Need Help with Bearings for Shaft

[Codered741]

It doesn't seem necessary to use a tapered roller bearing for this application. You are already supporting the shaft with the pillow blocks to eliminate the angular misalignment of the shaft, so you really just need to support the axial thrust of the shaft. A thrust ball bearing will work fine for this, and be simpler and cheaper.
A)The tapered roller bearing is not being used for misalignment, it's for thrust only and will last for years.

Also, if you use the tapered roller, you may have to worry about the shaft actually lifting up from the radial forces of the pulley. (Not likely from the stepper, but still a potential problem.)
A)the weight of the antenna and the pillow block set lock screws will prevent any torsional stresses from lifting the antenna.

I agree that the Tapered Roller will work fine for a long time. I am merely pointing out that it is probably overkill, and IMO not necessary.

Tapered rollers, when used in opposing pairs, are great at resolving multi-directional forces. Eg: Bearings placed with opposing tapers at either end of the shaft will keep the shaft aligned in all directions, because of their ability to resist both thrust and radial forces. Of course this setup requires some method of pre-loading the bearings, thereby making the setup slightly more complex.

In the OP's scenario, using a steel ball or needle thrust bearing will resolve any forces at play, be somewhat simpler to implement, all while saving a few bucks.

The point about lifting the shaft is, as I mentioned, unlikely. But I felt it was worth mentioning while on the topic of bearing design.

-Cody

 

[Oso Rojo]

Wow guys, I went off to work with no response and I get home a few hours later and there's pages, Thanks!

The antenna and shaft should weigh less than 10 lbs and I'd like to get under 5 pounds. The issue is going to be mass. The stepper motor is being used to be able to precisely aim the antenna. This is the advantage of direct drive. The mass will become an issue when the antenna has to change directions. The lighter the structure, then quicker the stopping and changing directions.

I had though about using gearing between the motor and the shaft, but then I have backlash issues or the price goes way up to get anti backlash gears. For the $100 for the gears, I can get a lot of motor.

The main purpose of the bottom bearing is to keep the weight of the shaft off the motor and to keep the shaft from drifting down out of the pillow blocks.

For now, this is a proof of concept and will only operate indoors, so waterproofing is not an issue.

I think that addresses all the questions. I'll look again in the morning after I've got some sleep.

Oh, one more thing, the rotational speed of this thing will not be much more than 20 rpm.

 

[Oso Rojo]

I would recommend a belt reduction, at least 2:1, no more than 4:1 without an idler, for the stepper, rather than a direct drive.

If I use timing pulleys with a cogged belt, will there be backlash when reversing direction?

 

[Rbeckett]

No backlash unless the belt is very loose. I have none on my power feed.

Bob

 

[Richard King]

Here another idea. Take theirs off and put on yours.

http://www.amazon.com/Amplified-Dig...374712929&sr=8-1&keywords=rotating+tv+antenna

 

[Oso Rojo]

Interesting, It might be worth the $40 to dissect it and see what it does.

 

[Richard King]

Interesting, It might be worth the $40 to dissect it and see what it does.

When I was a kid, we had a cabin where we had lousy TV reception and my Dad had a store bought antenna rotator, I believe he bought it at Radio Shack Rich

 

[Oso Rojo]

There are a number of TV and light wait rotors available today. The problem is these don't duplicate exact bearings each time. For my project I need to be able to get about 1 or 2 degree accuracy and they just don't have that resolution capability. So this leaves me with either using a stepper motor and count the steps in software, or use a shaft encoder which will also need some custom software/hardware to keep track of the correct readings.

 

[SEK_22Hornet]

Wow guys, I went off to work with no response and I get home a few hours later and there's pages, Thanks!

The antenna and shaft should weigh less than 10 lbs and I'd like to get under 5 pounds. The issue is going to be mass. The stepper motor is being used to be able to precisely aim the antenna. This is the advantage of direct drive. The mass will become an issue when the antenna has to change directions. The lighter the structure, then quicker the stopping and changing directions.

I had though about using gearing between the motor and the shaft, but then I have backlash issues or the price goes way up to get anti backlash gears. For the $100 for the gears, I can get a lot of motor.

The main purpose of the bottom bearing is to keep the weight of the shaft off the motor and to keep the shaft from drifting down out of the pillow blocks.

For now, this is a proof of concept and will only operate indoors, so waterproofing is not an issue.

I think that addresses all the questions. I'll look again in the morning after I've got some sleep.

Oh, one more thing, the rotational speed of this thing will not be much more than 20 rpm.

As for anti-backlash gearing, for what you are doing the simplest and cheapest solution would be a cog belt drive (otherwise known as a timing belt) - look at the XL series - it should handle what you are doing just fine - precise ratios are very easy, just like gears - for example, a 2 to 1 drive use could use a 64 "tooth" pulley on the shaft and a 32 "tooth" pulley on the motor. Check out http://www.qtcgears.com/qtc_redo/econobelt/XL-Timing-Belt-Pulleys-Aluminum.htm. If these look a little too light, you can always go up to the L series.