I was told I should ask here. Please note I am not an engineer and I apologize if this isn't an appropriate place, as I suspect this is for engineers to exchange ideas (though I found no "rules" at all).

With no expectation of a high degree of precision, what should I do to mestimate (measure/estimate) the force required to completely close the lid of a large box? It is hinged and latches, much like the hood of a car, and has gas charged struts holding it open. It doesn't take much to get it down - I think the struts only slightly offset gravity - but from there it takes considerable force to fully close/engage the latch mechanism (significant friction plus a strong spring). It is too difficult for the local seniors to get closed, but it is important for theft prevention.

I'd like to put a small motor inside that grabs the latch and pulls it closed that last inch. I realize I'll have a lot to do to design the "grab" and also to consider things like pinch protection... but for now I just want to see if a suitable motor exists. What I don't know how to do is calculate the force.

Can I simply stand on a scale and take the delta when it latches, along with the distance the latch traveled? Intuitively - for my non-engineering brain anyway - I can't really see how the distance matters (if the force is enough to move it an inch isn't it enough to move it a mile?), but the definition for the term "foot pounds" suggests that shorter distance requires less force. So let's say my scale tells me I weigh 200 lbs and when I press on the hood at the point it latches it says I weigh 100 lbs, and it traveled an inch... does that mean I need a motor rated for 8.33 foot pounds? Or am I completely off base?

Thank you for your indulgence.

  • $\begingroup$ "foot-pound' refers to torque, not linear distance travelled. Your motor is pulling linearly, so it needs to apply a certain force and maintain that force for a while. Your problem is converting the motor's torque to linear force via gears, pulleys, etc. $\endgroup$ Dec 1 '17 at 17:28
  • $\begingroup$ Using a motor for this will be tricky to design. Have you considered simply adding a bar/lever out from the lid (perpendicular to the hinges). The longer you make this the more mechanical advantage you will have and the less effort required. $\endgroup$
    – atom44
    Dec 1 '17 at 17:29
  • $\begingroup$ Thank you @mg4w. For a variety of reasons, I can't modify the outside of the box. There are plenty of practical one-off solutions (10 seniors leaning on it at the same time), but this is a problem with this setup across multiple locations so I'm hoping to come up with an elegant - and marketable - solution. Can we ignore the trickiness of design for a bit and see if it will be possible based on the force required? If there's no motor small enough to do the job, then I'll just stop now. $\endgroup$
    – bcsteeve
    Dec 1 '17 at 17:54
  • $\begingroup$ Thanks @CarlWitthoft. Correct that it is travelling linearly, but don't worry about how the conversion will occur. At this point I want to know if a motor exists that can do the job (ie. small and powerful enough... it is the "powerful" I'm currently working on). According to wikipedia, "foot pound" is the energy required to move a 1 lb object 1 foot. So doesn't that translate into needing less torque for moving something a shorter distance? $\endgroup$
    – bcsteeve
    Dec 1 '17 at 17:56
  • $\begingroup$ @bcsteeve "a little knowledge can be dangerous" Yes in units of work performed, you can use "foot-pound," but when you see that unit in reference to a motor, or any rotary action, it's talking about torque. $\endgroup$ Dec 1 '17 at 18:15

I would suggest that a good way to estimate this sort of thing is by using sand bags (or similar). Just pile them on the lid untill it closes and then weight them.

Having said that it sounds like your real issue is a bit more nuanced and the real problem is getting the latch closed and there might be better ways to approach this than just adding a powered actuator. To me it sounds like a less stiff latch is an all round better solution.

Another option is to change the relationship between the rate of the gas struts and the weight of the lid. As you will be aware a car trunk usually closes itself with relatively little effort so it may be that using different struts (or changing their mounting position) or adding weight to the lid or just lubricating the hinges will solve the problem.

In response to comments.

100lb seems like a lot to close a latch, but if it is designed to have someone lean their weight on it then maybe. The difficulty here though is that when you lean on something to close it, especially in this sort of situation is that dynamic forces and momentum come into play and the 'effort'required to operate a 'sticky'mechanism can be hard to quantify.

My initial instinct in approaching this problem would be to just add a bit more weight to the lid and see if that helps.

  • $\begingroup$ The lid goes up and down just fine. It really is the latch that's the problem, as you say. What I want to do, however, is come up with an add-on solution for what is already there, rather than changing what is there. I think the sandbags would be the same as what I proposed, no? I went and tried it, and my guess was bang on - it took near exactly 100lbs off my weight to get the lid to latch. Is that just too much for a smallish actuator to handle? $\endgroup$
    – bcsteeve
    Dec 1 '17 at 18:23
  • $\begingroup$ I should note that the latch is as stiff as it is (ie. the spring that must be overcome) for reasons the designers deemed necessary. For liability reasons, I don't think I should go about trying to modify that. The solution I'd like to come up with should represent a stronger pull, not an easier job. If that makes any sense. $\endgroup$
    – bcsteeve
    Dec 1 '17 at 18:26

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