Is a bushing even needed at low loads and low rpm?

[gulfcoastfella]

I've got a shaft that rotates at about 1 rpm and handles a radial load of approximately 25 lbs, a thrust load of 17 lbs, and a bending moment of about 150 lb-in. The shaft and joint will be submerged in lakewater. A shaft diameter of 1.5 in would be nice, but in general a larger shaft would be preferable to a smaller one. I was looking at bushing options, but I'm starting to wonder if I need a bushing of any kind at all (considering the low loads and very low rpm). It seems that a no-bushing solution would also go hand in hand with a larger shaft diameter.

 

[moejoe15]

If not a bushing then what?

 

[gulfcoastfella]

The polished shaft would sit in a polished hole of the same diameter running through the support structure. Basically, the shaft itself would be the bushing.

 

[Q_Goest]

Could you make the part with the hole in it out of plastic? Metal on plastic is much better than metal on metal. There are a lot of different plastics with fillers to improve lubricity. I'd check something like McMaster Carr if this is a 1-off.

 

[gulfcoastfella]

Could you make the part with the hole in it out of plastic?

I've read in several places that plastics, while good for humid and splashed applications, will break down when submerged for an extended period.

Metal on plastic is much better than metal on metal.

Isn't metal on metal essentially the makeup of a brass or steel bushing anyway?

 

[Q_Goest]

Hi Gulf, My experience is in dry running applications with a high PV. Your application has a low PV and has water to 'lubricate' it, so the environment should be much more forgiving. Bushings might be bronze but they have lubricants and polymers to keep them from galling. Other bushings are straight plastic. Depending on the plastic, they should work very well especially in water where you gain some lubricity. I'd suggest looking for a plastic that has low water absorption and added lubricants such as oil, Teflon, molydisulfide, etc...

There is also bearing grade machinable bronze but personally, I'd try the plastic first.

 

[pantaz]

What material is the shaft? What material is the "hole"? Daily run-time? Expected service life? What is your acceptable MTBF?

Generally speaking, bushing material is softer than the shaft, so any wear occurs on the bushing. Also, replacing a worn bushing is a much easier fix than replacing a shaft or repairing an elongated hole.

 

[gulfcoastfella]

Thanks Q_Goest, that's some great advice. I had read in some engineering texts that plastic can break down in submerged applications, but I think they were speaking about plastics in general.

I've found that Teflon is generally able to run submerged, and I found a http://www.igus.com/wpck/default.aspx?pagenr=7103&C=US&L=en" offering iGlide bearings, some of which are rated for submerged application as well. This just goes to show that engineering texts are generalizations, and if you find someone who offers something out of the ordinary, then by all means use it if it applies.

Also, replacing a worn bushing is a much easier fix than replacing a shaft or repairing an elongated hole.

That's a great point Pantaz.

 

[moejoe15]

Your hole is the bushing, not the shaft. I would use one if I could. One thing you can use is what they've used in the past. Oil soaked wood bearings or even Lignum Vitae if you can find it which is naturally oil soaked and was used for bearings for many years until modern materials took over. No maintenance and waterproof too.

http://www.appropedia.org/Oil_Soaked_Wood_Bearings_(Practical_Action_Technical_Brief)