How to identify and confirm the hose specification when replacing a hose assembly
Hydraulic hoses and couplings are used in many different applications. Because hoses are flexible, they can move, bend and dampen pressure peaks and vibrations, solving many technical problems. They may look simple, but they are carefully designed components. Industrial specifications such as SAE and ISO are widely used in design and testing. Despite all this care, a hose assembly remains a wear part that needs to be replaced after a certain period of time. In this article we give some tips on what you should pay attention to when choosing or replacing a hose assembly.
Most hoses are made up of layers. Seen from the inside out, these are the inner tube, one or more reinforcement layers and then the outer cover. Each layer has its own function:
The inner tube transports the liquid. In many cases that liquid will be a hydraulic oil, but nowadays many biodegradable liquids are also used. Make sure that the inner tube is resistant to the relevant liquid, as well as to the temperatures reached, as these are often high.
The reinforcement layers consist of steel or synthetic threads and are applied in a crosswise or spiral woven bandage. They ensure that the hose can withstand pressure. In most cases the number of layers is printed on the hose (2ST, 1SN, 4S, 6S) Beside the number of inlays the material and construction are important. Hoses with spiral inlays are preferred in case of frequent and/or high-pressure peaks.
The outer cover, which is made of rubber or plastic, protects the hose against external influences. Depending on the application, it must be resistant to high and low temperatures, wear, chemicals, (animal) fats, weathering and ozone.
Couplings and adapters
Couplings form the connection between the machine and the hose. Usually they are crimped to the hose by means of a ferrule. In this way, the hose coupling forms a leak-proof join between the hose and the connection to the adapters of the machine. The ferrule not only secures the hose but also seals it against external influences.
Hose couplings and ferrules are available in both steel and stainless steel. In aggressive environments, stainless steel may be preferred. However, steel couplings are now also provided with a corrosion-resistant protective layer.
There are many different types of adapters in use on machines. They are provided with an internal and/or external thread. Flanged connections are also common. When choosing the right hose couplings, you should not only make sure that they fit well, but also that the adapter and the hose coupler can be connected to each other without leakage.
The inside diameter of the hose must be chosen in such a way that speed at which fluid flows remains within the recommended range. If the hose is too small, noise, unnecessary pressure and power loss can be the consequences.
In case of replacement, the inner diameter can usually be determined by the imprint on the hose. This is indicated in inches (1/4", 3/8", 1/2", 3/4" etc.) or in millimetres (DN06, DN10, DN13, DN20 etc.). The dash size is also often printed. This number represents the number of 16 parts of an inch. For example, -8 corresponds to 8/16 (i.e. 1/2" or DN13). Size tables often also show the outside diameter of the hose. This can help to identify a hose. Due to wear and tear, this is not equally reliable in all cases.
The length must be long enough to allow proper and tension-free mounting on the machine. A hose that is too short may not bend or stretch sufficiently under the influence of pressure impulses. Hoses that are too long can rub against each other or against nearby components or get stuck.
Know your pressure values
The hose assembly must be able to withstand the pressure in the system. The hose is often marked with Bar, mPa, or PSI to indicate the allowable working pressure. The relationship between them is as follows: 1 mPa = 10 bar. 1000 PSI = 69 bar. 1000 PSI is also indicated as 1K. 6000 PSI (~420 bar) is then indicated as 6K.
The most commonly used abbreviation for working pressure is WP (working pressure). In addition to the permitted working pressure, the burst pressure (abbreviated as BP) is sometimes also indicated. If this pressure is reached, the hose bursts – the clue is in the name... It goes without saying that this value must never be reached. There is often a factor of 4 between burst pressure and working pressure. However, hoses for cleaning also have a 1:2.5 ratio.
A permissible working pressure also applies to the hose couplings. Due to the available space, this is not mentioned on the coupling but can be determined by means of tables. In general, the highest values can be achieved with hose couplings with flat-sealing O-rings or with flange couplings.
The maximum working pressure of the assembly is equal to the lowest value of the hose coupling or of the hose. In many cases, the permitted working pressure of the hose assembly is engraved on the sleeve or indicated on a label that is permanently attached to the hose.
Finally
Always ask questions when choosing replacement hoses instead of simply duplicating the original hose. How did the existing hose fail? Are there any signs of wear on the cover or temperature cracks? Does the machine produce pressure peaks or is it more of a static application? Make sure you find a replacement hose that best fits the application requirements. If in doubt, talk to an expert. It’s always better to be safe than sorry.
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