Environmental considerations in the selection of pneumatic cylinders
Making the correct choices and considering all aspects of the operating environment when specifying a pneumatic cylinder are crucial if an acceptable degree of reliability is to be achieved and costly downtime minimised, says Rebecca Hammes of Parker Hannifin.
A standard pneumatic cylinder consists of an end cover, cylinder, piston, piston rod and front gable, along with a piston rod bearing and scraper seals. Their general design adheres to ISO standards and they are intended to meet the great variety of performance requirements found in a multitude of demanding industrial and factory automation applications. Aluminium is typically used for the cylinder and end caps to provide the necessary strength whilst keeping weight down and chrome plated steel or stainless steel are utilised for the piston rod.
The selection process for a pneumatic cylinder has two main strands. Firstly the actual operating performance and parameters, and secondly the operating environment. Covering both of these effectively is key to arriving at the optimal choice for a given application. Considering the operating performance requirements first, the design engineer will need to gather some basic application information including the available air pressure and the desired speed and cylinder power. Additional factors to consider include the maximum theoretical force – keeping in mind that this value needs to be 50-100% greater than the basic force needed in order to account for effects such as friction plus potential lateral loads on the piston rod and braking effects at the end of
the stroke. Cylinder manufacturers’ specifications usually contain these values or the guidance to allow them to be easily calculated.
With the first stage covered, the environmental factors must be taken into consideration; sometimes these are somewhat overlooked when selecting a pneumatic cylinder. They are important if design engineers want to optimise operation and maximise working life and reliability.
These factors may include ambient temp-erature, vibration, and operating media or chemicals and other fluids or materials present.
For a standard cylinder, fundamental performance characteristics are often specified for an operating temperature range of –20°C to 80°C; however, some more challenging applications exist beyond these limits. As a result, some pneu-matic cylinder manufacturers produce product variants which, with the help of specific higher performance lubricants and a special sealing materials, address applications in either extreme low temperatures down to –40°C, or higher temperature up to 150°C.
Other aspects of the operating environment from one application to another can vary enormously and need to be considered before making a final product selection. For example, in a dry ‘non-aggressive’ environment, a cylinder can be used that does not need to have its surfaces protected; in such environments, a chromium steel piston rod will normally suffice. Conversely, in humid operating envir-onments, maybe with risk of salt spray, all external cylinder surfaces will need to be coated, and the piston rod should be made of stainless steel. Further special operating environments exist such as those that are ATEX-rated where equipment is used in potentially explosive environments, or for applications in food and beverage production, where specific certification of the pneumatic cylinder is required, and where special ‘clean’ design features that reduce the amount of entrapment points for bacteria exist. A further challenge in food production environments is the frequent washing of the work area which can lead to damage to static and dynamic gaskets and seals. As a result, engineers must select a cylinder that is self-lubricating and therefore designed to run dry.
In other applications, deposits on the piston rod can cause issues – for example sugar sediment in food equipment, or resins, asphalts and even cement in other applications. To alleviate this problem, options exist to install metal scraper rings as opposed to plastic or rubber versions with the piston rod seal to remove the residue before it can cause damage to the cylinder assembly.
The challenges for pneumatic cylinders are as numerous as the sectors in which they are needed to operate. Downtime costs money and lost production so must be avoided for anything other than routine maintenance. Hence careful selection based on operating requirements and environmental factors must be considered at the outset.
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