Hatch Struts...Formula?

[48Rob]

Hi all,

Trying to figure out what size struts I need without having to drill 20 different holes, and try 10 different struts...guess I should have paid more attention in math class

This drawing was posted by Andrew,



I've been studying it, but haven't found the common denominator (if there is one) that I can use to apply to my hatch size.

Is there a formula?

Rob

[mikeschn]

Andrew,

Can you come up with a shock layout for the Generic Benroy?

Mike...

[Guy]

Dear Rob and Mike,

Here is a link to easylift systems build your own site. Please note they will, for free, tell you exactly which spring to use for a particular application.

http://www.easyliftsprings.com/contact/buildyourown.html

[angib]

Is there a formula?

Aaaiiieee! You mean, like, a formula for world peace? 'Cos that's easier than a formula for hatches and gas springs.

The diagram above works with the NAPA gas spring that was quoted in the original message (sorry, lost it!). The main variable for positioning the gas spring is how far you want the hatch opened. To make the hatch open through a bigger angle, you move the whole gas spring arrangement nearer to the hatch hinge. To help work this out, here are two more diagrams with the gas spring moved 1" and 2" nearer the hinge. I've had to redraw the original so here they all are.

Original:


Moved 1" nearer the hinge:


Moved 2" nearer the hinge:


To save working out angles, I show the 'opening' dimension of the hatch compared to its length, so:
- the original gives an opening 111% of the hatch length (ie, 61"/55");
- the moved-1" gives an opening 116% of the hatch length;
- the moved-2" gives an opening 124% of the hatch length.

If that helps. But maybe not.....

Andrew

[grant whipp]

If you want to know the proper way to mount a galley lid prop system - whether it is the star-lock prop, spring prop, or gas-strut prop - all you need to do is have a look at the rear lift-up door on almost any camper shell / pick-up truck top cap. The mounting point on the lift-up door (in our case, the galley hatch lid) is always only a few inches away from the hinge, and the fixed pivot point on the wall is usually close to the length of the prop (in the open position) down the wall from the hinge.

While in theory (and in actual practice) the method shown in Andrew's drawings WILL work, there are several fundamental problems when put into actual practice. First, spring props and gas struts don't like working in that 90º operating swing (actually, the props don't care - it's the stress on the mounting points that cause all the problems down the road) ... they work best in the 60º or less range, and if you study the aforementinoed lift-up doors, you'll see something in the 45º or less range. Second, notice that in the closed position, the pressure from the spring/gas-charge is actually pushing the hing components apart. When mounted in the proper manner, the pressure should push the hinge components together, thereby adding the additional benefit of helping to hold the lid closed.

Most galley lids are 44-48" hinge-to-threshold, and here is the mounting that has worked for me for nearly 20 years:
Pivot point on the lid - 5" down the rib from the hinge
Pivot point on the wall - approx. 17-1/2" down the galley wall from the hinge, and in just enough to clear the lid framing.
I build a fairly light weight lid, and on a 4' wide teardrop I typically use 45-55# gas struts, 18 to 18-1/2" long in the open position. Most retail outfits will let you exchange for a heavier set if you need to.

One other thing to keep in mind if using gas struts is to keep the gas cylinder on the down side. When the seals fail (and they WILL fail), nasty fluid leaks out, and if that cylinder is on the up side, that fluid will run down the rod and drip all over your counter-top - better to keep the it contained on the prop as best as you can.

Good Luck on all your projects! In the meantime ...

CHEERS!

Grant

[Joanne]

Is there a formula?

Aaaiiieee! You mean, like, a formula for world peace? 'Cos that's easier than a formula for hatches and gas springs.

The diagram above works with the NAPA gas spring that was quoted in the original message (sorry, lost it!). The main variable for positioning the gas spring is how far you want the hatch opened. To make the hatch open through a bigger angle, you move the whole gas spring arrangement nearer to the hatch hinge. To help work this out, here are two more diagrams with the gas spring moved 1" and 2" nearer the hinge. I've had to redraw the original so here they all are.

