Allan straight-link valve gear is a symmetric reversing valve gear used on 19th-century steam locomotives, where two eccentric rods drive a straight slotted link whose die block transmits motion to the valve spindle. The straight link itself is the defining component — it slides vertically as the eccentric rods raise or lower it, while the die block stays fixed in space and picks off the blended motion. The design fixes the constant-lead problem of Stephenson gear by splitting movement between link and die. Hundreds of British and colonial railways, including the LNWR Crewe-built fleet, ran Allan gear into the 1900s.
Inside the Allan Straight-link Valve Gear
The Allan gear sits between the Stephenson link motion and the Gooch fixed-link as a compromise. Two eccentrics on the driving axle — one set for forward gear, one for reverse — drive eccentric rods that pin to the top and bottom of a straight slotted link. The die block inside that link couples to the valve spindle through a radius rod. Slide the link up, the lower eccentric dominates and the engine runs forward. Slide it down, the top eccentric takes over and the engine reverses. Mid-gear sits halfway with both eccentrics cancelling, giving short cutoff for economical running.
The trick is that BOTH the link AND the die block move when you notch up. In Stephenson gear only the link moves and lead steam grows with cutoff — bad for high-speed running. In Gooch gear only the die moves and lead stays constant but cutoff symmetry suffers. Allan splits the movement roughly 50/50 by linking the reversing reach rod to a bell crank that simultaneously raises the link and lowers the die. You get nearly constant lead across the cutoff range, which is what makes the gear acceptable for fast passenger work.
If the bell crank ratios drift off — say the lift arm gets bushed loose by 1.5 mm at the pin — the lead-constancy property collapses and you'll see uneven exhaust beats, hot valve chests at short cutoff, and the locomotive losing power above 40 mph. The slot in the straight link must run dead parallel to the die-block faces within 0.05 mm over its length, or the die binds at one end of travel and slips at the other. That's the failure mode you find on tired heritage engines — slot wear opens to a taper and the slip of the link goes erratic.
Key Components
- Straight slotted link: The defining part. A flat steel bar with a precision-machined parallel slot, typically 200-350 mm long on full-size locomotives. The slot width must match the die block within 0.05-0.10 mm clearance — tighter binds, looser knocks audibly at every reversal.
- Die block: A hardened steel block sliding inside the link slot, pinned to the radius rod that drives the valve spindle. It picks the blended motion from whichever eccentric currently dominates. Typical face hardness is 55-60 HRC; below that the slot polishes a groove within 50,000 miles.
- Forward and backward eccentric rods: Two long rods from the eccentrics on the driving axle to the top and bottom of the link. Length must be matched within 0.5 mm or the valve events differ between forward and reverse running — readers who notice asymmetric exhaust on a restoration usually find a bent eccentric rod.
- Lifting bell crank and reach rod: The reversing lever in the cab pulls the reach rod, which rotates a bell crank that simultaneously raises the link AND lowers the die block (or vice versa). The 1:1 split between link lift and die drop is what gives Allan gear its near-constant lead.
- Radius rod: Connects die block to valve spindle through the combination lever. Length sets nominal valve lead — typically 1/16 inch (1.6 mm) on a slide-valve locomotive. Get this length wrong by 1 mm and you'll never find a valve setting that doesn't waste steam.
- Eccentric sheaves: Two cast or forged sheaves keyed to the driving axle, set 90° plus or minus the lap and lead angle apart from each crank throw. Throw is typically 5-6 inches (125-150 mm) on a standard-gauge passenger engine.
Where the Allan Straight-link Valve Gear Is Used
Allan gear had a specific niche — British and colonial railways from the 1850s to roughly 1900, particularly where simplified manufacture mattered. The straight link is far cheaper to machine than Stephenson's curved link, which is exactly why Alexander Allan promoted it at Crewe Works. You see it on preserved engines today, on miniature locomotive models, and occasionally on industrial steam plant.
- Heritage railway preservation: Highland Railway Jones Goods 4-6-0 No. 103, preserved at the Riverside Museum Glasgow, runs Allan straight-link gear and operated under steam during the 1959-1965 BR heritage tours.
- Colonial broad-gauge locomotives: Indian State Railway BESA-class 4-6-0s built by North British Locomotive Company used Allan gear well into the 1920s on metre-gauge mainline service.
