Both speed reducers and gearboxes are devices that change the speed between a working machine and a prime mover. A speed reducer is a device that converts the prime mover's speed to a fixed speed and transmits it to the working machine (generally reducing the prime mover's speed to achieve a constant transmission ratio). A gearbox is a device that converts the prime mover's speed to multiple speeds and transmits it to the working machine (achieving a variable transmission ratio).
I. Reducer
A speed reducer is an independent, closed-loop transmission device between a prime mover and a driven machine. It is used to reduce speed and increase torque to meet operational requirements. Speed reducers are compact in structure, highly efficient, transmit motion accurately and reliably, are easy to use and maintain, and can be mass-produced, making them widely used.
Working principle of speed reducer
Speed reducers are generally used in low-speed, high-torque transmission equipment. They reduce the speed of electric motors, internal combustion engines, or other high-speed power sources by meshing a gear with fewer teeth on the input shaft with a larger gear on the output shaft. Ordinary speed reducers may also have several pairs of gears that work on the same principle to achieve the desired speed reduction effect. The ratio of the number of teeth on the large gear to the number of teeth on the small gear is the transmission ratio.
Basic structure of a speed reducer:
A speed reducer is mainly composed of transmission parts (gears or worm gears), shafts, bearings, a housing, and accessories. Its basic structure consists of three main parts: (1) a combination of gears, shafts, and bearings; (2) a housing; and (3) speed reducer accessories.
A gear, shaft, and bearing assembly where the pinion and shaft are integrated is called a gear shaft. This structure is used when the gear diameter is not significantly related to the shaft diameter. If the shaft diameter is d and the gear root circle diameter is df, then this structure should be used when df-d ≤ 6～7mn. However, when df-d > 6～7mn, a structure where the gear and shaft are separate parts is used, such as a low-speed shaft and a large gear. In this case, the gear and shaft are circumferentially fixed with a key connection, and the shaft components are axially fixed using shaft shoulders, bushings, and bearing caps.
The gearbox housing is a crucial component of the reducer, serving as the base for the transmission parts and requiring sufficient strength and rigidity. The housing is typically made of gray cast iron, but cast steel housings can be used for reducers subjected to heavy loads or impact loads.
Gearbox accessories
In order to ensure the normal operation of the reducer, in addition to giving sufficient attention to the structural design of the gear, shaft, bearing combination and housing, we should also consider the reasonable selection and design of auxiliary parts and components such as oil filling and draining of the reducer lubrication oil sump, checking the oil level, precise positioning of the cover and housing during machining and disassembly and maintenance, and hoisting.
Most gearbox housings are made of medium-strength cast iron, while heavy-duty gearboxes use high-strength cast iron and cast steel. For small-batch production, steel plates can also be welded together. Gearbox housings are required to have simple shapes and smooth surfaces. For ease of installation, housings are often made in a split configuration, with the split surface usually coinciding with the axial plane.
Features of Commonly Used Speed Reducers
▲Single-stage helical cylindrical gear reducer
▲Single-stage cylindrical worm gear reducer
▲Two-stage helical cylindrical gear reducer
▲Two-stage cylindrical gear electric motor reducer (coaxial type)
▲Two-stage helical cylindrical gear reducer (shaft-mounted)
▲Cycloidal pinwheel reducer
▲ Harmonic gear reducer
▲ Planetary gear reducer
General assembly steps of speed reducer
Install base → Assemble input shaft → Assemble intermediate shaft → Assemble output shaft → Install each shaft → Engage and rotate → Assemble top cover → Assemble top cover → Assemble bolts → Assemble end cover.
II. Transmission
A transmission is a mechanism used to change the speed and torque from an engine. It can change the transmission ratio between the output shaft and the input shaft in a fixed or progressively increasing manner; it is also called a gearbox. A transmission consists of a gear-changing mechanism and a control mechanism; some cars also have a power take-off (PTO) mechanism. Most transmission mechanisms use ordinary gears, but some use planetary gears. If the speed of the transmission output shaft can be continuously varied, it is called a continuously variable transmission (CVT); otherwise, it is called a stepped transmission (PST).
Working principle of transmission
Mechanical transmissions primarily utilize the speed reduction principle of gear transmission. Simply put, a transmission contains multiple sets of gear pairs with different gear ratios. The gear shifting behavior when a car is driving involves operating a mechanism to engage these different gear pairs. For example, at low speeds, gear pairs with higher gear ratios are engaged, while at high speeds, gear pairs with lower gear ratios are engaged.
2.1 Stepped Transmission
Tower gearbox
Two conical pulleys are fixed to shafts I and II respectively, and the transmission belt can be moved to three different positions on the pulleys. Because the diameter ratios of the two conical pulleys for each gear stage are different, when shaft I rotates at a constant speed, shaft II can achieve three different speeds by changing the belt position. This type of gearbox mostly uses flat belt drive, but V-belt drive can also be used. Its advantages are smooth transmission and simple structure. However, it is relatively large in size and inconvenient for speed changes.
Sliding gear transmission
A sliding gear is movable on a shaft, and the torque it transmits is transferred to the shaft via a key or spline connection. Gear meshing achieves speed change. This type of gearbox is convenient for changing speeds, has a compact structure, high transmission efficiency, and is widely used, but it cannot use helical gears.
Clutch gear transmission
Helical gears or herringbone gears can be used to ensure smooth transmission. If a friction clutch is used, speed changes can be made during operation. Its disadvantages are that the gears are constantly engaged, leading to faster wear, and the clutch occupies a larger space.
2.2 Continuously Variable Transmission
Some machines need to continuously change their operating speed to adapt to changing working conditions, which requires the use of continuously variable transmissions (CVTs). CVTs come in various types, including mechanical, electric, electromagnetic, and hydraulic. Mechanical CVTs have advantages such as simple structure, good transmission performance, wide applicability, convenient maintenance, and high efficiency, and are therefore widely used.
▲Roller-type flat disc continuously variable transmission
▲Rhomboid continuously variable transmission
Advantages and disadvantages of continuously variable transmissions (CVTs)
Advantages: Simple structure; slippage between transmission components during overload can prevent machine damage; smooth and noiseless transmission; easy and smooth continuous speed change, etc.
Disadvantages: Cannot guarantee an accurate transmission ratio; low transmission efficiency; large overall dimensions; small speed range;
