An Eccentric Crank is a circular disc or collar mounted on a rotating shaft so that the disc's geometric centre sits offset from the shaft's axis of rotation. That offset, called the throw, converts uniform rotary motion into reciprocating linear motion as a strap riding the disc's outer diameter pushes and pulls a connecting rod. The throw equals exactly half the stroke. We see it on stationary steam engines running 100 to 400 RPM driving valve gear and feed pumps — a Stuart Turner 10V model engine is the classic small-scale example.
Eccentric Crank Interactive Calculator
Vary eccentric throw and shaft speed to see stroke, reciprocating travel, cycle rate, and animated slide motion.
Equation Used
The eccentric throw e is the offset between the shaft axis and the eccentric disc center. Because the disc center moves one throw to each side of mid-travel, the driven slide stroke is exactly Stroke = 2 * e. Shaft speed only converts that geometric stroke into cycle rate and average sliding speed.
- One shaft revolution produces one full reciprocating cycle.
- Throw e is the offset from shaft axis to eccentric disc center.
- Stroke is pure geometry and equals twice the throw.
- Speed outputs ignore friction, clearance, and rod obliquity.
How the Eccentric Crank Works
The Eccentric Crank does the same job as a regular crankshaft — convert rotation into reciprocation — but it does it without breaking the shaft. That matters when the shaft has to pass straight through the mechanism to drive something else further down the line, like a flywheel, a governor, or a second cylinder. You bore a hole in a disc, offset that hole from the disc's centre by the throw distance e, slide the disc onto the main shaft and key it. As the shaft turns, the disc's outer diameter wobbles in a perfect circle of radius e around the shaft axis. A two-piece eccentric strap clamps around that outer diameter and rides it like a bearing. The strap connects through an eccentric rod to whatever you're driving — typically a slide valve or a pump plunger.
Stroke equals 2 × e. That is not a guideline, that is geometry. If you machine the throw to 12.5 mm the stroke will be 25.0 mm, every time. The bore-to-throw tolerance has to be tight — we hold the eccentric-to-shaft fit at H7/k6 or better, because any slop there shows up as a hammering noise at top-dead-centre and accelerates strap wear. The strap-to-disc clearance is the other critical fit: too tight and you cook the bronze, too loose and the rod knocks. A typical running clearance on a 75 mm eccentric is 0.05 to 0.08 mm with a film of steam-cylinder oil.
What fails first? The strap. Bronze or babbit straps wear oval over thousands of hours, and once the clearance opens past about 0.2 mm on a small engine you'll feel the knock through the bedplate. The second failure mode is the keyway — if the key shears or the grub screw backs off, the eccentric rotates on the shaft and the valve timing walks, which on a steam engine means the engine quits making power and starts venting live steam straight to exhaust.
Key Components
- Eccentric Disc (Sheave): Cast-iron or steel disc bored off-centre by the throw distance e. Mounts on the main shaft with a key and grub screw. The outer diameter is ground to roundness within 0.02 mm and surface-finished to Ra 0.4 µm so the strap can ride it as a plain bearing.
- Eccentric Strap: A two-piece bronze or white-metal collar that wraps the disc OD and is bolted together around it. The strap rides the disc with a 0.05 to 0.08 mm running clearance and converts the disc's wobble into a linear pull-push at the rod end.
- Eccentric Rod: Steel rod, often forged with an integral fork or eye, that links the strap to the driven element — slide valve, pump ram, or feed-pump plunger. Length sets the geometry of the linkage but does not affect stroke.
- Main Shaft: The drive shaft passes continuously through the disc bore. This is the whole point of the Eccentric Crank — the shaft is unbroken, so it can carry a flywheel, governor drive, or second eccentric on the same axis.
- Key and Keyway: Locks the disc's angular position on the shaft, which sets valve timing on a steam engine. A square or rectangular key in a milled keyway, typically 6 mm × 6 mm on a 25 mm shaft. If this lets go, timing walks and the engine stops making power.
Who Uses the Eccentric Crank
You find the Eccentric Crank wherever an engineer needed a short reciprocating stroke driven off a continuous shaft. The stroke is fixed by the throw, which makes it ideal for valve gear and small pump drives where stroke length is a design constant, not a variable. The mechanism does not handle large loads gracefully — strap pressure climbs fast as throw increases — so you don't see eccentrics driving connecting rods on the main piston of an engine. They drive accessories.
- Steam Locomotion: Stephenson valve gear on a Class A4 LNER locomotive uses two eccentrics per cylinder — one for forward gear, one for reverse — sliding on an expansion link to vary cut-off.
- Stationary Steam: Stuart Turner 10V model engine drives its slide valve from a single eccentric on the crankshaft, throw of 1.5 mm giving a 3 mm valve travel.
