27 Replies Latest reply on Aug 31, 2016 1:22 PM by Dave Bear

# Designing drive belt and cogs with defined cog position....

Hi all,

I suppose this one is for those with great engineering or mathematical skills.

I need to design a matching pair of cogs that will be driven by a rubber belt.

The cogs are separated by a defined dimension that cannot be changed. The cogs also need to be able to maintain an exact 180 degree of opposite positioning as they are for opposing camshafts. So, basically when the keyway of one cog is true north so to speak, the other must be true south.

The maximum outside diameter of the cog (teeth) can only be 25mm.

A third cog of equal design could be added as a tensioner if trigonometry makes this design any easier. If not, I was just going to design a basic adjustable tensioner pad later.

I'm thinking perhaps the drive belt itself would have a 2 - 3mm back for the teeth to form off of also?

I have mucked around with this for two days now and cannot get all of these things to line up. If anything, the outer diameter of the cogs could change by 1 or 2mm max to solve the issue but it didn't help me. Attached is a rough draft, obviously each item would need to be created as a separate part. Any help, wisdom, links, tips, pdf or whatever is extremely welcome.

Kind regards,

Dave.

• ###### Re: Designing drive belt and cogs with defined cog rotation....

You may want to download the actual timing belt pulleys/sprockets and get the width of the belting, not sure how you can hold the center distance and the 180 deg rotation, unless there is a timing belt that is currently being used, that timing belt must have a certain pitch and teeth spacing for that to hold out 100%..

• ###### Re: Designing drive belt and cogs with defined cog position....

Hi John,

This all started from a tutorial that some guy created but never finished and never did any of the hard yards. I've managed to solve so many issues and created many many parts on my own (with the help of members here) and I don't wanna give up now. The whole thing is a custom job and not relative in anyway to any known true design. So, with that in mind, it's now all about learning how to engineer things to suit the situation...........

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

As John recommended, timing belt is an option since it's hard to control the cog position due to the rubber belt slipping - If you decide to go with timing belt, there will be lots of work to pick the right belt length and pulleys so you can line up the two cogs in the right position

Another option is using gears and I guess you'll not like it since they'll take all space you have

• ###### Re: Designing drive belt and cogs with defined cog position....

Look at this, it was posted earlier.

• ###### Re: Designing drive belt and cogs with defined cog position....

Hi Tony,

I came across that gem by Gianluca a few days ago but unfortunately I couldn't get that to work in this scenario either. I would also prefer a belt drive as opposed to a chain drive but thank you.

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Like Elmar Klammer mentioned above - the selection is based on HP - once you know that then you need to look at the correct Gates chart - and then use what Gianluca Mattaroccia uploaded the other day, you don't need to worry about the belt teeth detail at all, it'll be a huge slow down for your computer, however if you're going after visuals then by all means you'll want to model all the detail, till then have a wonderful ....

• ###### Re: Designing drive belt and cogs with defined cog position....

Hmmm, food for thought John,

I couldn't get Gianluca's approach (chain drive) to work, and stated by himself below he has used a different approach for this sort of situation before anyhow. I'd much rather belt drive and I can't let the timing move hence the teeth.

One thing you've got me wondering now though? Will my camshafts still rotate within the assembly if I go down this route? I've just remembered I thought I read somewhere that belts don't animate....

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Hi Dave,

Go to PartView™ System Belt Products Catalog | Gates Corporation this will give you all the drawings for industrial cogged belting, including hubs and sprocket. That will give you available sizes for your application. The Design Flex® Pro™ Software lets you also calculate horsepower rating and noise levels etc. The timing is simple. Use taper lock bushings that you can orientate as you like on a smooth shaft. Your system is simple if you run only in one direction. With dual direction you will need to run two belts side by side with the belt tensioner on opposite sides. That way you can control back lash when rotation reverses. Otherwise you timing goes out of sync when the slack side would switches over to the other side. The syncro belts stretch very little and should affect your timing little.

Elmar

• ###### Re: Designing drive belt and cogs with defined cog position....

Thanks Elmar,

I'll definitely give that one a look at tomorrow, it sounds promising, thank you.

This is a single direction only situation also.

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

I have solved similar problems in the past with a slight different procedure than the one posted by Tony Cantrell above. You need to work with equations to make it possible but it is not hard to achieve. I am very busy at the moment but I will try to post something for you as soon as possible.

• ###### Re: Designing drive belt and cogs with defined cog position....

Hi Gianluca,

If you get the time I'd very much appreciate it, thank you!

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Dave, is a 1:1 ratio a requirement for the project?

I am just thinking of a shortcut for this project.

• ###### Re: Designing drive belt and cogs with defined cog position....

Yes, both cogs will be identical in ratio but it is crucial that I have symmetry in the teeth I guess so that I can maintain the 180 degree opposite cog positioning (keyway).

