By Benjamin Shobert
Edited by Kenneth Korane
Composite cylinders offer fluid-power users an alternative when traditional metal-body cylinders won't do. But they also carry a reputation for being pricey and having a limited range of sizes and configurations. A new generation of cylinder tubing, however, lets designers take advantage of high-performance composites, along with lower costs and greater design flexibility.
Composite cylinders outperform their steel and aluminum counterparts in several significant ways. For instance, composites resist a wide variety of corrosive liquids, powders, and gases. And manufacturers can custom compound resins to address particular environmental concerns. For example, a manufacturer could develop cylinders that are inert to caustic or acidic exposure.
Composites are also much lighter than the metals used in typical fluid-power cylinders. In fact, composites are similar in weight (0.068 lb/in.3) to exotic and expensive light metals that cost significantly more.
Unlike metal cylinders, high-strength, filamentwound composite tubing resists impact without deforming the cylinder wall. This impact resistance makes composites ideal for applications where moderate-velocity debris damages or destroys metal cylinders.
And composite cylinders made of thermally resistant thermoset-epoxy resins are suitable for continuous operation at temperatures ranging from –100 to 225°F.
However, from a historical perspective, composite cylinders have several inherent limitations that discourage widespread use. Conventional composite cylinder tubing relies on gel-coating to produce a low-friction ID surface. This gel-coat resin typically contains a suspended hard lubricant such as MoS2, graphite, or PTFE to offer a degree of longterm lubricity. After the resin hardens, continuous fiberglass filaments are wound over the gel-coat tubing and the resulting bilaminate material is cured and fabricated.
The gel-coat material, while relatively smooth, comes with several limitations — in particular, brittleness — which make it subject to failures ranging from simple delamination to gas permeation.Slightly misaligned rod and piston assemblies are a leading cause of delamination. Combined with high cycle rates, this results in repeated impacts to the cylinder wall which weaken the bond between the filament windings and gel-coat. This often separates the liner from the backing and leads to cylinder failure. External impacts and wide temperature variations also cause liner delamination in gel-coated tubing.
Gas-permeation failures can be found in gas boosters where the gas under pressure penetrates the gel-coat and creates an effect similar to cavitation that cracks and buckles the coating.
To prevent such failures, a new type of linerless cylinder tubing has been developed that eliminates the gel-coat. The interior wall of linerless cylinders is the wear surface, so delamination and gas permeation are no longer concerns.
A proprietary manufacturing method produces a smooth, ultrahard finish, but without secondary processing steps as required for gel-coating. Thus, one immediate benefit is fewer manufacturing steps and lower costs.
The inner surface has a hard and slightly rolling texture. It uses a resin with a smooth surface finish (5 to 15 in.) The resin matrix incorporates a lubricant which gives the resin layer a low coefficient of friction, and lets the lubricant migrate to the wear surface for low-friction performance over the life of the cylinder.
The smoother surface finish and better lubrication give linerless cylinders a significantly lower coefficient of friction compared with gel-coat designs. In rapid-cycling applications, for example, gel-coated cylinders tend to become hot while linerless cylinders remain at ambient temperature. Cooler, lower-friction operation contributes to longer seal life. Seals in gel-coated cylinders tend to show wear early in their cycle life, while those in linerless cylinders generally show no attributable wear even after 1 million cycles.
The new design addresses several other limitations of composite cylinders. For instance, it eliminates process-related regulatory concerns. To manufacture gel-coated composite cylinder tubes requires a manual spraying operation. This spray typically uses a styrene monomer — an increasingly regulated substance due to new MACT (Maximum Achievable Control Technology) legislation soon to be mandated by the EPA. Linerless cylinders do not rely on this process, so the product's future is not in jeopardy and manufacturers enjoy lower costs and fewer hassles.
The new design also opens doors to more types of products. Cylinders often require modifications to the edges or ID, but this is not cost-effective with gel-coated tubing. Fabricating an ID groove, for example, is difficult due to the highly brittle gel-coat. Scrap rates are usually high because damaged gelcoats cannot be easily or economically repaired. Linerless cylinders, however, are suitable for edge and ID modification, with lower fabrication costs and fewer rejected assemblies.
Linerless composite cylinders can also be manufactured in smaller sizes. This is because as the ID and OD get smaller, so does the portion of the tube wall bearing the stresses. Small IDs are not practical with gel-coating because the brittle coating tends to crack during filament winding.
Fluid-power engineers should keep in mind that linerless composite tubing was designed not only as an alternative to traditional composite cylinders, but also to replace conventional honed and plated metal tubing. It eliminates many problems associated with metal cylinders, such as piston stick-slip, especially in applications where the piston remains static for extended periods. In addition, the tubing will not corrode in hostile environments and is inherently thermally stable.
Linerless composite cylinders are suited for a range of applications, such as pneumatic-valve actuators, hydraulic actuators for aircraft, dump mechanisms for railroad cars, waste-water treatment flow controls, and replacement for aluminum cylinder tubing.