Dog box
The term dog box is used in motor sports . It is the name for a gearbox with special synchronization .
Basically, you have to first mention the different types of gearbox structures in order to get a rough overview.
Synchronized H gearbox
This is the most common gearbox installed in normal street legal vehicles. The gears are synchronized using so-called "synchronizer rings", which are usually made of brass.
When a gear is engaged, the shift fork pushes the shift sleeve against the synchronizer ring , which then presses against the gear wheel. The synchronizer ring acts like a brake pad and thus adjusts the speed of the input shaft to the next gear, so that it can then be engaged properly by the shift sleeve sliding over the synchronizer ring onto the gear wheel. Shift sleeve, synchronizer ring and gear wheel are made very filigree and usually have many "teeth" (approx. 20-30) through which the frictional connection is established. The teeth themselves are chamfered conically at the tips in order to achieve greater shift comfort. The ratio between “tooth” and “free space” is approx. 50:50, so that when a gear is engaged and there is a frictional connection, the teeth fit together without play.
The advantages of this synchronization are the high level of comfort and the long service life (with sensitive shifting).
Disadvantages of this synchronization: The transmission needs “time” to shift, the synchronization wears out very quickly with aggressive shifting, and the shifting speed and shifting comfort are heavily dependent on temperature and oil viscosity .
Dog box gear
In principle, any synchronized H gearbox with new gear sets and shift sleeves can be converted to a Dog Box gearbox. Contrary to popular belief, this transmission will continue to shift with the normal H profile. The main difference is that the gearshift sleeve no longer has many small “teeth”, but mostly only 4-6 large ones, as does the gear wheel. The ratio between “tooth” and “free space” is around 30:70 in order to avoid the risk of “tooth on tooth” during the switching process.
The synchronizer ring is omitted and this synchronization takes place suddenly. The teeth mesh and "the passage is inside". The teeth are not conical, but flat and are ground on the sides, which means that the gear wheel and gearshift sleeve “pull together” under load to prevent the gear from jumping out. Due to this hard type of “synchronization”, the wear on the edges of the dog “teeth” is very high.
In "cheap" Dog gearboxes, the dog teeth are manufactured on the gear wheel, which requires the entire gear wheel to be replaced if it is worn. In Motorsport Dog transmissions, these are designed to be exchangeable on the gear wheel, which significantly reduces the cost of the revision.
In contrast to the synchronized gearbox, where slow gear changes increase the service life, the gear change in the Dog Box gearbox must be quick (which does not mean force!) In order not to unnecessarily stress the teeth.
The shifting process itself can take place without a clutch, for this only the load has to be briefly removed from the transmission; this can be done via the control unit with a load interruption or by simply "lifting the gas".
The advantages of this transmission are the high shifting speed and (which is also a disadvantage) the fact that the gear can always be engaged.
Disadvantages are the higher maintenance effort, somewhat less comfortable gear changes, clacking noises during load changes, which are caused by the design-related space between the "dog teeth", and the fact that the gear can always be engaged, which is what happens when shifting (also so-called "money shifts" e.g. B. 1-2-1 or 4-1) can quickly cost an engine by overspeeding.
Sequential transmission
We are talking about a dog box gearbox, which is "upgraded" to a sequential gearbox by a shift drum .
Sequential, i.e. continuous, does not represent the arrangement of the gears in the transmission, but the order of the gears in which they are shifted. In most cases, this cannot be easily converted into a standard H gearbox.
With this type of transmission, shifting is no longer carried out using the familiar H-profile, which requires two pivot points on the transmission for shift cables, but rather, as is known from the rally , for example , with simply "pushing or pulling" the gear lever , which is attached to a pivot point on the transmission leads. When the lever is moved, the shift drum in the gearbox, which has millings with which the shift forks are pushed into their position, is always turned a little. The aisle is selected and the aisle is engaged simultaneously . This assumes that the millings in the roller must be calculated and manufactured absolutely perfectly in order to carry out this interaction smoothly. The design of the transmission does not matter, e.g. B. 3-shaft gearbox or 2-shaft gearbox with countershaft, etc. The shift drum is matched to each gearbox.
The advantage of this gearbox is the very high switching speed and the fact that shifters are not possible, since the profile of the roller only enables the gears to be engaged one after the other.
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Disadvantages, similar to the above with the Dog Box gearbox, are the clicking noises of the dog rings when changing loads and gears, as well as the higher maintenance requirements. The neutral position can only be reached by shifting the transmission through. In addition, the gearbox cannot be shifted smoothly when the engine is not running and this should be avoided, as “tooth on tooth” can prevent the gear from being engaged. So you can often see that racing cars have to be rolled in order to be able to shift through the gears with the engine stopped in order to get into neutral.
Sequential transmissions are built into most motorcycles.
In principle, one can say that a light clutch friction disc has a positive effect on the switch behavior and wear, since the mass to be "synchronized" decreases.
There are also so-called “sequential shifters”, which take on the function of the shift drum and are often driven with dog box gears. In these, specially milled discs take on the role of the roller and enable the gear to be engaged by pulling and pushing the selector lever. These must be manufactured for every type of gearbox and are therefore not universal. The disadvantage here is that inaccuracies are caused by the shift cables, which means that they do not shift as precisely as fully sequential transmissions. The use of a sequential shifter in a gearbox with synchronizer rings is possible in principle, but makes no sense because of the time required for synchronization.
The typical humming or howling noises of racing gears come from the straight teeth used in racing, which are used there because of the lower losses of the non-existent shear of the helical teeth.
Helical gears run very smoothly and comfortably, but have the disadvantage that they also create an axial load on the gear shafts and the housing and the efficiency drops somewhat due to the above-mentioned shear forces.
Depending on the design of the gearbox, the shafts are either supported by tapered roller bearings or, if the axial force cancels one another, by ball bearings . The use of straight-toothed gear wheels in principle always allows the use of ball bearings.