Slide valve motion study

With the steam, exhaust ports, cylinder, and steam chest seat modeled, now we have to consider the size and motion of the slide valve, which will regulate the steam admission into the cylinder.

Meyer talks about this extensively (mostly about relating the motion of slide valve to the eccentrics), but the subject can be compressed into the following three “rules”:

1: “Steam must be admitted into the cylinder at one of its ends only
at one time.”

Below is a cross section through the cylinder and the slide valve. The blue solid is the slide valve. Its arc in the middle is the exhaust cavity. The two tubes curving out left and right are the steam ports, terminating at each end of the cylinder. The trapezoidal cavity in the middle is the exhaust.

Ok! Let’s take a look at the slide valve motion:

The first picture shows the 2 “legs” of the valve covering up both steam ports. The second shows the slide valve at its extreme travel (exact distance is not known yet, since I haven’t designed the eccentrics), exposing only 1 steam port (left) while covering the other. Looks like the slide valve meets rule 1.

2: “The valve must allow the steam to escape from one end of the
cylinder, at least as soon as it is admitted into the other end of the cylinder.”

To satisfy this rule, the distance between the inner face of the valve’s legs must satisfy the following equation:

distance = 2 x Width(bridge) + Width(exhaust)

Plugging in the “as-virtually-built” measurements:

2*(0.625) + 1.25 = 2.5in

The picture below shows the measurement of this distance satisfying the 2.5in rule.

What this means visually is that the valve should allow the steam at one end of the cylinder be evacuated (via the exhaust cavity and port) before allowing the steam to enter the other end of the cylinder. This is important to eliminate backpressuring the piston.

The picture below shows that the left steam port would be exhausted before the steam is admitted into the right side. Looks like this rule is satisfied!

3: “The valve must cover the steam ports so as not to allow the
steam to escape from the steam chest into the exhaust port.”

This rule is satisfied when the exhaust cavity is not greater than the length given in the above equation. As you can see, it is not! So, this rule too is satisfied.

Visually, in any of the pictures above we can see that at no time can the steam from the chest escape into the exhaust.

So, indeed, the slide valve satisfies all three rules!

4 thoughts on “Slide valve motion study

  1. Hello!
    Just curious as i am learning about steam locos who are “Meyers” and “Lingenfleter” mentioned in your posts?

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