Lifting of coils with the bore in the vertical orientation could give rise to safety issues if the coil integrity is compromised during the slitting and packing operation. Coil telescoping (whereby the inner wraps of the coil spiral out) is known to occur during lifting, which could pose as a serious threat to the safety of personnel involved. In this type of incident, the coil straps are also broken when their breaking strength is exceeded and the whole coil would unwrap itself at an elevated position. Back tension is applied to the strip while shearing wide strip into narrower slits; this allows sufficient radial pressure to be built up within the bulk of the narrow coils. Upon unloading, the radial pressures at the innermost and outermost wraps decrease to zero but the bulk of the inter-wrap pressure within the coil remains largely unchanged. The interwrap frictional forces developed within the coil enable the coil to retain its integrity under its own weight. It is found that the radial pressures developed within the slit coil play the most crucial role in providing sufficient frictional resistance to support the weight of the coil wraps during lifting with the bore in the vertical orientation. In addition, the inter-wrap pressures near the footprint of the mechanical lifting device, near the bore, have the most significant influence in preventing coil telescoping

Tu, C. V.

Yuen, W. Y. D.

University of Queensland eSpace

5th Australasian Congress on Applied Mechanics, ACAM 2007  
10-12 December 2007, Brisbane, Australia 
Lifting of Steel Coils in Bore-Vertical Orientation           
 
C.V. Tu and W.Y.D. Yuen  
 
BlueScope Steel Research,  
BlueScope Steel Limited, Port Kembla, Australia 
 
Abstract: Lifting of coils with the bore in the vertical orientation could give rise to safety issues if the 
coil integrity is compromised during the slitting and packing operation.  Coil telescoping (whereby the 
inner wraps of the coil spiral out) is known to occur during lifting, which could pose as a serious threat 
to the safety of personnel involved.  In this type of incident, the coil straps are also broken when their 
breaking strength is exceeded and the whole coil would unwrap itself at an elevated position. 
Back tension is applied to the strip while shearing wide strip into narrower slits; this allows sufficient 
radial pressure to be built up within the bulk of the narrow coils.  Upon unloading, the radial pressures 
at the innermost and outermost wraps decrease to zero but the bulk of the inter-wrap pressure within 
the coil remains largely unchanged.  The interwrap frictional forces developed within the coil enable 
the coil to retain its integrity under its own weight. 
It is found that the radial pressures developed within the slit coil play the most crucial role in providing 
sufficient frictional resistance to support the weight of the coil wraps during lifting with the bore in the 
vertical orientation.  In addition, the inter-wrap pressures near the footprint of the mechanical lifting 
device, near the bore, have the most significant influence in preventing coil telescoping.   
Keywords: bore-vertical, inter-wrap friction, lifting, safety, steel coils, straps, telescoping. 
 
1 Introduction 
In many applications, narrow steel coils are required for further processing to produce various steel 
products.  These narrow coils are normally obtained by slitting full-width coils,  as produced from the 
steel manufacturing plants.  Due to the large aspect ratios (coil diameter to coil width ratio), it is often 
more convenient, and more stable, to store and handle these narrow coils with the bore in the vertical 
orientation (rather than in the bore-horizontal orientation). 
Lifting of the coils with the bore in the vertical orientation could give rise to safety issues if the coil 
integrity is co mpromised during the slitting and packing operation.  Coil telescoping is known to occur 
during lifting, which could pose a serious safety concern.  In this type of incident, the coil straps are 
also broken when their breaking strength is exceeded and the whole coil would unwrap itself at an 
elevated position. 
When shearing a wide strip into narrower slits, back tension is applied to the strip, thus allowing the 
radial pressure to be built up within the bulk of the narrow coils.  Upon unloading, to satisfy the 
boundary conditions at the innermost and outermost wraps (in the absence of the outer circumferential 
straps), the radial pressure would decrease to zero but the bulk of radial pressure within the coil 
remains largely unchanged.  The radial pressures in the coil, together with the inter-wrap friction, 
enable the coil to retain its integrity. 
This paper presents an analysis of the build up of the radial pressures inside the coil during coiling 
(from the applied coiling tension), and their influence on the integrity of the coil during its lifting in a 
bore-vertical orientation.  It is found that the radial pressures developed within the slit coil after coiling 
play a crucial role in providing sufficient frictional resistance to support the weight of the coil wraps.  In 
addition, the inter-wrap pressures around the footprint of the mechanical lifting device, near the bore, 
have the most significant influence in preventing coil telescoping.   
The circumferential steel straps may provide some degree of restraint at the outer wraps.  However, 
the radial coil straps offer only secondary contribution in maintaining the coil integrity during handling. 
 
