A shielded, grease-lubricated ball bearing can be compared to a centrifugal pump having the ball-and-cage assembly as its impeller and the annulus between the stationary shield and the rotating inner race as the eye of the pump.
Shielded bearings are not sealed bearings. With the shielded type of bearing, grease may enter the bearing readily, but dirt is restricted by the close fitting shields. Bearings of the sealed design will not permit entry of new grease, whereas with shielded bearings grease will be drawn in as the bearing cage assembly rotates. The grease then will be discharged by centrifugal force into the ball track of the outer race.
If there is no shield on the backside of this bearing, the excess grease can escape into the inner bearing cap of the equipment bearing housing.
Single-Shield Bearings
Plants applying best-in-class practices today consider the regular single shield bearing with the shield facing the grease supply (Figure 1) to be the best arrangement. Their experience indicates this simple arrangement will extend bearing life. It also will permit an extremely simple lubrication and relubrication technique if so installed.
This technique makes it unnecessary to know the volume of grease already in the bearing cartridge. The shield serves as a baffle against agitation. The shield-to-inner-race annulus serves as a metering device to control grease flow. These features prevent premature ball bearing failures caused by contaminated grease and heat buildup due to excess grease. Further, warehouse inventories of ball bearings can be reduced to one type of bearing for the great bulk of existing grease-lubricated ball bearing requirements. For other services where an open bearing is a must, as in some flush-through arrangements, the shield can be removed in the field.
Double-Shielded Bearings
Some manufacturers still subscribe to a different approach, having decided in favor of double-shielded bearings. These usually are arranged as shown in Figure 2. The housings serve as a lubricant reservoir and are filled with grease. By regulating the flow of grease into the bearing, the shields act to prevent excessive amounts from being forced into the bearing. A grease retainer labyrinth is designed to prevent grease from reaching the inner side of the bearing.
On equipment furnished with this bearing configuration and mounting arrangement, it is not necessary to pack the housing next to the bearing full of grease for proper bearing lubrication. However, packing with grease helps to prevent dirt and moisture from entering. Oil from this grease reservoir can and does, over a long period, enter the bearing to revitalize the grease within the shields. Grease in the housing outside the stationary shields is not agitated or churned by the rotation of the bearings and, consequently, is less subject to oxidation.
Furthermore, if foreign matter is present, the fact that the grease in the chamber is not being churned reduces the probability of the debris contacting the rolling elements of the bearing.
On many pieces of equipment furnished with grease-lubricated double-shielded bearings, the bearing housings usually are not provided with a drain plug. When grease is added and the housing becomes filled, some grease will be forced into the bearing. At this point, any surplus grease will be squeezed out along the close clearance between the shaft and the outer cap because the resistance of this path is less than the resistance presented by the bearing shields, metering plate and the labyrinth seal.
Open Bearings
High-load and/or high-speed bearings often are supplied without shields to allow cooler operating temperature and longer life. One such bearing is illustrated in Figure 3.
If grease inlet and outlet ports are located on the same side, this bearing commonly is referred to as "conventionally grease lubricated." If grease inlet and outlet ports are located at opposite sides, we refer to "cross-flow lubrication." Figure 4 shows a cross-flow lubricated bearing.
Lifetime Lubricated, 'Sealed' Bearings
Lubed-for-life bearings incorporate close-fitting seals in place of or in addition to shields. These bearings are customarily found on low horsepower equipment or on appliances, which operate intermittently.
A large petrochemical company in West Virginia has expressed satisfaction with sealed ball bearings in certain equipment applications as long as bearing operating temperatures remained below 150° C (300° F) and speed factors DN (mm bearing bore times revolutions per minute) did not exceed 300,000. Close-fitting seals can cause frictional heat and loose fitting seals cannot effectively exclude atmospheric air and moisture, which will cause grease deterioration.
Procedures for Regreasing Equipment Bearings
Rotating equipment bearings should be regreased with grease, which is compatible with the original charge. It should be noted that the polyurea greases often used by the equipment manufacturers may be incompatible with lithium-based greases.
Single-Shielded Bearings
To take advantage of single-shield arrangements, Phillips Petroleum developed three simple recommendations.
* Install a single-shield ball bearing with the shield facing the grease supply on equipment having the grease fill-and-drain ports on that same side of the bearing. Add a finger full of grease to the ball track of the backside of the bearing during assembly.
* After assembly, the balance of the initial lubrication of this single-shielded bearing should be done with the equipment idle. Remove the drain plug and pipe. With a grease gun or high volume grease pump, fill the grease reservoir until fresh grease emerges from the drain. The fill-and-drain plugs then should be reinstalled and the equipment is ready for service.
It is essential that this initial lubrication not be attempted while the equipment is running. It was observed that to do so by pumping action would cause a continuing flow of grease through the shield annulus until the overflow space in the inner cartridge cap is full. Grease then will flow down the shaft and into areas where it is not wanted. This will take place before the grease can emerge at the drain.
