Gas Spring/Hatch question

[bbarry]

I'm getting ready to buy gas springs for my hatch. It weighs approximately 40 lbs at the estimated attachment point. If I get two springs rated for 50lbs, will this be too hard to close?

That's as close a guess as I've been able to make using McMaster's guide and a little Midwestern mental engineering.

Anyone have thoughts?

Brad

[planovet]

You might want to go a little bigger. You don't want the hatch closing on you if a stiff wind/downdraft comes along. My hatch weighed 85 pounds (+/-). I got 150# struts from McMaster and they work great. I can actually open AND close the hatch with one finger. I probably could have gone with the 125# but I didn't want to return the others.

[bbarry]

You might want to go a little bigger. You don't want the hatch closing on you if a stiff wind/downdraft comes along. My hatch weighed 85 pounds (+/-). I got 150# struts from McMaster and they work great. I can actually open AND close the hatch with one finger. I probably could have gone with the 125# but I didn't want to return the others.

Hmm...maybe I don't understand this force thing very well. If your hatch weighs 85lbs total, the force necessary to lift it on an arm some length down the hatch should be less, yes? The hinge will be supporting some of the weight. If that's the case, you have 300lbs of force acting to lift less than 85lbs? I'm confused!

Brad

[8]

Not building one myself yet, I don't know how they are rated, so I can only say to follow the mfg specs. Of course, that does not help. But I still have physics thoughts on the topic. I don't know things like...do struts exert the same force for the entire length the travel?

Does planovet mean that his hatch weighs 85 pounds total instead of the force pushing down at the strut attachment point is 85 pounds? It's a big difference.

It's all leverage. 85 pounds could easily exert 300 pounds of pressure when the hatch stick straight out.

I interpret your wording to say that the force exerted at the attachment point of your hatch strut is 40 pounds. If that is the case, either you have a really light hatch or the attachment point is far away from the hinge. But then you would need a strut with a lot of travel.

As for the hinge supporting some of the weight...I'll try to give an example. Say the hatch weight is evenly distributed and is three feet hinge to bottom, and say the strut attaches one foot from the hinge. Then, when the hatch is sticking straight out, the weight is supported by the strut. Actually, MORE than the total weight of the hatch is on the strut, and the hinge is holding the hinge end DOWN, not up. Think of it like holding a hammer straight out in your hand loosely. Your index finger is the fulcrum and the heel of your hand is holding it down keeping the heavy end from rotating down. It makes more sense if you think about a heavy hammer. Now if the strut attachment point is two feet from the hinge, then the hinge supports some and the strut supports less but you need a really long strut.

I thought I read somewhere you can get the force applied by the mounting point by attaching a stick there pointing down and let the weight press against a bathroom scale located about where the other end should attach. Most likely, the value will change based on how far open the hatch is and the more it is horizontal, the more force needed to hold it open.

[48Rob]

My hatch is around 50-60#, and I ended up using 100 or 120# struts.

I tried lighter ones, and they held it open, but the slightest bump would bring them down.

Rob

[bbarry]

It's all leverage. 85 pounds could easily exert 300 pounds of pressure when the hatch stick straight out.

I interpret your wording to say that the force exerted at the attachment point of your hatch strut is 40 pounds. If that is the case, either you have a really light hatch or the attachment point is far away from the hinge. But then you would need a strut with a lot of travel.

I thought I read somewhere you can get the force applied by the mounting point by attaching a stick there pointing down and let the weight press against a bathroom scale located about where the other end should attach. Most likely, the value will change based on how far open the hatch is and the more it is horizontal, the more force needed to hold it open.

8, welcome to the forum! I hope you find lots of useful information and people on here like I have!

I did exactly what you talked about in your post. I took a piece of scrap and placed it under the hatch approximately where I wanted to mount the spring. It ends up being about 20" out from the hinge. The other end on a bathroom scale read about 40lbs.

I don't think that a hatch could ever exert more linear force that it actually weighs. I will agree that a 85 lb hatch could probably exert 300 ft-lbs of torque (twisting force) acting directly on a pivot point.

I think I know my problem. I've been calculating linear force as if the spring were stationary and simply lifting the weight upward. In fact, the spring will be mounted so as to face down/out first. It will press outward on the harch attachment point and require significantly more force to raise the hatch by applying force in this direction. Duh!

Please forgive the crudeness of the drawing, it's the best I can manage on my laptop with Paint.



This would explain why folks like Rob and Planovet got good results with the larger capacity springs. Thanks for talking me through it everyone! I think I'll give the 125lb springs a try and see how that works.

Brad

[8]

I don't think that a hatch could ever exert more linear force that it actually weighs. I will agree that a 85 lb hatch could probably exert 300 ft-lbs of torque (twisting force) acting directly on a pivot point.

It depends on the mount point of the spring. If it is off center and closer to the hinge as it is in your drawing, then it will. Try the same bathroom scale setup, and with the hatch sticking straight out hang a weight on the bottom of the hatch. If the weight is say 8 pounds (gallon jug of water) then the scale reading should increase possibly three times that if the mount point is one third of the way from the hinge.

In the end, I would over-rate them for reasons others have mentioned. A wind could blow it down. And if it is too much, you can always add a weight to the bottom to compensate. Another option is to use the springs just for ease of lifting and add a stick of some kind to latch it open so no wind could possibly blow it closed.

[Juneaudave]

I think the struts on mine are 100's and they work fine when it is warm, but I need to use a prop when its cold. Seems like getting something a little heavier than you think you need is reasonable for a variety of reasons...especially if you are just guestimating the strut location...

[madjack]

...errrrr, have ya seen these threads...........
http://tnttt.com/viewto ... 8203#88203
http://tnttt.com/viewtopic.php?t=11210
...lotssa intersting/helpful info here.......
http://tnttt.com/viewto ... s&start=15
madjack

[bbarry]

...errrrr, have ya seen these threads...........
http://tnttt.com/viewto ... 8203#88203
http://tnttt.com/viewtopic.php?t=11210
...lotssa intersting/helpful info here.......
http://tnttt.com/viewto ... s&start=15
madjack

Yup, read through most of these, thanks! I'm pretty sure how I'm going to mount the springs, just wasn't clear on the force needed, but it makes more sense now.

As an aside, SUV hatches mount in both orientations, depending on the model. Here's a Jeep Commander hatch set up similar to how I'm going to mount mine (close to the hinge on the body, and further out on the hatch. Interestingly...I have seen ZERO automotive springs mounted with the plunger up. I buy cars for a living and literally look at thousands of vehicles a month. I'm guessing this is done for a reason (lubrication of the seal?). So, mine will be going in plunger down as in this photo.



Thanks for everyone's help, I'll post pics over on my build thread once I have them up and in.

Brad

[planovet]

I have seen ZERO automotive springs mounted with the plunger up. I buy cars for a living and literally look at thousands of vehicles a month. I'm guessing this is done for a reason (lubrication of the seal?).

Yep, that's the reason. Here is an exerpt from the McMaster Carr website:

Note: All gas springs contain pressurized nitrogen gas as well as hydraulic fluid that lubricates the seal and provides slam-resistant damping at the end of the stroke. To avoid loss of force, you should store and install gas springs rod-end down.

And I mounted mine like you plan to. It works great.