Posted by Fluid Gas Handling Team
on Thursday, August 31, 2017
PTFE convoluted tubing is used when customers need a very flexible tube that can withstand constant flexing, for high-temperature fluid handling applications and for applications where other wires and cables need protection. As a protective tube, the convoluted tubing is used as a harness for easy routing and/or as protection from crushing. Convoluted tubing is available in many types of plastics, but for this blog, we will be discussing mostly PTFE with a few references to FEP and PFA convoluted.
Fluoropolymer convoluted tubing was originally developed to provide a light-weight, non-flammable, corrosion resistant alternative to the heavier metal interconnects in aircraft, allowing airlines to achieve a weight savings of 40%-80% over conventional materials such as aluminum, stainless steel or brass. The benefits of the fluoropolymer convoluted tubing shaped its future as the material of choice for the aircraft industry and eventually spilled into almost any industry needing a flexible, chemical and heat resistant tube.
How is convoluted tubing made?
When convoluted was first manufactured, a smooth bore tube was used and a wire was pulled down onto the tubing to form the convolutions. Once all of the tubing had been convoluted with the wire, the wire was attached to the end of the tubing to keep the convolutions formed. Forming the convolutions manually was time-consuming and cumbersome, and in large quantities, not cost effective. Eventually, the technology was acquired to automate the process.
Convoluted tubing uses twice the material of a standard smooth bore tube, creating a tube reinforced within itself to handle higher pressures and offer increased flexibility. Convoluting the tube increases the flexibility of a tube by adding hoop strength to the circumferential strength to resist collapse, which results in a tube capable of turning corners and winding between machinery.
One of the unique characteristics of PTFE is that it has a memory. It can remember the sizing dimensions it was originally extruded to meet. This becomes a distinguishing factor between PTFE and FEP convoluted tubing when putting a cuff on the end of the tube. Because the PTFE can remember its original dimensions, it naturally reverts back to its original dimensions when heated and the cuff is formed. This allows the operator to size the cuff on a die and achieve a surface so smooth that you cannot tell it was ever convoluted.
FEP convoluted tubing is extruded as a convoluted tube. When adding a cuff, sometimes the operator needs to apply additional stress on the tube to pull the convolutions out. Although a fairly smooth cuff can be achieved, you will always be able to see a remnant of the lines inside of the tube that mark where each convolution ended and began.
Styles of convoluted tubing
PTFE convoluted tubing is available in a wide variety of styles.
- Low Profile – Has a larger inside diameter for increased flow
- Heavy Wall – Produced with a heavier wall to handle higher pressures or vacuums
- Close Convolution – Increased convolutions per inch to create a tighter pitch (the tighter the pitch, the more flexible the tubing)
- Compressed Convolutions – Very small convolutions for the ultimate flexible convoluted tubing; this tubing handles very low pressures
- Reverse Convolutions – Convolutions are angled in the opposite direction to alleviate problems with twisting
- Wire-Wrapped Convoluted – Wire reinforcement is available to achieve a tighter bend radius and create a tube with stronger crush resistance; wire is available on the I.D., O.D. or both
- Mil-Spec Convoluted – Mil-Spec tubing is available to meet aerospace requirements of AMS-T-81914 Specifications
The pictures below are just a few of the more common styles.
Watch the video
For more information on convoluted tubing and other flexible tubing options from Parker, watch the video.
End styles for cuffed convoluted tubing
When ordering convoluted tubing, there are just as many end styles as there are types of convoluted.
The images below are a visual explanation of the differences.
Ordering PTFE convoluted tubing
Ordering convoluted tubing is never standard. Each manufacturer has his own "standard" convoluted tubing. Some are based on the aircraft specification AMS-T-81914 and others came from the specifications of particular customers when the product was developed. If you order a 5/8" convoluted tubing from Supplier A and a 5/8" convoluted tubing from Supplier B there is a good chance that you won’t be comparing apples to apples. In some product lines, 5/8" refers to the inside diameter and in others, 5/8" refers to the outside diameter. In other cases, it refers only to the cuff dimensions of the tubing and has nothing to do with the internal diameter or the external diameter of the tubing. Because of all these variations, buyers often have a difficult time ordering the correct product.
Parker has loaded an RFQ form to the Support Assets tab on parker.com to eliminate potential misunderstandings, "Convoluted Tubing RFQ
." Other literature is also available to show the differences between the different styles of convoluted.
Convoluted tubing is available in bulk, in cuffed pieces and in tubing assemblies. When higher pressures are needed, PTFE convoluted is over braided with stainless steel or polypropylene. And when static dissipation is a problem, PTFE and PFA convoluted tubing are available in static-dissipative designs to reduce static electrical charges.
Convoluted tubing is just one of the options Parker offers when customers need flexible tubing for their applications. Other tubing includes corrugated, retractable coils and formed tubing. For more information on these designs, contact Parker Parflex Division
Article contributed by Janine Kruit, marketing services manager at Parker Parflex Division.
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