When I finised my telescope, I determined that I had about 10 lbs of force on the very front of the telescope to balance. Many years ago (November 1999 Sky and Telescope), I found an article by Tom Krajci called A Balancing Act for Dobsonian Telescopes. In this article, Tom describes how to balance a telescope with springs. He needed one spring on each side, I needed two to do the job on my telescope.
I followed all Tom's instructions and the spring that the calculations showed would do the job, showed some signs of counterbalance, but did not counter balance the scope. I was disappointed. Later analysis showed I calculated the spring K value wrong. I had calculated inches per pound, INSTEAD of pounds per inch.
So, Barbara and I took a trip back to Iowa in early March, 2011 to see all our folks. My dad has always been a huge part of my telescope building efforts. In discussing the equations and the performance of the spring to counter balance my telescope, I found out he is a genius on levers, which is what this counter balance system is all about. He had great suggestions, which I implemented upon returning home.
One thing my dad suggested is not to let the cable fold back on itself, basically rolling over the top. He indicated you loose a lot of work from the spring when you allow this to happen. At his suggestion, I added screws to catch the cable to keep the cable pulling the spring strongly. With these screws, the cable never rolls over the top and back upon itself. Also, I moved the screws out as close to the edge of the altitude bearing as I could, thus increasing the lever distance and getting more pull from the springs.
The above picture shows the cable guide screw I added to the left side of the telescope's balancing system.
In my calculations, I needed to balance 900 in-lbs. To do this, I used two springs with a wire size of .54 and .63. The .54 wire spring yeilded a spring constant (k value) of 1.53. The .63 wire spring had a k value of 1.79. Together, their sum was 3.32. Putting these values, along with a distance of 10.25 inches for R into Tom's equations, yeilded a 476 in-lbs force for each spring set. With this force on each side, I was well above the 900 in-lbs I needed to balance my scope. Operationally, the balance is perfect.