Balancing a Radial?

[stevehuckss396]

Is there a rule of thumb for figuring out the weight of the bob weight for balancing a 9 cylinder radial. On a conventional V8 i think it's the half the reciprocating weight and all the rotating weight equals the weight of the bob weight. Any idea what it is for a radial?

 

[leerkracht]

look the article

balancing of radial engines part 1 (google search)

 

[stevehuckss396]

Very helpful Thank you!

 

[Peter Twissell]

Attached is my spreadsheet for calculating radial engine balance.
The approximations 'half reciprocating weight plus all of rotating weight' are probably ok for most model engines.
My spreadsheet calculates the motion of all the moving masses, including the path of each slave rod, with options for compensated / non compensated slave rod locations at the master rod.
The spreadsheet generates a graph showing the total moment of all the masses. Enter a value for the counterweight and the graph shows a circle superimposed on the total moment graph. Adjust the value to get the clsoest match possible.
Please feel free to use and distribute this and if you find any errors, let me know.

 

[petertha]

Your spreadsheet looks very interesting & obviously lots of work, Peter. Thanks for sharing. I'm going to input my 5-cyl to see how it compares.
Just a quick question, how does it work? LOL
I made engine specific inputs as red. Then are we inputting a 'guestimate' counterbalance mass & then iteratively evaluating (possibly balance error?) to some convergence value?

 

[stevehuckss396]

I'm with Petertha. This sheet looks extremely powerful. With a little tutoring on what is being asked for input and what is being output this could be very useful. Is there any way we could get you to type something up?

 

[Peter Twissell]

I'll sit down and write a user's guide tomorrow.
I'm trying to cover a model aircraft with tissue tonight!

 

[stevehuckss396]

No rush!

Maybe you can put a description of what the inputs are and output numbers are telling us right into the spread sheet at the bottom so we can scroll down for quick reference. This looks like a great tool thank you so much for offering to us.

 

[Peter Twissell]

Attached is a new copy of the spreadsheet with added comments and formatting.
The green cells are all the required inputs. The cell to the left of each input has a comment, which is displayed when you place the cursor on that cell and describes the input. For example, for my engine, the master rod Center of Gravity is input as 12.726/177, because its CoG is located 12.726mm from the crankpin axis and the centre to centre length of the master rod is 177mm. This gives the cell value 0.07 (displayed to 2 decimal places).
The object is to minimise the value in the Balance Error cell (orange).
How does it work? - The spreadsheet calculates the location of CoG of each of the moving parts at intervals of 5 degrees of crankshaft rotation, through a full 360 degree rotation.
Next, each of those locations are multiplied by the part mass to give the moment of that part about the crankshaft axis. These moments are then added up to give the total moment of the moving parts at each crank angle interval.
The total moments are then compared to the counterweight moment at that crank angle to give the balance error. The figure displayed in the Balance Error cell is the maximum of all those balance error values.
In my engine, the Balance Error is 15218gmm, the equivalent of a 15.218kg mass orbiting at 1mm radius. This seems large, but in a 120kg engine, it is tiny, generating a free vibration amplitude of just over 0.1mm (15.218/120).

 

[awake]

Peter, what a wonderful tool! I don't have any plans to build a radial engine at this time, but who knows down the road. I applaud your willingness to share all of your hard work!

 

[Peter Twissell]

Thanks for your kind comments. I have to say I enjoy putting together a spreadsheet as much as any other part of the design and manufacturing process. It costs me nothing to share it.

 

[minh-thanh]

Peter Twissell
A useful spreadsheet when I do radial engine .
Thanks for sharing !

 

[petertha]

So if I understand correctly, once the engine specific parameters are entered, you enter a counterbalance mass, it computes a corresponding balance error number. If you plotted a bracketed range of corresponding values (like attached I made of your data) it would appear something like so & the minimum can be visualized. Is that the routine? If so, I can see something like 'solver' function going straight to the answer with a bit of jiggery, but this quick & easy.

15.218kg mass orbiting at 1mm radius
I don't quite get the dimensions of this parameter (1mm). Is it just the format of the calculation?

Lastly, does the counterbalance mass not also have to it's COG or maybe I missed how it was treated in the calculation.

 

[Peter Twissell]

Petertha
Yes, that's what it does.
I've never used the solver function in Excel. I actually created this in open office calc, but I assume it probably has a similar function available.
By all means, adapt it to your liking.

15218 gmm or 15.218kg at 1mm radius is the equivalent offset mass at the point where the difference between the counterbalance and the other moving masses is greatest.

The counterbalance mass is given at the crank throw radius. This makes it easy to measure of you don't have the facility to calculate it in cad.
When balancing a crankshaft by the empirical method, weights are attached to the crankpin, so I used that standard.