Dual-ratio drive shifts on-the-fly

May 11, 2006
Planetary drives reduce the speed and increase the torque hydraulic motors deliver to sprockets, wheels, and drums.

The GFT 45 T2/T3 combines twoand three-stage planetary drives in a compact package.

They're generally used to propel construction, agriculture, and forestry equipment.

Conventional planetary drives usually come in either two or three-stage configurations. Two-stage drives have reduction ratios from about 16:1 to 60:1 and typical three-stage ratios range from 60:1 to 350:1.

The GFT 45 T2/T3 from Bosch Rexroth, Lake Zurich, Ill. (boschrexroth-us.com), combines a two and three-stage planetary drive in one compact package. The dual-ratio unit functions as a two-stage drive for low-ratio (high-speed) operation and the third stage engages for high-ratio ( lowspeed) movements. It generates maximum intermittent torque of 45,000 N-m (33,188 lb-ft) and maximum output speed is approximately 100 rpm. According to Bosch Rexroth engineers, the dual-ratio design holds four major advantages:

  • Wide operating speed range.
  • Two different reduction ratios in a housing about the same size as a single reduction ratio drive.
  • The ability to shiftonthe-fly between reduction ratios.
  • It operates at high speeds without external cooling.

The GFT 45 generates a high ratio through an input stage that disengages when the vehicle requires low-ratio output. The high ratio is approximately four times that of the low ratio. The planetary input stage is inside the supporting axle — not in the gearbox cover as is normally the case with three-stage driving gears. This arrangement lets the stage engage as needed. Input is on the sun wheel, and the output on the planet carrier.

Two spring-applied, hydraulically released clutch packs engage and disengage the input stage. One clutch connects the freely rotating ring gear to the supporting axle, the other connects the sun wheel shaft and planet carrier.

Each clutch pack has a hydraulic release port. This gives four possible combinations for clutch position and operating mode:

  • When the vehicle is stationary and no hydraulic pressure on the clutches, spring force engages both clutch packs. This blocks the input stage and brakes the wheel hub via the gearbox. Thus it serves as a static emergency/parking brake.
  • If pressure is applied to both clutch packs, the drive is in the free-wheel condition and the gearbox is idling.
  • With pressure routed to the inner clutch pack and none to the outer one, the drive operates in the high-ratio, lowspeed mode. This disengages the clutch between the sun wheel and planet stage. The ring gear remains connected to the supporting axle, fixed to the housing, and the planet input stage reduces engine speed by roughly a 4:1 ratio.
  • If pressure is routed to the outer clutch pack and none to the inner clutch pack, the drive operates in the low-ratio, high-speed mode. The clutch between-the ring gear and housingdisengages, and the clutch between the sun-wheel input and planet carrier engages. The outer clutch pack releases and lets the input-stage ring gear rotate, while the inner clutch pack secures the input shaft to the ring gear.

The nonpositive action of the clutches means the gear can be changed in motion and under load. The planetary drive's wide operating speed range is further enhanced when used with a variable-displacement hydraulic motor.

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