Help with auto-closing a latch 4

[smartiep]

First, I have to express that I'm not an engineer and I struggled to find an appropriate place to ask questions like these. I hope this is the place.

Right now I simply have an idea to address a problem with a product I own. I know it frustrates other owners, so ultimately I'd like to market a solution. I'm at a VERY early stage of just seeing if this is possible and what might be the way to tackle it.

In summary, there's a heavy lid which is supported by gas struts and when closed there is a latching mechanism holding the lid shut tight. This latching mechanism is very stiff and requires a lot (some would say too much) force to fully close. I envision installing a motor that will somehow "grab" the lid and pull it the final 20mm to its fully latched position. It takes very little force (thanks to gravity vs. the gas struts) to lift and lower the lid, but that final 20mm requires about 450N (measured by adding weights to the lid until it latched and summing those weights).

At this point I'm not overly concerned with how the "grab" will take place. I'm initially concerned with if there's going to be a suitable motor for the task, which will fit in the very limited space. But I can't even begin to search for such a motor until I understand the spec requirements of said motor. My limited knowledge is the problem here and I'm hoping for some help.

I carefully measured the latch mechanism and put it into a 3D model, which is below. Not shown are the strong spring (between points A and B) and the coil spring which bias both of the rotating armatures in the clockwise direction. The right armature is the latch - and what we're primarily concerned with - and the left one is the catch/release mechanism.

I'm envisioning a motor along the same axis of rotation as the right armature, which engages the edge near point C and forces it counterclockwise into the locked position (~47 degrees).

Like I said, it takes 450N of downward force to overcome the friction of that armature, the strong spring, and all other applicable forces. I have no idea how to translate that into torque required for a motor doing what I describe. I'm sure it is pretty basic, but I get lost simply at the units (Newtons vs. Newton-Meters).

Can someone provide some help? Is there more information needed? I am most interested in solving my problem as stated rather than looking for clever "have you considered doing this?", as at this point this is more a learning exercise than a practical application. But I thank all input

 

[handleman]

Umm, question. You stated there is a "strong spring" between A and B. Its purpose appears to be twofold: intentionally supply some resistance to latching, and pop the lid up somewhat once the release is pulled. Just replace the strong spring with a weaker one.

 

[smartiep]

So, if you took a screw driver with round shank similar to the bar that engages the latch, and pushed it into the latch as if the lid was closing, would it still take something like 450 N / over 100 lbs of force to latch the latch?

I actually tried exactly that. I failed to measure. I just didn't successfully find a way - which what was at hand - to take the measurement. But I'd guess "yes"... it is very hard to engage. I'm a pretty strong guy and while I could do it, I probably couldn't do it 5 times in a row Let me put it this way: if I remove the spring, I can push the screwdriver through with on more force than it would take, say, to open a lever style door handle. It feels to me like the vast majority of the force is coming from the spring.

With the unmarked release lever over by A pulled out of the way, does it still take ~ 450 N to move the latch to the closed ( but now un-latched) position ?

Yes... or very near it. The torsion spring on the release lever is not very strong.

With the A-B spring removed, do all the links rotate freely?

Fairly. The release lever is biased with the torsion spring and there is friction on the other one (but otherwise free). Perhaps I should attempt to measure each component's contribution? Or does that matter?

If you insert dimes, pennies or small washers into the open spring coils with the latch latched, and then release the latch, is the spring loose on hooks A and B ? With the spring stuffed with dimes and washer does the latching force decrease significantly, perhaps even to a satisfying degree?
If so, the Tmoose wedge-o-matic© is available for a mere 20% of the gross sales.

Hey now! Don't over-solve the problem If it doesn't have a motor and some electronics giving it some mystery and a satisfying "click", what are people going to pay for?

 

[smartiep]

If you are not tied to the idea of a motor you might consider some short ratchet strap style tie downs like these

But then I'd need a motor to pull the straps Unless you mean attach them on the outside, and that's not practical. I can't do anything to the outside of the box. I have a small amount of room on the inside in "front" of and below the latch.

As a one-off... if this were just my box and I was too weak to close it... I'm totally on board with your thoughts here. I'm sure there's plenty of low-tech, low-cost solutions on a one-off basis. There's guys that have welded a bar to the top for leverage, for example. Its just not what I'm looking at doing.

 

[smartiep]

Umm, question. You stated there is a "strong spring" between A and B. Its purpose appears to be twofold: intentionally supply some resistance to latching, and pop the lid up somewhat once the release is pulled. Just replace the strong spring with a weaker one

New explicit criteria: No changing the spring!