Original:

<snip those beautiful drawings>

To save working out angles, I show the 'opening' dimension of the hatch compared to its length, so:
- the original gives an opening 111% of the hatch length (ie, 61"/55");
- the moved-1" gives an opening 116% of the hatch length;
- the moved-2" gives an opening 124% of the hatch length.

If that helps. But maybe not.....

Andrew

Hi Andrew,

I wish I had paid better attention in my college math classes. I started out in mechanical engineering but moved to computers instead.

Now, as you move the lower attachment point closer to the hinge, isn't there a greater "lever effect" on the strut? Doesn't the strut see more apparent weight on it? I don't want to get into a full mechanical analysis of the operation of the hatch, but I'm wondering if there is a practical range where the strut mounts should be located?

Joanne with her dunce cap on

[angib]

Now, as you move the lower attachment point closer to the hinge, isn't there a greater "lever effect" on the strut? Doesn't the strut see more apparent weight on it?

Joanne, you're absolutely right. Moving the strut nearer to the hinge means that you would need a higher-poundage strut to hold up the same weight of hatch.

Grant, the logic to putting the gas strut cylinder-up/rod-down is that virtually every strut will damp itself as its reaches full stroke, as long as it's this way round - it's why car trunk lids ooooze into the fully-open position rather than simply banging open. This may be a country-specific thing - gas struts very rarely fail here, maybe because we don't get the extremes of temperature that you do.

I can see the sense in both points you make about fixing the strut the opposite way round to what I've shown - I would go and correct all the drawings above, but then subsequent posts wouldn't make any sense!

Andrew

[48Rob]

Thanks to all for the detailed explanations and drawings!

I'm going out today to look at a truck topper...

Somewhere between 45 & 60 degrees, seems pretty simple now!



Rob

[Ira]

My brain can't absorb all of this:

As I've said before, I'm using two poles/broomsticks stuck into flag holder brackets.

[grant whipp]

Ira!

Nothing wrong with broom sticks ... they've worked on teardrops for 70-odd years ... as long as you don't camp where the wind blows hard and you haven't devised a safety catch for the lid when it wants to sail.

This stuff ain't rocket science (though I suspect there are those who would like you to THINK it is). It's really all a matter of "copy & adapt" - find an application that has already been engineered and proven successful in other applications, copy it and adapt it to your application. Heck, you've already done that all over your teardrop, why stop now ... ;-} ;-} ...?

CHEERS!

Grant

P.S. Andrew, the gas props I use dampen in both directions. All the GOOD ones do, now. It's those pesky spring-props that make you have to hold on to your lid as it reaches the top. And if you use star-lock props, well, it's Power by Armstrong all the way up AND down (if you don't want it to slam shut, that is ... ;-} ;-} ...!).

[doug hodder]

I don't know if it is the temperature variations or what...but I've had toppers on all my trucks back to the mid 80's...and before that they had funky springs in sleeves...but anyway....on all of my gas shocks...I've had to change them at least once on all of my toppers....but when they fail...you'll know cuz it will fall and hit you in the head....I use a broomstick till I can get a new pair....Doug

[Nitetimes]

I don't know if it is the temperature variations or what...but I've had toppers on all my trucks back to the mid 80's...and before that they had funky springs in sleeves...but anyway....on all of my gas shocks...I've had to change them at least once on all of my toppers....but when they fail...you'll know cuz it will fall and hit you in the head....I use a broomstick till I can get a new pair....Doug

Yep, me too. And the ones on cars are worse because the piece they are holding up is heavier. I had an '84 Caravan that the hatch props went bad on, talk about a headache, I carried a broomstick for that one too but it sucked when you'd get in a hurry and forget about it, gave me a headache and a sore neck for a week one time.

[emiller]

I just make sure to put mine about 6 to 8 inches from the pivot point, any further for me doesn't lift the hatch far enough. The only draw back from attaching them closer to the pivot point is you use a stronger gas shock. I have 150# on my first tear and 120# on my second.

[48Rob]

Got the struts installed, no more prop rods




Rob

[Steve Frederick]

Got the struts installed, no more prop rods




Rob

Love that Tear!!