Simulation of Reversing crank and rods when moving the cut off control from FF to FB. Graph of collar pivot position versus reversing crank cut off angle.
British Caprotti valve gear designed by Associated Locomotive Equipment in the 1950’s for Steam Locomotives improved on the original 1920’s design. See Patent GB631895A” Improvements in Valve Gear for Reversible Steam Engines”, for an explanation of how it works and GB170855A “Valve gear for reversing steam engines” for the Caprotti original.
Two steam locomotives exist today – BR Std Class 8 Duke of Gloucester 71000, and BR Std Class 5 73129 both have the valve gear fitted.
In this video the operation of the reversing crank and telescopic rods is simulated. The reversing crank is geared to the control in the cut off cab. The operation of this red wheel can be seen in the video “On the footplate of 71000 Duke of Gloucester” (www.youtube.com/watch?v=GHuYeYttBb8). The video shows the gear wound from Full Fowards through to Full Backwards. The movement of the scroll nuts C.036 and C.037 along the screw shaft C.684 causes the nuts to twist. These are coupled to the cams shown in other videos here via bars in the holes. The movement causes the cams to spin (or phase) relative to each other, and this controls the steam cut off.
Mid gear is when the slot at the end of the reversing crank is 2.25 degrees off the vertical. At the ends of the screw thread the motion of the nuts is stopped. This causes the inner reversing rod C.585 and outer reversing rod C.592 to telescope on their internal springs. As the nuts move closer to each other the cut off is notched upwards. The smallest quantity of steam is admitted to the cylinders when the nuts have the largest distance of separation. This is the condition when the engine is moving at higher speed.
Completed with Autodesk Inventor 2016 Dynamic simulation.

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