Precision rolling for wind turbines

The Islamic Republic of Iran has made great strides in generating energy from renewable sources, mainly wind power. According to Energy Ministry official, the country has the potential to generate 15,000 megawatts of wind energy.

The ministry plans to set up wind turbines with a capacity to generate 1,650 megawatts of energy by March 2014. Because the country's wind turbine industry is continually increasing, the size of the towers including height, diameter, and plate thickness, is growing too. The towers large size and rolled sections can't be kept in stock easily. And for this reason, wind tower production requires a lean, continuous-flow environment, with sections moving quickly between cutting, rolling, welding, and then painting.

The precision rolling process forms these massive sections. Due to the unique demands of the wind business, rolling machines are now operating at previously unheard-of levels of productivity.

Choose the right machine

One rolling machine running intensively over two or three shifts every working day can produce about 6,000 rolled sections a year, enough for 200 towers. Any unscheduled shutdown can be a disaster. Ideally, one rolling machine can produce one conical section in less than 30 minutes. This time includes aligning, rolling, and tack welding.

Speed is the name of the game. A machine with insufficient horsepower for rotation either produces insufficient torque, which creates quality problems, or insufficient speed, which lengthens production times. In the wind business, both are simply unacceptable. Therefore, in this specialised field, plate rollers usually require above-average horsepower to drive the rolls.

Because of their high volume and speed requirements, most wind tower production facilities use fixed-geometry, four-roll systems. Fixed-geometry machines have lower rolls that are positioned in a certain way for a given design. They can be moved up and down, either with a linear slide (straight motion) or a swing arm (curved motion). On a fixed-geometry machine, the lower rolls always move up and down in the same pattern. The rotating top roll cannot be repositioned. Though fixed-geometry machines may have less flexibility than variable-geometry designs, they do offer high precision and speed, which are paramount for a wind tower operation.

For shops that do not specialise exclusively in wind towers - those who take on overflow work from other tower manufacturers, for instance - a three-roll, variable-geometry machine may be an option. These machines allow operators to move the lower rolls horizontally and the top roll vertically. This allows a shop that usually does, say, pressure vessel work to take on occasional work involving thicker rolled sections (2-inch or more). A variable-geometry machine allows operators to move the rolls to adjust for this increased thickness. In many of these cases, the higher versatility of the variable-geometry machines compensates for their slightly lower output rate.

Rolling basics

Selecting a machine that matches the required roll geometry and thickness minimises the flat portions at the beginning and end of the rolled sections. The rolls grab the lead-in plate edge and, once the plate is in the correct position, commence prebending to minimise the flat portion at the start of the roll. This operation takes considerably more power than rolling; typically a machine that can roll 1.25-inch plate may only have enough power to prebend 1-inch plate.To prebend, rolls act like a press brake in reverse. The bottom rolls rise, pushing the plate against the top roll to create the initial bend. After rolling, the rolls do the same thing at the plate's trailing edge. The problem is that during each bending operation, the rolls must hold onto the plate somewhere, and these pinch points happen to be at the very leading and trailing edges of the rolled plate.