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Old 11-23-16, 04:44 PM   #1
trw92a
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Locking differential for NASA Challenge Rover

I'm helping my brother design a part for his Nasa rover challenge project

https://www.nasa.gov/roverchallenge/home/index.html if you want to check what it's about.

It's a 3 wheeler (2 front, 1 rear) with an open differential for the front wheels. We were talking about how most rovers failed the obstacle course *generally* because of lack of grip and no crawler gear for the hills. Even rovers with lower gears struggled with grip due to wheel slippage with open differentials.

A limited slip diff may be too complicated and costly, but is anyone aware of mechanical locking differentials available for sale? Or open differentials (for pedicabs and other trikes, assuming they're not 1 wheel drive)?

The wider wheelbase plus the weight of two riders makes having a solid axel (no diff) not an option because of the detriment to agility and handling
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Old 12-01-16, 05:15 PM   #2
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Originally Posted by trw92a View Post
I'm helping my brother design a part for his Nasa rover challenge project

https://www.nasa.gov/roverchallenge/home/index.html if you want to check what it's about.

It's a 3 wheeler (2 front, 1 rear) with an open differential for the front wheels. We were talking about how most rovers failed the obstacle course *generally* because of lack of grip and no crawler gear for the hills. Even rovers with lower gears struggled with grip due to wheel slippage with open differentials.

A limited slip diff may be too complicated and costly, but is anyone aware of mechanical locking differentials available for sale? Or open differentials (for pedicabs and other trikes, assuming they're not 1 wheel drive)?

The wider wheelbase plus the weight of two riders makes having a solid axel (no diff) not an option because of the detriment to agility and handling
What kind of surface are you running the vehicle on? If it is on sand or loose dirt, then I'd look back at using no differential.

For forward only, using a dual ratchet should work. Consider an axle that attaches to a chain and freewheel on the right, and a chain and a freewheel on the left. Unfortunately you would lose the ability to go in reverse.

You might be able to modify it so that you have a set of reversing gears like a lathe uses. So, your drive would be like:

Cranks ==(chain)==> Idler Shaft ==(2 chains)==> 2 independent freewheels on stub shafts. ==(lathe reversing gear set)==> Stub Axles.

Thus, for reverse, throw your two reversing gears, and continue to pedal forward. For forward, the ratchet allows slippage when turning.

PowerCranks use a smooth bearing ratchet. A bit different style if you don't want the clicking.
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Old 12-01-16, 05:46 PM   #3
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You want a Torsen differential.

Locking differentials are generally appropriate for high torque loads (4x4 or drag cars), which you won't have.

Find the smallest one you can, something off an old Audi quattro might be ideal.
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Old 12-02-16, 07:56 AM   #4
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Originally Posted by trw92a View Post
I'm helping my brother design a part for his Nasa rover challenge project

https://www.nasa.gov/roverchallenge/home/index.html if you want to check what it's about.

It's a 3 wheeler (2 front, 1 rear) with an open differential for the front wheels. We were talking about how most rovers failed the obstacle course *generally* because of lack of grip and no crawler gear for the hills. Even rovers with lower gears struggled with grip due to wheel slippage with open differentials.

A limited slip diff may be too complicated and costly, but is anyone aware of mechanical locking differentials available for sale? Or open differentials (for pedicabs and other trikes, assuming they're not 1 wheel drive)?

The wider wheelbase plus the weight of two riders makes having a solid axel (no diff) not an option because of the detriment to agility and handling
I think Higgins welding in Minnesota might have something you can work with, i've been looking at their stuff recently, looking into changing my one wheel drive delta trike to two wheel drive
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Old 12-02-16, 12:20 PM   #5
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? Rear (1) wheel drive FTW
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Old 12-02-16, 04:29 PM   #6
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There are some German Made Delta Recumbents , which drive both rear wheels but You are also needing to steer with those front wheels

Make some Ball type universal joints in your Machine shop .

Or How about Using wheel chair wheels up front, with the handrails in place ?
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Old 12-02-16, 04:49 PM   #7
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Some trikes have a standard differential that can be easily adapted. There was a surplus company that sold similar miniature differentials from small industrial equipment, but I've lost the link at the moment.

The problem with a standard differential is that both wheels will drive on a hard surface, but it is easy for one wheel to break free and spin on a loose surface.

On a 4x4, there usually is a differential between the front and rear, so a single wheel breaking loose could stop the vehicle cold.

A cheap option... say you have 2 "engines" (two people pedalling), then you could give one person control of the front end, and another control of the rear end, and thus give full-time power (limited to what a single person could put out) to each axle.
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Old 12-06-16, 07:33 PM   #8
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A simple low weight design could use 2 freewheeling front wheels on a common powered axle(no reverse).

