In maintaining equipment or machinery, no one would
argue that the simplest components are nuts and bolts.
Similarly, most recognize that incorrect use of these
basic components can lead to maintenance headaches
at best, disasters at worst.
The concept of nuts and bolts, and fastening them
properly, dates back to the infancy of man''s mechanical
ingenuity. Through most of the industrial revolution, as
machinery and the structures that housed them grew
more complex, the technology of bolting them together
stuck in place. However, over the past several
generations the physics that describe these fastening
devices were more closely studied, resulting in improved
means of measuring and applying measured force,
consistently and correctly, with manual torque
wrenches.
By the early 1970s the ambitions for larger and more
complex buildings and more intricate machinery, along
with the desire to build all these things more
productively, led to the development of the hydraulic
torque wrench. The design of this tool translated the
power of linear hydraulics into an arcing motion to
generate the torque necessary to turn nuts and bolts.
Now construction, maintenance, and machine assembly
crews can more safely generate thousands of
foot-pounds of repeatable torquing force to fasten or
loosen very large bolts within a confined area or a
precarious location. This same power can be used to
break loose heavily corroded bolts for disassembly.
Design Enhancements
In recent years, the demands have increased on the
simple bolting operation. In the construction of nuclear
power plants, processing facilities, underground tunnels,
and aircraft, the concern has been on record keeping
and traceability to verify safety and environmental
reliability of the project. The control and measurability
of hydraulic torque wrenches have helped make this
level of inspection possible.
These complex and critical functions depend upon bolts
tightened within the specifications of the bolt''s
material, the material being joined and the function to
be performed. If a bolt is tightened too little, vibration
can cause a supporting member on a bridge to come
apart or pipeline flange gaskets to leak. Tighten a bolt
too much and the stress can cause the bolt to
eventually break.
Recent developments in versatility have made
hydraulic torque wrenches an even greater
asset for reliable bolt assembly. The in-line
wrench is one of the newest developments in
hydraulic torque wrenches. Basic elements of the
wrench are the drive gear, drive pawl, body -- which
contains the hydraulic cylinder -- and reaction device.
The in-line wrench features a ratcheting wheel with a
hexagonal hole cut to the size of the nut that it has to
fit. The ratchet wheel, drive pawl, and lever arm are
part of a replaceable cassette that is exchanged for
different bolt sizes. The change-out can be
accomplished without using tools.
With previous hydraulic wrench design, the reaction arm
was clamped onto adjacent bolts during the bolting.
The body of the in-line wrench has a reaction pad,
which pushes against any adjacent solid object to
provide a reaction point.
The centerline of the hydraulic cylinder forms a 90°
angle with the centerline of the wrench arm with the
cylinder at midstroke. This design provides maximum
torque at cylinder midstroke for each hydraulic pressure
setting. For maximum safety, the reaction forces are
contained in-line to prevent the wrench from pulling or
twisting off the bolt under load.
Swivel couplings allow the hoses to rotate for easier
positioning of the wrench. Low body height, high
torque-weight ratios, and new flat designs allow use of
the wrench in restricted areas too tight for manual
methods. This sleek profile pays off in greater freedom
of machine and structural design for architects,
engineers, and product designers. Wrench bodies can
fully enclose the wrench arm to prevent interference in
operation by objects or debris.
The variety of pump designs to deliver pressure to
these wrenches means even more versatile positioning
and usage. Electric powered pumps are the common
choice, especially for interior applications. In dangerous
environments compressed air can be used.
Manufacturers are continually developing lighter weight
pumps for use in hard to reach areas.
Regardless of power source, a pump-mounted relief
valve enables accurate torque adjustment and precise
repeatability. Digital gauges can provide the highest
level of accurate measurement of the pressure delivered
by the pump for greater control.
Power Factors
The power of a hydraulic wrench is determined by three
factors -- hydraulic fluid pressure in the cylinder,
cylinder position arc, and wrench arm length. Example:
An Enerpac SQD 50 torque wrench with a maximum
torque capacity of 3,550 ft. lbs. will generate 2,100 ft.
lbs. at 7,200 psi. An SQD 100 using the same pressure
will generate 4,600 ft. lbs. with a maximum of 7,360 ft.
lbs. at 11,600 psi.
Hydraulic powered torque wrench productivity is based
upon the flow of the pumping unit, the volume required
to extend and retract the hydraulic cylinder, and the
degree of turn of the nut per stroke of the cylinder.
Using a pump with a greater flow rate can increase the
speed of operation for a hydraulic torque wrench. In
selecting a pump, use the performance curve as a guide
in determining the flow rate for estimating torque
wrench speed. When sized correctly, a hydraulic torque
wrench can cut many hours from typical bolting
operations.
The hydraulic torque wrench is the tool for applying the
designer''s specifications for torquing bolts. The
precision called for by today''s projects is one of the
prime features of hydraulic torque wrenches. At an
accuracy of +/- 3%, hydraulic torque wrenches are
clearly superior over sledgehammers and striking
wrenches, which have no control, or even over
pneumatic impact wrenches, which have limited torque
control.
