For a direct simple-harmonic D slide valve required to uncover a selected steam port fully, maximum displacement from mid-position is port width plus outside lap. Nominal total travel is therefore 2(Pw + Lo), and direct eccentric throw is Pw + Lo.
D Slide Valve and Port Travel
Explore classical full-port D slide-valve travel while keeping lead and inside lap in their proper timing roles.
Equation Used
- Direct eccentric motion gives valve displacement equal to eccentricity under the simple reference.
- Selected outside lap is symmetric at both steam edges.
- Nominal travel is sized to uncover a steam port by its full selected width.
- Angular setting, rod angularity, valve events, flow, pressure, wear and ratings are excluded.
Inside lap is reported separately as exhaust-edge geometry; lead contributes to selected linear advance, not travel.
Steam edges, exhaust edges and valve travel
The D valve slides across two cylinder ports and a central exhaust passage. Selected port width and lap values set the drawn edges; orange indicates admission and blue indicates connection to exhaust. The ideal stem displacement spans the selected nominal travel. At the start of the cycle, displacement is outside lap plus selected lead. Inside lap changes the cavity edges. The chest and bridge dimensions are illustrative, and the colors show open connections rather than calculated steam flow.
What the calculator determines
Select port width, outside lap, inside lap and lead. The calculator reports nominal full-port travel, direct eccentric throw, selected linear advance at dead center (outside lap plus lead), and selected inside lap.
Classical full-port reference
half travel = Pw + Lo; total travel = 2(Pw + Lo); direct throw = Pw + Lo; linear advance = Lo + lead.
Selected-input arithmetic
For a 1.25 in port and 0.25 in outside lap, nominal full-port travel is 3.00 in and direct throw is 1.50 in. With 0.03125 in lead, selected linear advance is 0.28125 in.
Reference geometry only
Actual valve events require eccentric angular advance, crank position, rod angularity, port/bridge dimensions and the complete valve gear. Inside lap changes exhaust timing. This model does not predict cutoff, release, compression, pressure, flow, power, efficiency, wear or suitability.
Questions
Why is lead not added to travel?
Lead is the opening at dead center and contributes to linear/angular advance; it is not extra full-port width.
Why is inside lap not added?
Inside lap belongs to the exhaust edges and affects release and compression timing.
References & Further Reading
- Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), page 71, figure 238: D slide valve with lap. Wikipedia contributors. Slide valve. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
