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Q & A: spring-loaded tailgate

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Most recent answer: 04/14/2013
Q:
Hi, I need to calculate the torque i need in 2 torsion springs set on 2 hinges to be able to hold the weight of a steel door that opens from 0 degree vertical to 90 degree horizontal (like a pickup truck tailgate). The door weigh about 100 lbs and is 5' wide x 2' tall. I want to be able to open the door smoothly without having to hold its weight. Can you give me some help on this? I can send you drawings of the door and the hinges. Regards, Michel Valiquette
- Michel Valiquette (age 35)
Boucherville, QC, Canada
A:
The easy part of the calculation is to figure out the maximum torque needed. Measured around the hinge, the center of mass is about 1 foot away from the axis. The weight is 100 lbs. When the tailgate is horizontal that gravity force is at right angles to the line from the center of mass to the axis. So the torque is 100 ft-lbs. As the tailgate opens an angle  θ  from the vertical, the torque is just sin(θ)*100 ft-lbs.

So that means that if you want the spring-loaded hinge to just balance the torque, its own torque would have to go from zero to 100 ft-lbs as the gate opened.  The spring-loaded hinges I'm familiar with don't change that much as they open 90. Probably you could find some whose torque would approximately balance the gravity torque for a half-open tailgate. You'd have to pull a bit to get it partly open, and it would fall a bit once it was mostly open.

In principle you could rig up some torsion springs to do a better cancellation over the whole range, but it's not clear how good a cancellation you need. To get nearly perfect cancellation, you could put a 50 lb weight on a pulley so that the weight got pulled up the same distance the top of the tailgate moved down. That might get in the way of things, however.

Mike W.

(published on 04/14/2013)

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