Inertia Tensors in Crankshaft

[JCReynolds79]

Hi all,

I am getting a bit confused when trying to balance a crankshaft automatically in Pro/E.

I know how I want to balance it, i.e. sum of all the mrx values for each 'slice' of the shaft in the vertical and horizontal planes should be zero.

I have done this manually by interrogating each slice of shaft and putting it into a spreadsheet so I know it balances.

But in Pro/E I want to make use of the Optimisation tool...this means understanding the Mass Properties information.

When I do a mass props on the shaft, I am not sure which bit of information I should be looking at. I have been using Ixy and Ixz as my horizontal and vertical plane respectively. Ixx would be about crank axis.

But there are tensor values about the selected Coord Sys and also about the CoG with respect to the selected Coord Sys. The values all differ and it also all differs if I choose a different Coord Sys.

I am confused as to which value I should be aiming to zero, as per my calcs...

I dont understand why they differ depending on the location of the CS if the CS is located somewhere along crank axis. In the hand calcs, no matter where you take moments, along crank axis, they always sum up to the same answer.

Hope someone can help clear this up.

Thanks in advance. s_div],.OggiPlayerCo

Regards,

Jon Reynolds

 

[seymours2571]

Hello Jon,

If you are interested in specific inertia values about a particular axis or coordinate system then you will need to refer to the values provided for the selected CSYS. Refer to the line that says:

"INERTIA with respect to ??? coordinate frame"

The second set of values is the inertia tensor at the center of gravity but oriented to correspond with the selected coordinate system.

The principal inertia tensor is located at the center of gravity, but oriented such that all off diagonal terms are zero which provides the maximum MMOI values about the oriented coordinate axis. The rotation matrix provides means of orienting the principal frame with the selected coordinate system.

By selecting a different coordinate system you are changing the reference from which the inertia values are calculated. Hence, the reason for the change in values per change in coordinate system reference.

One way you could balance the shaft statically would be to optimize the design such that the COG lies on the axis of the shaft. For dynamic balancing I would need to break out the books again. It has been some time since I have done dynamic balancing.

Hope this helps.

Steve

Stephen Seymour, PE
Seymour Engineering & Consulting Group

http://www.seymourecg.com

 

[JCReynolds79]

Steve,

Thanks for your reply, that helps a lot.

I think I am probably not using the correct tool for the job.

I discovered that I can do a dynamic analysis in the mechanism module. So I can basically rotate the shaft at a certain speed and get force reactions at the crank axis.

But the results from the connection reactions are a little weird too... again, not quite what I expect and hard to tell which is the right value to be using.

I can do the dynamic balance by hand calc no problem, but I am just wanting to get Pro/E to do the hard work, plus going to and fro measuring chunk of my model and adjusting them by trial and error with the hand calcs is very tedious and definately seems not necessary considering the power of Pro/E.

Cheers.

Regards,

Jon Reynolds