Joy’s hydraulic shifting eccentric changes a steam engine’s valve-drive setting by moving the eccentric across the shaft. A central block is keyed to the shaft, and pistons on its opposite sides work in cylinders within the eccentric. Supplying oil to one side or the other moves the surrounding eccentric relative to that block. The animation shows this setting motion in an end view that follows the shaft.
Joy’s Hydraulic Shifting Eccentric Interactive Calculator
Set lateral travel x, total available stroke S, a perpendicular centre offset b and a one-way shift time. The tiles describe the selected travel; playback sweeps through the stroke from that position. Dimensions are illustrative, not settings for a historical engine.
Equation Used
The shaft centre is (0,0), and the eccentric centre moves along (q,b). Throw e is the distance between those centres; φ is measured from the positive lateral axis. This angle is not a valve lead or crank-angle setting. The model assumes straight lateral travel with fixed perpendicular offset. Available stroke and offset are entered assumptions, not dimensions taken from Hiscox. The animation uses a smooth prescribed shift over time t in each direction, without a hydraulic pressure or load calculation. If x exceeds S, geometric result tiles show -- and motion is held at the end stop; S/t remains the reference speed.
The eccentric moves around a shaft-mounted block
The keyed block turns with the shaft. Its two pistons fit the opposing cylinders formed inside the eccentric. Oil passages through the shaft and pistons allow pressure to be applied to either side. The relative movement changes the position of the eccentric centre and therefore the motion delivered by its strap and rod.
In the animation the shaft is the viewing reference, so the block and piston heads remain still while the eccentric slides. The blue and gold passages identify the two oil connections; their colors do not report pressure. Covers are removed to reveal the internal arrangement. The external oil-control equipment, eccentric strap and valve train are omitted.
Calculate centre position and throw
Let x be travel measured from the left end of a total stroke S. The signed lateral setting is q=x−S/2. A fixed perpendicular offset b places the eccentric centre at (q,b) relative to the shaft centre. The centre distance, or eccentric throw, is e=√(q²+b²), and its direction is φ=atan2(b,q).
Moving from one end to the other changes the sign of q. Equal distances on either side of the midpoint give equal throw, but different centre-vector directions. At the midpoint q=0, throw is b; it does not fall to zero. This construction illustrates the geometry of a laterally shifting eccentric without claiming the dimensions or valve settings of a particular engine.
The average speed for a full one-way shift is S/t. Actual oil flow, force and shifting time also depend on piston area, pressure, restrictions, leakage and load. Those effects are outside this calculator.
Compare the two ends and the midpoint
For S=60 mm and b=10 mm, x=0 gives q=−30 mm, throw 31.6 mm and centre-vector angle 161.6 degrees. At x=30 mm, q=0, throw is 10 mm and the angle is 90 degrees. At x=60 mm, q=30 mm, throw is again 31.6 mm and the angle is 18.4 degrees.
With a one-way time of 3 seconds, the average shift speed is 60/3=20 mm/s. The animation completes each smooth end-to-end movement in that time. Its instantaneous speed varies during the movement. The selected-travel tiles remain tied to the input slider; the drawing reports its current animated position.
Hydraulic reversal of a steam-engine valve drive
Hiscox’s entry 270 describes the keyed block, opposed pistons and oil passages as a means of reversing an engine. A historical account of David Joy’s work also illustrates fluid-pressure gear fitted to the locomotive Sussex, with separate oil channels and control equipment.
The common feature is motion across the shaft within the eccentric assembly. The shape of the block and the detailed oil circuit vary between drawings. The end-view proportions here are an explanatory example, rather than a manufacturing drawing of either historical arrangement.
Setting geometry and valve timing are separate
Changing eccentric position changes both the centre distance and its direction in this model. It does not by itself determine steam admission, exhaust, cut-off or which way an engine will run. Those events also require the main-crank reference, eccentric rod, valve, ports and lap.
The calculation uses ideal straight guides and rigid parts. It does not size oil passages, establish operating pressure or specify clearances. The animation shows the setting movement only, with shaft rotation suppressed by the chosen viewing frame.
Frequently asked questions
Why do the pistons stay still while the disc moves?
The view follows the shaft. The piston block is keyed to it, while the cylinders belong to the surrounding eccentric. The drawing shows their relative movement.
Does the eccentric slide along the shaft?
The setting motion shown here is transverse to the shaft axis. Oil acts in the opposed internal cylinders; a helical spline or separate axial sliding disc is not part of this illustration.
Is the centre-vector angle a valve lead angle?
No. It is the direction of the eccentric centre from the chosen lateral axis. Valve lead and events need the complete valve gear and a main-crank reference.
What happens at the midpoint?
The lateral setting q becomes zero. The perpendicular offset b remains, so the throw equals b. Midpoint travel does not establish neutral gear.
Why are some results blank?
Selected travel x is greater than the available stroke S. Adjust either value to put the setting inside the stated end stops.
Does the time input change playback?
Yes. It sets the duration of each prescribed end-to-end shift. S/t is the average speed for that stroke, not a prediction from oil pressure.
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