What Goes Up.....

[Colorado_Carter]

Who ever said "What goes up-must come down" was sooooooo wrong. There might have been a slight over estimation on my part as well.

So I ordered up some sweet gas struts for the hatch-I wanted to have them installed prior to making final adjustments and installation of handles. I did some research then decided more is better and then ordered something different. Long story short, I installed them and started to rip out the hinge (one screw needed reset-no big damage) when I tried to close them. They didnt even budge or pretend like they want to close-I even gave them a once over to see if there was a lock on them somewhere.

I ordered a columbia strut CS2000-150 which is a 20" extended strut with 150lb force at extension. They also carry struts with 120, 110, 90, 60, 30, etc.

Below is the best pics I have of the hatch interior structure. I don't have a total weight (est. 50-65lbs??) or the weight at the point of attachment. This is what I think I need to determine. Does anyone have any suggestions on size?

I plan on taking a luggage scale and measure the weight of the door at the point of attachment and at the middle of the hatch at height. Does anyone know how I would use this information.

[48Rob]

Carter,

Grant posted a really helpful chart somewhere here on the board about locating lifts and weights.
Sorry, I'm not sure where it is though.

Rob

[KCStudly]

Here is a link to McMaster-Carr's "About Gas Springs" page. If the explanation part doesn't open automatically just click on it at the top of the linked page.

There are also some of my own thoughts and some sketches for adapting linear actuators to TPCE's galley hatch starting here (scroll down a bit until you see the CAD profile images of the hatch area). The actuators are a little different, but behave very much the same when figuring force ratings and extension arrangements. As this is written, the discussion continues into the next page, with some design changes. It is all just at the design stage right now and my situation has some specifics based on the actuators that found their way to me, but you may find my thoughts on strut arrangement, hatch CG, dwell points and vectors (lines of force) helpful. (The force numbers currently estimated don't really take the direction of gravity vs. the strut angle into full consideration, but should be fairly close as a snap shot in time.)

Measuring actual force required directly at your strut attachment points in the direction that the strut is angled, and then rounding up some to account for wind and wear is probably the very best way to figure the power factor, but then you will have already decided on extended and collapsed length (er go throw, too), and mounting locations.

Don't underestimate wind force. We had a couple of small man way hatches with those gas struts used to access the roof at work and even on a small 2x3 foot hatch we had to up the strut rating 20-30 lbs per side to keep those thin sheet metal hatches open in typical wind conditions. I posted a link to a wind force calculator in another discussion on hatch struts somewhere; let me take a look. Yup, here it is. In this case we were talking about the wind trying to push a hatch over backward, but the same calculator can be used to figure the force of the wind trying to close a hatch.

Can you collapse the struts at all by hand in free air, or by leaning on them when pressed against a hard surface? Did they come shipped compressed? If so, then it would seem that they should close again with the proper application of force (i.e. no secret catches or handshakes required ).

[nevadatear]

Doing some organization. Swapping out white baskets for purple i fond at the dollar store to match our theme.

[Colorado_Carter]

Alright I have some more information. Using a luggage scale I measured the max. weight at the point of contact of the strut with the hatch at a couple different angles. I came up with about 55-60 lbs at different angles. I also figure I need to take into account the weight of the skin which will be 33-76lb depending on the thickness (hopefully the larger number (.04 or .06).

If I estimatethe final weight around 60lbs-70lbs(including skin, maybe some speakers?) and assume a buffer of +25% to account for wind and what not a gas strut with about 90lbs should do the trick.

What does everyone think about this?

[bdosborn]

Carter,

Grant posted a really helpful chart somewhere here on the board about locating lifts and weights.
Sorry, I'm not sure where it is though.

Rob

Here it is:

Hatch Struts...Formula?

Bruce

[WhitneyK]

There is also a seller on ebay that goes by the name "Strut-your-stuff-here" that was very helpful when I was researching this for mine. PM me if you would like his phone number, he's up in the state of Washington.

[PaulC]

Alright I have some more information. Using a luggage scale I measured the max. weight at the point of contact of the strut with the hatch at a couple different angles. I came up with about 55-60 lbs at different angles. I also figure I need to take into account the weight of the skin which will be 33-76lb depending on the thickness (hopefully the larger number (.04 or .06).

If I estimatethe final weight around 60lbs-70lbs(including skin, maybe some speakers?) and assume a buffer of +25% to account for wind and what not a gas strut with about 90lbs should do the trick.

What does everyone think about this?

Have thought about the fact that two 90lb struts have the potential to hold open 180 pounds. I use two 50lb struts on my hatches and have never had a problem.

