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Mechanics Corner: Tips to prevent and fix hydraulic hose failure

Vinny Endres Published on 02 September 2015

hydraulic hose

Hydraulic hose failures are dreaded by everyone for many reasons. Hopefully these tips can help you identify what you have, fix it properly and prevent failures in the future.

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Hose isn’t just hose

It might all look similar from a distance, but the specific hose you choose is important. Working pressure, bend radius, operating temperature, liner material and cover type can all influence this decision. All hoses meet a variety of standards from various organizations (ISO, DIN, SAE, EN), and simply finding a hose that meets the same standards as the failed hose is generally the easiest and safest route to take.

However, if you wish to take the time to understand the different standards and how they apply to the system the hose is on, you can determine what other hoses may work if the proper hose isn’t available or determine a different hose that may hold up better.

What do all the numbers printed on the hose mean? At a minimum you will find the working pressure (PSI or MPa), the hose size (mm or dash size), possibly a standard it meets (SAE 100R12, for example), likely the brand and model, and the production date (yes, it actually can expire). A hose or fitting’s dash size is expressed in sixteenths of an inch (-08 equals one-half inch). Some hoses use metric measurements as well.

Fittings

ORFS, JIC, ORB, DIN, BSPP, NPTF, JIS – this is just a sample of the fitting standards you may encounter. Identifying what you have is the first step. Then you can decide if you should adapt to something else if desired. The best thing you can do is get a caliper, some thread gauges and a handbook from any of the major hose and fitting original equipment manufacturers (OEMs) and measure or compare what you have with the charts in the book.

O-ring boss (ORB) is typically used to go into a port; it has straight SAE threads with an o-ring seal. O-ring face seal (ORFS) is very common on prime movers and uses straight SAE threads with an o-ring seal in the face of the male fitting; these are expensive but very reliable.

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JIC 37-degree is also very common and uses SAE threads with a 37-degree flare that creates a seal when tightened (overtightening can damage the seal). National pipe taper (NPT) is still common as well, but is the style you need to pay the most attention to. British standard pipe taper (BSPT) is very similar to NPT aside from the flank angle and the thread form. They will thread together but not seal properly.

Hydraulic fittings generally use NPTF (national pipe taper fuel). The thread form is slightly different and allows for a “dry seal,” which requires no sealant, unlike NPT fittings. NPT also tempts people to use black pipe or galvanized pipe for their hydraulic systems. Most of these products are not rated for the pressures in a hydraulic system and should not be used.

Repairs

Proper repair procedures will reduce the potential for issues later on. Never splice an old hose or crimp a new end on an old hose. Always start with fresh hoses and fittings. It’s best to use a blade designed for cutting hose (such as a scalloped or wavy blade), as these reduce the amount of contamination in the hose.

A foam gun uses foam projectiles to clean the hose and is the best way to completely clean the hose. When you select the hose you need and you find the style fitting you need, make sure the two are compatible. Not all fittings are compatible with all hose, even within the same brands.

While fittings from one brand may “fit” on another brand’s hose, there is no guarantee that they will be compatible, and thus the best practice is to stick within a brand. Some hoses and fittings may require skiving, although it is becoming much less common with the newer products. The proper crimp specifications need to be known as well so you can use the proper die and do not over-crimp or under-crimp the fitting. Make sure the crimper you are using is able to crimp the fitting you choose.

All of this information can be found in the manufacturers’ product books. If both of the ends you will be crimping onto the hose are angle fittings, you need to make sure you have the correct orientation of the fittings in relation to each other, or the hose will end up with a twist in it when installed, leading to premature failure.

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Installation

The perfect hose will still fail if it isn’t installed properly. Make sure to route the hose in a way that avoids exceeding its bend radius. Avoid putting any form of a twist in the hose during installation. If the hose will need to move, route it so that it will bend versus being twisted.

Support the hose where needed with proper clamps or other fastening devices, but make sure to not restrict movement of the hose that may be necessary for normal operation. Securing the hose away from potential abrasion points is also important but not always possible. For areas with high abrasion, consider using a hose with a more abrasion-resistant cover, as well as additional abrasion products such as a Kevlar abrasion sleeve that can be fastened to the hose.

Taking the time to select the proper hose and fittings and build and install the new hose properly can be a real burden upfront, but it will pay for itself in the long run with reduced downtime and headaches. The manufacturers’ product books offer a wealth of information on this topic, and with some time to research and learn, you will be able to not only repair your hoses, but also redesign them to make life easier in the future. PD

PHOTO: When replacing or repairing hydraulic hose, working pressure, bend radius, operating temperature, liner material and cover type can all influence the outcome of your decision. Photo by Lynn Jaynes.

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