Whenever it is finally
decided to get factory automation applications up and running it seems there
always exists the impulse to take shortcuts so that the system can be put into
operation as quickly as possible. However, if you're specifying electric
rod-style actuators, the higher initial cost and more complex design of these
actuators versus fluid power cylinders means that taking the time for
additional engineering and analysis on the front end of the application design
can significantly reduce overall costs.
Following are 10 tips that should always be kept in mind to
ensure optimization of your electric rod-style actuator application.
1: Calculate Loads Precisely
The accuracy of an electric rod-style actuator is
dependent on matching the electric motor, the lead screw and the bearings to
the anticipated loads. Know the precise static and dynamic loads of the
application and match them to the peak and continuous load capabilities of the
actuator.
2:
Don't Calculate for Fluid Power
Oversizing actuators is considered inexpensive insurance
against not having enough power. Therefore, it is not uncommon for engineers to
build in a 2:1 safety factor on fluid power applications. Because electric actuators can cost significantly
more, oversizing is a more costly mistake. Sizing programs, graphs and formulas
available from actuator manufacturers make this task easier and more accurate
than in the past.
3: Factor in Duty Cycle
Duty cycle is defined as a ratio of operating time to
resting time of an electric actuator expressed as a percentage:
[% Duty Cycle = Operating Time/
(Operating Time + Resting Time)].
An actuator that is moving for two sec and stopped for
two sec has a duty cycle of 50 percent. Underestimating the impact of duty
cycle on an actuator can lead to overheating, faster wear and premature
component failure. Overestimating the impact of duty cycle can lead to higher
initial costs due to oversizing.
4: Know Force and Velocity
When considered together, force and
velocity requirements dictate the capabilities of motors, screws and nuts in
electric rod-style actuators. A common error is specifying a stepper motor to
save money when a servomotor may be more appropriate for velocity and force
requirements. As the speed of a stepper motor increases, its available force
drops, whereas servomotors are able to maintain their force even as speed
increases. Similarly, force and velocity requirements will dictate the type and
pitch of the lead screw - whether it is an Acme screw with a composite or bronze
nut, or a ball screw or roller screw.
5: Employ Proper Guides and Avoid Side Loading
A rod-style actuator is vulnerable to
damage if the extended rod is subjected to even moderate side loads. This tends
to occur when the actuator is out of alignment with the main load, causing
severe wear to the front bearing and damage to the lead nut. The mounting can
also be a factor. For example, an actuator rod will have a
tendency to run out of alignment at maximum stroke when it is mounted
with a clevis-type rod end, creating significant side loads on the rod and
support bearing. To avoid side loading, be sure the actuator rod has an
appropriate mounting and is either guided or precisely aligned with the load.
6: Set Critical Speed Limits
Higher operating speeds can often improve
manufacturing throughput, but in a rod-style actuator, lead screw critical
speed becomes an upper limit. Critical speed refers to the rotational speed
that excites the screw's natural frequency. When a screw reaches critical speed
it begins to oscillate or "whip." The critical speed limit is dependent on the
screw length and diameter. As the stroke length increases, the distance between
the support bearings increases, causing screw oscillation over a certain speed.
This oscillation prematurely wears the support bearings and can result in
vibration, noise and even catastrophic failure.
7: Match Peak Thrust to Actuator
The peak thrust needed
for the application cannot exceed the peak thrust that can be delivered by the
actuator. First determine what the peak thrust requirement is for the
application and then compare that to
the thrust curve for the selected actuator.
Thrust curves from actuator manufacturers show a combined peak and continuous
thrust capability. For most of the duty cycle, the thrust stays under the
continuous thrust curve. Make sure that the actuator's peak and continuous
thrust capabilities are properly matched to the motor; some motors can provide
more peak thrust than the actuator can withstand, causing stresses that will
lead to premature failure. It is equally important to perform necessary column
strength calculations and verify that the actuator is capable of providing the
required peak thrust without screw or rod buckling.
8: Factor in Environment
High temperatures can affect seals, lubrication, bearings
and motor life. Extremely low temperatures can also affect performance,
lubrication and wear. Contamination with oil, water or abrasive grit can
destroy seals unless the actuator has an appropriate IP rating. Since IP
ratings only address static conditions, dynamic conditions (vibration, heat,
cold, movement) also have to be considered.
9: Total Envelope Matters
Be sure to consider the overall actuator "envelope" - the
length and width of the actuator and motor when the rod is fully extended.
Failure to do so may limit the size of motor that can be used or require an
alteration in the layout of the application. Be aware that the overall stroke
length and actual working stroke length of an actuator will be different due to
the "dead length" needed to accommodate internal features such as lead nut, bumpers
and limiters.
10: More Than a
Footprint
The actuator may be specified with an inline or
reverse-parallel (RP) motor mount/drive system. While an RP system offers a
more compact envelope, it is not considered as efficient due to the addition of
belts or gears. RP motor mounts offer gear/belt ratios for a mechanical
advantage and inertia matching, plus an additional mechanical mounting option
of a rear clevis. Inline motor mounting, on the other hand, is considered to
offer a more efficient drive system and dynamic performance. In some cases it
is better to alter the application to make room for an inline motor drive than
to specify a reverse-parallel drive where its performance characteristics may
not be the best match (see graphic).
Aaron Dietrich is product
manager for Tolomatic's electric
product line.
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