Overlay steering gear
The Superimposed steering mechanism is used for steering of tracked vehicles (eg. B. excavators or tanks ). It is a successor to the "Cletrac" steering system with two planetary gears, which was developed in the early 1920s by the US Cleveland Tractor Company for their tractors with chain drives . The Cletrac steering is operated by two levers that act on the differential gear via steering brakes ( band brakes ) .
A superimposed steering gear is more complex to manufacture because it requires its own drive motor, but it has lower energy losses and is easier and more precise to operate. The way it works is similar to that of a differential gear , with the difference that the speed differences between the right and left drive axles are not set automatically, but are specified by a second drive motor.
construction
The superimposed steering gear essentially consists of the following parts:
- Two identical, single-stage planetary gears
- A counter-rotating gear
- A hydraulic pump driven by the vehicle's drive motor (similar to the oil pump in cars and trucks)
- A hydraulic motor driven by the hydraulic pump
- Hydraulic control valves
In the simplest form, the two planetary gears are arranged next to one another parallel to the drive wheels . The sun gears (inner gears ) of the two planetary gears are firmly coupled to one another by a shaft . This shaft is connected to the vehicle's engine and transmits the driving force. The two external drive wheels (drive wheels) of the chain drive are rigidly connected to the planetary gear carriers of the two gears (possibly via an additional back gear ). The two ring gears with internal gear rings each work via a simple spur gear on two shafts that are coupled via a bevel gear . The central bevel gear has its own drive, which is usually designed as a hydraulic motor. The hydraulic motor is controlled by hydraulic control valves that regulate the oil delivered by the hydraulic pump. The hydraulic pump is driven by the main drive motor of the vehicle.
functionality
Straight ahead
The drive motor drives the shaft with the two sun gears via the clutch and gearbox . These transmit the torque to the two planetary gear sets that rotate between the sun gears and the two stationary ring gears. Both drive wheels have the same speed. The tracked vehicle drives straight ahead.
Cornering
The ring gears are also driven for cornering, so that their rotation is superimposed on the rotation of the axles when driving straight ahead.
The ring gears are set in opposite rotation via the hydraulic motor and the bevel gear. As long as the speed of the ring gears is well below the speed of the main drive, one of the chains runs faster than the other. If the speed of the ring gears increases, one of the chains stops at a certain point and the other rotates at twice the speed. If only the ring gears rotate while the main drive is idle, the vehicle rotates when it is stationary.
The rotational speed of the planetary gears results from the difference between the speeds of the main drive and the ring gears.
The driver of the tracked vehicle controls the hydraulic control valves with a steering wheel. The hydraulic oil flows through one of three working circuits, depending on the setting of the control valves. One circle is designed for idling. The oil is only conveyed in a circle without reaching the hydraulic motor and without performing any work. The other two working groups are each intended for one direction of oil delivery. Depending on the setting of the control valves, the oil is driven through the hydraulic motor either in one or the other flow direction. This causes the hydraulic motor axis to rotate in one direction or the other. The hydraulic motor drives the counter-rotating gear on its outer bevel gear and there are different speeds and directions on the drive wheels. At full throttle, curves can be driven faster and tighter due to the increased oil delivery rate.