The present invention relates to a planetary gear control system and, more particularly, to a planetary gear control system having a plurality of different selectable gear ratios for the transmission of power between an input and an output. A retarder is engageable at least in some of the gear ratios.
It is desirable for a gear system to permit the speed ratio between input and output to be changed while the gear system is loaded. Preferably, a hydrodynamic torque converter with a bridging coupling is provided between a drive motor and the gear system to facilitate starting from a standstill. The retarder is generally a hydrodynamic brake.
A number of competing parameters complicate the coupling of a retarder to a gear system. First, it is desirable that the retarder be mechanically positioned at a stage in the gear train where speed changes coupled to it are of a continuous nature rather than having rapid discrete jumps. This requirement tends to favor coupling the retarder to the output rather than the input. Second, it is desirable that the retarder be as small as possible. When coupled to the output, the relatively slow output speed in low gears requires a physically large retarder to produce the required torque or moment. Thus, this requirement tends to favor coupling the retarder to the input rather than the output. But, when the retarder is coupled to the input, it is subjected to discrete jumps in speed as the gear ratio is changed. Finally, it is desirable to minimize the thermal and stress loadings on the couplings and brakes employed to change the gear ratio of the gear system.
U.S. Pat. No. 4,191,072 discloses a planetary gear system in which a retarder rotor is permanently connected to the input shaft. At every gear change, the retarder rotor makes the same jump in speed as the drive motor. This produces unpleasant gear-change jolts, particularly when changing between lower gears. Elimination of these jolts requires complicated controls. In addition, when hydrodynamic braking occurs during a gear ratio change, the couplings and/or brakes used to accomplish the changeover are loaded by the braking torque of the retarder.
A publication, VD1-Zeitschrift 1969, pages 333-338 describes a gear system having a retarder rotor connected permanently to the output shaft. This eliminates the retarder-induced jolts when the gear ratio is changed, but it requires a larger retarder since the rotary speed of the retarder rotor is low at low travel speeds and the retarder braking torque is correspondingly reduced. Although additional gears can be provided to drive the retarder at a higher speed, this increases production costs considerably.
U.S. Pat. No. 3,749,209 discloses a gear system in which a hydrodynamic torque converter is relied on for braking rather than a retarder. This arrangement is incapable of providing sufficient retarding torque, particularly in the lower gears.
U.S. Pat. No. 4,077,502 discloses a gear system in which a hydrodynamic torque converter can be employed for braking to a stop. This system requires drive motor input to the gear system during braking.
In a gear system with an integrated retarder, disclosed in German laid-open patent specification No. 25 21 831, part of the gear system forms a step-up gear ratio which drives the retarder rotor from the output shaft. In this system, the gear component which drives the retarder rotor is stationary in at least one forward gear. In some embodiments in this reference, the gear component driving the retarder rotates in a reverse direction in one of the forward gears compared to the direction in another forward gear. This means that the retarder is operable only with the use of a special braking gear. A coupling and/or a brake must be provided to switch the retarder in and out. Such a coupling and/or brake is subjected to substantial stress which causes it to wear out more quickly than would otherwise be the case.
A further disadvantage is that the gear system must be disconnected from the drive motor whenever the retarder is switched in. This means that a braking contribution from the drive motor cannot be used.
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