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Custom Gearbox

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kelleybe View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kelleybe Quote  Post ReplyReply Direct Link To This Post Topic: Custom Gearbox
    Posted: Jun/14/2017 at 12:00pm
Hello,

Our team is now 4 years strong, and we are looking to design a custom gearbox.  We want to know where to start.

We have access to Shigley's Mechanical Design, 10th Ed., and we will be working with Timken bearing company.

We have been finding different reports from 2012-13 Michigan baja, Akron, and some others, but we are having a hard time with what initial assumptions to make, like output torque to the wheels we want/need, CVT final drive ratio on low and top ends, shift points, efficiencies, losses, etc.

We had a senior design project last year that designed a gearbox, but their knowledge was not passed on, and I am the drivetrain leader this year.

Please help!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adam_Clark Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 2:14pm
If you search through the forums you will be able to find almost all the answers to your questions. Specific numbers are up to you and how you would like your car to perform. Apply what your schooling has taught you.
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Brad SXT View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brad SXT Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 10:16pm
You've already got the first part right. Start early.

You've got the second part right as well. You know the topics of some of the major assumptions you need to get started. We won't spoon feed you numbers.

What exactly are you looking for from us? You'll need to develop your own models for vehicle acceleration and gear/case/shaft loading.

Sounds like you're on the right track but there really is no way to short circuit the design work on this one.
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kelleybe View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kelleybe Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 8:53am
What's your opinion on investment casting vs. straight machining?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 12:06pm
unless your school has a professor working on casting processes or can get a sponsorship for it, your best bet will likely be straight machining. 

With straight machining, it is easy to get raw material and CNC time from a school lab or local machine shop, especially for something simple and easy like a gearbox half. run times are generally low, programming is easy, and usually they do not require 5 axis. the other advantage is that all the features get machined at the same time ( bearing bores, gear clearances, centering pin holes, etc) so is is much easier to locate the material as you just need to ensure the part fits inside the stock. 

With casting, you will need to construct a mold cast the part, then mount and locate the part on a machining center to machine your bearing bores and other necessary features. this is not very cost effective for a one off part. this being said, if you can get the casting process sponsored, I would go that way as from a design and cost look for high volume production (remember goal of competition is to design something you can make 5000 units of per year), casting will be a major advantage. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brad SXT Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 1:03pm
Originally posted by RLM RLM wrote:

this being said, if you can get the casting process sponsored, I would go that way as from a design and cost look for high volume production (remember goal of competition is to design something you can make 5000 units of per year), casting will be a major advantage. 


Casting offers no additional design points over machined. Design points are awarded for showing the engineering cycle for a component. This can be done for any material and manufacturing method. 

We CNC'd our gear case so we could spend our time engineering other components rather than trying to figure out casting from a logistic standpoint.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 1:43pm

Originally posted by Brad SXT Brad SXT wrote:

Design points are awarded for showing the engineering cycle for a component. 
This part I do agree with you. A good engineering design cycle is crucial to doing well in design at this competition, but part of that cycle should also include manufacturing cost analysis as the design goal of this competition is to "Each team's goal is to design and build a single-seat, all-terrain, sporting vehicle whose structure contains the driver. The vehicle is to be a prototype for a reliable, maintainable, ergonomic, and economic production vehicle which serves a recreational user market, sized at approximately 4000 units per year. The vehicle should aspire to market leading performance in terms of speed, handling, ride, and ruggedness over rough terrain and off-road conditions. Performance will be measured by success in the dynamic events which are described in the Baja SAEĀ® Rules, and are subject to event-site weather and course conditions." - Baja SAE Rules 2017 Section A1.2

That's why I disagree with you on this:

Originally posted by Brad SXT Brad SXT wrote:

Casting offers no additional design points over machined. 

In good product development you need to look at both the component performance, but also the marketability of the product. Seeing as this vehicle is supposed to be a prototype of a production vehicle, if the team can design in such a was as to reduce cost of manufacture and increase ease of manufacture, all while not impeding performance of the part or overall vehicle, does this not deserve additional design points? reducing the cost of a product will help with the marketability of the product and thus drive sales up. this is something that should always be looked at in the engineering cycle. as a starting point, getting something working is the first step, then comes, making it work better, then how can we make it work just as well, while reducing the overhead cost to make it. this is why casting ( especially in this application) if designed well, should command more points over an equally well performing fully machined gearbox. if you disagree with this, then please explain to me why no-one in mass production of atv's, or automobiles, uses a fully machined from billet transmission housing? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BlueDevil08 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 2:03pm
Originally posted by RLM RLM wrote:

this being said, if you can get the casting process sponsored, I would go that way as from a design and cost look for high volume production (remember goal of competition is to design something you can make 5000 units of per year), casting will be a major advantage.

There needs to be some follow up to Brad's comments.  I think the way that both Brad and I read your initial comment was that by casting your gearcase it automatically equates to more design points.  