No doubt, you (and everyone else above) are right. The manufacturer probably over-spec'd that spring and replacing it probably *is* the "right" solution. Simple, cheap... and completely unmarketable. Plus, people have been complaining about this issue for at least 5 years and the manufacturer has changed the box's design at least twice and kept the same latching mechanism with the same spring and the same issue... I have to assume that's for a reason. It may simply be to pop the lid. I removed the spring and had a hard time getting it open with just fingers. But it could be (for all I know) necessary for its fire rating or the internal emergency release requirements or something else.

 

[handleman]

Your reluctance to mention the actual application, combined with references to the manufacturer's reluctance to address the issue and your mention of fire ratings and emergency release requirements make me suspicious that you may open yourself up to lawsuits if you sell something to modify this.

That said, unless you have a lot of space, the motor you are looking for likely doesn't exist. Plus you would need some kind of engaging clutch because in order to open the latch you'd have to back-drive the motor. You may have better luck pulling opposite the spring with an automotive door popper solenoid - Commercially available with lots of pulling force. But you're not interested in that.

 

[smartiep]

Your reluctance to mention the actual application, combined with references to the manufacturer's reluctance to address the issue and your mention of fire ratings and emergency release requirements make me suspicious that you may open yourself up to lawsuits if you sell something to modify this.

The reluctance is solely based in focusing discussion... apparently that has failed. I've never understood some peoples' desire to know every mundane detail in order to answer a question. The actual application wouldn't mean anything to anyone here, and on the extreme off chance it does... yeah, I suppose I don't want to show my hand. But, fine...

It's a box. A purpose-built box for (primarily) remote monitoring equipment used in mining and other industries as they attempt to comply with environmental regulations. It is also used by community groups for storing sporting or other equipment, though it is overkill for that. It is fire, bear, vandal and weather resistant. I suspect it is bomb resistant too, by the looks of it. It is smooth on the outside (I think for bears and vandals). It is a big box, which is why it has to have an internal emergency release in case someone gets the idea of shutting someone inside it. They are usually green. Is there any other details you think I should volunteer?

That said, unless you have a lot of space, the motor you are looking for likely doesn't exist.

I don't have a lot of space. Because I don't fit (well) inside the box, it is difficult to measure accurately. Not impossible, but difficult. Let's say "inches" (and not many). That's primarily why I'm here. My non-educated thought is that I need a motor that is rated for 450N but I'm not really sure how the math works between 450N force pushing down and the rotational torque required to affect that... is it the same? Higher? Lower? Unless it is significantly lower, then yeah... I can't find any motors even close in the size range that might be possible. That's why I *thought* this was going to be an easy question/answer lol. I *thought* I provided more than enough details in the original post to get a "you need to find one rated for 450N" or "450 * pi divided by the radius of the moon's largest crater" or whatever.

Plus you would need some kind of engaging clutch because in order to open the latch you'd have to back-drive the motor.

Why would that be? There's that big honking spring that does that, no? But even so, motors go both directions pretty easily, don't they?

You may have better luck pulling opposite the spring with an automotive door popper solenoid - Commercially available with lots of pulling force. But you're not interested in that.

Correct... the point of THIS thread was simply to get an answer on the motor, not look for alternative options. That said, this thread has already been derailed. The electromagnet idea was interesting, and now I have something else to Google... thank you. I never said off-topic discussion couldn't be useful


My thought was that if the motor idea didn't pan out, then I would look into linear actuators. If that didn't work out, I'd throw out a "any other ideas" kind of plea. And that's where I'd hope to hear about door poppers and magic magnets and such. Just wasn't that far yet. I wanted to start with a motor. Why do I feel I need to apologize for that? Was it really THAT offensive of an idea?

 

[smartiep]

ps can door poppers pull?

 

[btrueblood]

You need 100 lbs of force from a motor, acting over 20 mm or roughly 1 inch of motion. Any DC gearmotor, driving a 1/4-20 threaded rod or similar, would give this much force and then some.

Yes, linear actuators can do this, one like below has more than enough poop:

https://www.amazon.com/Zowaysoon-El...512515047&sr=8-7&keywords=linear+actuator+12v

You didn't say what kind of power is available, but you can search on Amazon to find similar devices operating on voltages from household ac down to a few volts I'd think.

Alternatively, take the same gearmotor shaft that turns the linear actuator screw and instead attach a lever, with enough torque you can generate the required 100 lbs. force - torque = force x lever length is the equation to use.

 

[Compositepro]

The latch shown in your original post looks very much like the hood latch on a car. Their operation is strongly affected by lubrication, or lack of it, or by rust and wear. Just allowing the bar that enters the latch spin freely will greatly lower the forces of operating the latch. Friction is your problem.

 

[eignvctr]

What about a neodymium magnet? Depending on what kind of space you have to work in that could be a solution. You could choose one to simply cut in half the closing force and make that trade for a more difficult opening. If you want more assistance you could devise a way to slide the magnet out of the other magnet or ferrous material's proximity or rotate it to cause it to repel another magnet.

