bolted joints training slides

Bolted Joints Training Program

Agenda:

I. Bolted Joints – Definition

II. Bolted Joints – Design Objectives

III. Assembling Methods

IV. Mechanical characteristics of fasteners

V. Other influencing factors

VI. Quality Controls & Failure

VII. Patented technology (Clamp-load Assembling & Control)

VIII.Highlights

Bolted Joints - Definition

Detachable unions of two or more parts of any materials, assembled through the used of threaded elements i.e.: bolts, nuts, studs, threaded holes.

Bolted Joints – Objectives

• A bolted joint is a detachable union of two or more parts by means of screws and/or nuts.

• The fasteners and its tensile forces aim to hold the joint together fulfilling its functions and resisting the working loads that arise.

• A properly assembled bolted joint is obtained by a clamp load generated by the interaction of internal and external threads inducing tension in the screw and compression in the joint parts that are connected by the screw.

Bolted Joints – Objectives

The two basic premises in a bolted joint design, is to avoid:

1. Lateral slipping of connected elements;(Function of external loads and friction forces between parts in contact)

2. Disconnection of the parts being held together...(Function of external loads and clamping forces between parts in contact)

Bolted Joints – Assembling Methods

There are basically, 4 methods to assemble a bolted joint in which all of them

has one common objective which is to deliver a desired minimum amount

of clamp-load.

These methods are:

1. Torque Control

2. Torque + Angle

3. Yield control

4. Bolt Tensioning

No matter which of the 4 tightening methods you choose, it is always necessary to

rotate one of the fastening elements (bolt/nut). While rotating the fastener, you’ll

always have at least two parts of that fastener sliding against other surfaces…

2 Surfaces in Contact + Relative movement = ?Yes, equals to friction… and despite the friction “coefficients”

do not depend on the force/clamp-load, the friction forces do, and they are

translated into reaction against the rotation of the fastener per:

Thread-thread friction forces + head/plates friction forces… which, when multiplied

by their own distances from the center axial line of the bolt, will be called:

1. Torque under the head (Mk);

2. Torque on the threads (MGA).

The Total Torque (MA), which is the torque registered by the tool during the

assembling, can now be expressed as: MA = Mk + MGA


But how did Engineers converted torque into clamp-load in order to guarantee they would assemble the

parts?

Looking at the image at the bottom-left corner, we understand that: by rotating the bolt, the helical feature

of the threads will force the bolt to stretch and consequently, shrink the compressed parts.

A certain amount of “energy” is delivered by the tool and spent to:

• Overcome friction

• Cause deformation on the parts (fasteners & plates)

The amount of deformation, can be directly equated to the clamp-load

by utilizing the elastic modulus of the joint and its geometric

characteristics… but how much energy went into causing deformation

and how much energy went into overcoming friction????

Over the years, engineers have used load-cells, strain-gauges and

other methods to try and correlate these variables, but just to find out

that they are unique to each bolted joint. Hence, cannot be directly

equated….


Torque Control

The use of instrumented bolted joints, were crucial to start building table charts with

reference values of Torque x Tension relationship. These charts would show nut factor (K)

values of reference by type of fastener’s coating interactions but still, high scattered values

which in most cases.

At that point, little was known about individual thread and head friction

coefficients, but engineers came up with the following formula to correlate

assembling torque and assembling clamp-loads

MA = F.D.K

where: F is the clamp-load

D is the fastener diameter

K is the nut factor (commonly assumed as 0.2)

Now, we have Torque controlled assembling strategies and great focus on

Torque measurements….

Torque + Angle

Yield Control

A next evolution on the Torque + Angle tightening strategy,

came with the advance on the tools and torque x angle data

collection which made possible to program certain tools to,

during the assembling process, calculate the variations on

torque and angle values and estimating the point when the

curves achieve and passes the Yield point of the joints.

