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Gear Drive
Nov 26, 2018

Gear Drive

 

Gear transmission refers to the device which transmits motion and power by gear pair, which is the most widely used mechanical transmission mode in all kinds of modern equipment. Its transmission is more accurate, high efficiency, compact structure, reliable work, long life.

 

Characteristic

In various transmission forms, gear transmission is the most widely used in modern machinery. This is because gear drives have the following characteristics:

 

1) high transmission accuracy. As mentioned earlier, belt transmission can not guarantee the accurate transmission ratio, chain drive can not achieve a constant instantaneous transmission ratio, but the modern commonly used involute gear transmission ratio, in theory is accurate, constant. This is not only a key requirement for precision machinery and instruments, but also an important condition for reducing dynamic load and realizing smooth transmission under high speed load.

 

2) wide scope of application. Gear transmission transmission of the power range is extremely wide, can be from 0.001W to 60000kW, circumference speed can be very low, can also be up to 150m/s, with transmission, chain drive are difficult to compare.

 

3) can realize parallel shaft, intersecting shaft, staggered shaft and other space between any two axes of transmission, which is also with transmission, chain drive can't do.

 

4) reliable work and long service life.

 

5) high transmission efficiency, generally 0.94 ~ 0.99.

 

6) higher requirements for manufacturing and installation lead to higher costs.

 

7) strict requirements on environmental conditions, except for a few cases of low speed and low precision, generally need to be placed in the box cover to prevent dust and scale, but also need to pay attention to lubrication.

 

8) it is not suitable for transmission of two shafts far apart.

 

9) vibration reduction and impact resistance is not as good as belt drive and other flexible transmission.

 

Types

There are many types of gear transmission, according to different classification methods can be divided into different types.

 

1.According to the transmission ratio

According to whether the transmission ratio of a pair of gears is constant, it can be divided into fixed transmission ratio and variable transmission ratio gear transmission.Gear transmission mechanism of variable transmission ratio gear is generally non-circular, so it is also known as non-circular gear transmission, it is mainly used in some machinery with special requirements.And the gear in gear transmission mechanism of constant transmission ratio is circular, so it is also called circular gear transmission.

There are many types of gear transmission with fixed transmission ratio. According to whether the axis of rotation of the main and secondary driving wheels is parallel, it can be divided into two categories, namely, plane gear transmission and space gear transmission.

 

2.According to tooth profile

According to the shape of tooth profile curve, it can be divided into involute gear drive, cycloid gear drive, circular arc gear drive and parabolic gear drive.Involute gear transmission is the most widely used.

 

3.In terms of working conditions

According to the working conditions of gear transmission, it can be divided into closed gear transmission, open gear transmission and semi-open gear transmission.Open gear transmission gear teeth exposed, dust is easy to fall on the tooth surface;Closed gear transmission gear teeth closed in the box, can ensure good working conditions, widely used;Semi - open gear transmission than open gear transmission working conditions are better, the large gear part immersed in the pumping pool and a simple protective cover, but there are still external invasion.

 

4.According to the hardness of tooth surface

According to the hardness of tooth surface, it can be divided into soft tooth surface gear transmission and hard tooth surface gear transmission.When the hardness of two wheels (or one of them) tooth surface is less than or equal to 350HBW, it is called soft tooth surface drive;When the hardness of tooth surface of both wheels is >350HBW, it is called hard tooth surface drive.Soft tooth surface gear transmission is often used for the general low and medium speed gear transmission which does not require too much precision, and hard tooth surface gear transmission is often used for the gear transmission with strong bearing capacity and compact structure.

Although there are many types of gear transmission, involute spur gear transmission is the simplest and most basic one.

 

Failure Mode 

Gear transmission is to rely on the meshing of teeth to work, gear teeth are directly involved in the part of the work, so the failure of gear mainly occurs in the gear teeth.The main failure forms include gear tooth fracture, tooth pitting, tooth surface wear, tooth surface adhesion and plastic deformation.

 

1.Tooth broken

There are usually two kinds of gear tooth fracture: one is due to repeated bending stress and stress concentration caused by fatigue fracture;The other is due to sudden severe overload or impact load action caused by overload fracture.Especially, gear made of brittle materials (cast iron, hardened steel, etc.) is more prone to tooth fracture.Both types of fracture start on the tensile stress side of the tooth.Increasing the radius of tooth root transition Angle, improving the mechanical properties of materials, reducing the surface roughness to reduce stress concentration, and strengthening the treatment of tooth root (such as shot peening, rolling extrusion) can improve the fracture resistance of tooth.

