How to Size a Linear Actuator for Your Application?

For example, an applied load of 25 lbs may be well below the maximum rating of the linear actuator, but also may not be sufficient if the critical speed is exceeded. What happens to the actuator under a higher load? Does the linear actuator stop functioning completely, or is the life shortened significantly? Also, in some cases, the actuator's capability exceeds the specified load rating and has a built-in safety factor.

Due to the multiple components that make up the assembly and the intended application of the actuator, there are many aspects to consider. For example, the maximum design limitation may be due to the loading of the lead screw nut. In other cases, the controlling term may be a radial bearing limitation of axial thrust, or the selected motor option is insufficient to drive the linear actuator at the rated load and speed.

Here are some considerations and logical steps to follow when sizing a linear actuator:

Many actuators utilize lead screw or ball screw drive mechanisms. To determine which option is best for your application, you must consider load, speed, efficiency, back drive capability, cost, life, and repeatability requirements.

For higher loads and higher efficiency requirements, ball screws are usually the better choice. For a given load requirement, it is possible to use a ball screw with a smaller diameter than would be required using a lead screw.  

Accuracy

The accuracy of a linear actuator is the positional tolerance from point A to point B. For example, when a motor driver sends a signal to an actuator to move 1", the accuracy is the actual distance moved minus the theoretical difference. If high accuracy is required, a precision rolled lead screw with a lead accuracy of 0.0003"/inch (accuracy Also available in sizes up to .0001"/inch), or precision rolled ball screws are also suitable.

In this case, the ball screw is usually class C7 (some sizes can be as high as class C5). Because errors in the precision rolling process are linear, they can be eliminated by changing firmware or using linear/rotary encoders to create a closed-loop system of continuous position feedback.

Repeatability

Repeatability is a measure of the distance or accuracy with which a linear actuator returns to a specific point from either direction of travel. Tolerances are largely dependent on the nut and the type of clearance (axial clearance) between the nut and the screw or ball screw. Higher bi-directional repeatability requirements will benefit from lead screws with anti-backlash nuts. Various anti-backlash nut designs are available. In addition to providing zero backlashes, these lead screw nuts are compensating and will maintain their anti-backlash characteristics throughout the life of the actuator.

Another best option for high repeatability is a ball screw assembly with selectively fitted ball nuts to minimize backlash. In this case, the balls are sized to match the ballscrew and housed in the nut to achieve near-zero backlash conditions and meet application requirements. Backlash can be reduced to 0.0002" (5 microns) when using this method.

Ball screws are typically 90% efficient. The downside to this higher efficiency is the back drive. Backdrive is caused by a load pushing the screw or nut axially to create rotational motion. Ball screws require brakes to prevent the system from back driving and maintain their position when power is turned off.

If back driving is a consideration because the system needs to self-lock during a power outage, there is back pressure in the system, or a static load condition exists, then a lead screw is a better choice. Lead screws with efficiencies below 50% are self-locking, eliminating the higher cost of brakes needed to prevent shaft rotation. Space constraints may also prohibit the installation of brakes in the design.

In some applications, it is necessary to back-drive the assembly. In this case, a ball screw or lead screw assembly with greater than 50% efficiency will provide this function.

The number one cause of bearing and ball screw failure is lubrication. Insufficient or improper application – or simply using the wrong lubricant can cause actuator failure. External influences that affect lubrication, such as contamination, can also lead to lubricant degradation. Therefore, the type of lubrication should be carefully considered, including thickener, viscosity, and application.

Consult an application engineer when making lubrication selections. A variety of industry-specific options are available, including medical, aerospace, and semiconductor grades. Dry lubricants such as PTFE and ceramic coatings can be used on lead screw assemblies that use plastic nuts when the use of grease is not permitted or should be avoided.

Most linear actuators have a safety factor in the design load rating. A typical safety factor is 1.5 to 2.0, but the required safety factor may vary by application.

- What is the maximum load?

- What is the top speed?

