How do I calculate the offset or eccentric size for a steam engine? i.e If the stroke is say 2" what offset in the lathe chuck is required to open the passages at the correct time in the steam chest ? Would like a simple answer if this is possible!
The size of the eccentric is determined by the valve travel from one extreme to the other.
You're more likely to go wrong by making the travel too large, 'cos then the valves will damage the ends of the valve chest.
The timing of the valve is determined by the angular position of the eccentric. I can't give you a figure for this 'cos it will depend upon the type of valve gear. (Most about 90, but Stephenson's is ISTR 120)
As to the size of the offset in the chuck, that's an interesting geometrical exercise - when I did it, I omitted from the analysis the crucial point that all the jaws of a 3-jaw move together! Congratulations on having a go yourself!
No simple answer ..... it depends upon the dimensions of your valve ports and the "lap" of the valve.
Recommend you get yourself a book such as "valves for steam locomotives" by Lake & Reidinger from TEE publishing if you want to know more. Probably more info than you need but it will put you on the right road to solving your question.
Alan
Get yourself a copy of Don Ashton's book on Stephenson valve gear. It has recently been reprinted and also has his book on Walshearts valve gear with it. This will tell you all you need to know about the design and setting of Stephenson valve gear.
Camnden Miniature Steam Services have it. Tel: (01373) 830151 or try
Maurice Hood wrote:
For some reason I have lost the original letter, but hopefully I can attach this to an answer. I am sure the writer was looking for a simple method and surely there is one - as used by various model clubs such as the Gauge1MRA. 1. The eccentric displacement needs to be 1/2 the travel of the valve rod. The valve rod travel needs to be the same as the valve cavity length. 3. This needs to be the distance between the outside edge of one inlet port and the oposite edge of the exhaust port. For instance, most G1 engines have 1/16 inlet, 1/8 exhaust, 1/16 gap. The valve cavity is therefore 1/4" and the eccentric displacement
1/8". The valve length overall will be 1/2(as used by LBSC) although the G1 experts tend to extend that to .522". This is obviously simple slip eccentric measurements but it lays the foundation for any modifications. Dicky Boast--------------------------------------------------------------------------
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