For most projects, the necessary amount of torque for
each fastener will be calculated in advance by the
designer. Some cases will need an estimate, such as
equipment maintenance, when specifications do not
exist. One method is to tighten a sample of bolts, and
then using a pocket calculator to find the mean and
standard deviation.
The best approach, when possible, is through prior
experience. Continue to use the same level of torque if
proven to be satisfactory. If not, then increase or
decrease the torque by 10% and then record the
amount. Repeat these incremental changes until
experimentally the proper level of torque is reached. If
there is no prior experience to draw from, refer to the
table available from the manufacturer of the fasteners.
With the ideal of achieving the highest level of
accuracy, there are a number of factors that can affect
hydraulic fastening inherent to threaded bolts. Before
covering these, first consider the basic anatomy of a
bolting operation.
The purpose of a bolt is to bring two pieces of material
together. The clamping force a bolt exerts on the joint
is the preload in the bolt, generated by torquing the
bolt -- causing the bolt to rotate and to tighten down or
loosen up. Because of the resistance of the bolt''s
threads against the threaded grooves in its hole, the
bolt is literally stretched. Since it wants to return to its
original condition, the bolt with the help of its head or
nut, clamps both materials together.
Stress increases on a straight line for the bolt and must
stop when the yield is reached -- or the point at which
permanent deformation in the bolt takes place, possibly
leading to breaking of the bolt. Not enough clamping
force allows the nuts to vibrate loose, causing flanges
to leak or structural parts to detach. Too much
clamping force leads to gasket damage, bolt galling, and
flange damage.
Fastening is one part of the bolting equation. Equally
important, or of possibly greater importance for
maintenance operations, is the sometimes formidable
challenge of disassembly.
Removal of corroded bolts is often an inexact science -- all too frequently approached with tools of questionable
performance and safety. Often, frustration leads to
cutting the bolt head or nut. Significant torque -- generally 150 to 200 percent of tightening, sometimes
more -- is needed for bolt removal as opposed to
fastening.
Torque calculations run up against the real world, and in
this world the torque-preload relationship and accuracy
can be impacted by what has been referred to as the
"nut factor." One of the major determinants is the
lubrication of the nut surface. Steel on steel of course
can create tremendous friction and resistance, versus a
bolt that has been lubricated. About 70 to 90% of the
energy required to tighten a bolt is to overcome friction
in the joint. Tremendous contact pressures reaching
250,000 psi can be reached to overcome friction.
Small changes in friction will result in large changes in
bolt tension. Changes can be large enough to cause
bolts to be tightened below specification or past safe
load limits despite the application of the same torque
for the same design fasteners. Hole sizes and burrs on
the threads can also affect friction.
Lubrication will not only play an important role in
overcoming friction, but will also have a direct effect on
the bolt''s ductility. More ductile bolts can be stretched
beyond yield before failure tension, calling for a
reduction in torque.
Friction is just part of the picture. Dozens of variables
exist, ranging from the parts surface finish, the bolt
hole, the fit of the wrench on the nut or bolt, and the
number of times a fastener has been used. Other
variables affecting torque are in depth research
associated with lubricants. Those include the type used
as well as its temperature.
With all of these factors affecting torque, hydraulic
torque wrenches generally remain the most productive,
most accurate method of tightening bolts, particularly
large bolts. Bear in mind hydraulic wrenches can handle
bolts as small as 1/2 inch hex.
The key to achieving hydraulic torque wrench accuracy
is to reduce the effect of the variables with a basic list
of operating steps:
- Adequate training and crew supervision in using
hydraulic torque wrenches.
- Making sure fasteners are in reasonable shape.
Wire-brush threads if dirty and rusted. Chase
threads with a tap or die if damaged.
- Use of hardened washers between the nut or bolt
head and the joint members.
- Use of clean and fresh lubricants. Apply
consistently, the same amount to the same
surfaces by the same procedure.
If the nut or bolt cannot be run down by hand, the threads may need to be cleaned or chased.
- Holding wrenches perpendicular to the axis of the
bolts.
- Adequate reaction points are used to prevent the
tools from twisting or cramping as a result of
cocked or yielding reaction surfaces.
- Tightening multiple fasteners from the center of
the pattern toward the free edges if the pattern
is rectangular. Work in a cross-bolting pattern on
circular or oval joints.
- Most importantly, keeping thorough records of the
tools, operators, procedures and lubricants. This
is particularly important for maintenance
purposes, both in terms of consistency and
clarity.
Though the above indicates that bolting is still an art,
hydraulic torque wrenches are 30% more accurate than
mechanical torque multipliers, air tools, electric
nutrunners, and other more traditional torquing
technologies. While the leading aversion to using toque
wrenches is their cost, the accuracy the user buys
pays off in greater reliability of bolted joints and greater
peace of mind. Moreover, hydraulic torque wrenches
provide your project the power, versatility, productivity
and documentation to meet the demands of
construction, equipment assembly, and maintenance.