A Single Eccentric Variable Valve Throw is a steam-engine valve gear where one adjustable eccentric on the crankshaft drives the slide valve, and the eccentric's throw — its offset from the shaft centre — can be changed manually or under load to vary valve travel. Stuart Turner's launch engines and many small donkey engines use this layout. Shortening the throw reduces port opening and shifts cutoff earlier, cutting steam admission per stroke. The result: one engine that runs efficiently at light load and still delivers full power when you open the throw.
Single Eccentric Variable Valve Throw Interactive Calculator
Vary the eccentric offsets for min, mid, and max settings and see the resulting slide-valve travel.
Equation Used
The total slide-valve travel is twice the eccentric offset because the eccentric centre moves from one side of the shaft centreline to the other during a full revolution.
- Direct eccentric strap to valve rod motion.
- No rocker ratio or linkage amplification.
- Valve is centered at mid-stroke.
Inside the Single Eccentric Variable Valve Throw
The principle is simple, the execution is fussy. A single eccentric is a disc bolted to the crankshaft with its centre offset from the shaft axis. As the shaft rotates, the eccentric strap riding on it moves back and forth — that linear motion drives the valve rod which pushes the slide valve across the steam chest. The throw of the eccentric, meaning twice the offset between disc centre and shaft centre, sets the total valve travel. Make the throw bigger and the valve uncovers more port; make it smaller and the valve barely cracks the port open before reversing.
Variable throw means the eccentric is built in two parts — a fixed hub keyed to the shaft and a sliding sleeve or adjustable carrier that lets you reposition the eccentric centre relative to the shaft centre while the engine is running or stopped. On a Stuart 10V donkey engine the adjustment is by hand on a stationary engine; on more sophisticated layouts a governor or a hand wheel pushes the carrier through a screw thread. You're effectively changing the valve travel and the steam admission timing in one motion. Lap and lead — the geometric overlap of the valve face on the port and the small early opening before top dead centre — change too, which is why this gear is never just a power knob.
When tolerances drift, the engine tells you. If the eccentric strap clearance opens past about 0.05 mm on a 50 mm eccentric the valve rod develops a perceptible knock at each reversal and admission timing wanders by 2-3° of crank angle. If the carrier locking screw backs off under vibration the throw creeps shorter mid-run and you'll hear the engine soften and lose RPM with no throttle change. Worn keyways on the fixed hub let the whole eccentric rotate slightly on the shaft, shifting angle of advance and skewing cutoff between forward and reverse strokes — the engine starts running rough on one half of the cycle while the other half stays clean.
Key Components
- Fixed Hub: The keyed inner section bolted to the crankshaft. It carries the angular reference for the eccentric and sets the angle of advance — typically 90° plus lap angle from the crank pin. Keyway fit must be a light interference; any clearance lets the hub creep on the shaft and shifts timing 1-2° per running hour.
- Adjustable Carrier or Sleeve: Slides radially across the hub on a dovetail or T-slot, repositioning the eccentric centre relative to the shaft centre. This is the part that varies the throw. Travel is usually 3-12 mm offset, giving valve travels of 6-24 mm depending on the engine. A locking screw or clamp holds the setting.
- Eccentric Strap: The bronze or cast-iron ring riding on the eccentric outside diameter. Bore clearance must sit at 0.025-0.050 mm on a 50 mm eccentric — tight enough that there's no knock, loose enough that it doesn't seize when the strap warms up under steam.
- Valve Rod: Connects the strap to the slide valve through the steam chest gland. Length is set so the slide valve sits centred over the ports at mid-stroke. Get this length wrong by even 0.5 mm and you skew lead between admission and exhaust strokes.
- Slide Valve: The D-shaped valve riding on the port face. Its lap — the overlap on the steam port at mid-position — combined with the eccentric throw determines cutoff. Typical lap is 3-6 mm on small launch engines.
- Locking Screw or Clamp: Holds the carrier at the chosen offset. Must be torqued to spec — usually 8-12 Nm on small engines. Vibration backs this off if it's not wired or held by a Nyloc; that's the most common single failure on these gears.
Industries That Rely on the Single Eccentric Variable Valve Throw
Single eccentric variable throw gear shows up wherever an operator needs a power adjustment on a small steam engine without paying for the complexity of Stephenson's or Walschaerts' link gear. It's the engineer's compromise — one moving part where link motion has a dozen, but you give up smooth running reverse and continuous infinitely-variable cutoff under load. You see it on launch engines, donkey engines, classroom demonstration engines, and a number of late-Victorian portable engines where the owner wanted some way to throttle steam economy across the working day.
- Marine Auxiliary: Stuart Turner 10V donkey engines on Thames sailing barges at Maldon, used for bilge pumping and winch work where the operator dials throw down for slow steady running and up for short lifting bursts.
