Axle mounts- Chain tensioning

[Jeff Sharpe]

First, a hearty thanks to all of you bigger teams for the support you've provided, and continue to provide, to us smaller rural teams. Hopefully it is helping raise the level of competition for all of us!

Todays dilemma is how to tie our 5/16" axles to the frame. We are using a simple skid-steer 4-wheel drive approach with 3/8"chain and sprockets. Last year we simply drilled holes through our Aluminum angle frame and used shaft collars to keep the axles and wheels in place. The wheels were direct tied to our sprockets and just spun on the stationary axles.
Chain tension was an on-going problem however. We used half-links as the chain stretched far enough but thought you all probably had some better ideas.

thanks again in advance! J#

[Patrick Wang]

A simple solution to this issue is to instead of simply drilling holes, instead cut slots.

By cutting slots at at least the pitch of your chain, you can ensure that you can slide the axle to the most tensioned positioned for the chain, then secure the mounts for the axle in the tensioned position.

For this solution you would have to make axle bearing blocks that accept the shaft and then bolt into the slots on your angle. instead of simply drilling into your angle.

[Matt Reiland]

I would definately invest in some bearings or at least oil impregnated bushings to reduce the drive line friction.

We have had good luck tensioning the chain using a 'jack' screw setup where you tighten a screw that pushes up against a block that is holding the axle. It will work extremely well

[Bduggan04]

Do you really have enough traction to need 3/8" chain. That stuff weighs almost 5 times as much as 1/4" chain. For every 3 feet of 3/8" chain you have, it weighs a half pound more than it would with 1/4" chain (3 feet 3/8"=.658lbs., 3feet 1/4"=.141lbs.). If you have the extra weight it may be acceptable, but if you're overweight in three weeks I would reevaluate the need for such a thick chain.

[Jeff Sharpe]

Matt- Are you suggesting that the 5/16 bearings provided with the wheels are not good enough and that we should let the axle shaft spin as well? Thanks for the input. J#

[Matt Reiland]

Whoops, Sorry, I didn't see in there the mention of the bearings in the sheels. I am so used to all of our sprockets and gears welded to all of the shafts. I would however still consider using a jack screw method for tensioning, it is very resistant to loosening up.

[maxgebhardt]

remember that the bottom of the chain will sag as you go forward, so either shell out some big bucks to msc for a spring loaded tensioner (bad idea), or take some sheet metal, and cut slots out, put an idler sprocket (not nylon), with as few teeth as possible on an axle, and use nuts on the ends, so you can shove it up as needed, and tighten the ends against slippage. note: the msc tensioners are $100+ and my team is poor, so we shy away from big numbers, but if you have the cash, they are the better option.

[Al Skierkiewicz]

Jeff,
An arrangement that will work for the whole season is to make a sub-chassis that the drive motor(s) mounts on. You can use a slot arrangement to mount this to the main chassis. The simple addition of a bolt and locknut (the bolt is mounted so as to pull back on the motor mount) arrangement will allow you to tension the sub-chassis as the chain stretches (break in). You loosen the mounting hardware, take up the slack with the tension bolt and then tighten the mounting hardware to hold everything in place. Use bushings to support all shafts that the chain engages. The bushings are available from Small parts and are worth the money to insure smooth running. Use the shaft collars to hold the shafts in place against the bushings and add a little oil every once in a while to keep everything moving.

[Rickertsen2]

MSC sells premade spring loaded tensioners that work quite well.

[Jim Meyer]

Be careful with spring loaded tensioners. Spring loaded tensioners are designed to work on the slack side of the chain. This works great when you never change the drive direction of the chain. Since most FIRST robots do change the drive direction of the chain, you need to tension both sides.

We've found it far easier to either have an adjustable fixed tensioner or adjust the center distance between the sprockets. The easiest tensioner (used by our OCCRA robotics team) is simply a piece of PVC (1") bolted to the frame next to the chain with a 1/4" bolt and large washer. You just loosen the bolt and move the piece of PVC to take up any slack.

[Cory]

Originally posted by Jeff Sharpe
First, a hearty thanks to all of you bigger teams for the support you've provided, and continue to provide, to us smaller rural teams. Hopefully it is helping raise the level of competition for all of us!

Todays dilemma is how to tie our 5/16" axles to the frame. We are using a simple skid-steer 4-wheel drive approach with 3/8"chain and sprockets. Last year we simply drilled holes through our Aluminum angle frame and used shaft collars to keep the axles and wheels in place. The wheels were direct tied to our sprockets and just spun on the stationary axles.
Chain tension was an on-going problem however. We used half-links as the chain stretched far enough but thought you all probably had some better ideas.

thanks again in advance! J#

If you are using 80/20, or any kind of extrusion, you can simply slide any kind of bracket, or mount you make when the chain needs tensioning

/me loves 80/20

Cory

[Jeff Waegelin]

Before, we've always used U-bolts in a slot to hold our shafts in. This year, we're going to make plastic pillow blocks to replace the U-bolts, but we're still going to have an adjustment slot for tensioning.

[Jeff Sharpe]

Thanks all for the input. We've decided to put the motor/gear/sprocket assembly on a 1/4"Al plate base which bolts to our chasis. To tighten the chain we'll unbolt and add washers until we reach a point where we can add another 1/2 link. J#