In this video, learn about the function of the conrod, and how the critical loading cases occur in real life.
- [Instructor] So, we want to understand a little bit more about the conrod, what is the background to this component we're going to be doing analysis of, so I'm particularly interested in the engineering background. The things we're going to be looking at are the material properties, how is it made, the fabrication process, what are the loading actions that we're going to be applying to the conrod, and what are the boundary conditions that we're going to be using in the analysis. Now, the background story to the conrod. It's actually a very large component, it's nearly 19 pounds, so it's not even going to be something used in the biggest pickup truck that we have.
In fact, the application's going to be an internal combustion engine, probably in some earthmoving equipment or something like that. It's a 4030 steel and it's drop-forged as part of its forging process. The drop-forging, well, this is an analogy that she will be a proper full-sized drop-forging machine, but it's the same kind of principle. Heat, pressure, lot of action there to form the conrod into the shape that we want. But for the final machining, the inside bearing for the little end and the big end, they're going to be machined out.
Also, the rough forging is going to be trimmed out, so quite a lot of action required to actually create our conrod. So, looking at some details of the big end, it's going to be split to allow assembly. That's usually a laser cut and it's a simple splitting operation. We're also going to have cap bolts tapped into the conrod, and this holds the cap in place. Now, we're going to ignore all this and we're just going to do crude modeling, so we're not going to have the cap bolts, we're not going to have the regions in here, we're just going to have a simple representation.
And our justification for this is that we're focusing on the little end, that's where we're going to be looking for key stresses. We're essentially going to ignore the big end, other than there's a simple load transfer path. So, looking at the power cycle for a typical internal combustion engine, there we have the induction, where we're bringing in the fuel-air mixture. Compression, where we're now squeezing that fuel-air mixture and the valves are closed. We have ignition, again the valves are closed, this is the power stroke.
The gases are burning rapidly and this is forcing the conrod down into against the crank. And finally, we have the exhaust stroke, where the burned gases are being pushed out, the exhaust valve is open. Now, for us, we're going to pull out critical load cases. The first one is the ignition stroke. In the ignition stroke, we've got the power being applied at the little end, and then we have the reaction through the crank and that's going to build us up a compressive load path. For the exhaust stroke, now we have a tensile load path, the mass of the piston, and the piston assembly is being flung outwards, that's pulling on the top of the little end there, that's actually applying a tensile load.
But the conrod itself is also accelerating, so we also got to introduce the inertia body loads of that conrod, force equals mass times acceleration of the conrod. Again, everything is going to be reacted out as a tensile load path through the crank pin. Now, in a full project analysis, we'd be looking at the full 720-degree cycle. We'd also be looking at off-axis inertia loads, so not working actually, imagine a crank angle over slightly. We'd be looking at something which is going to basically be producing bending loads.
Now, we're going to be ignoring that for this particular analysis application. Now, what we're going to concern ourselves with fatigue life, we're going to assume it dominates. So, fatigue life is going to be a high stress amplitude, which is what we're calculating, then we're going to be assuming that that stress amplitude is oscillating. So, we have positive and negative stresses, so for example, if we calculate a stress here, in one case, in the compressive case, or a stress here in the tensile case, we're going to assume those are going to be cycling.
If they're cycling, we could be talking about 30 years of operation, say, for a typical vehicle. That's many millions of cycles building up here and it's that continuous, repetitive operation which is going to be creating the fatigue damage. So, our requirement is to make sure we keep below what's called the endurance stress. If we look at this graph here, it's a graph of stress amplitude, which is what we're going to be calculating, which is a stress amplitude here, against number of cycles to failure.
This is what's called an S-N Curve, it's derived experimentally, it's on the log-log scale, and you can see, we could look up a particular value of stress amplitude and say that's the life that we can achieve. What we're aiming for is to get below this kind of knee here and say this stress is so low that theoretically, we'll blow this endurance stress. In that particular case, theoretically we've got an infinite life. In practice we never quite get an infinite life, but that's going to be our target, to get stresses below that level.
So, the ideal load path would be, we'd put in, for example in the tensile load case, bearing at the little end here and it would be opposed by bearing at the big end. So, a load diagram here, tensile load, a pull, and a pull like that. Now, again, with the big end, we're going to cheat somewhat. We're going to have a simplification, we're going to say instead of an equal and opposite bearing load, we're just going to super glue this down or weld this down to ground.
Again, as we mentioned before, that's going to give not very accurate stresses around the big end, but a whole focus is on the little end and adjacent regions here in the conrod body. So, we're going to be applying a load and reacting out. Now, there are more sophisticated ways of actually applying this type of analysis, but for the moment, we're just going to keep things simple and use this type of approach here on the right. So, that's the conclusion, that's the background story to how we're going to set up the analysis.
- Setting up Simulation properties and defined views
- Preparing the geometry
- Setting up a local coordinate system
- Splitting surfaces
- Defining the constraint and the loads
- Running analysis
- Contour control
- XY plots