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A rodless cylinder is a pneumatic component capable of moving a load in a linear path with compressed air. Whereas a traditional pneumatic cylinder uses a rod to push or pull the load from the piston, a rodless cylinder moves the load alongside the piston.
This article will provide you with all the relevant information on rodless air cylinders, what they are, how they work, and a buying guide!
Table of Contents
- What is a Rodless Cylinder?
- Types of Rodless Cylinders
- Rodless Cylinder Applications
- Rodless Cylinder Buying Guide
- FAQs (Frequently Asked Questions)
What is a Rodless Cylinder?
Rodless, cable, band, or magnetically coupled air cylinders are types of air cylinders that are selected where a footprint issue prevents the use of a traditional lower cost, but much longer footprint, rodded type air cylinder.
An 8″ stroke tie-rod type cylinder will be more than 16″ long when the rod is in full extension, and likely quite a bit more when the end caps and cylinder mounts are taken into consideration.
A meter-long rodded air cylinder will need more than 2 meters of footprint for installation. Allowance must be made for the rod to extend from the end of the cylinder to its full reach. Sometimes there just is not enough room on a machine for that overall length.
An application may also need to have a cylinder load move within the overall cylinder length. That cannot happen with a typical rodded cylinder. Perhaps there is a need to use an air actuator to move a load that is some distance away from the cylinder itself.
All of these types of scenarios have one solution in common; the rodless air cylinder. Whatever your application calls for in terms of rodless cylinders, here is more information about your options.
Types of Rodless Cylinders
There are three main rodless cylinder types, which all refer to how the piston is connected to the carrier to allow movement of the load:
- Rodless Band Cylinders
- Cable Cylinders
- Magnetic Cylinders
Rodless Band Cylinders
Band cylinders, also known as slotted cylinders, are so named as it is a type of zip-locked band that keeps the compressed air within the barrel of the cylinder to do the work, even though the cylinder carriage on the outside of the cylinder barrel is mechanically connected to the piston on the inside.
The carriage of a band cylinder is propelled along the outside length of the cylinder barrel as the piston assembly inside reacts to flowing compressed air.
The linkage that connects the cylinder carriage to the piston assembly travels along with a slot that runs the full length in the band cylinder barrel.
Contained within this linkage is a device that separates the two band seals, one of which is on top of the barrel slot, the other seal which is inside the cylinder barrel.
How Do Rodless Cylinders Work? – Band Cylinders
So how do rodless cylinders work? The graphic below is meant to illustrate the concept of how the rodless air cylinders work, not to present an accurate reproduction of the engineering involved. The actual cylinder construction is quite a bit more complex than as shown.
As the carriage moves, the two band sealing strips are alternately opened in front and then closed behind the moving carriage, regardless of the direction of travel.
The seals on the pistons inside the cylinder barrel press the inner band seal tight against the barrel of the cylinder, preventing air from leaking out there.
The carriage on top of the band cylinder will have a wiper assembly at either end which will both remove any debris from the carriage path, and press the top seal tightly against the outside of the slot in the cylinder barrel, stopping any compressed air from escaping there.
Even though there is a perpetual hole created where the bands are separated to allow the carriage/piston assembly to move, clever engineering design keeps most of the compressed air inside the cylinder to do work.
Common components of a band cylinder are:
- end caps
- cylinder barrel
- cylinder piston
- mechanism for connecting carriage to piston
- sealing bands / strips
Band Cylinder Benefits
One of the many benefits of band cylinders is that the cylinder barrels can be sized appropriately to carry a load.
If the band cylinder barrel/carriage is of sufficient size, or if the barrel comes with integral bearing rods to sufficiently support the side load, the tooling from the carriage can cantilever well off to one side of the cylinder, as well.
For example, a lengthy band cylinder could be installed adjacent to a non-motorized conveyor. When it was time to move an item along the conveyor, an arm could extend from the band cylinder to intersect the item and move it along.
