Maintenance: Filter check, Part Two

Dirt hasn’t changed much over the centuries, but the close internal clearances on many of the components in today’s high performance off-road equipment can’t tolerate it as well as older machines. Nowadays, cleanliness and contamination control are crucial to long equipment life and good maintenance. And the only thing that stands between your equipment and ruinous amounts of dirt and contamination is your filtration program. Last month we looked at lube oil, fuel and exhaust filters. Now in part two of the series we’ll examine the technology behind air, hydraulic, and breather filters.

    Consider this: In the course of burning one gallon of diesel fuel a typical engine will consume more than 8,000 gallons of air. And should that engine inhale as little as a teaspoon of dirt over any length of time, it will soon begin to show signs of accelerated wear and premature failure.

    That makes your machines’ intake air filtration a critical first line of defense against contamination. To enable off-road machines to breathe cleanly amid all the dust kicked up on a jobsite, most manufacturers offer three layers of air filtration. This starts with a pre-cleaner – a cyclone separator or multi-tube pre-cleaner that drops out or ejects 75 to 95 percent of the heaviest airborne particles. Next downstream is the main filter, which stops 99.9 percent of the remaining dust, followed by an inner, secondary or safety filter.

    There is some misunderstanding about what these different layers of filters do, says Tom Miller, director of air product management for Donaldson. “The safety filter isn’t a polishing layer or part of a two-stage filter like some people think,” Miller says. “It doesn’t add to the filtration capacity. It’s there in case there is a failure of the primary filter – either a filter that had a breach in it or a filter that was installed wrong. It also protects the induction system when the primary filter is being changed.”

    Air filter maintenance is also sometimes misunderstood. Filtration experts all describe their worst nightmare: a guy pulling a primary filter off an engine, knocking it on a fender to loosen up the dust – or blowing it out with an air compressor – and then re-installing it. Although not uncommon, this practice is guaranteed to send that engine to the rebuild shop much earlier than necessary. Knocking dust out of a filter or blowing it out with compressed air immediately allows dust to accumulate on the clean side of the filter. And blowing compressed air onto a filter may remove a lot of dust but it also forces some dust more tightly into the fabric of your filter media, thus increasing your restriction.

    There are several types of pre-cleaners. The tube-type pre-cleaners found on haul trucks collect dirt in a bowl that you simply empty when full. But the two-section designs that scavenge by an ejector muffler or rotating vane type pre-cleaners don’t require any maintenance, Miller says.

    The first rule of air filter maintenance is the less frequently you change it, the better. Every time you open the compartment up you introduce contamination. Some people change the primary air filter when they change the oil filter, Miller says, but you really shouldn’t. Wait until the restrictor gauge tells you it’s time to change and not before. You can’t tell when a filter is clogged just by looking at how dirty it is. And if you change air filters too frequently you’re actually putting more contamination into the engine since the filter is least efficient when it’s brand new. “Service by restriction,” Miller says, “not by scheduled maintenance.” The safety filter, as a general rule, only needs changing during every three or four changes of the primary filter.

    One of the biggest changes in heavy equipment over the past decade is the move from gear-driven hydraulic pumps to piston pumps. The new pumps give your joysticks and implement levers great feel and load sensing capabilities, but that precision requires pumps with high pressures, closer tolerances and cleaner hydraulic fluid. Variable displacement piston pumps require higher cleanliness than fixed displacement piston pumps. And applications of more than 3,000 psi require cleaner fluid than those under 3,000 psi. Plus the growing popularity of attachments on excavators, backhoes and skid steers also provides new opportunities for contaminants to enter the hydraulic stream.

    The measurement used to gauge the cleanliness of your hydraulic fluid is the ISO 4406 standard that describes the amount of particulate matter in the fluid. The code refers to particle counts at 4-, 6- and 14-micron levels and is expressed as a ratio. The key thing to remember, says Frank Scherbing with Caterpillar’s filter and fluids sales support group, is that every time you go up a level, say from 16/13 to 17/14, you double the amount of contamination in the 6- and 4-micron range. Conversely when you go down a level, you increase your fluid cleanliness by 100 percent.

    With proper filtration, hydraulic fluid has a much longer life than lube oil and well maintained fluid could last indefinitely, says Bert Elfers, director of product management for Baldwin Filters. To maintain it properly an oil analysis should be performed every two to six months depending on the severity of the application. Filter changes should be performed on the basis of pressure drop and the cleanliness of the oil, he says.

