ISO 15552 pneumatic cylinders


ISO 15552 is a specification for pneumatic cylinders. ISO 15552 specifies two classes of hydraulic cylinders for industrial use. The piston rod sealing type distinguishes the two classes: a compression seal or an end face seal.

ISO-15552-PNEUMATIC CYLINDERS

The latest ISO-15552-2:2004 specification defines standardized pneumatic and nonstandardized, nonpneumatic, mechanical, and spring-operated cylinders for fluid power systems in industry and other sectors. A compressed air cylinder offers many advantages to its applications, like being lightweight, easy to transport, and providing long service life without deterioration in the friction area due to lubrication with oil or grease.

The cylinder is defined in ISO 15552-2:2004 by methods for mechanical or pneumatic cylinders:

Cylinder Seal Types as defined in ISO 15552-2

The 2nd method of definition leaves the choice for the type of seal on individual applications. A method is described, applicable, or suitable for cylinders with a narrow piston rod seal aperture. The definition allows minimal design freedom but provides safety and reliability.

ISO 15552 is a series of international standards covering the usage of hydraulic systems (pneumatic, mechanical and electrical). It specifies parameters and test methods to guarantee that hydraulic systems meet their intended performance specifications in their intended application. It takes into account environmental protection, health, and safety requirements.

ISO 15552 is the successor to:

The latest version of ISO 15552-1:2002 is available from Avimatic (formerly Bosch Rexroth).

Pneumatic and mechanical cylinders are used for various industrial applications, but in many cases, safety and cost advantages make electric or pneumatic control systems more attractive. On safety, CPC has calculated that electric control over a pneumatic cylinder is at least five times safer than manual control. There are also many reasons why an operator may prefer to use electric power over a pneumatic cylinder:

Practical electric power systems provide a much greater degree of automation than manual systems with pneumatic cylinders. Pneumatic systems have several drawbacks.

Cylinders can fail for several reasons. The most common cause is the contamination of the oil or air fed to the system. Hydraulic fluid should ideally be as pure as possible and must not contain any solid or liquid particles or other contaminants that might damage the system elements. The cylinder seals must be free from excessive wear, and the pistons must not be excessively worn either (which may lead to premature seizure in use). All cylinders are fitted with one or more pressure relief valves that can release excessive pressure safely before damage to components causes a catastrophic failure. This, in turn, could cause injury or damage.

The stroke of a cylinder is defined as the longitudinal distance covered by the piston when it completes an entire cycle. This can be calculated using:

The stroke is also known as travel or displacement. For example, if the cylinder in a hydraulic press with a 10 mm bore has a stroke of 500 mm (or 0.5 m), the piston will move through 500 mm of its travel between fully retracted and fully extended positions. This can be described in terms of the speed at which it travels, which would be ten × 500 / 0.5 = 2500 mm/s.

With pneumatic cylinders and associated pumps, the speed at which the piston moves is a fluid power system design issue. Designs with high-speed pistons and larger cylinder bores can be more efficient but may be harder to repair or replace or have shorter service life due to wear occurring at higher speeds. In some applications, it is desirable to have a relatively slow-moving piston to maintain good run-up and cooling of the shaft bearings. The rate of piston movement will also be influenced by any air venting through the cylinder walls (as described above) and the operation of valves in the cylinder head.

The pressure developed in a pneumatic cylinder depends upon the size of the bore, the force exerted by the piston, and the air pressure available to fill it. Material or tension will reduce the operating point for a given stroke length. However, this may be at the expense of reduced working life. Reducing stroke size may minimize cylinder bore size, which may not be desirable for an increased distance between valves or other components in a system.

If there is too little air pressure about the bore and piston force size, there will be insufficient force to overcome friction within the system and move the cylinder fully. This is known as losing prime.

Lacking prime can occur for many reasons.

In addition, a pneumatic cylinder may lose prime if excessive air pressure is introduced into the cylinder. Priming the cylinder by forcing air through it may be useful, but this should be performed slowly to avoid losing prime.

The cross-sectional area of any piston (where ‘cross section’ is defined as the area of any plane drawn through the piston at that position.) must not be too large, or it will not have sufficient internal volume to provide adequate lubrication for wear protection. It will not have enough volume to maintain pressure in use, so there will be no pump performance.

Sizes of the piston are usually specified in one of two ways. Most commonly, the size is “pounds per square inch” (PSI). This is not a particularly significant figure, as it is not a true measure of pressure but rather relates to the force exerted by the piston that can be measured on specialized equipment such as a Torrington tester or shock wave analyzer. The correct unit for describing cylinder forces is more relevant for this purpose than PSI – it is force per unit length/area, which also applies to hydraulic cylinders.

More commonly, operating pressures are indicated in terms of international standard atmosphere conditions (ISA).

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