Pkg Solutions
Quality Glossary
The McKee formula for Compression Estimation
The style common method of calculating a grade compression failure load (CFL, also
sometimes noted as Box Compression Test Strength or BCT), is the McKee formula. This formula was developed in the
1960's and is named after one of it's main inventors. Shown below is the
shortend form of the formula, which is normally used in such calculations.
It should be noted that the McKee formula is being superseded by more
accurate formulas developed in recent times.
Accuracy of the McKee formula
An
extract form "Box compression analysis of world-wide data spanning
years" by Urbanik and Frank:
Until recently, the state of the art of box comĀpression estimation was the
equation by McKee et al. (1963) or various
modifications, with the assumption that Eq.
provided an estimation that was accurate to within about 6% on
average and 15% for the majority of
single-wall boxes. In reality, we see
that for a broader data set of
single-wall boxes, the McKee equation
systematically overestimates compression strength. It
provides an estimate within about 11%
of the true box compression value on average, and the strength
estimates of many boxes are off by
more than 20%. The estimated strength of
double-wall boxes and tube constructions can be off by
even more.
The shortend version of the McKee formula is:
CFL = 5.3 * ECT *
where:
ECT = edge crush test in Kpa/cm
T = thickness of the board in cm
Z = perimeter of the box in cm
CFL = Case failure load in Kpa
Or in SI-Units:
CFL = 53 * ECT *
where:
ECT = edge crush test in N/mm
T = thickness of the board in cm
Z = perimeter of the box in cm
CFL = Case failure load in N
The general full form of the McKee equation is:
CFL = a * ECT
b * {
}
a-b
* Z
2b-1
where:
a & b = constants determined by experimentation
ECT = edge crush test
Z = perimeter of the box in cm
CFL = Case failure load in N
D = bending stiffness of the board
=
The Geometric Mean of the Machine Direction and Cross Machine Direction
bending stiffnesses
CFL(lbs) = 2.028 * ECT
0.746 * {
}
0.254
* Z
0.492
For more information see:
Predicting
Box Compression Strength
BOX
COMPRESSION ANALYSIS OF WORLD-WIDE DATA SPANNING 46 YEARS
In its turn ECT can be calculated from the CCT (corrugated crush test) of
the different components of the board in accordance with the following
formula:
ECT = 0.86 * (CCT1 + d * CCTF + CCT2)
CCT1 = Kpa/cm edge crush resistance of a corrugated strip of liner 1
CCTF = Kpa/cm edge crush resistance of a corrugated strip of fluting
medium
CCT2 = Kpa/cm edge crush resistance of a corrugated strip of liner 2
d = the draw factor of the board's fluting