    The ISO cleanliness level is determined by the most sensitive component in the system, Elfers says. A simple gear pump and cylinder typically requires only a 10-micron return line filter to maintain the required cleanliness level of 19/17/15. On the opposite end of the spectrum, servo valves require levels on the order of 16/14/11 with 4-micron pressure filters and kidney loop filtration circuits to polish the oil. Factors such as pressure and the type of fluid used (such as petroleum based vs. fire resistant) will affect the required cleanliness level, Elfers says.

    To gauge the cleanliness of hydraulic oil you need a particle count as part of the oil analysis. A particle counter is an expensive and specialized piece of equipment and not something most contractor shops would own, but many equipment dealers and oil labs are set up to give you particle counts as a part of their scheduled oil sampling.

    Greg Skeels, Caterpillar’s engineering manager for the filters and fluid support group, says it’s best to take your hydraulic oil sample after the engine and hydraulic system are up to operating temperature, ideally at lunchtime after a half day of work. That way the fluid has been well circulated and it’s warm enough that elements of the oil’s additive package will be fully dispersed and not give you a false count as can sometimes happen with cold hydraulic fluid.

    Knowing the cleanliness of your hydraulic fluid is key to choosing the right filter and evaluating the effectiveness of your hydraulic filters and contamination control procedures. Anytime you open the hydraulic system, whether it’s to simply replace a hose, repair a pump or motor or put in a new hydraulic cylinder, you need to switch from your regular hydraulic filter to what’s known as a clean-out filter.

    A clean-out filter looks like a regular filter but filters to a finer level, typically 6 to 10 microns and is left in place on the machine for 250 hours or until the desired cleanliness is achieved, Scherbing says. Then you would put on a regular filter with a rating of around 25 microns, he says.

    Once the desired fluid cleanliness is achieved, there is little benefit to continuing to use the finer clean-out filter as it will only load up more quickly and have to be replaced more often, Elfers says.

    Paul Ewing, national accounts manager for off highway at Luber-finer, subscribes to a different philosophy, recommending the 6- to 10-micron filter be left on the machine as a permanent filter.

    A kidney loop machine does much the same thing, except it is an off-machine device that pulls fluid out of the system, filters it and pumps it back in, much like a kidney dialysis machine.

    Choosing the best hydraulic filter for your needs requires that you balance the cost of the filter with the benefits. “When you get to some ultra high efficiency hydraulic filters you’re working with fully synthetic media and they’re fairly expensive to produce,” Scherbing says.

    Hydraulic systems are sensitive to pressure drop and contamination. As a result there are a large range of hydraulic filters using the more expensive synthetic media, Elfers says. Cellulose filter media can have equal efficiency down to about 10 microns and cost less than synthetic media but will typically have a higher pressure drop. But for the additional cost, the synthetic media will allow you to filter to a finer level without compromising flow. In practical terms that means you will enjoy cleaner hydraulic fluid and extended component life. Ewing says he has had customers who have documented a 50-percent reduction in hydraulic system wear particles by switching from a 25-micron cellulose filter to a fully synthetic 10-micron filter.

    There are a number of compartments on a piece of equipment that require breather filters, and while in the past these may have received scant attention, today they’re coming under closer scrutiny.

    About three years ago Caterpillar conducted some tests that showed breather filters could be a major source of contamination in certain machines and applications. In some cases the breather filters weren’t much more sophisticated than just some steel wool in a canister. “That takes nothing out of a dusty environment,” Skeels says. “And then if you think about a hydraulic system heating up and cooling down and filling cylinders and emptying cylinders, the amount of air that gets sucked into that tank is huge. So not having the right breathers has a significant impact on the overall system cleanliness.”

    One of the keys is to have a breather filter that has a micron rating as small as the primary filter for that compartment. “Otherwise you’re just wasting your time,” Ewing says.

    In some cases desiccants, chemicals that absorb and trap moisture are recommended for breather filters. But not in every case. If your OEM specs call for a hydraulic fluid that emulsifies water in the oil, then the oil takes care of the water. If your spec calls for a non-emulsifying oil, then a desiccant breather may be beneficial and reduce the water removal chores put on the regular filter. If you’re using OEM branded or approved filters, you’re fine, but if not check with your filter provider to make sure the filters you use are appropriate for the type of fluid that you’re using.