A Valve Motion Eccentric is a circular disc fixed to a steam engine's crankshaft with its centre offset from the shaft axis, driving a strap-and-rod linkage that converts rotation into the reciprocating motion needed to operate a slide valve. The offset distance, called the throw, defines half the valve travel. It exists to time steam admission and exhaust to the cylinder without using a separate cam profile. On a typical 0-6-0 locomotive the throw runs 50–75 mm, giving valve travels of 100–150 mm at the steam chest.
The Valve Motion Eccentric in Action
The Valve Motion Eccentric, also called the Eccentric for valve gear in locomotive shop manuals, works by mounting a machined disc — the sheave — onto the crankshaft so its geometric centre sits a fixed distance away from the shaft's axis of rotation. That offset distance is the throw. As the crankshaft turns, the centre of the sheave traces a circle around the shaft axis, and a split bronze strap clamped around the sheave's outer diameter is forced to follow that circle. Connect a rod to the strap and you have converted continuous rotation into pure simple-harmonic linear motion. No cam profile, no gear train, no cycloidal complexity. Just a circle offset from another circle.
The geometry matters because steam engines need precise valve timing. The angle between the eccentric's offset direction and the crankpin — the angular advance — sets when the slide valve opens and closes relative to piston position. A typical setting puts the eccentric 90° plus the lap-and-lead angle ahead of the crank, often around 120–135° for forward gear. Get this angle wrong by even 3–5° and the engine loses power, runs rough, or in the worst case admits steam during exhaust and back-pressures itself to a stall. On preserved engines you'll see fitters checking this with a tram and a dial indicator at the valve spindle, not by eye.
Failure modes come from two places. First, the strap-to-sheave running clearance: too tight and the bronze picks up and gallops, too loose and you get knock that hammers the eccentric rod ends. The accepted clearance is 0.001 inch per inch of sheave diameter, plus a thou. Second, sheave key shear — if the gib-head key loosens, the sheave rotates on the shaft, valve timing walks out, and the engine starts hunting. You diagnose that by chalking a witness mark across sheave and shaft and watching it move shift to shift.
Key Components
- Eccentric Sheave: The cast-iron or steel disc keyed to the crankshaft, machined with its outer diameter offset from its bore by the throw distance. Typical throw on a stationary mill engine runs 30–80 mm; on a small model engine 3–6 mm. Bore-to-shaft fit is H7/h6 with a tapered gib-head key carrying the torque.
- Eccentric Strap: A two-piece bronze or whitemetal-lined strap that clamps around the sheave OD and rides on it as a plain bearing. Running clearance is 0.001 inch per inch of sheave diameter — too tight picks up and seizes, too loose knocks. Lubricated by trough oilers or a worsted-yarn siphon feed.
- Eccentric Rod: The connecting rod between strap and valve gear. On Stephenson link motion two rods connect to opposite ends of the expansion link; on Walschaerts gear one rod drives the link directly. Rod length is fixed during commissioning to set valve lead — typically 1/16 inch (1.6 mm) on a small loco.
- Gib-Head Key: Tapered steel key locking the sheave to the crankshaft. Must transmit the full valve-gear torque without shifting. A loose key is the most common cause of timing drift on preserved engines — re-fit if witness marks show any rotation between sheave and shaft.
- Oil Trough or Siphon Feed: Continuous lubrication is mandatory because the strap is a hydrodynamic bearing under reversing load. SAE 30 mineral oil at 0.5–2 mL per minute is typical. Lose the feed and the strap blues within 20 minutes at 200 RPM.
Real-World Applications of the Valve Motion Eccentric
The Eccentric and Straps for Valve Motion shows up everywhere a slide valve or piston valve needs driving from a rotating shaft. It's the dominant valve-actuation method across 150 years of steam practice — locomotives, traction engines, mill engines, marine compounds, model engineering — and it still appears in modern reciprocating compressors and some specialty pumps. The reason is simple: it's mechanically robust, indifferent to dirt, and tunable in service.
- Steam Locomotives: Stephenson link motion on a Great Western Railway 5700-class pannier tank uses two eccentrics per cylinder — one for forward gear, one for reverse — both keyed to the driving axle.
- Stationary Mill Engines: A Pollit & Wigzell horizontal cross-compound at Ellenroad Engine House drives its main slide valve via a single eccentric with adjustable strap on each cylinder.
- Traction & Showman's Engines: Burrell showman's road locomotives use an eccentric and strap for valve motion on the high-pressure cylinder, with the throw set to give 4½ inch valve travel at full gear.
