The team had a long series of adventures, some good and some bad, on the road to developing a means for the robot to lift the cap ball and place it on the vortex. Because this end game activity is worth 40 points, we gave its accomplishment a high priority.
After some internet research and some playing with Lego parts, we decided to implement a scissor lift with a servo-operated pair of arms to grab the cap ball and raise it aloft. We decided to build the lift out of greenheart wood because of its toughness and ease of drilling and tapping. We used brass tubing bushings along with machine screws and lock nuts for the joints. By cutting the brass tubing slightly longer than the distance through two scissor legs, we could prevent over tightening from making the joints bind.
For the servo, we found a digital servo with a heavy duty frame and geared shaft that promised to have plenty of torque and the desired range of motion. It was expensive, but has served us very well. The part is a TM-D645MW-400 Servo Gearbox from ServoCity.com.
We had intended to fabricate the ball lift arms out of greenheart as well, but several teams at the Smithville meet were using pieces cut from an inexpensive tennis racket. The arm shape that can be made this way is almost perfect for the application.
Our original greenheart scissor lift worked OK, but we became concerned that it was not strong enough to survive the excitement of competition. So we changed to a composite formed with an extruded aluminum shape from the hardware store with a carefully cut greenheart core.
For the lift actuator, we benefited from the burgeoning 3D printer industry in an unusual way. Because they are used in 3D printers, lead screw actuators are available from China via Amazon.com for an amazingly inexpensive price. We purchased a lead screw, bronze follower nut, two pillow block bearings and a motor shaft coupler for about $17! Pairing this with an AndyMark EveRest 40 motor provided more than enough power to lift a cap ball high into the air. In fact, the screw generated such force that we had to add limit switches to cut off the motor if it tried to push beyond the range of motion allowed.
To improve the tracking between the two sides of the lift, we added a couple of shafts and long throw linear bearings to prevent the draw bar from twisting to the side.