Bearing Clearance for a white metal plain bearing with grease lubrication 2

[ajr78]

Hi, I am after basic rule for diametrical bearing clearance on a white metal plain bearing lubricated with grease.

I am aware of basic rules (1.5 mil per inch of shaft) for oil lubrication that give a clearance of around 0.34 mm or 0.013". I do not know if similar such rule for grease lubrication exists?

The actual bearing is on a vertical shaft of 230 mm (9") rotating at 150 RPM. It is around 450 mm or 18" long. The grease is an ISO 100 -150 cST at 40 deg C. Radial load is estimated at a maximum of 19 kN giving a bearing average pressure of 0.18 N/mm2 or 26 psi.

 

[3DDave]

Oil tends to either flow to fill the gap or be forced to fill the gap - grease will be squeezed from the gap and what will remain is whatever oil film that can creep back from where the grease is forced to. Since the oil film is created by creep, there doesn't need to be much clearance. I would not expect a long life from this.

 

[electricpete]

> shaft of 230 mm (9") rotating at 150 RPM.

Surface Speed = pi*D*freq = (pi* 0.23m/rotation)] * [(150 rotation) /(60*sec)] = 1.8 m/sec

Neale's "Lubrication and Reliability Handbook" states

A.7 Plain bearing lubrication
Greases - Use restricted to operating speeds below 1 - 2 m/sec​

That's a little fuzzy to me. As I read it, the maximum speed for grease lubrication of plain bearings is somewhere between 1 and 2 m/sec. So your 1.8 m/sec might be near or above that maximum.

I've never seen any large grease lubricated sleeve bearings myself. I have no idea how clearance would be selected.

=====================================
(2B)+(2B)' ?

 

[TugboatEng]

The issue I see is that as the clearance is increased, the contact pressure also increases. The lubricant in grease and oil is the same, ISO 100 so there shouldn't be any different there. The problem with grease is that you don't have flow so there is no cooling. This will limit speeds, especially with white metal bearings.

 

[electricpete]

ok, another reference from Neal, once again section A.7. This has a lot more info related to grease lubrication of sleeve bearings

Neale's "The Tribology Handbook"

A7 Grease, wick and drip fed journal bearings
​

There's a lot there and I didn't really digest it, but it seems like a good reference for designing grease lubricated journal bearings. Interesting to see figure 7.3 which allows you to estimate apparent grease viscosity from on base oil viscosity and shear rate (I presume shear rate is related to surface speed divided by clearance).


=====================================
(2B)+(2B)' ?

 

[electricpete]

> The lubricant in grease and oil is the same, ISO 100 so there shouldn't be any different there.

I think that thought process is valid for elastohydrodynamic lubrication in rolling bearings, but not necessarily for sleeve bearings. I posted above a correction for viscosity due to non-newtonian behavior of grease.

> The problem with grease is that you don't have flow so there is no cooling. This will limit speeds, especially with white metal bearings.

I agree the cooling raises some questions. Knowing the apparent viscosity and shear rate you can estimate heat generated from viscous friction. It's a fairly long bearing with L/D=2 which means you don't get as much benefit from axial transfer of heat to the shaft above/below as you would with a shorter bearing. I guess to decrease heating we'd decrease base oil viscosity (and effective viscosity) and increase clearance, but you still have to meet a minimum viscosity to prevent boundary lubrication and a maximum clearance to avoid excess vibration. I wonder if grease NLGI number (penetration) plays into that at all.. I didn't see that mentioned in connection with apparent viscosity.

=====================================
(2B)+(2B)' ?

 

[TugboatEng]

I was surprised to see OP mention viscosity. Most greases are NGLI 2 regardless of viscosity. I switched over to Mobil greases as they're the only one that puts the viscosity in the name and saves me a bunch of time parsing through datasheets.