- Marine Reciprocating Engines: Triple-expansion engines like those on the SS Shieldhall use an eccentric per cylinder for the bottom-end of the Stephenson gear, sized for valve travels of 6–8 inches.
- Reciprocating Air Compressors: Atlas Copco and Ingersoll Rand industrial piston compressors still use eccentric-driven valve unloaders on capacity-control circuits.
- Model Engineering: 5-inch gauge live steam locomotives built to LBSC designs use silver-steel eccentrics with bronze straps, throws around 5/16 inch (8 mm).
The Formula Behind the Valve Motion Eccentric
The fundamental relationship is between eccentric throw and valve travel. Throw is half the peak-to-peak linear displacement of the strap, so valve travel is exactly twice the throw — assuming a rigid rod and no gear-link reduction. At the low end of typical practice — a model 5-inch gauge loco with 5 mm throw — you get 10 mm valve travel, which only just clears port width plus lap. Nominal mid-sized practice is 50 mm throw for 100 mm travel, the sweet spot for full-size narrow-gauge and small standard-gauge locos. Push the throw to 90 mm for a heavy mill engine and you're at 180 mm valve travel — generous port opening and strong steam flow, but the strap-to-sheave bearing area now carries significant inertial load and you must size the strap accordingly.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tv | Instantaneous valve travel from mid-position | mm | in |
| e | Eccentric throw (offset of sheave centre from shaft axis) | mm | in |
| α | Crankshaft angle from top dead centre | deg | deg |
| φlead | Angular advance of eccentric ahead of crank (90° + lap-and-lead angle) | deg | deg |
| Tv,max | Maximum (peak) valve travel = 2 × e | mm | in |
Worked Example: Valve Motion Eccentric in a preserved narrow-gauge 0-4-0 saddle tank
You are sizing the valve motion eccentric for the rebuild of a 2-foot gauge Bagnall 0-4-0 saddle tank running at the Statfold Barn Railway. The slide valve serves cylinders of 7 inch bore × 12 inch stroke, port width 1¼ inch (32 mm), steam lap 5/8 inch (16 mm), exhaust lap 1/16 inch (1.6 mm), lead 1/16 inch (1.6 mm). You need to fix the eccentric throw and confirm valve travel across the operating range — the engine works between 80 RPM (yard shunting) and 280 RPM (mainline running on the demonstration line).
Given
- Port width = 32 mm
- Steam lap = 16 mm
- Lead = 1.6 mm
- Required peak valve travel Tv,max = 2 × (port + lap + lead) mm
- Operating speed range = 80 – 280 RPM
Solution
Step 1 — fix the required peak valve travel from port geometry. Full opening must clear the steam port, plus you need lap to give early cut-off and lead to ensure steam admission before dead centre:
Step 2 — solve for eccentric throw. Throw is half the peak travel:
Step 3 — check valve velocity at the nominal running speed of 180 RPM (mid-range cruising). Peak valve velocity occurs at mid-stroke:
At the low end — 80 RPM yard shunting — valve velocity drops to vvalve,low = 2π × (80/60) × 0.050 = 0.42 m/s. The strap is barely loaded, lubrication is forgiving, and you can get away with a worsted siphon feed. At the high end — 280 RPM mainline — peak velocity climbs to vvalve,high = 2π × (280/60) × 0.050 = 1.47 m/s. That's a 3.5× jump in peak inertia load on the strap from low to high speed, which is why the strap on a fast-running loco is always a heavier section than on a slow mill engine of similar throw.
Step 4 — set angular advance. With 16 mm lap and 1.6 mm lead at 50 mm throw, the lap-and-lead angle is arccos((lap + lead)/e) measured from valve mid-travel, giving an eccentric advance of roughly 90° + 21° = 111° ahead of the crank for forward gear.
Result
Eccentric throw is 50 mm, giving 100 mm peak valve travel — comfortably above the 99. 2 mm minimum, with eccentric advanced 111° ahead of the crank. At 80 RPM the valve oozes back and forth at 0.42 m/s and the engine pulls smoothly with generous cut-off — fine for shunting. At 180 RPM (nominal) you're at 0.94 m/s and the engine is in its sweet spot. Push to 280 RPM and peak valve velocity hits 1.47 m/s, which is where strap inertia and oil-film breakdown start to matter — you'll want a forced-feed mechanical lubricator rather than a siphon. If your measured valve travel comes up short of the predicted 100 mm, three causes dominate: (1) a worn eccentric rod brass at the gear end allowing lost motion, typically 1–3 mm slop you can feel by hand; (2) the gib-head key partly sheared and the sheave rotated on the shaft, showing as broken witness marks across the keyway; or (3) bent eccentric rod from a previous over-pressure event, which you confirm by laying the rod on a surface plate and checking with feeler gauges.
