Mechanical transmission

Transmission with belt in an industrial facility

The mechanical transmission is a mechanism in charge of transmitting power between two or more elements within a machine. They are a fundamental part of the elements or organs of a machine, often classified in one of the two fundamental subgroups, the transmission elements themselves and the clamping elements.

In automotive, the term "transmission" It applies to the entire mechanical transmission system, including the clutch, gearbox, drive shaft (for rear-wheel drive vehicles), differential, and final drive shafts. However, colloquially it is also used to refer simply to the gearbox of a vehicle.

History

Ancient Transmission

Interior view of the windmill Pantigo. From the bottom to the top: cover grill, brake wheel, brake and balancín. The mill is located in James Lane, East Hampton, Suffolk County, Long Island, New York

Old crane with gears located in the port of Seville

Poles driven factor

Transmission by pulleys and drive belt of an old sewing machine

Internal mechanism of a Kyocera printer

Although some archaeological remains have been found (such as the Antikythera mechanism) which attest that at least in the II century B.C. C. toothed gears were known in ancient Greece, there is no evidence to show that the peoples of antiquity had gears in their machines, mainly composed of simpler elements such as levers, pulleys or lathes.

Wooden-carved worm gears in China have been found in a royal tomb in the Chinese city of Shensi, dating to AD 50. C. It would not be until the III century when in ancient Rome gears began to be used in mills and pumping systems used in mining.

The first transmissions included units to transfer rotation between shafts arranged at right angles and different types of gears, and were used in wind or river mills, and in devices drawn by draft animals, which powered machines such as hydraulic pumps, grinding equipment or cranes. It is possible that this technology was preserved in the Islamic world until the 13th century.

At the beginning of the Renaissance, the rudiments of mechanical transmissions were already available in Europe, used in clocks for public buildings such as cathedrals. Prominent figures such as Leonardo da Vinci (who designed a primitive gearbox with a conical piece and another cylindrical) or the Danish astronomer Olaf Roemer studied different types of mechanical transmissions in their treatises.

Already in the 19th century, figures such as the British Robert Willis (considered one of the first mechanical engineers) or Christian Schiele, developed the technology necessary to produce the precision gears that allowed the creation of all kinds of machines linked to the improvement of the steam engine and the rise of the industrial revolution, the mass production of watches, the manufacture of instruments highly accurate scientists, or the design of the first internal combustion engines. At this time, it was common for the machinery of industrial plants to be driven by pulley and belt transmission systems, arranged on a series of horizontal shafts suspended from the ceiling of the workshop, which in turn were driven by a steam engine or a water wheel.

The beginning of the XX century saw the progressive popularization of automobiles with internal combustion engines, in whose transmissions (first with belts, then with chains and finally with shafts) the gearbox became a must-have. Improvements in metallurgy led to the creation of the first modern manual transmissions, and as early as 1903 French industrialist André Citroën invented the innovative double worm gears. The first synchronized manual gearbox appeared in a Cadillac in 1929, and in 1940 General Motors introduced the Hydramatic system, the first fully automatic transmission to use a hydraulic torque converter. For the rest of the century XX, some new systems appeared, such as the continuously variable transmission developed by DAF in the Netherlands. Its Variomatic transmission was used in various vehicles built by DAF and Volvo up to the 1980s, and subsequently found application in light motorcycles.

In the field of railways, the progressive replacement of steam engines by diesel locomotives on non-electrified lines, led to the appearance of diesel-hydraulic transmissions and diesel-electric transmissions (which are not strictly transmissions mechanical), due to the difficulty of transmitting large traction power using conventional friction clutches.

At the beginning of the XXI century, the growing tendency to replace vehicles with internal combustion engines by electric motors, allows one to venture the progressive disappearance of mechanical gearboxes in automobiles, due to the ability of electric motors to adjust their torque by changing the intensity of the electrical current supplied to them. In fact, the European Union has planned that by 2035 all cars sold in its territory will be electric.

In any case, mechanical transmission systems will continue to form part of numerous machines that move objects preferably linearly, in which exact synchronism must be ensured between different moving parts, such as printers and 3D printers, cranes and construction machinery, robotics, and all kinds of machines industrially dedicated to the processing or packaging of various products, especially food.

General considerations

Simple reducer gear

Transmission of a windmill used as a hydraulic pump

In the vast majority of cases, these transmissions are carried out through rotating elements, since the transmission of energy by rotation requires much less space than when it is carried out by translation.

A mechanical transmission is a way of exchanging mechanical energy different from pneumatic or hydraulic transmissions, since to perform its function it uses the movement of solid bodies, such as gears and belts. transmission.

