Stephenson valve gear

Hi all.

Have had my 1-1/2 traction engine running on air, seems ok.(Paxman)

But getting to big for my britches started reading the Allchin book and it seems to me that the setting of the valve differs between the two.

Pax saying "lap of valves 0.05"

Allchin "At dead centre at each end, the edge of the inlet port should show as a thin black line.

Going to make a new slide valve, so was hoping the " Lads" could put me on the right track

Thanks Vernon

Sala Kahle

Can't see why you are wanting to make another slide valve.

The Paxman has a 0.6" long valve and the overall width of the ports is 0.5" so that will give you the 0.05" lap at each end as Greenley (not Paxman) says.

Eccentrics look to give approx 1/4" of movement and the inlet slots are 5/64" wide.

Therfore when eccentric is in mid position there will be 0.05" of valve beyond the port, when eccentric is at full throw it will have moved 0.125" which is equal to the lap (0.05" ) plus Port (0.078" ) so the inlet port will be fully opened.

Now what you are possible getting confused with is the "lead" of the valve which if the eccentric is set to lead the crank by 90deg plus say 8-12 deg will show a hairline opening when the crank is a TDC not the eccentric.

J

Edited By JasonB on 29/03/2012 17:13:42

Just looked at the Alchin drawings and that has a lap of 0.086"

Thanks for that Jason.

Making a new valve, cause the old one leaves a little to be desired.

Will make the "new" one to the same specs, just try and do a better job this time.

Once again thanks for your time and trouble. Penny just droped,think I needed to shift between forward and reverse.