Help with custom transmision and gear CAD.

[ZeroValue]

I have been trying to design a custom two speed transmission for our robot. I was planning on using the AndyMark Gen1 transmission as a base model, then replacing the gears with 20 degree pressure angle gears so that we can get the reduction that we wanted. My idea was for a 4:1 shift with the highest being around 7:1 and the lowest being 28:1. I have tried experimenting with the spur gear feature in inventor but I am unable to get it to work. Does anyone know of a guide for this or can anyone help? greatly appreciated.

[=Martin=Taylor=]

Originally Posted by ZeroValue

I have tried experimenting with the spur gear feature in inventor but I am unable to get it to work. Does anyone know of a guide for this or can anyone help? greatly appreciated.

The spur gear maker-thingy sucks in Inventor 2008 and 11. If you have access to Inventor 10, it works great there (or at least this has been my experience).

If you don't like the design accelerator you could always download the i-gear from the FIRST CAD Library. It always worked for me (I <3 i-sprocket).

[Greg Needel]

One of the first things I do when designing a transmission is similar to you I take a look at the ratios I want to achieve and then make a spread sheet of the available gears and their pitch diameters. Using this info I can play with gear sizes to obtain the desired ratios.

When going to CAD the transmission I recommend not doing the teeth features to start and just making cylinders that have an outside dimension of the pitch diameter. This makes the components much easier to model and will accomplish the goal of having proper geometric alignment, even if it doesn't look as pretty.

The other thing you should note about 2 speed transmissions is you need to have enough size on the face of the output gear to have a dog (or any other shifting mechanism) engage the face of the gear. Alot of this has to do with how compact you are trying to make your transmission as it will limit the ranges of your shift. Without doing any calculations I suspect a gearbox that has a 4 to 1 shift will have to be rather large even using 20DP gears just due to the sizes required for the gears so their center to center distances align properly. Typically 2 speed transmissions are more in the range of 2-3 times a shift. So you will have to decide if you need that speed range.

If you need any further help, please continue to ask. We all started where you are. Just don't give up we are here to help.

[JVN]

I typically design using cylinders at PD (as Greg described). I end up changing my gearing approximately 100,000 times during each design process so these approximations make my life way easier.

Once I've finally finished the design and I want to drop in "real life" models I go to www.bostongear.com and use their BostSpec2 service to download the models for the gears I'm using. (We don't typically make our own gears, we just buy Boston Gears and modify them.)

Check out www.bostongear.com they have some great stuff.

Then again... lately I just use AndyMark products as my base. They are great and easy to mod for our uses.

Hope this helps!
Regards,
John

[Nikhil Bajaj]

AndyMark now has a 4:1 high/low ratio option for their shifters, it's an optional gear set. It works very well once you have the original shifter (Super Shifter, or Gen 1) set up.

Our drive train uses the guts of the Super Shifter but one of our drive module plates is the same as the side plate of the Super Shifter (we CNCed it to match but also hold our wheel and tensioner shafts.) If you are really excited about building your own shifting transmission, that's awesome, and there are plenty of people on this site (myself included) who would be happy to offer feedback and help.

However, if you just want a drivetrain that has 28:1 low speed and 7:1 high speed, I'd recommend taking an AndyMark shifter with the optional gear set, and then finishing your ratio with some nice cheap sprockets, which would make it easy to get another 6/7 speed reduction on top of that, so you get 24:1*(7/6) = 28:1.

That said, building custom gearboxes is often very rewarding, and I'd say you can't really make a wrong decision going either way.

[,4lex S.]

I have tried experimenting with the spur gear feature in inventor but I am unable to get it to work. Does anyone know of a guide for this or can anyone help?

It took me a while to get design accelerator working, but I might have some useful advice. First of all, accelerator only functions in assembly mode (as far as I know). Second, Inventors calculations may be the problem. I would suggest just accepting any conflicts it might have due to the calculations. I am sorry if I am diagnosing the obvious (I don't know how much inventor experience you have). If I haven't diagnosed your problem effectively, some clarification as to where you are getting caught up would be helpful.

[roboticWanderor]

Originally Posted by Greg Needel

One of the first things I do when designing a transmission is similar to you I take a look at the ratios I want to achieve and then make a spread sheet of the available gears and their pitch diameters. Using this info I can play with gear sizes to obtain the desired ratios.

When going to CAD the transmission I recommend not doing the teeth features to start and just making cylinders that have an outside dimension of the pitch diameter.

Originally Posted by JVN

I typically design using cylinders at PD (as Greg described). I end up changing my gearing approximately 100,000 times during each design process so these approximations make my life way easier.

That's how I worked out the spacing and gearing of my dual CIM gearbox that I designed. Getting the drawings off of the suppliers website might be your best bet in terms of inventor, however [Solidworks fanboy] SolidWorks Toolbox feature has a great gear, sprocket, etc creator that works very well, so if you are looking to make your own drawings that is your best option [/fanboy]

be sure to check your math and measurements and use the tools available to you, such as the JVN calc. Happy CADing!

[kramarczyk]

We built this exact set-up for overdrive with AM parts as Nikhil suggested.

AM SS with 4:1 span and 48:17 output into a 24:22 chain reduction (AM sprockets of course).

It works fine on the long breaks, but be advised, it can be expensive from a power consumption standpoint (especially with a skid steer). If you map your floor speed to power and efficiency you'll get the picture. Since I already have the plots I've included them.

[Andy Baker]

Originally Posted by kramarczyk

We built this exact set-up for overdrive with AM parts as Nikhil suggested.

Not only did Mark's team use this setup, he was the friend who suggested this ratio as an option. Last fall, at the ARC event in Warren, Michigan, Mark and I were talking about the need for a wider reduction between low and high.

A good number of teams used this option on their Shifters last year.

Thanks to Mark for making this suggestion.

Sincerely,
Andy