Crankshaft Design (WORK IN PROGRESS!)
 "Inline 3 Crankshaft and Balance Shaft")
When I did my degree a few years ago in Automotive Engineering, for my final year project (on which I based my dissertation) I undertook a 'Crankshaft Design Package' project, which was suggested to me by a company as a suitable final year assignment.
The idea was to develop a 'package' or tool with which one could input data requirements for a particular application and the output would be a 3D CAD model of a crankshaft which would be almost ready to use (see example right). Obviously it is impossible to design a generic package that will accomodate everyone's preference for manufacturing technique, style of counterweights and so on, so the task was set about with the frame of mind of demonstrating more what I could do, as far as demonstrating a knowledge of crankshaft design and applying it through suitable means to create a tool that will create a range of crankshaft configurations automatically just from the user inputting necessary data.
The project was quite successful. I had a CAD model which would automatically regenerate into a range of configurations based on input from a spreadsheet. The spreadsheet was where the user input information about the crank design, calculations were performed and data output into the CAD model to drive it accordingly. Journal sizing, fatigue factor of safety (derived from stress analyses) and balancing were all taken into account. The range of configurations was I3, I4, I5, I6, V6 60° V6 90° V8 cross-plane, V8 Flat-plane, V10 and V12.
So this page is all about crankshaft design. It is really only the basic hand calculations that form the backbone of crankshaft design. These days, modern simulation and computing take crankshaft design to a much more advanced level than is feasible by hand calculation. But nevertheless it is always very useful to know the basic maths behind any computer simulation I think.
The information I am trying to provide is what I would have liked to have found when I was trying to understand it as a student. I am still learning myself so I feel I must add a small disclaimer that the information on this page (as with any page on my site) really should not be taken as gospel. I like to think I am informed and do not just write what I think is how it should be, but research and understand things before I write. Please, if you read something you do not feel is correct, send me a comment on the feedback page.
Basic Procedure
Basically, I broke the design down into three main areas:
Start by calculating the minimum projected area (journal width and diameter) necessary to support the loads imposed on the crank from combustion and inertia loads. Then work out the maximum and minimum stress in the worst location, which is the extreme fibre on the cross section between journal fillets, known as the critical section. From this, the Fatigue Factor of Safety can be calculated and then journal sizes may need or be desired to be adjusted.
Once the strength is satisfactory, balancing can be calculated to eliminate unwanted vibration.
Please use the links either in the list above or in the navigation bar to the left to follow through the pages for the various stages.
