# Practical low-tech real world method for measuring how much force to close a lid

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?

• "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. Dec 1 '17 at 17:28
• 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. Dec 1 '17 at 17:29
• 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. Dec 1 '17 at 17:54
• 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? Dec 1 '17 at 17:56
• @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. 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.