Preventing air cylinder failure
May 1st 2010

John Hill,marketing services manager for Parker Hannifin’s Pneumatics Division, looks at the potential problems of pneumatics in industrial applications and gives simple guidelines for addressing these issues to maintain and maximise efficiency

Pneumatics has traditionally been the technology of choice for many factory production processes, such as materials handling and small parts assembly. Now, with the introduction of closed loop control, precision pneumatic components and high speed electronics, pneumatics technology is finding far wider application, competing directly with electrical and hydraulic closed loop circuits.

Pneumatics cylinders generally represent the heart of most systems, and are expected to function reliably under often tough conditions. Similarly, engineers now expect cylinders to be both to be easily interchangeable and available in a myriad of different configurations.As a result, growing complexity is increasing the potential areas for problems and failures to occur; especially if cylinders are incorrectly specified, fitted or used.

However, by taking a methodical approach to system design and build, and to the analysis of problems if they occur, it is possible to ensure that cylinders perform at optimum levels boosting uptime and productivity.

Pneumatic cylinder failure typically results from five different conditions: side load mounting, contamination, poor lubrication, out of sync cycle rates and operation in excess of component limits.

Side load mounting

Piston rod side load mounting can be caused by misalignment, off centre loads, rod deflections, imprecise tie rod torque and long stroke action, resulting in, at best, excessive seal failure, bushing wear or galling of the piston rod and, at worst, bending or complete breaking of the piston rod.

Accurate installation is key to overcoming the problems caused by side load mounting. For example, it is essential to ensure that the load to be moved is precisely aligned with the cylinder’s centre line, while also checking that the cylinder rod is mounted inline with its mating machine component in both the extended and retracted positions to ensure efficient operation and extended cylinder service life.

It is always important to select a mounting method that absorbs force on the cylinder’s centreline, such as a rear end cap mounting style for piston rods that are loaded primarily in compression mode. On the other hand, if the piston rod is loaded primarily in tension, a front end mounting is generally the best option.

In any case, it is vital that all cylinder mountings are monitored periodically using sensors for signs of rod run out or deflection and a stop tube or rod alignment coupler being installed to optimise cylinder performance.

Contaminants

Whether it is from the air supply itself or from the external environment, debris, water and oil can cause metal surfaces to rust and seals to age prematurely, reducing operating efficiency and increasing maintenance requirements and costs. However, there are various solutions that can be used to prevent contamination. For example, filtering the compressed air supply and using metallic wipers can help to overcome particulate contamination, while the use of stainless steel rods and fasteners or special coatings and epoxy paints prevents damage from water vapour. Equally, plant engineers should always select seals that are compatible with any oils used in the application to reduce the damaging effects of such contaminants.

Poor lubrication

It goes without saying that correct lubrication is critical for efficient operation. Adding a lubricator to the air preparation system or installing an injection lubrication system or non-lube cylinders can all overcome the problems associated with lack of lubrication. Shock absorbers, pressure regulators and flow control systems can also help if high speed is not a major concern.

When properly specified, installed and maintained cylinders should be leak free, however even the smallest problem can cause leakage. For example, environmental conditions, such as temperature, moisture content or dry running can cause wear to seals and, thus, leakage. In this instance, it is usually worn seals and torque inaccuracies that are causing the problem and, therefore, need to be remedied.

Operating over component limits If an application causes a cylinder to operate at rates over its limits, with regards to pressure, load and energy, then failure is almost always inevitable.When cylinders run at overload capacity, seals are subjected to higher stress and friction rates, causing rods to bend or break and actuators to fail. In addition, if the system contains speed control or energy absorbing devices, pressure spikes can also occur that are two or three times above normal system pressures.

To resolve these mechanical issues, operators should ensure that all rod accessories are firmly mounted by threading all the way down or by using spacers or shims. Increasing the rod-thread size or using a studded rod end can be equally effective in overcoming the problem.

Summary

Additional problems can arise from the materials used in the cylinder construction itself. For example, an all stainless steel cylinder distinctly differs from a stainless steel body cylinder. Additionally, miscalculating kinetic energy in performance equations can literally cause a cylinder to shatter. However, if plant engineers, operators and maintenance personnel know what to look for in terms of unit design and operation, while adopting best practice maintenance procedures, equipment can keep running at peak performance, ensuring efficient, reliable and cost effective operation.