[modeleng] Re: Mouland Valve Gear

Ron,

The lead (and lap) comes from the fact that the drive for the valverod 
comes from below the slide. Imagine Hackworth gear with the slide 
directly above the wheel centre, and with the slide vertical, which 
would be the mid-gear position. The return crank describes a circle, but 
the top of the rod from the return crank to the slide runs up and down 
in a straight line. Any other point on the rod describes an ellipse. If 
you choose the right point to take off the valve drive, the minor axis 
of the ellipse is equal to 2x (lap + Lead). This motion is 180 degrees 
out of phase with the piston, and corresponds to the drive obtained from 
the   combination lever in Walschaerts' VG.

Angling the slide introduces another component at +/- 90 degrees to the 
piston, again corresponding to part af Walschaerts', but this time from 
the expansion link.

The straight line slide is an approximation to a curve and of course 
standard Hackworth suffers from any vertical movement of the locomotive 
on its suspension. I believe that Joy valvegear is related to Hackworth 
but has a curved slide, probably because the links are shorter and the 
approximation to a straight line won't work any more.

I hope this is clear, I could show it with a sketch on the back of an 
envelope much quicker than type it (an probably clearer too!)

Regards,

JohnP

Ron Head wrote:
> Hi John (and Peter)
> 
> One thing that's always puzzled me about Hackworth gear is - 

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