MAINTENANCE • MACHINE TOOL SPINDLES
In servicing machine tool spindles it is essential to analyze the operating troubles and probable causes of the improper performance or malfunctions of the spindle, especially if the ball bearings have failed prematurely.
Causes and conditions that indicate or lead to ball bearing replacement are:
Out-of-roundness of work; Impossibility of maintaining tolerances; Existence of noise and chatter; Production of rough or poor finishes; Accidental damage to machine spindle; “Brinelling” of bearings; Entry of coolant and contamination into bearings; Faulty lubrication of bearings.
Before the spindle is removed from the machine, a check should be made to ascertain whether the trouble is with the spindle and the bearings or the result of improper tooling, roughness in the machine slides, irregular feeds or some other such cause.
pressed to the shoulder with a uniform pressure. However, if the bearing sticks and requires excessive force at any point, it may have been cocked . Continued pressure on the bearing ring that was not squarely is likely to scrape and seriously damage the bearing seat. The source of trouble, which can be a burred or tapered seat, should be determined and corrected before proceeding.
Fretting corrosion in the bearing bore can be the result of loose shaft fits. This action produces iron oxide causing free iron to be removed from the shaft. This material gets into the bearing and being abrasive causes wear in the bearing, produces noise and engenders premature bearing failure.
Misaligned bearings can be the cause of early fatigue failures. Bearing misalignment can be caused by housing bores that are not in line with each other or when the bearing seats on the shaft are not concentric. Bearings may also become misaligned if the shaft or housing shoulder is out-of- square. When considerable misalignment is present, the load track veers from one side of the raceway to the other. Highly misaligned loads could force the balls over the edge of the raceway shoulder, causing the ball surfaces to be creased and resulting in premature fatigue failures.
When the cause of damage or failure of ball bearings is not immediately apparent, there is a tendency to suspect faulty bearing materials or defective workmanship in the product; but the materials and the manufacturing methods in the production of ball bearings, through many years of research, have reached such a state of refinement that defects in bearings are extremely remote. In most cases, ball bearings fail from preventable causes, such as faulty mounting practice, incorrect shaft and housing mounting fits, improper or unsuitable lubrication and intrusion of foreign matter.
Foreign matter – coolant, dust, dirt or other contaminants – pressed to ball bearings is the most common cause of bearing failure. Such a condition may be caused either by the entry of dirt or moisture, by the improper handling of the bearing during the mounting procedure or from inadequate sealing of the shaft and housing. Severe cases may cause abrasion and wear of the revolving parts, generally resulting in excessive axial looseness in the bearing. Visual evidence of this would be dull, gray discoloration of the raceways and the balls. Under such conditions ball bearings become noisy and inevitably fail to perform satisfactorily.
Bearing replacements without corrective measures often result in the repetition of the trouble. It becomes very important, therefore, to determine accurately and completely, the cause of the improper functioning of the ball bearings.
Ball bearings should be pressed onto a shaft with the mounting force applied only to the inner ring. Similarly, when bearings are mounted in a housing, the mounting force should be applied solely to the outer ring. Mounting dents or ‘brinells’ high on the raceway are caused when the mounting force is applied to a ring which is not being mounted. They should be seated solidly against the shoulders by means of an arbor press and a tubular drift. It is important that both ends of the tube are faced square and that all corners are broken to avoid flaking. The tubes must be clean and free from scale, both inside and out, to avoid the possibility of dirt falling into the bearings.
Lubrication is essential for the proper operation of ball bearings. Greases and oils are both used over a considerable range of speeds and operating temperatures. The choice of lubrication should be made after the careful consideration of the factors involved.
Generally, ball bearings operating at moderate speeds require relatively small amounts of lubrication. Too much lubrication, accompanied by churning and excessive torque, is manifested by temperature rise and intense over-heating. Lack of lubrication causes a rise in operating temperature, coupled with a whistling sound when running. Unless such conditions are corrected, the rings, balls and cage may be seriously damaged. Discoloration of the bearing parts is evidence of excessive operating temperatures.
In mounting ball bearings, particular attention should be given to the mounting fits. If the bearing seat is of the correct diameter and without serious taper, the bearing can be
MAINTENANCE MACHINE TOOL SPINDLES