  
2 Stability of Bore-Vertical Lifting  
A typical narrow coil to be lifted with its bore in 
the vertical orientation is shown in Figure 1.  It 
usually comprises a circumferential strap and 
four radial straps.  The lifting device consists of 
a mechanism to allow the feet of the lifter to 
expand under the bore to support the inner 
wraps before lifting.  In the case of failure, the 
inter-wrap frictional force is insufficient to hold 
the load which would then have to be supported 
by the straps.  If the straps break, almost all the 
coil wraps would telescope out and the situation 
can be more vulnerable for multiple-coil lifting 
with the dunage (spacers between coils) not 
correctly positioned, hence exerting excess 
load on the bottom coil. 
On this problem there are several aspects 
required to be investigated, including the 
frictional characteristics of the material, the 
strength of the straps, back tensioning capacity 
of the slitting machine and the relaxation of the 
radial pressure within the coil after its re lease 
from the coiling mandrel. 
2.1 Evolution of Stresses in Coiling  
The role of inter-wrap frictional force is the most important factor in the safe lifting of coils with the bore 
in vertical orientation.  There are two extreme scenarios:   
(i) If the coil is loosely wound, the inter-wrap friction is low and the radial straps are the primary load 
bearing elements for the coil weight.  
(ii) If the coil is wound very tightly with high coiling tension and assuming that the circumferential 
strap can maintain the outer wraps tight, the coil would behave largely as a solid cylinder (with a 
hollow centre) and there should be little load imposed on the radial straps since the inter-wrap 
friction is high enough to support the weight of the coil. 
Most practical situations lie somewhere between these extremes.  For safe operation, the inter-wrap 
friction should play the primary role  in providing the coil integrity since numerous operational issues 
with radial straps may reduce their load carrying capacity during lifting.  These include mis-crimping, 
non-uniform radial strap spacing, strap damages or seal flattening. 
For a given coiling tension, during the build up of the coil, the addition of every new wrap would 
impose some stresses on the previous wraps.  The radial and circumferential stress components 
within the coil can be calculated if the wrap and mandrel characteristics are known (e.g. inter-wrap 
compressibility, mandrel stiffness etc.)  After the collapse of the mandrel for coil removal, the inner-
most wrap and the outermost wrap will be free from external forces and there is a redistribution of the 
stresses within the coil.  The resulting radial pressure is the s tress component of interes t in bore-
vertical lifting.  
A computer model based on the analysis of Cozijnsen & Yuen [1, 2] was used to calculate the stress 
components from which the inter-wrap frictional force distribution can be estimated.  As an illustration,  
assume that the radial pressure and tangential stress distributions are uniform across the strip width, 
their profiles before and after removal of the coil from the mandrel for typical coiling condition 
(thickness h=1.90 mm, bore radius Ri=254 mm, ratio of the outer and bore radii Ro/Ri=3.0 and a 
constant coiling tension, sT, of 6.0 MPa) are shown in Figure 2.  After the mandrel is removed, the 
inner-most and outermost radial pressures must approach zero (boundary conditions) and the radial 
pressure as well as tangential stress distributions must be redistributed to satisfy the new condition.  
The tangential stress in the outermost wrap, if not released, would retain the coiling tension as shown 
in Figure 2(b).  Note that these stress distributions depend highly on the radial compressibility of the 
Figure 1 
Typical lifting arrangement for narrow coils. 
  
coil, which is governed by the surface characteristics of the strip.  For our calculations, compressibility 
typical of metallic coated strip has been adopted. 
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.0 1.5 2.0 2.5 3.0
R/Ri
Ra
di
al 
Pr
es
su
re
 (M
Pa
)
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Ta
ng
en
tia
l S
tre
ss
 (M
Pa
)
Radial Pressure
Tangential Stress
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1.0 1.5 2.0 2.5 3.0
R/R i
Ra
di
al 
Pr
es
su
re
 (M
Pa
)
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
Ta
ng
en
tia
l S
tre
ss
 (M
Pa
)
Radial Pre ssure
Tangential Stress
 
 (a) (b) 
Figure 2 
Calculated distributions of Radial Pressure (PR) and Tangential Stress PT in the coil 
(a) before and (b) after its removal from the mandrel. 
 
