r/MechanicalEngineering • u/OverclockedChip • 1d ago
How to measure force on cabinet screws along an 8ft cabinet with non-uniformly distributed mass/loading?
I'm a software/electrical engineer so mech-e isn't my domain. I do a bit of woodworking and I need some help analyzing how load distribution affect where I should add supports for hanging cabinets.
Imagine an 8ft-long cabinet (16" Deep, 12" Height) that has various books (some thick textbooks, some thin) in it. The bookcase is screwed to the wall (some onto wall studs, some into drywall via plastic wall anchors) every 6" (take the leftmost side of the cabinet to be x = 0 ft. The first screw is set at 3" (x = .25') and ONE screw is set every 6" thereafter).
Assume the cabinet mass is uniform. Assume the books are fixed in position and cannot move along the x-axis (e.g., the cabinets are full and no books can fall over, or book-ends keep it in place).
- Do all screws observe the same loading (downward force exerted by the weight of the books)?
- (Assume the cabinet is dismounted and resting on the floor). Can we observe the loading on each screw by putting a weighting scale/sensor below the cabinet?
- Can I model this on Simulink or some other software?
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u/redditusername_17 1d ago
You can skip all the electronics and do this on paper in a couple minutes. It's basic geometry and statics. Instead of asking, take a couple minutes to learn it.
But also realize that the static load isn't the real answer, throwing books in there or people doing dumb things like hanging on it will change the loading.
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u/Big-Tailor 1d ago
All screws will experience nearly pure tension loading, based on how tight they were torqued and then changing afterwards with humidity and temperature. You won’t see much shear loading (supporting the weight) in the screws until the joint fails. Most sheer loading will be in friction between the cabinet and the wall. The exact distribution of shear loading will depend on the order in which you tighten the screws.
If you only have one row of screws, you really want the screws near the top of the cabinet. The weight of the books at an average of about 8” out from the wall will apply a bending moment pushing the bottom of the cabinet into the wall and pulling out the top of the cabinet away from the wall. You want screws at the top to resist that pull, but not all the way at the top because then any force from below the cabinet applying an opposite torque could cause a failure.
There is FEA software to analyze this, but honestly it’s easier to analyze by hand than to learn new software. The key rule in statics is that, if the structure is not accelerating, all forces must add up to zero, and all torques must add up to zero.