Source
r/AskEngineers•
SomeoneElse899

How can I determine how I thin I can make a plain bearing?

Mechanical

I've got an situation were I need to make a plain bearing really thin, but I can't find anything online, or in any books I own, about how thin I can make it. Does anyone have any suggestions for me? The shaft is around 5" and will be rotating at less than 5RPM. There will be over 100,000 ft-lbs going thru this shaft which will be applied in a way that the bearing can see up to 140,000 pounds of side load.

Posted Mar 5, 2020, 9:24 PM UTC

18
Jakes1967•

How can I determine how I thin I can make a plain bearing?

Passive Load torques, Active Load Torques, type of material, type of bearing, mass - all come to mind.

2
SomeoneElse899OP•

I understand how the last three would play a factor in wall thickness, could you give me an idea how the first two affect it too? I'm guessing it might have something to do with contact stresses, but I'm not sure how.

1
Jakes1967•

It's been a number of years, so you'll have to forgive any rustiness. Also this excludes Moment of inertia etc.

Factors such as resistance, friction, heat, high and low spots, balance (yes, even at 5 rpm - due to loading/torque), duty cycle, load variances, shaft alignment, even down to how the shaft is driven.

Somewhere there is a formula for reduction in bearing thickness and increase in bearing width. Roller bearings usually accommodate/encompass this very well.

1
TugboatEng•

You can apply babbitt directly to the bearing housing. That's about as thin as you're going to get.

2
DevonPine•

Plain bearing will not work well at 5rpm I don't think, as you won't have any lubrication there at all. It'll just be grinding away. I think you really need some type of rolling element bearing.

1
SomeoneElse899OP•

They work, because we've been using them on this application for 20 or so years now (very small build numbers though). We use molykote which is a paste more than it is an oil, and from my understanding is design for very high loaded like in my case. We barely have any room for a plain bearing, so any roller bearing is going to be out of the question.

One components had some issues so the shaft needed to get smaller. The engineer that made the decision to make the shaft smaller didn't account for all other products that mated with it. Were finally addressing those issues and it looks like we're running out of room for the bronze bushing.

1
DevonPine•

Fair enough, today I learned.

2
SomeoneElse899OP•

TBH, I don't know how a lot of our stuff works. The math says no, but reality says yes.

1