For a quick calculation of the force needed to lower the bridge, you can use this equation.
F is the vertical force, in pounds, required.
D is the distance, in inches, that your device contacts the bridge, measured from the end edge.
F = 21 + D * 0.75
So if your contact point is 1" from the end, you would need 21 + 1*.75, or about 21.75 pounds of force.
If your contact point is 12" from the end, you would need 21 + 12*.75, or about 30 pounds of force.
Once you know this number, you can select the right motor and gearing combination to give you that force at the end of your lever arm (remember the motors are mostly rated in oz-in so you have to convert).
If you are using a pneumatic, remember that you need to determine the vertical component of the force, based on the angle it contacts the bridge.
(i.e. 50 pounds at a 45 degree angle is only about 35 pounds vertical).
As noted below, the “starting point” is around 21 pounds and may be a little less. Bridges vary and the underside construction has an impact.
Use this as a guide and design margin into your system.
The forumula doesn’t add any margin, but depending on some assumptions it could be a high starting value. One of our early designs was pushing about 10" up the ramp with about 28 pounds vertical, and that was “just” enough.
The calculations say it takes about 28 - 29 at that point.
From some of the posts in other threads, it seemed that some teams were not sure at all what it took to lower the bridge - I was giving some guideance.
the torque created shifts as the bridge moves. This formula happens to provide a built in quantity that partly allows for the shift. However, It dose not accommodate a shifting contact point (having the same point of contact for the entirety is hard to design and have strong enough to take the abuse of FRC).
*]your motors/pneumatic won’t be a constant force for all times used and will become weaker over time. The greater the buffer amount built in, the slower It will deteriorate and the easier It will be to compensate.
Make sure that whatever force you are designing to push down on the bridge with can be counteracted by the weight of your bot. I’ve already seen several designs that can tip their own bot with their bridge manipulator before they move the bridge, especially if they try to tip an occupied bridge and it’s heavier than anticipated. Go carefully!
I’ve seen several teams desperate to add legal ballast to their bots while at the regional. It isn’t an easy last minute modification.