Originally posted by Brad SXT Brad SXT wrote:

Design points are awarded for showing the engineering cycle for a component...Casting offers no additional design points over machined. 

This should have been phrased that casting offers no additional design points over a machined case inherently.  It is up to the designer and ultimately the presenter to explain why casting was selected over machining from and engineering standpoint and have the necessary data, analysis, and testing to support the decision.  The same can be said for a team that chose machining over casting; it works both ways.  

Originally posted by RLM RLM wrote:

 Seeing as this vehicle is supposed to be a prototype of a production vehicle, if the team can design in such a was as to reduce cost of manufacture and increase ease of manufacture, all while not impeding performance of the part or overall vehicle, does this not deserve additional design points?

This is absolutely true and would be awarded design points IF AND ONLY IF the presenter explains this to the judges, has the necessary data, analysis, testing, etc. to support his/her claims.  If you did not see RIT's design finals presentations in Kansas or Illinois they specifically talked about designing to reduce cost without sacrificing performance and gave examples of areas across the entire vehicle of how and why they did it.

Originally posted by RLM RLM wrote:

then please explain to me why no-one in mass production of atv's, or automobiles, uses a fully machined from billet transmission housing?

Keep in mind that, as you stated earlier, these are prototype vehicles.  While you are correct that the production level ATVs, cars, etc. do typically use some form of casting throughout their production parts, at the prototyping stages of development they are not typically made from the same exact manufacturing process as is used for full production.  Producing tooling for a 1 off part or even a very low volume of parts may or may not be more cost effective over a fully machined housing; especially very early on in development as any design change will require a tooling modification or in worst cases a new tool.


Edited by BlueDevil08 - Jun/15/2017 at 2:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brad SXT Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 2:45pm
This is turning into one of the best discussions I've seen on the forum for quite a long time. 

Alex did a great job rephrasing what I was trying to say. There are no inherent design points associated with fancy widgets or different manufacturing processes and I don't want teams to get that impression. It's all about the design and engineering that goes into them.

On a side note, thanks for being on these forums RLM. You are one of very few people who still routinely respond to all levels of questions on here. I particularly like the $10 fee for spoonfeeding. Anybody pay up yet?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bracer Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 4:47pm
Designing a new gearbox is an overwhelming task to the unexpecting. Kudos to you for starting early! This should allow you to get most everything mostly figured out with minor tweaks for system integration.

I know in years past our team hasnt set output torque values, we have normally set our wanted too speed within reason and used that for calcs.

Depending on what cvt you're using, you should be able to find realistic values for the ratio ranges. Shift points of the car are all based on your vehicle, it takes a fair amount of time to tune in a cvt system to a car, and the differences it makes are crucial for a well performing system.

A quick and dirty way for getting a total efficiency is to calc out what your theoretical tip speed is, and then using a long stretch of flat land and a GPS or some other speed measuring device, find your actual top speed.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/16/2017 at 8:17am
Alex, Thanks for clearing everything up, makes much more sense now and I 100% agree with both you and Brad. I would still love to see some of the more advanced teams with tried and true gearboxes, push their designs further to a more mass manufacture style box. these are the teams which more or less run the same gearbox year after year, with very few tweaks because it works very well. 

I really like seeing that teams are starting to push their prototyping process further and further each year. I love seeing teams 3D print full size or scale models to verify clearances and simulate motion. Each year this competition pushed the limits further and further and I couldn't be happier about it. I just wish I could still compete. 

Brad, In response to your side note, no one has paid yet, but the number of PM's requesting spoonfeeding has seen a decline. Further testing is required. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kelleybe Quote  Post ReplyReply Direct Link To This Post Posted: Jul/28/2017 at 1:21pm
Update:

So I have done calculations based on wheel sizes, gradients, surface condition, acceleration and vehicle weights.  Basically it's a huge excel file that asks you to put in vehicle weight and vomits out a bunch of tables with different scenarios.  I want to take the  theoretical torque values from this and compare them to the physical max tractive torque of our 2016 car to see whether the theoretical MTT is within a reasonable value of physical.

For a MTT or more commonly Total tractive effort (TTE) test, I was thinking about using corner weights to weigh the car, then do a sled pull and just load it up with weights until the car gets stuck and use the horizontal component of force and use that to estimate the physical MTT at the wheels.  I feel like once I get over this road block...Wink...I know most of the other initial things I have to do to get the project...rolling. Cool  Or I could try and build a small dyno...but that would take too much time I think.

Any suggestions?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jul/28/2017 at 3:24pm
seems reasonable to me. 
dont forget to take into account the vertical component of the pulling force that will add (or subtract depending on angle of attachment chain) to the vertical force on the wheels of the vehicle.

Just a side note, having a roller dyno will also make cvt tuning a breeze down the road.
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