Formula for determining the placement of gas shocks on hatch

[Mike C.]

I have searched the board in attempt to locate a sticky or a thread that gives a formula, based upon the weight and surface area of a hatch, for locating gas lift shocks.

Our hatch is approx. 80" tall x 76" wide, in a "C" shape, narrower at the top, and we are trying to decide the size and placement of the gas shocks.
We have lifted it to the height we would like the shocks to lift it and the weight at the bottom of the C was approx. 35 lbs and the weight at the edge of the trailer is approx. 65 lbs. When we raise it to the height we would like it to be, there is approx. 48" from the peak to the vertical wall.



I was hoping there would be a mathematical formula that could incorporate size and weight and give me a close approximation as to the
placement and size of gas shocks.

I read the easylift post, Sonetpro's post and Steve Tripper's sticky and cannot see any formula. If anyone has an answer, the help would be greatly appreciated, but if not, I guess we will have to use the old trial and error method.

If you have questions as to the type of trailer, you could look at Miriam C.'s build album.

Thank you in advance for any help.

[WarPony]

Mike, I was doing the same thing for my hatch struts today. On yours, I think the 27" will look better but the 36" placement will be stronger and take stress off of the hinge when the wind is blowing hard.

Jeff

[tonyj]

I don't have any formula, just a reminder (that I forgot) to make sure the lower edge is high enough that a person wearing a brimmed hat doesn't unexpectedly hit his head on the corner.

My upper attachment on the hatch is about 6 inches from the hinge and doesn't provide nearly enough leverage even with 120# struts on each side (remember, I overbuild, so my hatch turned out heavier than even I thought it would be). I'll bet you will need to be 12-18 inches from the hinge.

Sounds like you'll need some input from one of our resident engineers.

The real problem may be the top corner of your cabinet. If the top strut mounting point is above the cabinet, the length of the strut may prevent a mounting point on the wall. Take a stick the length of the strut in the closed position and position it on the trailer wall to see where the upper and lower points will be. The strut may have to be mounted so the upper attachment point is even with, or lower that the corner of your cabinet. Depending on the closed length of the strut, the lower attachment may be below your counter.

Am I clear as mud on this?

[sdtripper2]

Here ya go M:


Formulas and planning for Struts
Good explanation on sizing struts
and tools for calculating the size
http://tinyurl.com/28c4qm
http://www.easyliftsprings.com/technical/tech_info.aspx
***

Steve T's (sonetpro) formula for struts
http://tinyurl.com/zsxpw

For all those interested:
Find lots more on hatches and struts here.
http://tnttt.com/viewtopic.php?p=140874

[Mike C.]

Thanks all for the input.

Steve, I think the tinyurl is what I was looking for, but I need to study it
for a little bit.

Again thanks for the help and suggestions , it is appreciated.

[Dooner]

Make sure that you take in account the length of the cylinder when in the closed position. The way I look at it, in your picture, the two ends will end up in almost the same spot when the hatch is closed.
Just something to remember.

[emiller]

McMaster Carr also has a simple formula.
http://www.mcmaster.com/ page 1137

[Mike C.]

Dooner, I was coming to the same conclusion. That is why I was trying to figure out where to place them to make them fit, and still lift.

emiller, checked out McMaster Carr site and copied the page. Really good info. What is strange is they compete with us in the industrial electrical markets, but I never thought of them for that kind of thing. Wonder if WW Grainger would have the same thing?

Anyway thanks to you both for the help.

[emiller]

Not sure if Grainger does, I don't think so because I use both companies. My rule of thumb is to put one shock pivot point six inches from the hinge of the galley. Either hatch or side wall.

[emiller]

looking at your design I would think your pivot points are too far from the hinge which will make your shocks travel too far and fully compress before your hatch closes. On my first tear I used small 8" 120# gas shocks. They worked fine except it put alot of strain on the hinge. The tear I have now has 15" 150# shocks and works better. I mounted six inches down from the hinge on the side wall and then lifted the hatch to where I wanted it open and thats where I attached the other pivot point.

[angib]

Mike,

I've tried to overlay on your photo (in red) where the mounting points for your strut would need to be. This is for a strut about 32" long (extended) and it's compressed to about 18" when the hatch is down.



But, there are two problems:

- Firstly you can see that in the down position the strut hits the top edge of the cabinet.

- Secondly, when the hatch is lowered, it looks to me like the hatch ribs will cover the top couple of inches of sidewall - so where I've shown the bottom strut mount, the hatch rib would hit it.

I can't see a way around these two problems - I think you are going to have to use a fixed-length pole on each side that either you just put them in place when you lift the hatch, or that are hinged to the hatch at the top and fit in sockets on the sidewall.

Andrew

[tonyj]

Or move the cabinet face frame back, or build cabinets that don't touch the side walls. Or put the attachment point on the sidewall inside the cabinet with a slot in the top of the cabinet. Or . . .?

[Keith B]

One fix is to not use struts but use solid supports...1. they will never leak and sag, 2. they are strong and you can place them further out on your hatch not putting so much pressure on your hinge, 3. the wind wont "accidentally" blow it shut or spring them open and 4. mine cost about $8 TOTAL to build... struts are like $30 a piece...
Mine are affixed to the hatch on 1/4" pivots, then lock in place into the sleeves mounted on the counter top... when not in use they swing back and store into the black light pole holders (used for boat anchor lights) and my hatch opens to 6'5" tall so most wont bump their head.
Costs: 1 piece of 1/2" EMT $3; 2 1/4" threaded inserts $0.75, light clips, $4; holders on cabinet welded out of some scrap 3/4" gas pipe, misc. nylon washers and 1/4" ss bolts.

[Miriam C.]

Mike,

I've tried to overlay on your photo (in red) where the mounting points for your strut would need to be. This is for a strut about 32" long (extended) and it's compressed to about 18" when the hatch is down.



But, there are two problems:

- Firstly you can see that in the down position the strut hits the top edge of the cabinet.

- Secondly, when the hatch is lowered, it looks to me like the hatch ribs will cover the top couple of inches of sidewall - so where I've shown the bottom strut mount, the hatch rib would hit it.

I can't see a way around these two problems - I think you are going to have to use a fixed-length pole on each side that either you just put them in place when you lift the hatch, or that are hinged to the hatch at the top and fit in sockets on the sidewall.

Andrew

Thanks Andrew, Miriam here and there has to be struts, or I have to cut the hatch down. It is just too heavy for one person to put poles up and hold it. I can't even put it up there high enough.

You are right and the only place to put struts is in the area I marked. The hatch rib touches the cabinet and the electrical inlet will be in the way of anything there anyway.

There is a possibility I can cut the cabinet shelf off on that tower and lower it but the inlet might still be in the way. I will measure some more.

Thank you all for your helpful suggestions.

[emiller]

Using solid supports are good but don't forget I've seen many a hatch blow over in a wind gust. So make sure what ever you use secure your hatch props.