Typically, the transmission changes the speed of rotation of an input shaft, resulting in a different output speed. In everyday life transmissions are commonly associated with automobiles. However, they are used in a wide variety of applications, some of which are stationary. Primitive transmissions comprise, for example, reducers and right angle gears in wind or water mills and steam engines, especially for pumping, grinding or hoisting (norias).

In general, transmissions reduce improper high speed, low torque rotation of the prime mover output shaft to a lower speed with higher torque, or vice versa. Many systems, such as the transmissions used in automobiles, include the ability to select one of several different ratios thanks to the use of a gearbox, a set of gears and speed reducers that allows the ratio of rotational speed and torque to be regulated. moment between two rotating devices, such as the engine and wheels of a vehicle. In these cases, most ratios (usually called "gears" or "shifts") are used to reduce engine output speed and increase torque; although higher ratios can be overdrives which increase output speed.

Its most common use is in motorized vehicles, where the transmission matches the output of the internal combustion engine to the drive wheels. Said motors must operate at a relatively high rotation speed, an inappropriate circumstance when the vehicle must start, stop or drive at low speed. The transmission reduces the highest engine speed to the slowest wheel speed, increasing torque in the process. The transmissions are also used in bicycles, in stationary machines and wherever it is necessary to adapt different rotation speeds and torques.

Often a transmission has multiple gear ratios (or simply "gears") with the ability to shift between them as speed varies. This switching can be done manually (by the operator) or automatically (by a control unit). It is also usually possible to control the direction of rotation (forward and backward). Also, there are single ratio transmissions, which simply change the speed and torque (and sometimes the direction) of the engine's output.

Transmissions are also used in naval, agricultural, industrial, construction and mining equipment. In addition to conventional gear-based drives, these devices often employ hydrostatic drives and adjustable-speed electric drives. Alternative mechanisms include torque converters and power transformation, such as diesel-electric transmission and hydraulic machines. There are also hybrid configurations such as automatic transmissions used in the automotive industry, which use a valve system to change gear by varying the pressure of a fluid in response to engine revs, vehicle speed, and throttle opening.

Single and multi-ratio transmissions

Main gear box and rotor of a Bristol Sycamore helicopter

Gears of a five-speed gearbox + back of the Volkswagen Golf 1600 (2009)

Selected change box of an Amphicar, with optional transmission of the propeller

Tansmission of a tractor with 16 marches forward and 8 backwards

Scheme of a turbine, with its multiplier transmission system

Often called reducers because they slow down (although sometimes designed to increase) slewing speed, simple transmissions sometimes include a right-angle redirection of the slewing axis (such as for example, it is common in helicopters, see the image). They are often used in agricultural equipment, where a main engine serves as a horizontal PTO for various systems driven by vertical shafts, such as cutting devices on mowers, mechanisms for spreading seeds or fertilizers, or endless screws that allow load grain into silos or trucks. More complex equipment, such as combines and snowplows, have drives with outputs in more than one direction. Also helicopters use a split torque gearbox, where power is taken from the engine in two separate directions for the main and secondary rotor.

Gearboxes in a wind turbine convert the slow, high-torque rotation of the turbine into a much faster rotation of the electric generator. They are much larger and more complicated than gearboxes in farm equipment. They weigh several tons and usually contain three stages to achieve an overall gear ratio of 40:1 to over 100:1, depending on the size of the mill blades. Larger mills have to spin slower, but all generators have to spin at similar speeds of several thousand rpm. The first stage gearbox is usually a planetary gear, given its compactness and ability to spread the huge torque from the turbine over a greater number of low-speed shaft teeth. The durability of these gearboxes has been a concern. serious problem for a long time.

On the contrary, multi-ratio transmissions arose in applications that require the availability of different speed reductions, necessary especially in the case of vehicles in order to regulate the delivery of torque. an internal combustion engine, very different when it should start and when it works at full capacity. To do this, there are gearboxes, devices that allow the relationship between the turns of the engine and those of the wheels of the vehicle to be varied (these relationships are called "gears"). Thus, at start-up, when the wheels must turn very slowly, a mechanical gearbox allows a small gear driven by the engine to mesh with a larger diameter gear connected to the wheels, so that the wheels turn much slower than the engine (known as first gear). On the other hand, once the vehicle is launched, the rotational speed of the engine and the wheels are the same, and the engine crankshaft can be directly connected to the wheels (what is usually known as fourth running or direct).

