Gear is often in the key parts of mechanical equipment, so the wear failure of gear will bring great losses to the equipment, gear failure is often caused by the failure of gear teeth. In this paper, we share four cases of gear transmission failure and solutions.

1.Introduction of reducer and gear

1.1 type of reducer

Reducer includes worm gear reducer, harmonic reducer and planetary reducer. Among them, the worm gear reducer has the function of reverse self-locking, the deceleration is relatively large, the input shaft and output shaft are not on the same axis, nor on the same plane, so the transmission efficiency and accuracy of this type of reducer are not high.

Harmonic reducer mainly transfers power by means of controllable elastic deformation of flexible components, with high accuracy, but the flexible wheel has short service life, poor ability to bear impact force and low transmission rate.

Planetary reducer has compact structure, small return clearance, high precision, long service life and large rated output torque, but its cost is too high.

Gear is often in the key parts of mechanical equipment, so the wear failure of gear will bring great losses to the equipment, gear failure is often caused by the failure of gear teeth. In this paper, we share four cases of gear transmission failure and solutions.

1.Introduction of reducer and gear

1.1 type of reducer

Reducer includes worm gear reducer, harmonic reducer and planetary reducer. Among them, the worm gear reducer has the function of reverse self-locking, the deceleration is relatively large, the input shaft and output shaft are not on the same axis, nor on the same plane, so the transmission efficiency and accuracy of this type of reducer are not high.

Harmonic reducer mainly transfers power by means of controllable elastic deformation of flexible components, with high accuracy, but the flexible wheel has short service life, poor ability to bear impact force and low transmission rate.

Planetary reducer has compact structure, small return clearance, high precision, long service life and large rated output torque, but its cost is too high.

1.2 gear related knowledge
The accuracy of gear generally considers economic factors, but the accuracy level of gear has a great influence on the generation of noise and backlash, and the noise generated by high-precision gear is smaller.

For the gear width, increasing the gear width can reduce the unit load under constant torque. When the torque is constant, the noise of the pinion is greater than that of the big gear. Increasing the gear width can also increase the bearing capacity of the gear.

Small pitch can ensure as many teeth contact at the same time as possible, which can reduce the transmission noise and improve the transmission accuracy. Smaller pressure angle can reduce the noise and increase the accuracy of gear operation.

In the design and manufacture of gear, we should pay attention to gear error, including pitch error, tooth direction error, etc. the elimination of these errors can improve the accuracy of gear and ensure the efficiency of reducer operation.

When the assembly is not concentric, it will lead to the unbalanced operation of the shafting. If the high-precision gear is unbalanced, it will have a great impact on the accuracy of the gear operation. The effectiveness of the reducer will also be damaged.

The tooth surface hardness is also an important factor affecting the efficiency of the reducer gear operation. Low tooth surface hardness will make the reducer gear operation failure.

2 Failure forms and solutions of Reducer Gears
2.1 failure of gear broken teeth

The failure caused by gear broken is the most serious situation in the whole mechanical engineering, mainly including overload fracture, fatigue fracture, random fracture, etc. for overload fracture, it refers to the tooth broken when the gear is subjected to one or more serious overloads. In this case, the fracture of the gear is generally located at the root of the gear, and the fracture is generally flat and rough.

Fatigue fracture refers to the gear in a long time and high cycle state, which makes the root of the tooth produce fatigue cracks and fracture. The fracture can be divided into fatigue fracture surface and final fracture surface. There is no plastic deformation in the fatigue fracture area, and the fatigue fracture may occur at the end of the gear. Random fracture usually occurs in the root of gear, but special cases also occur in other parts.

There are many measures to prevent gear fracture, such as increasing the fillet radius of the tooth root, eliminating the machining tool marks in this part, so as to reduce the stress concentration of the tooth root, increasing the stiffness of the shaft and support, so as to reduce the local stress, and carrying out cold treatment such as shot peening and rolling on the gear teeth, so as to improve the tooth surface.

2.2 the tooth surface is worn and cannot be driven

In the process of gear transmission of gear reducer, the wear of tooth surface is inevitable. The wear of tooth surface also includes adhesion wear, abrasive wear, abrasion, corrosion wear and burn. For adhesion wear, lubrication is mainly referred to. If the lubricating oil layer is complete and the thickness is quite thick, the contact between metals will be reduced and no wear will occur.

If the temperature and pressure of oil film are the same, the higher the viscosity of oil, the smaller the wear. In the case of low speed, heavy load, extreme temperature, the oil film may be damaged. In this case, the wear occurs on most gear surfaces except for the pitch circle. This kind of wear can be improved by improving the hardness of the tooth surface, reducing the roughness of the tooth surface and increasing the viscosity of the oil.

Abrasive wear and abrasion mainly refers to the unclean lubricant with impurities or the grinding debris caused by the contact between metals during operation, which causes the gear of the reducer to wear and make the transmission work invalid.

If the gear is in good alignment and the scratch is not caused by the micro convex body encouraged on the pool surface, the wear of the whole gear will extend to most areas. Corrosion and wear refers to the chemical reaction between some substances in lubricant and metal on tooth surface, which causes metal corrosion.

2.3 tooth surface pitting and bonding

In the process of gear transmission, the contact stress of each point on the contact surface presents a pulsating cycle change. After a period of time, cracks will be generated due to the metal fatigue on the contact surface, and the metal spalling and pitting will be formed when the cracks expand. This situation can be prevented by improving the tooth surface hardness and reducing the surface roughness.

