1. The gear’s construction and function

Gears can be broadly classified into two categories: the tooth ring and the wheel body, notwithstanding their varied forms and sizes resulting from their many machine purposes. The following categories comprise common cylindrical gears: disk gear, sleeve gear, internal gear, shaft gear, sector gear, and rack. Disk gear is the most commonly used type among them.

A ring or rings may be present on a cylindrical gear. The typical single-ring gear is easily manufactured; nevertheless, the shoulder frequently affects the small tooth ring of double or triple gears, restricting the manufacturing techniques that can be utilized.

2. Precision requirements of cylindrical gear

 The manufacturing accuracy of the gear itself has a great impact on the working performance, bearing capacity and service life of the whole machine. According to the use conditions of the gear, the following requirements are put forward for the gear transmission:

(1) Motion accuracy – it is required that the gear can accurately transmit the motion, and the transmission ratio is constant, that is, the Angle error of the gear in a revolution does not exceed a certain range.

(2) Smoothness of work – the gear transmission movement is required to be smooth, and the impact, vibration and noise are small. This requires limiting the change in the instantaneous speed ratio when the gear rotates to be small, that is, to limit the Angle error in a short period.

(3) Contact accuracy – When the gear transmits power, in order not to make the contact stress too large due to the uneven distribution of load, causing premature wear of the tooth surface, which requires that the tooth surface contact should be uniform when the gear is working, and ensure that there is a certain contact area and meet the requirements of the contact position.

(4) Tooth side clearance – When the gear is required to be driven, there is a certain gap between the non-working tooth surfaces to store lubricating oil and compensate for the dimensional changes caused by temperature and elastic deformation and some errors in processing and assembly.

3. Gear materials

Equipment should be utilized in compliance with the suitable material’s working conditions. The machining performance and service life of gears are directly impacted by the material selection.

• • General gear selection for low and medium carbon alloy steels, such as 20Cr, 40Cr, 20CrMnTi, etc., and medium carbon steels, such as 45 steel.
• • 38CrMoAlA nitride steel is a suitable material for important gears with higher requirements. Other materials that can be used for non-force transmission gears include cast iron, plywood, and nylon.

4. Heat treatment of gears

In gear processing, two heat treatment processes are arranged according to different purposes:

1) Blank heat treatment: arrange pre-heat treatment normalizing or tempering before and after the tooth blank processing, the main purpose is to eliminate the residual stress caused by forging and roughing, improve the machinability of the material and improve the comprehensive mechanical properties.

2) Tooth surface heat treatment: After tooth shape processing, in order to improve the hardness and wear resistance of the tooth surface, carburizing hardening, high-frequency induction heating hardening, carbonitriding and nitriding heat treatment processes are often carried out.

5. Blank gear

Bar material, forging, and casting make up the majority of the blank form of gear. Bar materials are utilized for small-diameter gears with a straightforward construction and minimal strength requirements. Gears with a diameter larger than 400–600 mm are often cast blanks, but multipurpose forgings are also frequent when the gear demands high strength, wear resistance, and impact resistance. For large and low precision gears, the gear teeth can be cast directly to minimize machining; for small and complexly shaped gears, new processes like powder metallurgy, hot rolling, cold extrusion, pressure casting, precision casting, and precision casting can be used to produce tooth billets with gear teeth, improving labor productivity and conserving raw materials.

The selection of machining scheme of tooth blank

For shaft gear and sleeve gear, the processing process is basically similar to the general shaft and sleeve gear, the processing process of disc gear is mainly discussed.

The processing technology of the tooth blank mainly depends on the structure of the gear body and the type of production.

(1) A large number of mass production of tooth blank processing

When a large number of medium size tooth billets are processed, the process plan of “drilling one pull one tool car” is used:

1) Drilling or reaming with blank outer circle and end face positioning;

2) Lacon;

3) The outer circle, end face, grooving and chamfering of the coarse finishing car on the multi-tool semi-automatic lathe are positioned with holes.

This process scheme can be composed of assembly line or automatic line because of the use of efficient machine tools, so the production efficiency is high.

The “car pull a car” process plan is frequently used in the processing of teeth billets produced in batches.

1)       To the outer circle of the tooth billet or wheel placement, fine turning outer circle, end face, and inner hole; 2) Support the spline hole or pull-hole using the end face; 3) Aligning the fine car’s outer circle and end face with holes.

This solution can be achieved by horizontal lathes or turret lathes and broaching machines. It is characterized by stable processing quality and high production efficiency. When the tooth blank hole has steps or the end face has grooves, it can make full use of the multi-tool on the turret lathe to carry out multi-station machining, and finish the tooth blank machining on the turret lathe at one time.

Gear tooth processing method

The tooth shape machining of gear ring is the core of the whole gear machining. There are many processes in gear processing, which are for the shape of the gear processing service, the purpose is to finally obtain the gear that meets the accuracy requirements.
According to the machining principle, the tooth shape can be divided into forming method and developing method. The forming method is the method of cutting the tooth surface with the forming tool that matches the shape of the tooth groove of the gear to be cut, such as milling teeth, drawing teeth and forming teeth grinding. The developing method is the method of cutting the tooth surface according to the meshing relationship between the gear tool and the workpiece, such as hobbing, gear shaper, shaving, grinding and honing.
The choice of tooth shape machining scheme mainly depends on the precision grade, structural shape, production type and production conditions of the gear. For gears with different precision grades, the commonly used tooth shape machining scheme is as follows:

1) Gear with grade 8 accuracy. Gear hobbing or gear shaper can meet the requirements. For hardened gear can be used: roll (insert) tooth – tooth end machining – quenching – correction hole processing scheme. But the machining precision of tooth shape should be improved before quenching.
2) 6-7 precision gear. For hardened gears can be used: rough hobbing – fine hobbing – tooth end machining – fine shaving – surface quenching – calibration datum – gear honing.
3) Gear with grade 5 accuracy or above. Generally used: rough hobbing – fine hobbing – tooth end machining – quenching – calibration datum – rough grinding – fine grinding. Grinding is the machining method with the highest precision and the smallest surface roughness value in tooth profile machining at present, and the highest accuracy can reach 3-4 levels.

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