Rebuild to spin again

Theoretically, a machine tool spindle can be repaired an infinite number of times and remanufactured no matter how extensive the damage.

“In extreme cases, the damaged components can be remanufactured to the point of remaking the whole spindle,” said Zeljko Cvitkovic, engineering manager of spindle remanufacturing/production at SKF USA Inc. in Lansdale, Pennsylvania. “The determining factor when considering a spindle rebuild is usually cost. Ideally, customers pay less than half the cost of a new spindle to repair an existing one. Sometimes the cost will be higher, but if the repair cost is estimated to exceed the cost of a new spindle — and it is readily available — then it makes sense to buy a whole new spindle.”

GTI’s Johnny Johnson works on a Bryant spindle. Image courtesy of GTI Spindle Technology

Tom Hoenig, president of GTI Spindle Technology Inc., concurred that the general rule is to repair a spindle when the cost is about 50% of a new one. The Manchester, New Hampshire-based company, which has two other U.S. facilities, provides repair, remanufacturing and rebuild services for spindles and other machine components and is the exclusive North American reseller of spindles from Italian manufacturer Peron Speed International.

However, he said COVID-19 caused the 50% threshold to go much higher.

“We have had customers pay over the price of a new spindle for a repair because of the shortages and slow delivery from OEMs,” Hoenig said.

Nonetheless, those supply disruptions have increased the lead times to obtain new spindles, which can be devastating to a manufacturer that doesn’t have a backup spindle when needed.

“Spindle failures can lead to a whole production line shut down and the associated high costs of lost production,” Cvitkovic said. “On top of that, a new spindle may not be available from the original equipment manufacturer for months.”

He said even if a repaired spindle costs as much as a new one, SKF USA can deliver it in significantly less time, allowing the equipment to be up and running faster.

“The OEM price and delivery of the new spindle sets the bar for everything,” Cvitkovic said.

Pull Before Failure

One way to reduce the repair cost is to be proactive and have a spindle serviced while it’s still spinning instead of waiting until it’s “torpedoed,” Hoenig said.

“If you’re pulling a spindle before it locks up, you just saved yourself 50% of the repair cost because you didn’t let it run to failure where it seizes and where bearings tend to spin inside and wreck all kinds of components,” he said. “The electrical field will be burned out because you tried to drive bearings that are locked up. If you wait till it stops running, you’re really doing yourself a disservice.”

Not only does the cost go up, but the lead time increases. A torpedoed spindle might need to have components reground and chrome-plated, housing surfaces checked and fixed and components remanufactured or reverse-engineered to make new ones, said Lisa Bailey-Beavers, vice president of sales and marketing for GTI Spindle Technology. Regardless of how extensive the damage is, the company brings spindles back to OEM specifications or better.

A spindle is shown prior to and after being repaired. Images courtesy of SKF USA

Cvitkovic said although some end users assume that a spindle repair involves only taking the spindle apart and installing new bearings, that is rarely the case.

“In some situations, spindles are run to failure and come to a screeching halt,” he said. “When that happens, other components get damaged, not just the bearings.”

Cvitkovic recommends that end users be proactive and watch for changes in performance to help determine when a spindle is starting to fail. For example, the spindle may begin to sound different when rotating, the surface finish on parts may look degraded or the spindle may be too hot when touched.

“If you ignore those signs,” he said, “that’s what could increase the cost and the time to repair.”

To know when a spindle is going to fail, Hoenig said his company has a division called GTI Predictive Services that provides predictive maintenance. While GTI repairs spindles at its facilities with rare exceptions, the company does a full analysis of functioning spindles at customers’ plants.

“It could be 100, 300 machine tools so that we know at any given time — in real time — what condition their spindles are in and whether they are coming to a failure mode,” he said. “They’re never down without knowing that something is going to fail ahead of time.”

Predicting spindle failure centers around wireless vibration analysis that is either conducted manually by a technician about once a month or monitored continually online, Hoenig said. In addition to reducing the cost of repairs, predictive maintenance services allow customers to minimize downtime by planning when to shut down a machine to change a spindle.

If a manufacturer does find itself in a scenario in which it’s critical to get a damaged machine spindle up and running as soon as possible, he said GTI offers a quick turnaround option for emergency repairs.

“The customer can pay a premium to be pushed to the front for speedy delivery,” Hoenig said.

Main Causes

One common and obvious reason for spindle damage and failure is operator error.

“The operator may exceed the capacity of the spindle or write the program incorrectly and cause a hard crash,” Cvitkovic said. “In our inspection process, we evaluate everything to determine what needs to be repaired or replaced.”

Shown is the spindle assembly room at GTI Spindle Technology. Image courtesy of GTI Spindle Technology

He said another primary cause of spindle failure is contamination. The contamination, which can be from coolant, chips or grinding swarf, works its way into the spindle, causing premature failure.

When contamination issues are found during spindle evaluation, Cvitkovic said SKF USA discusses the problem with the customer and recommends ways to improve sealing and upgrade spindle performance. SKF USA also can change a grease-lubricated spindle to an oil-lubricated spindle or replace standard steel bearings with hybrid ceramic bearings to extend life or increase speed. He estimates that approximately 10% to 15% of spindle repairs involve upgrades.