Gears are mechanical components with teeth on their periphery, designed to continuously engage and transmit motion and power. They mesh with another toothed component to change the speed, torque, and direction of the power source. In the late 19th century, the principles of gear cutting and the emergence of CNC machines and milling cutters based on these principles accelerated the development of gear production. At the same time, the smooth operation of gears received significant attention.

Gear transmission is one of the most widely used forms of power transmission. It can transmit motion and power between any two shafts, providing smooth, reliable, efficient, and long-lasting operation with a compact structure. The range of transmission speeds and power is extensive. Manufacturing gears requires specialized CNC machining shops to achieve the required dimensional and installation precision.

Gears have a wide range of applications and are often used in conjunction with bearings as essential components in various mechanical equipment, such as CNC machines, airplanes, ships, fans, and more.

Manufacturing Process of Gear

  1. Forging of Blanks: Hot die forging is a widely used process for manufacturing gear blanks. Wedge cross-rolling technology is especially suitable for producing blanks for complex stepped shafts. It offers high precision, minimal post-machining allowances, and high production efficiency.
  2. Normalizing: The purpose of this process is to achieve the desired hardness for subsequent gear-cutting operations and prepare the structure for final heat treatment, effectively reducing heat treatment deformation.
  3. CNC Turning: To meet the precision requirements of gear machining, CNC machining centers are used for the machining of gear blanks. Mechanical clamping and non-regrinding cutting tools are employed, allowing for synchronous processing of hole diameter, end faces, and outer diameter in a single setup. This ensures the perpendicularity of the inner hole and end face while maintaining small dimensional variations in mass production. Consequently, gear blank accuracy is improved, ensuring the quality of subsequent machining of gear.
  4. Gear Hobbing: Radial hobbing technology is widely used due to its high efficiency and ease of achieving the required tooth profile and tooth direction modifications.
  5. Heat Treatment: Gears require carburizing and quenching to ensure their excellent mechanical properties.
  6. Grinding: Precision machining is performed on parts of the gear, such as the inner bore, end face, and outer diameter of the shaft, which have undergone heat treatment. This process improves dimensional accuracy and reduces form and positional tolerances.

Types of Gear

Spur Gear (Straight Gear)

Spur gears have teeth that are perpendicular to the axis of the gear. They typically have an equal number of teeth, and their operation involves the mutual engagement of teeth between two gears, transmitting motion from an input shaft to an output shaft.

Advantages of spur gears

  • Simple structure, relatively low manufacturing complexity, and cost.
  • High transmission efficiency, increasing the output power of the transmission.

Disadvantages of spur gears

  • They tend to generate significant noise and vibrations during operation.
  • The relative movement speed between the teeth of two gears is relatively high, which can lead to increased wear and tear.

Helical Gear

Helical gears have teeth with a slant or spiral shape, forming a conical configuration. They achieve automatic variable speed and torque transmission through the engagement of two conical gears.

Advantages of helical gears

  • They can withstand greater loads during the transmission process.
  • The relative movement speed between the gear teeth is lower, reducing wear and noise on the gear surfaces.

Disadvantages of helical gears

  • They have lower efficiency compared to spur gears.
  • The structural complexity and manufacturing challenges can result in more manufacturing defects, leading to reduced efficiency and lifespan of helical gears.

Double Helical Gear

Double helical gears have opposing helical teeth with grooves in between and are used to transmit power between parallel axes.

Advantages of double helical gears

  • Reduces axial forces.
  • Provides smoother transmission and engagement.
  • Lowers noise levels.
  • Offers self-centering capability.
  • High load-carrying capacity.

Disadvantages of double helical gears

  • Challenging to manufacture.
  • High cost.

Spiral Gear

Spiral gears are used to transmit motion between non-intersecting and non-parallel axes. They operate based on the engagement and rolling of helical teeth with sloped surfaces.

Advantages of spiral gears

  • Capable of withstanding significant radial and axial loads.
  • Exhibit lower levels of vibration and noise, ensuring smooth and reliable transmission.
  • The rolling engagement reduces sliding between the gears, enhancing transmission efficiency.

Disadvantages of spiral gears

  • Relatively complex to manufacture, requiring specialized machining processes and equipment.
  • Larger in size.
  • The complexity of manufacturing and the need for specialized processes result in relatively higher production costs.

Internal Gear

Gears with teeth on the inside circumference are referred to as internal gears. Typically, in internal meshing gear mechanisms, the gear located on the gear ring is the internal gear.

Advantages of internal gear

  • Requires less assembly space, making it suitable for machines with space constraints.
  • Achieves transmission efficiency of over 95%.
  • Offers high torque transmission capacity and stability, capable of handling significant loads.
  • Gear machining precision can reach GB5 or even GB4 levels, meeting high-precision transmission requirements.

Disadvantages of internal gear

  • Generates relatively high noise during operation.
  • Demands precise machining and strict assembly processes.
  • Requires lubrication and cooling with lubricating oil.
  • Overloading and excessive loads on the gears can reduce their lifespan.

Straight Rack

Straight rack transmission is a mechanical transmission method based on the engagement between the teeth of a large gear (rack) and a small gear (pinion) to transmit power.

Advantages of Straight Rack

  • High transmission efficiency, typically between 98% and 99%.
  • Smooth operation.
  • Long service life due to high requirements for tooth profile parameters and surface quality.

Disadvantages of Straight Rack

  • Design and manufacturing of straight rack transmission are relatively challenging.
  • Can produce significant vibration and noise at high speeds.

