Drift from USATC S160

Steve

I will try and find the information Don Ashton published in the Model Engineer. What I do know from checking out the valve gear for my 5" gauge N class is that there is that some designers knew a lot more about designing valve gear than the text books tell you. When I built my 5" gauge class 2 I was unhappy with the published design for the model valve gear. I managed to get a copy of the full sized valve gear drawing and designed the gear based on the full size dimensions using the method set out in Don Ashtons original valve gear design book. The scaled down full size design and the calculations using the formulae Don Ashton provided corresponded very well. When it came to the N class I again followed the Don Ashton provided formulae but found the results did not correspond with the scaled down full sized valve gear. I therefore used a computer simulation programme (which is accesible from Don Ashton's web site) and started to adjust dimensions to match the scaled down valve gear. This improved the valve events a great deal. The main adjustments were the link back set and the length of the eccentric rod. Using the computer programme with the full size dimensions results in the same valve events tabulated on the full size drawing. My conclusion from this was that someone with a very good understanding of valve gear design set out the original valve gear. The fore gear events are quite even but the back gear events are not so good. The events are best at about 40% cut off in fore gear - probably where the engine would do most of its work.

I suspect Harold Holcroft was the brains behind this but we will never know.

 

I agree in that with the normal layout (in fore gear, the radius rod in the lower half of the expansion link) the lifting link and expansion link would both be swinging in much the same plane resulting in little die slip. Will have to get round to a serious study of Don Ashton's website.