I was very interested in the recent article by Mike Tilby regarding hydrostatic transmission, in large part because hydraulics have always been a bit of a mystery to me.

I was trying to be sure I understood the fundamentals and was being caught up short by the diagram in “Box 1”.  I finally discovered why things were not adding up correctly.  I really would have liked that diagram to include the fundamental equations, but I suppose maybe those aren’t necessary for the learned readers.

What I concluded was that the text included in the box was correct, but the diagram’s final output value was not.  Assuming the pressure in the large cylinder is 255 psi and the diameter of the cylinder was 5in, then the output force should be:

F = p * A   and   A = pi * ( 5 / 2 )^2 or 19.63 sq in

F = 255 * 19.6

F = ~5006 lbs and not the 1265 shown in the diagram.

This then agrees with the text’s statement of 100x input force of 50lbs

Ron