15.12.1 Introduction
The problems with spindles designed for small wheels is somewhat different to those for larger wheels in that nearly all wheel bond types, including most superabrasives, are limited by bond strength to operating in the speed range of 35 to 60 m/s. Taking 100 mm (4) diameter as the cutoff for this discussion, the limitations are in flexibility to handle a wide range of wheel diameters and maximum wheel speeds for very small wheels as those used in, for example, the fuel injection industry. The operating range under consideration is therefore 6,000 to 200,000 rpm.
15.12.2 Rolling Bearing Spindles with Belt Drive for Small Wheels
The simplest and oldest designs of spindles are grease-packed, angular contact-bearing spindles, belt-driven similar to the larger cylindrical wheel counterpart discussed above except the bearings
are always at least ABEC 7 or 9 (ISO class 4 or 2) and the belts are multiribbed. Spindle nose runout is maintained at 2 pm or better. Grease-packed bearings are capable of running up to about 30,000 rpm [Bryant 1986] or 500,000 DN [Nakamura 1996]. Similarly, belt drives are available up to about 30,000 rpm [Bryant 1986], although the preferred limit is nearer 10,000 rpm [Aronson 1998]. With variable speed control, this arrangement provides a compact flexible arrangement for dedicated internal and universal grinders performing internal and face grinding with conventional wheels in sizes in the range of 100 mm down to 25 to 50 mm in diameter.
15.12.3 High-Speed Spindles for Small Wheels
For speeds faster than 30,000 rpm, various design considerations need to be incorporated.
Use of direct drive ac asynchronous induction motors with frequency converters are mostly used to eliminate the need for belts. The drive will usually have a closed loop tachometer, thermistor monitored temperature control, and overload protection [Precise n. d.]. Spindle horsepower curves need to be closely married to the rpm requirements as each drive will have a particular operating range in terms of rpm and torque. Controls are now available that can extend the constant horsepower range over a much wider speed range. Bryant [1992], for example, provides just two spindles to cover the entire range from 8,000 to 70,000 rpm.
15.12.3.2 Dynamic Balancing of High-Speed Spindles
Dynamic balancing of all rotating components and in situ balancing to better than 1.2 mm/s displacement is critical to spindle life and grind quality [Gamfior n. d.].
15.12.3.3 Oil-Mist Lubrication for High-Speed Spindles
Oil-mist lubrication to replace grease packed bearings can increase the DN value of steel ballbearings from 500,000 to 1,000,000. Oil mist lubrication requires an oil-mist source and a compressed air source (Figure 15.35). The air needs to be carefully dried to prevent rusting of the spindle and must be filtered to 5 pm or better to prevent contamination of the bearings. The oil — mist at a pressure of about 1 bar provides positive air displacement to the unit, which, coupled with a labyrinth seal, prevents coolant ingression. Also note, even with the current oil-mist systems, when the spindles are run over extended periods of time a long length of pipe connection should be used (>1 m) to facilitate air-oil mixing.