10 Tips for Specifying Electric Rod-Style Actuators

Whenever it is finallydecided to get factory automation applications up and running it seems therealways exists the impulse to take shortcuts so that the system can be put intooperation as quickly as possible. However, if you're specifying electricrod-style actuators, the higher initial cost and more complex design of theseactuators versus fluid power cylinders means that taking the time foradditional engineering and analysis on the front end of the application designcan significantly reduce overall costs.

Following are 10 tips that should always be kept in mind toensure optimization of your electric rod-style actuator application.

1: Calculate Loads Precisely

The accuracy of an electric rod-style actuator isdependent on matching the electric motor, the lead screw and the bearings tothe anticipated loads. Know the precise static and dynamic loads of theapplication and match them to the peak and continuous load capabilities of theactuator.

2:Don't Calculate for Fluid Power

Oversizing actuators is considered inexpensive insuranceagainst not having enough power. Therefore, it is not uncommon for engineers tobuild in a 2:1 safety factor on fluid power applications. Because electric actuators can cost significantlymore, oversizing is a more costly mistake. Sizing programs, graphs and formulasavailable from actuator manufacturers make this task easier and more accuratethan in the past.

3: Factor in Duty Cycle

Duty cycle is defined as a ratio of operating time toresting 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 fortwo sec has a duty cycle of 50 percent. Underestimating the impact of dutycycle on an actuator can lead to overheating, faster wear and prematurecomponent failure. Overestimating the impact of duty cycle can lead to higherinitial costs due to oversizing.

4: Know Force and Velocity

When considered together, force andvelocity requirements dictate the capabilities of motors, screws and nuts inelectric rod-style actuators. A common error is specifying a stepper motor tosave money when a servomotor may be more appropriate for velocity and forcerequirements. As the speed of a stepper motor increases, its available forcedrops, whereas servomotors are able to maintain their force even as speedincreases. Similarly, force and velocity requirements will dictate the type andpitch of the lead screw - whether it is an Acme screw with a composite or bronzenut, or a ball screw or roller screw.

5: Employ Proper Guides and Avoid Side Loading

A rod-style actuator is vulnerable todamage if the extended rod is subjected to even moderate side loads. This tendsto occur when the actuator is out of alignment with the main load, causingsevere wear to the front bearing and damage to the lead nut. The mounting canalso be a factor. For example, an actuator rod will have atendency to run out of alignment at maximum stroke when it is mountedwith a clevis-type rod end, creating significant side loads on the rod andsupport bearing. To avoid side loading, be sure the actuator rod has anappropriate mounting and is either guided or precisely aligned with the load.

6: Set Critical Speed Limits

Higher operating speeds can often improvemanufacturing throughput, but in a rod-style actuator, lead screw criticalspeed becomes an upper limit. Critical speed refers to the rotational speedthat excites the screw's natural frequency. When a screw reaches critical speedit begins to oscillate or "whip." The critical speed limit is dependent on thescrew length and diameter. As the stroke length increases, the distance betweenthe support bearings increases, causing screw oscillation over a certain speed.This oscillation prematurely wears the support bearings and can result invibration, noise and even catastrophic failure.

7: Match Peak Thrust to Actuator

The peak thrust neededfor the application cannot exceed the peak thrust that can be delivered by theactuator. First determine what the peak thrust requirement is for theapplication and then compare that tothe thrust curve for the selected actuator.Thrust curves from actuator manufacturers show a combined peak and continuousthrust capability. For most of the duty cycle, the thrust stays under thecontinuous thrust curve. Make sure that the actuator's peak and continuousthrust capabilities are properly matched to the motor; some motors can providemore peak thrust than the actuator can withstand, causing stresses that willlead to premature failure. It is equally important to perform necessary columnstrength calculations and verify that the actuator is capable of providing therequired peak thrust without screw or rod buckling.

8: Factor in Environment

High temperatures can affect seals, lubrication, bearingsand motor life. Extremely low temperatures can also affect performance,lubrication and wear. Contamination with oil, water or abrasive grit candestroy seals unless the actuator has an appropriate IP rating. Since IPratings 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" - thelength 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 analteration in the layout of the application. Be aware that the overall strokelength and actual working stroke length of an actuator will be different due tothe "dead length" needed to accommodate internal features such as lead nut, bumpersand limiters.

10: More Than aFootprint

The actuator may be specified with an inline orreverse-parallel (RP) motor mount/drive system. While an RP system offers amore compact envelope, it is not considered as efficient due to the addition ofbelts or gears. RP motor mounts offer gear/belt ratios for a mechanicaladvantage and inertia matching, plus an additional mechanical mounting optionof a rear clevis. Inline motor mounting, on the other hand, is considered tooffer a more efficient drive system and dynamic performance. In some cases itis better to alter the application to make room for an inline motor drive thanto specify a reverse-parallel drive where its performance characteristics maynot be the best match (see graphic).

Aaron Dietrich is productmanager for Tolomatic's electricproduct line.