Welded wire mesh is commonly used to reinforce concrete structures. Indeed, the first recorded use was on a bridge construction project in France, back in 1875. Today, it is used in vast quantities throughout the world, and in order to meet demand, mesh is now produced by fully automated machines, like those manufactured by the Swiss company, Schlatter. Production involves fast and accurate feeding of wire from very large reels to automatic welding stations, a process that requires both speed and precise positioning. Requirements such as these can only effectively be met by servo technology.
Schlatter’s type MG mesh welding machine is of a modular design and tailored to produce flat and round wire reinforcing meshes in a variety of sizes. The machines are computer controlled and can be combined with different loading, feeding, straightening, cut-to-length and pay-off units to build a fully automatic production line.
Wires can be welded crosswise together in virtually any alignment; the mesh screen geometry is entered via a process master system, which can be checked on screen. Using parameters entered for the mesh width and wire cross section, the system calculates the individual welding values for each node. The system is very flexible and is geared to just-in-time production for direct delivery of meshes from factory to construction site.
In operation, wire is fed from a reel weighing up to 5 tonnes, mounted with its axis in the vertical position, via an accumulator to the unwinding module. This is fitted with Lenze type MCA geared, asynchronous servomotors from the company’s ‘L-force’ product portfolio. The model chosen for this machine was the MCA21 with a rated torque of 25Nm (300Nm peak). The gearbox is a Lenze hollow shaft mounted helical unit with a locking bush connection on the output and internal force fit connections, all of which minimises backlash and wear.
The MG machine processes hot rolled steel wire of up to 12mm diameter. Processing the steel while it is still hot considerably improves productivity, but there is a downside - it has a much rougher surface than cold rolled steel. Oxidisation scale, together with the surface ridges pre-formed into the wire, were imposing very high loads on this machine. When passed through the feed unit, this uneven surface tended to generate abnormally high amplitude vibrations, which had a detrimental effect on the service life of the servomotors.
The unwinding module operates between 2 and 5.5m/s, depending on the diameter of wire being processed. At these speeds, any tangles in the wound coils result in snatches that impose large shock loads on the servomotors, adding to the loads already caused by the surface induced vibrations.
When premature servomotor failures were first reported by an Italian customer, the Swiss OEM, in conjunction with its servomotor supplier, moved swiftly to despatch a team to the site and set about solving the problem once and for all.
Measure and remedy
Measurements carried out at the Italian customer’s site by Lenze revealed a clear reason for these premature failures: acceleration seen by the geared servomotors was exceeding the maximum permissible by no less than a factor of eight! Indeed, acceleration values of up to 16.3g (100m/s2) were measured at the motor and the gearbox, which had been designed to tolerate shock levels of just 2g - normally adequate for most machine applications. Using experience gained from its development of vibrators for the pre-cast concrete industry, Lenze’s team, together with engineers from Schlatter, proposed eleven measures to increase the mechanical strength of the installed servo units.
The team initially focused its attention on the motor fasteners. The long axial motor stator fixing bolts were protected from vibration by applying resin half way along the bolt length, and all fasteners had thread locking adhesive applied to them. The motor bearings were secured with O-rings and an alternative lubricant was specified. The plug terminals were replaced by a single terminal box, the terminals all being secured by an adhesive, while the motor cables were upgraded with extra fibreglass insulation to prevent chafing. Finally, the motor environmental rating was increased to IP65, essentially to protect them from dust ingress.
With this comprehensive package of measures, Lenze was able to increase the mechanical strength of its asynchronous servomotors for this particularly demanding application to the satisfaction of all concerned.