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

 

[Nescius]

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.

 

[smartiep]

Thanks for your response.

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.

 

[MacGyverS2000]

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

 

[smartiep]

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?

 

[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?)?

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.

 

[Nescius]

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.

 

[MacGyverS2000]

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

 

[Tmoose]

"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.gif

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.jpg

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.

 

[1gibson]

Do you have room to grab it with an electromagnet?

 

[MintJulep]

How about simply less strong springs?

 

[smartiep]

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.

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.

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?

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:

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.

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...

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.

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

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.

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.

 

[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.

 

[Nescius]

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.

 

[smartiep]

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:

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.

Good luck.

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


Thank you BrianE22 for the non-derailing response

 

[3DDave]

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.

 

[Tmoose]

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.

 

[MintJulep]

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.

 

[1gibson]

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.

 

[hendersdc]

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

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.