Reverse gear

Principle of the reversing gear of a machine tool. The machine is driven by a flat belt transmission. The drive belt can be placed either on pulley 1, 2 or three. A shifting device for the belt is available for this purpose. The pulley 1 is connected to the bevel gear I via the shaft α. The pulley 2 rotates freely on the shaft α (idle). The pulley 3 is directly connected to the bevel gear III. The bevel gear K directly drives the output shaft β.

A drum rotates with the drive shaft α, on which two belts are placed: one crossed and one parallel. The belt pulleys 2 and 5 are firmly connected to the output shaft β. The belt pulleys 3 and 4 run freely on this shaft (idling). With a shift fork 1, the belts can be moved together into positions 2 + 3 (output direction V), 3 + 4 (idling) and 4 + 5 (output direction R).

Reversing gears are "manual gearboxes in which only one direction of rotation can be switched". Such gears are required if a drive system is to provide two equivalent directions of rotation, but the reverse cannot be generated by the drive machine. A further distinction can be made between pure reversing gears and change ‑ reversing gears. A pure reversing gear is characterized by the fact that the direction of rotation can only be changed with the ratio one. Otherwise there is a reversing gearbox.

Principles

The reversal of the direction of rotation can be generated by the following gear types:

Gear drives are used particularly frequently in industry because they can transmit power with high levels of efficiency . The losses in gear drives are made up of the gearing, bearing, sealing and other losses. These losses are particularly low compared to other types of gearbox. In addition, gear drives are characterized by a very high power density , as they transmit the peripheral forces with the normal force . Non-positive gears only transmit the peripheral force with the frictional force . ${\ displaystyle F_ {N}}$ ${\ displaystyle F_ {N} \ cdot \ mu}$

Variation of the number of spur gears

The first basic idea for reversing the direction of rotation by means of gear drives is to switch between an even and an odd number of spur gears. The fact that each spur gear pairing results in a reversal of the direction of rotation is adopted.

Countershaft design

In the first principle for reversing the direction of rotation, five spur gears are arranged in a countershaft design . Since all shafts are fixed in the housing, countershaft transmissions can be assigned to stationary transmissions. The countershaft design is used, for example, in synchronous transmissions, powershift transmissions, group transmissions and reversing transmissions as a structural solution. In reverse gearboxes in countershaft design, one pair of spur gears always meshes directly with one another and the rest of the gear meshes via an intermediate gear that is mounted on the countershaft . When idling, the spur gears are rotatably mounted on the shaft by needle bearings . One of the spur gears is connected to the shaft by a synchronizing unit, thus switching the direction of rotation of the output. Another possibility for controlling the power flow is the axial displacement of the gears , but in this case the helix angle of the gears must not be greater than 10 °.

Turning heart

In addition to the countershaft design, the number of wheels can be varied with a turning heart. This is rotatably mounted on the drive or driven wheel and has a lever that enables synchronous, counter-rotating and idling. While the axes of the drive and driven gear are fixed in a translatory manner, the axes of the intermediate gears can be moved on a circular path due to the turning heart . The intermediate gears are always in mesh. While the input and output gear is only connected via an intermediate gear in synchronism, it is connected in counter-rotation via two intermediate gears.

A disadvantage of this principle is that, due to the design, the reversing gear can only be shifted at a standstill and without load. Another disadvantage is that the forces of the intermediate gears that arise in tooth contact require large holding forces due to the long lever . In addition, the principle is prone to temperature distortion and vibrations . Compared to the countershaft design, the lower number of spur gears and the simple nature of the gear lever construction of the reversing heart are advantageous.

Bevel gears

The switchable reversal of the direction of rotation can also be implemented with bevel gears. A variant of this gear principle consists of three bevel gears. Additional bevel gears and journals can be integrated in order to tap moments for power split. Because the axes of the bevel gears are stationary in relation to the housing, bevel gear reversing gears are classed as stationary gears. So that the input and output axes are identical, two bevel gears are usually coaxial and are connected to one another by an orthogonal intermediate bevel gear . The intermediate bevel gear does not have to be orthogonal to the bevel gears, but this results in the highest degree of efficiency. It is crucial that the axes of rotation intersect at one point with straight and helical bevel gears.

