Precision lubrication for high-speed spindle bearings

Converting from oil-mist to air/oil lubrication clears workplace air and conserves oil.

By William Mitchell

Precision lubrication is the lifeblood of the high-speed spindle bearing. This topic is relevant for review because there has been a great deal of development effort put forth on behalf of bearing lubrication delivery systems over the last 10 to 12 years. These new systems offer choices and benefits not previously available to user of high-speed spindles.

Historic perspective

For years, machine tool builders have tried to develop spindles for higher speeds. This effort improved surface finish and increased stock removal rates. In addition, they have had to meet requirements for increased spindle bearing life, higher load ratings, increased bearing stiffness and tighter tolerances on internal bearing parts.

In the early 1930s, oil mist was developed as a means of distributing lubricating oil to spindle bearings. Only a small amount of oil is required, but flow must be continuous. The "bearing-inch" concept originated sometime in the late 1930s to early 1940s. The concept states that adequate lubrication requires 0.01 fluid ounce of oil per hour per inch of bearing. Therefore, a four-bearing spindle using a single-row size 105 bearings (approximately one in. diameter) requires:

q = ID*N*C

= 0.9843 x 4 x 0.01

= 0.0394 fl. oz. per hr.

where q = oil flow (fl. oz. per hr.)

ID = bearing diameter (in.)

N = number of bearings

C = constant = 0.01 fl. oz. per hr. per bearing-inch

Though the bearing-inch concept only estimates the flow rate, it did give the spindle design engineers a working standard that proved itself successful on a vast number of spindles during the ensuing years.

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Air and oil

During the past 45 to 50 years, the air-oil mist lubrication system, as well as improvements in bearing and spindle design technology, have allowed spindle bearings to be operated at much higher speeds than previously thought possible.

The success of air-oil mist technology came with a price, however. It proved, over time, that fugitive oil mist affects the factory environment. Further, not every oil can be converted to mist, so the blends available were limited. Also, air consumption was high and costly. Oil misting demands special air filtration because air passes through the bearings along with the oil. In many cases, the air requirement was as high as 10 to 15 cfm per spindle. This flow was required because early spindle designers felt a need to use air as a coolant. In addition, the air sealed the spindle's output shaft to help exclude contaminants.

The air consumption with oil misting is significant. For example, in a factory with 20 machines, the air consumption is 200 to 300 cfm. This is a high volume, and costs to operate and maintain the air system are significant.

New lubricant delivery system

Technological advances have overcome some of the difficulties.

In air/oil injection technology, the oil isn't converted to a mist first. It's injected directly into an air stream. A positive displacement pump moves the oil from a reservoir to a mixing valve, which mixes it with air. The mixture is fed through a small tube and nozzle, or and orifice, directly into the bearing, which then receives a fine spray of oil.

These new systems offer choices and benefits not previously available to user of high-speed spindles.

– William Mitchell

Unlike oil-mist, air/oil injection produces a two-phase flow in the distribution tubing. The oil moves along the wall, pushed by the air moving down the center. This stands in direct contrast to the flow of mist.

The results are immediately obvious. The only mist the system produces is within the bearing itself. One also may use an enhanced oil to improve bearing life,something that's impossible with oil-mist units. One can expect less fugitive oil and a clearing of the work environment.

Oil distribution lines can be thinner for ease of routing. The required air volume can be reduced significantly. For those cases requiring air for bearing cooling or sealing, a bypass can provide increased airflow.

The quantity of oil a bearing needs is quite small because any oil injected into the spindle reaches the bearing.

Oil reclamation

As good as these improvements are, they don't go far enough. The final piece of the puzzle is to collect and recycle or dispose of the oil after it has done its job.

With some minor mechanical modifications, most spindles are candidates for an oil reclamation unit. New spindles can be manufactured with provision for air/oil or oil-mist reclamation.

Conversions

Sometimes, it's possible to convert spindles designed for oil-mist to an air/oil system. It usually depends on the method used for distributing oil-mist inside the spindle.

If it has lubrication reclassifier orifices or interior nozzles at the bearing locations, there's a good chance that a conversion is possible. If the spindle uses so-called "cavity" lubrication, wherein oil-mist is blown into its body, then be wary. More investigation is necessary.

There are many variations in spindle design. Each must be taken as a separate case. It's often necessary, and advisable, to have the lubrication equipment supplier, the spindle OEM or spindle rebuilder review the spindle and give recommendations regarding the conversion.

Remember the basics

The bearing-inch has been, and continues to be, a good benchmark for oil quantity. Each supplier of lubrication equipment may have variations in the way oil quantity is calculated. It is important to check the calculation variables. It is better to err with too much oil than too little. Most ball bearings are reasonably tolerant of higher oil flow.

Know your spindle. Check with the tooling manufacturer or spindle rebuilder to find the bearing specifications. In addition, and just as important, find out exactly how the oil is directed to the bearings.

Don't force the system to do more than lubricate the bearings. For example, if an air seal is needed, design an appropriate seal. If extra air is needed for cooling, make separate provisions for it.

Several lubrication equipment manufacturers offer air/oil systems. One such air/oil conversion panel assembly is shown in Figure 1. It contains the necessary hardware for a conversion. More than one hundred of them are in operation around the country.

When applying these panels to a machine designed for air/oil lubrication, set the oil output to 0.03 cc every three minutes. This results in a total consumption of 0.6 cc per hr. Alternatively, use approximately one-half the bearing-inch requirement for a four-bearing, 35,000-rpm spindle.

For a spindle designed for oil-mist lubrication, set the oil flow to 0.05 cc every three minutes. This results in a total consumption of 1.0 cc per hr. That figure corresponds to approximately 85 percent of the bearing-inch recommendation for a four-bearing, 35,000-rpm grinding-spindle.

When one contrasts these oil quantities with those recommended by some machine tool builders, which often suggest as much as 10 to 11 cc per hr., it's clear that far too much oil is being used. It's also readily recognized that bearings are extremely tolerant of varying oil quantities.

William Mitchell is principal consulting engineer at William A. Mitchell Assoc. He can be reached at (802) 885-5068.

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