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
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
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
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.
im not testing though. we wanna make a full size, then correct it, and we need a bearing-of-sorts to hold it
Ok, so your final design plans will be wood?
maybe, maybe not. instead of questions, any ideas?
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
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.
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.
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.
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.
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.
Also, concerning brushes, we used a 23" diameter outer helix combined with and offset by 180 degree inner spiral that about 12" in diameter and a piece of 2.5" PVC in the middle spinning on a CIM attached to an old BaneBots transmission. This was quite enough to get the ball up our four revolutions to the top (about 4’) using no traction tape or brushes, just slick PVC against the orbit balls. I should add that we lost our single sad orbit ball on Saturday so we had to use a basketball stand-in for some of our testing. The diameters are the same but the basketball is much much heavier than the orbit ball and our setup was still enough to move the much heavier ball up our helix. Granted the basketball is much stickier than the orbit ball but it’s weight should be enough to offset any benefits top be had by it being stickier.
We’re using ABS (black plastic sewer pipe) for our rollers, and we have a wood robot they are mounted to. They are mostly all chain driven.
We discovered that the 3" ABS caps that some stores sell are flat on the ends, and also they are a pretty close fit to the inside of 4" ABS pipe, as well as to the outside of 3" abs pipe, so the 3" caps can be used for two different sized rollers.
There is a neat thing called a “flanged radial ball bearing”, and it can be used in a chain sprocket, or put into a hole drilled in that ABS end cap. When the bearing is supported on the end of a bolt, then you have a complete roller, and it’s pretty easy to make it driven by a motor/transmission by #35 chain (same as the drive wheels use).
http://photos.project1726.org/albums/userpics/10009/normal_IMG_0100.JPG
this is the cap with a sprocket installed, and an FR8ZZ bearing installed. This is the type of sprocket that came in the kit, and it needs to have the hole slightly enlarged (a half round file will work) to make the bearing fit. You can drill the 1-1/8" hole in the cap with a “Forstner Bit”, and it should give a good fit for the bearing in the end cap for the other end of the roller.
I think everyone answered your question. But may I suggest using like poly ethylene or some other plastic instead of copper for the out side? depending on how much copper, you might be raising your center of mass unnessarily by putting on too much weight. Make sure to cut plenty of holes in the outside too. 