In a turn, the inner wheel provides drive and the outer wheel can overrun it. If the inner wheel slips, the outer will drive the unit.
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Old 12-07-16, 10:58 AM   #9
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Of course since the way 2 wheel steering works best is each wheel turns at a different Radius .

so the design with 2 freewheels will be both inboard with separate drive axles and Universal joints at the individual wheels
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Old 12-07-16, 11:03 AM   #10
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A simple low weight design could use 2 freewheeling front wheels on a common powered axle(no reverse).

In a turn, the inner wheel provides drive and the outer wheel can overrun it. If the inner wheel slips, the outer will drive the unit.
Now that is clever!
I'm impressed!
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Old 12-12-16, 02:44 PM   #11
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Originally Posted by CliffordK View Post
Some trikes have a standard differential that can be easily adapted. There was a surplus company that sold similar miniature differentials from small industrial equipment, but I've lost the link at the moment.

The problem with a standard differential is that both wheels will drive on a hard surface, but it is easy for one wheel to break free and spin on a loose surface.

On a 4x4, there usually is a differential between the front and rear, so a single wheel breaking loose could stop the vehicle cold.

A cheap option... say you have 2 "engines" (two people pedalling), then you could give one person control of the front end, and another control of the rear end, and thus give full-time power (limited to what a single person could put out) to each axle.
Small correction, most 4x4 vehicles have locked and unlocked center 'diff', when it's locked, assuming open diffs front and rear, you'll have (1) front and (1) rear powered wheels.

In an 'all wheel drive' or 'full time' type system, if the center diff is not locked you'd be correct, (1) wheel front or back, assuming no technological aids such as computer vectoring, braking and who knows what else.
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Old 12-12-16, 02:47 PM   #12
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Originally Posted by trw92a View Post
I'm helping my brother design a part for his Nasa rover challenge project

https://www.nasa.gov/roverchallenge/home/index.html if you want to check what it's about.

It's a 3 wheeler (2 front, 1 rear) with an open differential for the front wheels. We were talking about how most rovers failed the obstacle course *generally* because of lack of grip and no crawler gear for the hills. Even rovers with lower gears struggled with grip due to wheel slippage with open differentials.

A limited slip diff may be too complicated and costly, but is anyone aware of mechanical locking differentials available for sale? Or open differentials (for pedicabs and other trikes, assuming they're not 1 wheel drive)?

The wider wheelbase plus the weight of two riders makes having a solid axel (no diff) not an option because of the detriment to agility and handling
Depending on the type of diff it is, you may be able to modify it to be 'locked'. Some 4x4 guys weld the diffs, which effectively makes it 'locked'. It's cheap and easy. Not sure how similar the diff you are using is to full size vehicle.

http://www.instructables.com/id/One-...-Differential/
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Old 12-12-16, 03:07 PM   #13
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What were the locking hubs that people used to have on their vehicles.. where one would have to step outside and twist a lock on the hubs (what a pain). I suppose that was only on the front hubs.

I could imagine building two sleeves with ratcheting pawls similar to what was described above. Then if they needed to be locked, manually insert a locking pin.
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Old 12-14-16, 02:55 PM   #14
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https://www.warn.com/truck/hubs/hubs.jsp
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Old 12-14-16, 04:53 PM   #15
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The rules are pretty strict about not using commercially available parts. Fold up in a 5x5x5 foot container that has to be able to be carried by 2 people for a set distance and be 15 inches above a flat surface with passengers on board. Need a lot of design coordination and weight savings depending on your initial vehicle concept.

That being said, consider a torsen limited slip design for maximum flexibility and reliability plus consider the stated turning radius. Your tire design, tread pattern, etc will be important to navigate the course conditions they have setup. I just glanced at the rules and probably missed a lot of the details/fine print, but wish you luck on your project. It certainly will be a challenge if you have to design and make almost all the parts (except the bearings and grease/oils).
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Old 01-09-17, 04:46 AM   #16
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Maybe the vehicle is already finished by now, but I think it's an interesting question that gave me some (wild) ideas. First, slippage is always going to be an issue with a front wheel driven vehicle going uphill because of the weight transfer to the rear. So with two driven wheels at the front and steering with the rear wheel (the sensible lay-out imo) you'll need to have a lot of the weight of the pedallers on the front axle.

My first idea is not to use a differential at all. Have each of the pedallers drive it's own front wheel. They will have to synchronize when going in a straight line, but that can't be that hard if they are next to eachother and they feel the resistance of slippage when they go out of sync. In corners one will just have to pedal harder or shift to a higher gear.

You might also want to consider a viscocoupling to connect the wheels. It's simpler and cheaper than a Torsen and they are used in different industrial devices with different kinds of oils with their own required properties. It will be quite difficult to find the right properties of the oil and the plates to get it right for this application. But maybe that's part of the fun.

Another possibility might be to connect the wheels with a CVT like coupling, rubber belts and conic pulleys. You could mechanically adjust the cones with the steering movement, so power output of both pedallers could be 100% while cornering and climbing. You could also use an of the shelf NuVinci CVT for bikes and connect it's normal control lever to the steering, but that would probably break the rule about commercially available parts.
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