Machine element or mechanism – Gearing – Interchangeably locked
Patent
1998-09-10
2000-04-18
Ta, Khoi Q.
Machine element or mechanism
Gearing
Interchangeably locked
74331, 74337, F16H 3093
Patent
active
060501529
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a motor vehicle gearbox comprising two concentrically mounted input shafts, arranged to be driven alternately, and two lay shafts driven by the input shafts, the lay shafts having gears meshing with gears on an output shaft, and at least one gear being rotatably mounted on each lay shaft and lockable to its shaft by means of engaging means meshing with a common gear on the output shaft.
Double lay shafts are used in gearboxes in which it is desired, for a given gearbox length, to obtain more incremental gear speeds than what is practically possible in a conventional gearbox design with one lay shaft, and, in so-called power-shift gearboxes, i.e. gearboxes with double input shafts, each driven by an individual clutch. In such gearboxes, the gear positions are preselected, whereafter the actual shifting is done by releasing one clutch and engaging the other clutch. There are two main types of known gearboxes with double lay shafts. In one type, gears on the two lay shafts engage, in pairs, common gear on the outward shaft. In the second type, each lay shaft gear engages an individual gear on the outward shaft. Common to these two types, which are shown and described in SE 8003158-6, is that the lay shafts are the same radial distance from the main shaft. Both have advantages and disadvantages. The advantage of arranging pairs of lay shaft gears meshing with a common gear on the output shaft is that the gearbox can be dimensioned with minimum length for a given number of gear speeds. The disadvantage is however, that the lay shaft gears in the respective pairs must be the same size, which limits the freedom in selecting incremental between gear speeds. The second type provides maximum freedom in regard to the selection gear speed steps, but requires, for the same number of gear speeds, that the axial dimension of the gearbox be larger.
Gearboxes of the first type, which are particularly intended for heavy vehicles, such as trucks and other work vehicles, which require a very high gear ratio in the lowest gear speeds, have very small diameters, due to the design of the gearbox, for the lay shaft gears for the lowest gear speeds. When these are releasably mounted on their shafts they must be mounted on needle bearings, which, for the small diameters in question, result in thin gear hubs and consequently limited torque which can be transmitted.
SE-A-8700583 shows and describes a gearbox which has lay shaft gears for second and third gear speeds releasably mounted on their shafts and engaging a fixed gear on the main shaft. The lay shaft gear for first gear has, however, a gear ring cut directly in the lay shaft, which means that it can be made with a very small diameter and still transmit a large torque. This design requires, however, that the releaseable gear for first gear be mounted on the output shaft.
The purpose of the present invention is to achieve a gearbox of the type described by way of introduction which, while retaining the advantage of minimizing dimensions described above, provides greater freedom as regards the selection of incremental gear speeds and placement of the releasable gears for the lower gear speeds.
This is achieved according to the invention by virtue of the fact that corresponding gears mounted on the lay shafts and which share a common gear on the output shaft, each have different numbers of gear teeth.
Such a design can be achieved according to the invention in practice, on the one hand, by disposing the lay shafts at different radial distances from the output shaft and, on the other hand, by employing so-called "addendum modification" of the gears which provides for a different number of gear teeth to give greater flexibility in selecting gear increments, as subsequently discussed.
In the previously known gearboxes with opposing gears of equal size on the lay shafts, the gear speed increments between subsequent gear speeds, for example between third and fourth, were exclusively dependent on the difference in gear ratio between the primary gear rin
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AB Volvo
Ta Khoi Q.
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