Seals protect spindles from contamination

Spindle units in machine tools have rotating and nonrotating components. Bearings in the stationary housing support the rotating spindle. The support bearings may be based on solid rolling elements, such as angular contact ball bearings, cylindrical roller bearings or tapered roller bearings. They may use a liquid, as in hydrostatic or hydrodynamic bearings, or a gas, as in aerostatic or aerodynamic bearings. The support bearings may even levitate the spindle in a magnetic field (magnetic bearings).

All of these bearings have precision components that contaminants can easily destroy. In addition, the inside of a spindle unit often houses the motor, tool-change mechanism, sensors and cooling systems. All internal elements must be protected from the spindle environment. Also, bearing lubrication should not be allowed to leak into the cutting zone, where it can contaminate coolant as tramp oil.

Spindles usually operate in hostile environments. They often rotate at high speeds, get hot and are exposed to chips, dust and smoke. There also may be coolant at high pressure and flow rates in the work zone. A critical component of spindle design—the seal—keeps these contaminants out of the spindle even under the harshest operating conditions.

If the spindle speed is only a few thousand rpm or so, a contact seal is probably sufficient. A sliding contact seal uses a soft material, such as a polymer or even cloth preloaded against a polished surface, to keep contaminants out. At low spindle speeds, the sliding contact does not generate much heat through friction. But in spindles that operate at higher speeds, the friction-generated heat becomes a problem, and a noncontact seal is required.

One type of noncontact seal is a labyrinth seal (Figure 1), named after the maze-like structure in Greek mythology designed by Daedalus to hold the Minotaur. Essentially, a labyrinth seal creates a long, narrow, torturous, open path that inhibits the passage of contaminants to the spindle interior and prevents small oil leaks, such as air-oil bearing lubrication.

Courtesy of S. Smith

Figure 1. In this schematic of a labyrinth seal, the seal is comprised of the black, red and green rings. The black and red rings are attached to the spindle and rotate with it. The green ring is attached to the housing and is stationary.