BankShot Battery and Motors for Lift Arm Issue

I need some assistance. I’m the coach of a small team of 3 boys who are competing in the BankShot events.
Their robot is a Scoop/Bulldozer like design; so there is an arm that holds the scoop and lifts it up and over the end zone to drop them.
Their first event, everything went fine, but afterwards, while prepping for an upcoming event, it seems the arm no longer lifts right. The design has two motors attached to either side of two long pieces and the motor rotation is what makes the arm go up and down. We started noticing that when the battery gets to about 3/4 from full, the motors begin to really struggle to life the arm, especially if there are 2-3 balls in our scoop. At full charge, the motor lifts the arm with no issue. Can anyone offer advice on what might going on?

Thanks,
Dower

You probably made some seemingly minor changes to the arm, to make it stronger, but that also made it heavier, and that little bit of weight is making a difference. Another possibility is that something got twisted and is rubbing somewhere, or a piece of string or hair is wrapped around an axial.

If the problem is fixed with a full battery, then the simplest solution is to get more batteries and go into each match with a fresh battery. My teams always try to use a battery that is at 100%, and we don’t rely on the battery icon, but rather go to the System Information menu on the brain to get the exact voltage. The bigger and more complicated your robot gets, the faster it will go through batteries. If you are participating in Programming Skills then using fresh batteries is even more important for getting consistent results.

Other alternatives include using a lower gear ratio for the lift, or attaching rubber bands to the lift that are stretched when it is down and then shrink (releasing energy) as the arm lifts.

Dower,

It sounds like the scoop is attached directly to the motors. That will provide very little power, a lot of speed, and will be hard to control.

Have the boys look at the Clawbot (building instructions with the kit) and see how the motor attaches to the arm. If they use the same gearing, and two motors, it should easily be able to lift the weight of 6 or 8 balls, plus the scoop.

Good luck
Steve