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Mechanics Corner

A Journal of Applied Mechanics and Mathematics by DrD, # 2

cMachinery Dynamics Research, LLC, 2015

Box Tipping

It is a common practice for manufacturers to ship their products in packing crates that arestrapped down on pallets for handling. There is often concern about the stability of thispackage as it is handled in transit to the purchaser. For this problem, we understand thatthe manufacturer wants to perform a simple test on each package shipped to assure thatit will not tip over in transit. The test will consist of tipping the package slightly to theleft and placing a block under the right edge of the pallet. The block is then quickly pulled

out and the question is whether or not the package will fall over to the right. The answerdepends upon the amount of the initial tip to the left and the location of the center of massof the combined packing crate and pallet.

It is clear that the falling box impacts the oor, causing an impulsive distributed load toact on the bottom of the package. This will apply both an impulsive upward force and animpulsive moment to act on the box. Since the actual distribution of the force is unknown(and unknowable), an impulsemomentum approach to this problem is not likely to get veryfar. There is, however, a much simpler energy analysis available. Go over to the PDF formore details.

Energy Based Analysis of Tipping PotentialConsider a box, strapped to a pallet, as shown in the gure below. The assembly overalldimensions are b h, and the composite center of mass is located at (u; v)as indicated.

b

h

u

v

Fig. 1 System Denition

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Four states are considered as shown in Fig. 2 below. The initial condition of the test is thesecond state shown in this sequence.

LC o RC

= 0LeftCriticalCond.

TestInitialCond.

Impact RightCriticalCond.

g

Fig. 2 Four System States

Left Critical Condition

The left critical condition is the limiting position to avoid tipping the box to the left. Itis apparent that the initial test condition, = o must be such that o LC. In the leftcritical condition, the center of mass is directly over the left edge. Thus

LC+ arctanv

u

=

2

or

LC=

2

arctan v

u

The initial test condition must not raise the box any more than this; if it does, the wholeassembly will fall to the left.

Test Initial Condition

In the test initial condition, the box is at rest but the center of mass is elevated. Thepotential energy of the box is

VTIC=M g (u sin o+ v cos o v)

with respect to the position= 0. The initial kinetic energy of the box is

TTIC= 0

because the box is at rest. This is where the test begins.

Impact

When the box is released from rest in the initial condition, it falls to the oor where impactoccurs. The impact is a complicated process, involving a distributed impulsive loading onthe base of the pallet and acting for a very short, but indeterminate, time interval.

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From the initial release down to the instant of impact, energy is conserved. Therefore,

TTIC+ VTIC=TImpact Initial+ VImpact Initial

Because this position is the reference state for potential energy

VImpact Initial = VImpact Final = 0. Thus

TImpact Initial = VTIC=M g (u sin o+ v cos o v)

Real impacts always involve the loss of some energy, but the amount is dicult to quantify.In the limiting case, energy is conserved in an impact, so that

TImpact Final TImpact Initial =M g (u sin o+ v cos o v)

Right Critical Condition

The right critical condition is reached when the box is on the verge of tipping to the right.At this condition, the center of mass is directly over the right edge of the pallet as shown in

the fourth image of Fig. 2. At this position, the angleRC

is such thatRC+ arctan

v

bu

=

2

or

RC=

2 arctan

v

bu

In the right critical condition, the potential energy of the box is

VRC=M g [(b u) sin RC+ v cos RC v]

and the kinetic energy is

TRC 0

where the equality represents the limiting condition.

Resolution

The whole question comes down to, "Is the total energy at the end of impact sucient toreach the right critical condition?" If it is, then the box will tip over; if it is not, the boxcannot tip. Thus, to assure that the box does not tip,

TImpact Final < VRC

M g (u sin o+ v cos o v)< Mg [(b u) sin RC+ v cos RC v]

u sin o

+ v cos o

v

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