This reference maps two selected lead values to stop angles using θ = 2(90°+λ). It does not calculate a complete reversing gear, valve events, engine direction or safe operating procedure.
Linear actuator checks for Simple Reversing Gear Mechanism
An actuator-driven version of simple reversing gear mechanism should be reviewed for stroke, linkage angle, and peak force. The mechanism may feel light through most of the travel and still bind or spike in force at one position.
When comparing a cylinder to an electric actuator, convert the discussion into stroke, force, speed, duty cycle, and installed length. Matching one specification is not enough.
If the application needs synchronized motion, feedback should be treated as a requirement rather than an accessory. Two open-loop actuators wired together can drift apart when friction, load, or mounting geometry differs between sides.
- 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.
Reversing-Gear Selected Stop-Angle Mapping
Explore the preserved selected lead-to-stop-angle mapping with a bounded reversing lever and quadrant.
Equation Used
- The declared θ=2(90°+λ) mapping is applied exactly.
- Input order has no meaning; the values are sorted.
- The lever sweep is illustrative and bounded by the mapped stops.
- Valve events, loads, clearances and engine operation are excluded.
The two lead inputs are sorted, so swapping them does not change the mapped stop band.
Lever, pivot and selected angular stops
The animation shows one finite reversing lever sweeping between two mapped stop rays on a fixed quadrant. A connected output link follows the lever tip illustratively; no valve gear or steam distribution is modeled.
What the calculator determines
Enter two selected lead values in either order. The calculator sorts them and reports the corresponding low and high mapped stop angles, midpoint and angular span.
Selected angular mapping
a=min(λ1,λ2); b=max(λ1,λ2); θlow=2(90°+a); θhigh=2(90°+b); midpoint=(θlow+θhigh)/2; span=θhigh−θlow.
Worked Example: Simple Reversing Gear in a heritage steam-launch reversing-gear rebuild
You are setting up the simple reversing gear on a recommissioned 1908 Simpson Strickland 4 hp single-cylinder launch engine being refitted to a 22 ft mahogany picnic launch on Coniston Water. The engine has a 2.5 in bore and 3 in stroke, a flat slide valve with 1/8 in lap and 1/16 in lead, and a 1 in crankshaft. You need to determine the correct angular spacing between the ahead and astern stop pins, and check the friction-collar drag torque is adequate at slow harbour idle, nominal cruise, and brisk running.
Given
- Lead (linear) = 1/16 (1.59) in (mm)
- recc = 0.625 (15.9) in (mm)
- Crankshaft diameter = 1.0 (25.4) in (mm)
- Eccentric strap mass = 0.45 kg
- Operating speed range = 120 / 350 / 600 RPM (idle / cruise / brisk)
Solution
Step 1 — convert linear lead into angular lead. With eccentric throw recc = 15.9 mm and lead = 1.59 mm, the lead angle is approximately:
Step 2 — apply the stop-spacing formula at this nominal lead. This is the angle you mark out between the ahead and astern stop pins on the crankshaft collar:
Step 3 — check the bracketing range. If you accidentally set the lead low at 2° (lead block slipped during fitting), the spacing collapses to:
At 184° the valve barely cracks open on dead-centre and at 120 RPM harbour idle the engine will labour and may stall when you ask it to pull away from a mooring. If lead is set high at 8°, spacing opens to:
At 196° the valve is wide open before the piston reaches dead-centre and at 600 RPM brisk running the engine kicks hard on the first stroke after reversing — you'll feel the launch lurch as you go astern. The 191.4° figure at 5.7° lead is the sweet spot: clean starting at 120 RPM, smooth pull at 350 RPM cruise, and no hammer at 600 RPM.
Step 4 — friction-collar drag check. The torque needed to accelerate the eccentric strap from rest through the slip angle in roughly half a revolution at 350 RPM is approximately:
10 N·m on a 1 in shaft is the standard heritage-launch setting and gives positive engagement without locking the eccentric solid.
Result
Set the stop pins 191° apart on the crankshaft collar, with the friction collar adjusted to 10 N·m break-away torque. At 120 RPM harbour idle the engine starts cleanly off the mooring, at 350 RPM cruise it pulls evenly with no roughness, and at 600 RPM brisk running there is no audible kick on reversal — that's the operating window the gear is designed for. If your engine refuses to pick up after reversing, suspect a glazed or oil-soaked friction collar first, before you start drifting stop pins. If you hear a sharp metallic clack on every reversal but the engine runs fine, the driving lug has mushroomed and is making contact with the stop face on a worn corner — pull the eccentric and dress the lug square. If the lead has drifted by more than 2° between rebuilds, check the stop pins for shear or for the collar having spun on the shaft taper, because that single fault accounts for most timing complaints on simple reversing gear.
Mapping only
The formula is preserved as a selected geometric transformation. Eccentric geometry, lap, lead definition, cutoff, valve travel, link position, loads, clearances, direction change and engine behavior are excluded.
Questions
Does this predict valve timing?
No.
Can the two inputs be reversed?
Yes. They are sorted before the stop-angle mapping is applied.
References & Further Reading
- Wikipedia contributors. Steam engine — Valve gear. Wikipedia
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