Dave

• ###### Re: Designing drive belt and cogs with defined cog position....

Maybe not clear on the timing - like Elmar mentioned you would need to use a Tapered Bushing anyway without a keyway, you would only need orientation line marks on the ends of the shafts or make a fixture to align the shaft to 180 degs and then tighten up all the pulleys and timing belt, remove assembly fixture and you should be ready to go....

• ###### Re: Designing drive belt and cogs with defined cog position....

Or keyway the opposing shaft to suit the alignment of the belt teeth? Same principle yes?

• ###### Re: Designing drive belt and cogs with defined cog position....

Dave please post an assy drawing with the pair of cogs in desired location. It will help speed up the process of those who intend to help. Thanks

• ###### Re: Designing drive belt and cogs with defined cog position....

I don't have any designed Gianluca, this is the step I am at.

All I know is what the centre's of my 2 camshafts are which is 73.16mm. The camshafts themselves are yet to have whatever revolves, keyways, extrusions etc. needed to facilitate the drive cogs added to them.

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Thanks Dave Bear, then the problem is simpler than you think. If you are using a timing belt on a 25mm cog I do not think you will have slip problem if you use a keyless bushing like the one in the picture.

So what I would do is this:

1. Model the first cog with a keyway
2. Model the second cog with a machined housing to allocate the keyless bushing.
4. Create a belt assy, follow my tutorial. You do not need to draw the cross section of an ansi chain but a the base of the timing belt (I thin rectangle)
5. Set up Pitch diameters on both and create a belt with a desired length. Make sure you consult a catalog to know the tooth pitch to use for your belt. Then make provision for a self-tensioner, idler or any other requirement you have. Define the stroke for the idler and make sure you create belts with lengths you can actually buy.
6. Model on tooth on  belt you just created and create a chain component pattern or curve driven pattern
7. Go back in your assy and see where your job done (if you insist in using keyways on both cogs you can now see what's the angle shift for your second cog. Then go to the machine shop and broach a new keyway in the second cog based on the angle offset you have measured with the model.

Let me know if you have questions

• ###### Re: Designing drive belt and cogs with defined cog position....

Thanks Gianluca, sounds like a plan!

I believe the keyless bush you've supplied the picture of is possibly what Mike referred to as a Ringfeder, so there's another vote for that option. I appreciate you putting some thought into it. I've got plenty to trial and test tomorrow so hopefully something will work out.

Thank you!

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Not sure this answers you question about a trigonometry method but here are some methods to get the job done;

-Multitude of ways; SW has many ways to do things, I tend to skip the extra fluff (cut corners) to just get the job done as well as it needs! Sure I would love more automatic methods, but so many things are unique and never duplicated in my line of work.

- Model Math; I have learned since I am not a degreed engineer or have a mind for math I use SW to layout geometry, define the absolutes and work backwards from there. This still 'manual' and requires designer to engage the input and output numbers.

- Belt estimation; since I've always had the luxury of some fudge factor (adjustable idler/tensioner) I get things close and let the absolute not be so absolute...

- Use Pitch CL; disregard the belt teeth, etc. and simplify the layout with belt length using 'model math'

SW has tons of tools but no one can tell you exactly how to apply them all...

Hope it helps, Jason

• ###### Re: Designing drive belt and cogs with defined cog position....

Hi Jason,

I've tried many different things, some automated, some manually drafted but I just can't get all components to tie into each other no matter how I configure them.

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Oh Yeah,

'Path Length Dimension'; it can be trick to work with but might help position your idler?

Mutable sketches; leave 'pulleys' in one sketch and draw the belt and define idler position over top in separate sketch...

• ###### Re: Designing drive belt and cogs with defined cog position....

For minor adjustment offset keys would work once timing is set.

• ###### Re: Designing drive belt and cogs with defined cog position....

This seems trivial to me. Let me ask a few questions to make sure I'm not missing the hard part

1. Do the parts have to be identical, or can the keyways be clocked differently?

2. Are you married to the shaft key, or can you move to a ringfeder or some similar?

3. Can the cogs be in 2 parts?

4. This may be obvious, but can I assume you are using a standard timing belt?

• ###### Re: Designing drive belt and cogs with defined cog position....

May seem trivial to a mechanical engineer Mike, but to a novice, it's doing my head in..........

1. Thinking about it just now, the keyways could be clocked differently, yes.

2. Shaft key was just the initial concept but other think tapered bush may be the way to go.

3. Not exactly sure what you mean by this Mike? A split cog?

4. No, not standard, nothing on this engine is standard unfortunately.

Dave.

• ###### Re: Designing drive belt and cogs with defined cog position....

Sorry, that seemed arrogant. It's just that when I think I know the answer it's usually because I'm missing the hard part.

• ###### Re: Designing drive belt and cogs with defined cog position....

No worries..........