* Relubrication may be done while the equipment is either running or idle. (It should be limited in quantity to a volume approximating one-fourth the bearing bore volume.) Test results showed that fresh grease takes a wedge-like path straight through the old grease, around the shaft and into the ball track. Thus, the overflow of grease into the inner reservoir space is quite small even after several relubrications. Potentially damaging grease is kept from the stator winding in motors. Further, since the ball-and-cage assembly of this arrangement does not have to force its way through a solid fill of grease, bearing heating is kept to a minimum. In fact, it was observed that a maximum temperature rise of only 20° F occurred 20 minutes after the grease reservoir was filled. It returned to 5° F rise two hours later. In contrast, the double-shield arrangement caused a temperature rise of more than 100° F (at 90° F ambient temperature the resulting temperature was 190° F) and maintained this 100° F rise for more than a week.
Double-Shielded Bearings
Ball Bearings
* Completely fill the cavity adjacent to the bearing. Use necessary precautions to prevent contaminating this grease before equipment is assembled.
* After assembly, lubricate stationary equipment until a full ring of grease appears around the shaft at the relief opening in the bracket.
Cylindrical Roller Bearings
* Hand pack bearing before assembly.
* Proceed as outlined for double-shielded ball bearings.
If under-lubricated after installation, the double-shielded bearing is thought to last longer than an open (non-shielded) bearing given the same treatment because of grease retained within the shields (in addition to grease remaining in the housing from its initial filling).
If over-greased after installation, the double-shielded bearing can be expected to operate satisfactorily without overheating. This will be as long as the excess grease is allowed to escape through the clearance between the shield and inner race, and the grease in the housing adjacent to the bearing is not churned, agitated and caused to overheat.
It is not necessary to disassemble equipment at the end of fixed periods to grease bearings. Bearing shields do not require replacement.
Double-shielded ball bearings should not be flushed for cleaning. If water and dirt are known to be present inside the shields of a bearing because of a flood or other circumstances, the bearing should be removed from service.
All leading ball bearing manufacturers are providing reconditioning service at a nominal cost when bearings are returned to their factories. As an aside, reconditioned ball bearings generally are less prone to fail than are brand new bearings. This is because grinding marks and other asperities are now burnished to the point where smoother running and less heat generation are likely.
Open Bearings
Equipment with open, conventionally greased bearings generally is lubricated with slightly different procedures for drive-end and opposite-end bearings.
Lubrication procedures for drive-end bearings.
* Relubrication with the shaft stationary is recommended. If possible, the equipment should be warm.
* Remove plugs and replace with grease fitting.
* Remove large drain plug when furnished with the equipment.
* Using a low-pressure, hand-operated grease gun, pump in the recommended amount of grease or use one quarter of the bore volume.
* If purging of system is desired, continue pumping until new grease appears either around the shaft or at the drain opening. Stop after new grease appears.
* On large equipment, provisions usually have been made to remove the outer cap for inspection and cleaning. Remove both rows of cap bolts. Remove, inspect and clean cap. Replace cap, being careful to prevent dirt from getting into bearing cavity.
* After lubrication, allow the equipment to run for 15 minutes before replacing plugs.
* If the equipment has a special grease relief fitting, pump in the recommended volume of grease or until a 1-inch-long string of grease appears in any one of the relief holes. Replace plugs.
* Wipe away any excess grease that has appeared at the grease relief port.
Lubrication procedure for bearing opposite drive end. If bearing hub is accessible as in smaller equipment with large couplings or drip-proof equipment, follow the same procedure as used for the drive-end bearing. For fan-cooled equipment, note the amount of grease used to lubricate the shaft-end bearing and use the same amount for commutator-end bearings.
Motor bearings with grease inlet and outlet ports on opposite sides are called cross-flow lubricated. Regreasing is accomplished with the equipment running. The following procedure should be observed.
* Start equipment and allow to operate until normal equipment temperature is obtained.
Inboard bearing (coupling end).
* Remove grease inlet plug or fitting.
* Remove outlet plug. Some equipment designs are equipped with excess grease cups located directly below the bearing. Remove the cups and clean out the old grease.
* Remove hardened grease from the inlet and outlet ports with a clean probe.
- Inspect the grease removed from the inlet port. If rust or other abrasives are observed, do not grease the bearing. Tag equipment for overhaul.
* Bearing housings with outlet ports. First, insert probe in the outlet port to a depth equivalent to the bottom balls of the bearing. Then, replace grease fitting and add grease slowly with a hand gun. Count strokes of gun as grease is added. Finally, stop pumping when the probe in the outlet port begins to move. This indicates that the grease cavity is full.
* Bearing housings with excess grease cups. First, replace grease fitting and add grease slowly with a handgun. Count strokes of gun as grease is added and stop pumping when grease cavity is full.
Outboard bearing (fan end).
* Follow inboard bearing procedure provided the outlet grease ports or excess grease cups are accessible.
* If grease outlet port or excess cup is not accessible, add two-thirds of the amount of grease required for the inboard bearing.
* Leave grease outlet ports open and do not replace the plugs. Excess grease will be expelled through the port.
* If bearings are equipped with excess grease cups, replace the cups. Excess grease will expel into the cups.