Figure 3 presents typical radial pressure and tangential stress distributions over a range of coiling 
tensions used in the slitters of interest (h=1.90mm, Ri=254 mm, Ro/Ri=3.0).  It should be noted that the 
radial pressure is mostly near the maximum value at the position corresponding to the limit of the 
lifters feet, typically at 1.40 times the inner coil radius based on the traditionally-used lifter.  This 
conveniently allows the use of peak radial pressure, PM, in the calculation of wrap slippage and the 
complication of radial pressure reduction near the bore can be ignored.  Thus a simpler relationship as 
shown in Figure 4 showing the variation of PM against coiling tension sT can be used instead. 
More general results over a wider range can be obtained for tension up to 50 kN/m and for strip 
thickness from 1.0 to 5.0 mm.  These are plotted in Figures 5 and 6, with the coiling tension expressed 
in kN/m (instead of tensile stress in Mpa) for practical reasons.  For clarity, pressure distributions such 
as those shown in Figure 3 will not be plotted here and only the peak pressure will be shown.  As 
mentioned earlier, the radius of the lifter feet falls within the region dominated by the peak pressure.  
The pressure difference between the peak value and that at the lifter feet is less than 5% over the 
practical range of tension (e.g. less than 30 kN/m). 
 
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
1.0 1.5 2.0 2.5 3.0
R/R i
R
ad
ia
l P
re
ss
ur
e 
(M
Pa
)
-20
-15
-10
-5
0
5
10
15
20
Ta
ng
en
tia
l S
tr
es
s 
(M
Pa
)
Coiling 6 MPa
Coiling 12 MPa
Coiling 18 MPa
Radial Pressure
Tangential Stress
18MPa
12MPa
 6MPa
 
Figure 3 
Distributions of PR and PT after mandrel removal 
(h=1.90 mm, Ri=508 mm, Ro/Ri=3.0, R/Ri=1.40 is 
at the limit of the lifter support). 
Figure 4 
Peak Radial Pressure, PM, versus Coiling Tension 
(h=1.90 mm, Ri=508 mm, Ro/Ri=3.0). 
 
 
0.0
0.5
1.0
1.5
2.0
2.5
0 5 10 15 20
Coiling Tension (MPa)
Pe
ak
 R
ad
ia
l P
re
ss
ur
e 
(M
Pa
)
  
Figure 5 shows the variation of the calculated peak radial pressure in the coil after mandrel removal 
with the coiling tension, and the calculated peak radial pressure is also plotted against the strip 
thickness for a given coiling tension in Figure 6.  Through curve fitting, the peak radial pressure 
PM (MPa) can be expressed as a function of the strip thickness, h (mm), and coiling tension FT (kN/m): 
PM=a/hb,  (1) 
where a=FT(FT/900+1/20) and b=FT(FT/9000+1/400)+1/2. 
Equation (1) can be used to calculate the peak radial pressure for a given coiling tension and strip 
thickness with reasonable accuracy within the thickness range of 1 to 5 mm and coiling tension up to 
30 kN/m. 
Combined with the results presented in the next section, Figures 5 and 6 can be used in determining 
the appropriate coiling tension to provide a suitable radial pressure to support a coil of a given outer 
diameter if the frictional coefficient is known.  It should be noted, however, that these results depend 
on the mandrel and strip properties assumed in the calculations. 
 
Figure 5 
Calculated Peak Radial Pressure after  
Mandrel Removal versus Coiling Tension. 
 
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1.0 2.0 3.0 4.0 5.0
Strip Thickness (mm)
Pe
ak
 R
ad
ia
l P
re
ss
ur
e 
(M
Pa
)
50 kN/m
40 kN/m
30 kN/m
20 kN/m
10 kN/m
5 kN/m
 
Figure 6 
Calculated Peak radial pressure after Mandrel 
Removal vs Thickness for various Coiling Tensions. 
 
2.2 Coil Wrap Support  
This section addresses the issues related to integrity of the coil when it is lifted with the bore in a 
vertical orientation.  If the radial straps are ignored, the frictio nal force developed from the radial 
pressure plays the most important role for coil stability.  On the other hand, if the interwrap friction is 
insufficient, the load would have to be partially supported by the radial straps and their breaking 
strength would become critical to maintaining the coil integrity. 
 
2.2.1 The Role of Friction  
The following analysis considers the lifting of a coil vertically from the bore region, with the load 
bearing capacity of the radial straps not taken into account, and the circumferential strap assumed to 
be sufficient to hold, at least, the outer coil wrap in position.  At a radial location R along the coil, 
knowing the radial pressure PR, the maximum frictional holding force FR is given by  
RR PRWF mp2= , 
where W is the width of the coil and m is the frictional coefficient between the wraps (at R). 
The mass of the portion of the coil from radius R to the outermost radius RO is: 
 
)( 22 RRWM OR -= pr , 
 
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 5 10 15 20
Coiling Tension (kN/m)
Pe
ak
 R
ad
ia
l P
re
ss
ur
e 
(M
Pa
)
Strip 
thickness
1 mm
2 mm
3 mm
4 mm
5 mm
  
where r is the bulk density of the material.   
The frictional force between the wraps at radius R must be able to support this mass to avoid 
telescoping, i.e.: 
gMF RR > ,  
where g is the gravitational acceleration. 
As we move inward towards the bore, the mass of the coil wraps required to be supported increases 
and at the same time the circumference 2pR reduces.  Just under a critical radius R*, the frictional 
force can no longer support the weight and slippage at R* will occur as shown schematically Figure 7.   
 