- Does the maximum load and speed occur at the same time, or does the linear actuator only experience higher loads at slower speeds? (It helps to understand that linear actuators aren't overdesigned for situations that don't happen at the same time.)

- What is the duty cycle? (If it's very low or very high, this will affect the size and components used in the actuator.)

It is easy to design for a higher safety factor, but doing so increases the cost considerably.

All of the previously mentioned considerations are directly related to the cost of the linear actuator. Review the requirements to determine which are critical to the application and successfully meet the goals of each axis. It may be worth customizing your actuator for the features and benefits you need so you don't pay for something you don't need, or worse, don't get what you need.

Are linear actuators designed for static or dynamic load ratings? If the load is dynamic, does that apply to all speeds? Maybe not. If plastic nuts are used, the Pressure Velocity (PV) may be exceeded. In this case, a different nut material may be required, such as higher PV plastic, bronze nuts or ball nuts. Angular contact bearings can increase the axial thrust rating.

- Static Load - The maximum thrust load (including impact) that should be applied to the retaining nut assembly.

(Note: Static loads can be reduced by selecting additional end-finishing options and screw-mounting hardware.)

- Dynamic Load - The recommended maximum thrust load that should be applied to the screw and nut assembly while it is in motion.

Stroke length is often overlooked. Depending on the unsupported length, the rotational speed of the screw or ball screw should not exceed the critical speed of the shaft. Critical speed is the rotational speed in RPM that excites the natural frequency of the lead screw. Resonance occurs at the natural frequency regardless of lead screw orientation (vertical vs. horizontal). Critical speed can be affected by lead screw straightness and assembly alignment. The maximum recommended speed is 80% of the calculated critical speed. Proper selection of end support, diameter, and screw lead will improve results.

The operating environment is a key consideration when selecting the correct linear actuator for your application. Harsh conditions, dust, and dirt can seriously affect the service life and may render the actuator inoperable. Linear actuators with a covered design or optional bellows will protect the guide system or ball screw assembly. Sealed bearings should also be used in these cases. The lubrication of these parts can attract debris from the environment. It's a brilliant idea to protect the rolling elements so they continue to move freely.

Wide temperature ranges or extremely cold temperatures can also affect actuator life and performance. Climate-compatible materials will ensure proper functioning. High humidity applications can also cause problems for linear actuators. If these conditions exist, make sure the selected linear actuator is rated to withstand the factors.

Rails for linear actuators can take many forms. Rolling element linear bearings, traditionally known as profile guides, are one of the most commonly used guide systems due to their load-carrying capacity and ease of installation. Rolling element linear bearings have two load capacity specifications, dynamic load capacity, and static load capacity.

Even after determining the load requirements, attention should still be paid to the orientation of the linear actuator in the application (horizontal, vertical, inclined). Loads can be on the axis of the guide system or offset to create a moment load condition. Determines whether the load is centered or offset on the actuator's shaft, creating a moment load (rotational force) on the system. If the load is deflected, what is the reaction force produced? Pitch, Roll, and Yaw are the three rotational forces around the X, Y, and Z axes. Each of these has its limitations.

Consider how the linear actuator is mounted. If the linear actuator is used vertically, the rolling rotational force on the carriage may be pitching when the linear actuator is used horizontally. The pitch force on a linear actuator used vertically may be the yaw force if the same actuator mount is rotated 90 degrees.

If higher loads are required, increasing the rail size, adding a second bracket block, using a longer bracket block, or changing the mounting orientation can all help keep the assembly from exceeding the maximum load limit. Consult a Helix Applications Engineer to determine the best linear actuator rail system and mounting for your application.

These are the steps and considerations for determining and selecting a linear actuator. If you want to order linear actuators, please contact us.

UG Controls is a professional custom valve actuator manufacturer. We use our engineering expertise and industry experience to continuously improve our products, striving to provide efficient solutions and competitive prices. UG is also a global supplier of highly engineered actuators and accessories to the Oil & Gas, Mining, Chemical, Pharmaceutical, Water & Power, Food & Beverage, and general industrial markets.