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Great Spindle Torque/Power Debate

Last Updated: Feb 26, 2013 09:56AM EST
An article by Bruce Tillinghast, Walker Machinery

In the past couple months I have run into a hornet’s nest of debates about HP/Torque/Thrust - all pertaining to material removal for difficult materials like Inconel or Titanium. The main issue being that we don’t all talk the same talk, so to speak.  The Japanese builders say it one way, the Italian manufactures another, the German conglomerates another, the Korean contingent another and the Americans even another. 

If this is consistently the case, then we are doing our customers a disfavor and all should be cognizant of this when we talk power!  I will leave axis thrust for another debate but the most telling specification difference I have seen is the heart of the machine tool: the spindle drive/motor/gearbox!  Whether on a Mill or a Lathe, it appears to be a constant debate, even though its capabilities define the performance of a machine tool. Often, spindle specification is expressed as hp “S3-30%, 60min” or KW S5min. What do these figures mean?  

These terms represent the relationship between power, torque, and speed when choosing a spindle for an application. For example, is a spindle rated for 20hp with 1,500-rpm base speed better than a 10hp spindle with a base speed of 500 rpm for cutting titanium?

 Between zero and base speed, the power increases in a linear fashion, while the torque is a constant. This is the rated torque of the spindle and is available at the base speed of the machine. Note that with a vector drive, full-rated torque is available from zero speed, while motors driven by an inverter will have zero torque at zero speed.
Between a base speed and the maximum speed of the motor, the power is a constant, while the torque decreases--remember that power is proportional to torque times the speed. This is called the rated power of the spindle.
* For some motors, the rated power may decrease beyond a certain rpm

It is important for machine tool users and sellers to understand the power-torque-rpm curve, since the cutting processes have to be designed so required power/torque at a speed is less than the available power/torque. When the process exceeds the available power and torque, it overheats the motor  -eventually burning it out. Yes burning it OUT! This is not a good thing as they cost a lot of money!

Based on different duty cycle ratings, rated power and torques have different values. It is important to distinguish the different ratings so the application is designed for a machine's capabilities. The International Electro technical Commission--IEC--has released a duty cycle rating standard.

Every motor has a continuous-duty rating for power and torque--an S1 rating. S1 is based on reaching thermal equilibrium on a sufficiently long duration. Since most motors in machine tools--axes or spindle motors--are used in a non-continuous duty cycle, the power/torque capability is higher than the continuous duty cycle, since the motor heats up differently. A standard specification--IEC 34--describes the capability of a motor based on a set duty cycle.
 
The ratings are S1 through S9.
The four ratings for machine tool spindles are S1, S3, S6, and peak load ratings:
* S1 -- Continuous duty rating: Constant load with duration long enough for motor to reach thermal equilibrium.
* S3 -- Intermittent periodic duty type without starting: A sequence of similar duty cycles at constant load separated by no-load--zero spindle speed--conditions.
* S6 -- Continuous operation--periodic duty type: A sequence of similar duty cycles at constant load separated by no-load--but continuously running--condition.
S3 and S6 ratings are expressed as power available for a given percentage of load period in a given cycle duration. When no cycle time duration is specified, a 10-minute time applies as default.
* 15hp S3-30%, 60min -- S3 rating for the spindle is 15hp available when the spindle is under constant load for 18 minutes--30 percent of the 60-minute cycle.
* 10KW S6-60% -- S6 rating of the spindle is 10 kW when the spindle is used under a constant load for 6 minutes--60 percent of a 10-minute cycle.
* Peak load rating -- The instantaneous power available for a very short burst, such as entering a cut, or for accelerating the spindle to speed.

Knowing machines' ratings - whether a lathe or a mill - offers two advantages. First, it allows a one-to-one comparison when considering a machine purchase. Second, it lets the design of the cutting process' required power/torque match the machine's rpm availability.
  
*Data for this article was extracted from an article from 2007 by Jairam Manjunathaiah, PhD
And from standard specification--IEC 34.

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