Joy’s valve gear takes its input from the engine connecting rod. A short forked link, a frame-mounted correcting link and a lever working through an adjustable curved guide turn that motion into valve-rod travel. The animation keeps the joints connected and the rods at fixed lengths as the crank turns. Change the guide angle to compare the motion on either side of mid-gear.
Joy’s Valve Gear Interactive Calculator
Set crank radius, guide inclination, selected crank angle and speed. The tiles report the selected crank position and full-cycle travel; the animation continues from that angle. All dimensions scale together with crank radius. Steam-port events and cut-off are not calculated.
Equation Used
C is the crank pin and X the crosshead. P lies on the connecting rod; the short link PA has intermediate pickup Q. D is a fixed anchor. B slides in the curved guide of centre N, and J is the valve-driving end of the lever QB. V lies on the horizontal valve axis. Each position is found from these length and guide constraints. The illustrative proportions follow the Muncaster-derived geometry documented in Simon Fraser’s simulation; crank radius scales the assembly. Valve displacement is measured from the midpoint of its cycle extrema. Piston displacement is measured from its leftmost position. Playback runs at one fifth of entered rpm. These results describe ideal geometry, not locomotive valve settings.
From the connecting rod to the valve
The engine crank moves the crosshead through the main connecting rod. Pickup P lies partway along that rod, so its path combines horizontal reciprocation with the rod’s changing inclination. It is calculated from the crank and crosshead positions, rather than prescribed as an ellipse.
The short link PA connects this pickup to correcting link AD, which swings about fixed anchor D. Point Q on PA drives the long lever QJ. A block B partway along that lever must stay in the curved guide. The lever’s upper end J drives the horizontal valve rod through a separate connecting rod.
Tilting the guide changes the constraint on B and therefore the valve motion. The guide remains at its selected inclination during each crank cycle. Rods occupy separate axial layers where their outlines cross.
Rigid links and a curved guide
The crosshead follows Xx=Cx−√(L²−Cy²), with its axis through the crank centre. Intersections of two circles locate the correcting-link joint A and the guide block B. These intersections enforce fixed rod lengths and the circular guide path simultaneously. Points P, Q and J then follow their fixed ratios along their respective rods.
The valve end lies on a fixed horizontal axis. Its connecting rod gives Vx=Jx−√(R²−(yv−Jy)²). A complete revolution provides the extreme valve positions; their difference is full travel, and their average defines zero valve displacement.
The model uses a main connecting rod 8.875/1.5 times the crank radius, with its pickup 3.5/8.875 of the way from the crosshead. Correcting-link length is 5.125/1.5 radii, and the short link is 2.25/1.5 radii, with Q one third of the way from P. Distances QB and QJ are 2.404/1.5 and 2.854/1.5 radii. Guide curvature and valve connecting-rod length both use 6.875/1.5 radii. The fixed pivots and valve axis retain the same scale.
Compare guide settings and crank positions
At the default 25 mm crank radius, piston stroke is 50 mm. Pause playback and turn the crank-angle control through a revolution to compare piston and valve positions. The valve follows its own linkage-derived motion; it does not simply copy the piston.
Move the guide from +12 degrees through zero to −12 degrees. Full valve travel and the relationship between valve and piston position change. A zero-degree guide can leave residual valve motion, so it should not be read as a guarantee that every steam port stays closed.
Doubling crank radius doubles all lengths and displacement results while preserving the shape of the motion. Changing rpm alters playback speed. At zero rpm the drawing stays at the selected crank angle, while full-cycle travel can still be calculated.
A valve drive without a separate eccentric
Hiscox illustrates Joy’s gear for both vertical and horizontal engines in entries 257 and 258. In each, a pin on the engine connecting rod supplies the motion, and changing the slotted-guide setting controls the valve drive.
This drawing uses a horizontal arrangement. Its proportions follow the Muncaster-derived layout documented by Simon Fraser, with a uniform scale set by the crank-radius input. It is a geometric demonstration rather than a drawing for manufacturing or adjusting that model.
Motion alone does not determine steam events
The correcting link and curved guide combine motions from the connecting rod into the valve drive. Finite rod lengths make the resulting displacement more complex than a scaled sine wave.
Admission, release, compression and cut-off need the valve and port geometry, lap, centring and crank reference. None of those events is inferred from guide angle alone. The diagram omits steam passages, bearing fits, flexibility, inertia and wear. It cannot establish operating loads, fuel savings or service intervals.
Frequently asked questions
Does the gear use a driving eccentric?
No. Its input comes from a point on the engine connecting rod. The correcting link and guided lever transfer that motion to the valve rod.
Do the links stretch in the animation?
No. Circle intersections and the crosshead constraint keep the connected rods at their specified lengths throughout the cycle.
Does guide angle equal cut-off percentage?
No. Guide angle changes the linkage motion. Cut-off requires a valve-admission event related to piston travel, with the valve and ports specified.
What does zero valve displacement mean?
It is the midpoint between the maximum and minimum valve-rod positions for the selected guide setting. It is not a statement that a port is open or closed.
Why does some motion remain at zero guide inclination?
The crank, connecting rod and correcting linkage still move. The finite geometry can transmit residual motion through the guide; steam admission would also depend on valve lap.
Are these dimensions valve-setting instructions?
No. They define a scaled geometric example. Use the drawings and operating specifications for the actual engine when determining its valve settings.
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