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[David L]

OK, I'm cerainly no mathmatician here but wouldn't the total force exerted be divided by two (even if inequitably for some reason) when calculating the pressure applied to one shelf, since clamps are not prejudice and don't exert thier energy in one direction only?

David L

[Tim R]

John: It turns out each bracket is rated at 300 lbs, so it should work out. I will reinforce the bottom with more plywood.

David: As Sir Newton pointed out, for every action (force) there is an equal and opposite reaction. So, it the stick exerts 7 lbs on one end, it must exert 7 lbs on the other, else it would be in motion, accelerating either up or downward (not what we want ☺).

[David L]

John: It turns out each bracket is rated at 300 lbs, so it should work out. I will reinforce the bottom with more plywood.

David: As Sir Newton pointed out, for every action (force) there is an equal and opposite reaction. So, it the stick exerts 7 lbs on one end, it must exert 7 lbs on the other, else it would be in motion, accelerating either up or downward (not what we want ☺).

Grasshopper Tim, that may not be true, I'm not talking about Sir Issac Newton and his laws of the universe, again, I must state that I'm not a mathmatician, a structural engineer or even a very smart person, however, seems to me that since you have a measuring gauge at only one end of the energy emitting utinsil, it is absorbing the collective force from both ends of the stick, If you were to put a scale at both ends of the stick at the same time would each scale read 7lbs. or 3.5lbs? (Keeping in mind that an adjustment would need to be made so that the length of the stick, in the bent position, would be the same as it is with only one scale as this could change the amount of force emitted.)
The other thing to consider, and I think Tony addressed it already is that when a shelf is rated, it is rated as load bearing down and not load bearing up. I think all this may be pointless as we have already determined that your shelf rating should be sufficient and that weights will need to be added to the top shelf, however I find it very interesting (if nobody else does) and it certainly has my wheels spinning!

David L

[michaeld]

I am a structural engineer, plus I slept at a Holiday Inn last night. Tim R is correct; the forces at each end are equal and opposite (e.g. 7 pounds downward on the lower shelf and 7 pounds upward on the upper shelf). Make sure the upper shelf brackets are not like the ones I have - a small upward force will disengage them - big trouble!

Mike

[michaeld]

deleted duplicate post

[David L]

Thanks Mike for clearing that up. My pea sized brain is having a hard time accepting that. But if you're a structural engineer then that means somebody paid a whole lot of money for you to learn that this is true, so I will have to trust your education and go with you on this one. But what if... ah, nevermind!

David L

[michaeld]

I've only built one guitar, but I've learned that common sense, ingenuity, and patience are way more useful than an engineering degree when it comes to lutherie!

Mike

[michaeld]

I've only built one guitar, but I've learned that common sense, ingenuity, and patience are way more useful than an engineering degree when it comes to lutherie!

Mike