If the band cylinder were installed with the tooling extending forward from the carriage, the tooling can move in and out of enclosed spaces. I have seen band cylinders used in this manner for parts pick-and-place from injection molding machines.
Two band cylinders connected carriage to carriage can provide a reasonably priced X-Y actuator for work that requires these axes. Add a third cylinder to these two in a vertical plane, and you have an X-Y-Z axis machine for fairly precise pick and place, or to lay down a glue bead in a given pattern on a workpiece, another application I was involved in.
Band Cylinder Mounting Style
Band cylinders are mounted in a variety of ways, with end cap foot mounts being most common for single cylinder installation. As the barrel length of the band cylinder increases, there will be a need for incremental barrel supports, the spacing determined by the load and the style of the barrel.
If the load to be moved is large, or needs to be cantilevered far off the centerline of the carriage, a smaller band cylinder can be coupled with external slide rods that can widen the center of gravity and virtually remove the load from the band cylinder carriage entirely. This is the best of scenarios for a band cylinder, as even though (depending on the manufacturer and style) they can themselves carry loads, band cylinders are prone to rapid wear if the load is not well within the design parameters of the cylinder.
Rodless Cylinder Proximity Switch Ready
Band cylinder barrels are now typically manufactured with a slot into which proximity switches can be installed for position sensing.
It is fair to say that all band cylinders leak, with some manufacturer brands leaking more than others.
If a band cylinder is the right actuator for your application, part of the cost of doing business with this style is that compressed air will be consumed not just by doing the work but by bleeding to the atmosphere through leaks. Band cylinder sealing strips leak when they are new, and leak even more when they are worn.
Cable cylinders are pneumatic air actuators (cylinders). They are rodless air cylinders too but don’t use a band-type sealing method as band cylinders do.
Rather than having a piston rod sticking out one or both ends of the air cylinder-like standard rodded air cylinders, cable cylinders use a cable that enters either end of the cylinder barrel and is attached to either side of the internal piston.
How Do Rodless Pneumatic Cylinders Work? – Cable Cylinders
So, how do rodless pneumatic cylinders work? The cable exits the cylinder barrel through the cylinder end caps, winds around a sheave or pulley on either end of the cylinder barrel, and joins outside the cylinder barrel in a non-supported carriage.
When compressed air enters the cylinder the piston moves from end to end. The cables which are attached to either side of the piston and extend out the ends of the cylinder, move as well. Depending on the direction of the piston both the carriage and any on-carriage tooling move towards one end of the air cylinder or the other.
The barrel of the cable cylinder is usually round, though, as long as the piston would move easily, there’s no reason why it couldn’t be elliptical or octagonal.
Barrels can be made of steel, aluminum, or composite materials and can be many feet in length. As the length of the barrel increases, this creates a need to provide barrel supports to ensure that the cylinder barrel stays straight.
This is an end cap in the center of which is a hole through which the cable passes from the end of the cylinder. The cable will pass through the hole, and seals at the end around the cable will help ensure that compressed air doesn’t escape easily.
Cable cylinders are notorious for leaking through these seals as if there’s ever any sideloading of the cable it would tend to wear these end cap seals quickly.
The end caps can be cushioned to decelerate the piston and the load. Depending on the load, however, it may be better to have the carriage and load stopped externally with shock absorbers or a fixed stop than using in-cylinder cushions.
This is the sheave or pulley which is attached to the end cap on both ends of the cable cylinder. The cable, attached to both sides of the piston inside the cylinder, travels through the end cap gland seals, around the sheave, and is ultimately attached to a carriage.
This depicts the cable. One of the significant benefits of the cable cylinder is that the cable can be relocated elsewhere via a series of sheaves, and, when the piston inside the cable cylinder barrel moves with incoming compressed air, the carriage will move regardless of how far away from the cylinder it travels. The carriage travel could even move tangentially to the orientation of the cylinder itself.
