Advantages Of Hydraulic Tensioning


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Schaaf GmbH Hydraulic Bolt Tensioning Technology

The advantages of generating pure axial pre-tension using pressures up to 4000 bar (SSV and HM)

Torsion-free generation of tension force – Economical use of bolt material capacity
A bolt is designed to hold two or more components together. Axial bolt elongation is the only force at work here. Torsion, which is generated, due to friction in the threads and in the support surface, by using torque wrenching methods, means only unnecessary weaking of the components. This requires stronger bolt connections. This disadvantage is no longer existing when using SCHAAF SSV and HM to generate the axial pre-tension. This increases the economic utilisation factor. Axial tension force has been attained by using SCHAAF SSV and HM for bolts with a thread diameter from 8 mm to over 800 mm.

Maximum accuracy
Since the pre-tension is purely axial and only varies by -4% maximum, bolts can be tightened up to the yield strength of the material used. An integrated displacement transducer even allows tensioning beyond the elastic limit.

Increased safety
Due to the purely axial bolt tensioning force applied and as the safety factors given in bolt calculation instructions come down to a minimum, bolts can be higher tensioned for the same bolt thread cross section. The connection is more secure.

Frictionless application of tensioning force
Due to the purely axial bolt tensioning force applied, the main nut (SSV) or the locking nut (HM) can be threaded without effort or friction up to the flange. This completely eliminates the problem of chewing of the threads with a fine pitch or of bolt connections made of austenitic steel. For large bolt connections the calculated total torque is dependant, by more than 90% on the amount of friction. Estimated coefficients of friction for torque from µ = 0.08 to µ = 0.35 (a tensioning difference of around 400%) are now a thing of the past.

Capability to start simultaneous tensioning at multiple points
„Crossover-tightening“ of several bolts at several points is no longer necessary, since SCHAAF SSV and HM offer the possibility of tightening all the bolt connections simultaneously with exactly the same tensioning force. This also applies when increasing the tensioning force.

Start of tensioning independently of tensioning path
As opposed to thermal screw starting procedures, SCHAAF SSV and HM make it possible to eliminate different gaps between individual components. Each bolt can be tensioned with exactly the same force and not over the same distance!

Quickest test capability at any time
SCHAAF SSV and HM make it possible to apply a residual tensioning force by means of test force stress application in order to determine if any settling of the screw may occur, and if so, to eliminate it.

Universal application
With a compact unit and the ability to interchange the component parts relating to the size of the bolt, SCHAAF SSV can also be used for other thread sizes.

Saving of time
SSV and HM will dramatically save time required for tensioning applications. This effect increases for higher tensioning forces, larger bolt thread diameters and also the installation of several SCHAAF SSV or HM. Time savings up to 99% are possible! It is no longer necessary to apply large torque by hand. There is a number of handling features available, designed to make work easier with SCHAAF Bolt Tensioners and Hydraulic Nuts, especially when it comes to large dimensions.

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Advantages Of Hydraulic Tensioning

  1. 1. Advantages of hydraulic tightening for bolt connections schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  2. 2. Purpose of a bolt Connect at least 2 parts dismountable together glueing, welding, Friction µ riveting Prevent take-off by elongation of the bolt and compression of the parts Sealing effects Transmitting shear-forces by friction Weight-reduction: Use of high-strength bolts with high and exact pretensioning force schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  3. 3. Different models Screw-head forms default values for pretensioning force Thread-size + pitch Standard series, fine series… Thread forms metric, buttress, trapezoidal, round… Strength categories: 8.8, 10.9, 12.9 Surface treatening Friction coefficient Forms Necked down bolts – large elongations for only little loss of pretensioning! Shoulder studs Special bolts …. schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  4. 4. Bolt loads Axial operating force Optimum load: Only axially Shear-force Bending moment Torque Exspecially during mounting Partly during operation Often pretensioning to 90% of yield stress for highest load capacities of bolts schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  5. 5. Tightening methods I method advantages disadvantages Tighten- ing factor Tightening by cheap, available Large deviations in pretensioning force 2,5-4 hand (socket everywhere Inapplicable for large thread-sizes wrench) Torque-controlled Little more expensive Smaller deviations, inknown influence 1,7-2,5 with torque wrench Available of friction Inapplicable for large thread-sizes Thermical with Very precise Only for large clamping length and 1,05-1,2 measuering of special screw forms (flat heads) elongation Ultrasonic measuring or exact length (Ultrasonic) measuring for each bolt schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  6. 6. Tightening methods II method advantages disadvantages Tighten- ing factor Controlled by precise, Extensive equipment necessary 1,2-1,4 yieldstress of Almost independent torque-angle from friction Impact wrench Applicable for large Large deviation in tightening torque due 2,5-4 (electric/air driven) sizes to design Adjustment is inaccurate, requires measurement of Actual pretensioning force Counter-torque has to be taken by user or external support hydraulic High forces achievable Accessories needed (Pump/Hose…) 1,2-1,6 Detection of actual force possible Tensioning several bolts at a time schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  7. 7. Tightening Factor Tightening Factor kA according 1 to SCHAAF remaining clamping relation K By Changing the flux of 0,8 forces during pressurization and 0,6 releasing pressure a loss of force takes place 0,4 Depending on clamping length relation l / d 0,2 Depending on thread- 0 pitch p 0 2 4 6 8 10 12 14 16 18 20 clamping length relation l/d → kA = 1 / K ≈ 1 - 2 large pitch small pitch (Empirical data) schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  8. 8. Calculation base Precondition for exact dimensioning is the notice of operational FB loads!! FV Tensioning diagram: Calculate pretensioning force from operational loads Compression and elongation Clamping force has to be > 0 during operation schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  9. 9. Calculation base Depending on tightening method the mounting force can be determined: FVM= kA x FVmin kA = tightening factor (1,05-4) FVmin = Min. pretensioning force schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  10. 10. Calculation base Thread-Friction Head-Friction schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  11. 11. Calculation base Torqueing methods lose a lot of torque for overcoming friction inside the thread and below the head: MA=FVM x (d2/2 x tan (φ+ρ‘) + µk x dk/2) Thread-friction Head-friction schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  12. 12. Example Using a torqueing method: Thread Pitch Torque MA Friction Tensionin transforming torque to size D P [Nm] coeff. µ g force FVM axial force by means of [mm] [mm] [-] [kN] inclined plane 36 4 2000 0 3142 Due to friction only little (100%) axial force Friction coefficient has 0,08 522 to be estimated (17%) Additional torque load inside th screw 0,25 188 (6%) Result: 120 6 64500 0 67500 Little transforming rate from (100%) torque to pretensioning force (abt.. 90% loss) 0,08 5510 For large sizes the torque can (8%) not be reached by hand Abt. 300% difference 0,25 1870 Large deviations when fricition is unknown (3%) schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  13. 13. Preconditions for hydraulic tightening Thread-size and pitch are known Protruding bolt end is long enough Design is modified for little losses: Only few separation planes No washers necessary. If necessary: place tool directly on washer Large clamping length schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
  14. 14. Conclusion Advantages of using hydraulic tightening methods Friction coefficient has not to be estimated Only axial load can be set to the bolt – no torque load due to friction Bolt size can be reduced Force depends only on the piston area and pressure of the tensioning device – no inaccuracy Special solutions possible schaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007