What is Torque?

Torque is a “turning” or “twisting” force and dif fers from tension, which is created by a straight pull.
However, we use torque to create a tension.

HOW?
(Diagram A) As the nut and bolt are tightened, the two plates are clamped together. The thread angle in
the bolt converts the force applied into tension (or stretch) in the bolt shank. The amount of the tension
created in the bolt is critical.

WHY?
A bolt tensioned properly works at its optimum efficiency and will resist coming undone. However, if the
tension is too low, the nut could vibrate or work loose. If the tension is too high (overstretched), the bolt
could break. Ever y bolt has a correct optimum torque/tension figure for each fastening application. It is
important to have these figures available so that the end product will be safe, efficient and economical.

HOW DO WE MEASURE TORQUE?

(Diagram B) Torque is the result of multiplying the value of Force applied by the Distance from the point of
application.

Comparing the two examples, please note that the same Torque result can be achieved with a lower Force
if the Distance from the nut/bolt is increased.

Another factor that affects applied torque when using torque wrenches is if it is “length dependent,” which
means that the actual torque applied to the fastener varies if the hand position on the wrench is varied
(even if the wrench is preset). This occurs if the pivot point of the wrench mechanism is not coincidental
with the point of application of torque.

THE IMPORTANCE OF TORQUE CONTROL?

The reliability of machine parts subjected to fluctuating loads and stress depends on the fatigue strength
of the materials. A threaded fastener, however, relies upon an elastic interaction between the mating
components. Its objective is to clamp parts together with a tension greater than any external force trying
to separate them. The bolt then remains under almost constant stress and is immune to fatigue. If the
initial bolt tension is too low, the fluctuating load in the shank in much greater and it will quickly fail.
Reliability, therefore, depends on correct initial tension and is ensured by specifying and controlling the
tightening torque.

DIAGRAM A:

Diagram B

Torque = force x lever length of wrench:

Force of 20 lbs. x 1 ft. = 20 lbf.ft or Force of 10 lbs. x 2 ft. = 20 lbf.ft
 Force of 20 lbs Force of 10 lbs

Tightening Bolted Joints with Torque Tools

These general guidelines are to help identify potential pitfalls relating to the
tightening of bolted joints.

Use a calibrated Torque Tool: Ensure that a calibrated torque tool is used and
that a torque value is specified on the tightening specification. Be aware that certain
automatic tightening tools, such as impact wrenches, can result in significant
variations occurring in the torque value and the bolts preload. A calibrated torque
tool should therefore be used for the final tightening operation or inspection.

Specify the correct tightening torque: Whenever feasible, specify the tightening
torque based upon actual test results rather than a theoretically calculated value.
Experimental determination of the tightening torque can be established by
measurement of bolt extension, strain gauges or by the use of a load cell embedded
in the joint

Specify a Tightening Sequence: The majority of joints consist of more than one
bolt and bring together surfaces that are not completely flat. The sequence of
tightening bolts can have a major influence on the resulting preloads. With
such joints, consideration should be given to specifying the sequence in which the
bolts are to tighten. Because the joint surfaces compress, tightening one bolt in the
vicinity of another will affect the preload generated by the first bolt tightened.

A good tightening sequence ensures that an even preload distribution is achieved in

the joint (See Dia. A). Since joints containing conventional gaskets have a
comparatively low compressive stiffness, bolt preloads in such joints are particularly
sensitive to the tightening sequence. Based on experience, if the bolts are in a
circular pattern, a cris-cross tightening sequence would normally be specified. For
non-circular bolt patterns, a spiral sequence starting at the middle
would normally be specified (See Dia. B).

On critical joints, a tightening pattern that tightens the bolts more than once can be
specified to ensure an even preload distribution.

