"one of the points which we should care about "

1. GKN Land SystemsIdentifying Damage To Elastic Couplings:
Failure Modes & Photo Examples
Cornell Pump Company, November 2014

2. 2
PLUG-IN STYLE COUPLING GENERAL FORM

3. 3
Visual Failure Mode Identification: PVN, PVA, PS Models
Plug-In Style Coupling Models PVN & PVA
With Rubber Bonded to Center Ring
PS Model With Bushings
and Clamping Ring

4. 4
Visual Failure Mode Identification – Mode Interrelation
Event Strings and Interrelation of Multiple Visual Cues:
The next slides will highlight examples of visual cues which are indicators of events
that have occurred in a flexible coupling’s rubber element due to environmental
conditions and system related forces exerted on the coupling such as:
 Applied torque
 Shock torque
 Torsional vibration levels
 Oscillation (reversing torques)
 High ambient temperature
 Heat conduction though shafting
 Oil contamination
 Exposure to solvents and cleaning agents

5. 5
Visual Failure Mode Identification – Mode Interrelation
Event Strings and Interrelation of Multiple Visual Cues:
Visually identifiable features related to coupling failures can provide an indication of
the types of events that have occurred prior to the rubber element failing – normally
this means the working area of the rubber that was stressed and stretched while
transmitting system torque has in some way separated/sheared with a mechanical
fuse effect thus stopping further transmission of torque in the driveline.
NOTE: In the majority of cases these visual indicators are highly interrelated in a
string of events such that one event which gives a certain visual cue leads to second
or third events producing other visual cues (domino effect). It is uncommon to have
only a single effect produce a singular visual indicator.
For these reasons it is suggested you consult GKN Stromag for further analysis.
This includes possible dissection and chemical testing to determine more
conclusively what chain of events has most likely caused the rubber element to
fail/shear.

6. 6
Visual Failure Mode Identification – Force Break
Torque Overload: system torque exceeds the coupling’s maximum torque
capacity TKmax (typically > 300% of nominal torque rating)
 Shock torques – momentary load spikes
 Driveline system lockup – rubber element shears as planned design of
mechanical fuse effect to protect driveline components
(examples: pump stoppage from debris intrusion, seized bearings)
 Visual Cue: 45° tears all in one direction, distributed around the face of the
shear plane (surface where the two sections of rubber have separated or
sheared)
 High Shock Torque Visual Cue: separation/removal if drive teeth on plug-in
style couplings (PVN, PVA, PS)

7. 7
Visual Failure Mode Identification – Vibration Break
Vibratory Torque Overload: high alternating torque (system oscillation) exceeds
the coupling’s admissible alternating torque capacity
TKW (in units of Nm or lb-ft)
 Visual Cue: 45° tears in both directions from high amplitude oscillations
forming “X” shaped tear patterns distributed around the face of the shear
plane (surface where the two sections of rubber have separated or sheared)

8. 8
Visual Failure Mode Identification – Vibration Break
Excessive Power Loss: vibration energy in the system exceeds the
coupling’s permissible power loss
PkV (admissible damping power in Watts)
 Damping in the rubber absorbs vibration energy and converts it to heat
energy, and this heat must be dissipated by cooling
(also linked to high ambient temperatures that restrict cooling)
 If more vibration energy is absorbed by the rubber than can be released
by cooling, the rubber melts from the inside out at the working zone
(shear area) of the rubber element where it is most stretched under load
 Visual Cue: face of shear plane without repeating tear patterns and few
45° tears in one direction at random locations; inside of shear plane area
melted from the inside out with voids in the rubber from gasification
(essentially a melted ring where the rubber has been vaporized)
 Example Causes: engine issues such a cylinder misfire, governor
hunting on older engines, trouble maintaining load (sensor faults, fuel rail
supply issues, water contamination in the fuel, air feed restriction, etc...)
and therefore the engine cycles its speed repeatedly when loaded

