advantages of hydraulic tensioning
DESCRIPTION
Schaaf GmbH Hydraulic Bolt Tensioning TechnologyThe 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 accuracySince 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 forceDue 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 applicationWith 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 timeSSV 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.TRANSCRIPT
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Advantages of hydraulic tightening for bolt
connections
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Purpose of a boltConnect at least 2 parts dismountable together
glueing, welding, riveting
Prevent take-off by elongation of the bolt and compression of the parts
Sealing effectsTransmitting shear-forces by friction
Weight-reduction:Use of high-strength bolts with high and exact pretensioning force
Friction µ
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Different modelsScrew-head forms Thread-size + pitch
Standard series, fine series…Thread forms
metric, buttress, trapezoidal, round…
Strength categories: 8.8, 10.9, 12.9
Surface treateningFriction coefficient
FormsNecked down bolts – largeelongations for only little loss of pretensioning!Shoulder studsSpecial bolts
….
default values for pretensioning force
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Bolt loadsAxial operating force
Optimum load: Only axially
Shear-force
Bending moment
TorqueExspecially during mounting
Partly during operation
Often pretensioning to 90% of yield stress for highest load capacities of bolts
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
method advantages disadvantages Tighten-ing factor
Tightening by hand (socket wrench)
cheap, available everywhere
Large deviations in pretensioning forceInapplicable for large thread-sizes
2,5-4
Torque-controlled with torque wrench
Little more expensiveAvailable
Smaller deviations, inknown influence of frictionInapplicable for large thread-sizes
1,7-2,5
Thermical with measuering of elongation (Ultrasonic)
Very precise Only for large clamping length and special screw forms (flat heads)Ultrasonic measuring or exact length measuring for each bolt
1,05-1,2
Tightening methods I
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Tightening methods IImethod advantages disadvantages Tighten-
ing factor
Controlled by yieldstress of torque-angle
precise, Almost independent from friction
Extensive equipment necessary 1,2-1,4
Impact wrench(electric/air driven)
Applicable for large sizes
Large deviation in tightening torque due to designAdjustment is inaccurate, requires measurement of Actual pretensioning forceCounter-torque has to be taken by user or external support
2,5-4
hydraulic High forces achievableDetection of actual force possibleTensioning several bolts at a time
Accessories needed (Pump/Hose…) 1,2-1,6
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Tightening Factor
Tightening Factor kA according to SCHAAF
By Changing the flux of forces during pressurization and releasing pressure a loss of force takes placeDepending on clamping length relation l / dDepending on thread-pitch p
→ kA = 1 / K ≈ 1 - 2
0
0,2
0,4
0,6
0,8
1
0 2 4 6 8 10 12 14 16 18 20
clamping length relation l/d
rem
aini
ng c
lam
ping
rela
tion
K
large pitch small pitch
(Empirical data)
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Calculation base
FV
Precondition for exactdimensioning is the notice of operational loads!!
Tensioning diagram:Calculate pretensioning force from operational loadsCompression and elongationClamping force has to be > 0 during operation
FB
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Depending on tightening method the mounting force can be determined:
FVM= kA x FVmin
kA = tightening factor (1,05-4)FVmin = Min. pretensioning force
Calculation base
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Thread-Friction
Head-Friction
Calculation base
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
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)
Calculation base
Thread-friction Head-friction
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
ExampleUsing a torqueing method: transforming torque to axial force by means of inclined plane
Due to friction only little axial forceFriction coefficient has to be estimatedAdditional torque load inside th screw
Result: Little transforming rate from torque to pretensioning force (abt.. 90% loss)For large sizes the torque can not be reached by handLarge deviations when fricitionis unknown
Thread size D[mm]
Pitch P[mm]
Torque MA
[Nm]Friction coeff. µ[-]
Tensioning force FVM
[kN]0 3142
(100%)
0,08 522 (17%)
36 4 2000
0,25 188 (6%)
0 67500 (100%)
0,08 5510(8%)
120 6 64500
0,25 1870(3%)
Abt. 300% difference
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
Preconditions for hydraulic tighteningThread-size and pitch are known
Protruding bolt end is long enough
Design is modified for little losses:
Only few separation planesNo washers necessary. If necessary: place tool directly on washerLarge clamping length
www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007
ConclusionAdvantages of using hydraulic tightening methods
Friction coefficient has not to be estimatedOnly axial load can be set to the bolt – no torque load due to friction
Bolt size can be reducedForce depends only on the piston area and pressure of the tensioning device – no inaccuracySpecial solutions possible