Allan straight-link valve gear uses two eccentric drives, a straight slotted link and a sliding die block to transmit motion to a steam valve. During reversing adjustment, the link and the valve-rod suspension move in opposite directions. Explore that arrangement in the animation and compare the two entered adjustment contributions. The numerical balance index does not calculate steam cutoff or prove constant valve lead.
Allan Straight-link Valve Gear Interactive Calculator
Compare the two reversing-arm contributions and explore a constrained straight-link mechanism. The percentage index measures input balance, not actual valve lead.
Equation Used
The two controls compare adjustment contributions. Link share and Die share normalize the entered values by their sum. Input difference subtracts Die drop from Link lift, and Balance index decreases with the magnitude of that difference. These numbers are not physical die displacement or valve lead.
- The controls are percentage inputs for comparing the two illustrative adjuster arms.
- The die moves opposite to the straight link, as in Allan gear.
- Equal input values give zero difference; this does not establish constant valve lead.
- Positive input difference means the Link lift value is larger; negative means Die drop is larger.
The balance index compares the two entered percentages. It is not a physical valve-lead, steam-cutoff or port-timing calculation.
Running motion and reversing adjustment
The two eccentric rods connect the rotating axle to the ends of a straight slotted link. A die block engages the slot and connects through a radius rod to the valve drive. Changing its position relative to the link changes how the eccentric motions contribute to valve movement.
Allan’s arrangement shares the reversing adjustment between the link suspension and the valve-rod suspension. They move in opposite directions, allowing a straight link. This adjustment is distinct from the cyclic movement that occurs as the axle turns: the die block is not fixed in space throughout the engine cycle.
Parts to follow
- Two axle eccentrics and rods: supply the two cyclic motions.
- Straight link and die block: combine those motions at the selected relative position.
- Reversing shaft and suspension arms: adjust link and valve-rod positions oppositely.
- Radius rod and valve spindle: carry motion toward the valve.
The animation first demonstrates adjustment, then turns the axle at that setting. Its fixed illustrative rod lengths are not a drawing for a particular locomotive. If the chosen proportions exceed the linkage’s illustrated range, the display reports that condition instead of presenting a valid running geometry.
Understanding a steam-engine valve drive
The straight-link arrangement is useful to study alongside Stephenson and Gooch link motions. Each changes the valve drive by changing how two eccentric motions reach the valve rod. The straight slot is a visible distinguishing feature of Allan’s arrangement, but a straight slot alone does not define the full geometry.
A restoration or model build needs the correct engine drawings, eccentric settings, rod lengths, suspension geometry and valve dimensions. Those determine port opening and valve events. This demonstration does not assign a gear to a named preserved locomotive or provide replacement clearances, material hardness or a machining specification.
What the percentage outputs mean
Let L be the nonnegative Link lift input and D the nonnegative Die drop input. If L + D is greater than zero, Link share = 100L/(L + D) and Die share = 100D/(L + D). If both are zero, both share outputs are zero.
Input difference = L − D, in percentage points. Balance index = max(0, 100 − |L − D|). It is an illustrative comparison of the entered numbers, not an efficiency or a measured property of the gear.
Equal values give a 100% index, even when both values are zero. Pairs with the same proportional split can have different differences and indices: 60/40 and 30/20 both give 60% Link share, but their differences are 20 and 10 points. This is why the index must not be interpreted as valve lead or design quality.
Valve lead describes admission-port opening at a piston dead centre; cutoff concerns when admission ends during a stroke. Determining them needs actual valve displacement relative to crank position, valve lap and port geometry. Neither is derived from these two input percentages, and zero input difference does not prove constant lead.
Worked example: comparing two contributions
Enter Link lift 60% and Die drop 40%. Their sum is 100, so Link share is 60% and Die share is 40%. Input difference is 20 percentage points, and Balance index is 80%.
With Link lift 40% and Die drop 60%, the shares reverse and Input difference becomes −20 points. Balance index stays 80% because it uses the absolute difference.
With both inputs at 50%, the shares are 50/50, the difference is zero and the index is 100%. These are numerical comparisons of adjustment inputs; they are not millimetres of travel or a steam-cutoff setting for a locomotive.
Comparing link motions
| Arrangement | Characteristic reversing adjustment | What still needs calculation |
|---|---|---|
| Stephenson | Moves a curved expansion link relative to the valve connection. | Events depend on eccentric and suspension geometry. |
| Gooch | Shifts the radius-rod connection relative to a link with fixed mean suspension position. | The curved link and radius-rod geometry must be compatible. |
| Allan | Shares opposite adjustments between a straight link and valve-rod suspension. | Full rod geometry and valve dimensions determine actual events. |
Lead variation, reversing effort, wear and operating speed cannot be assigned universal numerical limits from these names. A complete geometry calculation and the actual engine specification are needed.
Allan gear questions
No. It only means the two entered values are equal. The index has no valve lap, port dimensions or physical lead calculation.
No. It is the numerical difference between two percentage inputs, expressed in percentage points. Physical die motion depends on the complete linkage.
The formula subtracts the absolute input difference from 100. Zero minus zero is zero, so the index is 100 even though both normalized shares are reported as zero.
No. The reversing adjustment and cyclic running motion are different. The die remains in the link slot while the mechanism transmits motion toward the valve.
No. Actual motion and port opening depend on geometry, lap and lead. This percentage calculator does not establish cancellation or a safe engine operating condition.
No. Cutoff is a steam-admission event relative to piston travel. The input shares are not a steam-cutoff calculation.
No. Use the correct engine drawings and operating instructions. The two-input comparison does not diagnose wear, set valve timing or prescribe regulator and cylinder-cock operation.
References and further reading
Hermann Haeder, A Handbook on the Steam Engine, valve-gears discussion, page 230 describes the straight link and opposite link/block adjustment. The historical treatment discusses steam distribution from the mechanism’s geometry; the percentage index on this page is not that analysis.
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