Many hoses benefit from built-in support.
By Larry Peres
Over the past five years, rubber hoses have become an increasingly important part of our product line here at RL Hudson. As Hudson’s applications engineer specializing in hose technology, I am charged with the task of making sure our customers receive hoses of the highest quality that will perform as needed in service. During my 25 years of experience in the rubber and hose industries, I’ve learned that good hose performance has much to do with the materials used in manufacturing.
As you may know, many rubber hoses are composed of three layers: an inner “tube” that contains the fluid being conveyed; a middle layer of reinforcement to help the tube withstand both internal pressure and external forces; and an outer cover for further protection and better aesthetic appeal. I’d like to focus here on the reinforcement layer. First I’ll briefly describe the materials most commonly used for reinforcement, then I’ll outline the various methods used to apply these materials during the hose manufacturing process. I’ll close with a bit of information about testing.
TYPICAL REINFORCING MATERIALS There are several options when it comes to hose reinforcement. Polyester, rayon, Kevlar®, cotton, and nylon yarns are commonly used, though some hose designs may utilize wire. The decision as to which reinforcing material is most appropriate depends on the demands of the application.
Polyester and rayon, for example, are most widely used to reinforce curved hoses (such as radiator hoses). Both offer moderate tensile strength compared to Kevlar, and both can undergo moderate shrinkage.
Kevlar is the trade name for a high-strength, low-weight, flexible fiber marketed by DuPont. You’ve probably heard of it; it’s the fiber used in bullet-resistant vests! Because of its high tensile strength, Kevlar is ideal for use in high-pressure hoses. Of course, this does come at a price; Kevlar is about 30 percent more expensive than polyester yarn.
On the other end of the cost spectrum, cotton is a low cost reinforcement material. It only offers minimal strength properties, however, so it is not widely used outside of low-pressure hose applications. As cotton gets wet, it initially gains strength, but because it does not dry, it eventually succumbs to fungal activity and deteriorates.
Unlike cotton, nylon does not absorb moisture readily, and it is highly resistant to the development of fungus. Nylon offers good strength and elongation properties, as well as resistance to impact, abrasion, and fatigue. Wire also finds usage as a reinforcement material. Steel wire is typically used when fabrics alone will not provide adequate reinforcement, such as in high-pressure industrial or hydraulic hoses.
Applying reinforcement: Depending on the needs of the application, hose reinforcement can be applied in several ways. These include knitting (Figure 1), which creates a repeating rectangular or square pattern over the rubber tube, and braiding (Figure 2), which creates a diamond pattern.
MODES OF APPLICATION Just as there are a variety of reinforcing materials, there are also a number of ways in which these materials may be applied. These methods include knitting, braiding, spiraling, wrapping, and weaving.
As the name implies, knitting involves the stitching of yarn into an interlocking grid. An example of this is shown in Figure 1. Note that knitting tends to create a repeating rectangular (or sometimes square) pattern. Knitting is mostly used in formed hose applications, such as radiator hoses, heater hoses, and fuel filler hoses (low pressure). Knitting has some key advantages over other reinforcement methods. For one, it radiuses (bends) better in the forming stage (when the hose is slid onto a mandrel to be shaped and vulcanized). Knitting is especially useful for hoses that will face lower burst pressures.
For hoses that must withstand higher burst pressures (over 200 psi), braiding is a better bet. Examples of such applications include high-pressure air, water, steam, and twin welding hoses; air conditioning, power steering, and fuel injection hoses; and hydraulic hoses. As is shown in Figure 2, braiding creates a distinctive diamond pattern over the rubber tube. Braiding can be applied using a horizontal or vertical braider, and for comparably sized hoses, braiding can be faster than knitting.
Whereas both knitting and braiding came to be used for hose reinforcement in the early 1900s, spiral reinforcement did not become common until the 1950s. Spiral reinforcement is applied horizontally by two opposing and oppositely rotating spindle decks. Because they are simple in design, these decks can move very quickly, making the application of spiral reinforcement a relatively fast process.
Wrapping involves the application of fabric reinforcement spirally to hose held on a rigid mandrel. Wrapping typically features multiple layers (plies) of fabric, with the orientation of each successive ply reversed for greater overall strength.
Finally, a specialized loom can be used to weave a seamless textile jacket. Such woven jackets provide strong but lightweight and flexible reinforcement for applications such as fire hose. In some cases, two jackets may be used for even greater reinforcement. In many fire hoses, the outer jacket also serves as the cover (i.e. an outer rubber cover is not added).
BURST TESTING Since one of the primary reasons for reinforcement is to prevent the hose from succumbing to internal pressures, testing is required to see if indeed the reinforcement meets the specification and will prevent the hose from rupturing when pressurized in service. We at RL Hudson conduct burst tests using our own hydrostatic tester with chamber.
This tester features a 48” long tank and a dual pump system that allows both 0-1000 psi and 0-3000 psi testing. Our tester also enables us to conduct proof testing of couplings and clamps, gauge outside diameter (O.D.) swell, and measure length change in service. Hose tests are detailed in the American Society for Testing and Materials (ASTM) document D 380.
WRAPPING IT UP There are many decisions to be made when thinking about hose reinforcement, and this article has been but a brief overview of the topic. If hoses are important to you, however, you may be interested to know that we at RL Hudson are currently hard at work on a Rubber Hose Handbook. Following in the footsteps of our O-Ring Design & Materials Guide and our Shaft Seal Design & Materials Guide, our new hose guide will feature material profiles, manufacturing details, discussion of sample applications, troubleshooting tips, and more. Until our Hose Handbook is complete, do not hesitate to call us at 800-722-6766 if we can help you with a hose application.