Be cautious with the use of plain washers: Clearance between the bolt shank
and the washer hole can result in relative lateral motion occurring. It can change the
friction surface from nut and washer, to washer and joint surface during tightening.
This affects the torque-tension relationship and will lead to large variations in
preload. In some situations, such as to cover slots or to reduce the surface pressure
under the bolt head, plain washers are traditionally specified. In such circumstances,
ensure that they are of sufficient thickness and hardness and that they are a good fit
to the bolt shank.

Flange Headed Bolts: On relatively soft materials or when high tensile bolts are
used, consideration should be given to the use of flange headed bolts and nuts. Such
fasteners reduce the surface pressure under the nut surface reducing the amount of
preload lost due to embedding. Due to of the larger diameter bearing faces, generally
a higher tightening torque is required because more torque is dissipated by friction.

Gaskets: Conventional gaskets are non-elastic; this results in a reduction in the bolts
preload over time. The majority of such non-elastic condition usually occurs shortly
after installation. This usually causes relaxation in a bolt. To reduce the effect of such
problems, re-tightening of the bolts is frequently completed after allowing a period of
time to elapse after initial tightening.

Embedding: Embedding is a plastic deformation that occurs in the threads of the

fastener and in the joint itself. It is caused by high stresses generated by the
tightening process. Such embedding results in a loss of bolt extension and hence
preload. Typically, preload loss due to embedding is in the region of 10%. It increases
with the number of joint surfaces being clamped and with the roughness of those
surfaces. High surface pressures under the bolt head can also be a cause of
excessive embedding.

This can be due to the use of high tensile fasteners in relatively soft materials.
Hardened washers or the use of flanged fasteners can reduce such effects. Caution
should be exercised in the use of short bolts clamping several interfaces together. In
such joints, the small amount of bolt extension can be significantly reduced by the
large amount of embedding, which can be anticipated.
Joint Relaxation in Fastened Joints

Joint relaxation occurs to some extent in all fastened joints and is caused by the
surfaces of the part embedding or by "soft parts", such as gaskets, collapsing under
the clamping force. For correctly designed joints, relaxation is small and can be
virtually ignored. However, relaxation is a particular problem on joints where gasket,
or parts such as spring washers, are present. On these joints it can take a long time
before the joint settles and results in a reduction in clamping until the condition has
stabilized.

The physical phenomenon for the collapse of material is called "creep." All material
creep to some extent. For example, glass is a liquid and creeps over time. That is
why an old glass window may show "waves" at the bottom of the window.

The creep is most obvious and dramatic right after the force has been applied. In
many tightening applications the majority of the creep, reducing the clamp load (and
sometimes static torque), appears within the first 10-50 milliseconds.

The techniques that can be used to reduce the effect of creep are;
1. Torque the fastener, undo the joint and retighten.
2. Redesigning of the joint (for example, replace soft gaskets with sealing
compound).
3. Torque the fastener, wait briefly and then apply again (can be repeated in several
steps).
4. Low RPM of the tool at final clamp down.

With a given design, action 3 above is quite commonly used. It is especially easy to
achieve if the application is tightened with a fixture spindle in a station but with the
introduction of sophisticated air and electric hand tools it becomes more and more
common even for those. However, the negative ergonomic ramifications should be
considered if the tool is not suspended or supported.
Selecting the Power Tool to Suit the Joint

To help ensure that you achieve the torque accuracy required of your application, it
is important to select a power tool with low scatter and low mean shift. Joint stiffness
varies, so it's necessary to make an allowance for it to achieve the proper torque.

Fitting screws into steel without interlining components is one example of a hard joint
with rapid tightening. For this application, the power tool needs to have a quick
clutch that interrupts the torque at the preset level.

Another example is a soft joint with gaskets and washers, or a long screw, requires a
screwdriver with a strong motor to attain short tightening times.

If there are subsidence or plastic deformations in the joint, then you should opt for a
clutch type power tool or a tool with a hydraulic pulse mechanism. The joint would
then have time to settle as you are assembling.

Joints with self-locking elements in the thread or clamped nuts are becoming
increasingly common. If you work with joints like this, we recommend a screwdriver
with a shut-off clutch.

