The term "speed reducer" might sound unfamiliar to many. In fact, speed reducers function as a means of matching speeds, transmitting torque, and changing the direction of motion between a prime mover and a working machine, and are widely used in modern machinery. First, it's essential to understand the three core components of mechanical equipment: the drive unit, the controller, and the speed reducer. In this course design, you will encounter three two-stage speed reducer design schemes: a two-stage cylindrical gear reducer, a two-stage bevel-cylindrical gear reducer, and a two-stage worm gear reducer. Through this course, you will master the ability to design simple transmission devices.
In speed reducers, the core transmission component is the gear drive. Gear drives are widely used in industrial products, vital to a nation's industrial lifeline, from spacecraft to watches and toys—they are virtually ubiquitous. Gear drives constitute a large family with many types, each with its own characteristics. The type and form of the gear drive directly determine the characteristics and application range of the speed reducer. Based on the arrangement of the gear axes, they can be divided into parallel-axis gear drives, intersecting-axis gear drives, and staggered-axis gear drives. First, we will introduce several common types of gear drives to illustrate their characteristics.
Spur gears
Spur gears are among the most common gears used in actual production. The contact line on their tooth surface is a straight line parallel to the gear axis. Because the teeth simultaneously enter and exit mesh along the entire tooth width, they are prone to impact and noise, resulting in relatively poor transmission smoothness. However, they are easy to machine and achieve high precision. In external meshing, the two gears rotate in opposite directions, while in internal meshing (ring gear and gear), the two gears rotate in the same direction.
helical cylindrical gears
Helical gears are cylindrical gears with helical tooth lines. The contact line of the gear is a straight line with an angle βb to the axis. The teeth gradually transition from one end to the other from the start of meshing to disengagement, resulting in smooth transmission and low noise. This meshing method also reduces the impact of gear manufacturing errors on the transmission, allowing for the transmission of higher loads and making it suitable for high-speed applications. When a pair of helical gears mesh, if one gear is left-handed, the other is right-handed. The sum of the helix angles β1 and β2 of two helical gears on parallel shafts is 0. If the sum of β1 and β2 is not 0, a staggered-axis helical gear transmission is formed.
Herringbone gear
Helical gear drives generate axial forces. To eliminate the influence of these axial forces on the shaft system, one gear is made into a symmetrical helical gear with opposite directions, thus eliminating the axial force. The tooth profile resembles a herringbone pattern. Herringbone gears are difficult to manufacture and have high processing costs, but they have advantages such as high transmission power, high reliability, high precision, and strong load-bearing capacity, playing an important role in heavy machinery fields such as aerospace and shipbuilding.
Gear and rack drive
Rack and pinion drives consist of gears and racks. The rack's tooth profile is a straight line, equivalent to a cylindrical gear with an infinite pitch circle radius. Its working principle is to convert the rotational motion of the gear into the reciprocating linear motion of the rack, or vice versa. For example, the steering system of an automobile is achieved through rack and pinion drives.
bevel gears
Bevel gears are used for gear transmission between two intersecting shafts, allowing the output shaft direction to be changed. A bevel gear has a cone as its pitch surface, with the teeth cutting along the cone. Bevel gears are classified as spur, helical, and curved, with spur gears being the most common, while helical gears have been gradually replaced by curved gears. The shaft angle can be any angle, with 90° being the most common. Spur bevel gears have lower manufacturing precision and produce more vibration and noise during operation; therefore, their circumferential speed should not be too high.
worm gear drive
Worm gear drives are used to transmit rotary motion between intersecting shafts, typically with an intersecting angle of 90°. Compared to cylindrical gear drives, they are less efficient, making them suitable for low-power applications. The transmission ratio range can reach 8-100, and they can achieve self-locking. Because the worm teeth are continuous helical teeth with numerous meshing pairs, they experience low impact loads, smooth transmission, and low noise.
