Clever hydraulic concept for self-propelled forage harvesters

Clever hydraulic concept for self-propelled forage harvesters
Marginal Column

November 2017


On the Series 8000 self-propelled forage harvesters, John Deere is using a new hydraulic concept developed in conjunction with Bosch Rexroth for the header and feed roll drive.

The combination of the hydrostatic circuits for the header and feed roll drive allows a stylish implementation of two key additional functions on the forage harvester - the sharpening function for the knives on the cutterhead and the cutterhead braking function. Both functions utilize the fact that the header pump and the cutterhead are attached to the same drive. This enables the header pump to be used as a hydrostatic drive or for braking the cutterhead. During harvesting, the cutterhead is powered by a belt drive. This guarantees high efficiency of the machine’s main drive.

Header drive

The header drive is a hydrostatic circuit with axial piston variable pump (A) in swashplate design, including electronic proportional control (EP) and an axial piston constant motor (B) in bent axis construction. In addition to the flexibility in terms of positioning of the hydrostatic units, this adjustable drive allows easy adaptation to different header types as well as electronic speed control, for example for different harvesting speeds. In case of overload, the hydraulic system has automatic torque limitation thanks to the high-pressure relief valves on the pump (A). Reversal of the header is possible by swiveling the pump to the other side.

Feed roll drive

The feed roll drive is a hydrostatic circuit with axial piston variable pump (C) in swashplate design and an axial piston variable motor (D) in bent axis design from Bosch Rexroth. Both units have electronic proportional control (EP). As the cutterhead runs at a constant speed, the feed roll defines the cut length of the crop. To cover the entire cut length range, in addition to the pump the motor has continuously variable adjustment. In turn, this allows continuously variable adjustment of the cut length. In case of overloading, the system is equipped with a “hydraulic clutch”: Opening the high-pressure relief valves on the pump limits the torque in the drive to a pre-defined value. This prevents damage to the drive system.

A further advantage of this system is the possibility of reversing the feed roll drive independently of the cutterhead. This enables foreign particles to be removed from the feed rolls even when the cutterhead is stationary. To do this the pump (C) is moved over center to the other side and thus the direction of rotation of the feed roll motor (D) is reversed by the reversal of the oil flow.

Feed roll drive quick stop

As the feed roll drive is part of the closed hydrostatic circuit, it can be stopped by “reversing the high-pressure side”. To prevent any foreign particles that get in from damaging the cutterhead, the pump (C) is swiveled as quickly as possible from the maximum swivel angle Vgmax to zero and, at the same time, the motor (D) is adjusted from its current displacement Vg (depending on speed/cut length) to the maximum displacement Vgmax. This causes a brake pressure to build up on the opposite side of the closed circuit. The motor, which is powered by the inertia of the feed roll, is stopped within the required braking time. The patented open loop control for this system is extremely reliable and operates without additional hydraulic or mechanical brake valves. The weight of the machine is reduced and wear of the components is prevented.

Brake function of cutterhead

As soon as the driver leaves the driver’s seat, the cutterhead must be stopped within a defined time to prevent potential injuries. This function is achieved by switching over the valve block (E). The header pump (A) is connected to the feed roll pump (C) to form a new closed circuit and the feed roll motor (D) is swiveled to Vg = 0. As mentioned above, the header pump (A) is attached to the same drive as the cutterhead, i.e. it is connected directly to the cutterhead. The mass inertia of the cutterhead thus drives the header pump (A) and the feed roll pump (C) is swiveled to Vg = 0. This causes a brake pressure to build up, which stops the cutterhead. A pressure relief valve on the feed roll pump (C) limits the maximum brake pressure. The brake power can be adjusted using the swivel angle of the header pump (A). To stop as quickly as possible, the header pump is set to the maximum swivel angle. The cutterhead is therefore stopped without any additional components.

Knife sharpening on the cutterhead

To sharpen the knives, the valve block (E) creates a new hydrostatic circuit with the feed roll pump (C) and the header pump (A). For this function, the header pump (A) connected to the cutterhead drive now operates as a motor and is driven by the feed roll pump (C). Overcenter movement of the feed roll pump (C) onto the other side reverses the direction of rotation of the cutterhead, i.e. it rotates backwards. The grinding stone for sharpening the knives is moved into position. No further components are required for this function.

Intelligently linked

The header and feed roll drives on the 8000 Series self-propelled forage harvesters from John Deere intelligently combine the two hydrostatic circuits. This enables the weight to be reduced and makes the machine layout simpler, not only reducing costs but also improving the reliability of the forage harvester. The patented quick stop function in particular shows how close cooperation between the machine manufacturer and supplier can result in innovative, application-specific solutions.

Header drive circuit Magnifier

Header drive circuit: Variable pump (A) with fixed motor (B), feed roll drive circuit (with quick stop function): Adjustment pump (C) with variable displacement motor (D), circuit for cutterhead brake function and sharpening the knives: Variable pumps (A + C) connected via valve block (E)