What do lifting chains, bladdertype accumulators, and hydraulic hoses all have in common? 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.
Hoses can leak from around their ferrules and show signs of abrasive wear, both of which are early warning signs that replacement would be prudent. But even under these conditions, estimating their remaining service life is virtually impossible.
Beyond the fact that their service life is finite and difficult to estimate, hydraulic hoses have other issues when compared to metal tubing, including:
- Hose expands and elongates under pressure. This flexing requires extra volume and adds to machine cycle time.
- Hose typically has a limited operating temperature range.
- Their requirement for regular replacement makes hoses a source of contaminant ingression.
- Hose is more expensive.
Despite these disadvantages, hoses are essential components of most hydraulic systems because the alternative conductor - metal tubing - cannot be used where:
- There is limited space (particularly in mobile equipment).
- There is relative motion between mating components.
- Noise and vibration need to be suppressed.
However, hose is often substituted for tube out of convenience. This is because a hose assembly can usually be fabricated and installed faster than a tube assembly can. The additional labor cost associated with metal tubing can make hose appear to be the cheaper solution. However, 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 H&P in May 2009 (http://bit.ly/HP509Infrared) illustrates:
An industrial hydraulic system was designed to operate at maximum of 1200 psi and 120°F. Zinc di-chromate coated steel tube distributed fluid from a 600 l reservoir to many stations around the plant. Over the years the system had been added to (without increasing its cooling capacity) to the point it was overheating in summer months.
Because the system operated well 10 months of the year, management did not want to upgrade the cooling system. So one of the maintenance workers suggested painting the hydraulic system’s tubing. Before proceeding, the maintenance team applied electrical tape to two hydraulic tubes, and measured the difference in temperature between the taped and untaped areas with an infrared camera. They found the taped areas on the tubes were 7°F cooler than in the untaped areas.
The tubing was painted flat white to match the rest of the hydraulic system. A week later, the system was running 10°F cooler. This may not sound like much, but it meant the hydraulic system could operate through the two hottest months of the year without overheating. It also deferred, if not eliminated, the need to increase installed cooling capacity.
Managing hose replacement
Despite the reliability of tube, and for reasons already explained, hoses are necessary in most hydraulic machines. And unlike tube, hoses are a maintenance item. So in a condition-based maintenance environment, how can you proactively manage hydraulic hose replacement?
Two considerations 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 replacement of hoses that had failed in service on their hydraulic shovels had resulted in machine availability falling to as low as 65%. In this case, when a multi-million dollar shovel stopped, so did a multi-million dollar fleet of haul trucks. Downtime was a major cost. So the client implemented a hydraulic hose replacement program. This involved changing out every hose on the machine every 18 months.
So when a shovel was down for planned maintenance, a portion of the hoses was replaced — beginning with the oldest. It worked; machine uptime was soon back above 90%. These large diameter, multi-spiral, hydraulic hoses aren’t cheap. But hose expense paled to insignificance when compared to the cost of the downtime it prevented. So by any economic measure, the hose replacement program was a success.
But it did have a fundamental flaw. It was treating all hoses the same. It was highly unlikely that each hose was equally responsible for an equal proportion of the in-service failures and downtime. So I advised my client to study historical data, and that he should expect to find a relatively small percentage of hoses were responsible for the majority of in-service failures and downtime.
In fact, the available data revealed that less than 20% of the hoses on the machine were responsible for nearly 90% of the failures. Armed with this data, the hose replacement program was optimized to reduce unnecessary hose change-outs.
Hydraulic hose and tube are not interchangeable; they are different tools for different jobs. The benefits of hose— which make it necessary for 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 tube cannot.
Brendan Casey has more than 20 years experience in the maintenance, repair, and overhaul of mobile and industrial hydraulic equipment. Contact him at firstname.lastname@example.org or visit www.hydraulicsupermarket.com.