Choosing the right linear actuator for single or multi-axis positioning systems can be difficult for most engineers and designers. Although, rotary actuators might seem like the simpler choice in terms of design and integration, it generally is not.
Rotary Actuators vs. Linear Actuators
If traditional systems of motion control like rotary actuators are used in applications where fewer linear actuators could work, the resultant design is said to be ‘over engineered’. An over engineered design is expensive, bulky, and functionally, not as effective. It is important to choose components that complement the design and bring about the desired effect with minimum cost and effort.
If you are unsure about linear positioning systems, it is important to consult an expert who can guide you in terms of design and the actual functioning of the application. This guide is written with the intention of helping engineers look for a suitable linear motion positioning system for a device. The following steps and subsequent posts should help you in choosing the correct linear actuator for your device.
What is a Linear Module?
The term linear module is often used interchangeably with the term linear actuators. Linear modules or linear systems consist of several different components like linear actuators, tables, motors, and drives.
Choosing the correct linear actuator is no mean feat. It can help make or break the application. In the following blog posts, we will discuss the various factors that help in choosing the correct linear actuator.
Guide to Choosing the Correct Linear Actuator
The steps below will help you in choosing the correct linear actuator for single or multi-axis positioning systems:
- Application Arrangement & Outline :
This is a primary step that will dictate the outcome of the process, and outline the steps that follow. Here, you will have to decide on the number of axes of motion. In-depth knowledge of the product and the desired results will enable you to choose the correct linear system arrangement. You can choose between one, two, and three axis configurations.
- Mounting of Linear Actuators (Orientation Factors) :
Mounting linear actuators to deliver precise movement is an important step. This step is dependent on the arrangement discussed above. In the basic one axis arrangement, this step is straightforward. It gets complicated when mounting linear systems in multi-axis arrangements. The following factors need to be taken into consideration
The direction of each axis needs to be considered. It will depend on whether the different axes move in sync, or they move independent of one another.
Integration of moving carriages and rails will determine the mounting method to be used.
Depending on the movement required, the different axes can be integrated vertically, horizontally, or in an inclined fashion. The mounting of the linear actuators will depend on the direction of travel.
The angle of the mount will have to be considered in terms of the horizontal.
Here, the direction of the movement of the axes will dictate the correct mounting method to be applied in an application.
Mounting of Linear Actuators (Mechanical Factors) :
Other than the orientation of the axes, the mounting of the linear systems will also depend on the mechanical factors of the application. Below are enlisted some of the mechanical factors to take under consideration:
The mechanical properties supporting the linear system play an important role in the mounting process. The rigidity of the support system, as well as flatness of the surface makes a difference to the mounting system utilized.
The spacing between the support points also plays a role in determining the mounting method employed for linear systems.
- Multiple Center of Gravity Positions :
The center of gravity of the load that is moved keeps changing as it moves across the axes. For every position, the center of gravity with respect to the datum (home) point on the different axes needs to be calculated. As the load/ object travels across the system in accelerated or decelerated movements, its center of gravity relative to a coordinate will keep changing. So, when mounting the linear systems, the different positions and calculations of center of gravity of the object at various points with respect to all the axes will need to be calculated.
Calculating the center of gravity at various positions, in the application and averaging the calculations will assist you in employing the right mounting methods.
- Load Bearing Capacity :
Calculating the mass or geometry of the load at various points on the axes with respect to the coordinates will help determine the load bearing capacity of the system. Stationary loading factors, and rapid moment loads, are other factors to be considered when determining the load capacity. The integrated linear systems, engines, and gearboxes act as overhung loads. This is also a determinant in calculating the load capacity.
- Stroke Lengths :
The stroke length of the actuator for each axis needs to be determined to provide the right direction and magnitude of movement. The extent of movement is largely dependent on the type of actuator used. For example, when using industrial ball screw actuators, the ball screw will determine the length of stroke. The stroke length cannot exceed the length of the ball screw. On the other hand, linear actuators utilizing belt drives have no limit on the stroke length. Theoretically, the stroke length can be infinite but in practical application, the length rarely exceeds 10 meters.
- Precision & Repeatability :
Most applications that require movement have definite needs for precision and repeatability. These are two important factors during the design and conception stages. The repeatability and accuracy of every linear actuator is defined upon production. You will need to speak to your manufacturer or supplier and purchase an actuator that fits your specific requirements. Generally, linear actuators with ball screws are more precise compared to those with belt drives.
- Traverse Speed :
The ball screws and bearings used in the linear actuator are the primary determinants of traverse speed and time. Typically, belt-driven linear actuators are faster than their ball screw, and linear bearing counterparts.
- Acceleration & Deceleration Rate :
In multi-positioning systems, acceleration by itself is not a crucial problem. The effect of acceleration on the load is what needs to be taken into consideration. In any automated system where linear motion systems are used, one also needs to consider the implications of deceleration on the load. For start-stop, or pick up and drop systems, acceleration, and deceleration needs to be factored in when choosing the right linear actuator.
- Cycle Time & Fatigue Life :
The fatigue life of a component will determine the life of a system. In addition, the fatigue life is dependent on the cycle time. Higher the frequency of the cycle, more will be the wear and tear; therefore, the fatigue will be higher. Hence, the actuator will have to be chosen depending on your expectations of the life of the actuating system. For example, if you consider two systems that need to function for the next 12 years, but the cycle time of one is double than the other. We can understand that the same linear actuator will not be able to provide the same results in the same budget. Two different actuator systems will need to be employed for both the test scenarios.
- External Forces :
When designing a system, we need to factor in accidents, extreme cases, emergencies, and the human factor. In case of an emergency, a load might need to be stopped or moved to another position abruptly. To incorporate this functionality into your actuator system, the components will have to be altered accordingly.
- Environmental Factors :
The linear system will have to be chosen according to the environmental factors. You need to consider the natural atmosphere of the location, as well as the application environment. The external atmosphere includes moisture, dust, and temperature. The application environment will include the ruggedness of the application, internal temperature, chemical exposure, abrasion, corrosive environment, physical cleaning, etc. You will also have to consider the level of vibration experienced within the application. For example, the actuators used in military applications are very different compared to those used in factories, or laboratories. In medical facilities, noise levels could be a problem, so the linear actuator will have to be chosen accordingly. Depending on the environment, you will have to choose the actuator, as well as the various seals, coatings, and casings that can be used to protect the equipment from damage.
When choosing a linear actuator, it is also important to consider the different electrical drives and systems that come into play. Most automated machines utilize complex components and electrical systems. Hence, it is important to choose a linear actuator that is compatible with the various electrical systems, and components.
Information and knowledge about linear actuators will assist you in choosing the other support components as well. It will also help you in incorporating the correct mounting methods, mounting arrangements, stacking of linear actuators, integrated drives, and affixation methods.