Selecting an electromechanical actuator (EMA) requires understanding the parameters for what you need it to do. However, there are only two basic types: ram-style and rodless. There are strengths and weaknesses to both and defining your operating conditions will help clarify which will be the appropriate choice for your needs.
In ram-style units, a motor turns a threaded screw, which causes a mating threaded nut to travel the length of the screw, causing the inner ram to move in and out of the actuator housing. These actuators are well-suited for moving heavy loads in harsh environments while still offering high accuracy and notable reliability. They are commonly used for high inertial loading applications where the loads are being pivoted or tipped. These actuators also have the flexibility to move quickly as well.
The notable limitation of ram-style actuators is the capable stroke length. This is due to the fact that the rod is unsupported outside of the actuator housing once extended. This unsupported extension can diminish the capable column loading of the rod, and in severe circumstances, result in buckling.
Rodless actuators are better suited than ram-style for applications that require a longer stroke. Rodless actuators come in two types: screw driven and belt driven.
For the screw driven types, a motor turns the screw which causes a mating nut (attached to a mounting carriage) to travel along the length of the screw. In some rodless styles, such as Hunt Valve Actuator’s, the screw is supported at appropriate intervals, allowing higher travel speeds at longer stroke lengths. The length of the stroke is usually only limited by the available length of the screw stock material, but longer custom lengths can be achieved by joining screw sections through specialized manufacturing processes
Screw driven actuators are typically used in applications that require high loads, a high degree of accuracy and reliability, or longer strokes.
Belt driven actuators use the rotational energy of a motor to turn a pulley which drives a belt. The belts are usually constructed of a polymer composite with steel wire reinforcements. Much like the screw driven counterpart, a belt driven unit uses a carriage that is attached to the belt as a means of transferring linear movement to the load. The key advantages of a belt driven system are the capability to travel at higher speeds over longer stroke lengths than screw driven units, while still maintaining a high degree of positional accuracy.
In most cases, belt driven units are a more economical choice than screw driven units.