Dear Carl,
The lid still opens like this __|. You don't want it to go past 90 degrees, or you would be lifting some of the dead weight of the cover, plus the force of the struts. Each cylinder comes with a closed length and an open length, and those will set where they need to be mounted on your tub and engine cover. You want the cylinders position so that when the cover is closed, they are directing their expansion (i.e. lift) upward. If the cylinder is not vertical when closed, you have to increase the lift numbers below by the dividing the sine of the angle into the numbers below. I have a worked example below that assumes the cylinder lifts straight up when closed.
I'll try to work an example on how to figure out the strut location and strength. On the weight side, if we assume that the engine cover weighs XXlbs, and has a center of mass some distance from the hinge Y. For simplicity sake, if the mass is uniformly distributed (so Y is half the length L), then if you are lifting the cover from the far point (L), you will need to lift Y/L of XX, i.e. 50%, since half the weight is in your hand and half at the hinge. If you want to offset all of weight, you would need a lift cylinder of XX/2, if it were lifting from the far point (L). As you move the lift point closer to the hinge, the lift force goes up, by the ratio of the distance between the lift point and the hinge. If you were to lift at the center point, I think that the the lift force would be XX * (Y+Y/2)/L, or XX*3/4. If the weight is unequally distributed the math is a little more complicated, but I think that this will work as a first pass, since you aren't trying to offset all of the weight anyway.
This is why you are finding short, but strong ones for cars; they are used to lift hatchbacks from very close to hinge, and thus have to lift nearly all of the weight of the hatchback.
If you assume that the current dead lift on the cover is 100lbs, then by the math above, it would estimate that the cover would weigh 200lbs in total. So a pair of lift springs about a quarter of the way out from the hinge would need to lift a combined 7/8ths of the weight, or 175lbs, or two struts of 87.5lbs. You have lots of choices in that weight range. If you aren't adding a latch, you will need to offset less than 100% of the weight to keep it from bouncing open.
If you have a shell on your pickup, you can see how how the struts offset the weight of the rear window, and how the geometry works out.
If you need only a temporary fix, parking it under a tree, and using a rope and pulley might be simpler...
I think that this math is correct, but feel free to chime in, if I have missed something.
All the best,
Peter
So here is what I kinda know... the lid is VERY heavy... I figure you are lifting around 60 to 80lbs and you are trying to get it over your head. The lid on the 1850 is big, and it houses a HUGE radiator that is filled with oil. Plus the steal that makes it up.
The actuator looks interesting but so far I see only 100lb max.
I have found a website with the type that go in cars and you can get up to 250lbs.... Actually you can get higher, but they are very short (Does this matter?)
What I am getting at is that I am thinking this is a lot of weight, but maybe I am missing the physics here.
So.... Here is a design question......... Lets say I am looking at the PT from the drivers side. The lid lifts like this \. So when I put in my cylinder, does it go in like \ or like /.