 
Figure 7 
Support of coil weight by the inter-wrap friction.   
 
0.00
0.20
0.40
0.60
0.80
0.00 1.00 2.00 3.00 4.00 5.00
a=(m P R)/(r gR o)
R
*/R
o
 
Figure 8 
Plot of R*/Ro versus parameter a. 
 
At the radial position R=R*: 
FR* =gMR* (2) 
From (2), R* is the positive root of the quadratic equation: 
012
2
=-+÷÷
ø
ö
çç
è
æ
oo R
R
R
R
a  (3) 
where  
a =m PR*/(r gRo), 
with PR* being the radial pressure at R*. 
The real positive root of (3) is the solution for R* with R*>RF where RF is the radius of the lifter feet.  
For R*<RF the wraps should be well supported by the lifter feet and slippage cannot occur.  Hence, 
aa -+= 12
*
oR
R
 (4) 
It should be noted that the width of the strip does not play a role here; only the radial pressure, 
frictional coefficient and outer radius have influence on the radial position for slippage.  Equation (4) is 
plotted in Figure 8. 
An expression for the minimum required peak radial pressure P*, below which slippage would occur, 
can be obtained: 
( )2* )/(1
)/(2 oFoF
o RR
RR
gR
P -=
m
r
 (5) 
  
For small a (corresponding to low frictional coefficient, low radial pressure and/or a large outer radius 
Ro), R*/Ro is high, indicating the critical location for wrap slippage is closer to the outer wraps.  Such 
situation is undesirable.  Figure 9 gives the critical radius for slippage for typical dimensions and 
condition of steel coil slitting (m=0.15 which is typical for un-lubricated metallic coated steel strip).  
Between Figures 6 and 9, it is possible to predict approximately if slippage is likely to occur for a given 
operating condition.  For other frictional coefficients, Figure 8 can be used.  For convenience, Figure 
10 illustrates the minimum required coiling tension (for m=0.15) for various coil dimensions. 
 
Figure 9 
Calculated R* versus the outer coil radius  
(m = 0.15). 
0
5
10
15
20
350 450 550 650 750 850
Outer Radius Ro (mm)
M
in
im
um
 T
en
si
on
 re
qu
ire
d 
(k
N
/m
) 5 mm
4 mm
3 mm
2 mm
1 mm
 
Figure 10  
Minimum coiling tension required for a range 
of strip thickness (m = 0.15).  
 
 
 
2.2.2 Coil Support by Radial Straps 
Once inter-wrap slippage occurs as discussed in Section 2.2.1, the coil integrity then relies on the 
holding strength of the radial straps.  The tensile strength of the strap material used is around 
900 MPa.  However, due to the variability of the crimping operation (crimping depth & number of 
crimps) and stress concentration at sharp bends (stress concentration factor found to be around 1.5-
1.8) during strapping, the effective breaking strength was found to be less than 70% of the tensile 
strength.  Extensive investigation into the performance and behaviour of coil straps has been 
conducted since they do contribute significantly to fail-safe coil lifting operation, however the 
discussion is beyond the scope of this paper. 
3 Conclusions 
For vertical bore narrow coil lifting, the coiling tension must be higher than a critical level to avoid inter-
wrap slippage / telescoping.  For given strip thickness, the outer coil diameter and the inter-wrap 
frictional coefficient have significant influence on this critical value.  The critical tension is higher for 
thicker strip due to lower resulting radial pressure between the wraps.  Similarly higher tension is 
required for larger outer coil diameter due to increased mass and for lower inter-wrap friction.  The role 
of radial straps is secondary but the use of appropriate straps can provide additional safety margin if 
slippage does occur. 
References 
[1] M. Cozijnsen and W.Y.D. Yuen, Stress Distributions in Wound Coils, Proc. 2nd Biennial 
Australian Engineering Mathematics Conference, July 15-17, Sydney, 1996, pp. 117-124. 
  
[2] W.Y.D. Yuen and M. Cozijnsen, Optimum Tension Profiles to Prevent Coil Collapses, SEAISI 
Quarterly, July 2000, pp. 50-59. 
0
100
200
300
400
500
600
0 200 400 600 800 1000
Outer Radius R o(mm)
R
ad
iu
s 
at
 S
lip
pa
ge
 R
* (
m
m
)
0.20 
0.4
0.60
0.80
1.0
1.2
Radial Pressure
(MPa)Bore Radius
Radius at
Lifter Feet


Lifting of Steel Coils in Bore-Vertical Orientation

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Lifting of Steel Coils in Bore-Vertical Orientation

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