Temperature control of a high-performance spindle: For the team developing a more effective spindle, one of the design challenges was an unexpected source of heat

Modern Machine Shop, Dec, 2001 by Jack McCabe

The pursuit of more productive machining invariably requires a re-examination of spindle technology. Spindle performance directly determines the efficiency of the machining process.

A shared interest in improving spindle technology united the companies listed on the facing page. One of the goals of their shared project, led by the National Center for Manufacturing Sciences (NCMS), was to double metal removal rates during the machining of cast iron and aluminum.

The team recognized that the component virtually dictating the size and weight of a spindle is its motor. Making a motor more compact increases the challenge of temperature control. In fact, two performance factors limiting the design of advanced spindles are both temperature-related. They are the bearing lubricant coking temperature and the melting temperature of the motor winding insulation.

As spindle speed and power increase within a compact design, remaining within these temperature limits becomes increasingly difficult. But practical solutions can still be found. The 75-hp machining spindle prototype illustrated in Figure 1 demonstrates this. Supported on roller bearings, this spindle proved capable of deep cutting in cast iron at 2,500 rpm and rapid machining of aluminum with power to spare at 12,000 rpm.

The motor that delivers this performance is no bigger than a one-gallon paint can. Heat dissipation throughout the spindle was the major obstacle to accessing this motor's full power. To overcome this obstacle, the spindle design incorporated major advances. Jet-lubricated roller bearings enable the spindle to operate at twice the speed originally specified by the manufacturer for this class of bearings. Also, a motor winding innovation overcomes a heat source previously overlooked by all motor designers.

Torque And Speed Capabilities

The 75-hp spindle demonstrated its high-torque capabilities by deck milling cast iron engine blocks and boring their eight cylinders at twice the metal removal rate accepted in current production. It demonstrated its high speed metal removal capabilities by milling an aluminum head at 12,000 rpm while delivering up to 95 hp. Relative to a comparable spindle currently used in production, the 75-hp spindle delivered 40 percent more torque at 2,500 rpm and 220 percent more torque at 10,000 rpm, while offering triple the controlled speed range and a 20 percent higher maximum speed.

Achieving this level of performance involved innovation and at least one surprising discovery.

Chilled water circulates through a jacket surrounding motor and bearings both. The jacket keeps bearing temperatures below 100[degrees]C and the motor winding temperature below 150[degrees]C. Test instrumentation confirmed the effectiveness of the jacket in cooling the bearings, but motor temperatures indicated a significant discrepancy between predicted and measured values. At a winding temperature of 150[degrees]C, the delivered torque was 30 percent less than the design torque, and the heat generated in the stator was 2 kW more than was forecast. The search for the source of this heat revealed the presence of an electromagnetic loss phenomenon identified as the proximity effect--a condition known in the transformer field but long ago discounted in motor design.

The Bearings

Bearing manufacturer Timken Company is quoted in NCMS's report, An Evaluation of the Design and Performance of Three Advanced Spindles. (Learn how to obtain this report at the end of the companion article on page 103.) According to a Timken source, "It is highly likely that the NCMS spindle is the highest speed tapered roller bearing spindle in the world." The bearing operates at a DN (mean diameter in millimeters times the top speed in revolutions per minute) of 1.4 million--very high for this type of bearing.

The nose and tail bearing designs are shown in Figure 2. Heat is generated in three areas of the bearing: between the rollers and the race that is secured to the shaft; between the rollers and the race that is secured to the housing; and between the roller ends and a "rib" that is secured to the housing. Most of the heat is generated on the races, but the highest temperature occurs in the rib-roller interface, a highly loaded small area with limited heat transfer opportunities.

Bearing system performance was validated on an instrumented rig. As Figure 2 shows, the design includes channels for circulating chilled water through the bearing housing. The bearings are modified 95- and 75-mm Timken "HydraRib" tapered roller bearings. In the initial design, the bearings were lubricated by an air-oil system. However, tests indicated that additional heat removal was needed above 6,000 rpm. To permit higher speeds, a jet lubrication system replaced the air-oil system, with slots machined in the outer race to eject hot oil that might otherwise become overheated.

At the inner race, little can be done to improve cooling. However, measurements showed that the maximum shaft temperate under the inner race never exceeded 90[degrees]F--a low temperature resulting from the high efficiency of the motor applied to this spindle. The cool shaft provides a heat sink at the bearing inner diameter to complement the cooling jacket at the outer diameter. The bearings' various cooling effects enable the spindle to run continuously under full load to 12,000 rpm.