What do lifting chains, bladder-type accumulators and hydraulic hoses all have in common? Well, they typically have only one mode of failure – and that’s catastrophic. One minute they’re working as they should, and the next thing you know, they’ve gone to hell.
Oh, sure, hoses can leak from around their ferrules and show obvious signs of abrasion, both of which are early warning signs that a change-out would be prudent. But even under these conditions, estimating their remaining service life is virtually impossible.
The Trouble with Hose
Beyond the fact that their service life is finite and difficult to estimate, other disadvantages of hydraulic hoses when compared to tubes include:
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They expand and stretch under pressure. This flexing requires extra volume and adds to machine cycle time.
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They typically have a limited operating temperature range.
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Their requirement for regular replacement makes them a source of contaminant ingression.
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They are expensive.
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There is limited space (particularly in mobile hydraulics).
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There is relative movement between machine components and superstructure.
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Noise and vibration need to be suppressed.
Despite the above disadvantages, hoses are a necessary feature of most hydraulic systems. This is because the alternative conductor – tubes – cannot be used where:
- There is limited space (particularly in mobile hydraulics).
- There is relative movement between machine components and superstructure.
- Noise and vibration need to be suppressed.
However, hose is often substituted for tube when it’s not necessary. This is because a hose assembly can usually be fabricated much faster than a tube assembly. And, the additional labor cost required to fabricate and install a tube can make hose appear to be the cheaper solution.
But this belies the fact that the same hose may need to be replaced many times over the life of the machine. This false economy is similar to buying the machine itself on initial capital outlay alone without considering its total life-of-ownership cost.
Tube is Cool
Hydraulic tubing has some compelling advantages of its own. One of these is its superior heat transfer, especially if it’s painted.
One aspect of heat transfer is thermal radiation. The total radiation from an object is the sum of its reflection, emissivity and transmission of heat through the object.
When hydraulic tubing is painted, it reduces its reflectance and increases its emissivity, enabling better heat rejection, as this case study published in Hydraulics and Pneumatics magazine1 illustrates:
An industrial hydraulic installation was originally designed to operate at 1,200 psi and at a maximum operating temperature of 120 degrees Fahrenheit (49 degrees Celsius). Zinc dichromate-coated steel tube distributes fluid from the 600-liter reservoir to the various stations around the plant. Over the years, the system had been added to (without any increase in installed cooling capacity)—to the point where it was now overheating in the summer months.
Because the system operated satisfactorily for 10 months out of the year, management did not want to spend the money necessary to upgrade the cooling system. So, one of the maintenance guys on staff who was familiar with the thermal radiation theory described earlier suggested painting the hydraulic system’s tubing.
Before proceeding, the maintenance team did a test. Team members applied electrical tape to two of the hydraulic tubes, and using an infrared camera, they measured the difference in temperature between the taped and untaped areas. They found the taped areas on the tubes were 7 degrees F (4 degrees C) cooler than the untaped areas.
This gave the maintenance team the confidence to proceed with the idea. Because the rest of the hydraulic system was painted flat white, the tubing was painted the same color.
And, the result? A week and 12 cans of spray paint later, the system was running 10 degrees F (5.5 degrees C) cooler. This might not sound like much, but the end result meant that the hydraulic system could now operate through the two hottest months of the year without overheating. It also meant that the need to increase installed cooling capacity was at least deferred, if not eliminated.
Managing Hose Replacement
Despite the superior reliability of tubing, for reasons already explained, hoses are a necessary feature of most hydraulic machines. And unlike tubing, hoses are a maintenance item, one which usually gives no warning of impending failure. So in a condition-based maintenance environment, how can you proactively manage the replacement of hydraulic hoses? Well, there are two considerations which should be foremost in your mind: the downtime cost of in-service hose failures, and NOT treating all hoses equally. Consider this example:
A couple of years ago, I worked with a mining client whose ad-hoc replacement of hoses which failed in service on hydraulic shovels had resulted in machine availability falling to as low as 65 percent.
In this company’s case, when a multi-million-dollar shovel stops, so does a multi-million-dollar fleet of haul trucks. Downtime is a major cost. So, it implemented a hydraulic hose replacement program. This involved changing out every hose on the machine every 18 months.
Whenever a shovel was down for planned maintenance, a portion of the hoses were changed out, beginning with the oldest first. The plan worked. Machine availability was soon back above 90 percent.
Of course, large-diameter, multi-spiral hydraulic hoses aren’t cheap. However, hose expense paled to insignificance when compared to the cost of the downtime it prevented. By any economic measure, the hose replacement program was a great success. But, it did have a fundamental flaw. It was treating all hoses the same. And, it was highly unlikely that each hose on the machine was equally responsible for an equal proportion of the in-service failures and downtime.
I advised this company to study the historical data. I also stated that it should expect to find that a relatively small percentage of hoses were responsible for the majority of the in-service failures and downtime. In fact, the available data revealed that less than 20 percent of the hoses on the machine were responsible for nearly 90 percent of the failures. Armed with this information, the hose replacement program was optimized to reduce unnecessary hose change-outs.
The Final Selection
Hydraulic hoses and tubes are not interchangeable. They are different “tools” for different jobs. The benefits of hose – which make it a necessary feature of most hydraulic machines – come at a cost. It has a finite service life and usually gives no warning of failure. This makes it a difficult maintenance item to manage. For these reasons, hose should only be used where tubing cannot.
Read more on hydraulic system best practices:
Top 5 Hydraulic Mistakes and Best Solutions
11 Simple Steps for Flushing a Hydraulic System
10 Hydraulic Reliability Checks You Probably Aren't Making
Reference:
1. Hays, D., “Little Things Mean a lot In Manufacturing”, Hydraulics & Pneumatics magazine, May 2009, pp 38-41.
