Adaptable Layshaft Type Gearbox Using A Planetary Gearset
A gearbox comprises parallel primary or input and secondary or output shafts in a casing
. An output gear train of two constant-mesh gearwheels is fastened to the secondary shaft.
An independent input gear train of four constant-mesh gearwheels and a first and a second clutch are mounted on the primary shaft. A third clutch is mounted on the secondary shaft. The first and third clutches respectively couple the input train to the primary shaft and to the secondary shaft.
Three-element planetary gearset concentric with the primary shaft comprises a sun gear fastened to the input train, a planet carrier coupled to the primary shaft by the second clutch and a ring gear fastened to the output train. Thus the gearbox features three gear ratios by respective combinations of pairs of the three clutches. This arrangement constitutes a basic gearbox.
More complex gearboxes derived from the basic gearbox comprise a supplementary gear train and a fourth clutch and/or a first brake and/or a second brake for respectively driving and/or immobilizing the sun gear and/or the planet carrier. This provides numerous additional forward and reverse gear ratios that are always well distributed and easy to control.
It is known to implement such gearboxes with a small number of gear ratios, for example two or three forward gear ratios--one of which is generally a direct drive--and one reverse gear ratio, with a primary shaft and a secondary shaft, a geartrain and a clutch for each gear ratio, and a separate special gear for the reverse gear ratio.
Such gearboxes are simple, rugged and efficient and of acceptable cost and overall size, but their performance remains somewhat limited and more often than not the reverse gear ratio is somewhat perfunctorily implemented by means of a dog typesystem.
Similarly, it is known to implement gearboxes with a larger number of gear ratios, for example four forward gear ratios and two or four reverse gear ratios, by combining two ranges of two gear ratios and one reversing unit, or with three forwardgear ratios and three reverse gear ratios, or again with five forward gear ratios and one reverse gear ratio, etc.
Such gearboxes are still rugged and efficient but entail multiple layshafts and clutches so that their complexity, cost and overall size increase out of proportion to the number of gear ratios if an adequate level of performance is to beachieved. Also, in dual range gearboxes changing the range entails a shifting sequence that is no longer a single operation but a double operation--that is to say involving the simultaneous control of four clutches instead of two--resulting in muchgreater variations in inertia and much more serious problems in controlling the shifting, while the steps in the gearbox are precisely the same in both ranges, imposing gear ratio distributions that are often inappropriate.
Finally, and in a general way, these known gearboxes do not have any braked neutral setting, that is to say a combination in which the input to the gearbox is free and the output is immobilized, corresponding to the specific case where the ratiobetween the output speed and the input speed is equal to zero. The advantages of this type of combination, which is lacking in the types of gearbox under discussion, are specifically to permit immobilization of the vehicle with the motor running,notably during momentary stops imposed by traffic conditions, without needing to constantly apply the brakes of the vehicle, to guarantee good control over and continuity of changes between the forward gear ratios and the reverse gear ratios onreversing, and so on, all these factors tending to increase significantly driver comfort.
The object of the present invention is thus a gearbox retaining the original qualities of simplicity, ruggedness and efficiency of the known gearboxes with few gear ratios mentioned above, but also featuring, with a minimum of components andacceptable cost and overall dimensions, an increasing number of forward and reverse gear ratios that are always well distributed, a braked neutral setting in addition to the conventional unbraked neutral setting, and shifting sequences that always entailsingle operations only from one end to the other of the range--that is to say from the highest reverse gear ratio to the highest forward gear ratio--lending itself perfectly to any kind of control system.
Depending on whether the speed ratio of the input train between the sun gear and the planetary gearset and the first clutch is less than, equal to or more than unity, the third gear ratio is respectively an overdrive ratio, a direct drive ratioor a less than one-to-one ratio, in this context direct drive meaning rotation of the planetary gearset as a whole.
This gearbox is the basic gearbox. In accordance with another feature of the invention, this gearbox is adaptable and has provision for implementing a whole series of highly standardized gearboxes with increasing numbers of gear ratios, bysimple addition of minimum additional components, as will now be explained.
By adding to the basic gearbox a first brake for immobilizing the sun gear and/or a second brake for immobilizing the planet carrier the gearbox is respectively provided with a fourth gear ratio and a first braked neutral setting by combinationof the first brake with the second and third clutches, and/or a reverse gear ratio and a second braked neutral setting by combination of the second brake with the first and third clutches, and a third braked neutral setting by combination of the firstbrake with the second brake.
When the gearbox has four forward gear ratios and one reverse gear ratio, for example, the first and third forward gear ratios and the reverse gear ratio are subject to slip whereas the second and fourth forward gear ratios are not subject toslip.
by: Bowman
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