A simple reversing gear is a steam engine valve-actuating arrangement that uses a single loose or slip eccentric on the crankshaft to drive the slide valve, allowing the eccentric to swing through a fixed angle on the shaft so the engine runs ahead or astern depending on which way the crank is started. It solves the problem of needing a second eccentric and full link motion on small launch and auxiliary engines. The eccentric drags against a friction collar until it fetches up against one of two stops set roughly 140°–160° apart. You get full reversing capability with one eccentric, one rod, and zero linkwork.
Simple Reversing Gear Interactive Calculator
Vary the lead-angle range and see the resulting loose-eccentric stop spacing and animated slip arc.
Equation Used
The loose eccentric stop spacing is calculated from the lead angle using theta = 2 x (90 + lambda). Enter the low and high lead angles to get the corresponding stop-angle range for the ahead and astern stop pins.
- Uses the article's stated stop spacing formula.
- Lead angles are entered in degrees.
- Minimum and maximum lead inputs are sorted before calculation.
- This is a valve-timing geometry estimate, not a steam-flow model.
How the Simple Reversing Gear Actually Works
The simple reversing gear — also called a loose eccentric or slip eccentric reverser — sits on the crankshaft and grips the shaft only by friction. When you bar the engine over in the ahead direction, the friction between the eccentric bore and the shaft drags the eccentric round until its driving lug strikes the ahead stop pin. From that moment on, the eccentric is locked angularly to the crank and drives the slide valve through the eccentric rod and valve spindle exactly as a fixed eccentric would. To go astern, you stop the engine, bar it over the other way, and the eccentric slips round on the shaft until it fetches up against the astern stop. The two stops are set so the angle of advance is correct for whichever direction the crank is turning, typically 90° plus lead either side of the crank pin.
The geometry is fussy and you need to get it right or the engine will not start cleanly. The total slip angle between stops sits in the 140°–160° range for most launch engines — 2 × (90° + lead) is the textbook number, with lead usually 3°–6°. If you set the stops too close together, you lose lead and the engine runs sluggishly off bottom-dead-centre. Set them too far apart and the eccentric overshoots, the valve opens late, and you get a hard kick on the first stroke. The friction collar must grip hard enough to drag the eccentric round against the inertia of the eccentric strap and rod — typically 8 to 15 N·m of break-away torque on a 1 in shaft — but not so hard that the eccentric refuses to slip when you reverse.
Failure modes are predictable. If the friction collar glazes or oils up, the eccentric will not drag round and the engine will not pick up in the new direction — you bar it over and nothing happens at the valve. If the stop pins wear or shear, the slip angle opens up and timing drifts stroke by stroke. And if the eccentric bore wears oval, you lose grip in one direction only, so the engine reverses fine but will not run ahead, or vice versa. Every one of these is fixable on the bench in an afternoon.
Key Components
- Loose eccentric (sheave): A circular disc bored eccentric to the crankshaft by the throw distance — typically 12 to 25 mm for small launch engines. The bore is a sliding fit on the shaft with around 0.05 mm diametral clearance, gripped only by the friction collar.
- Friction collar or spring washer: Provides the drag torque that pulls the eccentric round when the crank turns. Usually a phosphor-bronze ring or a coiled spring sleeve, set to give 8–15 N·m of break-away on a 1 in shaft. Too tight and the eccentric will not slip; too slack and it will not drive the valve.
- Driving lug or dog: A radial projection on the eccentric that strikes the stop pins. Made hard — case-hardened steel at 55–60 HRC — because every reversal hammers it. Mushroomed lugs are the single most common rebuild item on a 50-year-old launch engine.
- Stop pins (ahead and astern): Two hardened pins pressed into the crankshaft or a collar fixed to it, set 140°–160° apart. The angular spacing equals 2 × (90° + lead). Get this wrong by more than 2° and the engine will start hard and run rough.
- Eccentric strap and rod: Standard split-bronze strap running on the eccentric, transmitting motion to the valve spindle. No different from a fixed-eccentric installation — the strap does not know the eccentric is loose.
