Advantages Of Hydraulic Tensioning

www.schaaf-gmbh.comschaaf_englisch_schraubenverbindung_kat6.ppt · 12.07.2007

Advantages of hydraulic tightening for bolt

connections


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


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


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


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


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


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)


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


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


Thread-Friction

Head-Friction

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)

Calculation base

Thread-friction Head-friction


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


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


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

Advantages Of Hydraulic Tensioning

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