Rodless Cylinders Information
Rodless cylinders are linear devices that use pressurized fluid to move a load within power transfer operations. A rodless cylinder should be used if the footprint of the area is small, when the load needs to be moved some distance from the cylinder itself, and when the load must move within the length of the cylinder.
They are suitable for long-stroke applications because they are protected from bending, piston binding, and uneven seal wear. Rodless cylinders are used in a variety of material handling, loading, feeding, lifting, and web cutting applications. They are also used in sliding carriers, conveyors, and spraying equipment. They are made of aluminum, steel, stainless steel, or plastic and driven by an electric motor, pneumatic or hydraulic assembly, or electrohydraulic pump.
Electrical - Electrical actuators uses electrically driven motors to drive the actuator to a desired position using an outside power source.
Hydraulic - Hydraulic actuators convert fluid pressure into motion in response to a signal. They can be used when a large amount of force is required to operate a valve.
Pneumatic - Pneumatic actuators adjust valve position by converting air pressure into linear or rotary motion
Electrohydraulic - Electrohydraulic actuators are self-contained electrically driven hydraulic pumps used to drive cylinders or actuators.
For more information on linear actuators, see Engineering360's Linear Actuators guide.
Guided cylinders are directed by attached rods or rails.
Unguided cylinders are directed by external hardware associated with the application.
Types of Coupling Styles
There are several basic coupling styles for rodless cylinders.
Band cylinders or direct-coupled cylinders are connected to the cylinder flange with a table, stage, or other moving element. Their name is derived from the zip-locked type band that keeps compressed air within the barrel of the cylinder. The carriage, which moves the load, is on the outside of the cylinder barrel and is connected to the piston on the inside. In this style, the carriage is moved along the outside length of the cylinder barrel as the piston assembly inside reacts with the actuation method. The piston assembly runs the length of the band cylinder barrel. Two band seals are present; one is located on top of the barrel slot and the other is located inside the cylinder barrel. When the carriage moves, the two band sealing strips are alternately opened in front and closed behind the moving carriage. This happens regardless of the direction of travel. An advantage of band cylinders is that the barrels can be easily sized to carry a load. Plastic or stainless steel sealing bands are used along the axis of travel.
Rodless cylinder. Video Credit: Gimatic
Magnetically-coupled rodless cylinders are moved by a magnetic field and enclosed to prevent the ingress of contaminants. The carriage is not mechanically attached to the piston inside the cylinder. The carriage is attached via a magnet on the piston in the rodless cylinder and a similar high-strength magnet will be part of the cylinder carriage. The attraction between the two magnets allows the carriage to be propelled along the outside of the cylinder. This design prevents contaminant entry into the cylinder. It is important to select a cylinder which will provide the correct coupling force between the carriage and the piston to support the mass of the load and the speed the load needs to be moved. The advantages of magnetically-coupled rodless cylinders include their compatibility with low pressure hydraulic services and that the re-coupling of the carriage to the piston simply requires placing the carriage over the location of the piston inside the cylinder. The disadvantages include a higher cost, and uncoupling at high speeds or hard deceleration.
Magnetic cylinder. Image Credit: All About Air Compressors
Cable cylinders pass cables from the ends of a gland seal to a pulley that connects to the carriage. In turn, the carriage is attached to the cable from the opposite cylinder end. When the actuation mechanism (i.e. compressed air) enters the cylinder, the piston and the attached cables move from one end of the cylinder to the other. The direction of the carriage depends on the direction of the piston. Cable cylinders save space, and the carriage can be located some distance from the actual barrel and piston. This is unique to the cable cylinder. Cable cylinders are also inexpensive per inch of stroke.
Componets of a cable cylinder. Image Credit: About Air Compressors
Specifications for rodless cylinders include maximum stroke, carriage load, operating pressure, operating temperature, bore size, breakaway pressure, and mounting style.
Maximum stroke is the maximum distance that the shaft travels from the fully retracted position to the fully extended position. This will determine the length of the cylinder and therefore the area needed for the actuator.
Carriage load is the load carrying capacity that the system requires. The cylinder system should be able to safely and efficiently move the actuated load.
Operating pressure and operating temperature are full ranges that will be present in the area surrounding the rodless cylinder.
Bore size is measured in English units such as inches (in) or fractions of an inch, or metric units such as millimeters (mm) or centimeters (cm).
Breakaway pressure is the pressure at which rodless cylinders slip or break away.
Cylinder length can be many feet in length. As the length of the cylinder increases, more supports need to be added to ensure that the cylinder stays straight.
There are several mounting options for rodless cylinders. The mounting option should be selected based on the design of the cylinder, load specifications of the mount, and the environment (i.e. surrounding space and conditions) the system will be placed in.
Clevis mount. Trunnion mount. Lug mount.
Clevis or eye attachment connects the cylinder to the extended end of the piston via threads.
Double end mounts have a nose and rear which contain threaded bosses for nut attachment.
Flange mounts are brackets placed on the cylinder
Floating mounts are for more convenient installation of the cylinder.
Foot brackets are flanges that rest underneath the cylinder.
Lugs are short blocks with holes that attach to the side of the cylinder and allow mounting to another surface.
Face mount have threaded holes on the front face for attachment.
Nose mounts are threaded for mounting through a hole with a nut on the other side.
Rear mount have tapped holes or mounting flange on the rear.
Threaded mounts are designed to mount to bolts via threaded holes in the cylinder head or cap.
Trunnion mounts are specially designed mounting blocks which can be located at the cap, end, or an intermediate location along the cylinder.
Many products provide special features.
Air/oil tandem provides smooth, hydraulic-like motion. This combination is only used in pneumatic cylinder.
Bumpers can cushion the impact at the ends of a stroke.
Linear position feedback can be analog or digital to provide continuous output of position.
Home, limit, or position switches have switched outputs that can limit travel.