To reverse the direction of rotation, the bevel gear rotatably mounted on the drive pin is connected to the drive shaft. In order to maintain the direction of rotation, the input and output shafts are connected to one another directly via the coupling . In this case the translation is trivially equal to one. In addition to the two directions of rotation, a central position of the coupling is also possible, which separates the power flow between the input and output shafts. As a rule, this type of transmission can only be switched when the vehicle is at a standstill and without load, since the wheels are usually connected with a dog clutch. This clutch is operated by a hand lever or an electric motor.

If a small axial offset has to be compensated, so-called hypoid gear pairs or bevel helical gear pairs are required. However, the efficiency of the reversing gear decreases with increasing axial offset, since the geometry continues to resemble a worm gear. In addition, a cardan shaft combination must be integrated for the direct connection of the input and output shafts. Large axial offsets, on the other hand, can only be achieved with helical screw drives. The efficiency and the transmittable torque are maximized if the translation of bevel gears is one and the required translation is then introduced. The reason for this is that bevel gears can only implement gear ratios with very poor efficiency.

The disadvantages of bevel gears are the additional sources of error compared to spur gears. Bevel gears are particularly sensitive to deviations from the point of intersection of the pitch cone angles. These deviations lead to one-sided wearing or clamping and ultimately to wear, unfavorable noise behavior and heating. In addition, bevel gears are more difficult to manufacture compared to spur gears due to the significant hardening distortion and deflections in the case of a flying pinion. On the other hand, the advantage of bevel gears is that the input and output shafts can be selected to be coaxial.

Epicyclic gears

For reasons of uniform loading, there are no epicyclic gears with fewer than three planets. However, the exact number of planets is irrelevant for the transmission ratio. In the case of a stationary web, an epicyclic gear unit becomes a stationary gear , in all other cases mixed forms of stationary and epicyclic gears are possible. For combinatorial reasons, there are nine possible states of motion, depending on the wave being held.

This can also be seen in Willis' sentence :

${\ displaystyle n_ {1} -i_ {12} n_ {2} - (1-i_ {12}) n_ {s} = 0}$

From the sentence it is easy to see that a reversal of the direction of rotation can be set in epicyclic gears. In addition, it follows from the sentence that a single reversal of the direction of rotation by holding a shaft is always associated with an unequal gear ratio for both directions of rotation. Therefore, through a clever interconnection of several epicyclic gear stages, it must be ensured that the gear ratio is the same for both directions of rotation. Such interconnections are referred to as coupling gears. In the literature, a coupling gear for constant translation in both directions of rotation is shown.

The advantages of epicyclic gears are in particular that the power density is high and a comparatively small installation space is required. Compared to countershaft transmissions, the complicated manufacturability and the more difficult to control lubricating oil supply are disadvantageous. In addition, it is disadvantageous that at least two stages are necessary. In contrast, it is advantageous in many cases that the input and output shafts are coaxial.

Belt drive

Another principle for changing the direction of rotation is the force-fit transmission of power through belts. These are used in some machine tools. In the principle there are three belt pulleys on the drive shaft, of which the two outer ones are twice the width of the inner pulley. The outer disks are rotatable and the inner disk is fixed. These pulleys are connected to the belt drum on the output shaft by an open and a crossed belt. While the open belt maintains the direction of rotation of the input and output, the direction of rotation is reversed by the crossed belt. That drum is just as wide as the sum of the widths of the discs.

To switch the direction of rotation, both belts are shifted at the same time. Depending on which belt is pushed onto the fixed central pulley, the rotation is reversed or maintained. One is also idle possible if both belts are on the outer panes.

So that the belt is centered on the pulleys, they have an arcuate profile. The centering is located on the output side to protect the traction means . By changing the drum / fixed disk diameter, this gear principle can also be used as a reversing gear.

The main advantages of belt drives are their simplicity and the low costs for parallel and crooked waves. In addition, these gears are very quiet and have a favorable elastic transmission behavior, which is very useful for absorbing impacts and for damping . Furthermore, there is overload protection due to the belt break. The disadvantages, on the other hand, are the high bearing load due to the required preload and the fluctuations in speed due to the slip with non-positive traction means. Another disadvantage is the need for tensioning devices, the sensitivity to temperature, moisture, dust, dirt and oil and the relatively large size . In addition, in the case of non-positive traction means, the reduced transferable peripheral forces with increasing peripheral speed are unfavorable due to centrifugal forces .

Friction gear

Friction wheels offer a further possibility of switchable direction reversal. In this approach, the drive shaft is floating and is perpendicular to the output shaft. Two friction disks are mounted on the drive shaft. The friction wheel mounted on the output shaft is located between these .