Depending on where extra pulleys are placed the weight moving capacity of the cable cylinder can be increased or lessened, and the carriage travel distance can be lengthened or shortened even as the piston strokes full length inside the barrel.
The carriage on a typical cable cylinder is fairly light-duty, meant only to attach the carriage to a load with a couple of bolts. In a typical cable cylinder, the carriage is basically unsupported. Therefore, the use of rails and bearings to carry and align the load, and the use of the cable cylinder carriage only to provide only linear motion, is normal.
If the cost of providing carriage support is higher than expected, consider using a band cylinder that, depending on the bore diameter, has integral support.
This depicts the piston. Different manufacturers offer different piston designs, yet all will have piston seals, and ‘O’ ring or ‘D” ring, or multiple rings around the circumference of the piston.
There may also be a magnet to allow piston position sensing, though, as mentioned before, you will need to get creative as to where to position the prox switches.
Cable Cylinders Benefits
The carriage moves within the length of the cylinder barrel. The benefit of this is space-saving when installing the air cylinder.
A typical NFPA type cylinder has a cylinder rod that protrudes from one or either end of the cylinder. The overall length of the cylinder will more than double the length of that rod. A 12″ NFPA cylinder will require an installation footprint of about 28″ depending on the manufacturer. A 12″ cable cylinder may need only a 16″ footprint.
Through the use of creative cabling, the cable cylinder allows the carriage that is being driven by that cylinder to be located some distance from the actual barrel and piston, unlike almost any other kind of air cylinder.
Another significant benefit of a cable-type air cylinder is its relatively low cost per inch of stroke.
Magnetically Coupled Rodless Cylinders
Magnetically coupled rodless cylinders are those where the “connection” between the piston inside the cylinder tube, and the carriage on the outside is made via magnets.
These types of cylinders are excellent in wash-down applications as moisture cannot enter the cylinder tube. They are somewhat limited in the load capacity of the carriages due to the relatively weak bond between carriage and piston.
Magnetically coupled cylinders are air cylinders that have been designed to fill a specific need! For some reason, the application will not allow a normally-connected air cylinder with the rod.
Unlike other types of rodless air cylinders, like band cylinders or cable cylinders which have the cylinder carriage mechanically attached to the piston, there is no mechanical connection between the external carriage and the piston inside the cylinder in a magnetically coupled air cylinder.
Instead, the magnetically coupled rodless air cylinder has a strong rare-earth magnet as a component of the cylinder piston. Another similar high-strength magnet will be part of the cylinder carriage.
The attraction between these two magnets is what allows the carriage to be propelled along outside the cylinder barrel without any mechanical connection between the two.
In the drawing above I focus on the piston inside the cylinder barrel, conceptualizing the magnet built-in as part of the piston assembly.
Each manufacturer of magnetically coupled air cylinders will have its own design, of course. This drawing is to show concept, not engineering details.
The sketch below shows the carriage on the cylinder to the left, and then an end view of the carriage, with the magnet as part of the carriage itself.
Coupling Force in a Magnetically Coupled Cylinder
Depending on the manufacturer, there may be a single magnet design, with one magnet only as part of the piston and in the carriage, or a different brand may have multiple magnets in each of the piston and carriage. Each design is unique to that manufacturer and to the design in coupling force.
A smaller magnetically coupled cylinder will have less coupling force due to only having room for smaller magnets. The design may try to overcome this by having more than one magnet in the piston and the carriage.
Magnetically Coupled Cylinder Mounting Styles
Mounting styles of the magnetically coupled cylinders vary too. The simplest is the cylinder with a threaded tang on each end. These tangs fit through a hole in another piece of equipment, and then a jam nut is threaded onto the tang which clamps the cylinder in place.
Different styles from different manufacturers offer variations on plumbing compressed air to the magnetically coupled cylinder, some having airports, in the end, others on the sides of the end caps.
