Due to manufacturing (Read bevel gear production process), installation errors, and dynamic impact, the gear will fail during use, resulting in the loss of the original function of the gear. Standard failure modes of gears include tooth surface wear, tooth surface bonding and galling, tooth surface contact fatigue, and broken teeth.
Teeth surface wear
Gear teeth surface wear is an inevitable physical process in the gear meshing process. Gear meshing transmits power through the interaction between tooth surfaces, so the gear meshing process will inevitably lead to wear of the gear contact surfaces.
Suppose the teeth surface of the gear is smooth and flat after wear. In that case, there is no macroscopic tear and damage, and the tolerances are within the allowable range; this kind of wear is called normal wear, and ordinary wear does not affect the normal function of the gear.
Suppose the lubricating oil contains hard particles, the contact parts of the tooth surface are poorly lubricated, or the dynamic impact is significant. In that case, the gear meshing process often leads to abnormal early wear on the teeth surface. At this time, the shape of the teeth’ surface changes, and the gear loses its normal function. Vibration and noise increase accordingly, and severe tooth surface wear will lead to permanent gear failure.
According to the different causes of wear, teeth surface wear can be divided into:
- Abrasive wear
- Corrosion wear
- Impact wear
Teeth surface gluing and galling
Tooth surface gluing and abrasion generally occur in heavy-duty or high-speed gear transmissions. It refers to the phenomenon that the tooth surfaces contact and “bond” under a certain pressure, and the metal falls off or transfers when the tooth surfaces separate. If the process occurs repeatedly, it will cause damage to the tooth surface, and the gear will lose its normal function, which is a relatively authoritarian state of wear.
The leading cause of tooth surface gluing and galling is the tooth surface oil film breakdown caused by poor lubrication.
The appearance of tooth surface gluing and abrasion is characterized by striped grooves on the tooth surface, especially at the tooth’s top and root. The gear’s vibration and noise increase significantly at this time.
If new gears are subjected to heavy loads or run at high speeds without sufficient running-in, local gluing may occur, resulting in tooth surface abrasions.
Teeth surface contact fatigue
Tooth surface contact fatigue refers to local metal shedding and damage on the tooth surface under cyclic pulsating shear stress.
During the meshing process of the gears, there are both relative rolling motions and relative sliding motions between the tooth surfaces. The alternating action of these two motions will cause the tooth surfaces to bear the cyclic pulsating impact force, resulting in contact fatigue.
According to the different parts of the tooth surface fatigue, tooth surface gluing and abrasion can be divided into:
- Pitting fatigue peeling
- Shallow fatigue exfoliation
- Fatigue spalling of the hardened layer
Broken teeth refer to the phenomenon that cracks appear at the gear’s root under cyclic bending stress, and the cracks continue to spread, resulting in the fracture of the teeth of the gear.
The force of the gear is similar to that of a cantilever beam. During the meshing process of the gear, the external load will generate cyclic bending stress at the root of the gear. When the cyclic bending stress is too large or the time is too long, it will cause cracks at the tooth root. When the external load exceeds the gear, The gear will break when the external force it can withstand is at its limit.
Excessive shock loads, severely unbalanced loads, and uneven materials can lead to broken teeth in gears.