A fluid power expert identifies potential failures and how to deal with them.
December 19, 2016
by Steve Gahbauer
There are plenty of ways for industrial hoses and their systems to fail. So how do you avoid trouble? By understanding why they fail.
“The purpose of troubleshooting is to identify the causes of failure and take action to correct the problems,” says Shane Monaghan, fluid power product manager with the industrial division of Gates Canada Inc., a manufacturer of power transmission belts and fluid power products in Brantford, Ont.
In a presentation to maintenance pros in Hamilton, he offered some practical solutions.
Monaghan identified the most common causes of premature hose failure as abrasion, burst, leaks, weep at the hose/coupling interface, coupling blow-off, coupling corrosion, cracks and twists, blisters and tube swell, and mushroom flare.
Eighty per cent of hydraulic hose failures are caused by abrasion, many of them the result of improper routing. Continuous rubbing results in deteriorated cover stock exposing wire reinforcement, which becomes weakened from environmental exposure and that leads to catastrophic failure.
There are three basic solutions:
Bundle hoses that flex in the same direction. Clamps, bent tube couplings, nylon ties, spring guards and sleeves keep hoses away from abrasion sources and exposure to non-compatible fluids.
Protect hoses with nylon and urethane sleeves and spring guards.
Change to a hose with a more abrasion-resistant cover.
Pinhole leak and/or hose burst occurs when pressure surges exceed the maximum operating pressure capacity, when the minimum bend radius is exceeded, or when reinforcement fatigue results from excessive flexing.
The remedy is to check the system’s pressure output and select a hose with the proper rating to handle the maximum pressure (including surges) of the application. If pressure surges are frequent, consider using a spiral, wire- rather than braid-reinforced hose.
Bursting at the coupling ferrule occurs when the hose isn’t long enough, or when there’s excessive bending and flexing. Other reasons are an over-crimped coupling or a wrong ferrule. Fix this by increasing the hose length to accommodate contraction under pressure or reduce stress at the coupling end.
Leaks are caused by damaged or worn out O-rings, damaged thread or seat, incompatible terminations or incorrect torque load. Look for O-rings damage caused by pinching during installation or possible material breakdown from heat or from fluid incompatibility. Check the threads and/or seat angle for damage. Any ding or burr can be a link. Threads could have been damaged if the coupling was misaligned during installation. Carefully replace and install.
Weep at the hose/coupling interface happens when there’s a bad crimp, excessive vibration, flexing or bending. If the hose assembly has been under-crimped or the stem improperly inserted, replace it.
Possible causes for coupling blow-off are under- or over-crimping, incorrect crimping dies, improper skive, incorrect fitting/hose combination, or insufficient hose slack in routing.
Solve these problems by modifying hose length and/or routing to allow for potential reduction under pressure.
Most hydraulic fittings are made from carbon steel and have zinc chromate plating that provides minimal corrosion resistance. ISO 9227 requires manufacturers to pass a 96-hour continuous salt spray test. Provide adequate protection for the coupling or ensure minimal exposure to corrosive elements. Other coupling materials such as stainless steel, brass or aluminum provide better resistance.
Hose cover cracks are caused by heat, ozone, undersized hose or reservoir, or flexing at very low temperature. An increase of 7.7 degrees C above the maximum temperature may decrease hose life by half. Seat cracks are due to over-torqueing, vibration or shock loading.
Tube cracks show up as brittle or cracked material.
Compare pump output to the hose size by using a flow rate chart. Select a hose type with a higher temperature rating. If that’s not possible, add cooling devices and increase the reservoir size.
Another frequent problem is hose twist, which occurs from improper installation or multi-plane bend. Twisting seven degrees may reduce service life by up to 90%.
Replace and reroute the hose to ensure that bending occurs only in one place. The use of bent tube or block-style couplings and adaptors may improve routing. When installing the assembly, hold the back-up hex nut to prevent it from turning and apply a twist. If male and female couplings are used on the same hose assembly, install the male first.
Cover blisters are caused by fluid incompatibility, gases under pressure, excessive vibration or condensation.
The solution is to replace the hose with one that’s compatible with the fluid, and bleed the system to get rid of trapped air.
The problem with flange head crack or separation is that flanges with a brazed or welded joint often soften the metal, which becomes a weak point. Replace the flange with a solid one-piece coupling that provides high-pressure reliability and increases the safety factor as well as burst ratios.
Monaghan notes a 3,000 psi coupling on a 3,000 psi hose does not always equal a 3,000 psi assembly. An often overlooked factor is the hose/coupling interface. Instead of designing just a hose or just a coupling, it’s necessary to design a system.
The next time you choose any hose and coupling, ask yourself whether the components have been designed for each other and validated together; whether you’re willing to accept an assembly with a guarantee for reduced impulse life and performance; and whether you’re prepared to risk catastrophic failure.
This article is condensed from a technical paper presented by Shane Monaghan at a meeting convened by the Hamilton section of the Society of Tribologists and Lubrication Engineers (STLE).