To change the direction of rotation, the opposite disc is pressed against the wheel. In addition, idling is possible if the drive axle is positioned in such a way that the friction wheel has no contact with both discs.

If the output shaft is also floating, it is possible to vary the translation. A friction gear reversing gear can therefore be implemented as a pure reversing gear and as a reversing gear. In addition to discs, cylinders, cones or balls can also be used as friction bodies. However, they all have in common that the moment at the contact points of the friction bodies is transmitted by tangential frictional forces. The maximum transmittable torques depend on the one hand on the coefficients of friction and on the other hand on the contact pressure. The coefficients of friction depend on the material pairings and the lubrication.

The advantages of the smooth start-up and noiseless operation are particularly evident in hoist construction . The disadvantages of friction wheels are the heavily worn contact surfaces and the resulting necessary contact security. Due to the wear and tear of the contact surfaces and the dependence on normal force and transferable torque, the contact pressure must be very high, which results in a high bearing load.

Further advantages of friction gears are the comparatively simple structure, the low maintenance costs, the overload protection due to slipping and the easy to implement stepless adjustment of the translation. Disadvantages are the unavoidable slip, the limitation of the service life and the limited transferable power.

Fluid transmission

The principles discussed so far are characterized by power transmission using a solid-state contact. However, there are also approaches with hydrodynamic power transmission . This is particularly useful for cranes, winches, shovel loaders, caterpillars and wheel tractors, rail vehicles and drilling field equipment.

Liquid transmissions according to Voith can be cited as an example in rail vehicle technology . These consist of two Föttinger converters, each with a pump, turbine and guide wheel. While the pump wheel is mounted on the drive shaft via the housing and the liquid imparts a swirl , the turbine wheel drives the output shaft by withdrawing this swirl from the operating medium. The power transmission therefore takes place between the pump and turbine wheel through the inertia forces of the filling medium, which in most cases is an oil. Because the stator backs up the oil flowing back from the turbine wheel through its blade shape, the torque acting on the output shaft is increased. If no torque conversion is required, the Föttinger converters can be replaced by Föttinger clutches. They are characterized by the fact that, unlike Föttinger converters, they do not have a guide wheel.

The change of direction of rotation takes place without jolts and wear, as the respective converter circuit is filled or emptied. When the engine is at a standstill, the reversing gear automatically switches to a neutral position, so that idling is also possible at high output speeds.

One advantage is that the maximum starting torque can be limited. The turbine wheel rotates about three percent slower than the pump wheel due to the fluid friction. This results in a high degree of efficiency of around 97%. Changing the dimensions or varying the operating point results in a considerably smaller housing and blade diameter with a comparable degree of efficiency. Because the blade diameter has the fifth power and the drive speed has the third power in the transferable power, turbo reversing gearboxes are particularly economical at high speeds. In addition, it is advantageous that the slip provides overload protection and that the force is particularly gentle.

Combinations

It is also possible to combine the principles already presented in order to obtain new solutions. An example of this is a boat reversing gear, which makes use of a helical spur gear stage and a sprocket stage :

To change the direction of rotation, the gear stage and shaft are connected by means of the coupling, in this way the power is transmitted via the intermediate shaft. The rotation is reversed because in a chain stage, in contrast to the gear stage, no reversal of the direction of rotation is introduced. By closing the clutch, the direction of travel is maintained.

One advantage of chain steps is that they, in contrast to non-positive solutions, transfer the power at every operating point without slippage. On the other hand, the low maximum circumferential speeds due to vibration and noise excitation during the intervention are to be assessed as a disadvantage.

Individual evidence

↑ Johannes Looman: Gear transmission: Fundamentals, constructions, applications in vehicles . 3rd revised and expanded edition. Springer Berlin Heidelberg, Berlin, Heidelberg 1996, p. 22 .

↑ ^a ^b ^c Joseph Jehlicka, Egon Martyrer, August Schalitz: Small lexicon: gearboxes and clutches . In: DVA small technical lexica . DVA Deutsche Verlags-Anstalt, Fachverlag department, 1964, p. 246 .

↑ Bertsche, B. (Bernd), Lechner, G. (Gisbert), Ryborz, Joachim., Novak, Wolfgang .: Vehicle transmissions : Fundamentals, selection, design and construction . 2., edited and exp. Springer, Berlin 2007, ISBN 978-3-540-30670-2 , pp. 66 .

