The invention relates to a gearbox switchable under load, which has an input shaft coupled to at least one further shaft, which extends parallel to the input shaft, via pairs of gearwheels to achieve different gear stages.
One gearwheel of each pair of gearwheels is rotatively fixedly connected to one of the two shafts and the other gearwheel of each pair of gearwheels is optionally rotatively fixedly connectable to the other of the two shafts by a respective friction coupling
Also, in the gearbox, the input shaft is rotatively fixedly connected to an engine shaft; mechanism to reverse the direction of rotation is provided in the torque flow from the input shaft to the output pinion; and mechanism for approximately simultaneously loading the friction couplings of the two lowest gear stages is provided and for starting purposes when driving forward and reversing, the torque flow through both friction couplings is maintained, at most, until an identical speed has been reached between the elements of the friction coupling of the lowest gear stage.
A gearbox of this type is known from DE 40 07 506 C1 which describes a simply designed gearbox switchable under load. The gearbox is similar in design to manual gearboxes and can be produced at reasonable cost. When starting, the relatively small friction couplings provided for changing gears are subjected to high thermal loads both in the forward and reverse gear. The thermal loads may reach critical values. Because of its length, a gearbox of this type is directly connected to the engine and is less suitable to be arranged transversely in the vehicle.
Here, an input shaft is connected to an outer coupling part which accommodates two inner coupling parts connected to separate coaxial shafts. When starting in forward gear, the gearwheels of the two lowest gear stages are engaged. Starting in forward gear is achieved by both coupling parts, and in addition to the switching sleeve of the lowest gear stage, a switching sleeve of the next higher gear stage is actuated at the same time, with the power flow being disconnected in one of the gear stages when the starting speed of the coupling of one of the gear stages has reached the input speed. Contrary thereto, when starting in reverse gear, only one of the coupling halves is loaded. Here, the double coupling is longer than the conventional coupling of a manual gearbox. Because of its design, the gearbox is less suitable to be arranged transversely in a vehicle.
When starting, both coupling parts are actuated, and the two shafts are connected rigidly by a switching sleeve. When reaching the slip-free condition at the coupling part of the lowest gear stage, the switching sleeve has to be disengaged, thereby generating a switching pressure.
It is therefore an object of the invention to provide a gearbox of the initially mentioned type which, on one hand, retains its design and preferably does without a separating coupling arranged between the engine shaft and the input shaft and, on the other hand, permits safe starting in both the forward and reverse gear.
The objective is achieved in every respect. Both when starting in forward gear and when starting in reverse gear, the couplings of the two first gear stages are loaded simultaneously and in a way which excludes thermal overloading. By distributing the pairs of gearwheels onto the two intermediate shafts it is possible to achieve a particularly short design.
As described in publications referring to the relevant state of the art, the friction couplings of the individual gear stages may be actuated electro-mechanically, with one axial actuating device alternatively loading the friction couplings of structurally adjoining, non-successive gear stages. However, it is also possible to use prior art individual hydraulic or pneumatic actuating elements to load the friction couplings of the individual gear stages. Within a very short space it is possible to use a five or six gear gearbox for incorporation into a transversely arranged engine/gearbox unit in a vehicle.
When the gearbox is driven in gear stages G5 and G6 at a high engine speed. When driving forward and reversing under overrun conditions in gear stage G1, the freewheeling unit remains closed. When reversing normally, it is necessary to bridge the freewheeling device.
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