An intro to torque multipliers
August 30, 2010 by Paul Dvorak
Filed under Fastening-Joining for Wind Power, Maintenance & operations
Or, how a small push can generate high torque
A torque multiplier allows generating a higher torque than a person would normally apply. The reaction arm is to the upper left. Norbar says all gears in its design rotate on needle roller bearings about hardened and ground journal pins. As a result, the devices have a torque multiplication accuracy of +/-4%, throughout the operating range.
There is not much room to maneuver in a nacelle 60m up so when bolt tightening is the task at hand, Norbar Torque Tools, Inc., of Willoughby, Ohio says a torque multiplier can be a better and safer alternative to a long wrench. A torque multiplier increases the torque that can be applied by hand. Of course, output power cannot exceed the input power, so the number of output turns will be fewer than the number of input turns. A brief equation shows how the mechanical parameters relate.
Power = torque x rpm
Handtorque multipliers use and epicyclic or planetary gear train with one or more stages. Each stage of gearing increases the torque applied by a factor of 5. Norbar Handtorque multipliers typically come in ratios of 5:1, 25:1, and 125:1.
In the planetary-gear design, a tech applies torque to the input gear or sun gear. The action rotates three or four planetary gears whose teeth are engaged with the sun gear. The outside casing of the multiplier (an annulus or ring gear) also engages the plant gear teeth, and normally rotates in the opposite direction to the sun gear. A reaction arm prevents the casing from rotating, and this causes the planetary gears to orbit around the sun gear. The planetary gears are held in a planetary carrier which also holds the output square drive. Therefore, as the planetary gears orbit around the sun gear, the carrier and square drive turns.
Without the reaction arm to keep the casing stationary, the output square will not apply torque.
Other reasons to use a torque multiplier include:
• It’s safer in the nacelle. Use of long levers can be dangerous. In the confines of a nacelle, long torque arms are impractical. Torque multiplier allows reducing the lever length or operation effort by a factor of 5, 25, or 125.
•Torque Multipliers are especially useful breaking loose stubborn or frozen bolts, and are widely used for a 10% verification of bolted joints.
• Accuracy of the torque value improves when applying torque smoothly and slowly. Torque multipliers do this by removing much of the physical effort from the tightening task.
The sketch shows input and output torques for a 25:1 torque multiplier.
Torque-output calculations are a matter of simple arithmetic with little risk of incorrect bolt loading due to conversion errors. Other manufacturer’s multipliers often require graphs or formulae to calculate the input torque to achieve a particular output.
Norbar says its comprehensive multiplier range of Handtorque torque multipliers includes standard products in up to 47,500 Nm (35,000 lbf-ft) devices and specials to 100,000 Nm (73,000 lbf-ft). Accessories include ‘nose extensions’ for reaching difficult to access bolts and torque transducers for highly accurate torque monitoring.
Norbar
Tools for Torque, Tools for Tension
November 27, 2009 by Windpower Engineering
Filed under Construction, Fastening-Joining for Wind Power, Maintenance, Materials, Mechanical Components, Wind Safety, Wind Turbine Installation
Wind turbine OEMs provide plenty of guidelines and instructions for tightening the bolts on their equipment. Each bolt gets a prescribed torque or tension. Bolts that fasten tower flanges, for instance, are often tightened to 2065 ft-lb. while foundation bolts are assigned a particular tension. Tension and torque, of course, call for different tools.
“We see specs for both tension on some turbine towers and torque on others. Whatever the characteristic, it depends on the OEM,” says Aztec Bolting sales engineer Adrian Keith (bolting.com).
Foundation bolts most often require applying a load with a tensioner. Briefly, the tensioner works like this: A worker hand tightens a nut on the bolt then places a socket around the nut. A bridge sits around the nut to support a hydraulic cylinder. Lastly, a puller screws onto at least three threads at the end of the bolt and until it contacts the piston on the hydraulic cylinder. The cylinder is energized by a pump which pulls the bolt to a prescribed tension. “It’s fairly accurate. The nut is then further tightened by hand, or with a short rod called a tommy bar. It does not take much effort. Then releasing the pressure lets the nut and bolt carry the load,” says Keith.
Most other places on the tower call for a torque and that requires a torque wrench. It can be manually applied with a long bar and some sort of indicator, but up tower, that method is dangerous. A hydraulic torque wrench makes more sense. The accompanying cutaway shows some internal detail in a design from Enerpac. In it, a hydraulic piston moves a ratcheting device about 20° maximum at a time. A worker would hit a spring-loaded switch to start the pump motor and monitor a pressure gage until it reads the pressure equivalent to a prescribed torque.
“In addition to loads, equipment dependability is extremely important,” adds Keith. “One thing we hear a lot is that an expensive crane is waiting because hydraulic tools are down. So a malfunctioning $5,000 pump, for example, could be delaying the work of a crane that costs $40,000 a day. The pump’s cost gets lost in the zeros.”
The hydraulic equipment is all heavily used and heavily loaded so it’s going to fail sometime. “Be prepared for it,” says Keith. Most often, he adds, a device’s seals fail from wear and then they can’t generate a required pressure, so keep a replacement set readily available.
