How would I go about doing this?

[Shashank R.]

Hi, right now I'm designing an intake system for FRC (FIRST Robotics Competition). Basically, there are the
totes:

(For one tote):
Dimension: 26.9" x 16.9" x 12.1"
Weight: 7.8 lbs

Now I'm designing something to suck the totes in, something similar to this:

See at the bottom the subsystem with the two wheels? Now in my design these will be able to articulate, but it will be held in place with a spring load. I want to figure out the maximum spring load I can put in so that the friction between the wheels and the end of the tote can still overcome the springs, but it has to be a strong enough load so that the wheels can still have enough friction and force to suck the totes in from either side. Sorry if this is a little confusing, so please ask any questions to clear things up. I'm not asking for any calculations to be done for me, but just some guidance on how I go about doing this.

Thanks, Shashank R.

 

[Shashank R.]

(UPDATE)

Please observe the robot in the bottom right hand corner:

It sucks in totes fed through the chute, but they open and close their arms. I'm trying to do the same, but without opening or closing the arms, but leaving the articulatable, held in with a spring load.