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# Help with auto-closing a latch4

## Help with auto-closing a latch

(OP)
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 :)

### RE: Help with auto-closing a latch

I know you indicated that you'd rather add a motor, but is there a reason the latch needs to be so stiff? A different spring might eliminate the need for a motor, but also reduce the motor requirements, if you still want it anyway.

### RE: Help with auto-closing a latch

(OP)

If I was designing the box, I'd agree. What I'm hoping to do is help owners deal with the thing they already have, not reinvent the thing. I'm thinking the spring was chosen for a reason. I'd *prefer* to leave it alone and only add to the design, rather than replace. But that's only if practical. If the numbers I'm mentioning are simply too onerous for a reasonably priced (say < $30 in volume) and small (few inches cubed) motor then I may have to look at replacing the spring or abandoning the project. ### RE: Help with auto-closing a latch This problem has been solved for years... take a look at automatic trunk latches (my Corvette has such a latch). I apply minimal force to get the latch to touch the mechanism, then a motor takes over and pulls it completely closed. Dan - Owner http://www.Hi-TecDesigns.com ### RE: Help with auto-closing a latch (OP) Thanks, but I'm trying to work with an EXISTING mechanism. The fact that some other mechanism does what I want is great for that mechanism, but how's that help here? Unless your suggestion is to buy corvette trunk latches and hope they happen to fit as a direct replacement? But sure, yes, that's a good example of what I'm trying to achieve. Now... how, given the parameters? ### RE: Help with auto-closing a latch Looks like a fun FBD to solve. I don't get the part about the motor that "engages the edge near point C"? Can you explain that some more (how does the motor shaft drive the edge?)? By "coil spring" do you mean a round torsion spring around the "B" link shaft? Torque is force times perpendicular distance between force and pivot point. If the spring put out 10 N and the distance from the point the spring is mounted to the pivot shaft were .01 Meters then the torque would be 10 x .01 = .1 NM. ### RE: Help with auto-closing a latch #### Quote (smartiep) What I'm hoping to do is help owners deal with the thing they already have, not reinvent the thing. I'm thinking the spring was chosen for a reason. I'd *prefer* to leave it alone and only add to the design, rather than replace. You're wanting to market a solution. Adding a motor, mounting hardware, wiring, controls, drilling holes, etc. are not minor things. I don't know what the nature of the "box" is, but the latch is probably an off-the-shelf item from another manufaturer, or a variation thereof. Don't be afraid to dig deeper, because the motor solution is already quite deep. I don't think MacGyverS2000 is advocating that you literally adapt Corvette parts, but rather some part that's already out there. It's solid advice, and where I'd start. If the goal is a sub$30 solution, it might be your only path.

Our friends in SE Asia make an astonishing variety of seemingly random stuff for even more astonishing costs. Astonishingly low, that is. Google every combination of keywords you can think of, improving your search terms as you learn more. You'll find places like Alibaba.com allow you to source some weird stuff. Start playing.

### RE: Help with auto-closing a latch

smart,

If you look at ANY automatic trunk latch, not just the Corvette's, maybe you'll get a better understanding how it's commonly done, and done on the cheap. It'll be a hell of a lot faster than us trying to teach you how to engineer it blind.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Help with auto-closing a latch

"This latching mechanism is very stiff"

I imagine you are loathe to reveal details to prevent sniping of your invention, without drawings and pictures or at least a much more detailed description, the interpretation is up to me.
https://media.giphy.com/media/m3kym20R9nXZm/giphy....

Does the lid latch if simply dropped from about 6 inches open, or slammed a bit?

I am speculating it is not the latch that is stiff, but some springs in the system.
Maybe the gas struts.

I
s there Maybe a safety spring built into the latch ?
http://www.autometaldirect.com/images/4/W-829_800....

Is there maybe a thick gasket under the lid?

Are there Maybe rubber bumpers that keep the lid at a certain height when closed, which must be compressed directly, or force the lid to bend in order allow the latch to, well, latch?

If so, reducing the spring rate or the required spring compression would offer an immediate reduction in the required latching force.

7 down 13 to go.

### RE: Help with auto-closing a latch

Do you have room to grab it with an electromagnet?

### RE: Help with auto-closing a latch

How about simply less strong springs?

### RE: Help with auto-closing a latch

(OP)

#### Quote (BrianE22)

Looks like a fun FBD to solve. I don't get the part about the motor that "engages the edge near point C"? Can you explain that some more (how does the motor shaft drive the edge?)?
I assume you can see the drawing (it sounded like Tmoose couldn't)? If so, you see the circle between approximately B and C? That's the axis of rotation for the existing armature. So my thought was to install a motor (how is left for future problem solving) about that same axis, with some sort of "plate" with an off-set pin protruding out until it can engage at C. The motor has to be pretty small diameter, however, to fit there without interfering with the A-B spring or the protruding feature at B. That's my big question... can I possibly find a high enough torque motor.

