Gas strut / spring size for floor trap door

1. Oct 2, 2012

smarchy

I am trying to design a similar situation and would appreciate if you could help with me with the math required to size a gas strut/spring. I've read a lot about it but I can't find a definitive source for the calculations required or the best placement of the strut. The final design intent is a trap door (in the floor) that will be flush with the floor. The SOSS integrated/invisible hinges will allow a minimal gap for the opening and allow the door to sit flush with the floor. I am worried about the weight required to open the door by a small handle. Thank you in advance for any help you can offer.

https://dl.dropbox.com/u/72370540/Feldstein%20-%20Trap%20door%20hinge%20sketch.jpg [Broken]

Last edited by a moderator: May 6, 2017
2. Oct 4, 2012

MachX

Hrmmm I dont know too much about the calculations of the gas spring. If you wanted to go high tech you could use a pneumatic actuator. Ive bought a few from clippard.com and they are great quality and can help with sizing/load/stroke. There are so many sizes and every valve and fitting on the planet. Of course you would need and air system OR you could hide a secret schrader valve somewhere to open the door with a bike pump, or a secret Co2 tank with a magical push button valve.... and you could plumb a pressure regulated exhaust valve to the opposite end of the cylinders so that the door shuts at a nice smooth speed. And you could even rig a tiny actuator plumbed to a button that will actuate a deadbolt from the inside so you can lock or unlock the door. These can be powerful enough that you could even mount perpendicular near the hinge base and it would still push the door open even at that angle. you might even be able to get away with one big powerful one mounted on 1 side of the door as long as the pivot on the door is good and strong. Thats just how I would do it :D might cost slightly more than gas though.

here is one of their calculators in case you are intrigued:

http://www.clippard.com/calculators/bore.asp

Last edited: Oct 4, 2012
3. Oct 4, 2012

MachX

I was thinking something like in the image below. Of course it depends on what you are using the door for and whether or not this is overkill. The good thing about pneumatics is with the right cylinder you have a lot of flexibility as far as mounting at many points and angles, and the pressure required can vary to meet the needed force as long as the cylinder clevis pivot at collapsed position falls below the frame.

I did manage to find an equation to determine gas spring mounting instructions as follows:

F1=(Fg x Lg / n x L1) x R
Where:
F1 = springs extension force
Fg = force of gravity of hatch at a given angle (45deg)
Lg = horizontal mount from pivot hinge to center of gravity.
n = number of springs
R = multiplied safety force of 1.2 for holding the door.
L1 and Lg determine placement
So if you have two 100lb springs @ 24" extended and and 16" compressed the equation would be:
L1 = Lg x R x Fg / (F1 x n)
45deg x 8feet / (2 x center of gravity when door is open)
Lg = 2.83 feet

L1 = 2.83 feet x 1.2 factor x 40 pounds/(100 pounds spring force x 2 springs) = 0.6792 feet x 12 inches/foot = 8.15 inches.

Therefore, mount the gas spring base end (piston rod end down) 8.15 inches from the hinge pivot pin on the side flange or opening to the hatch.

Add the 16-inch retracted length to the 8.15 inches to properly locate the upper end of the gas spring at least 24.15 inches from the hinge pivot. Increasing this latter distance increases the force to hold the cover open.

Hope this helps!

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Last edited: Oct 4, 2012