When to Use a Valve Motion Eccentric and When Not To
An Eccentric for valve gear is not the only way to drive a steam valve. Cam-driven poppet valves and direct-acting linkages each have their place. Pick by speed range, cut-off control needs, and how much maintenance the operator wants to do.
| Property | Valve Motion Eccentric | Poppet Valve Cam (Caprotti) | Direct Crosshead-Driven Valve |
|---|---|---|---|
| Typical speed range (RPM) | 50–500 | 100–1500 | 30–200 |
| Cut-off adjustability | Continuously variable via link motion or radial gear | Stepped, requires cam swap or shifted cam profile | Fixed by linkage geometry |
| Sensitivity to dirt and water | Tolerates wet/dirty steam — open mechanism | Sensitive — requires clean lubrication | Tolerates wet steam |
| Manufacturing complexity | Low — turned sheave plus split strap | High — cam grinding to ±0.025 mm | Low — simple links |
| Maintenance interval (typical) | Strap re-bed every 5,000–10,000 hours | Cam inspection every 2,000 hours | Pin/bush replacement every 5,000 hours |
| Steam efficiency at short cut-off | Good — 15–25% cut-off achievable | Excellent — sharp valve events | Poor — fixed events |
| Cost (relative) | 1.0× | 3–5× | 0.6× |
Frequently Asked Questions About Valve Motion Eccentric
Yes — same mechanism, different name conventions. UK and stationary engine practice tends to say Eccentric and Straps for Valve Motion to emphasise the strap as part of the assembly. American locomotive practice and most modern textbooks shorten it to Valve Motion Eccentric or simply eccentric. You'll also see Eccentric for valve gear in shop manuals when distinguishing it from feed-pump eccentrics or governor eccentrics on the same shaft.
Different eccentrics drive each direction, and they wear at different rates because forward gear sees the most service. The reverse eccentric strap is often the original from rebuild, while the forward strap has been re-bedded twice. A measurable 0.5 mm of additional clearance in the reverse strap shows up as knock under load because the rod reverses force at every dead centre and slams across the clearance.
Pull the reverse strap, blue the sheave, and re-bed to 0.001 inch per inch of sheave diameter. The knock disappears.
If you need continuously variable cut-off — for fuel economy and load matching — you need a link motion (Stephenson, Allan, or Walschaerts), which uses two eccentrics or one eccentric plus a return crank. If the engine only ever runs at one direction and one cut-off, like a mill engine driving a fixed line shaft, a single eccentric with no link is cheaper, simpler, and lasts longer.
Locos and traction engines always need link motion. Stationary mill engines often don't.
Almost always because you're measuring at the valve spindle on a Walschaerts or radial gear, not at the eccentric rod end. The combination lever and union link add motion derived from the crosshead, on top of the eccentric-derived motion. Predicted travel of 2 × throw applies only to a direct-drive Stephenson installation with the link block at full gear.
If you're on Walschaerts and measuring 110 mm with a 50 mm throw, that's normal — the crosshead contribution has added 10 mm. Check the design valve diagram, not the eccentric throw alone.
The 0.001 inch per inch of diameter rule scales down badly on small parts. For a model with a 25 mm (1 inch) sheave you'd compute 0.001 inch (0.025 mm) clearance, which is below what you can reliably measure with feeler gauges and below typical bronze thermal expansion at full steam temperature.
The practical floor is 0.0015 inch (0.038 mm) regardless of size. Below this, the strap will pick up the first time the engine runs hot. LBSC's writings on 5-inch gauge practice settle on 0.002 inch as the working number.
Yes — the mechanism is just a rotation-to-reciprocation converter producing simple harmonic motion. It works for feed pumps, lubricator drives, mechanical shaker screens, and reciprocating compressor unloaders. The constraint is stroke length: throws above about 100 mm get bulky, and the strap bearing area scales with sheave diameter, so high-load applications push you toward a slider-crank with a wrist pin instead.
If you need pure SHM at modest stroke and don't want to machine a connecting-rod big end, the eccentric and strap is the right call.
References & Further Reading
- Wikipedia contributors. Eccentric (mechanism). Wikipedia
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