Elevator counterweight / constant spring and ability to lift different number of tote

[lamk]

It's easy to figure out the counterweight/constant spring if we have only one single load like tubes used in previous year. Now we have to deal with loads that varies from 8 lb to 50 lb. How can we precisely move the elevator up and down and not fried the motor when holding a position. A linear electrical actuator(worm screw) with no back drive will be nice but we don't have the machine capability.