Gear reducers have a long history of application and are now technologically mature and standardized. With the development of industrial technologies such as aerospace, new energy, robotics, and medical devices, new requirements have been placed on reducers. These requirements mainly include simple and compact structure, short transmission chain, high power transmission, low noise, smooth transmission, and high operating accuracy. In today's industrial society, with increasingly sophisticated modern machinery, the demand for reducers continues to grow, providing a vast stage for their development and placing higher demands on students' understanding and research. So, what other forms of gear transmission can meet these requirements?
Planetary gear reducer
Students have already learned about planetary gear transmissions in their mechanical principles course. A planetary gear reducer consists of planetary gears, a sun gear, and an external ring gear. Planetary reducers are small in size, lightweight, have high load-bearing capacity, long service life, smooth operation, and low noise. They feature power splitting and multi-tooth meshing characteristics. They are suitable for applications in construction machinery, light industry and textiles, medical devices, instrumentation, automobiles, shipbuilding, and aerospace.
Harmonic gear reducer
Harmonic reducers achieve transmission by elastically deforming a flexible, thin-walled external gear and meshing it with a rigid internal gear. They consist of three main transmission components: a rigid gear, a flexible gear, and a wave generator. Harmonic reducers, with their advantages of large transmission ratio, zero backlash, compact structure, light weight, and high precision, play a crucial role in robotics and automation technology. my country began standardizing and serializing harmonic reducers in 1980; in 1985, it formulated a series of standards for small and medium-power general-purpose harmonic reducers, becoming the fourth country in the world to possess general-purpose harmonic reducer standards.
Cycloidal pinwheel reducer
The cycloidal pinwheel reducer is a planetary transmission. A double eccentric sleeve, offset by 180°, is mounted on the input shaft. Two roller bearings are mounted on the eccentric sleeve, forming an H-type mechanism. The central holes of the two cycloidal wheels serve as the raceways for the swing arm bearings on the eccentric sleeves. The cycloidal wheels mesh with a set of annularly arranged pin teeth on the pin tooth housing to form a low-tooth-difference internal meshing reduction mechanism. Unlike involute gearboxes, the cycloidal pinwheel reducer features zero backlash and high torque capacity, while maintaining a compact size, making it suitable for applications requiring high torque, low speed, and high precision.
RV reducer
The RV reducer, developed from the cycloidal pinwheel drive, is a two-stage reducer consisting of a planetary gear drive in the front stage and a cycloidal pinwheel drive in the rear stage. Since its market introduction in 1986, it has been widely used in industrial robots, machine tools, medical testing equipment, satellite receiving systems, and other fields due to its advantages such as large transmission ratio, high transmission efficiency, high motion accuracy, low backlash, low vibration, high rigidity, and high reliability. Unlike harmonic drives, whose motion accuracy significantly decreases with use, RV reducers are widely used in high-precision robot drives in many countries worldwide, and they are gradually replacing harmonic reducers in advanced robot drives.
In recent years, my country's industrial robot production and sales have both increased, and the manufacturing market has never been more in need of industrial robots than it is today. my country has been the world's largest consumer of industrial robots for eight consecutive years. Globally, Japanese reducer companies have made significant improvements in reducer structure, materials, processing and manufacturing technology, lubrication, noise reduction and testing, achieving a leading advantage in the industry. Representative companies include Nabtesco and Harmonic Drive. Although my country has made certain breakthroughs in high-end reducer design, materials, manufacturing, testing and experimental evaluation in recent years, a large number of RV reducers used in domestic robots are currently imported from Japan. For China's manufacturing industry to transform and upgrade, it must solve this "bottleneck" technical problem. The core factors restricting the localization of key components of industrial robots in my country are as follows: (1) The technology accumulation time is still short and the integration between industries is insufficient; (2) Some materials and core components are difficult to meet the performance requirements of components; (3) The processing equipment and processes are backward; (4) The testing and inspection capabilities need to be improved.
