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Linear Motion & Precision Technologies
Bolt-tightening Handbook
The SKF Group
The SKF Group is an international
industrial corporation owned by
SKF Sweden AB. Founded in 1907,
it operates in 130 countries and has
some 40000 employees.
The company has over 80 manufactu-
ring units throughout the world and a
network of nearly 20000 distributors
and retailers. SKF is the world leader
in the rolling bearing business.
SKF Linear Motion &
Precision Technologies
SKF Linear Motion & Precision
Technologies is an organization within
SKF which, as the name suggests,
is dedicated to the manufacture, sales
and service of linear motion products,
high precision bearings and spindles.
It serves the market through its
organization of 15 specialized sales
companies located in Europe,
North America and Japan.
In addition to the services provided
by these sales companies, product
and application support is available
worldwide through the SKF international
network.
Specialized Linear Motion
& Precison Technologies
Sales Companies
SKF Bearing sales
companies with Linear
Motion sales staff
Production facilities
Catalogue n° TSI 1101 AE
April 2001
Printed in France
© Copyright SKF 2001
The contents of this catalogue are the
copyright of the publishers and may
not be reproduced (even extracts)
without permission. Every care has
been taken to ensure the accuracy
of the information contained in this
catalogue but no liability can be
accepted for any errors or omissions.
Earlier catalogues with data which is
different than that contained herein are
no longer valid.
We reserve the right to make
changes required by technological
developments.
Introduction
4
Traditional tightening methods
7
Tightening with torque wrench
7
Tightening with heater rod
12
Tightening by mechanical elongation
12
Tightening with hydraulic bolt tensioners
13
Presentation
13
Features and benefits
14
Measurement devices for hydraulic tightening
16
Technical analysis of bolt-tightening
19
Comparison between torque wrench
and hydraulic bolt-tightening
26
Tightening of an existing bolted assembly
26
Design of a new bolted assembly
34
Simultaneous hydraulic
bolt-tightening
37
S i multaneous tightening of 100% of the bolts
37
S i multaneous tightening of 50% of the bolts
38
S i multaneous tightening of 25% of the bolts
39
Conclusion
43
Introduction
Without a doubt, bolted assemblies are the most commonly
used joints in mechanics.
These types of assemblies employ two basic elements:
• on the one hand, some kind of threaded component:
- screws and nuts,
- studs with nuts on one end,
- studs with nuts on both ends.
These components are sometimes used with diff e r e n t
kinds of washers
(Fig.1a below)
.
• on the other hand, some means for tightening.
These types of tightening means are the subject
of this Handbook.
In this document the word “bolt” will be used in a generic
sense to cover all three of the types of screwing
components mentioned above.
Although bolted assemblies at first appear very simple,
they cause several problems for design engineers,
assemblers, and maintenance departments.
Rough-dimensioning methods are too often used at
the design stage, leading to substantial oversizing
of all the components of the assembly, which does not
ensure assembly safety, quite the contrary.
In reality, the design of a bolted assembly requires
a methodical and rigorous approach, since mistakes
can lead to failures with often costly and sometimes
disastrous consequences.
Many surveys show that failures of bolted assemblies are
mainly due to the fact that they were not properly designed
(analysis, drawing, calculation, choice of components)
or implemented (tightening method, tooling, checking).
The surveys also show that among the possible causes
of assembly failure (overloading, improper design,
manufacturing defects etc.) the most frequent is poor
a s s e m b l y. Undertightening, overtightening and irregular
tightening alone cause 30% of all assembly failures.
Furthermore, in addition, 45% of all fatigue incidents are
due to poor assembly
(see Fig.1b below).
Screw and nut
Stud with nut on one end
Stud with nuts on both ends
Fig. 1a
Fig. 1b
: Primary causes of fatigue failure of bolted joints
4
Traction
Compression
Sealing
Shear stress
Spontaneous
loosening
Dynamic
loads
Fig. 2
Controlling bolted assemblies
It is fundamental to control the level of the tightening load, as
well as the accuracy of the tightening value, to ensure that
required performance of the bolted assembly will be achieved.
Complete control over the tightening conditions - from the
outset of the design stage - ensures the best use of the
b o l t ’s mechanical properties of bolts,
(see Figs. 3 and 4
b e l o w, and page 6).
Correct tightening of a bolt means making the best use of
the bolt’s elastic properties.
To work well, a bolt must behave just like a spring.
In operation, the tightening process exerts an axial pre-load
tension on the bolt. This tension load is of course
equal and opposite to the compression force applied
on the assembled components. It can be referred
to as the “tightening load” or “tension load”.
Depending on the application, the purpose of the tightening
load is multiple:
- ensure the rigidity of the whole assembly and make it
capable of supporting external loads due to traction,
compression, bending moments and shear;
- prevent leakage at seals;
- avoid shear stresses on the bolts;
- resist spontaneous loosening eff e c t s ;
- reduce the influence of dynamic loads on the fatigue life
of the bolts
(see Fig. 2 above)
.
Furthermore, all components (bolts and assembly parts)
must perform these tasks while remaining below the yield
point of their respective materials.
Uncontrolled
tightening calls
for oversized joints
Bolt-tightening is optimal when the bolt is properly
tightened: not too much, not too little! A bolt can fail just as
often - and even more so - when it is not tightened enough,
as when it is over-tightened.
Controlled tightening
allows optimised
joint sizes
Fig. 3
In this “Bolt-tightening handbook” and in the catalogue “Hydrocam Bolt Tensioners - Industrial Tightening Systems”,
engineering and design departments will find the theoretical and practical information they need to optimize bolted
assembly design and systems operators will find the information they need to control tightening.
5
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