- Industrial Pumps: Worthington duplex steam feed pumps used eccentric-driven pilot valves to sequence the main steam admission on alternating cylinders.
- Printing: Heidelberg platen presses used eccentric drives on the ink-train oscillator rollers to translate them axially while they rotated, blending ink across the form.
- Metalworking: Bench-top fly presses and small punch presses use a heavy eccentric on the main shaft to drive the ram, with throws of 6 to 25 mm depending on the press tonnage.
- Textile Machinery: Northrop automatic looms used eccentrics to drive the picking shaft and shuttle-box motions, where short, repeatable strokes at 180 picks per minute were needed off a continuous main shaft.
The Formula Behind the Eccentric Crank
The piston (or strap rod) position relative to the shaft centreline as a function of crank angle is what you actually need when you're laying out a slide-valve event diagram or sizing a pump's discharge profile. The basic relationship is simple harmonic plus a small correction for connecting-rod obliquity. At low throw-to-rod-length ratios — say e/L below 0.1 — the motion is nearly pure sinusoid and you can ignore the correction term. Push e/L above 0.25 and the obliquity term distorts the stroke noticeably, with the rod accelerating harder near top-dead-centre than bottom-dead-centre. The sweet spot for valve gear sits around e/L = 0.05 to 0.15, where motion is clean enough that you can time events off the simple sine and still hit valve openings within 1° of crank angle.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| x(θ) | Position of the rod end (strap pin) measured from the main shaft centreline along the rod axis | mm | in |
| e | Eccentric throw — offset of the disc bore from its outer-diameter centre. Stroke = 2 × e. | mm | in |
| L | Eccentric rod length, strap-pin centre to driven-end pin centre | mm | in |
| θ | Crank angle measured from top-dead-centre of the disc | rad or �� | rad or ° |
Worked Example: Eccentric Crank in a model marine triple-expansion engine
You're machining the eccentric for the high-pressure slide valve on a 1:8 scale model of a triple-expansion marine engine, the kind that powered Edwardian Clyde puffers. The shaft is 16 mm diameter running at 250 RPM nominal, with an idle range down to 80 RPM and a top end of 400 RPM. The valve needs 6.0 mm of total travel, and the eccentric rod is 90 mm long pin-to-pin. You need to confirm the throw, check the valve velocity at the operating range, and see whether obliquity will throw your timing off.
Given
- Stroke = 6.0 mm
- L = 90 mm
- Nnom = 250 RPM
- Nlow = 80 RPM
- Nhigh = 400 RPM
Solution
Step 1 — set the throw from the required stroke. Throw is half the stroke, no exceptions:
So you bore the disc 3.0 mm off-centre. Hold that to ±0.02 mm — any more and the valve will open early or late on alternating strokes.
Step 2 — peak valve velocity at nominal 250 RPM. Maximum velocity occurs at θ = 90° and equals e × ω:
78.5 mm/s peak across the valve face is comfortable — well below the 200 mm/s threshold where steam-port erosion starts to matter on a model.
Step 3 — compare across the operating range. At idle 80 RPM:
That's slow enough you can watch the valve crawl across the port if you pull the steam chest cover. At the top end of 400 RPM:
Still safe for a bronze valve on cast-iron seat with cylinder oil, but you'd want to check the strap clearance after 50 hours running because hydrodynamic film thins at higher speeds.
Step 4 — check obliquity. e/L = 3.0 / 90 = 0.033, which is well under 0.1, so the motion is essentially pure sinusoid. The valve event timing error from neglecting the correction term is under 0.1° of crank angle. You can lay out the valve diagram on the simple sine.
Result
Throw is 3. 0 mm and peak valve velocity at the 250 RPM nominal speed is 78.5 mm/s. That's the comfortable middle of the band — fast enough for crisp valve events, slow enough that the strap stays cool. At the 80 RPM idle the valve creeps at 25.1 mm/s, and at 400 RPM it peaks at 125.7 mm/s, which is the practical upper limit before strap heating becomes a 50-hour service issue. If your built engine measures a stroke other than 6.0 mm, the three suspects are: (1) the disc bore measured off the wrong reference face, giving e ≠ 3.0 mm — check with a height gauge on a surface plate, (2) the eccentric rotated on the shaft because the grub screw bottomed on the keyway instead of the shaft flat, or (3) the strap bolts are uneven and the strap is gripping the disc oval, which shows up as a stroke that varies between forward and reverse halves of the rotation.