Since the origins at the end of the XIX century of the automobile equipped with an internal combustion engine, different types of systems have been developed multi-ratio, such as manual transmissions (fully mechanical, in which the driver must choose for himself the most suitable gear ratio at all times) and automatic transmissions (equipped with a hydraulic torque converter and epicyclic gears, in which that an electromechanical or electronic system autonomously selects the most appropriate gear ratio at each moment based on the engine speed and the state of the accelerator pedal). The appearance of semi-automatic transmissions (based on mechanical gearboxes, but with an assisted gear change) and continuously variable transmission (with a belt mechanism that allows the gear ratio to be adjusted on time), has allowed the difference to blur. for the user existing between manual gearboxes (whose operation can be "automated") and classic automatic gearboxes, although internally their operating principles are very different.

In addition to ordinary gear-equipped transmission, hydrostatic transmission equipment and electric adjustable-speed systems are also common.

Types of transmissions

Manual transmission with front traction, which displays the gearbox and the final transmission incorporated in the same housing

Among the most common forms of transmission are:

• Bars in articulated mechanisms such as the classic biela-manivela mechanism of steam locomotives or rod systems.
• Cables, most of these only work on traction, although there are special cables to transmit other efforts such as torsion cables or syringes.
• Transmission chain.
• Straps or transmission bands.
• Gears, characteristic of gear boxes
• Friction wheels, which transmit perimeter movement, like the wheels of a vehicle.
• Friction discs, which transmit axial motion, like a clutch disk.
• Twisted shafts and shafts.
• Cardan joints and homokinetic joints.
• Leva tree.
• Ducks.

Motorized Vehicles

Motor and motor train from a front-wheel drive with engine

The powertrain (or powertrain) of a motorized vehicle is the set of mechanical components that provide power to the drive wheels. This set does not include the engine that generates the energy. Rather, the propulsion system is considered to include both the engine and the powertrain.

In motor vehicles, the transmission is usually connected to the engine's crankshaft via a pulley, friction clutch, or hydraulic coupling, in part because internal combustion engines cannot run below a specified rpm. given minimum (called idle) without coming to a complete stop. Transmission output is transmitted through the driveshaft to one or more differentials, which in turn drive the wheels. Although a differential can also include a reduction gear system, its primary purpose is to allow the wheels on an axle to rotate at different speeds as the direction of the vehicle changes (essential to prevent wheel slip when cornering).

Operation

Unpack the gearbox of an Albion car (doing 1910)

Scheme of a manual gearbox with the first gear enlarged

The function of the drive train is to couple the motor that produces the power to the drive wheels that use this mechanical power to rotate the shaft. This connection involves physically joining the two components, which may be on opposite ends of the vehicle and therefore require a long driveshaft. The operating speed of the engine and wheels are also different and must be matched to the correct gear ratio. As vehicle speed changes, the ideal engine speed must remain approximately constant for efficient operation, and therefore this gearbox ratio must also be changed, either manually, automatically, or through automatic continuous variation..

Depending on the way in which the gear ratio between engine and wheel speed is selected, there are four fundamental types of transmissions used in the automotive industry:

• Manual transmission: the driver must handle in combination with the clutch a gearbox with a certain number of gears, depending on the design of your gear system. The first models were not synchronized (i.e., to change the gear, the driver had to adjust the speed of the engine and the wheels by making a combination of pedals called double clutch), but then the synchronization between wheels and motor was achieved by stepping on the clutch, and finally the double clutch gearbox was developed (which reduces the loss of power by changing gear, by minimizing the time when the wheels are left unturned).
• Semi-automatic transmission: uses gear gearboxes equal to those of a manual transmission, but the clutch drive and/or the gearbox itself, are performed in a assisted way by a series of servoscanisms when the driver presses a button or moves the gear lever. He usually lacks the clutch pedal.
• Continuous variable transmission: it has a system of trunk and transmission belts that allow to automatically adjust the change ratio in response to the vehicle's gear and pressure on the accelerator. They lack change lever and clutch pedal.
• Automatic transmission: is characterized by using a hydraulic torque converter that replaces the clutch, and because the gears of the manual gearbox are replaced by an epicycloidal system. As in the case of continuous variable transmission, the driver has only to take care of regulating the desired gear of the vehicle using the pedals of the accelerator and the brake, and the management of the most appropriate exchange ratio is performed autonomously electromechanically or electronically. It loads the clutch pedal, and the change lever is used only to leave the vehicle parked, to start, to go backwards or to select the desired driving type.

Designs such as the robotic transmission (a conventional semi-automatic transmission, which can be fully managed by an electronic control unit) mean that the practical difference in use between the more advanced manual transmissions and automatic transmissions has practically disappeared.

Components

Some examples:

rear axle with final transmission of hypoid conic gear

The precise components of the drive train vary by vehicle type:

Transmission of construction vehicles, with total permanent traction