Floating D Valve

[JonRiley56]

All,

What is the rationale behind the floating D valve ? I can understand the idea that it will supposedly not become less effective with wear because it is being pushed up against the port surface by the steam pressure, but It seems like that may come at the expense of low pressure engine performance.

If I set the valve with it tight to the port surface it would seem that I am more likely to be able to run at low pressures because the steam is not being asked to hold the block against the surface, but only to force the piston up and down.

Thoughts ?

jon

[DetroiTug]

Hi Jon,

Compare the surface area of the valve and piston face. If there is enough pressure to turn the engine then there is enough pressure to hold the valve to the face. If the valve is 2" x 3" that is six square inches, at 5 psi, that is 30 pounds of force.

-Ron

[Lopez Mike]

My experience has been that every time the valve hasn't been free to float on the valve rod, it acts up, mostly by leaking at partial throttle. Things are always changing size with changes in temperature.

When the throttle is open it isn't so critical but at low pressures it can act up. On a new engine I regularly hand lap the valve flat with a sheet of 400 grit wet or dry sandpaper with the abrasive side up on a sheet of glass or a flat surface on one of my machine tools. Use light oil and don't let the grit get on your drill press table or whatever.

[DetroiTug]

I run mine tight on the valve rods on both cylinders. It hasn't caused any problems, but they are a problem to tighten and make them seal, especially if using lock washers, seems like the last little bit, cocks the valve away from the face. It takes a few tries. The way I check it is put a piece of hose on the exhaust for that cylinder and blow in to it while rotating the engine, and make sure it isn't leaking (excessively) anywhere.

-Ron

[marinesteam]

Take a look at the valve chest, rods & slide valve as a mechanical system and you will start to see why the valves "float"

On my York engine ,and it seems typical from the other designs I have seen, the valve rod is guided at each end. The guide points are running fits between the tail of the valve rod and a bind hole at the inside, top of the valve chest and also at the clevis end of the valve rod by a bush just above the gland. As noted in previous posts, the force created by the steam pressure on the large outside face of the valve is greater than the force created by the pressure from the ports. This imbalance of area is what creates the force pushing the valve onto the port face and will exist proportionally to the steam pressure.

A tight lateral fit of the valve onto the valve rod poses some problems.

1. The valve rod is over constrained which will lead to binding and premature /excessive wear on the glands or to the valve face. Three points of in-line support under a rigid beam is a poor choice mechanically. If all three points of contact aren't in line the beam will be forced to flex to make contact at all three points or the load will not be evenly distributed on all three points if the beam is straight. The lateral float of the valve allows the take up of any interference and prevents the valve rod from being forced sideways into the tail guide, gland bush or valve face. It also allows the valve to seal as it prevents the valve from being pulled off of the port face should the distance from the rod to the port face be longer than the distance of the valve face to it's through hole.

2. The float at the valve acts as a safety to allow slugs of water to pass through the valve preventing damage to the valve rod.

In other words: something has got to give. The easiest place for this from a design standpoint given the constrained valve rod is at point where the valve and valve rod meet.

The system can be made to work if all of the parts are in line but it's not ideal. Since the valve is held to the port face by the force imbalance there is really no need to add another constraint to force the valve onto the ports.

Cheers

Ken

[DetroiTug]

On the Tiny power engines, the valve rod is only guided on the packing glands, there is no guide on the far end. I've been running it like that all the way down to 5 psi for three years now, it hasn't caused any problems. When I first assembled it, one of the valves was lifting off the face. When I bought the partially machined engine, one of the cylinders had been machined slightly out of square. Put it on a face plate and dusted it and that fixed it. Any clearance on engine or valve motion is going to produce a clicking noise.

As long as everything is machined square, in-line and parallel, running the valve tight on the valve rod is not an issue. Regarding temperature/thermal expansion, the valve rod itself is typically 3/8" dia. and will deflect slightly. With the valve adjusted tight to the face and contacting fully during traverse, hasn't been an issue. But no two engines are identical. What works on mine may not work on yours?

-Ron

[marinesteam]

On the Tiny power engines, the valve rod is only guided on the packing glands, there is no guide on the far end.

Ron,

That is a different beast, As your valve rod isn't guided at both ends, in effect it's only constrained by two points, the valve and the gland, not potentially three. In this case the valve rod is not overly constrained. The valve is being held to the port face by the steam pressure and that end of the rod (and affixed valve) is better able to seek it's own position with less potential of binding. Any mis-alignment between the valve and gland will be magnified and show up as a translation at the clevis end of the valve rod. The downside is that unless the lateral clearance between valve and valve box is low the gland (or lower bushing) needs to resist all of the rotative forces being placed on the valve rod by the valve gear, which may have a detrimental effect on the longevity of the gland.

Cheers

Ken

[fredrosse]

For a slide valve engine, the valve needs to be held against the valve seat by steam pressure to work properly. In theory this force can be small, and "balanced" slide valves achieved this by venting the backside of the valve to low pressure, reducing (but not eliminating) the tendency for the steam chest pressure to hold the valve against its seat.

The slide valve also needs to be able to lift off its seat for pressure relief if some water gets into the cylinder when the engine is turning over. For this reason the valve stem attachment will typically allow the lifting of the valve off its seat, an important safety feature of the slide valve.

Unloading the steam force tending to push the valve against its seat by applying force through the valve stem is generally not a successful process, and will tend to wear the valve stem or its gland into an oval shape, quickly followed by main steam leakage direct to atmosphere. The friction of the valve on the valve seat is generally not so much of an issue with lightly lubricated slide valves operating at the pressures we steamboaters typically use.

[DetroiTug]

One consideration to my methodology, I'm running continuous steam oil with tallow. Which is more than likely the reason I'm not having any issue with it as wear is minimal. I pulled my engine apart a few months ago, internally other than some light carbon build up everything looks as tight as it did at initial assembly.

Also, the Dvalves in this engine are roughly 2" x 3" at 100 psi, that's 600 pounds of downward force. The 303 stainless valve rods are only 3/8" diameter. Oscillating impact on the adjusting nuts and rod overtime is going to keep increasing the endplay clearance.

Regarding hydraulic lock. These valves can still lift off the valve face. The valve rod will flex as mentioned above.

[Mike Rometer]

Oscillating impact on the adjusting nuts and rod overtime is going to keep increasing the endplay clearance.

I have had floating D valves running for over 20 years; made with no discernible end-float, and still showing no discernible end-float. The 'buckle' is made of the same material as the D valve, so expands at the same rate, therefore clearance needs be absolutely minimal, and remains so, If everything is made accurately there will be little to no movement of the valve against the buckle at any time, except perhaps should a hydraulic push the valve off, so no wear, and no increase in clearance. The only wear will be to the port/valve face and that just wears to a better seal.

The old adage was always slide valves wear in; piston valves wear out.

The only problem I've had with them in that time is that a locknut once worked loose with the obvious result. Sorted in half an hour.