Eccentric Crank vs Alternatives
The Eccentric Crank competes with the regular crankshaft and the Scotch yoke for the same job — turning rotation into short-stroke reciprocation. Each one wins in a different region of the design space. Pick on stroke length, load, and whether the shaft has to pass through.
| Property | Eccentric Crank | Conventional Crankshaft | Scotch Yoke |
|---|---|---|---|
| Typical stroke range | 3 to 50 mm | 25 to 500+ mm | 10 to 200 mm |
| Practical RPM ceiling | 400 RPM (strap heating limited) | 6,000+ RPM (rolling-element bearings) | 1,500 RPM (slot wear) |
| Load capacity at the rod | Low — limited by strap pressure | High — sized by big-end bearing | Medium — limited by yoke slot Hertzian stress |
| Shaft passes through? | Yes — main feature | No — shaft is broken at the throw | No — shaft ends at the pin |
| Motion profile | Near-sinusoid (e/L small) | Sinusoid + obliquity correction | Pure sinusoid |
| Service life of wear parts | 3,000-10,000 hr (strap relining) | 20,000+ hr (bearing change) | 2,000-5,000 hr (slot/peg wear) |
| Best application fit | Valve gear, accessory drives, small pumps | Engine main pistons, compressor cylinders | Pure sinusoidal output, small pumps |
| Build complexity | Low — bore offset hole, fit strap | High — forged or fabricated throws | Medium — slot machining critical |
Frequently Asked Questions About Eccentric Crank
The strap and disc don't expand at the same rate. A bronze strap on a cast-iron disc gains roughly 0.018 mm per °C per 100 mm of bore, so a 75 mm strap that runs 60 °C above the disc loses about 0.08 mm of clearance — which is the entire running clearance gone. You measured cold, but at operating temperature the strap is squeezing the disc.
Fix it by setting cold clearance at the upper end of the spec (0.08 mm not 0.05 mm) and verifying with a feeler gauge after a 30-minute warm-up run. If the strap is still hot, check that the oil feed hole in the strap actually lines up with the wick groove — a misdrilled oil port is the second-most-common cause.
A single eccentric gives one fixed valve timing. To reverse a steam engine you need to shift the eccentric's angular position on the shaft by roughly 180° minus twice the lead angle — and a single keyed eccentric cannot do that. What you're seeing is the engine running on the wrong side of the valve event in reverse: steam admission happens late, exhaust opens early, and you lose most of the expansion work.
This is exactly why Stephenson and Walschaerts valve gears exist — two eccentrics or one eccentric plus a return crank, linked through an expansion link, so you can shift the effective timing without re-keying the shaft.
Three questions. First, does the shaft need to continue past the drive point? If yes, eccentric wins by default — a crankshaft breaks the shaft. Second, what's the rod load? Above about 500 N continuous on a 50 mm eccentric, strap pressure starts cooking the bronze and you want a rolling-element big end on a proper crank. Third, what's the RPM? Above 400 RPM the eccentric strap struggles with hydrodynamic film maintenance — go to a crankshaft with a needle or roller big end.
For a typical 20 mm stroke metering pump at under 200 RPM and modest load, the eccentric is simpler, cheaper to make, and lasts longer.
The stroke = 2 × e relationship is exact, so a 0.4 mm shortfall on a 25 mm stroke is a 1.6% measurement or build error and the candidates are short. Most likely the strap clearance is showing up as lost motion at each dead-centre — 0.2 mm of clearance contributes 0.4 mm of total stroke loss because you lose half the clearance at each end of travel.
Check by measuring strap-to-disc clearance with a feeler gauge. If clearance is correct, the next suspect is the dial indicator setup — if you're measuring at the rod end and the rod is flexing under measurement-finger pressure, you'll read short. Move the indicator to the strap pin centreline.
Knock at top-dead-centre on a fresh build is almost always one of two things. Either the strap bolts are over-torqued and the strap is binding at the disc's high-spot side, then snapping free — that's a sharp metallic click synchronous with the shaft. Or the eccentric rod is hitting the slide-valve buffer or pump end-stop because your rod length is wrong by 1-2 mm and the strap pin runs out of geometric travel.
Pull the steam chest cover and rotate the engine by hand through one full revolution while watching for hard contact. If the strap binds at one angular position, back off the strap bolts a quarter turn at a time until it rotates freely with light drag.
Grease works for low-speed, low-load applications — fly-press eccentrics that cycle a few times a minute can run on lithium-complex grease for years. For anything above about 100 RPM continuous you need a flowing oil supply, because the strap-disc interface is a plain bearing and it relies on a hydrodynamic oil film. Grease can't replenish into the load zone fast enough at speed.
Running dry is asking for a seized strap inside an hour. Even brief dry running scores the disc OD and once that surface finish goes above Ra 1.6 µm you'll never get a stable oil film back without re-grinding the disc.
References & Further Reading
- Wikipedia contributors. Eccentric (mechanism). Wikipedia
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