FAILURE TO PERFORM

When a machine tool spindle fails to perform properly – or breaks down altogether – suspicion can turn to any number of causes. The detective work should begin with the basics. Was the spindle incorrectly assembled, stored, handled, or installed? Is the spindle being used in an application for which it was not intended? Did external conditions or influences contribute to the problem? From there the investigation can advance by focusing on defects in the spindle, which will provide the clues to uncover the root cause of failure and, ultimately, make the appropriate fix.

Normal fatigue wear of spindle components is a root cause of failure that can be anticipated over time and can potentially exact a heavy price. This makes it critical that attention be paid routinely to preventive maintenance as a safeguard to forestall performance problems before they can escalate. Spindle components typically subject to wear include the shaft; front, rear and main housings; bearings; stator; rotor; finger assembly; drawbar springs; spacer; encoder; and front shaft nut and cap. If any of these components exhibits signs of wear or damage, it should be repaired or, in the worst case, replaced.

Here are some other potential root causes of spindle failure, along with recommendations to help keep unscheduled downtime at bay:

Contamination. When contaminants invade, shafts and/or bearings will suffer. Contaminants include any foreign substances ranging from coolant and condensation to grinding swarf, chips, and debris from material being machined. Whether penetrating shaft or bearings, contaminants can find openings through worn or otherwise ineffective seals. Proper seal installation and maintenance is essential to minimize risks. Other tips: tool coolant should never be directed at shaft seals, while keeping an eye on non-direct splash back to the spindle, and any noticeable buildup of chips or debris should be monitored and removed regularly.

Crashes. Increasingly high-speed machining applications have paved the way for potentially more spindle crashes and debilitating downtime. The marketplace has responded with detection systems specifically engineered for spindles to monitor conditions and signal whether a problem may be in the works. Systems have been developed and equipped with various sensors to measure vibration in the spindle head, speed of the spindle, and temperature of the spindle’s main bearings and driving motor. And, should the spindle eventually go down, maximum operating levels for vibration, temperature, or speed can be logged for later analyses.

Overloads and Preloads. Signs of overload on a spindle’s bearings can include deep ball tracks and/or spalling (or surface failure) – and severe spalling in the fatigue areas of a bearing’s raceway can cause overheating. The culprits in overloading may be improper bearings, an excessive preload, the feed rate, belt tension, or even a combination. The upshot is that a load higher than a bearing’s capacity will proportionally – and sometimes dramatically – reduce bearing life.

In a similar way, bearings must have the proper amount of preload for maximum life and optimum performance. Too much preload will cause a bearing to fail, while insufficient preload will cause the rolling elements of a bearing to skid – resulting in poor tool performance and possible failure. Either way, a bearing’s preload must be properly set to realize long-life expectancy.

Lubrication issues. Supplying the right lubricant in the right amount at the right time is the overarching goal for spindles (and for all rotating equipment). For example, when lubrication is not doing its job, friction can become a spoiler by increasing wear, generating unwanted heat and higher operating temperatures, limiting speeds and power, and reducing overall energy efficiency. Lubricant can also be subject to breakdown in service and should be regularly monitored to confirm integrity. Excess oil is not the answer, either. This can also generate high friction and drag creating an overheating condition.

In short, lubricants should always be high quality, properly specified and stored for the application, and clean and free from moisture or other contaminants. Neither too much nor too little lubricant should be applied and lubricant containers and all transmission lines should be inspected to rule out pre-existing contaminants. In the case of air/oil lubrication systems, clean and dry air is imperative for prolonged spindle life.

Mishandling. A radial or axial impact to a spindle shaft can cause brinelling (or permanent “denting”) that will subsequently result in rapid deterioration of a spindle’s precision bearings. Never strike a spindle, even with a slight mallet tap on the shaft. External vibration, too, can be problematic by causing “false brinelling” that occurs when contacting points vibrate against each other to form a surface resembling a brinell mark. Spindles should be properly handled and protected from vibration and other potentially adverse influences to help keep the equipment performing as intended.

The overriding message is that there is no shortage of suspects “when good spindles go bad.” Spindle failures can also result from a variety of root causes such as imbalance, tool-change errors, improper repair, and many others. The good news is that sometimes a spindle will indicate a developing problem in its early stages if the user cares enough to watch for the signs.

As examples, unusual noises, increased operating temperature, shortened tool life, higher levels of rework or scrap, decreased performance, shaft run-out or end play, looseness, drawbar pull force, or abnormal vibration may suggest that the spindle should undergo a checkup. Best-practice maintenance procedures offer a practical first line of defense to help optimize spindle performance. Should problems be detected early, qualified expertise can often provide a big assist in rooting out causes and recommending remedial action. Preventing run-to-failure will help in many ways. For one, it makes troubleshooting failure modes possible and, second, it will usually save money on the repair.