Feed Screw Breakage

Practical Approach to Screw Breakage

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Practical Approach to Screw Breakage
And How to Avoid Failures
Jeff Myers
BARR Inc.
Virginia Beach, VA
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Background
Mechanical failure of a feedscrew can
result in a loss of thousands of dollars in
downtime & production loss.
This presentation will investigate the
factors that contributed to screw failure
and offer some recommendations on
how to prevent it.
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Background
The two most common modes of failure
in a feedscrew are:
Torsional Failure
Fatigue Failure
Most failures result as a combination
of both Torsional and Fatigue.
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Most Torsional Failures Occur In
The Feed Section
Primary Causes:
•
Cold start
•
Improper start-up procedures
•
High load or shock
•
Foreign material
•
Poor alignment with gearbox
Typical Feed Throat
The feed section typically has the smallest root
diameter and must transmit the majority of the torque.
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“Cold Starts”
If the extruder is first started when
the barrel and screw are too cold,
the viscosity of the polymer will be
too high and the screw will not
rotate.
When the motor is energized, high
and dangerous torques can be
applied to the screw.
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Stress-Strain Diagram
Stressing a material beyond
its elastic limit will result in
permanent deformation.
Fatigue Strength
Elastic
Range
Yield Point
Cracks can develop once the
fatigue strength is exceeded.
Stress
Strain
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The Impact of A “Cold Start”
On The Strength
Initial crack is formed at
the flight as a result of a
“Cold Start” or high impact
loading.
Failure can occur days
or weeks later as a result
of fatigue stress.
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Factors That Contribute To
Screw Failure
•
Screw Material Choice and Quality
•
Maximum Drive Power & Torque
•
Screw Geometry
•
Alignment & Barrel Supports
•
Wear
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Screw Material Selection
Material selection criteria should be
based on the application and not
necessarily the material cost.
Improper heat treating and stress
relieving can have a negative impact on
the material strength.
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Common Screw Materials
Yield
Strength
Sy (MPa)
Typical Hardness
(Rc)
Yield
Strength
Sy (psi)
Base Material
28-32
90,000
620
4140
28-32
110,000
759
4340
175,000
17-4 PH S.S.
33-36
1,207
Tool Steels
(D2 & H13)
2,070
58-60
300,000
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Calculation
Method
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Maximum Torque To Screw
T = 9,554 * (kw/rpm); Nm
T = 63,025 * (HP/rpm); in-lb
HP & kw = Maximum Motor Power
rpm = Maximum Screw Speed
At full Motor Power
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Maximum Shear Stress (Shaft)
J
r`
= * (D
4
– d
4
)
32
J
r
= ( * D
4
)
32
Polar Moment of Inertia
Solid Shaft
t = T * r
J
r
t ~ 75% Sy
T = t *J
r
r
Yield Strength for Torsion
D
Shaft with
Bore
d
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Maximum Shear Stress
Including Flight
J
f
= ((r
r
*H) *(H
2
*r
r
2
)] + (r
r
* H * d
2
))*N
12
H = F
w
* sin q
J
t
= J
r
+ J
f
r
H
D
s
D
b
r
r
d
r
T = t *J
t
r
Total Polar Moment of Inertia
t = T * r
J
t
N = # of flights
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Sample
Calculation
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Sample Data Input
Inputs (in)
Results
r
D
r
r
d
r
H
D
s
b
67,500
S
y Torsion
Polar Moment
of Inertia (in
4
)
33.25
J
t
Total
(J
r`
+J
f
)
8.21
J
f
Flt
25.04
J
r`
Shaft/Bore
25.13
J
r
Shaft
2.375
b
1.885
H
4.00
D
r
17.7
Helix Angle
315,000
Max Torque
(in-lb)
90,000
4140
Sy (psi)
1.00
# of Flt’s
“N”
1.00
Bore Dia
“d”
.500
Root Radius
6.00
Lead
.600
Flt Width
1.00
Feed Depth
6.00
Screw Dia
“Ds”
80
RPM
400
HP
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Torsional Strength of Section
3.56
1,122,000
w/Flt’s/ Bore
3.57
1,125,300
w/Flt’s
Safety Factor
Max Torque
(in-lb)
2.68
845,000
Shaft w/ Bore
2.69
848,230
Solid Shaft
In order to avoid failure, the maximum applied torque
needs to be less than what the section can safely handle
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Extruder
Alignment
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Alignment & Barrel Supports
Improper gearbox, feed throat and screw alignment
can cause the screw to flex resulting in wear and
fatigue failure.
+
-
A
Barrel Supports
Poor barrel support design can lead to barrel sag or
buckling causing extreme wear.
A
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Alignment & Barrel Supports
A sagging barrel will cause a straight
screw to flex once per rotation.
After many rotations and flex cycles,
the screw metal is weakened.
For two-stage screws, this type of
break often occurs in the first-stage
metering section.
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Fatigue Failure as a Result of
Extreme Screw Wear and Flexing
Break in first stage meter section
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Recommendations
to Reduce Screw
Failures
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Recommendation to
Reduce Screw Failure
Do not exceed the maximum strength of the
screw by applying basic strength calculations.
Reduce all sharp edges and use generous
root radii in critical stress locations.
Radius
Chamfer
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Recommendation to
Reduce Screw Failure
Increase surface finish on root. A reduction
in minute cracks and fine scratches can
improve fatigue strength by a factor of 2x.
Select the proper base and flight hardfacing
materials based on the application.
Use proper heat-treating and manufacturing
procedures to reduce residual stresses in the
base material.
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Recommendation to
Reduce Screw Failure
Use proper start-up and shut-down procedures
to reduce possible damage to the equipment.
Check the screw and barrel for wear on a
regular basis and repair or replace as needed.
Align barrel and gearbox when replacing the
screw or as part of a regular maintenance
program.
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Conclusions
A simple calculation can be used to determine the
mechanical strength of a feedscrew.
There are a number of factors that contribute to
screw failures. By understanding the basic causes,
the correct designs and materials can be selected
to reduce failures.
By adopting standard preventive maintenance
(PM) programs and start-up procedures, failures
can be reduced and/or eliminated.
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Questions
?