Design time can be reduced
while ensuring durability and
Reducing design time is critical in engineering
because the result is lower costs and faster time to
market. Design time often includes a number of
non-value-added activities such as re-design, over-design,
or scope creep that can be minimized by thoroughly
understanding all of the application criteria and verifying
calculations and analysis via parametric testing of components,
modules, and full assemblies with data acquisition
equipment, and proving out projected performance
results with testing.
Figure 1. Robot palletizing application using a linear motion system.
Capture as much of the pertinent application information
as possible in the beginning to avoid having to
go back and repeat portions, if not the entire design
process. Be wary and prepared for scope change. Use
theoretical calculations and analyses to determine the
best initial designs, and then compare them with test
measurements of the key performance attributes on
actual equipment. Confirm bench test results by performing
cycle tests under actual field conditions.
Identifying the Requirements
The first and very critical step of nearly every engineering
process is identifying the application requirements.
Each product may have a unique set of criteria that will
affect its performance. Using a checklist will help to
ensure the consideration of parameters that may otherwise
Key application information
data in a sample checklist may
• Load/speed (dynamic and
• Voltage: 12, 24, 36, 48 VDC,
110, 220, VAC
• Direction of load
• Stroke length
• Life/duty cycle
• End-of-stroke protection:
Clutch? Limit switches?
• How will actuator be controlled?
• UL, CSA, CE
• Other: Consult your actuator
application engineer for
additional design considerations.
The selection of the correct ball screw
assembly for a specific application can
require an iterative process to determine
the smallest envelope and most costeffective
solution. The design load, linear
velocity, and positional accuracy
requirements are used to calculate the
diameter, lead, and load capacity of the
suitable ball screw assembly. Individual
ball screw components can then be
selected based on life, dimensional constraints,
mounting configuration, and
A good place to start is by defining
the direction and magnitude of the
load. The system orientation can be
very important. With a horizontal orientation,
the drive load is equal to the
payload weight, times the frictional
coefficient. With a vertical orientation,
the drive load is equal to the weight.
Loads acting on linear bearings and
guides can be vertical loads, horizontal
loads, or pitch, roll, or yaw moment
loads, or any combination thereof.
Loads may also vary in their magnitude
The resultant load vectors at each
bearing must be established from the proper combination of the various load vectors to which the
linear bearing system is subjected, as life expectancy cannot
be estimated based on just the overall system load vectors.
The load that each linear bearing is subjected to is called the
equivalent load for that given bearing. The system is then
sized based on the sizing of the most heavily loaded bearing.
For more information on computation methods for an
equivalent load, refer to the linear bearing and guide suppliers’
Figure 2: Three-axis welding gantry application.
A ball screw assembly, for example, is intended to carry
axial loads, translating rotational motion to axial motion.
The ability of the ball screw to resist buckling under compressive
loads is called its column strength. The screw carries
an axial load that is effectively equal in magnitude and opposite
in direction to the load imparted to the ball nut — its
complementary part — and is related by the design geometries
to the driving motor’s torque. In general, the column
strength is the limiting design parameter because for longer
lengths, it can be much lower than the material’s actual compressive
strength. Since the free length-to-diameter ratio is
intimately related to column bucking, it follows that for a
given diameter, the axial load capacity of a ball screw is
dependent upon its free length.
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