Raising/opening a wooden toy chest door using Torsion springs or constant force springs 3

[Franko314]

My apologies since I am an EE trying solve a mech problem which I thought was easy and it's not. And it has definitely been decades since MECH 230 at NJIT.
The toy chest door/lid is a rectangle of 14" x 18.5" and weights 1.67Kg
The axis/hinge is a threaded rod 10mm in diameter. I attached a wooden disk (Dia. 1.75") sort of as a knob to open and close the lid. Very hard to do by
hand. I calculated the torque required by assuming 1.67Kg on the center of the rectangle times 9.25" = 38.61 N.M
Since my wooden disk had a diameter of 1.75" (44mm), I caluculated that I needed a force of 175.53 N applied on the knob to raise the lid.
I want the normal position of the lid to be in the up/open position. I connceted up 180N of gas springs using a S.S. rope to the outside of the disk (knob) It not only raised the lid, it almost knocked it off of its axis. So much for my tourqe calculation. I experimented and found out that I can do it with a 100 N gas spring. So I want my lid to stay up all the time unless someone closes the lid. Once it is closed the spring can not raise it because I used a kitchen cabinet door (push release) latch to prevent the spring from opening.

https://www.amazon.com/gp/product/B07G766JN1/ref=ppx_yo_dt_b_asin_image_o05_s00?ie=UTF8&psc=1

The idea is once someone pushes on the closed toy chest door, the latch will release and the toy chest door will go back up to the full open position.
I desperately need help on this. May be a ratchet design with a Pawl and missing teeth on the ratchet gear????

The gas spring was just to try things out. It's too big. I need to build this using Torsion springs. I don't know what size/specifications Torsion spring to use, given that I can raise the lid with 100N applied on the perimeter of the 1.75" dia. disk. How do I dampen the Torsion Spring so that it does not slam up. Last question can and I and wouldn't I be better off using constant force springs. Oh, I want the lid to open up slowly (10 seconds)
Thank you for any assistance or advice.

Not sure if this is allowed. For those of you who are consulting mechanical engineer (as well as others I guess), Obviously I am not doing this for a toy chest. The end product is being patented. I do/will need one or two real mechanical engineers to make this happen.

 

[3DDave]

You face two problems.

One is that you did not calculate the energy required to open the box lid. As the lid gets higher it takes less energy for every degree of opening, which means it requires less and less force as it opens. The nearly constant force of a gas strut means that it continues to provide the same amount of energy input throughout.

Gas struts can have internal orifices to control extension rate; check with a gas strut supplier.

Which leads to the second problem - a constant force spring is what you already have and it's not what you need. If you use one, you will need an external damper, particularly if you want it to move so slowly. A torsion spring can have decreasing torque as it unwinds, but still will require a damper to move so slowly.

Someone may make escapements to regulate such a thing but I don't expect one will be off-the-shelf to fit this design - example is the mechanism that controls the rate at which weights on grandfather clocks drop.

 

[Franko314]

Thank you 3DDave.
Did not think about the non-linear force.
Adding external damper is definitely not a problem. Looks like my best bet is to go with a Torsion spring. The question is how do I spec out the Torsion Spring? What Torque? If I have to, I can change my spec and go up in 5 seconds instead of 10

Any ideas as how build the catch and release mechanism. Is there a gadget out there that works better than kitchen cabinet catch

 

[desertfox]

Hi Franko314
Firstly I am struggling to understand why a door that weighs only 1.67kg would be hard to open by hand?
however to fit a torsion spring to open the lid the first consideration is the angle of movement from the door closed position to the door fully open position? That then determines the rotational movement of the torsion spring, after that the length of the torsion spring arms will depend on the forces you wish to apply to the lid and its position within the lid ( i assume you want the torsion spring fitted round the hinge axis).
You could make the spring strong enough to open the lid all the way or just take the weight and allow someone to open it by hand. I would need to see a lot more detail in order to design said spring but you should contact a spring manufacturer and they would probably design it for you and to protect the design ask them to sign a confidential agreement that they won't pass any details of the design on. (thats always still a risk but at some point you have to trust someone)



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[r13]

Please do not mix units, it is confusing. Below is the torque on the spring.

 

[TheTick]

You CAN calculate the force and energy required. I spent nearly four years solving this exact problem a hundred times over.

Start with a graph. Excel is great for this. Plot two curves:
[ol 1]
[li]Closing torque about hinge axis due to gravity vs angular position[/li]
[li]Spring torque vs angular position[/li]
[/ol]
5 degree interval is usually sufficient.

Wherever spring torque is greater, the spring is pushing the lid up. Points where the curves cross are resting points.

For torsion springs, the process is simple. Torque is linear, calculated from angular preload and spring constant.

