Eccentric Drive

Part of Steam Engine

How to convert rotary motion to reciprocating motion to operate steam valve gear, pumps, and other sliding mechanisms.

Why This Matters

A steam engine needs more than just the main piston to work. The slide valve that admits and exhausts steam must open and close at precisely the right moments in each stroke. A feed water pump must stroke back and forth to supply water to the boiler. An exhaust pump may be needed in condensing engines. All these mechanisms need reciprocating motion — back-and-forth sliding — derived from the rotating crankshaft.

The eccentric is the mechanical solution. It is a disc mounted off-center on the crankshaft. As the shaft rotates, the eccentric’s center traces a circular path around the shaft center, pushing a strap and rod back and forth. The total back-and-forth travel equals twice the offset distance (called the throw). A 1-inch throw eccentric produces 2 inches of total stroke.

Eccentrics are mechanically elegant — they are simple to make, reliable, produce any desired stroke length, and can be set at any phase angle relative to the main crank. Every steam engine uses at least one; compound valve engines may use three or four.

Eccentric Geometry

Key dimensions:

• Throw (eccentricity): Distance from shaft center to disc center. This is half the total stroke. A 1-inch throw gives 2-inch stroke.
• Disc diameter: Typically 3 to 5 times the throw. Larger diameter reduces bearing pressure on the eccentric strap.
• Width: Enough for the strap to bear without tipping, typically 1.5 to 2× shaft diameter.

Phase angle: The angular position of the eccentric relative to the main crank determines when the valve opens and closes relative to piston position. This must be set correctly during installation (see Setting Up section).

Calculating stroke for the slide valve: The slide valve needs enough stroke to uncover the steam ports fully while also covering the exhaust port. Measure your valve’s required travel and set the eccentric throw to provide that travel plus 1/4 inch margin.

Typical values for a small engine:

• Steam port width: 3/4 inch
• Required valve travel: 1.5 to 2 inches
• Eccentric throw needed: 3/4 to 1 inch

Making an Eccentric

Material: Cast iron is traditional and works well. Steel is stronger but harder to machine. Bronze or gunmetal can be used for small eccentrics.

Casting a split eccentric: The eccentric disc is made in two halves so it can be clamped to the shaft without removing the shaft from the engine. This is critical for a crankshaft that has been assembled into the engine frame.

1. Make a wooden pattern in two halves, each a semicircle with a semi-circular bore sized for the shaft. The bore center is offset from the disc center by the throw distance.
2. Cast the two halves in cast iron.
3. Machine the flat mating faces of each half accurately so they clamp together without a gap.
4. Drill and tap for four clamping bolts through both halves.
5. Machine the outer (eccentric) circumference round and smooth — this is the bearing surface for the strap.
6. Machine the bore to fit the shaft.

Key the eccentric to the shaft: A keyway cut in both the shaft and the eccentric bore, with a fitted key, prevents the eccentric from rotating on the shaft. This maintains the phase angle setting.

Alternatively, use a setscrew through the disc into a dimple in the shaft for a simpler but less positive lock.

Making the Eccentric Strap

The strap is a circular ring that surrounds the eccentric disc. It must be a close sliding fit — tight enough to not rattle, loose enough to rotate on the disc smoothly.

Construction:

1. Cast or forge a ring to surround the eccentric. Make it in two halves like the eccentric itself.
2. Machine the inner bore to match the eccentric outer diameter plus 0.002 to 0.005 inch clearance.
3. Line the inner bore with babbit metal or cast bronze inserts if bearing pressure is significant.
4. Add an oil hole and small groove around the inner face for lubrication.
5. Add an ear or lug on one side of the strap — this is where the eccentric rod attaches.

Oil supply: The eccentric strap must be lubricated. Drill an oil hole through the top of the strap. In operation, apply oil with an oilcan every few hours, or connect a continuous drip feed oiler.

Making the Eccentric Rod

The rod connects the eccentric strap to the valve (or pump mechanism). It must:

• Be long enough to allow the required travel
• Be stiff enough not to buckle or whip
• Have adjustable end connections to allow phase angle and valve travel fine-tuning

Construction:

• Forge or machine from 3/4-inch to 1-inch square or round iron bar
• Turn or file the ends to form simple eyes or fork ends
• One end connects to the strap ear with a pin; the other end connects to the valve rod with an adjustable clevis and pin
• The adjustable clevis allows the effective rod length to be changed by a few inches for fine-tuning

Setting Up the Eccentric

Setting the phase angle (angular position on the shaft) correctly is critical. Incorrect phase angle causes the valve to open too early, too late, or not fully — all degrading performance.

Procedure for a simple D-slide valve engine:

1. Set the piston at top dead center (TDC) — fully in.
2. Observe where the valve is: it should be centered over the ports (equal steam admission available on each side).
3. Mark the shaft position (scratch line on shaft and eccentric).
4. Rotate the engine to bottom dead center (BDC) — piston fully out.
5. Again observe the valve position.
6. If the valve covers equally at both positions, phase angle is approximately correct.
7. Fine-tune by rotating the eccentric slightly on the shaft and testing again.

Valve events to check:

• Steam admission begins just before TDC
• Steam admission ends partway through the stroke (cutoff — where the valve closes, allowing steam to expand)
• Exhaust opens near end of stroke
• Exhaust closes just after TDC (compression of residual steam, cushioning piston)

For a beginner engine, simply set the eccentric so the valve is centered at the midpoint of piston travel in each direction. This gives late admission but is safe and functional.

Multiple Eccentrics

Two eccentrics per shaft: Most double-acting engines use two eccentrics — one for each end of the valve, or one for each valve in a compound system.

Valve gear with reversing: Stephenson’s link motion uses two eccentrics (one for forward, one for reverse) with a curved slotted link between them. Sliding the link up and down changes the engine from forward to reverse and adjusts cutoff. This is the standard arrangement for locomotive engines and allows variable speed control as well as reversing.

For a stationary engine that only runs in one direction, a single eccentric suffices.

Maintenance

Weekly: Add oil to eccentric strap oil holes. Check strap for overheating — if too hot to hold, lubrication is insufficient or clearance too tight.

Monthly: Check clamping bolt tightness. A loose eccentric shifts its phase angle, disturbing valve timing.

Annually: Remove strap and measure bore for wear. If bore is worn oval by more than 1/32 inch, reline with fresh babbit or replace bronze inserts.

Signs of eccentric problems:

• Engine knocks rhythmically → loose eccentric or strap rattling
• Engine loses power → valve not opening fully, check phase angle
• Engine runs roughly in one direction → worn or incorrectly set eccentric