Face gear
The crown gear is an angular gear in which a cylinder gear (pinion) interacts with a crown gear. The pinion can be moved freely in the axial direction over the crown gear without affecting the contact pattern or tooth play. The classic pairing is straight-toothed and has a 90 ° axis angle. This means that the teeth of the classic crown gear stand up like the prongs of a crown.
The crown gear toothing is determined by the pinion geometry, pinion position and the translation. The pressure angle varies along the tooth width, thus creating a transition from the conical external toothing to the conical internal toothing.
Basically, any pairing is possible in terms of helix angle, axis angle and axis offset, so that the axes intersect or cross. Crown gears enable simple assembly, a free combination of several drives or the construction of a differential .
history
1 - water wheel
2 - drive shaft
3 - pinion wheel
4 - crown wheel
5 - bevel shaft
Crown gears were used in many different ways in antiquity and in the Middle Ages. However, they have been replaced by bevel gears , which are easier to calculate and manufacture. Their application can still be seen today in historical mills, where the crown wheel engages the crown wheel and drives the vertical shaft. The “forgotten” transmission was only rediscovered in the 1940s. The first foundations for the theory of gearing and the calculation of crown gears as well as for the machining (gear shaping) of crown gears were created in the USA. For example, face gears were used for rear axle modules that consisted of a hypoid set (conventional) and differential (face gear).
At the beginning of the 90s, extensive research work began in Holland, which improved the calculation methods and introduced the continuous hobbing of face gears as an economical manufacturing process.
construction
Crown gears, like bevel gears and worm gears, belong to the bevel gears . The axes of the pinion and crown gear can intersect or cross (axis offset) and are at an axis angle of theoretically 0 ° to 180 °, in practice mostly at right angles. The straight or helical toothed pinion is a cylindrical gear with an involute profile. This has the advantage that deviations due to manufacturing or operating conditions (temperature) only cause a change in the backlash with tolerable contact pattern changes.
The crown gear resembles a ring-shaped curved rack, with the pressure angle steadily decreasing from the outer to the inner diameter. In this way, the variable peripheral speed on the crown gear as a function of the diameter is compensated for in relation to the constant peripheral speed of the pinion on the base circle. The pointed outer teeth and the steep tooth flanks on the inner diameter limit the usable tooth width. Crown gears achieve similar efficiencies as spur gears. Practical gear ratios are covered from i = 1 to 15 for crown gears, whereas bevel gears only translate from i = 1 to 8.
There is little information about tooth geometry in the literature, in CAD systems and in calculation programs. There are specialized companies that can calculate this geometry and thus make noise and vibration improvements.
Applications and features
Crown gears are used in machine tools, in automobiles, in off-highway applications, in transportation, for energy generation and in medical technology.
A decisive advantage is the axial freedom of the pinion shaft, which is given by the constant geometry of the pinion. Therefore, an axial adjustment when assembling the pinion is superfluous, in contrast to the bevel gear set. The axial freedom makes the gearbox relatively insensitive to manufacturing and assembly errors and housing deformations.
Furthermore, it is not necessary to run in the pinion and wheel in pairs and they can be installed separately. This significantly reduces the manufacturing and storage costs. Furthermore, no axial bearing forces have to be absorbed in straight-toothed pinions.
Due to the constant pinion cross-section, head bearings of the pinion shaft are possible for increased stability and improved vibration damping. The interaction of a crown gear with several pinions (power split), which can serve as input or output, is also facilitated by the axial freedom.
Since 2013, more and more companies have been using face gears for their new drive systems. Due to the small installation volume and the temperature independence, the face gear is more used for electrical actuators in the automotive industry.
Crown gears also allow high gear ratios with just a single gear stage. For example, in the case of turntable drives with gear ratios of i = 10, for which high concentricity properties are required, the total division errors are halved in contrast to conventional two-stage gears.
The crown gear can also be named as a replacement for worm gears. With this composition, a helical pinion with a large helix angle (β = 45 °) and with a relatively large axial offset interacts with a crown gear. Such gears have less wear and tear with higher efficiency and a more compact design, although gear ratios of up to i = 100 are possible. Self-locking, such as with worm gears with large ratios, can be bypassed, which is a great advantage, especially with door drives that have to be pushed open by hand in emergency situations.
Another advantage over conventional angular gears is the freely selectable axis angle of the gear.
The further development of crown gear technology has also opened up new fields of application in the recent past, especially in the high-performance area.
Numerous other applications relate to differentials in motor vehicles.
production
For a long time, the manufacture of crown gears, in particular that of the crown gear, was the greatest obstacle to the economic use of the gear. It is only in the recent past that numerous efforts have been made to develop new, more economical manufacturing processes for face gear production. Hobbing and generating grinding are used to manufacture face gears for use in the high-performance area. Face gears that are used in auxiliary gears are mostly produced by non-cutting manufacturing processes in large series from plastics (e.g. by injection molding) and metallic materials (e.g. by drop forging, extrusion, sintering).
Individual evidence
- ^ Thomas Bausch: Innovative Zahnradproduktion, expert Verlag, 3rd edition, 2006, pages 90–98
- ↑ a b See in particular Fig. 3 and claim 1 in US Patent US2270567A of April 13, 1940 "Differential mechanism" by George Slider, Chrysler Corp.
- ↑ DE 69503385: Crown gear, crown gear and method and tool for manufacturing the crown gear.
- ↑ DE 69406444: "Tool for the production of a face gear which can interact with a helical gear, and a method for the production of such a face gear" by Anne Sijtsta, 1994.
- ↑ DE 103 08 800: crown gear differential or set of at least two crown gear differentials; " The invention relates to a crown gear differential. In such a differential, the side gears are designed as crown gears. Due to the structural design of the differential, a complete vehicle series can be equipped with the same differential regardless of the motorization of the individual vehicle. In order to vary the transmissible torque, the Differential, special differential gears can be used as required, which do not require a differential bolt. ", 2003
