this is just information that i will put out here
i have both the heavy duty & standard duty version of the 18 inch superjack
it pushes a 7 1/2 ft dish ( and these are fairly lightweight dishes) the hd version is much smoother in movement.
I will add more to this when i get home cause i think the arm shaft is larger on the HD version 2 inches as compared to the regular standard version 1 1/2 ( i think) i will verify when i get home.