Rear axle steering system for a mobile crane

1. A rear axle steering system for a mobile crane having at least one actively steered rear axle with wheels arranged thereon, comprising:

a hydraulic steering system including a first plurality of hydraulic steering cylinders, said first plurality of hydraulic steering cylinders including a centering cylinder and a second plurality of steering cylinders, a said centering cylinder being arranged for centering said at least one steerable axle in the event of an error in said hydraulic steering system; said second plurality of steering cylinders associated with said at least one actively steered rear axle for steering said actively steered rear axle in a desired manner;

a braking system configured for individually individual braking of each wheel of said at least one actively steered rear axle; and

a control system for actuating said braking system in the event of said error in said hydraulic steering system, so that a predetermined, desired steering position of said at least one actively steered rear axle is achieved by selective braking of at least one wheel of said at least one actively steered rear axle.

2. The rear axle steering system crane according to claim 1 , wherein said mobile crane additionally comprises a steered front axle; a setpoint angle sensor associated with said steered front axle; a first actual angle sensor and a second actual angle sensor associated with said actively steered rear axle; said second actual angle sensor taking over the function of said first actual angle sensor in the event of an error.

3. The rear axle steering system according to claim 1 , wherein said mobile crane additionally comprises a steered front axle; a first setpoint angle sensor and a second setpoint angle sensor associated with said steered front axle of said mobile crane; said second setpoint sensor taking over the function of said first setpoint angle sensor in the event of an error.

4. The rear axle steering system according to claim 3 , wherein said mobile crane comprises at least two steered front axles mechanically coupled to each other; said first steered front axle of said mobile crane being associated with a first setpoint angle sensor and at least said second steered front axle being associated with a second setpoint angle sensor; and a control system connected with both said setpoint angle sensors; and wherein in the event of a failure of one of the first or second setpoint angle sensors said at least one actively steered rear axle is steered by said other one of said first setpoint sensor and said second setpoint sensor.

5. The rear axle steering system according to claim 1 , additionally comprising a control device and wherein in the event of an error in said hydraulic steering system, said control system actuates said braking system by means of said control device so that a predetermined, desired steering position is achieved by selective braking of at least one wheel of said actively steered rear axle affected by said error.

6. The rear axle steering system according to claim 5 , wherein said control device is an anti-lock brake system control device, an electronic braking system control device or an anti slip regulation control device; and, in the event of an error in the hydraulic steering system, said braking system selectively brakes by means of said control device at least one wheel of said actively steered rear axle which is affected by said error, so that a predetermined, desired steering position is achieved, and wherein a braking action of said control device is replaced by said selective braking for steering said affected rear axle in a desired manner.

7. The rear axle steering system according to claim 1 , wherein said at least one actively steered rear axle includes at least two actively steered rear axles, each of said actively steered rear axles being operably coupled to two of said second plurality of steering cylinders.

8. A rear axle steering system according to claim 7 , wherein said hydraulic steering system of each actively steered rear axle comprises a shut-off valve installed on each of said first plurality of hydraulic steering cylinders of said rear axle; each of said first plurality of hydraulic steering cylinders having an inflow and an outflow for hydraulic oil; and said shut-off valve being adapted to inhibit a flow of hydraulic oil in one of the directions comprising said inflow to and said outflow from said first plurality of hydraulic steering cylinders.

9. A method of steering a mobile crane comprising:

providing at least one steerable rear axle including wheels and a hydraulic steering system with no centering cylinder for centering said at least one steerable rear axle after malfunction of said hydraulic steering system;

actively steering said at least one rear axle by said hydraulic steering system;

detecting an error affecting said steering process by means of said hydraulic steering system of said at least one actively steered rear axle; and

selective braking with a control system at least one of the wheels of said affected rear axle so that said affected rear axle is brought to a predetermined, desired steering position.

10. The method according to claim 9 , wherein said control system includes a steering computer and said error is detected by determining a failure of said steering computer of said at least one actively steered rear axle.

11. The method according to claim 9 , wherein said error is detected by determining a failure of an angle sensor associated with said at least one actively steered rear axle.

12. The method according to claim 9 , wherein said mobile crane has at least two actively steered rear axles and wherein said control system includes a first steering computer of a first actively steered rear axle and a second steering computer of a second actively steered rear axle, when said error is an error in said first steering computer, said second steering computer takes over the function of said first steering computer exhibiting said error.

13. A method according to claim 9 , wherein said mobile crane has at least two actively steered rear axles and wherein said control system includes a first steering computer of a first actively steered rear axle and a second steering computer of a second actively steered rear axle, wherein when said error is an error in said first steering computer, said second steering computer takes over the control for said selective braking of at least one of the wheels of said affected rear axle for achieving a predetermined, desired steering position of said affected rear axle.