The use of grease as a lubricant for our precision bearings on various spindle applications is becoming more popular due to the development of better ball bearing greases, simplification of design and elimination of the “human maintenance factor” which is frequently responsible for too much lubrication, not enough lubrication, or the wrong kind of lubrication. Prelubricating the bearings at assembly with the correct amount of the right grease and thus eliminating all grease fittings has increased precision production in many instances.
For successful lubrication, grease for ball bearings should have good mechanical and chemical stability with low torque characteristics. Two different types of grease, one soft and the other heavier, have proved to be suitable lubricants for machine tool spindle bearings. The “soft” greases have a worked penetration factor of about 300. The heavier grease has a worked penetration factor of about 200 and is of the channeling type. All greases show a very slight change in consistency after operation in a bearing. As the softer grease has a tendency to churn, particular attention should be given to the quantity packed into the bearing. Because the heavier grease is of the channeling type, the amount used is not critical.
Below a 400,000 DN value, which is equivalent to a 40mm bore bearing rotating at 10,000 rpm, either a light consistency grease or the channeling grease may be used. When using grease of a channeling type at low speeds, the bearing may be packed full and will operate at no appreciable rise in temperature. Bearings may also be packed full of the lighter grease, but a greater rise in temperature will be notice- able until the excess lubricant is expelled from the bearings.
At continuous speeds above a DN value of 400,000, the operating temperature is generally lower when the bearings are lubricated with a lower consistency grease. However, the grease quantity in each bearing must be limited. At these high speeds, an excessive amount of grease in the bearing may result in greatly increased operating temperatures, due to churning action. This condition, if uncontrolled, may lead to premature bearing failure.
The top graph in Figure 16 shows bearing temperature increase due to break-in procedure. The peaking temperature followed by the leveling off is a result of the new grease being worked and then stabilized for a particular condition of load and speed.
It is important that the peak temperature not exceed 100F above room temperature since the chemical consistency and characteristics of the grease can be permanently altered. Thus, the proper break-in procedure is to run the machine until the spindle temperature rises to 150 F and then turn it off to allow the grease to cool. Repeat until the spindle tem- perature stabilizes at a temperature below 130F.
The bottom graph in Figure 16 shows the typical tempera- ture rise of the bearing once the grease has been worked in for the specific speed and load.
Rise Above Room Temperature F
Grams of Grease in Each Bearing
Bearing Temperature Increase Due to Break-In Procedure
Typical Temperature After Break-In Procedure
Figure 16 – Temperature vs Time
Bearing Bore Size in MM
Figure 17 – Grease Quantity Chart