Explained: The Inner Workings of a Press Brake Hydraulic System

Press brake is a widely utilized bending machine that has achieved hydraulic efficiency. As a critical piece of equipment for sheet metal processing, it is irreplaceable and plays a decisive role in determining product quality, processing efficiency, and precision.

Typically, the press brake is an upper piston-style press machine comprised of several components, including a frame, sliding block, hydraulic system, front-loading rack, back gauge, mold, and electrical system, as depicted in Figure 1.

1. Left upright
2. Left oil cylinder
3. Oil tank
4. Right hydraulic cylinder
5. Ram
6. Worktable

The hydraulic system of a press brake operates by forming vertical downward pressure through the use of two parallel working hydraulic cylinders. This pressure drives the die on the bending beam, allowing for the completion of the bending process.

The hydraulic control system, which acts as the “brain” of the press brake, is responsible for controlling the synchronized operation of the bending process and the positioning of the hydraulic cylinders during full loading of the press brake machine.

In this article, we will explore the workings of the press brake hydraulic system.

Hydraulic System

For each bending motion, the typical bending process of the upper bending beam includes:

(1) Oil pump starting

The motor rotates in the direction indicated by the pump arrow, which is the clockwise direction, driving the axial piston pump. The oil is then discharged through the pipeline and into the valve plate and electromagnetic overflow valve before returning to the tank. When valve number 19 is closed, the oil in the lower cavity of cylinder number 20 is held in a fixed position.

(2) Downward movement

The fast descending motion of the press brake is generated by the bending beam, the self-weight of the accessories, and the pressure of the oil. During this process, the hydraulic cylinder does not have a rod cavity through the filling valve, and any back pressure produced by the rod cavity causes the oil liquid to return quickly.

The fast forward movement starts from the top dead center and, after a brief period of deceleration, the ram slows down at a specific distance from the bending plate. The descending speed of the ram is adjusted by valve number 18, and the rapid drop is initiated by the operation of the electromagnets No. 9 YV1, No. 24 YV6, No. 13 YV4, and No. 17 YV5.

The oil in the lower chamber of cylinder number 20 enters the tank through valves 19, 18, and 17, while the upper chamber oil of the same cylinder is injected through valve 21. When the ram reaches the limit switch, electromagnets No. 9 YV1, No. 8 YV2, No. 11 YV3, No. 13 YV4, and No. 24 YV6 start working, causing the ram to transition to its working speed.

If the ram is out of sync, valve number 15 will automatically correct it. The drop position of the sliding block is restricted by the mechanical block within the cylinder.

(3) Bending

The bending phase begins with the pressure buildup of the non-bar cavity.

The bending speed is limited by the quantity of oil supplied by the oil pump. On the other hand, it can be adjusted by the direction valve of the proportional valve.

At the same time, the direction valve also controls the synchronous operation of the bending beam and the positioning of the lower dead center.

The bending force is limited by the proportional relief valve to limit the pressure of the pump.

The corresponding values of speed, synchronization, positioning and pressure are all from the CNC.

The pedal switch or button controls the electromagnet working time, which includes No.9 YV₁, No.8 YV₂, No.11 YV₃, No.13 YV₄ and No.24 YV₆, which realize the joggle distance when the sliding block drop.

The speed of slide drop is adjusted by valve 16.

The ram is controlled up by No.11 YV₃ and No.24 YV₆.

The length of the working time of the same electromagnet can realize the moving distance of the ram.

(4) Pressure relief

The stress relief of the no-bar cavity begins when it reaches the bottom of the dead center, or after a brief holding time, allowing the material sufficient time to form and enhancing the dimensional precision of the parts. The pressure holding and pressure relief are performed by the proportional directional valve, which is controlled by the numerical control device.

In an effort to improve processing efficiency, the time required for pressure relief should be minimized. However, to avoid unloading impact on the entire system, it is necessary to extend the discharge time as much as possible. In other words, the pressure relief curve should be as smooth as possible, avoiding steep drops.

The optimization of the entire process is achieved through the use of the proportional directional valve.

(5) Master cylinder return

The pump flow and the hydraulic cylinder have a pressure area in the bar cavity, which determines the maximum return speed, which is typically close to the fastest speed. The return process requires synchronous operation, starting with the pressure reduction of the bar cavity and ending at the upper dead center.

At the moment of return, it is necessary to reset the pressure of the No. 8 YV2 electromagnet for 2 seconds, then the electromagnets No. 11 YV3 and No. 24 YV6 start working and the sliding block begins to return at a constant speed.

(6) Pressure adjustment of press brake

Valve No. 6 and No. 11, the high-pressure overflow valve and electromagnetic overflow valve, respectively, are primarily responsible for maintaining the rated power of the press brake. Meanwhile, valve No. 14 regulates the return force of the machine to prevent damage caused by overload.

The pressure within the hydraulic system can be monitored via the pressure gauge No. 7. The nitrogen pressure in accumulator No. 10 mainly controls the pressure necessary for the operation of valves No. 19 and No. 21.