Cheers
Paul

[Socal Tom]

I would think that the total force combined on the two struts would need to be about the same as the force required to lift the hatch. Since a 3/4 inch sheet of plywood weighs 75 lbs, and I think my hatch feels lighter than that, I'm going to say that the total force for two struts could be under 75lbs. 2X 50Lbs would probably work, but might require more force to close than you want. I would probably choose 2x40.
Tom

[KCStudly]

I would think that the total force combined on the two struts would need to be about the same as the force required to lift the hatch. Since a 3/4 inch sheet of plywood weighs 75 lbs, and I think my hatch feels lighter than that, I'm going to say that the total force for two struts could be under 75lbs. 2X 50Lbs would probably work, but might require more force to close than you want. I would probably choose 2x40.
Tom

If it were only that simple everyone would get it right the first time. You are forgetting to account for the relative position of the strut versus the center of gravity of the hatch; the leverage or teeter totter effect. To put it in simple terms, the hatch is the skinny kid sitting out at the end of the teeter totter, and the strut is the fat kid sitting in closer so that they can be balanced.

If your hatch is 75 pounds and 4 feet long, then that 75 pounds is acting, say about 2 ft from the hinge (more or less depending on shape, angle of slope, construction, attachments like lights, towel racks, etc.). Now if the struts are only located 1 foot away from the hinge, then they will see about twice the load due to leverage. Each strut would need to be at least 75 lbs, plus a little for wind, etc.

It gets even more complicated because the strut is not acting perpendicular nor vertical to the hatch CG; and the hatch CG, in most cases, is not acting perpendicular to the hinge relative to the ground. Here is kind of a tedious explanation of force vectors, but it gets the point across, and that is that a force applied in one direction may have to be larger than the force it takes to move something in a different direction.

So the fact that lots of people say they used 40 - 50 lb rated struts and have no problems seems to correlate with people who have built small 4 wides with relatively short (and therefore light weight) hatches. Maybe I'm mistaking.

On the other hand I have read lots of times that people underestimate the strut rating needed to lift their hatch.

I don't know, maybe all of my reading, studying, researching, designing and postulating on this subject could backfire on me when I actually get to the point of installing mine (if it comes to that). I will be the first to admit if I end up being wrong.

[wagondude]

I thought the struts were there to hold the hatch open and to assist with opening. That would require a lower force as opposed to having the struts make for an auto opening hatch. That would more than likely make the hatch hard to close. I had a Camaro that had struts on the hatch. They would not open the hatch, but held it up just fine. That all glass hatch would have been a bear to lift without them.

[Socal Tom]

I would think that the total force combined on the two struts would need to be about the same as the force required to lift the hatch. Since a 3/4 inch sheet of plywood weighs 75 lbs, and I think my hatch feels lighter than that, I'm going to say that the total force for two struts could be under 75lbs. 2X 50Lbs would probably work, but might require more force to close than you want. I would probably choose 2x40.
Tom

If it were only that simple everyone would get it right the first time. You are forgetting to account for the relative position of the strut versus the center of gravity of the hatch; the leverage or teeter totter effect. To put it in simple terms, the hatch is the skinny kid sitting out at the end of the teeter totter, and the strut is the fat kid sitting in closer so that they can be balanced.

I probably oversimplified, but I think many people over complicate this kind of thing.
When it comes to complicated math I like to estimate what I think it should be and then do a reality check before I trust the fancy math. I've found that my gut is usually not too far off of the right answer. In this case, my gut doesn't fit with needing 300lbs of force.
Tom

[KCStudly]

many people over complicate this kind of thing

Yeah and I'm usually at the front of that line. Where do I sign up?

When it comes to complicated math I like to estimate what I think it should be and then do a reality check before I trust the fancy math. I've found that my gut is usually not too far off of the right answer. In this case, my gut doesn't fit with needing 300lbs of force.

TLAR method works everytime 80/ct of the time.

But you are absolutley correct, 300 lbs does sound like a lot at first blush.

[Socal Tom]

OK, I think I have an idea. Place a bathroom scale on your counter ( level it first). Make a prop rod that will hold the hatch at the height you want it to be. Then Put the prop rod between the bathroom scale and the hatch, do your best to put the prop rod straight up. Now the weight on the prop rod ( as read by the scale) is basically the lift force you need. Do your best to mount the lift shock so it will be straight up an down when fully extended. If it has to be at a angle you at least know what the final upwards force needs to be.

Once you have the right lift force the rest should be reasonably straightforward.
Tom

[WhitneyK]

By George, you've got it!!!

That's how it was explained to me by the "strut your stuff here" guy.
The prop rod for measuring the weight needs to be at the location of where your strut / shock will be located on the hatch. For example, mine at the place where you lift it was 60#. However, where the struts / shocks needed to be mounted it weighed around 145#. Big difference! So, take that weight, in my case 145#, divide by 2 (number of shocks) = the poundage that each shock needs to be. So I would, in theiry, need 2) 75# shocks. But, he said to use the 80 or 90#er's for insurance (in case a wind caught it and tried to push it down).