Gear Classification and Accuracy

1. Classification of Gears

– Based on the type of motion transmission:

• Spur gears are used to transmit motion between parallel shafts, including straight spur gears and helical gears (Picture a, b, c, d, e).
• Bevel gears are used to transmit motion between two non-parallel shafts, usually perpendicular to each other, including straight bevel gears, helical bevel gears, and spiral bevel gears (Picture g, h, i).
• Worm gears, meshing with a worm shaft, are used to transmit motion between two perpendicular shafts with a large transmission ratio (Picture f).
• Rack and pinion mechanisms consist of a rack meshing with a gear and are used to convert rotary motion into linear motion and vice versa (Picture j).

– Based on the type of meshing, gears are classified into internal gears and external gears.

– Based on technological characteristics, gears are classified as follows: with hub and without hub; plain bore and splined bore; stepped gears (Picture e); and integral shaft gears (Picture i).

2. Accuracy

The accuracy of gears and worm gears is evaluated in accordance with the Vietnamese national standard TCVN. This standard specifies 12 different accuracy grades, ranging from grade 1 to grade 12, in which grade 1 represents the highest accuracy and grade 12 the lowest accuracy. In practice, only accuracy grades 7 to 11 are commonly used.

The accuracy of gears and worm gears is characterized by the following criteria:

• Transmission accuracy: This accuracy is evaluated by the angular error of a gear or worm gear after one revolution. This error is caused by inaccuracies in the manufacturing system. Transmission accuracy is very important for mechanisms requiring precise angular positioning, such as indexing drives of gear-cutting machines or measuring and counting mechanisms.
• Operating stability (running smoothness): Operating stability is evaluated by the cyclic error, defined as the average value of the transmission error, expressed as the ratio between the maximum deviation and the number of gear teeth. This characteristic represents the stability of the rotational speed of the transmission during one revolution of the gear or worm. Fluctuations in rotational speed cause dynamic loads, vibration, and noise in the transmission. Therefore, this accuracy is especially important for power transmissions operating at high speeds.
• Contact accuracy: Contact accuracy is evaluated based on the contact pattern (contact area and shape) of the tooth profile along the tooth length and tooth height and is expressed as a percentage (%).
• Tooth backlash accuracy: Tooth backlash is the clearance between mating tooth flanks in a gear transmission (the larger the gear, the larger the backlash). Backlash is not determined by the accuracy grade of the transmission, but by its function and operating conditions. For example, transmissions requiring precise angular positioning need small backlash, whereas gears used in high-speed turbines require larger backlash.

Based on this, four backlash classes for gear transmissions are specified:

• Zero backlash
• Small backlash
• Medium backlash
• Large backlash

Among these, transmissions with medium backlash are the most widely used in practice.