How you need to install the magnetically coupled cylinder in your application might determine which manufacturer you acquire the cylinder from, as not all have the cylinder mounting style that may be needed.
Magnetic Cylinders Benefits
Benefits of the magnetically coupled cylinder design include their design being leakproof, making them ideal to use in areas where compressed are leaks would be a problem. They can also be used for low-pressure hydraulic service.
Another great benefit of magnetic cylinders is that the re-coupling of the carriage to the piston simply requires moving the carriage to where the piston is located inside the barrel, making the process easy.
But of course, there are some drawbacks which include the high-speed carriage movement combined with rapid or hard deceleration can uncouple the carriage from the piston. The carriage has no mechanical connection to the cylinder itself, therefore, an external load and carriage guidance system are almost always required in these designs. And, they tend to be higher cost solutions over other rodless cylinders types.
There is no question that, in load-moving applications, the only solution is the magnetically coupled cylinder. Knowing this style of air cylinders is available may help make the machine design a little less complex.
Rodless Cylinder Applications
Rodless cylinders of all types, due to their many advantages are used in a wide variety of industries. Some of their most common applications include:
- Commercial sewing
- Food and beverage
- Optical sorting
- Spray painting
- Tire fabrication
- Warehouse lifting
Rodless Cylinder Buying Guide
What do you need to know to select the appropriate band cylinder for your application? Here is a checklist you must ask yourself:
- What is the weight and size of the load to be moved?
- Where will the load be in relation to the center of the carriage?
- What is the distance the load will be moved?
- What is the speed required in distance per second?
- What will stop the load/carriage at the end of stroke?
- Is position sensing required?
When selecting a rodless cylinder, other than the types that were previously described, you should also consider the following criteria to make sure you select the right one for your tasks:
- Air pressure: As the piston is driven by compressed air, there must availability of constant air pressure throughout the application.
- Bore Size: Bore size can be chosen according to the capacity range required for the application. For a cylinder to perform at maximum capacity, a larger bore size should be considered.
- Carrier Load: The carrier must be able to support and move the mounted load safely and efficiently.
- Cylinder length: This is the actual working stroke distance – the distance that the load will travel.
- Dead length: The length of cylinder that cannot be used due to internal components and room required for end-of-stroke. This is required to determine the working cylinder length required for application.
- Stroke speed: This is the speed at which the piston moves within the cylinder. This would depend upon the air pressure supplied at each port and the load.
Notable Rodless Cylinder Manufacturers
Some notable manufacturers to look out for are:
- Festo rodless cylinders
- Parker rodless cylinders
- Tolomatic rodless cylinders
- Miller rodless cylinders
- SMC rodless cylinders
- Numatics rodless cylinders
- CKD rodless cylinders
- Norgren rodless cylinders
FAQs (Frequently Asked Questions)
Rodless cylinders are commonly used in pneumatic applications requiring a compact installation with a wide range of stroke capabilities. They are most often used for automation applications that require positioning and can also run full stroke to index tooling, or position objects in material handling applications.
A piston driven by compressed air moves within a cylinder bore, very much like a conventional profile or tie-rod cylinder. The key difference is that motion is transmitted to the outside through the body length, rather than by a piston rod through an end cap.
Rodless actuators are devices used in many pneumatic applications that require a compact installation, offering a wide range of stroke capabilities. They have their the internal piston connected to an external carriage, by means of a magnetic or mechanical coupling system. Rodless actuators differ from a normal air cylinders in that no piston rod extends outside the cylinder body.
Linear cylinders work by moving an object or piece of equipment in a straight line, allowing you to move an object extremely accurately and repeatably if necessary. The primary reason for designing a linear actuator into a system is for the need to move a payload in a linear fashion rather than a rotary one.
If you have any questions regarding rodless air compressors, please leave a comment below, with a photo if applicable, so that someone can help you!