Protecting machine tool spindle bearings

Manufacturing Engineering, Jul 2001 by Hoeting, Steven

Redesigned seal excludes contaminants

The most critical components of a machine tool spindle are its bearings. Bearings support the shaft and provide accurate. smooth rotation. Foreign particles and fluids damage bearings, and ultimately shorten spindle life.

Seals placed between the shaft and stationary parts protect the spindle bearings. There are many seal designs, but none are perfect for every application. The main challenge is interfacing with a shaft that can spin, while still providing a good seal. Other factors, such as size constraints, temperature, humidity, environmental air pressure fluctuations, chemical exposure, and particle geometry, influence seal selection.

Historically, machine tool spindles employ rubbing-type lip seals for slower applications, and labyrinth-type gap seals for high-speed applications. In instances where environments are extreme, air purge is added to the labyrinth seals to continuously purge contaminants from the sealing area. In the mid1990s, safety concerns prompted machine tool builders to more completely shroud the working area of machine tools. This change, in conjunction with dramatically increased coolant flow rates and pressures, creates a situation where traditional seals are inadequate. Bearing contamination occurs within a relatively short time.

In 1996, Setco constructed a test apparatus that would enable the replication of in-service seal failures. A further objective was to build a foundation on which seal performance could be assessed. Among the features were:

* Instantaneous viewing of seal performance

* Flexibility with regard to seal size

* Variable coolant flow velocity and direction to replicate any conditions that would be seen in a working environment

* Allowance of any spindle mounting position

* Provide variable spindle speed and direction of rotation

* Variable internal chamber pressure

* Monitoring of environment, humidity, and temperatures

The test apparatus allows the mounting of a spindle fitted with the seal under test, so the seal is tested in conditions as similar as possible to those in its intended environment.

An initial set of benchmark test conditions were established based on conditions encountered in the field where seal failures had occurred. The scope was to test known seal configurations and evaluate them with regard to their acceptability under various field conditions to evaluate the seal failures to determine the root cause.

Tests are performed for every distinct combination of variable parameters. For example if shaft speed and air pressure were the only variables, there would be four pressure settings for each of the five speed settings, or 20 separate tests.

Since 1996, over 4000 tests have been conducted on internally developed seals as well as commercially available seals. From this battery of testing, a matrix has been developed rating commonly employed seal configurations and conditions relative to pass/fail criteria. The seal is considered to have passed only when no leakage was observed. Any leakage at all constituted a failure.

Various labyrinth and contact seals were evaluated and a rubbing lip seal is the only commercially available seal that passed. However, lip seals are speed-limited and tend to wear out. Labyrinths tend to follow a common theme; they leak at rest and at slow speeds. When speed increases to a critical point, centripetal pumping force of the rotor overpowers coolant spray.

Increased coolant flow raises the critical speed. A relationship between seal speed and coolant flow has been established for a common labyrinth seal:

Surface speed^sub critical^ (fpm) = 4.5 X coolant flow rate (gpm)

The formula is valid only for horizontal mounting. Gravity affects seal performance and ingress increases as the mounting angle goes above horizontal. With labyrinths in this environment, mounting position is not as large a factor in bearing contamination problems. Seals are flooded and labyrinths become useless. Coolant travels through the flooded labyrinth no matter how elaborate or tortuous the path. Liquid penetrates the seal unless assisted by a proper air-purge and changing the mounting position only changes the rate of ingress. Because many coolant sprays can flood the seal, the solution must be able to operate in a completely flooded condition.

Direct-spray coolant velocity is the most critical factor. Coolant traveling at high speed blasts through seals, regardless of how intricate the labyrinth. The second most critical factor is coolant flow rate. Excessive flow tends to completely submerge and overwhelm seals to the point of leakage.

Some positive internal pressurization (PIP) supported labyrinths do work. Very low pressure air at 1.0-3.0 psig (7-21 kPa), ported directly into the spindle, flows through its bearings, and out the seals. Air must be extremely clean and dry to not contaminate bearings. If too much pressure is applied, seals can overheat or even pop out of the spindle (depending on what labyrinth seal is used). Although it's a solution, PIP is unpopular among spindle users.