Apr 22, 2003

How Grease-Lubricated Bearings Function

Bearings and Lubrication

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

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

*               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

*               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

*               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.

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