Brown 182 depicts diagonal catch hand gear controlling the steam and eduction valves of a blowing or pumping engine, with the piston at the top and ready to descend. It is one state of the same mechanism shown in Brown 181–182. The catch operates valve handles; steam pressure moves the piston.
Diagonal Catch Valve Gear · Brown 182 State
Brown 182 shows the valve gear with the piston at the top and ready to descend. Compare cylinder forces and swept volume while watching the paired handles and four-valve changeover.
Equation Used
- Rod exits the top of the cylinder.
- Exhaust pressure is atmospheric.
- Pressure forces exclude load, gravity, friction and acceleration.
- Full-stroke admission for swept-volume estimate.
- One complete cycle contains an upstroke and downstroke.
- Valve changeover and catch clearances are illustrative.
The catch controls valves; it is not a ratchet lifting mechanism or a locomotive reverser.
Two valve pairs and a piston-operated catch
In Brown 181, lower steam and upper eduction valves are open. Steam enters below the piston while the upper cylinder exhausts, so the piston rises. Upper steam and lower eduction remain closed.
Near the top, the rising piston-rod tappet raises the lower handle. The catch holds that handle and closes the former open pair. It releases the upper handle, whose backweight opens upper steam and lower eduction. This is the state illustrated by Brown 182: the cylinder can now drive the piston down.
On the return stroke, the tappet operates the upper handle and restores the first state. The animation includes the paired handles, central diagonal catch, rod tappet shoulders and valve links. Its lower inset makes the four separate valve states visible. This page starts from the Brown 182 state.
Handle proportions, catch profiles and changeover timing are selected for explanation. The two valve groups use a close-before-open sequence in the illustration. Piston motion is prescribed rather than solved from steam pressure, inertia and load.
A valve-control mechanism, not a lifting jack
The primary reference identifies large blowing and pumping engines. The hand gear controls which end of the cylinder receives steam and which exhausts. It does not lift a toothed piston rod, and it is not a locomotive reversing quadrant.
The calculator therefore compares useful cylinder quantities: ideal force in each direction, full-admission swept volume, cycle period and mean piston speed. Sizing the catch itself requires its dimensions, material and valve loads.
Cylinder area, force and volume
With bore D and rod diameter d in millimetres, full piston area is A=πD²/4 and rod area is Aᵣ=πd²/4. The upper chamber has annular area A−Aᵣ because the rod exits at the top.
For admission gauge pressure p in MPa and atmospheric exhaust, upward force is pA newtons and downward force is p(A−Aᵣ) newtons. Divide by 1000 for kilonewtons. These are ideal pressure forces before gravity, external load, friction or acceleration.
At stroke S mm, full-admission swept volume per complete up-and-down cycle is V=(2A−Aᵣ)S/10⁶ litres. At n complete cycles/min, volume rate is Vn L/min, period is 60/n seconds and mean piston speed is 2Sn/60000 m/s.
This volume is measured at cylinder conditions. It is not boiler steam consumption, mass flow or free-air delivery. Clearance volume, compressibility, pressure variation and early cutoff are excluded.
A 250 mm bore and 600 mm stroke
The example uses a rod diameter 16% of bore: 40 mm. At 0.3 MPa gauge and atmospheric exhaust, ideal upward force is about 14.726 kN and downward force about 14.349 kN.
The full-admission volume is about 58.151 L per complete cycle. At 15 cycles/min, the corresponding volume rate is about 872.26 L/min, cycle period 4 s and mean piston speed 0.3 m/s. These figures describe the selected cylinder, not a historical engine rating.
Increasing rod diameter reduces downward force and upper-chamber volume without changing upward force. Increasing stroke increases swept volume and mean speed at a fixed cycle rate.
Keep valve sequence separate from force prediction
The source establishes the catch-and-handle sequence. It does not provide dimensions sufficient to calculate tappet impacts, catch stresses, backweight sizing or valve forces. The illustrated clearances are not a manufacturing design.
The ideal force outputs apply while the corresponding admission and exhaust paths are open. They are not instantaneous force predictions during trapped-steam changeover. Zero entered pressure gives zero ideal drive force even though the slow demonstration continues.
Animation playback is slowed for inspection. Use the reported cycle period and mean speed for the entered rate; do not time the screen animation as if it were an operating engine.
Diagonal catch hand-gear questions
What does eduction mean?
It means exhaust from the cylinder.
Does the catch pull the piston upward?
No. It switches the valve handles. Steam admitted below the piston supplies the upward pressure force.
Are Brown 181 and 182 different mechanisms?
They are two operating states of the same valve gear.
Why are upward and downward forces different?
The piston rod removes working area from the upper chamber.
Is the volume rate steam consumption?
No. It is ideal full-admission swept volume at cylinder conditions, without clearance or thermodynamic effects.
Does this calculate a safe catch angle?
No. Such a rating cannot be inferred from the historical schematic.
Primary mechanism reference
Henry T. Brown, Five Hundred and Seven Mechanical Movements, nos. 181 and 182: diagonal catch hand gear for blowing and pumping engines; lower steam/upper eduction during ascent, changeover at the top, and restoration on descent. The displayed cylinder dimensions and hand-gear proportions are explanatory selections.
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