Power Transmission - Select proper lubrication to extend bearing life

Greg Hewitt Plant Engineering and Maintenance
Posted 09-13-04

Adding the correct amount of lubrication at each re-lubricating interval is essential to achieve maximum bearing life. An incorrect re-lubrication quantity, whether it's too much or too little, can actually decrease bearing life.

Insufficient lubrication
Not using enough lubrication will lead to a breakdown, which may cause premature metal-to-metal contact between rollers and raceways. You also need to remember that grease acts as a barrier or seal, and aids in decreasing the incursion of contaminates into the bearing. When sufficient lubrication isn't present, the sealing ability of bearings is decreased. Contaminates can also more easily enter the bearing.

Excessive lubrication
Too much lubricant can cause its share of problems. For example, in a high-speed application, the bearing could actually overheat due to the amount of grease being pumped into the bearing. As the bearing continually churns excess grease, the continuous mixing generates heat within the bearing housing. If more lubrication is added, the bearing will eventually overheat and fail.

The majority of mounted bearings have seals that purge excess lubrication. Check with your mounted-bearing supplier to ensure the bearings in question have purgeable seals. These seals allow excess grease to pass through the sealed area and out of the bearing. This purging is spread out over time, but will help decrease the chance of over filling a bearing (Figure 1).

Re-lubrication quantity
There are two proper ways to determine the correct re-lubrication quantity. The first method is to contact the mounted-bearing manufacturer and request a recommendation, which is based upon your specific application. If that's not possible, your second option is to calculate the correct re-lubrication quantity using the following equations and examples:
To manually determine the correct re-lubrication quantity you must first obtain this information:

1. D=Bearing outer diameter (mm)
2. B=Bearing width (mm)
3. Q=Re-lubrication interval factor
• Use Q=0.002 if you have a weekly re-lube interval
• Use Q=0.003 if you have a monthly re-lube interval
• Use Q=0.004 if you have a yearly re-lube interval
The re-lubrication interval should've been determined prior to this exercise. Refer to the instruction manual supplied with the mounted bearing or see the column in the PEM February 2004 issue (Determining the Proper Re-lubrication Interval To Maximize Mounted Bearing Life).
4. Re-lubrication quantity equations:
• Grease Quantity (g)=D x B x Q or
• Grease Quantity (oz.)=(D x B x Q)/28.35

The following examples will better explain the steps to follow to calculate the exact re-lubrication quantity:

Example 1

210 series 1-15/16-inch set-screw ball bearing insert
Re-lubrication interval=weekly
Envelop dimensions, such as the outer diameter and width, should be given in the mounted-bearing manufacturers catalogue. For this example, a DODGE 210 series SC 1-15/16-inch ball-bearing insert was chosen.
1. Determine: D=Bearing Outer Diameter (mm)
D=90mm
2. Determine: B=Bearing Width (mm)
B=22mm
3. Determine: Q=Re-Lubrication Interval Factor
Given: Q=0.002 (weekly)
You now have all the needed information to use the following equations and determine the correct grease quantity:

• Re-Lube Quantity (g)=D x B x Q
• Re-Lube Quantity (g)=90mm x 22mm x 0.002=3.96 (g)
or
• Re-Lube Quantity (oz.)=(D x B x Q)/28.35
• Re-Lube Quantity (oz.)=(90mm x 22mm x 0.002)/28.35=.14 (oz.)

Example 2

22213K 2-7/16-inch spherical roller bearing
• Re-lubrication interval=monthly
For this example, a DODGE Imperial 2-7/16-inch insert was chosen. All needed dimensions were found in the manufacturer's bearing engineering catalogue.
1. Determine: D=Bearing Outer Diameter (mm)
D=120mm
2. Determine: B=Bearing Width (mm)
B=32.3mm
3.Determine: Q=Re-Lubrication Interval Factor
Given: Q=0.003 (monthly)

You now have all the information you need to use the following equations and determine the correct grease quantity:

• Re-Lube Quantity (g)=D x B x Q
• Re-Lube Quantity (g)=120mm x 32.3mm x 0.003=11.6 grams or
• Re-Lube Quantity (oz.)=(D x B x Q)/28.35
• Re-Lube Quantity (oz.)=(120mm x 32.3mm x 0.003)/28.35=.41 oz

Notice that all calculations deal with the bearing insert and not the bearing housing. In most cases, when determining the correct re-lubrication quantity, the housing style doesn't play a significant role. The amounts calculated in the two examples are sufficient to replenish the grease supply within bearing internals, and displace degraded lubricant.

Determining the correct re-lubrication quantity is essential to achieve maximum mounted-bearing life. It's estimated that 90 percent of all mounted-bearing failures are lubricant related. Proper re-lubrication quantities ensure that you're replenishing the bearing with fresh grease and purging the bearing of degraded grease and contaminates.

Taking the time to determine the correct re-lubrication quantity will increase bearing life and decrease maintenance time, which is associated with mounted-bearing failures.