Hydraulic Hose Components
Components
Hydraulic hoses are constructed of three main components, tube, reinforcement, and cover.
The inner most part of the hydraulic hose is the tube, wrapped up by reinforcement, which is usually surrounded with a cover.
Tube
The tube is the inner most lining of a hydraulic hose and comes into contact with the fluid. The compatibility between the medium and tube is most critical.
The inner tube contains the fluid and also protects the outer elements of the hose from the possible aggression of the conveyed fluid. Additional caution must be exercised in hose selection for gaseous applications where permeation may occur. Permeation or effusion is seepage through the hose pores resulting in loss of fluid. This may occur when the hose is used with fluids such as but not limited to; liquid and gas fuels, refrigerants, helium, fuel oil & natural gas. The material of the tube is chosen among a great number of synthetic rubbers. The chemical composition of the compounds should be selected to meet the requirements of the application.
Reinforcement
The tube itself cannot withstand the pressure of the conveyed fluid. The inner tube must be supported by the reinforcement to keep from stretching beyond its yielding point. The reinforcement material is the main component of the hose that resists internal or external pressures. The three basic types of reinforcement are Spiral, Braid, and Helical.
Spiral Reinforcement
Spiral reinforcement is typically steel wire and has four or six overlapping layers of reinforcement. Spiral wire hose can handle more severe applications with longer service impulse cycle life at extremely high pressures.
Braid Reinforcement
Braid reinforcement can be steel wire or textile, and can have single or multiple layers. Braided hose can handle low to high pressure impulse applications.
Helical Coil Monof lament Reinforcement
Helical coil monof lament reinforcement keeps the hose from collapsing in on itself when used in suction line applications. Helical hose can be rated to a full vacuum rating of 30 in Hg.
The pressure resistance of the hose must be higher than the working pressure. The safety factor is def ned as the ratio between the burst pressure and the max working pressure; for the hydraulic applications the safety factor is set to 4:1 by SAE and ISO standards, some special static applications might have safety factors as low as 2:1. For low pressure applications (up to 100 bar for example), textile reinforcements may be used. So nylon, rayon, aramid or polyester fabrics are woven, braided or wrapped around the tube. When the pressure gets higher stronger materials are needed and steel wire spirals or braids are used.
Wire braided hoses bear generally one or two layers of reinforcement while spiral ones have commonly four or six spirals (layers). Between each layer of braids or spirals an interlayer or friction layer is installed to create a bonding effect and to prevent frictional wear between the wires.
Each cycle in a hydraulic system damages the reinforcement to some degree and over time all hoses fatigue. In general, spiral hose constructions are better suited to high impulse heavy duty applications, which involve f exing and large pressure spikes. However, there are specialized braided hoses available like KURT TUFF that can achieve significant impulse cycle life in a f exing application with a tight bend radius. Helical coil monof lament reinforcement hose is found primarily in suction line applications and low pressure return line applications.
Cover
Environment, machines, and operators themselves can damage the reinforcement. The function of the cover is to protect everything it surrounds from these elements. There are several types of cover, each designed
depending on specific requirements: economy, safety, abrasion resistance, chemical resistance, etc.; evenTechnical GuideKURTHYDRAULICS For the Most Up to Date Information Visit www.kurthydraulics.com 157 aesthetics are features linked to the choice of the cover (e.g. color). Rubber cover can have wrapped finish instead of the smooth finish.
Braided covers are also used that are fabric and often impregnated with rubber adhesive. This is the best solution when minimum weight and heat dissipation are required. This solution is usually used on low or medium pressure hoses due to its relative weakness (e.g. R5 hose type). Coverless type is usually only used for stainless steel braided hoses, mostly PTFE hoses (e.g. R14 hose type).
Hose Identification (lay line) – With the exception of coverless type hoses, the entire length of hose shall be legibly marked with one or more stripes parallel to the longitudinal axis. Marking shall include, but not be limited to, the SAE hose specif cation number, including type designation where applicable, the hose dash size number, the fractional (inch) nominal hose inside diameter, and the date of manufacture, all repeated with the f first letter of each repeat not more than 760 mm (30 inches) from the f first letter of that preceding.
Electrical non-conductive 100R7, 100R8 and 100R18 thermoplastic hoses shall have an orange-colored cover. Also, in addition to the information required previously, the word non-conductive or electrically nonconductive shall appear in each marking repeat.
To determine a hose replacement size, read the lay line printed on the side of the original hose. If it is painted over or worn off, the original hose must be cut and the inside diameter measured. The hydraulic industry has adopted a measuring system using Dash Numbers to indicate hose and fitting size. The number that precedes the hose or fitting description is the dash size in increments of 1/16 inch, -5 = 5/16 inch, – 8 = 1/2 inch (8/16) and so forth. The dash size denotes the hose I.D. with the exceptions of the SAE 100R5, 100R14, and refrigerant hoses where dash size denotes hose I.D. equal to equivalent tube I.D. of the hose