This is the hand-operated eccentric-rod disengaging device shown in Brown’s movement 189. A short crank on a hand lever pulls a connecting link, raising the downward-opening notch, or gab, off the valve-lever pin. The calculator follows a closed linkage and compares its lift, clearance and ideal hand effort.
Eccentric-rod Hand Gear · Geometry Calculator
Inspect the hand lever lifting the open gab off its pin. Compare linkage scale, handle length, hand rotation and assumed lifting resistance.
Equation Used
- Eccentric connection and valve lever remain stationary during hand release.
- Selected linkage dimensions scale with pin diameter; they are not historical measurements.
- The continuous lower circle-intersection branch closes the linkage.
- Pin is circular and the notch opens downward relative to the rod.
- Assumed lifting resistance is independent of steam or valve load.
- Hand force acts perpendicular to the hand lever.
Manual hand gear illustrated by Brown, movement 189. A negative full-stroke gap means the pin is still overlapped, not a completed disengagement.
A lifting link releases the open gab
The eccentric rod normally embraces a pin on the valve lever with its downward-opening notch. Raising the rod separates those parts. In Brown’s movement 189, an upper hand lever is connected to the rod extension by a separate link.
The model holds the eccentric connection and valve lever at rest while the hand gear moves. The hand lever rotates about its own fixed shaft, its short crank lifts the connecting link, and the rod pivots upward about its stationary input connection. Every pivoted member retains its length.
The enlarged detail shows the notch moving away from the circular valve pin. A positive pin-to-mouth gap means the whole pin lies below the opening in this selected geometry. Autoplay raises and lowers the hand gear; it does not depict an engine being uncoupled under operating load.
What you can adjust
Pin diameter sets the scale of the illustrative linkage. Hand-lever length changes the operator’s leverage and attachment-point travel. Full hand rotation sets how far the rod is lifted. The downward lifting resistance is an independent assumed force at the gab.
This force is not automatically equal to steam pressure, valve force or eccentric-rod load. The model is useful for comparing ideal hand-gear geometry, rather than asserting a release force for an unspecified working engine.
Closed-linkage geometry and virtual work
Take the valve pin as (0,0), with positive y downward. For pin diameter d, the stationary eccentric connection is A=(−15d,0), and the hand shaft is D=(2.5d,−5d). The short hand crank has radius 2.5d. Its tip is F=D+(2.5d cosθ,−2.5d sinθ).
The rod extension E lies at the intersection of a circle of radius 20d about A and a circle of radius 5d about F. The continuous lower intersection is selected. If β is the rod angle, the gab center is B=A+15d(cosβ,sinβ). Gab lift is h=−15d sinβ.
The notch radius is 0.6d. Its mouth is aligned with the rod. In the rod’s frame, the fixed pin center has coordinates (15d(cosβ−1),−15d sinβ). The full-circle clearance below that mouth is h−d/2. A negative result means the pin is still partly within the gab.
For a hand length H and an assumed downward force W at the gab, virtual work gives ideal hand force Fhand=W(dh/dθ)/H when hand force is perpendicular to the lever. Handle arc is Hθ, with θ in radians. The peak is sampled across the selected stroke; no impact or friction force is included.
Compare a longer hand lever
At d=20 mm, H/d=10 and θ=35°, the handle is 200 mm long and travels through a 122.17 mm arc. The gab rises about 21.75 mm. Its full-stroke pin-to-mouth gap is about 11.75 mm.
Doubling the hand-lever length while keeping the linkage and angle unchanged doubles handle travel and halves the ideal hand force. Increasing the pin diameter scales all selected linkage dimensions and travel equally, while leaving the force ratio unchanged.
A short hand rotation can leave a negative clearance. The result and detail view show the gab still overlapping the pin.
Illustrative linkage dimensions
Brown’s drawing provides the arrangement, not engineering dimensions. The example uses fixed proportions scaled to pin diameter; these are not measurements of a historical engine.
The hand crank and the eccentric rod occupy separate axial planes, joined by the lifting link. The engine drive is stationary during this sequence. A real disengaging installation also needs axial clearance, support, pin and notch design, operating procedures and load information that this ideal geometry does not provide.
Hand-gear questions
Is this a spring-loaded automatic trip?
No. Brown’s movement 189 shows a hand lever with a connecting link that lifts the gab from its pin.
What does a negative pin-to-mouth gap mean?
The selected hand rotation has not lifted the mouth above the whole pin. The drawing still shows the actual position.
Why does the larger hand lever need less force?
It provides more hand travel for the same gab lift, reducing ideal force by virtual work.
Does the assumed resistance represent steam load?
No. It is a separately entered lifting resistance. The calculator does not derive it from an engine’s operating pressure.
Primary reference
Henry T. Brown, 507 Mechanical Movements, movement 189, printed page 50, described as another modification of the eccentric-rod disengaging device in movement 186. The drawing shows the upper hand lever, connecting link and open gab.
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