- Live steam model engineering: 5-inch and 7¼-inch gauge LBSC designs such as the 'Maisie' 4-6-2 specify Allan straight-link gear because the straight slot is achievable on a small lathe without a slotting attachment.
- Industrial mill engines: Some Robey and Marshall stationary horizontal engines used Allan-type straight-link reversing gear for rolling-mill duty where direction had to flip every 30-60 seconds.
- Stationary winding engines: Several Welsh slate quarry winders, including those at the Dinorwic complex, used straight-link reversing gear for the fast forward-reverse cycles of a hauling drum.
- Educational demonstration: The Manchester Museum of Science and Industry uses a sectioned Allan gear demonstrator to teach the geometric difference between Stephenson, Gooch, and Allan link motions.
The Formula Behind the Allan Straight-link Valve Gear
The practical question for a restorer or live-steam builder is this: how much valve travel does the gear deliver at a given reversing-lever position? Valve travel sets cutoff, cutoff sets fuel and water consumption, and it changes dramatically from full gear (long cutoff, maximum travel) to short cutoff near mid-gear. At full gear you'll see the maximum travel — the engine is making power but burning coal hard. Near mid-gear travel collapses toward zero and the engine coasts. The sweet spot for cruising is roughly 25-35% cutoff, where the die block sits about a third of the way up the link from centre.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tv | Valve travel from mid-position to full open | mm | in |
| re | Eccentric throw (half of total eccentric stroke) | mm | in |
| x | Die block displacement from link centre | mm | in |
| L | Half-length of the straight link slot | mm | in |
| θlap | Angular advance of the eccentric beyond 90° from the crank, accounting for steam lap and lead | rad | rad |
Allan Straight-link Valve Gear Interactive Calculator
Vary the 50/50 bell-crank split between link lift and die drop and see the resulting die drift and lead constancy.
Equation Used
The Allan gear trick is that the straight link and die block move in opposite directions. If the link lift and die drop are equal, the die block stays nearly fixed in space and valve lead remains nearly constant. Any mismatch appears as net die drift.
- Link and die motions are normalized as percent of the gear-change movement.
- The die moves opposite to the straight link, as in Allan gear.
- Zero net die drift represents the constant-lead condition.
- Positive drift means link lift dominates; negative drift means die drop dominates.
Worked Example: Allan Straight-link Valve Gear in a 5-inch gauge LBSC 'Maisie' Pacific build
You are setting valve travel on a 5-inch gauge LBSC 'Maisie' 4-6-2 with Allan straight-link gear. Eccentric throw is 9.5 mm, the slot half-length is 32 mm, and your steam lap plus lead gives a θ<sub>lap</sub> of 25°. You want to know what the valve travel will be at three reversing-lever positions: short cutoff cruising, nominal, and full gear.
Given
- re = 9.5 mm
- L = 32 mm
- θlap = 25 °
- xnom = 20 mm (nominal cruise position)
Solution
Step 1 — compute the cosine term once. It is constant for the gear, regardless of reversing-lever position:
Step 2 — at the nominal cruise position, die block 20 mm from link centre (about 62% of full travel):
That is half-travel of roughly 10.8 mm, or full travel 21.5 mm — comfortable for a slide valve with 25-30% cutoff. The engine cruises hard but economically here.
Step 3 — at short cutoff, die block 8 mm from centre (notched up tight):
Half-travel drops to 4.3 mm. Cutoff shortens to roughly 12-15%, the exhaust beats soften, and the engine coasts on expansion. On a 5-inch gauge live-steam locomotive this is what you'd use down a long descent — pretty, but you'll lose speed quickly if the grade flattens.
Step 4 — at full gear, die block at the slot end, x = 32 mm:
Full travel reaches 34.4 mm. This is starting position only — long cutoff (~75%), maximum power, maximum steam consumption. Hold a 'Maisie' in full gear above walking pace and you'll empty the boiler in three minutes flat.
Result
Nominal valve travel comes out at 21. 5 mm full travel (10.76 mm half-travel) at the cruise position. That is the value you set with feeler gauges at the steam chest. The range tells the real story — 8.6 mm full travel at short cutoff for coasting, 21.5 mm at cruise, and 34.4 mm at full gear for starting. The cruise figure is the design sweet spot. If your measured travel comes out 15% low, check three things in this order: (1) eccentric rod length mismatch — even 0.5 mm difference between forward and back rods skews the result asymmetrically; (2) die block clearance opened beyond 0.10 mm, which lets the die slip in the slot and lose effective travel; (3) reach rod pin wear at the bell crank — a worn pin reduces the geometric ratio between reversing-lever movement and die displacement, so the lever travels but the die does not.
