RUBY Direction Valve Steam Path

Executive Summary:
The valve is asymmetrical, with a more efficient fluid path when placed in the reverse direction mode.

Part of the reason that a factory condition Accucraft Ruby will go better in reverse than in forward is due to the fluid mechanics in the direction control block. With the direction control in the forward direction, the steam path is very different dimensionally than when in the reverse direction.

Physical Structure:
Direction control of the Accucraft Ruby series is based upon switching steam supply and exhaust paths through the use of a control valve. The control valve body is a rectangular machined brass block that resides between the cylinders and beneath the smoke box. Steam from the boiler always enters the valve through the top rear opening, and exhausts into the smokebox through the top forward opening. On each side of the valve body there are two pipes going to the cylinder valves. When the direction control is in the forward position, the front tube(s) supply steam to the cylinder valves, and the rear side tube(s) is the exhaust from the cylinders. In reverse, this is reversed. The valve body has an approximately 5 mm hole down the middle opening to the rear for the valve plug mechanism with the front of the through hole plugged.

The round valve shuttle (plug) is a machined metal plug with tight sliding fit to the body. In the valve shuttle there is a threaded rear hole for attachment to the direction control lever. This hole is not through into the steam space. This threaded attachment is used to adjust the initial depth of the valve plug into the valve body. The valve shuttle has two grooves machined into the circumference. The rear one is relatively narrow and approximately 1.7 mm in width and .4 mm in depth. The forward one is about 7.3 mm in width and 0.5 mm in depth. There is a single 1.7 mm hole from the rear groove bottom that makes a sharp bend and then through the front of the shuttle. This passage is +12 mm long.

Steam Flow:
When in the forward direction, the steam enters the rear groove. From there enters a sharp edged 1.7 mm hole that is perpendicular to the bottom of the grove. This entry hole can be in any radial position in relation the point where steam enters. The steam then goes through a 90 degree sharp angle and proceeds through the rest of the passage down the center of the valve plug to the front of the valve plug. From there it flows into the full center space of the 5 mm diameter valve body, and then into the forward pipes leading to the cylinder valves. When entering this forward space, it must fill this space, and transfer at right angles into the cylinder pipes. The exhaust from the cylinders utilizes the longer valve shuttle external groove in its path to the stack.

When in the reverse direction, the small groove and its restricted steam passage are not used. The valve shuttle is pulled back leaving the smaller groove behind the steam supply port, and the through passage is blocked. The longer groove then provides the steam passage from the supply to the cylinders. The exhaust utilizes the full 5 mm of the valve body to go from the cylinders to the stack.

Created August 12, 2004