 

2.Tooth pastry erosion

When the gear teeth work, the front meshing is under the repeated action of alternating contact stress, and a number of small cracks will occur on the tooth surface close to the pitch line.As the crack expands, small pieces of metal will flake off, a phenomenon known as tooth pitting.The extension of pitting on tooth surface will affect the stability of transmission, and generate vibration and noise, resulting in gear failure.Pitting corrosion is a common failure mode of well-lubricated closed gear transmission.Increasing the hardness of the tooth surface and reducing the surface roughness value can improve the pitting resistance of the tooth surface, open gear transmission, because the tooth surface wear faster, there is no pitting.

 

3.Tooth surface wear

When the gear tooth meshes, due to relative sliding, especially when the external hard particles enter the working face of meshing, the surface of the gear tooth will be worn.After the gradual wear of the tooth surface, the tooth surface will lose the correct tooth shape, seriously resulting in the tooth is too thin and broken, tooth surface wear is the main failure form of open gear transmission.In order to reduce wear, important gear transmission should use closed transmission, and pay attention to lubrication.

 

 

4.Tooth surface glue

In high-speed heavy-load gear transmission, the tooth surface of high pressure, high temperature, the lubrication effect is poor, when the instantaneous temperature is too high, will make two tooth surfaces of metals by partial melting, adhesion, when two tooth surfaces do relative movement, sticking to tear, and the tooth surface along the sliding direction of ribbon or the scars of large area, the transmission is not easy to form the oil film of low speed and heavy loading, friction heat is not big, but may also appear cold flow due to overloading.Lubricants with high viscosity or good adhesion resistance are used to reduce the surface roughness to form good lubrication conditions.Increasing the hardness of tooth surface can enhance the anti-bonding ability of tooth surface.

 

5.Plastic deformation of tooth surface

Soft tooth surface gear with low hardness, at low speed and heavy load, because the pressure on the tooth surface is too high, under the action of friction force, the metal on the tooth surface produces plastic flow and loses the original tooth shape.Improving the hardness of the tooth surface and adopting the lubricating oil with higher viscosity are helpful to prevent or reduce the plastic deformation of the tooth surface.

 

Design Criteria

Different forms of gear transmission failure are unlikely to occur simultaneously on a pair of gears, but they affect each other.For example, pitting corrosion on the tooth surface will aggravate the wear on the tooth surface, and serious wear will lead to the fracture of the tooth.Under certain conditions, the failure forms of tooth fracture and pitting are the main ones.Therefore, when designing gear transmission, the main failure forms that may occur should be analyzed according to the actual working conditions to determine the corresponding design criteria.

For closed soft tooth surface (hardness less than 350HBW) gear transmission.The lubrication condition is good, the tooth surface pitting will be the main failure form, in the design usually according to the tooth surface contact fatigue strength design, and then according to the tooth root bending fatigue strength check.

For closed hard tooth surface (hardness >350HBW) gear transmission, the pitting corrosion resistance is strong, the gear tooth fracture capacity is large, in the design and calculation.Usually according to the tooth root bending fatigue strength design, and then check the tooth surface contact fatigue strength.

Open gear transmission, the main form of failure is tooth wear.However, because the wear mechanism is relatively complex, there is no mature design and calculation method, so it can only be calculated according to the bending fatigue strength of the tooth root, increase the tooth thickness by 10% ~ 20% of the method of increasing the modulus, so that it has a long service life, to consider the impact of wear.

 

lubricating System

The lubrication mode of gear transmission mainly depends on the circumference speed of gear.

1) for open gear and low speed (v< 0.8-2m /s), light load and closed gear transmission which is not very important, manual lubricating oil or grease can be added regularly.

2) for v=2 ~ 12m/s closed gear transmission, use oil immersion lubrication.The large gear is immersed in the oil pool, and the oil is carried to the meshing surface by means of gear transmission.For cylindrical gear, the oil immersion depth should be 1 ~ 2 tooth heights, and the maximum oil immersion depth should not exceed 1/3 of the dividing circle radius of the large gear.The amount of oil in the oil tank is related to the transmitted power. The single-stage transmission is 0.35~ 0.7l /kW, and the multi-stage transmission increases exponentially according to the series.When the oil immersion depth of the low-speed gear in the multi-stage transmission is appropriate, but the high-speed gear fails to soak in the oil, a tanker can be used to supply oil to the high-speed gear.The depth of oil pool should not be less than 30 ~ 50mm in general, in order to prevent gear rotation will be oil pool bottom impurities stirred up, resulting in unclean lubricating oil, aggravating tooth wear.There should be sufficient oil in the oil tank to ensure heat dissipation.

3) for v>12m/s closed gear transmission, it is recommended to use oil injection lubrication to spray lubricating oil with a certain pressure to the gear tooth engagement surface.When w is less than or equal to 25m/s, the nozzle can be located at the tooth edge of the gear tooth.When v is greater than or equal to 25m/s, the nozzle should be located on the rodent side, timely cooling the teeth just engaged, and lubrication.Oil injection lubrication oil supply is full and continuous, suitable for high speed, heavy load of important gear transmission.