Corliss valve gear uses a rocking wrist plate to operate separate admission and exhaust valves at both ends of a steam cylinder. The admission drive can disengage at an adjustable point, allowing a dashpot to return the valve. Exhaust-valve linkages remain connected throughout the cycle.
Related actuator design tools
This model explains the historical wrist-plate valve linkage. Designing a separate actuator-driven linkage requires its own load, travel and timing analysis; the timing results here do not size an actuator.
For electrical sizing, measure or calculate the current under load. Long wire runs and undersized supplies can make a correctly selected actuator behave like an undersized one.
A crank or cam may tolerate continuous rotation through a difficult part of the cycle, while a linear actuator has defined end positions and internal limits. That difference matters when the mechanism needs repeatability or must not drive into a hard stop.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
Corliss Valve Gear Interactive Calculator
Explore the Corliss wrist plate and four valve drives. Adjust admission duration, stroke time and wrist swing. The results compare times; they do not calculate piston-stroke cutoff, steam-volume expansion or power.
Equation Used
- One forward and one return stroke per illustrated wrist cycle.
- Admission duration is an entered timing event, limited to one stroke.
- Link geometry is illustrative; all four valve-drive rods remain fixed in length.
- Governor and external eccentric drive are omitted; wrist motion is prescribed.
- Latch engagement and dashpot return are simplified event illustrations, not a force or impact simulation.
Admission valves release from their drive arms and return through dashpots. Exhaust valves stay mechanically connected. The displayed ratio is stroke time divided by admission time, not a thermodynamic expansion ratio.
Four valves around one wrist plate
The side view shows two admission drives at the top and two exhaust drives below. Four rigid rods connect their drive arms to the rocking wrist plate. Blue arms follow the plate. Gold admission-valve arms follow their drive only while the latch is engaged.
At the selected admission time the latch releases. The gold arm returns toward its closed position through a dashpot while the blue drive continues moving. On the next stroke the opposite admission drive repeats the sequence. The exterior view does not depict steam passages or valve-port opening area.
In a complete engine an eccentric supplies wrist motion and a governor sets the release point. Those assemblies are omitted here to keep the valve-drive and release action visible.
Admission control in steam engines
The separate admission and exhaust functions let steam admission end without requiring the exhaust drive to detach. The wrist plate distributes motion to all four valve drives.
This is an explanatory timing and linkage model. It does not establish engine output, steam consumption, allowable speed or the geometry of a particular historic engine.
Time fraction is not stroke fraction
The admission-time fraction is 100 t_admit/t_stroke, using admission time no greater than stroke time. The inverse time ratio is t_stroke/t_admit. Remaining stroke time is their difference, and wrist arc is twice its entered half-swing.
These are time calculations. Piston speed varies during a crank revolution, so 15% of stroke time does not generally mean 15% of piston travel. Thermodynamic expansion also depends on cylinder volume at cutoff and clearance. The displayed ratio compares stroke time with admission time; it does not measure steam-volume expansion.
Example timing comparison
A 200 ms stroke and 30 ms admission interval give 15% admission time, a 6.67:1 stroke/admission time ratio and 170 ms remaining. A 25° half-swing gives 50° total wrist arc.
If admission is set longer than the stroke, the calculation caps it at the stroke duration: 100% admission time, 1:1 time ratio and zero remaining interval. Increasing wrist swing changes the drawn drive-arm motion and arc result without changing the separately entered timing.
What is modeled and what is schematic
The rod endpoints are solved as connected fixed-length linkages at every wrist position. Admission-latch release is a timing event; the short return motion illustrates dashpot closure rather than predicting its dynamics.
The input ranges are mathematical exploration controls, not operating ratings. The model omits port dimensions, lap and lead, eccentric phase, steam forces, governor response, friction and impact. It should not be used as a construction drawing or engine-setting procedure.
Corliss-gear questions
Why are there four valves?
Each cylinder end has separate admission and exhaust functions. The illustration places admission valves above and exhaust valves below.
What happens when an admission latch releases?
The drive arm continues moving while the admission-valve arm returns through its dashpot.
Do exhaust valves release too?
No. Their drive links remain connected for both opening and closing.
Is the time ratio an expansion ratio?
No. It compares entered durations. Steam-volume expansion requires piston-position and cylinder-volume information.
Why does wrist swing not change admission time?
They are independent inputs in this model. The admission timing is prescribed rather than calculated from a governor and trip-cam geometry.
References
Hiscox, Mechanical Movements, Powers, Devices and Appliances, mechanism 252, printed page 74, wrist-plate arrangement.
Steam Engines, Machinery Reference Series No. 70, 1911, pages 24–27, Corliss valves, detachable admission gear and dashpot action.
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