Don’t Wait For Hydraulics Failure
EP Editorial Staff | January 13, 2017
Leverage predictive tools and techniques to keep hydraulic equipment operating at peak efficiency.
Industries spend countless dollars each year to repair hydraulic equipment. Many times, hydraulic-component failures don’t hit service-interruption triggers, i.e., specified hours of lost production or dollars worth of damaged equipment, and pose no safety or environmental impact. Thus, problems can go virtually unnoticed, until it’s too late. Traditional hydraulics maintenance practices are of the run-to-failure variety, meaning that a component is replaced and the machine is returned to service without any documentation as to what caused the problem, which, accordingly, can recur.
Consider the detail of a piston pump that is shown in Fig. 1. Cavitation resulting from a restriction on the inlet line caused the unit to fail catastrophically—to the point that the piston shoes melted. This failure could have been easily detected weeks in advance through one of three proven condition-monitoring approaches:
• making a thermal image of the pump inlet
• using ultrasonics to listen for cavitation bubbles
• placing a vacuum gauge in the inlet line of the pump and monitoring for minute changes.
These types of predictive technologies, coupled with practical reliability principles and strategies, can help plant personnel quickly identify failure modes in hydraulic systems, appropriately communicate that information, and prevent premature failures and unwanted downtime. The question is, if, as Merriam Webster defines it, “maintenance is the act of maintaining or the state of being maintained,” why are so many of today’s maintenance technicians spending so much of their time working on failed hydraulic systems after the fact?
Repairing and replacing equipment, changing out parts, turning knobs, and troubleshooting are examples of “failure based” activities. A more effective use of technician and machine-operator time would be for them to regularly engage in “improvement-based” activities such as looking and listening for problems before equipment fails. That, of course, requires predictive-maintenance (PdM) routes.
Build meaningful PdM routes
When it comes to building meaningful PdM routes for hydraulic equipment, we first must understand how these systems are supposed to operate and how they can fail to perform their intended function in a circuit, down to the component level. To paraphrase one industry rule of thumb, a clear operating context forms the basis for all of a plant’s reliability efforts.
The schematic in Fig. 2 details a typical operating context and how it applies to hydraulic-system reliability and service life. Let’s consider one item in the circuit—the hydraulic cylinder—in terms of predictive maintenance. It’s been designed to extend 12 in. in 8 sec., using 3 gal./min. (gpm) of pump flow, and operate at 1,200 psi. These design parameters represent four things that can be measured, trended, and compared to a target during this one machine function.
In the above scenario, what would be considered a functional failure and how could we measure it?
The hydraulic cylinder doesn’t extend at all, due to lack of pump flow to the cylinder, which is caused by an improperly set relief valve, allowing the pump flow to return to the reservoir. The situation could be a result of an operator adjusting the system’s relief valve to a setting below the 1,200 psi required to move the load and, thus, changing the entire machine operation.
As shown in Fig. 2, if a flow meter is installed on the discharge of the hydraulic pump and another flow meter installed on the return line of the relief valve to the reservoir, we can instantly see where oil is going. Without such monitoring devices, we play a guessing game with regard to a very simple problem that could result in many hours of downtime.
With effective PdM routes, during normal operation, a quick look at the pump-discharge flow meter would allow personnel to know whether the required pump discharge flow of 3 gpm is actually leaving the unit. Any variance in this number should prompt use of other monitoring devices to identify the root cause and develop a plan to deal with the problem before a component failure occurs. After all, decreased pump-discharge flow could have many causes, including the previously mentioned restriction on the pump inlet (cavitation). Monitoring can be done with a vacuum gauge located at the pump inlet. (Many people are unaware that a strainer/filter is typically located in the reservoir at the pump inlet. As the strainer becomes clogged over time from normal wear and debris, the vacuum pressure will rise, showing a potential problem.)
Another cause would be excessive internal wear of the pump. Over time, the pistons move in and out of the barrel, causing internal wear that opens the clearances and allows the internal leakage rate to increase. This movement of oil from high to low pressure will cause excessive heat that can be identified using thermal imaging. Trending the case temperature will reveal a slight rise over time and point to this potential problem. (Adding a flow meter to the case drain line of the piston pump would also allow personnel to see an increased leakage rate.)
The fact is that, unless a plant has condition-monitoring devices installed on its hydraulic systems, technicians and operators will always be in a reactive, or firefighting, mode.
Develop reliability scorecards
Given the fact that reliability in industry is based on measuring and improving, among other things, maintenance practices, equipment service life, and production processes, scorecards can be very effective tools.
Before a site can develop reliability scorecards, its reliability engineer, or team, and the production manager must first determine target-operating parameters. This will establish a baseline from which everything will be measured. While some facilities are willing to endure variances in their operating cycle times, some cannot due to tight production schedules and the need to produce specified numbers of products per hour.
Table I is an example of a typical reliability scorecard. In it, all operating parameters are spelled out in detail. Posted on the equipment to which it applies, this information should allow personnel to clearly understand what the machine is designed to do, as well as when it is not serving its intended design purpose and has functionally failed.
Such scorecards give operators and maintenance teams (including PdM technicians) the answers to four crucial questions:
• What do I check?
• What do I check with?
• Where do I check?
• What should I expect to read?
The increasing complexity of hydraulic systems and often-vague PdM checklists that many OEMs and suppliers recommend make the explicit information in these reliability scorecards ever more valuable for plant personnel.
Words to the wise
Sites that are truly seeking better ways to manage their hydraulic systems and, in the process, eliminate problems that could result in catastrophic failures, would do well to adopt these predictive-maintenance strategies. They can help increase a plant’s profitability by removing the avoidance costs of what it’s currently spending on repairs and send the savings back to the bottom line. MT
Information in this article was provided by Paul Craven, CFPHS. Craven manages one of Motion Industries’ (Birmingham, AL) repair shops in Pensacola, FL. Certified by the International Fluid Power Society as a Fluid Power Hydraulic Specialist, he has worked in the field for 25+ years. For more information, visit motionindustries.com.