Slippery Stuff

A few days ago, while topping off the hydraulic oil tank on my trusty old Crown reach truck, I had an unusual idea. My regular readers know I recommend knowing lubricant specs before changing to new g...

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June 1, 2008 by Jim Anderton, Technical Editor

A few days ago, while topping off the hydraulic oil tank on my trusty old Crown reach truck, I had an unusual idea. My regular readers know I recommend knowing lubricant specs before changing to new grades or types. Oil isn’t just oil, especially in modern lean-engineered faster-running equipment. Yet I had no idea what I was pouring into that reservoir, mainly because it was “only” hydraulic fluid.

No one thinks about hydraulic oils, and probably for good reason. Modern products are good enough to be foolproof and are generally cross compatible between brands, so it’s common to order and use them as a bulk commodity. Hydraulics is a different world, however. Unlike other lubes, the primary purpose of hydraulic fluid isn’t to lubricate, it’s to transmit power from one place (the pump) to another, namely the ram and/or hydraulic motor.

To do this, it needs to be incompressible. But what about entrained air? Bubbles and foaming are inevitable when any fluids are pumped and transferred. Hydraulic oils contain special additives to inhibit foaming, and common reservoir designs include perforated baffles that separate air from oil before it’s fed to the pressure side of the circuit. Of course, the fluid also has to lubricate the moving parts and inhibit oxidation and acid formation. And like any pressure lubricated system, getting suspended particles out of the oil is essential, by filtration and/or changing the fluid.

Unlike conventional machine lubrication, the pressure drop across the filter is especially critical since the oil is the power transfer medium. Unfortunately, standardized filter bases make it easy to swap in a filter with an incorrect micron rating or material. While it’s easy to say, “use the right filter”, what’s right? Cleaner is better, but a “beta” or micron rating that’s too tight will cause a significant pressure drop and may also increase fluid temperatures. Use a more porous media and pressure drop will be minimal, but fluid cleanliness suffers.

The simple, smart way is to use the equipment maker’s recommendation, noting the filter part numbers and the filtration specs. The specs are key, because you’ll eventually have to use an alternate to your main brand and your supplier will swear that his new line is a perfect interchange. How does he know? If you interchange by filtration specs rather than physical fit into the housing, you’re guaranteed to have OE-level performance. Running unfil- tered? I’ve seen it, and have talked to old millwrights who swear by frequent changes as a cleanliness strategy.

The old eyeball technique is bad news for modern close tolerance machinery; since the human eye can only see down to a 40-micron particle, the fluid is dead long before it’s visible to the naked eye.

While hydraulic fluids are relatively cheap, why do we assume it’s clean out of the barrel? A typical ISO 4406 21/18 (visit http://www.natrib.com/publications/articles/4Q01whitefield.php for an explanation) should be filtered right out of the drum before it sees the tank. Don’t have a filtration cart? Consider filling before the filter in the return line so the oil gets at least one pass before the pump.

Keeping hydraulic systems clean is a science in itself, and I’m as guilty as anyone of sloppy habits. After all, it’s not a press, just a 30-year old forklift. Of course, productivity is about parts out the door, not off the press, so everything, including materials handling, matters. So from now on, it’s clean fluid in a clean reservoir, always. Promise.


Jim’s Buzzword of the Month

Beta (bay-ta): In machine lubrication, the ratio of the number of particles entering a filter versus those leaving it, for a given particle size. For example, a five-micron filter with a Beta 80 rating will have on average 80 particles five-microns or larger downstream of the filter for every particle of the same size upstream. To convert to percentage efficiency, use: (Beta-1)/ Beta x 100. Beta is only one measure of filter effectiveness, but is a good way to compare filters and filtration systems.