Help with auto-closing a latch
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 :)
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
RE: Help with auto-closing a latch
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
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Help with auto-closing a latch
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
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
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
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
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
RE: Help with auto-closing a latch
RE: Help with auto-closing a latch
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.
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?
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:
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.
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...
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.
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.
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
RE: Help with auto-closing a latch
Good luck.
RE: Help with auto-closing a latch
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.
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
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
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
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
RE: Help with auto-closing a latch
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
RE: Help with auto-closing a latch
Yes... or very near it. The torsion spring on the release lever is not very strong.
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?
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
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
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
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
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?
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.
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?
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
RE: Help with auto-closing a latch
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
RE: Help with auto-closing a latch
RE: Help with auto-closing a latch
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Help with auto-closing a latch
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
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
RE: Help with auto-closing a latch
450N * centerline distance to the pivot axis = torque required.
Ted
RE: Help with auto-closing a latch
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.