Subject:Rodless air cylinders.

Rodless air cylinders.

Postdate:2009-08-13 07:03:13   Hits:4741

Rodless air cylinders.


An 8” stroke tie-rod type cylinder will be at least 16” long when the rod is in full extension, and likely quite a bit more when the end caps and mounts are taken into consideration.

A meter (3’ ) long rodded cylinder will need more than 2 meters (6’) of “footprint” for installation given that allwance must be made for the rod to extend from the end of the cylinder to it’s full reach.

There may the need to have a cylinder move a load within it’s own length?

Perhaps there is a need to move an item that is a remote distance away from the cylinder itself?

All of these types of scenarios have one solution in common; the rodless air cylinder.

And whatever your application calls for in terms of Rodless cylinders, here’s information about a variety of rodless cylinder options:
 




Band Cylinders

Band cylinders are so named as it’s a “zip locked” band that keeps the air within the barrel of the cylinder, even though the cylinder carriage on the outside is mechanically connected to the piston on the inside.

The carriage is propelled along the outside length of the cylinder barrel by the piston assembly inside.

The linkage that connects the cylinder carriage to the piston assembly travels along a slot that runs the full length in the top of 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.

’Band

As the carriage / piston assembly is driven along inside the rodless cylinder by the compressed air , the two band sealing strips are alternately opened in front and then closed behind, 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 press the top seal tightly against the outside of the slot in the cylinder barrel, preventing air from escaping there.

So even though there's 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.

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.

Common components of a band cylinder are:

  • end caps
  • cylinder barrel
  • cylinder piston
  • carriage
  • mechanism for connecting carriage to piston
  • sealing bands / strips

One of the many benefits of band cylinders is that the cylinder barrels can be selected to be load bearing. If the barrel is of sufficient size, it can be the load-bearing link between other stationary bearing surfaces within the machine’s infrastructure.

If the 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

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 carriage of the band cylinder to intersect the item, and then move it along without the cylinder itself interfering with the rollers of the conveyor.

If the band cylinder were installed with the tooling extending forward from the carriage, the tooling can move in and out of enclosed spaces. We’ve 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.

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.

Most band cylinder barrels are now manufactured with a slot into which proximity switches can be installed for position sensing.

It’s fair to say that all band cylinders leak, with some manufacturer's brands leaking more than others. If a band cylinder is the ideal choice for the 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.

What do you need to know to select the appropriate band cylinder? Here is a check list:

  • 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


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