To fully utilize the high performance of a quality power tool, it's important to know
the demands made on the power tool by different screwed joints. Here are some
examples that will help you select the kind of power tool you require.

Machine Screw - Hard Joint

Low resistance to turning until the screw head reaches its seating, after which
resistance rapidly increases. For rapid screwdriving with moderate torque accuracy
choose a high speed tool. For close torque limit or when the quality of thread is
uneven, choose a lower speed tool. This is also recommended for brittle material. The
most suitable clutch is a shut-off type. An alternative choice is a slip type.

Machine Screw - Soft Joint

Low resistance to turning until the screw head reaches its seating, after which
resistance slowly increases. A slower and more powerful tool should be used to seat
the fastener properly to minimize relaxation in the joint. Most suitable clutches: Shut-
off type. Slip type possible.
Self-Drilling Screw
The turning resistance gradually increases during drilling and thread cutting, but
more slowly than with self-threading screws. Choose a tool with speed over 1000
RPM. A slip type clutch is generally suitable but for thin material choose shut-off.

Machine Screw - Locking Element in Thread

Relatively high resistance to turning before screw head reaches its seating after
which resistance rapidly increases. Choose a tool with high torque from the lower
speed range. If rapid driving is required choose a larger tool with a higher speed.
Suitable clutches: Shut-off and slip type.

Thread Producing Screw

The turning resistance gradually increases as the screw is producing the thread and
reaches a maximum just before the entire hole is threaded. Choose a tool within
speed range 800 to 1300 RPM. Slip type clutches are generally suitable, but for thin
material choose shut-off type.

Wood Screw
The turning resistance gradually builds up as the screw is being driven and increases
rapidly when the screw head reaches its seating. The process varies considerably
with the degree of pre-drilling, different type of wood and sizes of screw. Choose a
low-speed tool, 400 to 800 RPM. Suitable clutches: Slip type and direct drive.
Selecting the Proper Torque Wrench

There are a wide variety of Preset and Adjustable torque wrenches and selecting the
shape and size for your application can be easy. However, understanding the variety
of torque delivery mechanisms of torque wrenches can be confusing.

A Preset torque wrench is similar to a person setting an alarm clock to signal the
achievement of a selected time. The wrench is preset to the required torque value of
the application and then the tool signals the user when torque is achieved. A preset
torque wrench must be preset using a torque analyzer. An “Adjustable” torque
wrench features a torque scale that allows you to see and adjust the torque setting.

There are three styles in which the wrench can signal achieving torque, either by a
“click,” “break,” or “slip.” Each of three wrench styles has a specific purpose and
utility. When you decide to spend the money for one, it’s important that you select
the one that will do the job properly and not generically.

Click Wrenches - Reset when pressure is released - length dependent.

“Click” wrenches are the most widely used torque product in the world. When the set
torque is reached, the tool typically emits a loud audible “click.” The operator can
feel the impulse from the tool and most break about 3 degree after set torque is
reached and then become positive. This positive action can cause over-torque
conditions. Proper use and training is required so that operators stop pulling the
moment the click sound is heard or felt. Resetting of the tool takes place when the
hand pressure is released. Work can then immediately continue on the next fastener.

Break-Over Wrenches - Reset at in-line position - length dependent.

“Break-Over” wrenches are essential to limiting the amount of torque applied to an

assembly or fastening. Upon reaching the preset torque value, the tool “breaks” at a
specific point along the tool’s length - usually at a pivot point near the tool’s head. It
typically deflects 20 degree or 90 degree on torque delivery; thus indicating torque
has been reached. Continuing force after the break will increase the torque applied
above the preset value. Many break-over wrenches require manual resetting, while

others have an automatic resetting feature.

