The Ross Yoke is an ingenious mechanism for transferring dual piston motion into rotational motion. It has the advantage of minimizing lateral forces acting on the pistons making for a more efficient, compact design.
The figure below shows a schematic of the Ross Yoke.
The pistons of the Stirling Engine are attached to A and C and reciprocate up and down as the flywheel rotates around an angle θ. Point A is connected to the expansion space piston (hot), and Point C is connected to the compression space piston (cold).
Points B and D are pin joints which allow for rotation and translational motion. Points E and F are pin joints which are fixed in place – they only allow for rotation.
By inspection we can write,
Now, it’s a bit of a pain to solve this equation every time you want to see the effect of changing dimensions. So to make my life easier I created an Excel sheet which allows me to input the dimensions of the various members of the mechanism and quickly calculate the resulting motion. Based on these results I created a simple procedure to determine the dimensions of a Ross Yoke according to the requirements of your design.
Design Procedure
1. Choose your desired z dimension (note that 2z is the distance between the pistons). Call this dimension Z.
2. Choose your desired stroke length (equal for both pistons). Call this length SL.
3. (Z/14) and (SL/4) become scale factors which will be used to calculate the remaining dimensions:
x = (Z/14)x25
y = (Z/14)x15
r = (SL/4)x1.45
d1 = (Z/14)x25
d2 = (Z/14)x15
Using these, along with your desired z dimension, you have the necessary dimensions of the Ross Yoke to give you the desired stroke length SL.
For example, let’s say we want a z dimension of 5″. So Z = 5″. And let’s say we also want SL = 1.7″. Then,
x = (5/14)x25 = 8.9″
y = (5/14)x15 = 5.4″
r = (1.7/4)x1.45 = 0.62″
d1 = (5/14)x25 = 8.9″
d2 = (5/14)x15 = 5.4″
Here’s a screen capture from the Excel sheet I used which shows a graph of the motion of A and C.
The phase difference is roughly 90 degrees for a large range of dimensions, as scaled in accordance with the given design procedure. The angular position of the hot side piston is 90 degrees ahead of the cold side piston position. For an alpha engine, 90 degrees corresponds to the phase difference for maximum power.
With a Ross Yoke drive, the resulting motion of the pistons is almost perfectly sinusoidal. The motion of the pistons at A and C is strictly up and down, with very little sideways motion, resulting in very little lateral forces acting on the piston cylinder walls.
Here’s an animation you might find interesting. This is courtesy of Matt Keveney: http://www.keveney.com/Ross.html.
Here’s a video of a large Ross Yoke mechanism I made out of wood:
This website by Zig Herzog analyzes some of the different drives for Stirling engines: