to enlightem me and to to tutor me step by step on gear design. Holding my hands up it feels like a form of dislexia as no matter how many times i read it i cannot fathom it, so here goes my story to it :-)

I love this forum, what i see is incredibly talented people prepared to give up time to educate and teach us to follow in their footsteps, this may be my downfall. The levels at which models are built and replicated do inspire me to work to the highest level regardless and i know some parts of my project i can achieve and other bits i.e. gears maybe not so quickly :-).
I purchased George's 300 engine plans recently, i did so in an effort to learn from them how to modify design create an engine to suit my personal end goals which is very similar in layout, this happens to be an AEC AV760. George has answered some of my queries and i love his work and help. Based upon what have i seen and read on this forum i hope to use Georges' plans to make up the engine block and to create similar detail and mods that are workable in machinery available to me (Mill, 2x Lathes, 3D printing, and a host of other stuff).

The engine have drawn so far has 5 gears on the front the 5th not being shown is below the crank driver for the oil pump. Looking at the drawings from the manual (i can scan and paste if allowed) i tried in my simple mind to create the gears in the layout shown and trying to get close matching the gears configuration, i figured i draw the circles and the teeth count will follow     but....reading the posts has taught me a tremendous amount and even if i dont understand it now i will keep on until i do.
So to all and sundry thank you and please dont give up on me yet, i will excel i am sure in other areas, i so look forward to recreating chris manns suspension layout he created on my tanker one day, this drives me on.
I have waited 60 years for this moment and yes its hard to learn some of the theory...oh ok almost impossible  but with the help as above i look forward to creating a working engine even if it takes me a long long time.
Cheers to all Bernie

[Vixen]

Hello Bernie

Would I be correct in thinking the AEC AV760 was the engine which powered the Routemaster double decker buses? You obviously have a deep rooted connection with this engine. Sounds like you have made a wise decision to base your model on an existing and proven design. Georges plans will provide a solid foundation from which to build your engine.

Scale or semi scale models are always going to be a compromise. You need to be prepared to change some internal details (such as gear separation distances) to match what gears etc, are readily available, while making the engine's exterior resemble the original.

I hope the forum members have and will continue to be able to support and help you. We are a small community, like an extended family. So dont let the bickering between certain members put you off. That's just family life in a way. We are all happy to try and help.

Mike

[Jasonb]

Bernie, if you can post a scan that would help this will also help us to get as close as possible to the same tooth counts so it looks correct.

Also as the position of the idler needs to float it is not ideal having the distributor gear's position from that. If you could post both the horizontal and vertical distances from crank shaft ctr to distributor ctr that would be good and also indicate which of the two could be moved slightly if needed.

Could you also confirm that your dimensions are metric which I have assumed they are as I don't know many full size let alone models with 12" crank gears. Also do you hope to cut your own or modify off the shelf gears. I assume you don't have any cutters so won't restrict your options by what cutters are available.

[steamer]

It's a lot better but the idler will need to be a whole number assuming MOD1 as at the moment you will end up with an odd number of teeth, is it possible to move No4 slightly to the left so the idler can be changed to 30PCD or further to the right so it can be 31PCD, this seems the easiest to move being just the distributor

Yes I mentioned it needs to be whole numbers of teeth