Cam-Over Wrenches - Reset automatically - non-length dependent

“Cam-Over” wrenches utilize a ball and lobe design that allows the tool to slip free
when torque is reached. Even if the application of force is repeated, the preset torque
value won’t be exceeded, eliminating the possibility of over-torque. These tools are
perfect for maintenance and production applications where over-torque conditions
are not tolerated. The use of cam-over wrenches takes the operator influence out of
the torque equation and offers more accurate and repeatable results than a standard
‘click’ type wrench.
Selecting the Proper Torque Multiplier

In nearly every heavy industrial application, turning threaded fasteners, nuts and
bolts is generally viewed by two criteria: (1) The need to fasten tightly enough to
prevent movement of parts and achieve a good seal without exceeding the fastener’s
elasticity level. (2) Successful removal of fasteners after long periods of
environmental exposure to harsh conditions.

Selecting the right heavy torque tool for the job is crucial. When it comes to turning
nuts and bolts, especially stubborn corroded ones, what could be more basic than
power?

You need controlled power. That is, “controlled torque,” which provides You need
controlled power. That is, “controlled torque,” which provides smooth torque, with
continuous rotation. Other wise, you could outdo yourself and strip threads, break
bolt heads or even cause personal injury.

Technicians are finding that the best solution for applying high torque today is with a
complete range of torque control products, including manual and powered torque
wrenches and torque multipliers. Mountz torque multipliers provide precision torque
control, making it easier and often safer to assemble and service-threaded fasteners
while reducing application problems and tool costs.

Mountz Torque Multipliers vs. Hydraulics Wrenches -

Hydraulic wrenches are notorious for their heavy ratchets, bulky compressors and
laborious operation. Hydraulic tools operate through a hydraulic ram that extends
and retracts, ratcheting the head. This is a long and tedious process that requires the
operator to activate and stand by the pump with a hand-held controller. Torque
Multipliers increase speed and productivity, as it is faster than a hydraulic wrench
and is less expensive. Designed to deliver smooth torque control, with “continuous”
rotation, these torque multipliers eliminate the cumbersome set up time and slow
ratcheting process of hydraulic wrenches.

Mountz Torque Multipliers vs. Impact Wrenches -

Impact wrenches are destructive by nature with its “hammering” design. These tools
are not ergonomically friendly to an operator and require a high maintenance budget.
Mountz torque multipliers are ergonomically safer than the harmful hammering of
impact wrenches and eliminates frequently costly repairs of impact wrenches.

Mountz Eliminator Torque Multipliers are available in hand, pneumatic and electric
models. These tools are the ideal solution for true torque control, speed, power,
physical ease, silence and safety
Establishing a Quality Torque Program
Experience, Quality and Reliable
In the manufacturing and assembly world, tightening, controlling, or measuring
torque fasteners is imperative for production efficiency. An inadequately torqued
fastener can vibrate or work loose: conversely, if the tension is too high, the fastener
can snap or strip its threads. Faced with these problems, manufacturers are realizing
that precise torque control can spell the difference between a safe, reliable, and
economical product and complete disaster.

Anybody who has to tighten a threaded fastener and needs to control, monitor, or
measure torque needs sophisticated torque tools. If manufacturers want to save
money, make their workplace safer, enhance product quality, or reduce their
exposure to liability; only specialized high-quality torque tools will get the job done
properly.

1. Pick the Right Torque Tool

A wide variety of tools are available to control or measure the torque applied to
fasteners, from electric screwdrivers to large industrial wrenches, analyzers, sensors,
and multipliers. These tools utilize calibrated torque setting mechanisms that may be
factory pre-set or user-definable. When the specified setting is reached, the tool
gives a visual, audible, or tactile signal. The anticipated production output, the type
of materials being joined, the amount of torque required, and the specified fasteners
determine the selection of tools for a given application. Lighter materials such as
wood or plastic may require only lightweight tools; likewise, heavy materials such as
steel may require stronger or larger tools. Tools should also have connection ports for
an RS-232 PC cable if torque data must be gathered.

