Sliding axle tension system

[ZeroValue]

This is my prototype (in cad) of a sliding tension system. It will fit inside a 2x1 aluminum frame piece. the cross screws are 8-32. Questions: Is it safe to pull on the tightening screws? how much should the "lips" go above and below the pocket in the aluminum to keep it from twisting?

[MrForbes]

If you used larger screws, like 1/4" or so, then it would be safer to pull on them. Expect #8 screws to bend pretty easily when used like that.

Also make sure that the screws can be removed easily if they do get bent.

[minisimon]

In addition to using large screws, you can increase the width of your hexagonal puller so that it almost touches the two sliders. Such a configuration won't attempt to bend the screw; only shear it, and the screw should be able to handle that better. You probably won't be able to use a simple hex standoff for the puller any more, but you seem to be up for some machining.

[joshy1323]

Originally Posted by minisimon

In addition to using large screws, you can increase the width of your hexagonal puller so that it almost touches the two sliders. Such a configuration won't attempt to bend the screw; only shear it, and the screw should be able to handle that better. You probably won't be able to use a simple hex standoff for the puller any more, but you seem to be up for some machining.

i agree completely

Originally Posted by ZeroValue

how much should the "lips" go above and below the pocket in the aluminum to keep it from twisting?

that depends on what you are using it for. what kind of force do you plan on putting on it that would make it twist?

It would be good to increase the screw size, but it's not necessary. Keep in mind, this is only my opinion. If you're tightening the chain enough to bend a screw, your chain is probably too tight. I'd suggest using a #10 screw, then you should be fine. Nice design.

[MrForbes]

I agree that it should not bend a #8 screw if everyting is just right. Unfortunately our friend Murphy makes things go wrong...and making the design less vulnerable to mistakes like overtightening the tensioners, seems to me to be good design practice.

[AdamHeard]

Increasing the screw size increases the mechanical advantage which will help it stay in tension better, and will obviously have stronger threads. Stronger threads makes them less likely to wear out.

I'd just suggest 1/4-20 or 1/4-28, I'd also buy a solid rod end or clevis rod end and use that rather than the custom piece you have (lile 6071K41 on mcmaster). It'll save time and eliminate machining ops.

[=Martin=Taylor=]

Well ideally you could calculate the diameter of the bolt you need.

Stress is equal to Force over area. If you increase the area of contact with the bolt you decrease the stress. (as someone already suggested)

But what IS the force? I'm not really sure.

The term "tensioning" is rather ambiguous in chain systems. According to the Diamond Chain company the deflection at the center of the chain should be between 2-3% of the center-to-center distance of the sprockets. But if the chain can deflect, it isn't really under tension is it? If there is no tension there is no force...

Something doesn't make sense...

[Al Skierkiewicz]

Remember that the forces on the cross screws are greatest when the tensioner is receiving forces from the chain in motion. You may not bend the screws while tensioning but I bet you will in a pushing match with anotheer robot.

[Al Skierkiewicz]

Remember that the forces on the cross screws are greatest when the tensioner is receiving forces from the chain in motion. You may not bend the screws while tensioning but I bet you will in a pushing match with another robot. BTW, nice looking design and CAD work.

[ChrisH]

Originally Posted by Al Skierkiewicz

Remember that the forces on the cross screws are greatest when the tensioner is receiving forces from the chain in motion. You may not bend the screws while tensioning but I bet you will in a pushing match with another robot. BTW, nice looking design and CAD work.

I agree with Al on this. But there is one other thing to consider. Screws need at least three threads worth of engagement to develop full strength, four is better. The hex shaped piece does not look to have enough thickness to have full engagement. You should also check the fastener lengths to ensure you are using a standard size.

Other than that it looks pretty good. By the way, it is customary in the design world to have others review your work as you are doing now. When your design decisions are questioned during the review process it is because the reviewers want to help you do a good job and have things turn out the way you want. So please do not be offended by any comments that you may disagree with.

Also, since all we are looking at is a picture, it is hard to tell the actual size of things. A few dimensions would help us evaluate just how much this design can take.

[Woody1458]

I'm confused as the where the wheel would fit. I see the axel going through but it seems the screw getting right in the way where a wheel would be.

[AndyB]

Originally Posted by Woody1458

I'm confused as the where the wheel would fit. I see the axel going through but it seems the screw getting right in the way where a wheel would be.

What you see goes inside a piece of 2"x1" aluminum. The wheel would be cantilevered off the side.

[Cory]

One issue not even addressed is that the only thing keeping the two bearings concentric is the axle. This is the reason we make ours have the two halves slide together. Then all the force is on the bearing housing itself and not side loading the shaft.

[DonRotolo]

Building on what minisimon and Cory have written, the side plates should touch in the middle (that is, fully enclosing the holding screws) and lock together, and then your tensioner would pull on that. Instead of machining the side plates out of huge chunks of aluminum, make a tube through which the holding screws would run, that are let into the side plates about half of the thickness. Maybe even make it a non-circular (outside) tube to allow the tensioner easy attachment and increase side load capacity.

Screws are great in tension, but less so in bending because the thread roots are huge stress concentrators. Also, imagine the screw does get bent - as squirrel wrote, how would you get it out?

If that doesn't make sense, PM me.

Very nice design! I think I'll be stealing it during build season!

Don