 

[MacGyverS2000]

The reluctance is solely based in focusing discussion... apparently that has failed. I've never understood some peoples' desire to know every mundane detail in order to answer a question. The actual application wouldn't mean anything to anyone here, and on the extreme off chance it does... yeah, I suppose I don't want to show my hand. But, fine...

We ask for the details because, as experienced engineers, we get tired of trying to solve poorly defined problems. We provide answers to the best of our abilities, then the OP comes back with another detail not in the original statement that changes the direction of the solution... and it becomes a circular problem/solution. I watched it happen in this thread. And once the "your solution isn't what I need, so why are you bothering me with it" attitude kicked in, I stopped caring about solving the problem... now I just sit back with popcorn until the next person gets aggravated.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[kjoiner]

I recall some Cadillacs (and maybe other cars) that had trunk lids with a mechanism that would pull them shut. Instead of "slamming" it shut, you pushed it down until a mechanism grabbed it and pulled it shut. My Mom almost got her fingers caught in one on a rental car so I'm not sure if they're still using them but it would perform the function although I'm not sure about the force. 450N (approx 100 lbf) is a substantial force.

As far as balancing the weight of the lid, check out Guden, Ace Controls and other damper companies to make sure the gas springs are not over sized. If using a torsion spring to balance the lid, check out companies like Lee Spring, etc.

There are a lot of slam latch companies that make latches that can hold the lid shut without excess friction etc. Rotary latches like those in automotive applications come to mind. Check out Eberhard mfg. and Southco mfg. for lots of options on slam latches.

Magnetic locks are simple, but they are not tolerant of separation between the magnet and the armature plate. They need to touch or you'll lose your holding force. Seco Larm makes lots of mag locks. I have a small one on my desk from another project (E-941SA-80Q) that holds 110lb at 24VDC and is only about 1-1/2" x 3". It pulls 120 mA at 24VDC

Kyle

 

[MintJulep]

I removed the spring and had a hard time getting it open with just fingers.

Well now you're on to something.

It's way easier to invent a push up to help open thing.

 

[IFRs]

I did not look at the drawing, but read the discussion. I see a small motor turning a short tapered shaft with threads cut into the shaft. To the lid you have added a threaded nut of some length or positioned in such a way to make it work, perhaps inside the spring. The lid comes down to its natural low but not closed position. The tapered shaft goes through the nut and contact is made between the nut and shaft. If needed, some action either lowers the nut or raises the tapered shaft as it begins to turn. The shaft engages the nut and draws it down with the lid. It won't take much torque to develop a large closing force. The motor is stopped by a torque limit or position switch. When it is time to open the lid the motor is activated in the opposite direction.

 

[hydtools]

The closing force causing the latch to rotate will act through the latch loop connected to the lid and in a direction colinear with the centerline of the slot in the latch plate. The torque or moment applied to the rotating parts is the product of the force causing rotation multiplied by the horizontal distance from the slot centerline to the rotation center of the latching parts. The torque required of a motor turning those rotating latch parts will be equal to that product. The finger feature of the rotating latch will have to pull the lid closed in place of an external push on the lid.
450N * centerline distance to the pivot axis = torque required.


Ted

 

[LittleInch]

smartiep,

here's my take on this having read the interesting discussion and looked at the drawing etc.

It's a latch mechanism which is existing and you don't want to fiddle about with too much. Ok I get that.
It's still interesting to see how the car designers have done it though.
something like this

https://newatlas.com/slamstop-car-door-closer/33924/#gallery

or this

https://auto.howstuffworks.com/power-door-lock3.htm

"power closure" seems to be magic search words


Your plan A was to add a rotational force (torque) to the main closure catch at around point C to rotate the catch 47 some degrees counter clockwise looking at your drawing.
Assuming point C is say 100mm from the axis and you need a peak of 450N to close this damned thing, you need around 450 x 0.1 = 45 Nm of torque if the axis of your motor is the same as the axis of the latch.

That's a lot for a small motor without lots of gearing which eats power also.

however you seem to have jumped straight to this without looking at it from an engineering view.

What you really need to power assist this latch is a force at point C or somewhere on the locking mechanism to rotate the latch. Now a motor could do it, but as said isn't the best. So consider other ways to impart the force.
Maybe attach a lingage to point C and a linera actuator
maybe a cable or linkage to point B and the linear actuator or maybe a small motor which winds in a wire from point B
Or if there is room, some sort of wedge which connects to the locking bar and goes left gradually forcing the latch closed. this could be powered by a worm gear. once locked it then needs to re-wind to allow you to open it.
Or maybe add a third ring to your latch with a gradual hook / circular wedge device so that it turns say 180 degrees to effect 20mm of movement and power that.
Lots of options, but you need to free your mind a bit fomr the motor concept as you first thought. IMHO.






Remember - More details = better answers
Also: If you get a response it's polite to respond to it.