In general, a distinction is made between single-column presses, double-column presses and double-column presses. A distinction is also made between:
- Presses with a fixed stroke
- Presses in which the stroke can be adjusted using two eccentrics that can be adjusted to one another
Furthermore, a distinction can be made between stiff presses with a high spring stiffness C of the press body and soft presses (low spring stiffness).
The actual pressing process is initiated by the following steps:
A flywheel is driven by an electric motor via a belt drive , from which the movement is transmitted to the clutch bell via a small and a large gearwheel (mounted on the eccentric shaft). During the intended travel movement, the inner clutch bell presses against the clutch plate equipped with friction blocks (wedged on the eccentric shaft), which then positively absorbs the movement of the gear. At the same time, the mechanically closed brake is pressed against the springs with the help of a piston ring and thus opened.
The connecting rods mounted on the eccentric press the tappet with the help of half shells. Compensating cylinders, which are located on the press, help the ram to open because the clutch is released at approx. 270 °. The brake is closed again with a time delay of 150 ms.
The electrical control of the eccentric press determines its area of application. According to trade association regulations ( UVV , BGV), inlay work may only be carried out on so-called "safe" presses. Inlay work is understood as the intervention in the tool in order to remove the raw part or the finished part.
Safety when pressing
For a safe control you need:
- OT (top dead center control)
A technical protective device, either as
- Two-hand control with simultaneity monitoring and contactor monitoring
- non-contact protective device (safety light curtain) type 4 according to EN 61496-1 with corresponding integration in the control (category 4 according to standard EN 954)
- Safe tool: The tool is designed in such a way that no hazardous areas can be reached. Openings or gaps through which workpieces are fed are therefore designed so that no parts of the body (fingers) can touch the hazardous areas.
- Shaft break protection
- Standstill monitor
All devices used must meet safety category 4. This is the highest security category!
All eccentric presses should be subjected to an annual UVV test. This is where the mechanical components and the electrical control are checked to ensure that they are working properly (see Industrial Safety Ordinance and trade association information BGI 724).
Clutches and brakes
Another division can be found in the clutches and brakes.
There are force-fit and form-fit couplings. The friction-locked clutch is usually provided with a friction lining similar to a car clutch with lamellas. The drive force of the flywheel is then transmitted via these lamellae. The clutch disc is usually actuated electro-pneumatically or electro-hydraulically and is spring-loaded. In the positive coupling z. B. pressed a conical shape into a counterpart and thus transmit the force. Nowadays, only non-positive couplings are used, as the positive-locking variant involves high safety risks and is therefore not permitted under the accident prevention regulations (in Germany, the employers' liability insurance association is responsible). Once triggered, a downward movement can no longer be stopped with a positive coupling. The form-fitting couplings that are still in use are subject to inventory protection and may continue to be operated, but only with "safe" tools where the danger area is not accessible.
As already described above, the clutch and brake lining are designed as a combination. It can either be coupled or braked. If the compressed air on the clutch valve breaks off in the event of a fault, the springs (therefore spring-loaded) push the combination back into the rest position immediately, i.e. the brake lining into the brake shell. The machine stops without any significant delay. In the past, when “safe” controls were not possible in terms of control technology, an additional brake was installed that was monitored electrically. This additional brake thus fulfilled the purpose of a "safe" control.
Press safety valve (PSV): To enable a downward movement, a compressed air valve is required. For eccentric presses with "safe" control, so-called PSV valves are prescribed, often also referred to as twin valves. Inside this valve there is a pressure compensator that monitors both compressed air channels of the valve for simultaneity. If there is a time difference, the valve is blocked and a message is sent to the controller. A defect in the PSV can never trigger the downward movement!
- Andreas Hirsch: Machine tool basics. Friedrich Vieweg & Sohn, Wiesbaden 2000, ISBN 978-3-528-04950-8 .
- Heinz Tschätsch: Practice of forming technology. Working methods - machines - tools. 8th updated edition. Springer Fachmedien, Wiesbaden 2005, ISBN 978-3-322-99433-2 .
- Stroke diagram. (PDF) accessed on January 18, 2016