Servo technology helps replace hydraulics with electromechanics

3 min read

TOX Pressotechnik GmbH is well known for its press drives that are used in the joining of sheet metals, commonly used in automobile and white goods production. Market pressure for flexibility, controllability and higher environmental standards led TOX to add to its range press drives with electromechanical operation.

As an alternative to press drives that generate high force with a pneumatic pressure intensifier and a hydraulic cylinder, the servo press drives use less energy, are easy to programme and set up, and do not require high pressure oil and air in the production environment. Standard TOX press drives are used in thousands of installations worldwide to deliver high axial forces used to give safe and speedy clinching, stamping and embossing. Forces up to 2000kN are available for joining materials and for other types of machine processes. Compact dimensions allow these press drives to be often installed onto robot arms. The enormous forces are a result of pneumatic pressure being converted into hydraulic force by an integrated pressure intensifier. Assemblies with customised strokes combine the advantages of pneumatics, giving fast strokes, with hydraulics, giving high force. Although the initial cost is higher, the alternative TOX ElectricDrive servo-driven spindles have customer benefits that have led to an increasing market acceptance. The ElectricDrive modular design is easily adaptable to individual machines with a press force capacity up to 400kN. It consists of a Lenze servo motor with a single-stage planetary reduction gearhead. This is connected by tooth belt to a planetary roller spindle which drives forward the working rod. Control is with resolver feedback to a Lenze 9400 servo inverter mounted in a cabinet. The servo-driven ElectricDrive is highly dynamic thanks to motors with short-term rating of 300% nominal torque combined with low inertias. This can reduce cycle times, and maintenance downtime is minimal with only an occasional re-lubrication of the roller spindle. Furthermore positional accuracy of the stroke is better than 0.01mm. A major attraction to plant operators is the ability to rapidly reset the press to suit different materials or processes. The Lenze servo inverter type 9400 is equipped with an integrated PLC which holds the software for the operating cycle. TOX store all the application software in the servo drive including a curve profile monitoring system, 500 process data records and about 270 processes. Resetting for a new workpiece is done in a moment with communication to the factory master controller by PROFINET bus or Ethernet TCP/IP. For integration of external sensors and actuators, there are analogue and digital I/O terminals on the inverter which can be increased almost infinitely by addition of a Lenze modular I/O system. It is becoming more common to find factories where hydraulic oil and compressed are banned leading to servo press drives becoming more and more the preferred choice. The automotive industry has proven to be a trailblazer in this respect, regularly using them in robot-guided joining tongs. In modern automotive factories the use of pneumatic/hydraulic press drives has fallen to about 20%, in part due to the higher weight of such devices. Aside from the automotive area, TOX ElectricDrives are used in assembly and special purpose machinery for operations such as inserting, riveting, press fits, crimping, piercing, punching and stamping. Typical strokes are up to 250mm with linear speeds around 200mm/s. Managing director of TOX Pressotechnik, Dr.-Ing Wolfgang Pfeiffer, explains the energy saving advantages of the servo press drives: ''Owing to servo technology, these motors are considerably more efficient than pneumatic/hydraulic drives and from a technological point of view, they are the best that is available''. The Lenze servo inverter also has an integrated safety option: "We need Safe Stop for robot-guided tongs because there may be people working in the vicinity of the robot. Other applications are fenced off with guards and therefore only need the Safe Torque Off safety function," he says. The Lenze servo inverter offers the highest safety levels demanded by customers meeting Performance Level ''e'' of EN ISO 3849-1 and "SIL 3" of EN/IEC 62061. Machine users also profit from the extensive resources that come with the 9400 servo inverter in that TOX not only provides a library of standard functions (operations such as crimping, joining, clinching, etc.) but also an additional library of special functions. This makes it possible to use process jumps or multiple conditions. The programmability and performance capability of the 9400 servo inverters also opens up further possibilities. With some additional engineering, customised interfaces can be created and the functionality of the servo press drive can be extended almost infinitely. ''The integrated PLC and the possibilities of extending the Lenze solution are the main reasons why we are using it in our high-end presses'' explains Dr.-Ing. Wolfgang Pfeiffer. "What convinced us from the very beginning was also the large, harmonised product portfolio and the readiness of Lenze to work together with us as a partner." TOX Presstechnik was quick to respond to market demands for a range of press drives with electric actuation. In choosing Lenze servo motors and drives, they achieved high dynamics and extensive functionality. Moreover, due to the integrated PLC, the servo press drive can cover a wide range of applications and can easily be adapted to a large variety of different tasks. As the electromechanical servo press drives do not need hydraulic oil or compressed air and work with permanently excited synchronous motors, they are especially energy-efficient, environment friendly and are practically maintenance-free. The operating costs are also correspondingly lower, with the result that the higher acquisition costs compared to conventional pneumatic/hydraulic press drives are amortised completely within just a few years. ''Low operating costs and a high degree of environmental compatibility in particular are becoming more and more important for users. They are increasingly basing their decision to buy not on the purchase cost but on a calculation of the total lifetime costs'', concludes Dr Pfeiffer.