"Cam-Over" wrenches utilize a ball and lobe design that allows the tool to slip free
when torque is reached. Even if the application of force is repeated; the preset torque
value won't be exceeded, eliminating the possibility of over-torque. These tools are
perfect for maintenance and production applications where over-torque conditions
are not tolerated. The use of cam-over wrenches takes the operator influence out of
the torque equation and offers more accurate and repeatable results than a standard
"click" type wrench.

2. Establish a Calibration Program

Calibration is fine-tuning the torque control process in a production environment.
Calibration should be checked periodically to determine whether torque tools are
operating at their proper settings. Many tools don't have a locking device, and users
may easily change their torque settings. When this happens, the tool falls out of
adjustment. A regularly scheduled calibration program enables quality control
personnel to correct divergence from proper settings, whether it's because of normal
slippage over time or because of adjustments to the tool. Begin by setting a
calibration interval initially based on severity of the application and the tool
manufacturer's recommendations. If the applied torque values are out of range, cut
the calibration interval in half and re-test the tools.

3. Preventive Maintenance
To maintain consistent accuracy, torque tools must be checked periodically for wear
or defective parts. A properly structured preventive maintenance program optimizes
tool performance and reduces unexpected downtime, thereby saving time and
money in the long run. Monitoring the number of cycles per day and total hours that
a tool is used is the most accurate way to establish proper maintenance intervals. It
is recommended that tools be serviced after 100,000 cycles, or if an inspection
reveals old or dry grease, parts that show signs of excessive wear, or loose screws
and bolts.
4. Use Torque Analyzers
Effective use of a torque analyzer is a fast and reliable method of calibrating torque
tools to their proper settings. Analyzers can also be used for quick tests on the line or
in the lab to determine whether torque tools are holding a given setting. They also
allow quality control inspectors to calibrate torque sensors and verify torque on
fasteners. A quality torque analyzer should have enough memory to record several
hundred readings, and it should store calibration data for multiple torque
transducers.

Too Loose or Too Tight? 10 Steps to Torque Control

In the manufacturing and assembly world, tightening, controlling, or measuring
torque on fasteners is imperative for production efficiency. An inadequately torqued
fastener can vibrate or work loose; conversely, if the tension is too high, the fastener
can snap or strip its threads. Faced with these problems, manufacturers are realizing
that precise torque control can spell the difference between a safe, reliable, and
economical product and complete disaster.

“Anybody who has to tighten a threaded fastener and needs to control, monitor, or
measure torque needs sophisticated torque tools," according to Brad Mountz,
President/CEO of Mountz Inc. "Likewise, if OEMs want to save money, make their
workplace safer, enhance product quality, or reduce their exposure to liability; only
specialized high-quality torque tools will get the job done properly."

The following 10 tips for achieving precision torque control will give OEMs vital
information needed to streamline their production processes:

1. Determine Torque Requirements

When determining correct torque specifications, the engineer must consider the
maximum load placed on the fastener, the strength of the material joined, and
whether the joint is hard or soft. A hard joint connects materials directly. In this case,
the fastener rotates very few degrees to develop full clamping force after it
encounters the material. Since a soft joint contains a gasket or involves compressible
materials, it requires additional tightening after the fastener makes contact, to
achieve full clamping force.

One recognized method is to perform a destructive test with a calibrated torque

control tool on One recognized method is to perform a destructive test with a
calibrated torque control tool on the actual material and fastener to be joined. An
evaluation is usually conducted with ten parts, ten fasteners, and a calibrated torque
control tool with a transducer. First the fastener is tightened to the point of failure,
then repeated several times to verify the consistency of the failure point. Now
another series of tests is begun whereby the joint is torqued to 75% of the failure
point. Depending on how the parts will be used, the tightening can be reduced by
any degree necessary. If parts on a machine are subject to heavy vibration, maybe
85% of the total force is necessary for good torque control.

