ENERPAC - Industrial Tools Página 415
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Torque Tightening Preload (residual load) = Applied Torque minus Frictional Losses Lubrication Reduces Friction Frictional Losses 10% converted to preload Lubrication reduces the friction during tightening, decreases bolt failure during installation and increases bolt service life. Variation in friction coefficients affect the amount of preload achieved at a specified torque. Higher friction results in less conversion of torque to preload. The value for the friction coefficient provided by the lubricant manufacturer must be known to accurately establish the required torque value. Friction points should always be lubricated when using the torque tightening method. 40% in threads 50% at nut face Select the Right Wrench Frictional Losses (dry steel bolt) Lubrication Reduces Friction Lubricant or anti-seizure compounds should be applied to both the nut bearing surface and the male threads. • When loosening a nut or bolt more torque is usually required than when tightening. Residual Bolt Load (tons) ▸ 11 10 9 • For general conditions it can take up to 2½ times the input torque to breakout. 8 7 6 • Do not apply more than 75% of the maximum torque output of the tool when loosening nuts or bolts. 5 4 3 2 1 0 Conditions of bolted joints 0 20 40 Torque Procedure 60 80 100 Torque (ft.lbs) ▸ (Applied load) 120 Lubricated with Moly grease (µ = 0.1) When torquing it is common to tighten only one bolt at a time, this can result in Point Loading and Load Scatter. To avoid this, torque is applied in stages following a prescribed pattern: Light oil (µ = 0.15) Dry unlubricated (µ = 0.2) Example of how a lubricant can reduce the effect of friction and convert more torque to bolt preload. Torque Sequence 1 Choose your Enerpac torque wrench using the untightening rule of thumb: • Humidity corrosion (rust) requires up to twice the torque required for tightening. • Sea water and chemical corrosion requires up to 21⁄2 times the torque required for tightening. • Heat corrosion requires up to 3 times the torque required for tightening. 1 1 5 12 5 8 9 8 Breakout Torque 4 3 3 4 4 3 10 6 2 7 2 7 11 6 2 Step 1 Spanner tight ensuring that 2 - 3 threads extend above nut. Step 4 Increase the torque to full torque following the pattern shown above. Step 2 Tighten each bolt to onethird of the final required torque following the pattern as shown above. Step 5 Perform one final pass on each bolt working clockwise from bolt 1, at the full final torque. Step 3 Increase the torque to twothirds following the pattern shown above. When loosening bolts a torque value higher than the tightening torque is normally required. This is mainly due to corrosion and deformations in the bolt and nut threads. Breakout torque cannot be accurately calculated, however, depending on conditions it can take up to 2½ times the input torque to breakout. The use of penetrating oils or anti-seize products is always recommended when performing breakout operations. 413
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