BEARING LUBRICATION

Bearing lubrication

Lubrication – coating the contact surfaces of the bearing shaft and housing with grease or oil – is one procedure used in almost every step of bearing storage and operation. With their protective coating against rust and corrosion, lubricants are applied to bearings during storage, during assembly and mounting, during operation, before removal, and after cleaning. Lubricants have four major purposes:

• Reduce friction and wear

• Dissipate heat

• Protect surfaces from dust and corrosion

• Help seals protect bearings

Reduce friction

Bearings are constantly moving during operation. Their moving races and rollers rub against each other as well as the housings around them. In application of high speeds and under heavy loads, bearings build up tremendous friction. Too much friction will wear down the surfaces by rubbing together. This results in premature failure and damage to the bearing, shaft and housing. Lubricants reduce friction. When applied on and around the entire bearing, lubrication smooths rubbing action and saves bearing parts from early failure Lubricants protect the shaft and housing in the same way.

Dissipate heat

Heat is caused by bearing friction. With shaft speed and load conditions also contributing to friction, it is not hard for overheating to occur. Heat wears down contact surfaces between the races and shaft and housing seats. By dissipating, or carrying away this heat, lubricants can prevent temperatures from reaching a point where they can cause severe wear and destruction.

Protect surfaces from dust and corrosion

The smallest amount of moisture, dirt or dust can cause the metal in bearing parts to corrode. Because bearings must be clean and smooth in order to operate properly, corroded bearings are useless. For this reason, bearing lubrication starts during the packaging process. Immediately after production, a new bearing is coated with oil or grease so it will not be harmed by moisture or dust that can accumulate during storage and handling prior to installation. Lubrication also protects against corrosion that can occur between the tight press-fit of a race with the bearing seat.

Help seals protect bearings

Seals installed on the shaft next to the bearing retain lubricants in the housing and prevent dirt or dust from getting in. Not only will these contaminants nick and scratch a bearing, they also wear down the shaft and housing. A thick coating of lubricant is a barrier to contaminants, assisting the seal in lubricant retention and dirt exclusion.

Lubricant selection

Lubricant selection depends on a combination of factors: the type of housing, operating temperature, operating speed and any particular requirement of that bearing type. In all cases, the best guide for proper selection of a lubricant is the recommendation of the vehicle’s manufacturer. There are two lubricant types – grease and oil. Due to the design

of equipment and the conditions under which it must operate, grease is the more widely used lubricant. Various types of grease are available and should be selected carefully. There are certain guidelines to follow when selecting the right lube for the job. For example, open bearings are only lubricated with a film of oil or light grease to protect them from corrosion before use. They must also be lubricated while running. Sealed and shielded bearings are grease packed from the factory and are sealed for life. The lubrication used in ball and other bearing types is usually a sodium or lithium based grease or oil. As a general rule, bearings run the coolest, and with the least amount of friction, when a minimum amount of the lightest-bodied lubricant that will keep bearing surfaces apart is used.

Use a heavier lubricant only if:

• Operating conditions require it;

• The load is too heavy for the lube;

• It is specifically called for in the application.

Heavy penetration grease will normally increase friction.

Wheel bearing grease specification

Here are some general recommendations in selecting the grease for wheel bearing uses. The grease should be smooth textured, consist of soaps and oils, and be free of filler and abrasives. Recommended are lithium complex (or equivalent) soaps, or solvent refined petroleum oils. Additives could inhibit corrosion and oxidation. The grease should be non-corrosive to bearing parts with no chance of it separating during storage or use. Using the correct amount of lube is essential. Failure to correctly lubricate the bearing or maintain proper lubrication may result in bearing damage, causing a wheel to lock.

To lubricate the bearing, force grease around the outside of the bearing; between the rollers, cone and cage. Pack more grease in the wheel hub. The depth of the grease should be level with the inside diameter of the cup. The hubcap should also be filled with grease.

Grease lubrication features

1. Reduces maintenance time. There are no minimum grease levels to maintain, so lubrication cycles are less frequent.

2. Is confined to the housing or bearing. This means a simpler seal design can be used to retain lubricant and exclude dirt.

3. Grease is more viscous than oil lubricants.

Oil lubrication

Since it is thinner than grease, oil needs more frequent lubrication intervals. Generally, oil is used to lubricate bearings in high temperature and/or high speed applications. It is used in heavy-duty fleet, automotive and agricultural vehicles. Gear drivers, for example, work well with oil lubricants. For hypoid gears, where both the gears and bearings used in the units require lubrication, S.A.E. grades No. 90 and No. 140 oil are normally used. For extremely cold operating conditions (around -40ºF to -60ºF), a lighter No. 80 grade is used. At high temperature and for heavy-duty applications, use the S.A.E. 140 grade. For extremely heavy loads and extremely high temperatures, S.A.E. 250 is recommended. Be sure not to mix vehicle motor oil with gear oils, as they could be incompatible.

Oil lubrication features

1. The correct amount of oil lubricant is easier to control than grease.

2. Oil lends itself more to the lubrication of all parts.

3. Oil flows better in lower and higher temperatures than grease.

General recommendation

Whatever type of lube – grease or oil – is selected, be sure to remember some very fine points that will help a bearing to continue performing well, or cause it to fail.

1. Never wash the protective lubricant off new bearings.

2. Use clean lubricants contained in clean, air-tight cans or drums. Store them in a cool, dry area. Dirt, dust or moisture in the lube can lead to eventual bearing failure.

3. Use only the lubricant called for in the job. No substitutes or interchanges (grease instead of oil, for example) should be made unless specified by the equipment manufacturer.

4. Do not overfill the housing. Too much grease or oil can seep out of overfilled housings, past seals and closures. The lube that escapes can collect dirt and cause damage to the bearings.

5. Too much lubricant can also cause overheating. This is particularly true of bearings running at high speeds, where the churning of the lubricant will cause the bearing to run too hot.