#### Quote (BrianE22)

By "coil spring" do you mean a round torsion spring around the "B" link shaft?
Yes round torsion spring, but not around the "B" link shaft... rather the shaft of the armature on the left (closest to A). That left armature is the release. It is biased by the torsion spring in a clockwise direction, holding the sprung right armature in place. A cable at the top of the release pulls it counterclockwise and the "B" link then rotates rapidly (due to the strong spring) clockwise.

#### Quote (BrianE22)

Torque is force times perpendicular distance between force and pivot point. If the spring put out 10 N and the distance from the point the spring is mounted to the pivot shaft were .01 Meters then the torque would be 10 x .01 = .1 NM.
OK, thanks. I suppose then if there were no gravity, or friction involved then this calculation would be easier. However, I measured a total of 450N required to completely latch, so I suppose that takes into account everything, right? And that's what I'd have to achieve with my solution, right?

If so, I can't find any motors even close. Like orders of magnitude off. So is this just not doable?

#### Quote (TMoose)

I imagine you are loathe to reveal details to prevent sniping of your invention, without drawings and pictures or at least a much more detailed description, the interpretation is up to me.
Not at all. I'm not worried about IP. This is an incredibly niche thing. Without my nuanced access to the market, it would be useless. Your post suggests you didn't see the drawing I posted and I thought I was quite liberal with the details. Of course I can't post every possible thing initially. Thank you for asking specific questions:

#### Quote (TMoose)

Does the lid latch if simply dropped from about 6 inches open, or slammed a bit?
It does not. As mentioned, it is supported by gas charged struts, so dropping it does nothing at all. Slamming it is difficult (said struts + air pressure) and no, I haven't been able to latch it by doing so.

#### Quote (TMoose)

I am speculating it is not the latch that is stiff, but some springs in the system.
Maybe the gas struts.
Well, the struts lose their mechanical advantage at a few degrees from closing, so I don't think they're an issue. The spring *is* the issue, which was stated in my initial post (was it not?)... and that is what I mean by "stiff". well, that + friction and sure, maybe the struts or...

#### Quote (TMoose)

Is there maybe a thick gasket under the lid?

Are there Maybe rubber bumpers that keep the lid at a certain height when closed, which must be compressed directly, or force the lid to bend in order allow the latch to, well, latch?
There is, but again that must be considered for the total force. It can't be changed. Again, I'm not so concerned with WHY it is hard to close, just that it IS hard to close.

#### Quote (TMoose)

If so, reducing the spring rate or the required spring compression would offer an immediate reduction in the required latching force.

#### Quote (MintJulep)

How about simply less strong springs?
Changing the spring was already addressed. I'm not dismissing it - and I've been experimenting with that - but ideally I'd like to leave that alone. I would like a bolt-on solution not requiring users to modify anything. As I said before, I suspect the spring is as it is for a reason. I wish to emulate a stronger person, not affect a weaker box.

#### Quote (1gibson)

Do you have room to grab it with an electromagnet?
That's novel. Do you think that would be powerful enough? How big does an electromagnet have to be and how much current is required? I'll have to look into that. Off the top of my head, I worry about it being too violent/sudden.

To those that say "go get some existing [thing]"... you didn't understand the post/requirements. There's pretty well zero chance that an existing auto-latch mechanism exists - in China or elsewhere - which is going to fit the bill and happen to fit in the space with perfectly lined up bolt holes, etc. If the point is to see how they work... well of course that's where I started. That's the whole point - to replicate that. But I have to do so with what I'm presented with. The manufacturers who utilize those have the benefit of having designed their "box" (trunk in their cases) to work with such a device. I'd hoped given the title of this place and the drawing I provided and specifications, etc... that I'd get better advice than that. What you're essentially saying is, "go back in time, have the people not purchase what they purchased, become that company and make it right the first time". The whole point here is a retrofit, not a redesign or a new product. I thought I was clear on that, and sorry if I wasn't.

If this simply isn't feasible, I'm certainly open to that. But if its just "difficult"... I've overcome difficult before. I've brought several products to market that are retrofit "add-ons" for various products to fix some design flaw (IMO) of some thing. I started this by saying I'm not an engineer, and that's true. I don't have specialized training. But I'm not an idiot, nor am I inexperienced. When/if the project progresses to the point that real engineering is required, I'll look to hire an appropriate person for the task. Who knows, it may be someone here! But at this VERY early stage, that is premature and I just want to see where the idea goes in terms of feasibility.

Thank you very much for your input.

### RE: Help with auto-closing a latch

Seems like the latch is misadjusted. Probably the user forum for that make and model has suggestions for fixing it. 100lbf is too high for a trunk lid.

### RE: Help with auto-closing a latch

#### Quote (smartiep)

I'd hoped given the title of this place and the drawing I provided and specifications, etc... that I'd get better advice than that.

