PVC to wood bearings??

[helgy8]

Our team is having problems figuring out what to use for bearings in our situation. We are trying to make an archimedies screw for holding the Orbit Balls with the motor driving it ontop, but we cant think of what to put on the bottom to hold it. It needs to have good hold because it needs to hold the pressure of the orbit balls being compressed by two inches on every side. please and thanx

[Daniel_LaFleur]

Originally Posted by helgy8

Our team is having problems figuring out what to use for bearings in our situation. We are trying to make an archimedies screw for holding the Orbit Balls with the motor driving it ontop, but we cant think of what to put on the bottom to hold it. It needs to have good hold because it needs to hold the pressure of the orbit balls being compressed by two inches on every side. please and thanx

Couple of questions:
1> Is the shaft PVC or wood?
2> When running the Archemedies screw are you turning the 'screw' or the center shaft (used as a beater bar)?
3> How much flex will the outer surface of your Archemedies screw have?
4> What is the force required to push a ball, with that much compression, up the screw?

I have concerns that your compression is too much and will damage the balls as well as increase the load on the motor(s) of the Archemedies screw. You may want to look into having the screw shaft compress somewhat, rather than doing all the compression on the ball itself.

P.S. Welcome to Chief Delphi and FIRST

[helgy8]

1>PVC for the shaft
2>Shaft
3>very little, made of copper piping
4>need bearings for testing first.
What should we use for a bearing

[waialua359]

If its just for testing, you dont necessarily need bearings.
Our prototype did just fine without them.

[helgy8]

Originally Posted by waialua359

If its just for testing, you dont necessarily need bearings.
Our prototype did just fine without them.

But what would work. not necessarily wood, but we need to hold the pvs while spinning

[Vikesrock]

Originally Posted by helgy8

But would work. not necessarily wood, but we need to hold the pvs while spinning

I believe he is suggesting that you could use something much more simple for a prototype, something like a thick piece of plywood with an appropriately sized hole may work just to test out an idea.

[helgy8]

im not testing though. we wanna make a full size, then correct it, and we need a bearing-of-sorts to hold it

[waialua359]

Ok, so your final design plans will be wood?

[helgy8]

maybe, maybe not. instead of questions, any ideas?

[DonRotolo]

PVC to wood is OK if well-lubricated (grease). Some greases will attack PVC, but most will not. Unlubricated it will generate too much heat and melt the PVC.

You may want to use a slippery plastic instead, like PolyEthylene. Think of the clearish-white cutting boards you see, they are PE or PolyPropylene (PP) and, maybe 1" thick (2 layers?) with some grease in there should be fine.

There are many, many other materials you can use, but if one is much harder than the other the soft material will wear out.

Don

[waialua359]

The reason I asked is because ideally for us, we dont plan on using a PVC to wood interface. Instead, we plan on using aluminum as provisions to utilize bearings is easy.
Attaching bearings directly to wood without some kind of plate or backing wont be as precise and may wear out.

[EricH]

Originally Posted by helgy8

maybe, maybe not. instead of questions, any ideas?

The reason we ask questions is so we can help you better.

If you have something like delrin available, you could conceivably make a ring out of that, though I don't know how tough it is to move delrin (which can be fairly slick) against other plastics, like PVC.

You could also, instead of compressing the balls, use some form of bristles to move the balls up. This has been done before, and it will eliminate some other concerns that have been brought up. If you have the 2006 Behind the Design book available, look at team 694's entry.

[Dick Linn]

Actually, wooden bearings can be very effective, and have pretty good pressure/velocity values. If you don't like that thought, and you can use a simple bronze bearing, then you are good to go.

[Daniel_LaFleur]

Originally Posted by helgy8

1>PVC for the shaft
2>Shaft
3>very little, made of copper piping
4>need bearings for testing first.
What should we use for a bearing

Depending on the length of your PVC shaft, it's going to bend ... possibly a lot. I'm not concerned about it breaking, though, as PVC is very resiliant, however,This will make coupling on the bottom (away from your motors) difficult since the angle for the coupling will change as your load changes. You may want to also look at the side loading of the drive motor ... most motors/gearboxes don't like side loads.

You may also find that the balls will get hung up in the 'middle' of the Archemedies screw if the balls at the top/bottom of the screw bend the shaft enough to take the load off of the balls at the center of the screw.

My suggestion for a bottom bushing is a flexible metal pin (maybe a spring?), greased up good, that is long enough so that the change in angle does not pull the shaft off of the pin.

I would also suggest something more rigid for the shaft.

[shawn1231]

Our team just finished our screw prototype. We, instead of making a "bearing" big enough to hold the PVC, drilled a hole in a flat cap and put a bolt through it which we then put into a piece of wood at the bottom with a hole big enough for the bolt. It worked for our prototype but I would highly recommend doing something similar. Not only is a bigger "bearing" going to be hard to find, or make, but you probably have some bearing in the shop you could use to fit a bolt.

Also about the compression, I think you will find that you are compressing the balls way too much. Aside from the pipe bending or whatever other problems this creates it's just not necessary. Our team finished our screw on Saturday and with no compression at all it works quite well.

I'd like to know the diameter on your center shaft and what exactly you have prototyped so far. I mean how have you actually implemented the screw, because I know the concept and how we did it but it is likely you did it completely differently.