Steam & Excursion > Steam Hobart-Allfree valve gear

While reading about New York Central steam of a 100 years ago or so I find a few references to Hobart-Allfree valves.

Anyone out there that can shed a light on this valve gear ?

Thanks

Probably exists in Patent filings only.

W.



Edited 1 time(s). Last edit at 12/04/16 14:39 by wcamp1472.

If you Google the name of the Valve gear, one reference that jumps up, is the American Mechanical Engineer periodical of about 1910.
It explains that Baldwin built an order of small compound mallets for a Mexican customer, equipped with the Allfree-variant.

In summary, the locos were non-superheated, and of conventional Mallet-articulated 2-6-6-2 engines.
The valve gear was primarily a conventional Walschaerts design, with additional levers and a separate exhaust valve and valve housing.
the separate levers opened an additional exhaust valve that held the exhaust port open for virtually the entirety of the exhaust stroke.

The conventional Walschaerts operates an inside admission piston valve, which has the exhaust rings part of the entire spool valve.  Thus, as the valve gear is shortened in travel ( 'hooked-up' in engineer jargon) not only is the admission port choked down to a small opening, the exhaust rings, likewise, close down the width of the exhaust ports.  Typically, there can be steam trapped at the head of the piston and some folks prefer to think of it as providing a 'steam cushion' to the thrashing pistons.

The Allfree design, added an additional exhaust valve, porting, and levers, that permitted the exhaust activity to virtually the end of the stroke.   

Another complication in design that was not functionally beneficial, economically justifiable and was obviated by the coming of the successful Schmidt superheater, IMHO.

It had no material or measurable benefit.  But was the child of an underemployed theorist about loco valve gears...

​W.

 

Thanks for the explanation.

Yes but it cut down on the back pressure increasing the operating differential across the pistons! Of course
adding 10 psi to the pops would do much the same thing but hey the engineer was kept busy.

Out