In the case of high speed, heavy load or lubrication failure, the gear surface will form local high temperature, and the contact surface will be glued. This situation is called gluing. In order to prevent gluing, the hardness and finish of the gear surface should be improved. For different types of gears, different steel grades should be adopted, and the correct selection of lubricating oil is an important measure to reduce gear failure.

2.4 failure caused by plastic deformation of tooth surface

Plastic deformation mainly includes ripple and ridge extension caused by rolling and hammering deformation, which is mainly due to the soft gear material, resulting in metal flow along the friction direction. The temperature and humidity changes in the working environment of the gear will also cause the plastic deformation of the tooth surface. Lubricating oil with higher viscosity can be selected to prevent the plastic deformation and improve the hardness of the tooth surface.

To find out the real reason of gear transmission failure of reducer can solve the corresponding problems, ensure the effectiveness of mechanical operation process, improve the operation efficiency, and promote the development of work.

1.2 gear related knowledge

The accuracy of gear generally considers economic factors, but the accuracy level of gear has a great influence on the generation of noise and backlash, and the noise generated by high-precision gear is smaller.

For the gear width, increasing the gear width can reduce the unit load under constant torque. When the torque is constant, the noise of the pinion is greater than that of the big gear. Increasing the gear width can also increase the bearing capacity of the gear.

Small pitch can ensure as many teeth contact at the same time as possible, which can reduce the transmission noise and improve the transmission accuracy. Smaller pressure angle can reduce the noise and increase the accuracy of gear operation.

In the design and manufacture of gear, we should pay attention to gear error, including pitch error, tooth direction error, etc. the elimination of these errors can improve the accuracy of gear and ensure the efficiency of reducer operation.

When the assembly is not concentric, it will lead to the unbalanced operation of the shafting. If the high-precision gear is unbalanced, it will have a great impact on the accuracy of the gear operation. The effectiveness of the reducer will also be damaged.

The tooth surface hardness is also an important factor affecting the efficiency of the reducer gear operation. Low tooth surface hardness will make the reducer gear operation failure.

2 Failure forms and solutions of Reducer Gears
2.1 gear failure caused by broken teeth

The failure caused by gear broken is the most serious situation in the whole mechanical engineering, mainly including overload fracture, fatigue fracture, random fracture, etc. for overload fracture, it refers to the tooth broken when the gear is subjected to one or more serious overloads. In this case, the fracture of the gear is generally located at the root of the gear, and the fracture is generally flat and rough.

Fatigue fracture refers to the gear in a long time and high cycle state, which makes the root of the tooth produce fatigue cracks and fracture. The fracture can be divided into fatigue fracture surface and final fracture surface. There is no plastic deformation in the fatigue fracture area, and the fatigue fracture may occur at the end of the gear. Random fracture usually occurs in the root of gear, but special cases also occur in other parts.

There are many measures to prevent gear fracture, such as increasing the fillet radius of the tooth root, eliminating the machining tool marks in this part, so as to reduce the stress concentration of the tooth root, increasing the stiffness of the shaft and support, so as to reduce the local stress, and carrying out cold treatment such as shot peening and rolling on the gear teeth, so as to improve the tooth surface.

2.2 the tooth surface is worn and cannot be driven

In the process of gear transmission of reducer, the wear of tooth surface is inevitable. The wear of tooth surface also includes adhesive wear, abrasive wear, scratch, corrosion wear and burn. For adhesive wear, it mainly refers to lubrication. If the lubricating oil layer is complete and thick, the contact between metals will be reduced, and wear will not occur.

If the oil film temperature and pressure are the same, the higher the oil viscosity is, the smaller the wear is. Under the conditions of low speed, heavy load and extreme temperature, the oil film may be damaged. In this case, the wear occurs on most tooth surfaces except the pitch circle. This kind of wear can be improved by increasing the hardness of the tooth surface, reducing the roughness of the tooth surface and increasing the viscosity of the oil.

Abrasive wear and abrasion mainly refers to the impurities in the lubricant or the wear debris caused by the contact between metals in the process of operation, which leads to the gear wear of the reducer and makes the transmission work invalid.

If the alignment of the gear is good and the scuffing is not caused by the micro convex body on the pool surface, the wear of the whole gear will extend to most areas. Corrosion wear refers to the chemical reaction between some substances in the lubricant and the metal on the tooth surface, resulting in metal corrosion.

2.3 tooth surface pitting and bonding
In the process of gear transmission, the contact stress of each point on the contact surface presents a pulsating cycle change. After a period of time, cracks will be generated due to the metal fatigue on the contact surface, and the metal spalling and pitting will be formed when the cracks expand. This situation can be prevented by improving the tooth surface hardness and reducing the surface roughness.

In the case of high speed, heavy load or lubrication failure, the gear surface will form local high temperature, and the contact surface will be glued. This situation is called gluing. In order to prevent gluing, the hardness and finish of the gear surface should be improved. For different types of gears, different steel grades should be adopted, and the correct selection of lubricating oil is an important measure to reduce gear failure.

2.4 failure caused by plastic deformation of tooth surface

Plastic deformation mainly includes ripple and ridge extension caused by rolling and hammering deformation, which is mainly due to the soft gear material, resulting in metal flow along the friction direction. The temperature and humidity changes in the working environment of the gear will also cause the plastic deformation of the tooth surface. Lubricating oil with higher viscosity can be selected to prevent the plastic deformation and improve the hardness of the tooth surface.

To find out the real reason of gear transmission failure of reducer can solve the corresponding problems, ensure the effectiveness of mechanical operation process, improve the operation efficiency, and promote the development of work.