Hydraulic system maintenance

Hydraulic system maintenance

Hydraulic system maintenance

When an assembly is in really bad shape, deciding to repair or replace can be a gray area. Cost is certainly important, but time might make the decision for you.

Back in 1986, Hydrotech Inc. supplied four central hydraulic power units (HPUs) to a tire manufacturer for several curing presses. Each HPU can provide hydraulic power to as many as four presses. After successfully serving the manufacturer for more than 25 years, they were put in storage when circumstances at the plant changed. Nearly three years later, though, the manufacturer needed greater capacity at one of its facilities.

However, because they were nearly 30 years old, the HPUs no longer matched up to the plant's more-modern presses. Furthermore, many of the components—including electric motors, hydraulic pumps, and other components—had worn out and were badly corroded. Without these operational presses, rubber products would be backed up on the production line because curing is the final step before the tires are tested. Downtime was the enemy, costing the customer money and delaying shipments to distributors.

Plant management had a choice: Either have new HPUs designed and built, or refurbish the existing ones. Because the original HPUs incorporate an unusual reservoir, designing and fabricating a new one would’ve delayed commissioning. However, Hydrotech designed and built the original HPUs using primarily standard components. They determined that if they could use the original reservoirs, the 30-year-old HPUs could be rebuilt to better than when they were new.

A Relic is Rebuilt

Hydrotech’s Service & Repair Division inspected the HPUs and determined that the reservoirs for all four units needed to be refurbished by dry-ice blasting and alkaline cleaning to renew their surfaces and prevent future corrosion. In doing so, it was able to make the central hydraulic units fully operational again.

Primary components were a different matter. After nearly 30 years, the HPUs were closer to technological relics than they were to the latest technology. Hydrotech consolidated the inner workings with smaller, more powerful hydraulic components. Newer electronic controls made many of the old hydraulic and pilot lines unnecessary, necessitating their removal. Circuitry was also modernized by incorporating multiple valve functions into manifolds.

Each hydraulic pump’s output line originally was plumbed directly to a check valve and unloading valve, with no output pressure feedback to the presses. Technicians installed a new relief valve assembly containing a pressure transducer for feedback to the work center’s PLC. The reservoir was also fitted with an electronic level-temperature switch. Together, these components provided real-time data on pump performance and to monitor hydraulic fluid level and temperature. For safety, a fire detection switch was installed above the motor-pump group to quickly alert the operator if a fire had started. This made the central HPUs more powerful and reliable, as well as safer.

Step-by-Step Procedure

As we do with any assembly such as this, the first step in our process was to completely dismantle the HPU, so as to individually test each component to determine if any can be rebuilt and reused in the reworked unit.

Reservoir—The reservoir is internally inspected for rust, broken welds, and other damage. Any evident damage that we believe compromises the integrity of the structure is repaired. Examples of this would be rusted drip pans and baffles that have broken loose. For this set of repairs, we found that one of the internal baffles had broken loose. It was welded back in place to ensure that the return oil was turned over before moving its way to the suction side of the reservoir.

Any modifications needed are resolved before sending the reservoir out for alkaline cleaning. This might include adding additional down lines, or brackets for components to be added. Next, any rusted areas are removed (cut out) and new steel welded back in. All welds of the reservoir are tested with a non-destructive crack detection penetrant fluid.

Pump—All internal hard parts are inspected for signs of wear or damage caused by cavitation, contamination, or high case leakage. Such problems usually indicate that changes to the system design are needed. All soft parts (seals) are replaced, as are hard parts needed to complete the repair. The pump is tested separate from the rest of the power unit to ensure good working condition, as well as to ascertain that it meets factory specifications.

The pumps we originally installed on these HPUs were Rexroth 1PV2V4-2X pumps that had been phased out in the mid-’90s. The outdated electric motors were replaced with more-efficient Toshiba EQP Global motors. The obsolete Rexroth pumps were replaced with Bosch Rexroth 130-cc VPV vane pumps.

Valves—Spools and bores of both the main and pilot stages are checked for wear caused by contamination and velocity sheering. If the HPU has onboard electronics, the electronics are bench tested to verify proper function. All parts are then cleaned, the seals replaced, and the valve reassembled and tested. The valve is then calibrated and tested to ensure it meets factory specifications and characteristic curves.

Heat exchangers—Heat exchangers are flushed out with brake wash or other quick evaporating cleaning solutions and pressure tested to check for pinhole leaks. If no leaks are found, the heat exchanger can be sent out for alkaline cleaning. Once returned, the heat exchanger is flushed out again, seals replaced, and pressure tested one final time.

The HPUs at the rubber plant used shell-and-tube heat exchangers. We found many of the tubes had blockages, calcium buildup, and other contaminants in the water supply. We made a joint decision with plant personnel that it was more cost-effective to replace the heat exchangers than to repair them.

Manifolds—We usually presume that no hydraulic schematic is available for any manifolds mounted with the HPU. We stamp all components on the manifold to ensure they are re-installed in the correct location after the manifold has been cleaned and tested. Next, the manifold is sent out for alkaline cleaning, then sent to a third party for florescent penetrant testing to check for cracks and fissions between the cavities within the manifold.

Once the manifold has been returned, all seals on the components are replaced, reinstalled on the manifold, and pressure tested as a complete assembly to verify proper function and ensure that no leaks exist.

Fittings and tubing—All fittings are inspected for cracks and damaged threads. All metal tubing in good condition goes through an alkaline cleaning process to ensure no surface rust or other damage. The original hoses and tubing showed signs of corrosion and leaks at the metal sealing surfaces. The decision was made by the customer to replace all the ferrule-lock fittings and tube and hose ends with 37-deg. JIC fittings.

Hoses—All hoses and end couplings are replaced with new.

All components found to be damaged beyond economical repair are replaced with new. Final assembly of the power begins once all components have been repaired and individually tested. Once components have been installed, the power unit is filled with hydraulic fluid and tested to ensure proper operation.

After testing and verification are complete, the reservoir is drained and wiped out with lint-free cloths and quick-evaporating brake wash. The entire assembly is then cleaned, prepped, and painted to specifications prior to being shipped.

Operational in Only Two Weeks

The entire process for the four 30-year-old HPUs took only two weeks from inspection to completion. Plant management’s decision to repair and upgrade the HPUs instead of replacing them averted the cost of buying a new reservoir and the engineering costs of a new design. More importantly, the decision avoided the downtime that would’ve resulted from having to engineer, manufacture, and assemble new HPUs from scratch.