This is in relation to something I mentioned in another thread, specifically about a storage ottoman I've designed and built. It's a large piece and in addition to hinges it must feature gas springs to help the user lift the top as well as keep it open.

At the moment I'm trying to figure out which gas lifts/springs to buy and have a choice of a number of parts each with a rated force/lift capacity. Since the lifts will attach closer to the pivot or fulcrum point of the lid, I expect there to be a mechanical disadvantage, which I'd like to calculate.

And seeing as I don't envision a career in applied mechanics, nor have I really any background in it, I'm hoping someone can help me through this if I provide a few numbers and measurements. With a quick peek at wikipedia, I believe a hinged top is most similar to a class 3 lever, one which the force applied sits between the pivot and the load. Although in this specific case the load is really the weight of the entire lever (lid) itself, not at additional object at the end (front) of the lever.

Here's an image of the ottoman frame with measurements. The most important one (from that image) is the distance from the pivot to the front of the top, which is 31" (the width of the ottoman frame).



The top weighs 26 lbs currently - that's without foam and upholstery. Maybe we can run two sets of calculations and make the second set with the weight at 30lbs

Now I suppose the missing piece of information is the force point. That's currently to be decided and it has to be derived from the lengths of the gas lifts available and the desired angle of the lid at its most open position.

I'm going to make a few sketches in the morning to show how this fits together and to try and decide on the force point. I've also got a photo of a commercial ottoman (smaller) with similar gas lifts in place. I may have to try a few different variations which will determine if the lift is applying any usable force in the closed position or not (greater assistance when opening from the closed position).

Here's a PDF from a manufacturer of gas springs showing some calculations:
http://www.acecontrols.com/PDF/Gas%20Spring%20Catalog-Revised%2005-2009.pdf