Choosing the Allan Straight-link Valve Gear: Pros and Cons
Allan gear competes directly with Stephenson and Gooch link motions. All three were patented within a 25-year window and all three solved the same problem differently. The choice between them depends on manufacturing capability, speed regime, and cutoff range required.
| Property | Allan straight-link | Stephenson link motion | Gooch fixed-link |
|---|---|---|---|
| Lead variation across cutoff range | Near-constant (±0.3 mm) | Increases with notch-up (up to +2 mm) | Constant (±0.1 mm) |
| Manufacturing cost (link machining) | Low — straight slot, planer or shaper | High — curved slot, slotter required | Medium — curved slot, but fixed |
| Suitability for high-speed passenger work | Good up to ~70 mph | Marginal — lead grows | Good — but heavy reversing effort |
| Reversing-lever effort | Medium — both link and die move | Light — link only moves | Heavy — die moves through full arc |
| Typical service life of link slot | 80,000-120,000 miles before reslotting | 100,000-150,000 miles | 60,000-100,000 miles (die wear) |
| Symmetry forward vs reverse | Excellent if rods matched ±0.5 mm | Excellent | Good but cutoff differs slightly |
| Adoption period | 1855-1905, mostly UK and colonies | 1842 onward, worldwide standard | 1843 onward, GWR and successors |
Frequently Asked Questions About Allan Straight-link Valve Gear
This is almost always reach-rod whip. The Allan geometry only delivers constant lead if the bell-crank ratios stay rigid through the full reversing-lever travel. Long, slender reach rods on tender locomotives flex under inertia loads at speed, momentarily shifting the link relative to the die.
Check rod deflection by clamping a dial gauge to the frame and pulling the reach rod laterally with hand force at running position — anything over 0.4 mm deflection means you need a stiffer rod or an intermediate guide.
Stick with Allan if the prototype had it. Walschaerts requires a return crank, expansion link, and combination lever that are harder to fit in 5-inch gauge frames than the simple straight slot Allan needs. The whole reason LBSC specified Allan on Maisie and similar designs is that you can cut the slot on a 4-inch lathe with a vertical slide.
Substituting Walschaerts also changes the prototype's character — the exhaust beats and notching behaviour differ noticeably to anyone who knows the engine.
True mid-gear should give near-zero valve travel and a coasting exhaust. If you're getting beats, the most likely cause is unequal eccentric rod lengths — the two eccentrics aren't cancelling because one is geometrically longer than the other, so the die block isn't sitting at the true cancellation point.
Measure both rods pin-to-pin with the engine on top dead centre. They should match within 0.5 mm. The second possibility is eccentric sheaves that have slipped on the axle — a 2° rotation will produce audible mid-gear beats.
Set the engine on dead centre, grasp the radius rod, and try to move it longitudinally. Any perceptible knock — anything you can feel as a discrete clack rather than a smooth resistance — means slot-to-die clearance has opened past 0.15 mm.
At that point cutoff symmetry between forward and reverse falls apart and you'll notice the engine pulling harder one way than the other. Re-slot to next standard die size rather than building up the slot with weld; weld repairs distort the parallel.
Yes, but you lose part of the gear's advantage. Piston valves want longer travel and shorter lap than slide valves to breathe properly at speed, and Allan gear's near-constant lead matters less when the valve has generous port openings anyway.
Most piston-valve locomotives moved to Walschaerts or Stephenson for that reason. If you must combine Allan with piston valves — some industrial locomotives did — bias the design toward longer eccentric throw (10-12 mm on a 5-inch gauge equivalent) to get adequate travel at the longer cutoffs piston valves prefer.
Full-gear travel is at maximum, and if your steam lap is small relative to the lap-and-lead angle θlap, the valve admits steam very early in the stroke. With a cold cylinder full of condensate, that first admission hits an essentially incompressible water slug and you feel it through the frames.
Two fixes: open the cylinder cocks for the first three or four revolutions to drain condensate, and never start at absolute full gear — notch back one position before opening the regulator. The Allan geometry gives you that flexibility because lead barely changes when you notch up one step.
References & Further Reading
- Wikipedia contributors. Allan valve gear. Wikipedia
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