pic: Stackerbox Mecanum Prototype

[gblake]

Originally Posted by JesseK

-- To tension the chains, simply mount the chains and slide the transmissions.
[/cdm-description]

JK,

Without an active tensioner, my hunch is that the Stackerboxes will inevitably slide in the directions the chain is pulling them.

I'm guessing that you will need something actively pulling the sliding motors/stackerboxes toward each other to keep the correct tension on the chains; perhaps a spring or one of those gizmos that has an eye bolt on each end and has an oval you twist to move the bolts closer or farther apart.

Also, to avoid having the chain tension creating a moment that wants to twist the shaft the drive sprocket is on, a bracket that wraps around the sprocket and supports both ends of the shafts could be what is at each end of a tensioning system.

Blake

[MrForbes]

Originally Posted by gblake

one of those gizmos that has an eye bolt on each end and has an oval you twist to move the bolts closer or farther apart.

The hardware store guys call them "turnbuckles"



(I know you knew that....)

[gblake]

Originally Posted by squirrel

The hardware store guys call them "turnbuckles"



(I know you knew that....)

Thank You! - I spent 30 minutes trolling through the (not-so-useful) Lowes web site trying to find/remember that word and avoid having to embarass myself; and I failed

Turnbuckle, Turnbuckle, Turnbuckle, Turnbuckle, ...

Blake

[EricH]

Originally Posted by gblake

JK,

Without an active tensioner, my hunch is that the Stackerboxes will inevitably slide in the directions the chain is pulling them.

Depends on attachment. I've never seen the tensioning method described fail. It's standard Kitbot tranny attachment. Not sure how Stackerboxes attach, but if it's similar, there should be no problem.

[s_forbes]

Originally Posted by EricH

Depends on attachment. I've never seen the tensioning method described fail. It's standard Kitbot tranny attachment. Not sure how Stackerboxes attach, but if it's similar, there should be no problem.

Key word is depends... Our rookie year we used slots in the frame (for the wheel axle bolts) to act as our tensioners, but they always loosened up after several matches. I know this strategy has worked for other teams before, but there was something funny about that first robot of ours...

Also note that the kit bot has an aluminum bracket bolted to an aluminum frame; lots of frictional force! Lexan may not act the same way and might slip, but I don't know for sure. A turnbuckle may be a very good idea! (or one of those cam shaped tensioners, like on the 968/254/60 drive)

[EricH]

Originally Posted by s_forbes

Key word is depends... Our rookie year we used slots in the frame (for the wheel axle bolts) to act as our tensioners, but they always loosened up after several matches. I know this strategy has worked for other teams before, but there was something funny about that first robot of ours...

That's part of your problem. I've always seen it with the trannies. 330 '05 had this, with KOP tranny (slots in base plate) attached to the frame with bolts going into slots in the frame.

Also, why would anyone use Lexan as the drive frame? (And don't tell me to see the 2007 design book--that team used a metal frame to support their drive.)

[JVN]

I believe Team 20 used Stackerboxes for their Mecanum drive this year. It seemed to work out pretty good for them. I may be mistaken though.

[s_forbes]

Originally Posted by EricH

Also, why would anyone use Lexan as the drive frame? (And don't tell me to see the 2007 design book--that team used a metal frame to support their drive.)

I was referring to the lexan part of the stackerboxes, not the frame rails. (as for lexan in drivetrains, you could see 190's robot in 2006 if you want a really neat example )

[AdamHeard]

I usually wouldn't use a COTS turnbuckle to tension chain as it just seems big and unweildy..... But, if you got one that is about the length between the inner bolts (to the center of the robot) on one stackerbox and the inner on the other, and the eyelets were about the clearance size for a #10, you could tension both chain runs by pulling the gearboxes towards each other.

EDIT: on second thought, get a turnbuckle with plain threaded ends instead. Make a block with a threaded hole for that end on one face, and a clearance hole for a #10 on the perpendicular face (not vertical). Put that block on one of the inner #10s on each of the stackerboxes, and hook up the turnbuckle to them.

[JesseK]

Originally Posted by s_forbes

Also note that the kit bot has an aluminum bracket bolted to an aluminum frame; lots of frictional force! Lexan may not act the same way and might slip, but I don't know for sure. A turnbuckle may be a very good idea! (or one of those cam shaped tensioners, like on the 968/254/60 drive)

A steel washer could be put in between the lexan face plate and the frame to create the same frictional force.

Now that I think about it, an aluminum or thick plastic cam as a locking mechanism could be useful once the chains are tensioned and the four transmission bolts are tightened. This may be the simplest and most lightweight option. Cam design is also an area of CAD I haven't explored yet.

Originally Posted by Water Bandit23

This might be a stupid question, but what type of metal is the frame made out of?

As shown & calculated it is 6061 T6 Aluminum. This reminds me, I need to make some adjustments and figure out how the cyllindrical plate spacers can be welded in such a cramped area...any ideas?

Originally Posted by gblake

Also, to avoid having the chain tension creating a moment that wants to twist the shaft the drive sprocket is on, a bracket that wraps around the sprocket and supports both ends of the shafts could be what is at each end of a tensioning system.

Blake

For the design I'm not worried about a moment on the output shaft, since such a force is usually the result of overtensioned chains or misaligned sprockets. I'd also like to note that the output shaft is already supported in 2 places by bearings and adding a third support is generally asking for too much complexity. A locking or permanent tensioning mechanism (as you suggested) combined with good build practices should circumvent this concern.

Originally Posted by AdamHeard

I usually wouldn't use a COTS turnbuckle to tension chain as it just seems big and unweildy..... But, if you got one that is about the length between the inner bolts (to the center of the robot) on one stackerbox and the inner on the other, and the eyelets were about the clearance size for a #10, you could tension both chain runs by pulling the gearboxes towards each other.

This seems like a great idea, but 1885's recent bad experience with tensioning systems that are dependent upon each other has put a bad taste in my mouth.

EDIT: on second thought, get a turnbuckle with plain threaded ends instead. Make a block with a threaded hole for that end on one face, and a clearance hole for a #10 on the perpendicular face (not vertical). Put that block on one of the inner #10s on each of the stackerboxes, and hook up the turnbuckle to them.

If implemented independently, this is probably the best way to deal with tensioning to make it zero-maintenance. I'll experiment with the cam system described above and this one for revision 2.

Thanks for the feedback guys, keep it coming!

[colin340]

i woul just move the stacker boxs cloeser to the wheel and go with a all gear drive Tran

[AdamHeard]

Originally Posted by colin340

i woul just move the stacker boxs cloeser to the wheel and go with a all gear drive Tran

In this case having chain as the final reduction is a good idea; provides a method of changing the final gear ratio without moving the gearboxes, also the frame can flex/bend slightly and still run.