- Slide valve and valve spindle: Receives the linear motion from the eccentric rod and admits steam to either end of the cylinder. Lap and lead are set by valve geometry, not by the reversing gear, so once timing is right the valve behaves identically ahead and astern.
Who Uses the Simple Reversing Gear
Simple reversing gear earns its place anywhere you need ahead-and-astern capability on a small or auxiliary engine without the cost, weight, and linkage complexity of Stephenson or Walschaerts gear. You will find it on steam launches, model engines, donkey engines, capstan engines, and small auxiliaries where the operator is happy to stop the engine to reverse it. It cannot give variable cut-off — that's the price you pay — but for fixed-cut-off duty it is unbeatable in simplicity.
- Steam launches: The Stuart Turner 5A and 10V launch engines as fitted to thousands of clinker and carvel launches use a slip-eccentric reverser as standard, and you'll see them on Lake Windermere steamboat-club hulls every summer.
- Model engineering: Cheddar Models Puffin and Proteus twin-cylinder marine engines use loose-eccentric reversing on each cylinder, sized for 3 ft to 6 ft pond yachts and small launches.
- Industrial auxiliaries: Steam-driven deck capstans on preserved coastal vessels such as the SS Shieldhall use simple reversing gear because the operator only ever wants full ahead or full astern at a fixed cut-off.
- Donkey engines and winches: Logging donkey engines of the Pacific Northwest era — Washington Iron Works and Willamette machines — used loose-eccentric reversing on the smaller line-pull winches where the choker-setter wanted simple, foolproof direction control.
- Heritage workshop demonstration engines: Single-cylinder shop engines at sites like the Bolton Steam Museum use simple reversing gear for direction-change demonstrations to visitors, because the mechanism is visible, audible, and easy to explain.
- Steam road vehicles: Early steam tricycles and small steam cars — the Locomobile runabouts of 1899–1902 — used variations on slip-eccentric reversing for the tiller-controlled reverse, before being superseded by Stephenson link motion on heavier models.
The Formula Behind the Simple Reversing Gear
The single number you actually have to set on a simple reversing gear is the angular spacing between the two stop pins. Get this wrong and the engine either won't start in one direction or will hammer itself to death in the other. The formula below gives the stop-spacing as a function of valve lead. At the low end of the typical lead range (about 2°) the stops sit at 184° apart and the engine starts soft but runs slow off-centre. At the high end (around 8°) the stops sit at 196° apart, the engine kicks hard on the first stroke and accelerates briskly but is rough at idle. The sweet spot for a launch engine is 4°–5° lead, giving 188°–190° between stops.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θstops | Angular spacing between the ahead and astern stop pins, measured around the crankshaft | degrees | degrees |
| λ | Valve lead angle — the angle by which the eccentric leads the crank past the 90° quadrature position so the valve opens slightly before dead-centre | degrees | degrees |
| recc | Eccentric throw — half the total valve travel, set by the eccentricity of the sheave bore | mm | in |
| Tdrag | Friction-collar break-away torque required to drag the eccentric round when reversing | N·m | lb·ft |
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.
Simple Reversing Gear vs Alternatives
Simple reversing gear is the cheapest, lightest, simplest way to reverse a steam engine, but it gives up a lot. Compare it honestly against Stephenson link motion and Walschaerts on the dimensions that actually matter for a working engine.
