Single Eccentric Reversing Gear

Inside the Single Eccentric Reversing Gear

The mechanism rests on one fact about slide valves — to reverse a steam engine, you must shift the valve's phase 180° relative to the crank. With one eccentric fixed on the shaft, you cannot rotate it on demand. So early engineers cheated. They let the eccentric slip on the shaft between two stops, or they disengaged the eccentric rod entirely from the valve spindle and drove the valve directly by hand or by a separate hook called a gab. That second mode is where the names slip eccentric gear and gab gear come from.

In fore gear, the driver drops the gab hook onto the forward driving pin and the eccentric pushes the valve through its normal cycle. To reverse, the driver lifts the gab clear, hand-bars the engine past dead centre into the opposite phase, and drops a second gab onto a back pin. The eccentric is now effectively driving the valve 180° out — not because the eccentric moved, but because the engine moved under it. If the slip stops are worn or the gab hook is bent more than about 1.5 mm out of alignment, the valve picks up late, the engine wheezes and stalls, and you cannot get a clean start in either direction.

The weakness is obvious once you operate one. There is no variable cutoff — steam admits for nearly the full stroke, fuel economy is terrible, and you cannot reverse under load. You must stop the engine, shift the gabs, and restart. That is why every serious locomotive after about 1842 moved to Stephenson's two-eccentric link motion, which gave both directions and variable cutoff from a single lever.

Key Components

• Single Eccentric: A circular disc bored off-centre and keyed (or slip-fitted) to the crankshaft. The throw — typically 50 to 75 mm on a small locomotive — equals half the slide valve's full travel. On a slip eccentric variant, the disc rotates freely on the shaft between two driving dogs set 180° apart.
• Eccentric Strap and Rod: A two-piece bronze strap clamps the eccentric and transfers its motion to a long rod. Strap clearance is held to about 0.10 mm — any more and the valve event smears at higher RPM. The rod ends in a forked or hooked fitting depending on the gear style.
• Gab Hook (or Slip-Link): On gab gear, a curved hook on the eccentric rod drops onto a pin on the valve spindle. Two pins exist — one for fore, one for back. The driver raises the hook with a hand lever to disengage. Hook-to-pin clearance must stay under 0.5 mm or valve timing wanders.
• Slide Valve and Spindle: A flat D-shaped valve sliding across the steam ports on the cylinder face. Lap is typically 6-10 mm and lead 1-2 mm on early designs. The spindle takes the motion from the gab and shoves the valve through 80-150 mm of total travel.
• Reversing Lever: A hand bar in the cab, pinned to a quadrant. Pulling it lifts both gab hooks together. There is no notching mid-position — it is fore, mid (off), or back. Mid-gear cuts steam entirely.

Real-World Applications of the Single Eccentric Reversing Gear

The gear belongs almost entirely to the 1820s-1840s pioneer era of steam, but you still see it on preserved engines, museum builds, model engineering projects, and a handful of stationary applications where simplicity beats efficiency. If you are restoring or building a pre-1845 locomotive replica, you will encounter this gear and you need to understand what it can and cannot do before you light a fire in the firebox.

• Heritage Railways: The replica of Stephenson's Locomotion No. 1 at Beamish Museum in County Durham runs original-style loose eccentric gear and demonstrates the hand-reversing procedure to visitors.
• Museum Locomotives: The 1829 Rocket replica at the National Railway Museum in York uses a near-period gab gear arrangement on its cylinders for accurate operation.
• Model Engineering: Live-steam 5-inch gauge builds of pre-Stephenson prototypes — typically Reeves 2000 castings or LBSC designs — use scaled slip eccentric gear for historical correctness.
• Stationary Steam Engines: Small workshop mill engines from the 1830s, like those on display at Kew Bridge Steam Museum, used gab gear because the engine reversed only at shutdown.
• Marine Steam (Early): Paddle steamers built before about 1840, including early Clyde-built vessels, used slip eccentric gear with hand-operated gab hooks to dock and undock.
• Education and Training: Engineering school demonstration rigs — Imperial College London and several US technical museums hold cutaway models showing how the slip eccentric reverses without two eccentrics.

The Formula Behind the Single Eccentric Reversing Gear

The valve's total travel is what the eccentric throw produces, and you size everything else from that. At the low end of useful throw — say 40 mm on a tiny model engine — port opening is shallow and the engine breathes poorly above a few hundred RPM. At nominal throw for a small full-size locomotive of this era, around 60-65 mm, you get clean port opening with reasonable fuel use at low road speeds. Push the throw past 80 mm and you over-travel the valve, hammer the cylinder ends, and burn steam. The sweet spot for a Locomotion-class build sits right at 60-65 mm throw with 8 mm lap and 1.5 mm lead.

Variables

Worked Example: Single Eccentric Reversing Gear in a Beamish-style Locomotion replica build

Your heritage steam workshop in Darlington is fabricating a working replica of an 1827 Stockton & Darlington pattern 0-4-0 with a single eccentric gab gear, and you need to set the valve travel and port opening for a cylinder with a 50 mm wide steam port. The eccentric throw is 60 mm nominal, valve lap is 8 mm, and you want to check what happens if a junior fitter sets the throw at 40 mm by mistake or oversizes it to 80 mm.

Given

• e_nom = 60 mm
• λ = 8 mm
• ℓ = 1.5 mm
• Port width = 50 mm

Solution

Step 1 — at nominal 60 mm eccentric throw, compute total valve travel:

Step 2 — compute maximum port opening at nominal throw:

This slightly over-travels the 50 mm port — the valve uncovers the full port and then slides 3.5 mm past the edge. That is exactly what you want, because it guarantees full port opening even with 1-2 mm of wear or thermal growth in the rod.

Step 3 — at the low-end mistake of 40 mm throw:

You only uncover 67% of the port. The engine will run, but it sounds breathy and short of steam at any road speed above walking pace — you can hear the throttling. A driver of one of these replicas will tell you the chimney sound goes from a clean four-beat bark to a wheezy hiss.

Step 4 — at the high-end mistake of 80 mm throw:

Total travel jumps to 176 mm. The valve hammers the back of its travel hard against the cover, you get a metallic knock at every stroke, and the gab hook pin loads spike past 200% of design. You will crack the gab pin within a week of running.

Result

Nominal valve travel sits at 136 mm with 53. 5 mm peak port opening — clean four-beat exhaust and full port uncovery on every stroke. At 40 mm throw you only get 33.5 mm port opening (67% of port width) and the engine wheezes; at 80 mm throw the valve hammers and the gab pin fails fast. The sweet spot is tight — you really do want 58-65 mm throw and not a millimetre outside that. If your measured port opening comes in below the predicted 53.5 mm on a fresh build, check three things in order: (1) the eccentric strap clearance has crept past 0.15 mm and is eating travel as backlash, (2) the gab hook is sitting proud of the pin by 0.5+ mm because the lift lever quadrant is mis-set, or (3) the eccentric key has spun in its keyway and the throw is actually 5-8 mm less than you think — pull the strap and measure shaft-to-bore offset directly with a vernier.

Choosing the Single Eccentric Reversing Gear: Pros and Cons

Single eccentric gear is the simplest reversing arrangement that works, and that is its only real virtue. Compare it to what came after on the same locomotives and the picture is brutal — but for a museum build, simplicity and historical accuracy outrank efficiency every time.

Frequently Asked Questions About Single Eccentric Reversing Gear