[Chris Fultz]

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.