Journal Bearing Design

[neweng22]

Hi Everyone,

I am helping a colleague with a equivalency mock exam and we cannot agree on the answer to this question:

A journal and bearing are to be designed for a shaft that turns at 250 rpm. Suppose
ISO VG100 (SAE Engine 30) is to be used as lubricant and the bearing length is to be equal to
1.2 times the diameter. If the no-load power loss is not to exceed 2.5 x 10-4 horsepower and the
diametral clearance is 0.0045 times the diameter, estimate the maximum diameter that can be
used for the journal, and the allowable temperature limit.

Any help would be appreciated, it's been forever since I've even cracked my Shigleys!

 

[Tmoose]

Is any radial load mentioned?

 

[neweng22]

No, no load is provided

 

[gruntguru]

All you are doing here is calculating viscous friction. You have power and rpm, so can calculate torque. On the friction side you will need to assume a running temperature for the oil and thus a viscosity. You can then derive a formula for friction torque in terms of bearing diameter. It will be something like:

Torque = r x A x Fn(t) (r=radius, A = brg surface area, t = clearance, Fn = "some function of")
= D/2 x (Pi x D x 1.2D) x Fn(0.0045D)

You can do the rest.

je suis charlie