Rodless Cylinders

Rodless cylinders were originally developed to overcome the space limitations of conventional actuators, providing relatively compact, space saving devices, capable of moving or positioning components or tools in assembly and automation systems.
To understand the advantages of a rodless cylinder, it is worth firstly considering a conventional cylinder with, for example, a stroke length of 500mm.
In the retracted position the length of the cylinder is 500mm, plus the length of the end caps, piston, nose bearing and piston rod, giving a total length of around 600mm; when the cylinder is fully outstroked, this length will increase to 1100mm. In many applications, however, such as compact automation assembly machines, it is impractical to incorporate a device with such a large footprint.
By comparison, a rodless cylinder of similar stroke has a length of only 600mm, enabling it to perform the same function as a conventional cylinder but without compromising on the space available.
As the name implies a rodless cylinder has no piston rod, being constructed with a slide table assembly mounted directly above the piston. The table and piston are connected either directly, using a slot cut along the top of the cylinder body, or indirectly, through magnets located on both the piston and table.
Directly Coupled Rodless Cylinders
The latest generation of directly connected rodless cylinders, such as SMC’s Series MY, make extensive use of aluminium and special resins and plastics to ensure that the inertial effects of the moving table assemblies are as low as possible and to improve overall performance.
Unlike earlier rodless cylinders, where connection between the table and piston was through a simple slot, with a rubber seal closing the area behind and ahead of the table as it moved, the latest rodless cylinders use a sophisticated seal belt, which reduces both air loss through the slot and maintains frictional losses to a minimum as the table slides.
The slot and rubber seal arrangement used in earlier rodless cylinders often meant that air losses could be as high as thirty per cent of the pressure applied, while dirt and moisture inevitably penetrated the piston barrel, resulting in reduced performance and product life.
To overcome these major problems SMC developed the mechanically coupled Series MY of rodless cylinders, where the extruded aluminium cylinder barrel has an open slot running along its length. The piston and slide table assembly are directly coupled by a yoke that extends from the piston through the slot to the base of the table and a stainless steel support ring, between the top of the yoke and the table. The cylinder barrel incorporates table guidance mechanisms within the extrusion, to reduce the overall dimension of the cylinder, plus slide bearing surfaces to ensure smooth operation with a minimum of friction.
A belt is then used to seal the slot as the table assembly moves. Unlike previous rubber seals, however, this is manufactured from a high performance resin and is carefully moulded to fit the exact profile of the slot in the cylinder barrel. Perhaps as importantly, the belt seal is automatically guided through a shaped groove in the top of the piston; this effectively breaks and re-closes the seal as close to the table assembly as possible, thereby reducing air losses to an insignificant level. (See diagram below).
Dust and other contaminants are prevented from entering the cylinder barrel by an NBR coated stainless steel dust band that passes over the top of the piston at the point where it locates in the base of the table. In addition, scraper seals are used to keep the slide area free of debris, to ensure that lubricants remain evenly distributed and to help guide the belt seal and dust band.
Directly coupled rodless cylinders can be mounted in a wide range of positions: vertical, horizontal, inverted and at an angle. They are capable of handling heavier loads and withstanding greater side forces than magnetically coupled devices (see below) and eliminate the possibility of the table becoming disconnected from the piston by a sudden shock or abrupt halt.
Indirectly Coupled Rodless Cylinders

In applications where air losses must be eliminated or where lighter loads are to be moved, magnetically coupled rodless cylinders can be used.

In these types of device, the piston and table assembly both incorporate powerful magnets, manufactured from Neodymium Ferrous Boron, a material with magnetic properties ten times greater than traditional permanent magnets. As a result, the direct connection, slot and seal arrangement in directly coupled rodless cylinders can be dispensed with completely. (See diagram below).

The cylinder barrel and table assembly retain the same integral guide mechanisms and slide and bearing surfaces as directly coupled devices, to produce a smooth, low friction movement. Internal air cushions can also be used to decelerate the table gently at the end of each stroke.

Magnetically coupled rodless cylinders are ideal for applications where relatively light loads need to be moved at low or medium velocities. They can, however, only be used in a horizontal or close to a horizontal plane, and are limited in operating speed by the strength of the magnets used to hold the table in position. In addition, for loads that project significantly beyond the edge of the table, it may be necessary to incorporate extra guides outside the envelope of the cylinder.

Typical Applications

Rodless cylinders were originally developed for use in all areas where space is restricted; they have, however, since become used in a wide range of applications, as they provide a simple, compact and reliable method of transferring, moving or positioning components or workpieces.
Typical applications include: Assembly automation, Packaging, Electronics, etc

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