Face Gear

Face gear refers to gears where the geometric profile of the tooth outline on the gear’s tooth surface is not composed of full circles and straight segments but is shaped through machining methods. This results in varying geometric shapes in the tooth grooves at different positions on the tooth surface, allowing for effective engagement.

Advantages of Face Gears

  • The geometric shape of the tooth surface on face gears is determined with high precision through machining.
  • Face gear teeth have ideal meshing characteristics as they are formed by different geometric shapes of tooth grooves on the tooth surface.
  • Face gear transmission offers a larger degree of overlap, which is important for increasing load-bearing capacity and ensuring smooth transmission.

Disadvantages of Face Gears

  • Different-sized and differently toothed face gears require various parameters for machining tools, which can increase the number of required tools and machining costs.
  • Due to the varying shape of the tooth width direction on face gears, there can be root cutting at the inner diameter and tip cornering at the outer diameter.

Straight Bevel Gear

Straight bevel gears have gear axes in different planes that intersect, and their toothed portions are conical in shape. The tooth surface is composed of a small conical surface near the hub’s center and a gradually expanding large conical surface. The line connecting these two conical surfaces with the axis forms a variable pitch cone surface.

Advantages of Straight Bevel Gears:

  • Relatively simple structure, leading to lower manufacturing costs.
  • The manufacturing process is relatively straightforward, with less machining complexity.
  • High transmission efficiency, good transmission precision, and smooth operation.

Disadvantages of Straight Bevel Gears:

  • Prone to wear.
  • Generates higher noise levels and has a shorter lifespan.

Spiral Bevel Gear

Spiral bevel gears are composed of two intersecting conical gears that intersect along a straight line, and their tooth profiles are spiral in shape.

Advantages of Spiral Bevel Gears

  • Smooth transmission with low noise.
  • High transmission efficiency.
  • Capable of bearing larger loads.

Disadvantages of Spiral Bevel Gears

  • Relatively challenging to manufacture.
  • Lower transmission precision compared to other transmission methods.

Worm Gear

Worm gear transmission is derived from the helical gear transmission with intersecting shafts. The small gear’s teeth wrap around the pitch cylinder surface for more than one turn, giving the small gear the appearance of a screw, known as the worm. The large gear is called the worm wheel.

Advantages of Worm Gears

  • Provides a significant gear ratio with a very compact structure.
  • Smooth transmission with minimal noise.
  • Inherently possesses self-locking properties. If the helix angle of the worm is greater than the equivalent friction angle between the gears, the worm gear transmission will exhibit self-locking.

Disadvantages of Worm Gears

  • High transmission friction losses, resulting in low efficiency. Typically, efficiency is only around 0.7 to 0.8.
  • Relatively high cost, as worm wheels are often manufactured using costly anti-friction materials like bronze, which increases production costs.

Gear Materials and Heat Treatment

Quenched and Tempered Steel

Steel Grades: 45, 35SiMn, 42SiMn, 37SiMn2MoV, 40MnB, 45MnB, 40Cr, 45Cr, 35CrMo, 42CrMo, etc.

Processes: Quenching or Normalizing:

  • Provides strength and toughness within the 220~300HBW range.
  • Precision gear cutting can follow heat treatment for accuracy.
  • Cost-effective, no need for specialized equipment.
  • Lower gear surface hardness, suitable for easier meshing.

Suitable for: General low-speed gears with moderate strength and precision requirements, especially large gears.

Processes: Surface Hardening (Induction or Flame Hardening):

  • High surface hardness with good core toughness.
  • Quick induction hardening process.
  • Prone to quench cracking.

Suitable for: High-load and compact gear applications.

Carburized Steel

Steel Grades: 20Cr, 20CrMnTi, 20CrMnMo, 20CrMo, 22CrMo, 20CrNiMo, 18Cr2Ni4W, 20Cr2Ni4A, etc.

Process: Carburizing and Quenching

  • High surface hardness with good core toughness (typically 56~63HRC).
  • Requires post-heat treatment gear grinding for precision.

Suitable for: Medium-sized and smaller gears requiring high load-bearing capacity, good impact resistance, high precision, and compact size.

Nitrided Steel

Steel Grades: 38CrMoAlA, 30CrMoSiA, 25Cr2MoV, etc.

Process: Nitriding

  • Achieves high surface hardness with good core toughness.
  • Minimal heat treatment distortion.
  • Extended nitriding process increases manufacturing costs.

Suitable for: Gears with relatively large and steady loads requiring a hard gear surface.

Cast Steel

Steel Grades: ZG310-570, ZG340-640, ZG42SiMn, ZG50SiMn, ZG40Cr1, ZG35CrMnSi, etc.

Processes: Normalizing, Tempering, and Surface Hardening

  • Lower strength compared to quenched and tempered steel.
  • Prone to casting defects.

Suitable for: complex-shaped large gears that cannot be forged.

Cast Iron

Steel Grades: Various types of gray cast iron, nodular cast iron, malleable cast iron, etc.

  • Economical material choice with excellent wear resistance.
  • Good casting and machining properties.

Suitable for: Gray cast iron and malleable cast iron for low-speed, light-load, and non-impact gears; nodular cast iron for gears subjected to larger loads and impacts.

In summary

gears are essential mechanical transmission components that achieve changes in speed and torque through the meshing of gear teeth. They offer advantages such as high transmission efficiency, substantial power transfer capability, high precision, strong reliability, and long service life.

Manufacturers need to understand how gears work and their various classes as well as how to choose the right type of gears to maximize productivity and efficiency by considering relevant considerations. For more information about gears, Contact us at KUSLA Rapid Manufacturing.

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