Rodless, cable, band or magnetically coupled air cylinders are types of air cylinders which 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 a 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:
And then there are band cylinders, information on which is just below.
Rodless Band Cylinders
Band 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 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.
The graphic above is meant to illustrate the concept of how the rodless cylinder works, 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 of 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 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 appropriate 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 none-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 axis. 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 work piece, another application I was involved in.
Band Cylinder Mounting
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 center line 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 manufacturers 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 atmosphere through leaks. Band cylinder sealing strips leak when they are new, and leak even more when they are worn.
Band Cylinder Selection
- 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
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 which enters either end of the cylinder barrel, and is attached to either side of the internal piston.
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.
More on that here: Magnetically coupled air cylinders