Hilchenbach, January 28, 2020

Energy-efficient direct drive for mechanical screwdown systems

Mechatronic solution for rolling mill stands

With an overall efficiency level of 98 percent, direct drives are extremely energy-efficient, low-noise and, because no gear unit is required, very maintenance-friendly. SMS group now launches this technology for mechanical screwdown systems in rolling stands.

Hot strip mills for steel and aluminum as well as heavy-plate rolling mills frequently use drive systems with forced-air ventilated asynchronous motors, gears, and mechanical brakes, despite their low energy efficiency and high maintenance requirements. Not only that, they are made up of many different components and take up a considerable amount of space.

Torque motors do not require a gear unit, as they can achieve high torque levels at low rotational speeds. This is because the motor has a high number of pole pairs thanks to the use of high-performance permanent magnets on the rotor. One indication of how torque motors have long since reached industrial maturity is that for many years now this has been the standard type of drive used for machine tools; it is utilized in a variety of different applications, including ship propulsion systems or as drive units in primary industries.

SMS group first applied the innovative direct drive for an edge scrap coiler for customer MMPZ-group in Miory, Republik Belarus. SMS group now also uses direct drives with a torque motor for mechanical screwdown systems in rolling stands. A direct-driven machine requires far fewer components, so customers benefit in many different ways:

  • Less energy consumption thanks to higher efficiency
  • Less maintenance thanks to fewer wear and spare parts
  • Better control dynamics thanks to fewer moving parts and a direct connection to the mechanical equipment

Customers reap these benefits not only while operating the plants but also when repairing or installing them, as costly and time-consuming adjustment and assembly work is no longer needed.

Direct drive replaces asynchronous motor and gear unit

With the conventional solution, an asynchronous motor (shown in red) uses a worm gear to drive a multispline hub with built-in adjusting screw. The system, which is installed on both mill housings, sets the preliminary positions of the roll sets by increasing or reducing the roll gap. In modern rolling stands, precise positioning is performed by the hydraulic gap control system.

The innovative integrated mechatronic design dispenses with the asynchronous motor and the worm gear. All required is a housing with a mounted-on braking device consisting of a multi-pole permanently excited synchronous motor - a torque motor – with integrated multi-spline hub and adjustment screw. The moving part, the multi-spline sleeve, is equipped with a strong permanent magnet. The stationary part consists of a stator of similar design as the stators in asynchronous motors. The stator is fed by a frequency converter, e.g. SMS group’s X-Pact® Drive. Highly compact designs are possible as the equipment is water-cooled.

Advantages of the compact design

The compactness of the system offers greater flexibility in terms of lower-cost stand design. With no worm gear unit, its mechanical efficiency is much higher. This means less energy is consumed, including only half of the usual power input. What's more, there are fewer moving masses, which in turn reduces mass inertia while improving the dynamics and control performance at the same time. The "active" components required to generate the torque, such as the permanent magnets and electrical steel stacks with windings (stator), are already integrated in those mechanical components (housing with bearings) that are needed in any case.

Unlike standard solutions, the drive unit itself is maintenance-free. Moreover, all the usual maintenance work on the worm gears (wear, gear lubrication, backlash, etc.) is no longer necessary.

Two components combined into one integrated system

In the solution described above, the electric motor was combined with the mechanical components to create a mechatronic system. Like with process engineering, two components were integrated into one system, thus providing significant benefits. This drive solution is a key element of the portfolio of SMS group not only as a plant engineering and construction company but also as a systems and solutions provider. Together with X-Pact® Drive, it is a perfectly harmonized drive concept that is borne out of in-house expertise.

Modernization projects

Both new and existing plants can be equipped with an electro-mechanical screwdown system with integrated torque motor. The adjusting screw can be retained or overhauled, depending on requirements. The old system is replaced by a multispline hub with magnets as well as a stator and housing with integrated brakes. This means the design is more compact and offers the advantages of a new screwdown system.