Valve Lap vs. Eccentric Angular Advance

[daves1459]

I'm in the process of re-springing the driver axles of my Disney type 4-4-0. The original leaf springs were made of spring temper silicon bronze. After adding over 50 pounds of lead in the cab, sand dome, and under the boiler casing the springs are flat and the frame is resting on the journal boxes. By observing the loco driver slippage on uneven track I'm convinced driver traction is being lost. So I decided to re-spring with steel leaf springs. Steel is 1 and 2/3s times stiffer than silicon bronze. Arithmetic says the steel springs should restore the proper position of the journal boxes.

While observing the traction issues I noticed that it took an excessive amount of throttle opening to launch the loco on a mild grade. After launch the loco would race requiring the the throttle to need to be closed in order to maintain a constant controlled acceleration. My previous experience with the design of Stephenson link valve gear is the symptom I was experiencing was the sign of excessive eccentric angular advance and/or excessive valve lap.

Using formula in Martin Evan's book "Model Locomotive Valve Gears" the current angular advance should be 25 degrees and cut off is 75%. O-K for the angular advance except the blue print value is given as 30 degrees. But, for the cut off at full gear 80% to 85% is needed for a smooth launch. So I wanted a second opinion. Google searching the internet I found a spread sheet program by Don Ashton that by plugging in my valve gear values gave 30 degrees advance and 75% cut off. By shortening the lap I can, via the spread sheet, increase the cut off to 80% and at the same time reduce the angular advance. My question to the readers is there anyone out there with actual experience with Mr. Ashton's spread sheet predictions by translating them into actual parts?

Thanks, Dave

[Steve Goodbody]

Dave,
Don Ashton, who died recently, is very highly regarded in the UK for his valve gear work. You may want to post your question on the Model Engineer forum for live steam locomotives (https://www.model-engineer.co.uk/) where you'll likely get some first-hand feedback.
Best regards
Steve

[daves1459]

Dave,
Don Ashton, who died recently, is very highly regarded in the UK for his valve gear work. You may want to post your question on the Model Engineer forum for live steam locomotives (https://www.model-engineer.co.uk/) where you'll likely get some first-hand feedback.
Best regards
Steve

Hello Steve,

Thanks for the tip. I did not know Don Ashton was a UK resident. I'll try the Mosel Engineer forum as you suggest.

Dave