Design Engineering

Preventing pneumatic tube degradation

By Randy DeForge   

Fluid Power Festo pneumatic tubes

Non-obvious causes of pneumatic tube degradation and how to prevent them.

Because pneumatic tubing is a common component in machines, many designers don’t give it the attention it deserves. In reality, choosing the right tubing is a critical aspect of machine design, and your selection process must consider factors that extend beyond the more “obvious” operating conditions. 

Given what’s at stake, it’s important to ask the right questions about your machine’s surroundings during the initial design process. Understanding both the obvious and less obvious conditions pneumatic tubing will be exposed to will inform the right choice of polymer – whether polyurethane (PU), polyamide (PA) or polyethylene (PE), among others.

 

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UV rays 

When it comes to tubing failure, 90 percent of customer complaints falls under environmental damage—including physical, chemical and microbial damage—compared to mechanical factors like rubbing or crushing. The reason for this disparity is due to the unforeseen dangers of certain environmental factors on the tubing. 

For example, adding tubing to a machine located near windows can expose the tube to damaging UV rays from sunlight. High-energy radiation—whether ultraviolet, X-ray or gamma radiation—will cause the tube’s macromolecules to split and lead to tubing deterioration. Such an oversight is completely accidental but can have long-lasting effects on not only the tubing’s lifespan, but also machine performance.

UV energy excites photons in plastics, a process that creates free radicals. In the presence of oxygen, these free radicals form oxygen hydroperoxides that compromise the backbone chain and weaken the structure. Called photo-oxidation, this process can also cause a color shift on the surface of the tubing.

While certain polymers, such as polytetrafluoroethylene (PTFE), have excellent natural UV resistance, adding certain stabilizers, absorbers or blockers to the plastic is an effective way to prevent UV degradation. For example, the addition of carbon black can provide sufficient protection for outdoor applications.

 

Hydrolysis 

In addition to UV rays, natural and artificial sources of water and moisture are other potential causes of environmental tubing damage. In general, ester-based PU tubes are susceptible to hydrolysis reactions that cause tubing degradation, while ether-based PU tubes offer greater hydrolytic stability, especially in humid environments. In general, it’s important to ensure the tubing used incorporates additives that resist UV radiation or hydrolysis, depending on your needs.

 

Heat and pressure spikes 

Heat can come from various sources, some of which may be more obvious than others. For example, perhaps you need to run tubing across a vehicle engine, in which certain components will become hotter than others. Throw a particular warm day into the mix, and the resulting high temperature may exceed what your tubing was initially designed to handle. Another common, though perhaps less obvious, source of heat is simple friction. If tubing runs up against components in a machine with high cycle rates, for example, then the frictional heat can build and cause the tubing to fail over time.

Much like temperature, pressure is a variable that requires you to consider conditions that may exceed the tubing’s normal range. For example, perhaps the machine you are designing incorporates a regulator that reduces a 200 PSI inlet pressure down to an acceptable operating pressure of 90 PSI. If the regulator fails, suddenly the whole system—your tubing included—will experience a damaging pressure spike.

In general, if you don’t design around the maximum temperature or pressure the tubing can handle, then any spike in these conditions can cause fatal “ballooning.” PA, also called nylon, tubes are ideal for standard applications with increased pressure and temperature ranges, and many can withstand pressure ranges over 290 PSI.

 

Chemical interactions 

Acids and bases can trigger chemical reactions in a tube’s polymer, causing its molecular structure to split, resulting in radial cracks. To avoid this structural deformation, it’s important to select a polymer that can withstand chemical exposure—such as perfluoroalkoxy alkanes (PFA), which can handle even the most aggressive acids and lyes. Perhaps the most obvious industry where chemical interactions are a concern is food and beverage, which typically utilizes equipment that must endure washdown and other caustic cleaning chemicals.

But unfortunately, not every scenario is this obvious. For example, if you’re running tubing near a machining tool that cuts metal, then it’s important to take into account the fact that some coolant may settle on the tubing. If that’s the case, chemicals within the coolant can react with the tubing material, leading to premature tubing failure.

In addition, standard PU tubing, if left in direct contact with electric wires or sensor cables in dark, humid environments, can experience chemical damage via phosphoric acid, which is found in phosphorus-based flame retardants. During this interaction, phosphorus esters diffuse out of the wires or cables, subsequently forming phosphoric acid on the tube’s surface. This reaction often occurs in cable channels, where PU tubes make direct contact with wires and cables.

 

Microbial and stress damage 

Outside of the food and beverage industry, many machine designers might not think about the dangers of microbes building up on the surface of tubing. But microorganisms and biofilms, such as bacteria and fungus, can inflict indirect damage on tubing—especially PU tubes—in the form of metabolic products. In addition to direct microbial degradation, damage can include acid attacks, enzymatic breakdown of plasticizers and an increase in the level of water contained in the plastic (hydrolysis).

In tubing, a polymer’s constituents provide a source of carbon or nitrogen for the metabolic process, ultimately causing failure in the form of chemical damage or stress cracks. Stress cracks can also occur due to the presence of polar organic substances, including alcohol, ester and ketones. 

The resulting internal stress caused by these substances reduces the polymer’s intermolecular forces due to the diffusion of the medium inside the tube. Making matters even more tricky, if tubing contact with the media that had initially caused the stress cracks suddenly stops, then the media diffuses out of the polymer—making it difficult to figure out the root cause of this damage.

 

Dynamic and mechanical damage 

Although the majority of tubing damage is the result of environmental influences, you must still factor in possible sources of mechanical damage. For example, a common engineering mistake is to assume a tube can handle sharper corners than it can take, especially in confined areas. But if the tube’s bend radius is reduced, even in static equipment, then the tube can easily kink, weaken and fail—especially if it’s also pressurized. 

From a design standpoint, it’s important not to assume your tubing can handle corners—no matter how small—as it can cause the tubing to kink or come up short. In other instances, once a machine begins to cycle, the tubing can snag on or rub against another component or surface. For dynamic situations like this, you’ll need to consider tubing materials that are more resistant to abrasion like PE tubing.

Each type of pneumatic tubing has its different strengths and weaknesses, all which must be balanced with the tube’s environmental and/or mechanical conditions. Keep in mind, however, that not all of these conditions will be obvious—making it extremely important to ask yourself the right questions during the initial design process of your machine. Considering all aspects of your tube’s operating environment will enable you to select the correct polymer and successfully avoid tubing damage, downtime and unnecessary costs.

 

Randy DeForge is the product manager (Air Supply & Accessories) for Festo Corp.

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