9. 9
Visual Failure Mode Identification – Misalignment Effects
Angular Misalignment:
 Visual Cue: ramped tooth wear uniformly angled toward front
or rear on plug-in style couplings (PVN, PVA, PS models)
Radial Misalignment:
 Visual Cue: tooth base cracks, outer/radial edge of rubber
element teeth abraded from offset compression pressure within
cast aluminum flywheel ring on plug-style couplings (PVN, PVA, PS models)
Axial Misalignments:
 Visual Cue: pump side protruding rubber teeth cut by inside edge of cast
aluminum flywheel ring on plug-style couplings (PVN, PVA, PS)
 Visual Cue: flywheel side face of rubber element discolored by heat conduction
from contact with engine flywheel on plug-in style couplings (PVN, PVA, PS)
 Visual Cue: bolt-though steel bushings torn out of rubber where hub flange and
clamp ring attach to rubber element (PS model)

10. 10
Visual Failure Mode Identification – Environment/Chemical
High Ambient Temperature – Heat Induced Power Loss Failure:
 Damping in the rubber converts vibration energy to heat energy, and this heat
must be dissipated by cooling. If the ambient temperure is so high that heat
cannot be removed by cooling, the rubber melts from the inside out at the
working zone of the rubber element (similar to excessive power loss failure)
 Maximum Allowable Ambient Temperature for Natural Rubber: 80°C or 176°F
 Visual Cue: shear plane edges without repeating pattern and few 45° tears in
one direction at random locations
Oil Impregnation/Contamination:
 Visual Cue: swollen rubber at surface from oil contamination
(example: engine crankshaft seal failure)
Chemical Contamination:
 Visual Cue: rubber surface shows features such as blisters, pitting, white dust,
or extensive surface cracking
(example: excessive use of cleaning agents entering vent holes)

11. 11
Load Induced Failure: Shock Torque
Reasons: TS,reg (t) > TKmax
or TS,irreg (t) > 1.5 TKmax
the machine locks up
sudden overload
Picture:
plain surfaces
no heating
cracking at 45° angle
hardening

12. 12
VA 421.65 W
 Crack is located near the
hub (inner ring)
 Damage at the surface
shows numerous 45°
cracks in the same
direction
 Wavy shear surface
Force Break: Torque Overload
12

13. 13
PS 42.70 W
 Damage to the highly
stressed bushing holes
(Angle approximately 45°)
 Flat shear surface with no
indication of melting
 No signs of heat input like
embrittlement or smearing of
the rubber
Force Break
13

14. 14
V 42.80 W
 Cracked near mounting
points
 Further cracks pass under
45° in the rubber element
 No signs of heat input like
embrittlement or smearing
of the rubber
Force Break
14

15. 15
Vibration Break
Reasons: Tw(n) > TKW
high alternating torque
Picture:
slowly cracking – lifetime reduction
zig-zag cracking edges
cracking at 45° angle
no heating
Vibration Break

16. 16
16
Common symptom during vibration break:
Shearing is located in the zone which is most stretched
during loads.
Vibration Break

17. 17
VN 35031
 Shearing is located in the
working zone which is most
stretched during loading
 Melting has occured from the
inside out (voids from
gasification)
 Rubber is very tender and
slimy in the shear zone
 No visible 45° tears
Vibration Break
17

18. 18
 Temperature
 Ozone, Oxygen
 Moisture
 Oil, Solvents, Cleaning Agents
External Influences
18

19. 19
Vibration Break: Heat Induced Power Loss Failure
Reasons: Pv(n) > PKV
hot environment
low air flow
internal rubber friction
result: heating
Picture:
internal melt down because of heat
cracking is clean and uniform
surface feels oily

20. 20
VN 43351
 Increased Shore hardness
of the material due high
ambient temperature
 Embrittled surface with 45°
cracks due to vibratory torque
load
Temperature Influence, High Ambient Temperature
20

21. 21
VN 43331
 Failure of the rubber-metal
bonding due to high temperature
of the connected gear shaft
transfering heat to center ring
T ≥ 110°C or 230°F
Heat Conduction
21

22. 22
[DOCUMENT NAME]

23. 23
[DOCUMENT NAME]