Determining the forces on the crankshaft

[mert1]

hi,

I am trying to calculate the shake, inertial and moment force acting on the crankshaft. The reciprocating motion of the piston+pin+small end of the conrod creates these forces. Still I need the right formula to figure out the correct numbers. Also, if these are for a 4 cylinder engine, do I need to multiply these force values with the number of cylinders?
Thanks in advance.

 

[GregLocock]

And um, is there any particular reason why the equations in a textbook, or those you were taught at university, don't apply any longer?



Cheers

Greg Locock

 

[mert1]

Sure they do. But the assumption is taking 1/3 of the mass of the conrod weight for reciprocating part and it seems to not fully correlate with the real life. So the rotational inertia seems to be off. Also for the moments, which I guess is shake force * x (center of recipro. mass to center of where ? is the question ). It would be great if you can give names for couple of books since mines dont seem to work. Thanks Greg.

 

[magnograil]

I went through this exercise recently when I pie cut the crank webs and had to come up with the balance mass/moment arm for different rods/pistons. The auto shop method of taking the piston/rod weight at the wrist pin with the rod held horizontal only gets close. It does not take into account the rotational inertia of the rod.
Start with the formulas for the geometry (crank pin relative to centerline, rod position and angle relative to crank pin and piston relative to rod) then use the F = ma and T = I omega formulas to solve for the reaction forces at the crank pin. You will have to use scales to find the rods mass center and the compound pendulum formula to find the rod moment of inertia.
Minimise the main bearing loads by varying the balance mass (at half the stroke distance) until the integration of the crank loads over a revolution are zero. That gives one cylinders loads.
Shift these loads by the angle of each crank pin and multiply by the distance from the engine reference center in 3D space. From this you can calculate the location and force required by balance shaft(s) to minimise vibration.
If you want to be elegant, add in the forces and moments of the valve train components.

 

[mert1]

Greg I see you are familiar with VSIGN. This is what I am using and trying to do a 3 WOT Driver left Ear SPL analysis. What I really need is to understand how the loadcase is generated for different engines. Since I see that even with the same engines the combustion torque and the rest of the forces (inertia, moment etc.) do not match. I guessed this is because of assumptions but could not go further.

 

[GregLocock]

Well, short of sitting down and calculating them I imagine these days we just drop the solid model into something like ADAMS Engine and then hope the predicted bearing loads and sidethrust loads make sense.

I still can't understand what your actual problem is, at least for simple cases.



Cheers

Greg Locock

 

[motorsportsdesign]

I have a program that plots the loads in Unigraphics. Takes about ten seconds to set-up and run. What would you like to know?

Jonathan T. Schmidt

http://www.schmidtmotorworks.com