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Spindles By (External) Design

Ed Zitney of SKF Machine Tool Services explains why understanding how a particular machine tool spindle design will benefit an application can provide a big assist in helping to realize its ideal operation and performance consistent with the job at hand.

Posted: September 2, 2011


Understanding how and why a particular spindle design will benefit an application can provide a big assist in helping to realize ideal operation and performance consistent with the job at hand.

The world of machine tool spindles is populated by a variety of designs whose proper selection will be influenced by application parameters and the complexity of the particular machine in which the spindle will operate, among other factors. In general, spindle configurations include belt-driven, coupled-driven, gear-driven, and motorized types – each offering overall advantages to meet application needs.

An overview of this universe:

Belt-driven spindles. These types historically have been a preferred choice for low-cost machine tools or those developing high torque. They typically incorporate fewer components in their design and can usually be repaired easily at lower cost. While belt-driven spindles have given way in many cases over the years to more sophisticated models, they remain a viable option for applications such as machine centers, lathes, and grinding machines.

Coupled-driven spindles. These types are quite similar to belt-driven counterparts. In this configuration, a coupling is affixed to a motor coupling to drive the spindle. Users will turn to this version when space may be limited for the spindle, even higher torque may be required, or for improved dynamic behavior – or the pick may be based simply on particularly favored design issues or a design preference.

Gear-driven spindles. These configurations use a separate motor that mates a gear from the drive motor to the spindle. Their design results in higher torque, higher power, and usually lower speeds.

Motorized spindles. For medium- and high-performance machine tools, the motorized spindle unit traditionally has been the way to go. Within this category there are two basic versions: asynchronous motorized and synchronous motorized spindles. These designs are in response to customer demands for especially high productivity and improved workpiece quality. They often will be used in machining centers, milling machine tools, grinding machine tools, and other high-performance applications demanding high speeds, increased power, and extended service life.

Asynchronous motorized spindles, usually associated with higher-speed machines, typically feature automatic tool changes and sophisticated electronics – and often they are water-cooled. Their primary advantage is that the spindle integrates its own drive motor. The absence of belts, gears, and couplings confers power efficiencies and savings, because no power is lost through transmission of a belt, gear, or coupling.

The asynchronous motorized spindle can deliver a very high horsepower rating and can be engineered to perform at various speed ranges. They normally are run by high-frequency drives that enable variable speeds to be achieved simply (as long as the operator knows the horsepower torque curve and operates within the parameters).

Permanent magnet synchronous motorized spindles have risen to represent the higher end of motorized spindle solutions. Advantages include much more horsepower and torque in much smaller packages and a capability to realize extremely accurate positioning. These types are highly complex (and costly) compared with asynchronous versions, but performance considerations have made them quite popular in tandem with today’s precision machinery.

As a general rule of thumb, the externally driven spindle designs (belt, gear, and coupled) will usually provide an “easy” solution offering the possibility to apply more powerful motors. Their speed, however, will be limited. The motorized spindle configurations, in comparison, can provide a more compact solution with a capability to deliver higher speeds and higher dynamics.

Spindles serve as the “heart” of any machine tool. They will determine the quality of the final product and govern the overall productivity and efficiency of a machine tool. Understanding how and why a particular spindle design will benefit an application can provide a big assist in helping to realize ideal operation and performance consistent with the job at hand.

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