The elevator tube travels 1.6". I used a scale and plotted the force to compress and stretch the springs that have the right force. It takes about 30 .lbs to fully extend and about 16 .lbs to compress. The original tube's length changes 1.7" from max compression to stretched. The original actuator is about 24" long and has 3" stroke.
Here's a simple graphic with some of the forces that has a nice "feel":
A little research on compression springs reveals that springs should be selected so their load is about 45% of their full capacity. I found a bag of 10 ea. 60 .lbs springs on Amazon for $18 that have 3.5" travel and should have them Wednesday.
Also, while testing, the motor became intermittent. Some strained and broken wires were found at the motor.
|Some shrink tube and a wire tie should keep the wire breaking in a bad place.|
|My first tube had +/- 2" of travel with about 14 .lbs of force. I rethreaded the inside rod so it was extending an inch less and put one of the smaller springs I made inside the larger spring to provide more force near the end of its travel.|
|In order to increase the rod travel a little more than the original design, I cut about 1/4 shoulder on one plug and on the sliding bushing.|