↑ Grote, Karl-Heinrich; Bender, Beate; Göhlich, Dietmar: Dubbel: Pocket book for mechanical engineering . 25th, revised and updated edition. Berlin, ISBN 978-3-662-54804-2 , pp. G125 .

↑ ^a ^b Pohlandt, Christian ,: Fundamentals of mobile working machines . KIT Scientific Publishing, Karlsruhe 2014, ISBN 978-3-7315-0188-6 , pp. II-25 .

↑ Winter, Hans: Machine elements: Volume 2: Gearboxes in general, gear drives - basics, spur gear drives . Second, completely revised edition. Springer Berlin Heidelberg, Berlin, Heidelberg 2003, ISBN 978-3-662-11873-3 , pp. 273 .

↑ ^a ^b Grote, Karl-Heinrich; Bender, Beate; Göhlich, Dietmar: Dubbel: Pocket book for mechanical engineering . 25th, revised and updated edition. Springer Berlin Heidelberg, Berlin, Heidelberg, ISBN 978-3-662-54804-2 , pp. G127 .

↑ ^a ^b Haberhauer, Horst 1950-: Machine elements: design, calculation, application . 18th, revised edition. Berlin, ISBN 978-3-662-53047-4 , pp. 571 .

↑ Bertsche, B. (Bernd), Lechner, G. (Gisbert), Ryborz, Joachim., Novak, Wolfgang .: Vehicle transmissions : Fundamentals, selection, design and construction . 2., edited and exp. Springer, Berlin 2007, ISBN 978-3-540-30670-2 , pp. 159-160 .

↑ Looman, Johannes .: Gear transmission: Fundamentals, constructions, applications in vehicles . 3rd, revised and extended Edition 1996, reprint with modified equipment. Springer, Berlin 2009, ISBN 978-3-540-89460-5 , pp. 351 .

^ Haberhauer, Horst: Machine elements: design, calculation, application . 18th, revised edition. Berlin 2018, ISBN 978-3-662-53047-4 , pp. 642 .

^ Haberhauer, Horst: Machine elements: design, calculation, application . 18th, revised edition. Berlin, ISBN 978-3-662-53047-4 , pp. 629-630 .

↑ Grote, Karl-Heinrich; Bender, Beate; Göhlich, Dietmar: Dubbel: Pocket book for mechanical engineering . 25th, revised and updated edition. 25th edition. Springer Berlin Heidelberg, Berlin, Heidelberg, ISBN 978-3-662-54804-2 , pp. G111 .

^ Haberhauer, Horst: Machine elements: design, calculation, application . 18th, revised edition. Berlin, ISBN 978-3-662-53047-4 , pp. 612 .

↑ Holzt, Alfred Conrad Udo: The school of the machine technician: The lifting and transport machines: Textbook for self-teaching in mechanical engineering . Ed .: Heepke, Wilhelm. Verlag von Moritz Schäfer, Buchhandlung, Leipzig 1911, p. 50 .

^ Haberhauer, Horst: Machine elements: design, calculation, application . 18th, revised edition. Berlin, ISBN 978-3-662-53047-4 , pp. 613 .

↑ Kickbusch, Ernst: Föttinger couplings and Föttinger gears: construction and calculation . Berlin, Heidelberg, ISBN 978-3-642-52434-9 , pp. 173 .

↑ Kickbusch, Ernst: Föttinger couplings and Föttinger gears: construction and calculation . Berlin, Heidelberg, ISBN 978-3-642-52434-9 , pp. 7 .

↑ Grote, Karl-Heinrich; Bender, Beate; Göhlich, Dietmar: Dubbel: Pocket book for mechanical engineering . 25th, revised and updated edition. Berlin, ISBN 978-3-662-54804-2 , pp. R53 .

↑ Grote, Karl-Heinrich; Bender, Beate; Göhlich, Dietmar: Dubbel: Pocket book for mechanical engineering . 25th, revised and updated edition. Berlin, ISBN 978-3-662-54804-2 , pp. G118 .

[1] Johannes Looman: Gear transmission: Fundamentals, constructions, applications in vehicles . 3rd revised and expanded edition. Springer Berlin Heidelberg, Berlin, Heidelberg 1996, p. 22 .

[:0-2] Joseph Jehlicka, Egon Martyrer, August Schalitz: Small lexicon: gearboxes and clutches . In: DVA small technical lexica . DVA Deutsche Verlags-Anstalt, Fachverlag department, 1964, p. 246 .