Find the range of acceleration so that the drums won't fall

[QuantumRose]

Problem:
https://drive.google.com/open?id=19PDuyshgP6u4U9mXU3BfrF031J8wL12B

Attempt: I tried to solve the range by listing out the Newton's 2nd laws of each drum

https://drive.google.com/open?id=1Eslg2Wy3orG41eywwm_O6qN4OQ3QN-bA
and from the equations of the drum 2, I get

https://drive.google.com/open?id=19MFda8L_YuDBlsOg1_76O3-w1rJjHhfW
Since drum 2 must stay on the bottom drums, so it will not fly up, thus

https://drive.google.com/open?id=1tpDM2zHyDQ84m73GC0EgcZsNz5Q4tq1z
I got the upper bound of the acceleration, but I don't know if it is correct.My question:
Although I got the seemingly correct upper bound of the acceleration, but I don't know how I can get the lower bound of the acceleration?

 

[phinds]

None of your images show up

 

[QuantumRose]

None of your images show up
View attachment 230886

I just fixed that, now it should be fine.

 

[haruspex]

got the seemingly correct upper bound of the acceleration, but I don't know how I can get the lower bound of the acceleration?

Each bound corresponds to the system falling apart. In which two ways can that happen? What can you say about the normal forces in those two cases?

 

[QuantumRose]

Each bound corresponds to the system falling apart. In which two ways can that happen? What can you say about the normal forces in those two cases?

Well, I think there are two cases, one is for the upper bound I solved(corresponds to the top drum flying up); the other case is for the lower bound(it corresponds to the situation when the top drum is falling down, i.e. N_1 and N_2 are not enough to support the top drum.)