For engineers designing systems requiring linear motion, the choice between rod type and rodless type actuators can be an important decision. Rodless actuators are more sophisticated and expensive than conventional rod type actuators, but also offer an array of advantages that affect system performance.
Understanding the inherent capabilities of the technology and specifying a solution that is best suited for a particular application is the key to a successful application of rodless cylinders.
Many engineers are more familiar with rod type actuators where the moving element or piston rod extends out of and retracts back into the body of the actuator. With rodless technology, the travel length or stroke is contained within the actuatorís overall length. This difference, while defining the fundamental distinction of the two technologies, is only the starting point.
The following are a series of design tips, application parameters, and technical points to consider when evaluating and specifying rodless or rod type actuators.
A rod type actuator requires the rod to extend out of the actuator body to achieve stroke. This requires space in addition to the actuator footprint to operate. With rodless actuators, space saving advantages of 20-50% in stroke lengths greater than 6 inches is common. For example, a typical rod type actuator with a 2-inch bore and 8-inch stroke will have an overall extended length of 21.75 inches. The equivalent rodless actuator overall length is only 14.3 inches because the stroke is contained within the actuator. This results in a 34% space savings.
Rodless actuators, in most cases, are constructed with some type of guide and support means. Bearing elements such as roller, ball or sliding are typically included within the actuator. Rod type actuators will usually include no provisions for guiding or supporting loads, which require the engineer to add these elements to the equipment configuration. Not only does this add complexity and expense to the system, it requires additional design time for the engineer.
Rodless actuators are designed as a self-contained structure.In many cases, the actuator becomes an integral part of the equipment. For example, in a door opening application, rod type actuators may require additional wheels or guides to support the door, whereas a rodless actuator may support the door directly, becoming an integral part of the equipment.
Long stroke applications
Rodless actuators excel in applications requiring long stroke lengths. Actuator strokes up to 24 feet are commonly available in standard product. Depending upon the type and manufacturer of the rodless actuator, strokes as long as 60 feet are achievable. Rod type actuators begin to lose their effectiveness in applications requiring long strokes. Unless the rod is sufficiently supported in some manner, accelerated seal and bearing wear will occur, leading to premature leakage and failure. With rodless actuators, guidance and support of the load is an integral part of the cylinder.
Equal force output in both directions
Rod type actuators have unequal effective areas, rendering the extension output force different than the retract output force. For example, a 2-inch bore rod type actuator with a 5/8 inch rod diameter exerts a theoretical force of 314 pounds of force at extension, but only 283 pounds of force at retraction at 100 PSI.This is a 9.8% reduction in force.If the actuator was specified with a 1-inch diameter rod, this reduction increases to 25%.
Rodless actuators have equal effective areas in both directions. For force and speed control, this means that both directions of travel behave and subsequently are controlled identically. Compare this attribute to rod type actuators where engineers must account for the differences in output force. This net effective area may be a concern if ignored, especially where speed control is required.
For design engineers using rod type actuators, there is often a need to use two pressure regulators, set differently, to assure equal force output as required. Rodless actuators eliminate this condition.
Greater control of moving elements
Another area of comparison between these two technologies is the mass of the moving elements. With a rod type actuator, particularly in longer stroke applications, the weight of the piston rod may become a factor. Moving and, perhaps more importantly, stopping the actuator requires careful consideration in the design of the system to accommodate the increased mass. With a rodless actuator, the mass does not change regardless of stroke length since the mass of the carriage mechanism and piston remain constant.
Another important factor often overlooked in actuator applications is deceleration. This requires an analysis of the mass of the load, including the mass of the piston rod, to adequately determine stopping distance and necessary deceleration devices. When applying auxiliary deceleration devices such as hydraulic shock absorbers on a rodless actuator application, the moving mass remains constant independent of stroke.
Both rod type and rodless type actuators may be mounted horizontally, vertically, inclined or upside down. However with rod type actuator applications other than vertical, careful consideration of load forces due to rod mass (in addition to application forces) is necessary to address moment loading. Because of the load supporting characteristics of rodless actuators, they accommodate flexible mounting configurations in addition to higher moment capabilities.
Rodless or rod type
When space savings, load guiding ability, equal force output, speed regulation, and weight savings are design factors, rodless actuators (pneumatic or electric) are often the optimum choice.There are many applications where rodless actuators can improve machine performance while making a positive contribution to overall weight and footprint of the equipment.
Understanding the differences of each technology puts the engineer in a position to make the best choice.