Hydraulic hoses are considered one of the major sources of maintenance intervention and unfortunately, accidents, as well. They are manufactured based on strict industrial standards, dictated by the SAE standards. As we may realise, hydraulic hoses are flexible ducts designed to carry high pressure fluid. To better illustrate this statement, a 3/8” ID hose delivering 20l/min of fluid at 140 Bar can be easily transferring 4.5Kw of power. Due to its construction and particularities of its applications, hydraulic hoses have a finite service life, which can be reduced by a number of factors.
From a maintenance perspective, little or no attention is usually paid to the hoses of a hydraulic system until a failure occurs. Hydraulic hoses are not designed to leak, but those of you who have worked in the field have seen it happen - and when they do, something is wrong. Leaks from high-pressure hydraulic lines are not just messy, they are dangerous. Leaks can create slip and fall hazards, fire danger, and they contaminate the environment. Leaks can also cause skin burns and, under high pressure, can penetrate the skin. The most common causes of leaking hoses are abrasions and improper assembly. If you work with hydraulic hoses, you should become skilled at inspecting hoses and fixing them when necessary.
What causes hydraulic hose failures?
1) External damage
Hydraulic hose manufacturers estimate that 80% of hose failures are attributable to external physical damage through pulling, kinking, crushing or abrasion of the hose. Abrasion caused by hoses rubbing against each other or surrounding surfaces is the most common type of damage.
You can help prevent abrasion by using hoses of the correct length and diameter, and by running the hose in the manner specified by the manufacturer. This includes ensuring the hose is supported and restrained by all provided hangars and/or brackets, replacing chaffing guards if they were originally installed but are now missing and paying attention to a damaged outer jacket, as this allows moisture to attack the exposed hose reinforcement (leading to rust and corrosion which could result in hose failure).
2) Multi-plane bending
Bending a hydraulic hose in more than one plane results in twisting of its wire reinforcement. A twist of five degrees can reduce the service life of a high-pressure hydraulic hose by as much as 70% and a seven degree twist can result in a 90% reduction in service life.
Multi-plane bending is usually the result of poor hose-assembly selection and/or routing, but can also occur as a result of inadequate or insecure clamping where the hose is subjected to machine or actuator movement.
3) Operating conditions
The operating conditions that a correctly installed hydraulic hose is subjected to will ultimately determine its service life. Extremes in temperature, for example high daytime operating temperatures and very cold conditions when the machine is standing at night, accelerate aging of the hose’s rubber tube and cover.
Frequent and extreme pressure fluctuations accelerate hose fatigue, e.g. rock hammer on a hydraulic excavator. In applications where a two-wire braid reinforced hydraulic hose meets the nominal working pressure requirement but high dynamic pressure conditions are expected, the longer service life afforded by a spiral reinforced hydraulic hose will usually more than offset the higher initial cost.
4) Abrasion protection
A primary source of hydraulic hose failure is abrasion resulting from cuts, friction caused by other moving hoses, or objects in the operating environment. Hose-to-hose abrasions are likely to show up where a length of hose travels through a boom or bulkhead, or along framework.
Cover material erosion can also be caused by non-compatible fluids such as toxic chemicals, acids, detergents and non-compatible hydraulic fluids. When exposed to air, the hose reinforcement is subject to rust and accelerated damage, leading to failure.
In most cases, abrasion problems can be spotted during routine inspections and replacement can be initiated before a major failure occurs. Bundling hose that flex in the same direction together can help solve hose-to-hose abrasion. Clamps, bent-tube couplings, nylon ties, clamp collars, spring guards and sleeving can be used to keep hoses away from the source of abrasion or exposure to non-compatible fluids.
You can protect the hose cover by using nylon and urethane sleeving and spring guards. An abrasion-resistant hybrid hose with greater abrasion resistance than any standard hose may also be available from hose manufacturers.
Made of special rubber/plastic compounds, the new cover materials have been tested to last as much as 300 times longer than standard rubber covered hoses. This feature increases service life, lowers maintenance, and eliminates the need for hose protectors such as guards, sleeves and bundling.
In cases where clamping is necessary to properly route the hose, choose good quality clamps capable of taking considerable stress for high-pressure lines. The I.D. of the clamp should be 0.8mm. smaller than the hose O.D. in order to provide a positive, non-abrading clamping action. Anticipate hose expansion and contraction of ±4%. Avoid clamping the hose to vibrating components, and never clamp a hose on its bending arc.
Avoid crossing hose lines, but when unavoidable, join the two lines at the junction point. Never clamp parallel high and low pressure hydraulic hoses together. This is because the differential change in hose length during pressure cycles will result in a seesaw action, which will damage to both hoses.
Careful consideration of the placement of hydraulic hose circuits can help ensure maximum equipment efficiency, minimum downtime and simplify maintenance procedures.
And finally a few words about hose and fittings leakage...
The Wrong Way to Find and Fix Leaks
What do you do when you find a leaking fitting? Find a wrench and give the fitting another turn? That extra turn could cause a greater leak or cause the fitting to fail entirely. Do not use your hand to find the leak. Use a piece of cardboard or wood instead. Hydraulic fluid is hot and can burn the skin. A pinhole leak, under pressure could actually inject fluid under your skin, causing poisoning, infection, and threaten life and limb. It can and has happened.
Test for Tightness
Before testing for tightness, shut the machine off and bleed hydraulic pressure from the line. If the fitting threads were to strip or a connection was to fail under pressure, injury or fire could result from the sudden release of hot oil. The usual cause of a leak at a fitting is improper assembly or damage. Make sure that:
> Both ends are clean inside and out, and that no physical damage has occurred;
> New seals are used and they have been cleaned and lubricated before installation;
> Fittings are not over-tightened, which can distort seals and ferrules, causing metal fatigue or cracking flared ends;
> Fittings are incompatible. There are many thread ends, and some may almost go together properly, but not quite.
Proper Assembly of Hose Ends are Important
Hoses that come apart under pressure can whip back with great force and release a lot of hot oil. If the failure occurs at the fitting, the usual reason is improper crimping, an incorrectly cut hose, or a ferrule that was not inserted into the hose all the way. If you assemble your own hoses, check your crimping dyes for wear. On some types of crimping machines, if the dyes become worn, the crimp is looser than it should be. Screw-type hose clamps are not to be used on pressurized hydraulic hoses.
People who work with any type of fluid piping system knows it takes clean, careful workmanship to prevent dangerous leaks. If you see a leak, report it. If your job requires you to fix the leaks, do it properly and safely.
By the Engineering Team, Hydraulic Distributors.
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