Calculate Horsepower to rotate an object resting on a thruust bearing

[jdkgml]

Hi...trying to determine if i need to consider both efficiency and c.o.f. of the thrustv bearing when determining required torque in order to obtain hp.

any insight appereciated.

 

[1gibson]

Only if it isn't negligible...

Post up some numbers

 

[flash3780]

Please post some more information about the bearing, and the speed of rotation.

 

[jdkgml]

Here is what i have done...

 

[vpl]

Is this a school assignment?

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[jdkgml]

No...not school...calculation at work.

Customer comment was concerning the 0.95 value for efficiency, wanted to know why that value is assumed conservative.

The only empirical value i got from the vendor was the c.o.f. - i am debating either removing the assumed efficiency completely or somehow converting the c.o.f. into an efficiency value to calculate accelerating torque.

Any ideas on how to transform a cof to an efficiency????????!!!!!

Thanks!

 

[moon161]

Efficiency= (total input - losses)/total input

It gets messy after that.

 

[1gibson]

With c.o.f. and bearing dimensions, you can calculate force required to rotate it and overcome that friction. With a bearing, seems like that should be a common parameter that the manufacturer would know. Is the bearing efficiency given in a % (not sure what that % would mean) or is it a torque value? Perhaps there is a chart available that shows RPM on X and frictional losses on the Y.

 

[GregLocock]

In my opinion having both an efficiency for the thrust bearing and a CoF is redundant.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MintJulep]

What is the physical meaning of efficiency for a thrust bearing?

Efficiency is output/input.

What is the "output" of a thrust bearing?

If you have cof, thrust and effective radius then you can convert that to torque that goes away and does nothing useful for your system.

 

[jdkgml]

kinda coming to same conclusion that they arev reduntant.

eff = 1-c.o.f.

thoughts?

 

[1gibson]

So the efficiency of a thrust bearing has no direct relationship to overall efficiency of the machine. It is only used as a method to calculate the friction generated based on thrust applied.

You input thrust in lbs, your output is loss due to friction in ft-lbs based on the thrust, thrust bearing efficiency, and RPM.

To translate that to overall efficiency of your machine is an entirely different matter. A 95% efficient thrust bearing could cause:
1% loss in overall efficiency of a 200hp machine
2% loss in overall efficiency of a 100hp machine.

That was the missing link in my head at least. Learn something every day.

 

[1gibson]

^^^ If the same thrust is applied to both machines.

 

[MintJulep]

eff = 1-c.o.f.

thoughts?

Wrong.

 

[zekeman]

Look, all you have to do is go to the mfr OF THAT BEARING or look at posted mfr data that gives the efficiency, perhaps vs thrust load, but it should be available.

It's that simple.

 

[jdkgml]

OK...thanks for comments...attached is the revuised calculation, let me know if you see any glaring errors.

thanks.

 

[GregLocock]

The glaring error is that I can't read through that lot and understand what you are doing. I am just as bad, but then I'm not the one asking for advice.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[flash3780]

I just perused through your calculations and I noticed a coefficient of friction of 0.0035... which is slicker-than-snot. Is your bearing made of teflon?

 

[racookpe1978]

One potential problem is your definition of terms: You're treating this "thrust bearing" as if it were a roller bearing or a lubricated sleeve bearing. (One that rolls, resiting the weight of the shaft and equipment.)

A "thrust bearing" has no "rolling resistance" per se, because it is unloaded (zero force) UNLESS it is resisting a linear force up or down the shaft. Then, the instantaneous thrust bearing resistance is going to depend on the instantaneous load axially in the shaft at that particular moment in time - NOT the weight of the shaft and rotating equipment that are being held in place by the roller bearings. As the turbine or pump comes up to speed, or as load changes on the pump, thrust bearing resistance changes.

 

[flash3780]

I had a bit of trouble following your calculations without a description of the problem, but does this help you?

(I was playing around with TeXmacs)