6-minute Solutions, Problem 48

[ezzieyguywuf]

I am trying to understand the solution presented to this problem. The way I worked through this is as follows:



where:



The problem I am having is in calculating P_{lat}. Per eq 54.131 in MERM, the lateral shaft load is a function of the lubricant pressure and the "projected shaft area", or "shaft diameter x journal length". The only problem is that the journal length is not provided. In the solutions provided in the Six-Minute Solutions book, their final equation for V-dot is equivalent to mine except they have a factor of "2xpi" in the numerator (I can post their equation as well, I just don't wish to violate copyright laws). In other words, the 6-Minute Solutions answer suggests that:



Is this true? If so why?

 

[bban]

From which version of 6MS (I'm using second edition( but do not recognize the question))

 

[ezzieyguywuf]

I also have the Second Edition. The problem statement is "A journal bearing has a shaft diameter of 1.5 in with 0.0015 in clearance....etc". It is on page 13.

 

[ezzieyguywuf]

Ah, I guess I should specify: I have the "Mechanical Systems and Materials Problems" version.

 

[J_MEC]

The 2pi factor in the equation in the solution is to convert revolutions to radians. 

P_lat is given to you as 750 lbf. You do not need to know the journal length to solve this problem. Equation 54-33 has P as power, not as pressure.

 

[ezzieyguywuf]

The problem statement says "a 750 lbf load is applied" - it does not specify that this is the lateral force on the journal. I guess I am just meant to assume this?

And yes, P=Power that was a typo on my part.

 

[Ramnares P.E.]

Is there a figure provided with the question?  Apologies but I only have the thermal/fluids version of the SMS.

 

[ezzieyguywuf]

There is not a figure provided. The entire problem statement is as follows:

A journal bearing has a shaft diameter of 1.5 in with 0.0015 in clearance. A 750 lbf load is applied as the shaft rotates at 30 rpm. The lubricant used has a specific heat of 0.42 BTU/lbm-degrees_F and a density of 0.0311 lbm/in^3. The friction variable is 3.50. The lubricant flow rate needed to keep the increase in temperature under 30 deg_F is most nearly...

Six Minute Solutions for Mechanical PE Exam Mechanical Systems and Materials Problems, Harriet G. Cooke, PE