2. Pick the Right Torque Tool

A wide variety of tools are available to control and measure the torque applied to
fasteners, from electric screwdrivers to large industrial wrenches, analyzers, sensors,
and multipliers. These tools utilize calibrated torque setting mechanisms that may be
factory pre-set or user-definable. When the specified setting is reached, the tool
gives a visual, audible, or tactile signal.

The selection of tools for a given application is determined by the anticipated

production output, the type of materials being joined, the amount of torque required
and the specified fasteners. Lighter materials such as wood or plastic may require
only lightweight tools; likewise, heavy materials such as steel may require stronger
or larger tools. Tools should also have connection ports for an RS-232 PC cable if
torque data must be gathered electronically.

3. Use Torque Analyzers

Effective use of a torque analyzer is a fast and reliable method of calibrating torque
tools to their proper settings. Analyzers can also be used for quick tests on the line or
in the lab to determine whether torque tools are holding a given setting. They also
allow quality control inspectors to calibrate torque sensors and verify torque on
fasteners.

"A quality torque analyzer should have enough memory to record several hundred
readings, and it should store calibration data for multiple torque sensors," said
Mountz.

4. Cooperation is Necessary
Orchestrating a successful torque program requires extensive teamwork in all
production related departments to assure consistent adherence to torque
specifications.

Production planners, supervisors, engineers, quality control technicians, and

assemblers must all work together to efficiently control the process. To avoid
problems, consult everyone whenever changes relating to the use or type of
fasteners are instituted.

5. Employee Training
Professional torque tool suppliers often offer personnel training sessions and
workshops. Topics to cover are basic torque theory, types of tools available, how to
operate specific tools, preventive maintenance, safety concerns, and job-related
ergonomics.

6. Employee Safety.
Worker fatigue and potential injuries can be avoided with safety programs and high-
quality tools. In critical applications where safety is an issue, the proper use of tools
can decrease the incidence of expensive lawsuits and product recalls.

To avoid accidents, tools and the work area should be inspected regularly. Worn
components should be replaced and unsafe conditions on the assembly line should
be rectified before injuries occur. Reducing worker fatigue must also be considered
towards achieving production line consistency and reducing repetitive motion
injuries. Torque control tools are available which improve ergonomics and reduce the
effort required for consistent tightening.

7. Establish a Calibration Program

Calibration is fine-tuning the torque control process in a production
environment. Calibration should be checked periodically to determine whether torque
tools are operating at their proper settings. Many tools don't have a locking device,
and their torque settings may be easily changed by users. When this happens, the
tool falls out of adjustment.

A regularly scheduled calibration program enables quality control personnel to

correct divergence from proper settings, whether it's because of normal tool slippage
over time or because of adjustments to the tool. Begin by setting a calibration
interval initially based on the severity of the application and the tool manufacturer's
recommendations. If the applied torque values are out of range, cut the calibration
interval in half and re-test the tools.

8. Preventive Maintenance
To maintain consistent accuracy, torque tools must be checked periodically for wear
or defective parts. A properly structured preventive maintenance program optimizes
tool performance and reduces unexpected downtime, thereby saving time and
money in the long run.

Monitoring the number of cycles per day and total hours that a tool is used is the
most accurate way to establish proper maintenance intervals. It is recommended that
tools be serviced after 100,000 cycles, or if an inspection reveals old or dry grease,
parts that show signs of excessive wear, or loose screws or bolts.

9. Torque Control Increases Quality Control

The precise control of torque is a key to quality assembly and can ensure that
products perform as expected. In many cases, companies spend a great deal of time
and money for disposal or repair of damaged parts during assembly, the result of
improper torquing. Worse yet, even if these products make it to market,
manufacturers are faced with customer dissatisfaction if they fall apart due to loose
screws or stripped threads.

10. Increase the Return On Investment

Tool suppliers should offer recommendations and answers to manufacturers' torque
control challenges. Will their tools enable assemblers to build quicker and with less
wasted motion? Will they give quality control inspectors more time to check parts
thoroughly? Will they reduce errors in the assembly process? And, will they save
money and time for the company overall?