- Heritage Launches: Simpson Strickland 4 hp single-cylinder launch engines on Coniston and Windermere picnic launches, where a hand wheel on the eccentric carrier lets the helmsman trim cutoff for fuel economy on long runs.
- Classroom & Demonstration: Stuart Turner Victoria and No.10 model engines used at the Bolton Steam Museum and the Internal Fire Museum of Power for showing students the link between throw, cutoff, and indicator diagram shape.
- Portable Industrial: Late-Victorian Marshall and Robey portable engines built before universal adoption of link motion, where a single shifted eccentric gave the threshing-yard operator a coarse but workable economy adjustment.
- Steam Air Compressors: Brotherhood single-cylinder vertical compressors at tramway depots like Crich, where the gear lets the engine run light when the air receiver is nearly full and pull harder when reservoirs draw down.
- Workshop Power: Robey and Tangye horizontal mill engines retrofitted with simplified single-eccentric gear for driving short line-shafting in small jobbing workshops where reversal is rare and load is steady.
The Formula Behind the Single Eccentric Variable Valve Throw
The piece of physics you actually care about is how valve travel changes as you slide the carrier. At the low end of the typical adjustment range — say 3 mm offset — the valve barely uncovers the port and the engine runs at heavy expansion, idling on residual steam pressure. At the high end — 12 mm offset on a typical small engine — the valve fully exposes the port, you get full admission for most of the stroke, and the engine pulls hard but burns steam. The sweet spot for steady running usually sits at roughly 60-70% of maximum throw, where you get clean port opening without wasteful late cutoff.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tv | Total valve travel — the distance the slide valve moves across the port face per half revolution | mm | in |
| e | Eccentric offset — the distance between the eccentric disc centre and the crankshaft centre, set by the carrier position | mm | in |
| Lp | Port opening — valve travel minus lap, the actual distance steam port is uncovered | mm | in |
| λ | Lap — slide valve overlap on the steam port at mid-position, set during build | mm | in |
Worked Example: Single Eccentric Variable Valve Throw in a 1925 Westinghouse air-brake test stand engine
Setting valve travel and effective port opening across three carrier positions on a recommissioned 1925 Westinghouse single-cylinder vertical engine being returned to service at a heritage railway air-brake test stand at the Didcot Railway Centre in Oxfordshire, where the engine drives a small reciprocating air compressor for charging brake reservoirs on preserved coaches. The engine has a slide valve with 4 mm lap and the variable eccentric carrier offers 3-12 mm of offset travel. The trustees want to confirm valve travel and port opening at low-load idle, nominal compressor running, and brisk reservoir-charging burst before the spring open day.
Given
- elow = 3 mm
- enom = 8 mm
- ehigh = 12 mm
- λ = 4 mm
- Maximum port width = 14 mm
Solution
Step 1 — at nominal carrier setting of 8 mm offset, total valve travel is twice the offset:
Step 2 — subtract the 4 mm lap to find the actual port opening at full valve travel:
This is a clean working setting. The port uncovers 4 mm out of a possible 14 mm — about 29% of full port — and cutoff lands near 50% of stroke. The engine runs steady, the indicator diagram shows a clean expansion curve, and steam consumption stays reasonable.
Step 3 — at the low end of typical adjustment, 3 mm offset for slow idle running:
Negative port opening means the valve never clears the lap. The engine cannot run at this setting — admission never happens. The practical low limit is wherever (e − λ) just exceeds zero, so on this engine you cannot drop the carrier below about 4.5 mm without the engine stalling. That's a useful number to know: half the carrier range is unusable on an engine with this much lap.
Step 4 — at the high end, 12 mm offset for maximum power:
Port opening of 8 mm out of 14 mm gives 57% admission, late cutoff, and the engine pulls hard but burns steam fast. Above this throw the strap travel starts hammering the steam chest gland and you lose oil seal integrity within a few hours of running.
Result
Nominal valve travel sits at 16 mm with 4 mm of useful port opening at the 8 mm carrier setting — a clean economical running point that matches a 50% cutoff. Across the range the picture is asymmetric: at 3 mm offset the engine simply will not run because the valve never clears the 4 mm lap, while at 12 mm offset you get 8 mm of port opening but pay for it in steam consumption and gland wear. The sweet spot sits at 7-9 mm carrier offset for steady compressor work. If you measure less port opening than predicted, three failure modes account for nearly all cases: (1) lap has grown beyond the design 4 mm because the slide valve face was lapped flat during overhaul without re-cutting the steam chest face, robbing 0.5-1 mm of effective opening; (2) the valve rod length is short by a fraction of a millimetre, biasing the valve towards one port and showing asymmetric opening between admission and exhaust; (3) the carrier locking screw has loosened under vibration and the offset has crept down 1-2 mm during the run.
