Gas Strut Calculator - round force up or down?

[campygirl]

I used the calculator to determine which struts to buy and with a 38.5" hatch length and a weight of 42,6 lbs, the opening force came to 72 lbs.
The length we need is 23.5". It only seems to come as 60 lbs or 90 lbs force when I look through the McMaster website. Even looking at slightly different lengths, they all jump from 60 lbs to 90 lbs.

We are inclined to go with the 60 lbs but our hatch has a a pretty tight curve and I am concerned it won't open high enough not to bump your head a lot

Any thoughts?

[Tom&Shelly]

I would try the stronger ones first. The danger of the weaker struts is that the hatch will tend to fall on your head. (Ask me how I know!) Also, in cold weather, they lose strength. (Again, ...) With the stronger struts, you can always add a little weight to the hatch, if necessary, to make them work the way you want to. (Add it to the lower end, and because of the lever effect it won't be nearly the difference between what you calculated and what the strut says.)

I believe McMaster will let you send back a pair that didn't work for you and trade them for the next size up or down.

We couldn't find a good match with them, and shopped around the interwebs. So you can probably get a close match to what you are looking for.

Tom

[campygirl]

The danger of the weaker struts is that the hatch will tend to fall on your head. (Ask me how I know!)

That is exactly my worry! Thanks Tom. Maybe we can just hang a cast iron pot from the hatch and call it even?

[Squigie]

Just don't let it uppercut you when opening.

[Hubert]

What is the stroke length of the strut you are looking at and what is the attachment point you chose? I get a lever arm of about 6.25" for the length and weight you gave, which would correspond to a spring with a stroke of a little under 7.5" if you use the 85% rule. If this is the case, I would try to change the geometry, maybe use a 24.5" spring with 11" of stroke if you can fit it. If you can increase the lever arm, you can use a weaker spring and reduce the force on the hinge, and the approximations made in the spreadsheet become a lot more tolerant to errors in weight and dimensions. Also, make sure the strut cams over so that it pushes the hatch shut when closed. If you can post a drawing or at least some pictures of what you are working with, you'll probably get some better advice.

[campygirl]

The spring I was looking at has a stroke length of 9.84"
Our calculated ideal attachment point was 8.3 inches from the hinge, but we thought it would work better if it was a little closer to 6.25"
If I move it 7", then the spring force required drops to 62 lbs so maybe we can work with that
I will try to draw it or sketch up a photo after work today

What do you mean when you say to make sure it "cams over'?

[Hubert]

Sorry for not being clear. I mean to lay it out so that in the closed position, the force of the strut pushes the hatch shut. If you extend the line between the two attachment points of the strut past the hinge, it should be below the hinge in the closed position. As you open the hatch, there will be a position where the line points straight at the hinge, then it will go above the hinge and push the hatch open. I think of this transition from one direction to the other as the point where it "cams over". Does that make more sense? If not, I can make a drawing later today.

I think you'll be better off if you use a longer lever arm if you have the room for it.

[campygirl]

That helps a lot!
Thanks you