Knowing that the joint will have very little clamp-load scatter

around the yield point, it is possible to program the tool to

stop after a certain threshold is met, which substantially

increases the capability of that assembly to the clamp-load.

This technology is mostly used with special alloys bolts of

high ductility that can sustain more plastic deformation

before snapping.

Bolt Tensioning

Mostly used in Oil & Gas and other industries utilizing large

bolts and with not many access restrictions during

assembling process. This “torque independent” method

consists of the use of an external kit and hydraulic pumps.

It also requires that portion of the bolt/stud is available to

be attached to a mandrill/puller and the tension is delivered

through hydraulic pressure that will make the puller stretch

the bolt/stud.

Once the desired stretch is achieved, the nut is rotated the

position and the pressure is release trapping that bolt

stretch in place.

Mechanical Characteristics of Fasteners

Once the minimum amount of clamp-load is determined and also the assembling method, it is

time to chose the size and property class of the fasteners.

Depending on the chosen

assembling method and other

characteristics of the fasteners

(friction coefficients for example),

a high scatter on the generated

clamp-loads may suggest the use

of fasteners up to 2X

larger/stronger than necessary to

guarantee joint reliability…

$$$ savings opportunities!!!

There are several factors that influence the behavior of bolted joints. Hence the

selection of plating, assembling methods, fastener size, property class and etc. must be

always specific for each individual joint even if two configurations look very similar to

each other. Some of these factors, not yet presented in this training can be listed below:

1. Plating of fasteners & other joint elements;

2. Assembling Tools & Methods;

3. Joint relaxation;

4. Work loads;

5. Etc.

Other Influencing Factors


The type of plating/painting on the fasteners and parts, have direct

effect on the values and scatter of the friction coefficients, which

will directly affect the achieved clamp-loads during the assembling

process utilizing torque strategies according to the equation:

MA = F.D.K

K, which is known as the nut factor, is directly affected by the

thread and head friction coefficients and also by geometric

characteristics of the fasteners.

Clamp-load variations due to variations on the nut factor can be reduced by utilizing controlled friction

coefficient types of plating.

The thread friction coefficients also affect the amount of axial load the fastener can absorb, as expressed in

the VDI 2230 per the following formula:

𝜎𝑀 =𝑣. 𝑅𝑝0.2

1 + 3.32.𝑑2𝑑0

𝑝𝜋. 𝑑2

+ 1.155. 𝜇𝐺

2


The type of assembling tools used in production, can affect the amount of delivered clamp-loads

proportionally to the degree of precision of the selected tool. I.e.: DC Electric nut runners can delivery

torque scatter on the order of < 1%, while pneumatic nut runners and click wrenches will present

scatter of > 15% on the torque values.

Example of DC Electric Nut RunnerPrecision < 1%

Example of Pneumatic Nut RunnerPrecision > 15%

Differences in tool’s precision must be taken into account in the design phase of the bolted joint!


Joint Relaxation can be a big challenge to the proper function of a bolted joint, mainly due to the

limited methods to measure and properly study the phenomena outside of mechanical lab

environments. While many companies rely on the measurement of Residual Torques to predict joint

relaxation, this correlation can be highly inaccurate and lead to inadequate actions on an attempt to

compensate clamp-load loss via loss on torques.

In this example, we are showing only ONE of the various

errors that can occur during the torque auditing of a bolted

joint. Other factors like: temperature, corrosion, socket

touching the plates, use of improper torque wrench, etc.

can highly influence the amount of residual torques

reported, promoting errors sometimes higher than 20% on

the torque readings and higher than 40% on the estimated

clamp-load values.


Workloads can influence the behavior of bolted joints by shifting force directions to different angles

and values from those predicted in the design phase.

