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September 6, 2002

9 Min Read
Technology Notebook: Supplemental oil filtration protects IMM hydraulic systems

Editor?s note: Jack Berg is president of Serfilco Ltd. in Northbrook, IL. He has extensive experience in filtration of hydraulic oil on a wide range of industrial equipment, including injection molding machines.

Injection molders tend to have characteristic responses to the idea of using supplemental hydraulic oil filtration to protect their equipment. One is, "Our systems are old. They leak a lot so we just replace the oil and put some absorbent around the machine." Others say their systems are new and include inline filters on the machines so there is no need for additional filtration. Some maintenance superintendents replace seals and pump packing or fix other sources of leakage to keep molding machines running.


The plastic injection molding machine above is typical of the type of equipment that can benefit from the supplemental filtration of hydraulic fluid.

Hydraulics Basics
A hydraulic system creates pressure by using a pump to build force against a noncompressible liquid. Through a series of valves opening and closing, and without manual involvement, the hydraulic system creates the back-and-forth sliding movement of a shaft, which, in turn, with cams, makes another part of the equipment go up or down, open or close.

We could say the hydraulic system works like the human body. For instance, it has a heart (pump) and lungs (breather), which allow the system to work. The pump, driven by a motor, is made with tight clearances to create the pounds of pressure needed to perform the task.

What Causes Hydraulic System Malfunction?
Hydraulic system failure very often involves contamination in the fluid. How does a brand-new system, flushed clean at the manufacturer's facility and filled with new, clean fluid, become contaminated? After all, isn't the fluid contained in a closed loop? Yes, but pumps involve gears and other close-tolerance components that move against each other and are separated only by the thin film of fluid in the system. Over time in operation, the action of metal against metal causes wear, which results in small (micron and even submicron-sized) particles to be suspended in the circulating fluid.

The breather, another source of contamination, allows atmospheric air to enter the fluid reservoir to replace the fluid as the fluid forces a shaft to move forward. As a shaft makes its reverse movement, the fluid returns to the reservoir and the air is pushed out of the breather.

Another source of contamination is the shaft. It moves out of the system during forward operation and is wiped clean by a seal on the way back into the system. Subjected to this continuous abrasion, the seal ultimately fails, causing fluid to leak out or abrasive particles to enter the recirculating fluid.

Therefore, all hydraulic systems include filters (sometimes referred to as strainers) installed in the circulating system of the fluid, and a filter is also installed in the breather to perform the same duties. But alas, nothing is perfect.

Although the human body may be able to operate under severe conditions for a limited or even an extended period, a hydraulic system subjected to ongoing adverse conditions simply self-destructs. A pump does not replace the wear caused by contamination, but rather continues to wear more and more from the effect of the previously created particles. It then fails to create the needed pressure due to slippage.


This is a portable filter system equipped with a large-capacity filter chamber for removing solid contaminants from hydraulic oil, and a coalescing chamber for removing water. The unit can be wheeled from machine to machine as needed.

Addressing Fluid Contamination
The logical answer to prevent wear and service downtime is to remove the particles as quickly as possible from recirculating hydraulic fluid. The challenge is to remove any particles from the pumps or valves that are finer than the thickness of the oil film, and most importantly, to do it before any damage occurs.

A well-designed hydraulic system includes a myriad of devices to remove the damaging particles. The question is: Will the filtration within the fluid system perform the needed cleansing or is additional filtration capacity desirable? Is particle removal worthwhile? When continuous, uninterrupted reliability is expected, the cost of downtime is the determining factor. In this scenario, more filtration is better.

It is important to determine how much space such a system occupies, the flow rate, and the particle-size retention of the filter media. Answers usually can be arrived at mutually by the user and the equipment supplier.

Portable Filtration
Some maintenance personnel use a portable filtration system?one that contains its own pump, filter chamber, filter media, and piping?that can be moved from machine to machine and become integral to the fluid system through the use of quick disconnects. Others employ hoses inserted into the breather opening of the hydraulic reservoir, or add a filter in a shunt system to provide additional filtration by processing a small quantity of fluid diverted from the pressurized piping.

A system individually piped to the hydraulic reservoir provides good results. The pump can be energized independently to remove particles even when the hydraulics are not operating. It may provide additional purification of the fluid from acid-removing media, or provide moisture reduction. Since the system is independent, as the filter flow rate is reduced or backpressure in the filter is increased, the performance of the hydraulic system is not affected.

