One maintenance company’s standard procedure for rechecking rotor blade bolts was to loosen then re-torque the 54 bolts on each of about 200 turbines. That’s a total of 10,800 blade bolts and a time consuming task. Some turbines require the task at three months after commissioning, and again in one year. One crew’s previous method for doing so used hydraulic wrenches. Although standard tools in the wind industry, they are cumbersome and work slowly. What’s more, bolt torque is usually determined by monitoring a pressure gage, which makes accuracy questionable.
Specs for the operation say to loosen each blade bolt and retightened to 258 ft-lb. Then each bolt is marked in a particular manner and turned an additional 120 degrees. The marking shows the angle has been completed.
When the crew chief started looking for a more efficient method, the suggestion came in to try an electric torque gun. One version of such a tool, the E-RAD 2000, was recommended by a representative from Alltite Total Bolting Solutions (alltite.net). “The device applies torque with ±3% accuracy and ±2% repeatability, and because it is electric, it can store the torque applied to each bolt in a file that can be downloaded later,” says John Smith, Alltite CEO.
The E-RAD version of an electric “pistol grip” torque gun is manufactured by New World Technologies Inc (radtorque.com) and is intended for high torque and angle measurements. “The gun uses an AC Servo brushless motor coupled to a patented 490:1 gear multiplier to generate the high torque,“ says New World Application Engineer Curt Kehler. “The E-RAD operates by precisely measuring and controlling the current output of the ac servo motor. This precise measurement and control allows for high degree of accuracy and repeatability. An internal CPU stores a torque-versus-current calibration curve so that when the motor reaches a required torque as identified by electrical current, the motor shuts off. In house tests show the gun gets better than ±1% of a required torque value.” To control the application of the additional angle, the gun’s ac servo motor drives an encoder which tracks and records 4,096 points on a 360° rotation of the motor drive.
The maintenance company ordered the torque gun with an option to define torque and the angle sequence. The defined settings are stored using an 8-channel selector switch on the gun’s control box. All defined settings can be locked out using a removable key so unauthorized personnel cannot change them. Basic data collection, which is included, records the set torque, set angle, and provides the actual torque applied and angle turned. To eliminate of user error, an LED indicator on the tool handle displays and a red light to indicate an incomplete torque cycle and a green light when the torque cycle completes.
The maintenance firm says the electric gun provided a return on investment within the first month because its speed allowed the crew to finish the maintenance job in half the time usually spent, and then it took only half the labor of a hydraulic wrench.