Don's Spring Counterweight Study
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COMMENTS

I consider the spring counterweight a 'last resort' solution after the weight above the mirror box has been minimized and the altitude bearing size has been increased to achieve balance WITHOUT the counterweight spring.

CONCEPT

The concept is quite simple and the counterweight spring system can be designed without using math.

CENTER IMAGE

• Point the telescope tube straight up and draw a line from the center of radius of the altitude bearing (+) to the spring attaching point on the mirror box.  Note that the moment arm is zero and the spring is pulling against the altitude bearing center of radius.

MOMENT ARM

• Move the telescope tube toward the horizon (RIGHT IMAGE) or past the zenith (LEFT IMAGE if your tube is designed to move past the zenith).  The moment arm is shown by the arrows.

SPRING TENSION

• Adjust the spring tension to counterbalance the weight of the tube assembly.

A spring and pulley system is clamped to the side of the mirror box to test the 'spring counterweight' concept.  The spring works with the tube between the horizon and the zenith.  The lower box is a pedestal. 

The spring and pulley system also works PAST zenith (my design).  Allowing the tube to move past zenith is a Jim Girard idea.  Jim Girard was Rose City Astronomers' past president and my mentor on this truss tube telescope.

The spring is a screen door spring and should be a metal spring.  Bungee cords are not a viable (long-term) alternative because they stretch over time (Peter Abrahams' comment).  The 'pulleys' are rollers for the screen door used with a sliding glass door.