Linear motion systems help reduce the hassle of component
selection and integrate into any machine design effortlessly. The Thomson
WM (Powerline) linear unit is designed to provide stiffness, rigidity and precision in
demanding applications such as machine tool automation and test and measurementbe secured firmly to the part that is being
machined or moved and may sometimes
be moved by robotic arms. If a carriage is
used, then multiple rail assemblies can be
deployed for multiple axis movement. The
rails also provide additional support for the
carriage and bearing blocks.
To deliver the required accuracy, rails must
be perfectly straight and smooth. Steel rails
and shafting, which are used in guidance
and support structures, are made to more
exacting standards and tighter tolerances
than plain bearings that are used in power
transmission shafting. Guide systems sup-
port the carriage and all the forces acting on
the attached load as it travels. Therefore, the
careful specification of the system plays a
critical success factor for rodless actuators,
even more so in high precision applications.
Linear bearings support lateral rather than
rotary movement. They travel back and forth
on the rails at a predetermined cycle rate.
For round rails, they could be ball bushings
or plain self-lubricating bearings that are en-
closed and supported in some type of block/
housing mounted to a bed.
Specifying linear motion
components
The most important factors to consider
when sizing, selecting, installing and op-
erating a linear system are the loads and
moments, speed, acceleration/deceleration,
required accuracy, duty cycle, parallelism,
rigidity and repeatability. Here are some fac-
tors to consider in relation to each:
- Load and moment – The loads are clas-
sified as radial, reverse-radial, lateral, re-
verse-lateral, axial and reverse-axial. Mo-
ment forces are classified as pitch, yaw
and roll. Analogous to an airplane, pitch
would be whether the nose is up or down,
yaw would be motion to the left or right,
and roll would be when the wing tips are
up or down. The guidance portion of the
system handles all of the loads and mo-
ments with the exception of the axial and
reverse-axial load, which are taken care of
by the thrust mechanism. - Speed – Speed is the rate at which a
moving object is able to move or oper-
ate and is normally referred to as the top
constant speed the object reaches. In-
creasing speed comes with higher pow-
er requirements, so one should carefully
consider the entire motion profile for min-
imum power consumption- Acceleration/deceleration – Acceleration/
deceleration refers to the rate at which
objects change in velocity over time. Ex-
cessive acceleration/deceleration yields
in unstable motion and excessive strain
on guide components due to induced
moment loads, all which can lead to less
service life of the system. - Required accuracy – Accuracy is how
closely the system moves compared to
a commanded position. It is a function
of many variables, including the accura-
cy grade of the components, installation
practices and mounting accuracy of the
machine base. - Duty cycle – Duty cycle is the amount of
“on time” versus “total time,” or it can be
defined as the number of reciprocating
motions per minute. This is one of the
most critical factors impacting the life of
the system. - Running parallelism – Maintaining paral-
lelism of the linear rails and shafts is criti-
cal to consistent performance and design
life. When the table moves, rail parallelism
will prevent binding in the bearings and
overall inaccuracies in the system. - Rigidity – Rigidity is important because
the system must be stiff or rigid enough
to prevent deformation or unintentional
movement. Undesirable deflections dur-
ing operation can result in production
errors. Preloading ball screws, fastening
components properly, torqueing fasten-
ers correctly, and having firm end sup-
ports on thrust mechanisms all contribute
to improved rigidity.- Repeatability – A linear system’s repeata-
bility relies on its components consistently
moving from one point to another and
back with minimal error. On servo systems,
for example, sensors, limit switches and
encoders provide feedback that can help
control errors and positioning.
With so many factors to consider, the
chances of mismatching motion control
technologies to one’s applications are high.
Linear component manufacturers provide
automated tools that help compare options
and manage trade-offs. Thomson Indus-
tries, for example, offers online tools that
help design engineers accurately size and
select linear motion systems and other com-
ponents. It provides an interactive series of
questions, starting with a comprehensive
analysis of motion control requirements,
quickly leading to an ideal solution for the
customer’s application.Whether you use an automated selection
tool or your own calculation, adequate at-
tention to selection of power components,
thrust mechanisms, and guidance and sup-
port systems can help you maximise the
precision and repeatability you get from your
production operations. While proper selec-
tion provides the foundation for success,
installation, operation and maintenance are
the building blocks.https://www.thomsonlinear.com/en/
indexFeature: Linear motion technology
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CNC and material handling applications require accurate and durable
linear motion systems. (Images courtesy of Thomson Industries, Inc.)