Gas springs or other compression/tension springs are dependent on the endpoints. Torque is the product of force and perpendicular to distance to axis. This distance will change as the lid opens, so distance needs to be recalculated/remeasured for each position. It's a triangle with two known legs and known interior angle, so it's solvable. Formulae available in Machinery's Handbook or elsewhere on interweb.

As for damping, many gas springs have built-in dampers. Some may be adjustable. Otherwise, you can add dampers as a secondary element.

Lastly, shop around. The product you want may well already exist. Try Rockler and Southco.

 

[moon161]

Gas struts

https://www.techbriefs.com/component/content/article/tb/supplements/md/features/articles/34117

push push or push to open latches

https://www.mcmaster.com/push-push-latch/push-to-open-grab-latches/

https://www.mcmaster.com/push-push

in-latch/push-to-open-grab-latches/

Simplified cald. the direction of the gas strut is not always vertical, but this can be a start.

 

[MintJulep]

Something like

https://www.sugatsune.com/product/heavy-duty-lift-assist-damper/

 

[MintJulep]

Or this

https://youtu.be/D20muyb6New?list=PLoRqbj6guG5e6vacpow7bXpa5RJaldf8x

 

[Franko314]

Thank you @desertfox
It is not hard to open a 2 Kg door by hand, unless you want to grab it by the hinge to do it. Effectively making your torque ZERO.

Thank you @r13 for calculating the Torque required. Sorry for mixing units. When I built this using a gas spring, I arrived at the same Torque number. I am not sure if I did the rest correctly. I needed to have my gas spring hanging vertically in the back. Since my hinge/axis of rotation is only 10mm in Dia, I would need a Gas Spring with a force of 780N if I were to strap the gas spring to the 10mm hinge/treaded rod axis. So I stuck a 44.5mm diameter disk to the end of my rod. This way I ended up needing 177N. It turned out that when I tried it, 177N was too much so I manually adjusted it to 100N. Someone explained earlier that I had not taken into account the energy.

Anyway the Gas Spring idea is not feasible, because it physically does not fit my requirements.. I need to go with a Torsion Spring.
QUESTION IS: DO I GO WITH THE 3.9 N-M Torque @r13 has calculated or since Torsion springs come with different length legs somehow that affects the Torque.

@TheTick Thanks for the explanation and making graphs. One point, I am not really building a wooden toy chest. I can't reveal what I am building. However, the Toy Chest example pretty much covers what I need to do. There are somethings that I cannot do like the good suggestion by @moon161 for the push-push latches. I can't install them in the front of the toy chest. There is no toy chest.

https://www.mcmaster.com/push-open-latches

These are amazing designs. Ever open up one of those Push-to-Open Roller Latches. Even after opening it, it's hard to figure out how it works.

I need to use a ratchet and pawl mechanism. The are some Pawl shapes that reverse themselves. They are installed on a ratchet gear that is missing half of it's teeth. You would have to open up a retractable extension cord or something like that to see it. If anyone knows of a ratchet and pawl pair that has that ability that I can buy, that would save me a lot of time. I am really resisting from bringing my Electrical Engineering brain into this and solve the problem with a small electromagnet. I would need a battery and I would not have a purely mechanical design. I am building something that has to be nearly maintenance free. Changing batteries is just no good. There has to be a purely mechanical was to move a pawl out of the way so that the toy chest door can go back up.

@mintJulep Thank you for your suggestions. As I wrote earlier, I am not really building a Toy Chest with soft opening and closing lid. We can just buy one of those at IKEA.

If anyone has a suggestion for a spring vendor/manufacture who may be willing to work with me to specify a Torsion Spring, I would appreciate it.

 

[desertfox]

Hi

Not sure where you are in the world but a google search will bring up loads of spring manufacturers, here’s one I found

http://holtsprings.co.uk/

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MintJulep]

It seems to be that you are fixating on a specific solution before you have fully assessed your actual (secret) requirements.

The company that I linked to makes many other products. Poke around for inspiration.

Generically, a "torque hinge" is a hinge with a torsion spring built in.

A torsion spring also (nor a torque hinge) will get you the slow-soft close feature that you desire.

 

[TheTick]

Your product is secret. Your problem is ordinary.

 

[moon161]

Your problem with a torsion hinge is that the spring moment is nearly constant but the moment from the weight of the lid is w*l*cos θ, as shown in my sketch above. As you found it will accelerate the lid more and more as it opens.

You could use various mechanisms with a linear or rotary driver that have the force diminish as it opens.

A real simple one is just a torsional spring that is relaxed (zero torsion) when the lid is vertical.

While you're at it make sure that a jack in the box isn't considered prior art ;-)

 

[r13]

Here is a simple solution. Some modification is required to keep the lid closed tight with the box. You can adjust the preload, T, to the level tat fits your application. You need two upper hinges (not shown). You'll also need a locking system, that consists of a circular track plate with slot and pin to guide the rotation of the lid, and lock it at 0° and 90° with latches.