SKF_Hydrocam_bolt_tensioners.pdf

Linear Motion & Precision Technologies

HYDROCAM Bolt Te n s i o n e r s

Industrial Tightening

Systems

Introduction

SKF ® has worked in the field of bolt-tightening by pre-load tension since the 1940’s.

At that time, design studies concentrated on the tightening of slewing rings,

where the SKF group’s subsidiary, RKS, is a leading specialist.

Since the 1940’s, SKF Equipements has continuously furthered its bolt-tightening experience

in every industrial sector and in many different types of applications.

Today, SKF Equipements offers two descriptive documents on the subject of

bolt-tightening: the “Bolt-tightening Handbook”, which discusses the technical aspects

of bolt-tightening, and this product catalogue: the “HYDROCAM ® Bolt-Tensioner -

Industrial Tightening Systems” catalogue.

Well aware that good tightening control is extremely important economically,

SKF Equipements offers both expertise and service to solve any problems involving

the tightening of bolted assemblies.

Screw and nut

Stud with nut on one end

Stud with nuts on both ends

Just what is a bolted assembly ?

A bolted assembly quite simply means the putting toge-

ther of at least two parts using one or several bolts. As

is shown in the figure above, these bolts can be made

up of screws and nuts, studs with nuts on one or both

ends, and possibly washers.

The design and implementation of a bolted assembly

requires a very strict methodological approach, for

errors can lead to costly and often disastrous failures.

Several studies have shown that incidents encountered

on bolted assemblies are most often due to improper

design of the assembly (analysis, design, calculation,

component choice) or poor implementation (tightening

method, tooling, inspection).

In the following document, the word “bolt” includes these

three types of screwing components.

Bolted assemblies are the most commonly used

connecting systems in mechanics. And although they

appear to be quite simple, bolted assemblies do pose

several challenges at many levels: design department,

assembly workshop, on-site, and maintenance.

Today it is known that of all the various causes of failure

(overloading, design flaws, manufacturing defects, and

others), the most frequent is improper assembly.

Tightening problems, whether insufficient tightening,

excessive tightening or heterogenous tightening, alone

account for over 30% of all bolted assembly failures.

Rough dimensioning methods are too often applied at

the design stage, which leads to unnecessary oversi-

zing. Such oversizing does not enhance product safety,

quite the contrary...

More specifically, 45% of all fatigue failures are estimated to be due to improper

assembly.

Therefore, the importance of the design of the bolted assembly and the means used

to tighten it are of utmost importance.

The “Bolt-tightening Handbook” clearly explains that “optimum tightening means a

bolt which is not too tight and not too loose” and shows how to achieve the best

tightening.

It reviews all the tightening methods and proves that the best tightening control is

provided by the Hydrocam hydraulic tensioner.

This “Product Catalogue” describes all the products and services that SKF offers

in order to enable you to better manage your bolted assemblies.

Both of these documents provide indispensable information on optimizing the

bolted assemblies that you are designing and on choosing the best tightening

method for your application. They describe the key aspects, which are:

• Quality

• A c c u r a c y

• Homogeneity

• Ease of use

These provide additional advantages such as:

• Reliability and safety

• Optimized dimensions

• Time savings

• Cost reduction in use,

control and maintenance.

In addition, this catalogue provides

other useful information such as

standard bolting dimensions, units

conversion, and the conversion of torque

into tension load...

As an example - did you know that

the SKF hydraulic tensioner can tighten

bolts from 5 to 500 mm ?

Please do not hesitate to contact SKF

for your needs. You may be surprised

by all the services SKF can provide

in the field of bolted assembly tightening.

What makes up the Hydrocam

bolt tensioner ?

A standard Hydrocam tensioner has a hydraulic body

which, using a hydraulic fluid, exerts a strong tension

load on the bolt through the brace screwed on that bolt.

In some tensioner types (HTC R), the body is screwed

directly on the bolt to be tightened. The body also rests

on the skirt in order to apply the reaction force on the

assembly to be tightened.

Prior to screwing the body-brace-skirt unit on the protru-

ding end of the bolt, a socket has to be placed on the

nut of the bolt. This nut can then be “turned down”

(screwed until the lower surface of the nut comes into

contact with the assembly bearing surface), by a tommy

bar, while the tensioner applies the tension load.

Brace

The brace is screwed on the protruding end of the bolt,

above the nut. It supplies the tension load exerted by the

hydraulic body on the bolt.

Hydraulic body

Using the fluid under pressure, the hydraulic body creates

the tension load on the brace by pushing on the skirt.

Skirt

The skirt is placed around the nut on the assembly to be

tightened. It transmits the compression force to the assem-

bly by reaction to the tension load applied on the bolt by the

hydraulic body.

Socket for standard nut (optional)

In the case of assemblies with standard nuts, a socket is placed

around the nut to effortlessly bring the nut into contact with the

assembly. For this purpose, the outer surface of the socket is drilled

with radial axis holes to hold the push rods. Turning down occurs

while the tension load is applied by the hydraulic body.

How a Hydrocam tensioner works

1 - The turndown socket is placed over the nut and the

hydraulic tensioner grasps the bolt.

2 - The brace/retraction unit is screwed onto the protruding

end of the bolt.

3 - After the hydraulic connections, the tensioner

is pressurized and applies the required tractive

force on the bolt.

4 - While the pressure is maintained, the nut is turned down

without loading, using the socket and the tommy bar.

5 - Their pressure is released and the piston is pushed back.

The tightening load is now exerted through bolt tension.

6 - The tensioner and the socket can be removed.

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