Good luck.

### RE: Help with auto-closing a latch

(OP)

#### Quote (3DDave)

Seems like the latch is misadjusted. Probably the user forum for that make and model has suggestions for fixing it. 100lbf is too high for a trunk lid.
Ok, I obviously wasn't clear because we keep dancing around this part of my original post:

#### Quote (Me)

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.

This isn't a one off. Nor is it a trunk... that was an analogy. It isn't a misadjusted latch. It could be argued that it is a product design flaw - I certainly think it is - but they ALL take a lot of force to close. I've only measured one, but I'm sure they're all around the same. The complains are consistent.

It doesn't matter what it is (its a box. With a lid. That latches.), or why it is the way it is. It doesn't matter if it should have used a different latch, or if the spring is too strong. What matters (to me) is that it is too difficult for a segment of end users to get the lid latched and I want to provide something to resolve the problem in the easiest way (to the end user) that doesn't compromise certain (undisclosed, and frankly irrelevant to the thread) criteria.

That's why I presented a specific engineering problem and was hoping for HELP on a VERY specific and preliminary aspect of the solution. I thought I was clear, but obviously not. I want to know how to determine the specs for a motor - as described - to see if this is feasible.

#### Quote (Nescius)

Good luck.
Indeed. I suppose it is just too difficult of an engineering problem.

Thank you BrianE22 for the non-derailing response :)

### RE: Help with auto-closing a latch

Simple answer: Use a motor that supplies enough energy over the time required to do the job. Easy-peasy.

If you want to really learn, buy a copy of "Machinery's Handbook." There is lots of information on designing mechanisms.

Let us in on who is complaining and what they are complaining about. I for one feel I am completely stupid, what with only having done airborne radar mechanics, B-52 tail gun gearbox redesign, a variety of ground military vehicle upgrades and modifications, and ship-board test sets.

### RE: Help with auto-closing a latch

Hi SmartieP,

I saw the cad model image, but must sheepishly admit I did not read the post thoroughly to attach a spring at A and B.

========================================

"The spring *is* the issue, which was stated in my initial post (was it not?)... "

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?

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 ?

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

=========================================

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.

### RE: Help with auto-closing a latch

2
Step 1 of problem solving is to understand the problem.

You have not done that yet. Rather you have jumped directly to step 27 "design an un-thought-out and overly complicated idea".

This rarely works well, but hey, maybe you'll get lucky.

A motor is the wrong device for what you think you want to do.

Saying that you have been clear is different from actually being clear.

There is lot's of good advice and suggestions from very experienced real engineers in this thread, but your attitude makes it unlikely that you will get much more.

Good luck.

### RE: Help with auto-closing a latch

Yes you can easily find an electromagnet with ~100lbf pull or more, the ratings are if it's already contacting a plate so it's more like a pull-off force than a pull-on force. But your measurement was slowly adding weight, and this force will be instant. Go bigger or use two, or get a new force required by dropping some weights from ~1 inch up, it might be ~75% of the slowly add weight. You could probably do it for ~\$150 or less in parts, the higher than expected cost may be offset by the simplicity.

### RE: Help with auto-closing a latch

If you are not tied to the idea of a motor you might consider some short ratchet strap style tie downs like these: https://www.mcmaster.com/#ratchet-straps/=1ajwryr

I am sure you can find cheaper ones, this is just and example. No electricity needed and the only modifications to your box are a couple of eyelets to attach the hooks to. These will easily generate more than 100 lbf tension.

### RE: Help with auto-closing a latch

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.

### RE: Help with auto-closing a latch

(OP)

#### Quote (TMoose)

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.

#### Quote (TMoose)

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.

#### Quote (TMoose)

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?

#### Quote (TMoose)

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?

### RE: Help with auto-closing a latch

(OP)

#### Quote (hendersdc)

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.

### RE: Help with auto-closing a latch

(OP)

#### Quote (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
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.

### RE: Help with auto-closing a latch

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.

### RE: Help with auto-closing a latch

(OP)

#### Quote (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.
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?

#### Quote (handleman)

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.

#### Quote (handleman)

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?

#### Quote (handleman)

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?

### RE: Help with auto-closing a latch

(OP)
ps can door poppers pull?

### RE: Help with auto-closing a latch

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-Electric-Linear-A...

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.

### RE: Help with auto-closing a latch

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.

### RE: Help with auto-closing a latch

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.

### RE: Help with auto-closing a latch

#### Quote (smartiep)

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

### RE: Help with auto-closing a latch

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

### RE: Help with auto-closing a latch

#### Quote (smartiep)

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.

### RE: Help with auto-closing a latch

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.

### RE: Help with auto-closing a latch

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

### RE: Help with auto-closing a latch

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/3392...
or this https://auto.howstuffworks.com/power-door-lock3.ht...

"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.

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