MOMENT OF INERTIA ON A CENTRUFUGAL PUMP 7

[robles2713]

HOW CAN FIND THE MOMENT OF INERTIA ON A NEW CENTRUFUGAL PUMP THAT I AM DESIGNING?
I NEED TO SIZE THE MOTOR CORRECTLY BECAUSE IS GOING TO BE A HIGH DOLLAR AMOUNT MOTOR.

 

[drawoh]

robles2713,

Moments of inertia should be explained in your kinematics textbook and/or in your machine design textbook. The Machinery's Handbook explains it too.

Is inertia really the primary load from your pump?

You are working on a project that requires some engineering training.

JHG

 

[vc66]

Mass moment of inertia or Area moment of inertia? I'm pretty sure you mean mass, but you don't elaborate.

Are you sure you have to design it? Centrifugal pumps are pretty commonplace. Take a look on Google, you may be able to find what you need.

As for finding the mass moment of inertia... as drawoh says, kinematics textbooks, Handbook of Mechanical Engineering, Machinery's Handbook, and Mark's should all have pretty good explanations.

V

 

[rb1957]

if you're designing it, i'd've thought that most drafting programs would include a mmoi calc/result

 

[PeterCharles]

If someone can design from scratch a centrifugal pump (which is beyond me) I'd have thought calculating the inertia would have been easy.

 

[Artisi]

Nothing like re-inventing the wheel.

 

[robles2713]

I think I can use our 3D CAD to do this calculations. So far all I can think of for cal MMOI is the main shaft and the impeller. We are planning on using three BAll BEARING to support the shaft, could these neglected on my calcs if is not important to have accurate numbers from my calcs? Anything else I should account for the MMOI?

 

[insult2injury]

I wonder how your customer would feel if he knew that you didn't know how to do this.

It's posts like these that make me cringe whenever I need to buy new equipment.

I2I

 

[vpl]

I NEED TO SIZE THE MOTOR CORRECTLY BECAUSE IS GOING TO BE A HIGH DOLLAR AMOUNT MOTOR

could these neglected on my calcs if is not important to have accurate numbers from my calcs?

If you're concerned about sizing the motor correctly due to cost, why do you feel it is not important to have accurate numbers from your calculations?

Another thing, your first question talks about the moment of inertia on a pump you are designing, but the statement quoted above is concerned with the cost of the motor.

Patricia Lougheed

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[electricpete]

For a single stage centrifugal pump, the pump rotating inertia is often relatively insigificant compared to the motor rotating inertia.

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[svanels]

HOW CAN FIND THE MOMENT OF INERTIA

Engineering Mechanics DYNAMICS, from Meriam and Kraige, Appendix B

We are planning on using three BAll BEARING to support the shaft,

If you want to include the bearings, use only the inner rings, don't forget the lock nuts and/or keys

If you succeed doing this by hand, compare the results with your 3D Cad, you could evaluate/quantify Electricpete's answer, which is also correct.

 

[Artisi]

What size pump/motor unit are you talking about?

 

[BillBirch]

Three ball bearings? Why? Loads will be difficult to determine and manufacturing accuracy will be a problem.

 

[Artisi]

nothing like a 3 bearing shaft for maintenance free problems - in your dreams.

 

[PeterCharles]

"nothing like a 3 bearing shaft for maintenance free problems - in your dreams."

Ain't that the truth, just seen some shafts fail for just that reason! My favourite arrangement, two double row self aligning bearings per shaft.

 

[electricpete]

We have several 100hp 3600rpm overhung pumps which have back-to-back angle contact bearings plus a single deep groove bearing (that makes three bearings).

See for example here:

http://www.omel.com.br/pdf/32.pdf

Roller bearings support the shaft on the pump side and back-to-back mounted ball bearings on the motor side. An option for a third bearing mounted on the motor side is available for very high suction pressures.

In the above case, it is a relatively short shaft. The back-to-back bearing acts like one bearing as far as radial stiffness goes. Also vertical pumps with long shafts may have more than 2 radial bearings.

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[Artisi]

Granted 3 bearings can be arranged on the one shaft as pointed out by electricpete but the 2 bearings, back-to-back refered to should be a matched ground set which in effect can be considered 1 bearing.
However, if someone said to me 3 bearings on 1 shaft without clarification I would assume 1 each end and 1 in the middle - which equates to problems.

 

[electricpete]

robles2713 - You did create quite a stir by saying you were desiging a pump, while at the same time asking a question which most would consider very basic. I think you would have gotten a better answer if you just asked your question without the mention of designing a pump.

Here is my take:

J = Polar mass moment of inertia about axis of cylinder or disk
J = Sum (m * r^2)
J = Int (rho * r^2 dV)
Shell volume element of width dr is: has volume dV = L * 2*Pi*r * dr

J = Int rho * r^2 dV = Int rho * L * 2*Pi*r *r^2* dr
J = Int rho * L * 2*Pi*r ^3* dr
J = rho * L * 2*Pi * [r^4/4]
J = rho L Pi r^4 / 2 [<= in terms of r]
J = rho * L *Pi * [d^4/32] [<= in terms of d]

If you have outer an inner radius of your part:
J = rho L Pi (Router^4-Rinner) / 2 [<= in terms of r]

Plug in consistent units and you will get polar inertia "J" or "W*k^2".

Beware of the oddball notation "G*D^2" which is based on diameter, rather than radius. It differs by a factor of 4 from W*k^2.

Example:
Assume solid steel disk has OD = 6", Length = 1.5"
Radius =6/2*inch = 3 inch
rho:=500*lbm/ft^3; # steel
J= rho* Length* pi* Radius^4 / 2 *(ft/12/inch)^5
J = .3834951971*lbm*ft^2


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[electricpete]

I guess I forgot to mention, if you have a complex geometry which can be broken up into sum of hollow disks/cylinders, than the total rotating inertia is the sum of the rotating inertia of those individual pieces.

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[drawoh]

electricpete,

Check out thread404-218303

Pounds mass is not good practise.

JHG