Keeping Your Spindle Healthy, Part 3 is the third in a three part series of our recommendations for preventing spindle damage and associated downtime. Last week we spoke about damage incurred when a spindle crashes. Prior to that we considered spindle failures that result at commissioning. This week we will look at the single biggest cause of premature spindle failure, contamination.

Spindle repair is very costly both in direct cash outlays as well as degradation of product quality and eventual loss of production. As one of the leading independent spindle repair shops we have seen a broad spectrum of preventable damage in our 26+ years of experience. So it is our hope that by sharing some of the failure modes that we’ve seen we can spare you the pain of a damaged spindle.

We will offer our insight from a repair shop point of view and invite you to send in any comments, anecdotes or corrections.


contamination, keeping your spindle healthy The most common cause of spindle failure that we encounter is a result of contamination. This shouldn’t be surprising. High speed spindles are particularly susceptible to contamination. They live in an environment where they are constantly immersed in cutting fluid, metal chips, grinding dust, stone dust, saw dust and other forms of grime. They most often operate at speeds that prohibit positive shaft seals such as lip seals. So it is no wonder that contamination related failures are prevalent. But even operating in these conditions there are simple steps that can help mitigate contamination caused failures.

 Labyrinth seals off of a CMS Router spindle.

Labyrinth seals off of a CMS Router spindle.

Labyrinth Seals

High speed spindles incorporate several different methods to keep out contamination. Labyrinth seals are the most common. These seals rely on a close fitting convoluted path to keep contamination from reaching the interior of the spindle. However, due to their high rotational speeds these seals are non-contact and so leave an ingression path for contaminates.

Besides the convoluted close tolerance fit, these seals also rely on the high speed shaft rotation to sling off the contaminate. However, when the spindle is stopped liquid can freely seep through these seals. In fact, the close fit aids wicking of contaminated cutting fluid into the interior of the spindle. Additionally, with some multi-axis milling machines, the spindle body can rotate to the point where the work head is pointing up further aiding the migration of liquid contamination to the spindle interior.

When stopping a spindle that employs a labyrinth seal it is important to stop all cutting fluid flow prior to stopping spindle rotation. Keep the spindle running for several minutes after stopping the tool coolant so the shaft can sling away and residual liquid. Additionally, cutting chips or grinding slurry that buildup around the shaft will greatly increase the likelihood of contamination migrating through the seals. Whenever possible clear all debris buildup from around the shaft prior to stopping rotation.

 New air knife seal on a CMS Brembana stone cutting spindle.

New air knife seal on a CMS Brembana stone cutting spindle.


Air Knifes

Air knives are another method often employed to prevent contamination from getting in the spindle; often in combination with labyrinth seals. These rely on constant air flow through a thin space between the stationary seal and the rotating shaft. They provide positive air pressure to the spindle interior preventing contamination ingression and typically work quite well.

However, these seals require constant air flow during operation and so can add some operational expense. Like labyrinth seals, they can be easily damaged. Debris can impact the seal affecting air flow and in some cases causing the seal to rub against the shaft.

When running a spindle that employs an air knife seal it is important to use only clean dry air. We recommend installing good filters and water separators just ahead of the air knife. We have received spindles where liquid contamination entered the spindle from the shop air used for the air knife seal.

As with any type of seal it is important to avoid spraying cutting fluid or air directly at the shaft seals. High pressure coolant can impinge near the seal opening and get past the air knife. Once past the air knife, the positive air flow carries contamination into the bearings. Also, as with other seals it is important to stop all coolant flow and allow the spindle to continue running with the air knife air on until all residual liquid is away.

 HST may be able to retrofit your spindle with high end IMPRO seals.

HST may be able to retrofit your spindle with high end IMPRO seals.

A number of specialty manufacturers provide high quality hybrid seals that utilize the best features of air purge and labyrinth. They may combine these designs with other unique features such as a positive seal when the shaft is not rotating. These seals have proved to be very effective. IMPRO is one such manufacturer. If your spindle is fitted with this type of seal we can replace it. These seals are rather delicate so it is our policy is to replace them with every rebuild. IMPRO seals are rather expensive and can add several days onto lead time.

If requested we may be able to retrofit your spindle with IMPRO seals.

Take A Ways

  • Always shut off tool coolant prior to stopping spindle rotation
  • Before shutting down, allow spindle to run long enough to sling off any residual liquid
  • Keep debris from building up around shaft seals
  • Only use clean dry air for air knife seals
  • Avoid spraying tool coolant directly at shaft seal

This is the last of our three part series reviewing some of our observations and experiences concerning spindle damage. We would love to hear from you. So please email us at

In the meantime if you have a spindle failure for any reason please feel free to contact us 1-603-483-0333. We answer the phone.