| Property | Simple Reversing Gear (loose eccentric) | Stephenson Link Motion | Walschaerts Valve Gear |
|---|---|---|---|
| Variable cut-off (expansive working) | No — fixed cut-off only | Yes — continuously variable from full to ~15% | Yes — continuously variable, very linear |
| Reversal procedure | Stop engine, bar over opposite direction | On the fly via reversing lever | On the fly via reversing lever |
| Number of eccentrics per cylinder | 1 | 2 | 1 (return crank) |
| Typical applications | Launches, model engines, donkey winches up to ~20 hp | Locomotives, marine main engines, mill engines | Locomotives, modern preserved steam, large marine |
| Build cost (heritage rebuild) | Low — single eccentric, two pins | High — two eccentrics, expansion link, lifting links | Highest — many precision links and pins |
| Typical maintenance interval | Friction collar every 5–10 seasons | Link pins every 2–4 seasons | Pin set every 1–3 seasons under heavy use |
| Power output suitability | Up to ~25 ihp practical | Unlimited — used to 4000 ihp+ | Unlimited — used to 7000 ihp+ |
| Operator skill required | Low — bar and go | Medium — must learn notch-up technique | Medium — same as Stephenson |
Frequently Asked Questions About Simple Reversing Gear
Almost certainly the eccentric bore has worn or been bored slightly oval, so the friction collar grips well in one rotational direction but slips continuously in the other. The eccentric drags round to the astern stop fine, but in ahead the collar is rubbing against the worn ovality and never quite locks the lug against the stop pin — so the valve timing wanders.
Pull the eccentric and check the bore with a dial bore gauge. If you see more than 0.03 mm out-of-round on a 25 mm bore, sleeve it or replace the sheave. A common quick check: bar the engine slowly by hand in ahead and watch the eccentric — if you see it judder rather than snap firmly to the stop, the bore is gone.
You can absolutely use simple reversing gear on a twin or even a triple, and Cheddar Models Puffin is the textbook example — a slip eccentric on each cylinder, completely independent. The catch is that both eccentrics must drag round at the same time when you reverse, which means both friction collars need to be set within ~10% of the same break-away torque. If one is appreciably tighter than the other, that cylinder will reverse first and the engine will sit on dead-centre with the cylinders fighting each other for half a revolution.
For engines above about 25 ihp the inertia of the eccentric straps becomes large enough that the friction collars need to be aggressively heavy, and at that point Stephenson link motion is mechanically simpler than a really stiff slip-eccentric setup.
Bronze rings give you a constant break-away torque that does not change with temperature or oil contamination — the friction is metal-on-steel and predictable. They wear slowly and need re-shimming every few seasons. Coil springs give you a more forgiving slip characteristic and self-adjust slightly as they wear, but they lose grip dramatically if oil reaches them and they fatigue if you reverse the engine more than ~50 times per outing.
Rule of thumb: if the engine reverses occasionally (a launch on a long lake) use bronze. If it reverses constantly (a capstan engine, a shunting donkey) use a coil sleeve and accept the maintenance trade.
The eccentric strap is binding on the sheave, or the eccentric rod is fouling the valve spindle gland at one end of its travel. The friction collar has enough torque to start the eccentric moving but not enough to push it through the binding point against the stop.
Check the strap clearance with feeler gauges — you want 0.05–0.10 mm circumferential clearance and free axial float. If the strap is fine, look at the gland: a freshly repacked gland that has been over-tightened adds enough drag at the rod end to absorb several N·m, and the eccentric stalls before it reaches the stop. Slack the gland nuts a quarter-turn and try again.
Probably not — they were tuning around real-world friction. The theoretical formula assumes the eccentric reaches the stop instantly, but in practice the strap inertia and gland friction mean the eccentric overshoots slightly when it lets go from the old stop, and you can compensate by setting the new stops a few degrees wider than theory.
Heritage builders like Simpson Strickland and Plenty routinely set stops at 195°–200° on launch engines running 4°–6° lead, because experience showed those engines started cleaner with the deliberate over-spacing. If your engine starts hard, try opening the spacing 2°–3° before you start chasing other faults.
On a launch engine under ~10 hp, leave it alone. The fuel saving from variable cut-off on a small intermittent-use engine is dwarfed by the rebuild cost and weight penalty of fitting a link motion. A Stuart Turner 5A burns maybe 4 kg of coal an hour at full cut-off and you'd save perhaps 0.5 kg/hr by notching up — measured in pence per outing.
On a working engine that runs all day under varying load — a launch in commercial passenger service, a donkey winch on a working coaster — the conversion does pay back, but the rebuild cost is substantial and the original simple gear is part of the engine's heritage value. Most museum trustees rule the conversion out on those grounds alone.
References & Further Reading
- Wikipedia contributors. Steam engine — Valve gear. Wikipedia
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