As, shown in the above diagram of forces, the residual clamp-loads

on a joint must be enough to resist the workloads in both lateral and

axial directions, avoiding additional tension on the fasteners due to

workloads. Additional loads, can trigger fatigue failure of the bolted

joints

Quality Controls & Failure Analysis

The quality control of bolted joints will always start on the manufacturing of the parts and the

performance of testing on the fasteners and parts being assembled. Common quality controls of bolted

joints and its elements are:

1. Mechanical properties (Tensile strength / Chemical composition);

2. Plating specifications (Corrosion resistance / Friction coefficients);

3. Dimensional inspections

4. Torque x Tension relationship (Friction coefficients – ISO 16047);

5. Vibration Resistance (DIN 25201-4 / DIN 65151);

6. Joint relaxation studies.

Testing of Bolted Joint Elements

Testing of Joint (simulated) and/or Bolted Joint Elements

Testing of Bolted Joint

While the testing of bolted joints elements is important, having good quality joint elements does not

guarantee you are failure free in the application as having out of specs elements doesn’t mean

your joint will fail.

Bolted joints can failure mainly in two circumstances:

1. During the assembling process: i.e.: thread stripping, yield/breakage of fasteners, cross

threading, fastener not completely settled in the joint etc.

2. Field failures, i.e.: fasteners coming loose, fatigue, hydrogen embrittlement etc.

Besides all failures must be individually analyzed, failures occurring by fasteners coming loose always

have one common factor in 100% of the cases: Low Residual Clamp-load in the application. Either

due to wrong design, wrong assembling parameters or even material issues. The fact is: when a

fastener has its right amount of residual clamp-load for the application, the joint will never come loose!

Quality Controls & Failure Analysis

Patented Technology – Clamp-load Assembly & Control(Torsional Angle Control)

Eliminates the need for instrumentation, external device exhausting lab testing.

Neglected for many years, the torsional angles of

fasteners, plates, sockets etc., where always

included in the total rotational angles used in the

calculations of bolted joints pre-load!


When separated from the total rotational angles, the remaining angle of

torsion of the bolt, has a direct correlation with the thread torque and its

thread friction coefficient, which then, can be correlated to the clamp-load

variations when we analyze it in the tighten – untighten – retighten torque

x angle curves in a bolted joint as in the below example.



The analysis of the Tighten – Untighten – Retighten

curve can be performed remotely and as results,

important characteristics of the joint can be extracted

from the torque values, fastener characteristics and

torsional angles measured in each one of the 3 steps

(tighten-untighten-retighten).

These parameters are:

1. Thread friction coefficients;

2. Head friction coefficients;

3. K Factor;

4. Joint stiffness;

From the above parameters, the assembling strategy is developed aiming

the targeted minimum clamp-loads!


Utilizing the same patent methodology, we have also created the ultimate

system that is capable of measuring Residual Clam-loads directly in the

application.


The systems consists of an electronic

transducerized Torque Wrench that captures

torque x angle data in high density of points,

allowing the user to enter “estimated values”

of thread friction coefficients and through the

use of graphic analysis and algorithms,

measures accurate residual clamp-loads

either in lab environments, production or field,

no matter when the joint was assembled and

with no need for any joint instrumentation.

1. Residual clamp-loads;

2. Residual torques (not operator dependent);

3. Thread friction coefficient;

4. K factors;

5. Pre-load loss;

Retighten curve used by for analysis of Residual Clamp-loads.

Highlights

1. Each single joint is different;

2. Final objective is Residual Clamp-load;

3. High clamp-load scattering is dangerous & expensive;

4. High precision assemblies require high precision control tools;

5. Good fasteners do not mean: reliable bolted joints;

Focus must be on:

CLAMP-LOAD, CLAMP-LOAD, CLAMP-LOAD

Thank You !

Visit our webpage at: www.pcltork.com or contact us directly at 850, W. University Dr. Suite B Rochester – MI 48307

Phone: +1.248.761.2884 or e-mail us @ [email protected]

Cutting Edge Fastening Analysis!

http://www.pcltork.com/

mailto:[email protected]

bolted joints training slides

Automotive