Choosing the Single Eccentric Variable Valve Throw: Pros and Cons
Single eccentric variable throw gear is one of three families you'll see on small to medium steam engines. The other two are Stephenson's link motion and Walschaerts' gear. The choice between them is rarely about ultimate performance — it's about complexity, cost, reverse capability, and how often the operator needs to change cutoff under load.
| Property | Single Eccentric Variable Throw | Stephenson's Link Motion | Walschaerts' Valve Gear |
|---|---|---|---|
| Cutoff range available | 20-70% of stroke, limited by lap and minimum useful throw | 5-85% of stroke, near-continuous | 5-90% of stroke, the widest and most linear |
| Reverse capability | Limited — requires stopping and resetting eccentric angle | Full reverse via shifting link block | Full reverse via shifting link block, smoother than Stephenson's |
| Part count | 4-6 moving parts | 10-14 moving parts | 12-16 moving parts |
| Build cost relative to single eccentric | 1.0× baseline | 2.5-3.5× baseline | 3-4× baseline |
| Adjustment under load | Possible with hand wheel or governor link, but coarse | Smooth and continuous via reversing lever | Smooth and continuous, best feel of the three |
| Typical application fit | Small launch, donkey, demonstration engines under 10 hp | Locomotives, marine engines, mill engines 10-2000 hp | Locomotives and large marine engines 50-5000 hp |
| Maintenance interval (typical heritage use) | Strap clearance check every 50 running hours | Link pin and die wear check every 200 hours | Multiple pin/bush checks every 200 hours |
Frequently Asked Questions About Single Eccentric Variable Valve Throw
Because total valve travel and useful port opening are not the same number. Valve travel is twice the eccentric offset, but the slide valve has to first overcome its lap before any port uncovers. If your engine has 4 mm lap and you set the carrier to 4 mm offset, you have 8 mm of valve travel but zero port opening — the valve rocks back and forth across the lap and steam never enters the cylinder.
The practical minimum carrier offset is roughly lap plus 1 mm, which gives a sliver of port opening for starting. Below that, no amount of throttle will make the engine turn over.
Three questions decide it. Will you reverse the engine under steam regularly? If yes, link motion every time — single eccentric reversing means stopping and physically resetting the eccentric angle, which is fine for a stationary donkey engine and unsafe for a launch backing off a pier. Do you need fine cutoff control under varying load? Link motion gives smoother adjustment. Is build cost or model size a constraint? Single eccentric wins — it's roughly a third the part count and far easier to machine accurately.
For a passenger-carrying launch over about 16 ft, fit link motion. For a static demonstration engine or a small auxiliary like a bilge pump drive, single eccentric is honest and adequate.
The angle of advance is wrong on one direction. A single eccentric is set with its centre leading the crank pin by 90° plus the lap angle for forward running. To reverse, you have to swing the eccentric to the mirror position past the crank — exactly the same angle on the other side. If you stop one or two degrees short, forward running is fine because that's the side you set originally, but reverse runs with skewed admission timing and you hear it as uneven exhaust beats.
Mark both forward and reverse positions on the shaft with scribed lines during initial setup and use a feeler against a fixed reference. Don't trust eyeballed angle.
Valve rod length is wrong, almost certainly. The slide valve should sit dead-centre over the ports when the eccentric is at mid-throw. If the rod is even 0.3-0.5 mm too long or too short, the valve biases toward one port and admission and exhaust events become asymmetric — exactly what you're describing.
Set the engine on dead centre, slacken the valve rod jam nut, and adjust the rod length until equal port opening shows on both sides when you rock the eccentric through full throw by hand. A pencil line on the valve rod against a steam chest reference is the easiest way to see equal travel.
Two common causes. First, lap has effectively increased because someone lapped the slide valve face during overhaul without reworking the steam chest face — the valve sits proud and overlaps further onto the port, shortening admission. Take a depth measurement of valve face proudness against the chest face; anything over 0.1 mm shows up clearly on the diagram.
Second, the eccentric strap may have measurable clearance — over 0.08 mm on a 50 mm eccentric introduces a delay between eccentric motion and valve motion at each reversal, which clips both ends of admission. Check strap clearance with a feeler at top and bottom of the eccentric throw; uneven clearance points to strap wear, even clearance points to general slop.
Yes, and it's been done since the 1880s — the Hartnell governor and the Pickering governor were both used to shift single eccentric carriers on small engines. The catch is the actuating force required. Shifting the carrier under load means overcoming the steam reaction on the slide valve plus friction in the carrier dovetail, which on a 6 in cylinder engine can be 20-40 N at the carrier. A small ball governor will not move it; you need a relay or a power-assisted linkage with a bypass piston.
For most heritage applications a hand wheel is honest and reliable. Automatic governing of single eccentric throw is a specialist build and rarely worth the complication unless the engine drives a load with wide swings, like a cyclic pump.
References & Further Reading
- Wikipedia contributors. Steam engine valve gear. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