However, some users choose not to energize the hydraulic reservoir filtration systems independently, instead energizing them only when the main motor starter of the machine starts the electrical motor that runs the hydraulic pump. This has two advantages: No one has to remember to turn the filtration on or off, so when the machine is running, the oil is continuously filtered.

Permanent Supplemental Filtration
A supplemental filtration system permanently mounted on the machine and piped to the hydraulic reservoir may provide low maintenance and reliable operation. By tying the system to the motor start function of the machine, the oil reservoir is filtered whenever the machine is running. Piping the system with shutoff-type quick disconnects at the filtration pump and the reservoir allow for easy maintenance access to both.


A machine-mount filter system is mounted directly to hydraulically operated equipment. Above, a filter assembly is mounted at the top of a molding machine. Below, the unit is mounted on the side of a machine.


Additional oil filtration may benefit the machine operator in an unexpected way by relaying to the maintenance department the true importance of clean oil in the machine. When the closed loop hydraulic system of a machine must be opened for maintenance, personnel will surely take greater care to prevent contamination of the system.

Monitoring, Analysis, and Supplemental Filtration
With today's sophisticated machine controls, in combination with the personal computer, maintenance personnel can carefully monitor the actual hourly service life of the hydraulic oil and the frequency of filter cartridge changes. One injection molder reports that a preventive filtration maintenance program should include a cartridge change at 720-hour intervals?actual time logged on the machine?or when the differential pressure between in and out on the filter chamber reaches 25 to 30 psig. This molder goes on to say that the filter system keeps his oil so clean that the pressure differential never reaches 30 psig, so he uses the 720-hour schedule to maintain his equipment.

When the filtration system is first installed, this interval should be cut in half and oil analysis should be performed frequently until a noticeable difference in the clarity of the fluid is detected. Using this procedure, the clarity of the oil can be monitored and the maintenance interval can gradually be increased until the 720-hour interval is reached.

Oil changes in the injection molding machine or other equipment incorporating a hydraulic system can become rare with an extensive program of regular oil analysis. It is a common misconception that hydraulic oils break down in service. While this is untrue, it is true that the anti-foam, rust, and oxidation inhibitors contained in premium grade hydraulic oils can be depleted at an accelerated rate if contaminants?such as particulate matter and water?are not routinely removed from the oil during uptime. Thus, coalescing systems with filtration capability are an asset.

Today's sophisticated oil analysis instruments can monitor the level of the various additives and signal when more are needed. Additional filtration not only saves the cost of hydraulic oil, but also downtime to change the oil is reduced.

The history of another injection molder illustrates the benefits that can be derived from supplemental filtration and close monitoring of the hydraulic fluid. At this firm, a new general manager learned that his predecessor had replaced the hydraulic oil in all six injection molding machines four years earlier. This maintenance expense cost the company $4312 (1540 gal or $2.80/gal). This plantwide oil replacement did little to minimize the hydraulic breakdowns that constantly plagued the shop. In nearly every case, the breakdown, or poor and inconsistent machine performance, was traced back to contamination in the hydraulic system.

The plant budget again presented the opportunity to change the oil in all of the machines. Although the new general manager made a similar expenditure, he chose instead to buy filtration equipment rather than change the oil. Each machine was equipped with a permanently mounted, external, 3-gpm filter unit. These filter units are wired and piped in place so that whenever the hydraulic system pump motor is running the filter unit is also operating, constantly filtering the machine's hydraulic reservoir.

Prior to installing the filtration units, the general manager tracked machine downtime using machine hour meters and a personal computer. He used the computer to monitor operational downtime with a numerical scale to predict future trouble. He determined when downtime reached an unacceptable level and it was time to overhaul a machine or replace it, and he found that the six machines averaged a total of two hours of downtime per month attributable to hydraulic problems.

During the first six months of operating the machines with the externally mounted filters, the downtime rate for hydraulic problems was cut in half. Throughout the next 12 months, the instances of hydraulic downtime continued to diminish, and the rate of hydraulic downtime leveled off at 18 minutes per month?an 85 percent reduction from the original rate. In addition to reducing downtime, the use of the filtration units eliminated the need for oil replacement in the machines.

Contact Information

Serfilco, Ltd., Northbrook, IL
Jack Berg
(847) 509-2900

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