pic: drivetrain idea (comments please)

[sdcantrell56]

I would also agree that directly driving the center wheel if at all possible is the best solution. As has been said even in some extreme case that you throw every chain, you will still be mobile and able to play the game. Also with this extra safety, you can run#25 chain or even timing belt without worrying that if one breaks you will be immobile. As far as using the kitbox in direct drive it can be as simple as having a new output shaft fabricated and making a hub for the center wheel or you could fab up completely new plates and only retain the gears form the kit transmission. Either way it is relatively easy to do and I would be willing to show you some designs I have done using kit stuff. Also I'm sure you could find a machine shop to fab up a new output shaft to keep cost down.

[=Martin=Taylor=]

Originally Posted by sdcantrell56

I would also agree that directly driving the center wheel if at all possible is the best solution. As has been said even in some extreme case that you throw every chain, you will still be mobile and able to play the game.

IMO if you're expecting to break #25 chain you are doing something wrong

Aside from the fact that even with 3 chains at least one wheel will always be powered if one chain breaks (I can attest to this having happened ).

Most of the off-the-shelf gearboxes (with the exception of the AM SS) require a second reduction after the gearbox, especially with 6" wheels. A second reduction will also allow you to "tune" your speed.

Just my 100 sense.

[AndyB]

I would recommend running your chain like so:

[] = wheel
s = sprocket
c = chain

[]s
  c
  ss  (GEARBOX HERE)
   c
[]ss
  c
  c
  c
[]s

Like so:

http://andyburchardt.com/robotics_re...0Drive%201.jpg

[sdcantrell56]

I never expect to break a #25 chain in the drivetrain and in fact we ran the entire season without ever having to replace the drive chains. However, for some people who seem to still be shy about using #25 chain and reaping the great weight savings, using it in a drivetrain where the center wheels are directly driven gives an even greater incentive to use the lighter chain.

[DonRotolo]

#25 chain is more sensitive to proper tensioning. #35 allows teams to be really sloppy and still quite effective.

As to the original question: I like the idea, very similar to our 2007 robot. We used a kitbot frame with a second level of frame at the height of the top of the wheels, primarily to get a flat platform.

Your implementation seems somewhat heavier than necessary. The inner rail doesn't add much strength, it can be 1/16" wall, while the outer rail will take a beating, count on bumpers to stiffen it and lend impact resistance.

You also need to consider cross-rails from outer to inner rail, and again from one inner rail to the other - that way, an impact's force is transmitted to all four elements, not just the one impacted.

If you can mount the gearboxes lower - gotta think about the sprocket wrap - you can end up with a flat (or nearly so) platform over which your manipulator can be built.

Don

[bmarick]

why did you put the gear box in between the back set of wheels?
Our team did a direct drive of the center wheel and then chain drive to connect the two outside wheels for power this worked great and we did not have to drop the center wheel.

[R.C.]

Originally Posted by bmarick

why did you put the gear box in between the back set of wheels?
Our team did a direct drive of the center wheel and then chain drive to connect the two outside wheels for power this worked great and we did not have to drop the center wheel.

As mentioned earlier, there are many different types of drivetrains and each one is unique. 4 wheel omni's, offset 4 wheel omni's, crab, rocker 6 wheel drive, direct drive 6 wheel drive, 2 wheel drive with casters, and the list goes on. It all depends on your team. We do the exact same thing this team does but we have a two stage frame:

We are able to make braces for the gearbox and we mount them lower so it IS NOT a 60 degree wrap, it is less than a 30 degree wrap. A direct drive does have its advantages and disadvantages.

[M. Mellott]

I would add a cross-brace (1x1 square tubing, 1/16" wall) to keep the frame strong and square...maybe two. They would also serve as additional mounting points for electronic boards and other devices.

As already suggested, using 1/16" wall tubing for the inner frame members is a good idea, but it may not be readily available in the 1x2 size you've chosen (not saying that it's not out there). In the case where you cannot find the 1/16" wall stuff, 1/8" wall U-channel would be a good option to save weight.

[JOhnch181]

for cross bracing are team used a heet of 1/2" plywood to create reidgity from side to side. It has the second benefit of giving you a flat surface.

[AdamHeard]

You'll really want to attach the frame members at more than just the ends. I would attach the two outer rails to each other between each wheel at a minimum, and since you're welding, it'll be real simple to do.

Originally Posted by JOhnch181

for cross bracing are team used a heet of 1/2" plywood to create reidgity from side to side. It has the second benefit of giving you a flat surface.

You can use just about any material, but if fastened in enough places, it creates a sheer-web that greatly increases the rigidity of the frame. Depending on the material, you may also need a centered crossbrace to keep it from sagging.

Originally Posted by M. Mellott

I would add a cross-brace (1x1 square tubing, 1/16" wall) to keep the frame strong and square...maybe two. They would also serve as additional mounting points for electronic boards and other devices.

As already suggested, using 1/16" wall tubing for the inner frame members is a good idea, but it may not be readily available in the 1x2 size you've chosen (not saying that it's not out there). In the case where you cannot find the 1/16" wall stuff, 1/8" wall U-channel would be a good option to save weight.

I have found 1/16" wall 2x1 to be very available.

[bmarick]

definitly the cross brace or extra bracing is going to be needed. Otherwise it will bow over time. Can some one explain the degrees of rap to me. I'm still a little new to the drive train systems.

[AndyB]

You want 120 degrees MINIMUM of chain wrap on each driven/driving sprocket.

[Daniel_LaFleur]

Originally Posted by bmarick

definitly the cross brace or extra bracing is going to be needed. Otherwise it will bow over time. Can some one explain the degrees of rap to me. I'm still a little new to the drive train systems.

A sprocket is basically a circlular object (360 degrees). You want the chain to wrap around at least 1/3 of the sprocket (120 degrees). I personally suggest at least 150 degrees wrap with no more than 180 degrees.

The more wrap you have the more forgiving your drivetrain will be, but you trade off that forgiveness with added drag and friction. Going less than 120 degrees of wrap invites the chain to jump off of the sprocket.