Effectively protecting machine tool spindles—which means primarily the spindle bearings that support the shaft—from contamination by coolant, moisture and chips has challenged some of the best and brightest engineering minds. Over the years, machine tool builders and spindle manufacturers have tried a number of seal designs to increase reliability and extend spindle life. The most common solutions include various designs using a mechanical seal in conjunction with an air purge/labyrinth seal around the spindle shaft.
Each seal type acts as a barrier to prevent contaminants from entering the spindle housing and thus protecting the bearings and other critical components from damage. This dual barrier works fine in machines that have relatively moderate speeds and open work zones.
Today, the challenges of preventing spindle contamination facing spindle manufacturers are compounded by higher feeds, speeds, coolant volume and pressure. Intensifying this is the trend to "dry floor" machine tool guarding which hides chip buildup around the spindle and masks the amount of coolant overspray.
Moreover, tighter guarding exacerbates the levels of heat and humidity inside the machine tool enclosure. This results in excessive condensation that can creep past the spindle seal and into bearings, again causing premature spindle failure.
If today's seal designs are to be successful, notes Tom O'Brien, director of engineering at Setco, four challenges must be met:
- The seal must protect the spindle in the dynamic or cutting mode when coolant pressure and volume are at their greatest.
- The seal must provide an equally effective barrier to contamination when the machine tool is in the static or low rpm mode because coolant and heat are still present.
- The seal performance must not be affected by chip buildup.
- The design of the seal must be simple and reliable.
To date no single mechanical seal has proven to be 100 percent effective in keeping out contaminants in both static and dynamic modes of spindle operation. Rubbing seals, for example, generally fit tightly around the spindle shaft and function well in the static mode, but they wear quickly and reduce spindle rpm in the dynamic mode. By contrast, seals designed to work in the dynamic mode— common air purge/labyrinth seals, for example, are generally overcome by coolant pressure in the static mode or fail if the air supply is shut off.
Setco, the Cincinnati-based manufacturer of precision spindles and slides, has introduced a patented air seal design that has outperformed all others tested in both modes of spindle operation. "The new seal combines the benefits of air and rubbing seals but eliminates the drawbacks common to each type," says Steve Hoeting, the seal's inventer.
Made of corrosion-resistant Viton material, the new air seal is really two seals in one, notes Mr. Hoeting, combining a unique geometric shape with a novel positive air system. This ensures that the spindle is protected from contaminants in both the static and dynamic modes. Customer production tests indicate that the new air seal can significantly extend spindle MTBF (mean time between failure).
During spindle operation, a positive air system of 10 to 15 psi—the typical requirement for auxiliary machine tool operations—provides a uniform outward flow of clean, dry air through the seal to prohibit contaminants from entering critical spindle components. A primary difference in the Setco air seal however, is in the way air is uniformly distributed around the 360-degree seal labyrinth.
The proprietary design eliminates uneven or spotty pressure zones. And even if the air supply should fail or power to the machine tool be shut off, design of the seal provides positive contact sealing against moderate contamination.
Further, to reduce risk of chip contamination and make the spindle more universal in its application—especially when difficult spindle mounting orientations are required—the new seal design eliminates the drain hole. The patented seal is compatible with grease or air/oil lubri-cation systems and high-speed spindle applications.