Crankshaft torsional vibration

barrti

Hi, I am attempting to model the torsional vibration of a simple 4 cylinder crankshaft (see file below). This model is not at all sophisticated - its more of a capability project.

The mode shapes are easy of course, but I am interested in trying to model the forced torsional vibration using timed combustion forces from pistons (not modelled yet).

This means I need a way of appling a force at a particular angular position say 5 degrees after the piston has reached top dead centre.

Another way is to apply a force at a discrete linear postion say when the piston is 10mm from Top Dead Centre.

I can apply a force at a particular time of course but I don't think this will be satisfactory for a few reasons.

I'm not sure what to do. Is there a way using Calculix within Mecway?

Cheers

Victor

I think it would be possible by writing a user subroutine for a load, such as a concentrated load. Section 8.4.2 (or 9.4.2) Concentrated load (cload.f) of the CCX manual says that node displacements for the whole model are available to the *CLOAD user subroutine.

Perhaps another way would be to use contact so that part of the crankshaft applies a displacement to a spring which produces a known force when it comes into contact. That might get ugly but at least you're not recompiling CCX and writing Fortran code.

barrti

Hi Victor
Thanks for you reply. I'll try your second suggestion first. If I use a simple spring that the piston can contact as it approaches TDC then this will simulate a piston compressing and expanding. This is a good fix for that part of the model.
So how would I get a spring to generate a force that simulates the combustion force? It really needs a. Actuator element rather than a spring that can trigger a force once the piston is in the right position.
Cheers

barrti

Victor will;it ever be possible to trigger forces or pressures based on position as well as time?

Victor

That's probably more of a question for Guido from CCX. If you can afford a few hours, you could have a go at compiling CCX which would be the first step to writing a user subroutine. There are step-by-step instructions along with the source code included with Mecway in the ccx\ccx_win64_source_2.15.zip file.

JohnM

I have some suggestions for you:

Start small - work out the details on a "single bay" model. Extending to 4cyl can be accomplished by applying the phase angle (180deg) and firing order(1-3-4-2).

Start with constant speed. To develop the pin loads you need to consider the mass of the piston, con rod, wrist pin, and rod/wrist pin inertias. There is also an offset to the wrist pin from the cylinder centerline, and this is important. I will bet you can find some slider-crank code on the internet that will be a good start for development.

Base everything on crank angle.

You will need a flywheel, this will dramatically influence your torsional response. In fact, if you spin up with a flywheel, you will find that you can regulate your timing issue with drag/damping, almost like a "Prony Brake". Remember that you are putting in power, you have to take it out!

This stuff is complicated, but fun! You are in for a long haul, and I can tell you that the industry tools developed 50 years ago are still being extended.

Good luck!

barrti

Wow thank you both. I am not home at the moment. I will investigate this when I am. Cheers