4. - Gear failures occur rarely
- A gear pair has not failed until it can
no longer be run.
This condition is reached when:
- One or more teeth have broken
away preventing
transmission.
- Teeth are so badly damaged
(worn-out) that unacceptable
vibration and noise are set up
when the gear runs.
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Gear failures
5. Gear failures
Gear failures can be any or combination of
a. Surface fatigue (pitting)
b. Wear
c. Plastic deformation
d. Tooth breakage.
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6. 6
DESTRUCTIVE (PROGRESSIVE) PITTING
INITIAL (CORRECTIVE) PITTING
DESTRUCTIVE (PROGRESSIVE) PITTING
THRU HARDENED GEARS
Surface fatigue is the
failure of a material as a
result of repeated surface
or sub-surface stresses
beyond the endurance
limit of the material.
SURFACE FATIGUE
7. WEAR
Abrasive sometimes called cutting
wear occurs when hard particles
slide and roll under pressure, across
the tooth surface. Hard particle
sources are: dirt in the housing,
sand or scale from castings, metal
wear particles from gear teeth or
bearings, particles.
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8. WEAR
• SCRATCHING is a form of abrasive wear,
characterized by short scratch-like lines in the
direction of sliding. This type of damage is
usually light and can be stopped by removing
the contaminants that caused it
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9. WEAR
• ADHESIVE WEAR results from high attractive
forces of the atoms composing each of two
contacting, sliding surfaces.
• Teeth contact at random asperities and a
strong bond is formed. The junction area
grows until a particle is transferred across the
contact interface.
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10. WEAR
• SCUFFING WEAR is the Falk term for an
adhesive type wear occurring at normal
temperatures where smooth burnished
appearing radial striations are observed in the
direction of sliding on the tooth surfaces.
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11. PLASTIC FLOW
• Plastic flow is the cold working of the tooth
surfaces, caused by high contact stresses and
the rolling and sliding action of the mesh. It is
a surface deformation resulting from the
yielding of the surface and subsurface
material
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12. BREAKAGE
• Breakage is the ultimate type of gear failure.
Bending loads on gear teeth usually cause the
highest stresses at the root fillets and at the
tooth profile/root fillet junctions. A gear tooth
is a cantilever plate with tensile stresses on
the contact side of the tooth and compressive
stresses on the opposite side.
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14. • viscosity.
– The viscosity must be high enough to maintain adequate lubricating
film between meshing teeth under all conditions.
• anti-wear and extreme pressure (EP) property,
• oxidation resistance,
• anti-corrosion property,
• anti-foaming property,
• demulsibility, i.e. good property to separate from water;
These properties are attained by use of suitable additives to
the base oil.
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Properties of gear lubricants
15. Lubricant selection for gears must be always be based on
manufacturer’s recommendations. The influencing factors
for lubricant selection are:
- gear type, speed, and speed reduction ratio;
- gear material and surface finish;
- operating temperature; and load characteristics.
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Gear lubricant selection
16. Lubrication application
Application of lubricant to meshing gear teeth is
preferably applied at the parting pair in which
the oil is used for cooling and at the same
time reduces amount of oil at the next
engagement cycle.
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