Spindle types: cooling, bearings, duty and what the power means

Four specifications decide whether a spindle suits your work, and the power figure is the least informative of them. Here is what to read first.

Cooling: air versus water

The two cooling routes compared
	Air-cooled	Water-cooled
Cooling method	Fan on the spindle draws air over fins	Liquid through a jacket around the stator
Installation	Nothing else to buy or plumb	Pump, reservoir, radiator or chiller, tubing
Noise	Distinctly louder — the fan is often the dominant sound	Quiet at the spindle; a pump elsewhere
Low-speed running	Cooling falls with fan speed, so slow running under load is the hard case	Cooling is independent of spindle speed
Failure mode	Dust-clogged fins and a failing fan	Blocked loop, failed pump, air lock, freezing, corrosion
Detail	Cooling page	Cooling page

Neither is universally better. Air cooling wins on simplicity and on machines that rarely spend long periods under heavy load at low speed. Water cooling wins on noise and on long, continuous, loaded runs.

Bearings and the speed ceiling

The bearings set the maximum usable speed more than the motor does. Two broad classes:

Steel rolling bearings — cheaper, tolerant of abuse, and generally the choice up to the mid-teens of thousands of rpm.
Ceramic hybrid bearings — ceramic balls in steel races. Lighter balls mean less centrifugal loading and less heat, which is what makes the highest speeds achievable.

We are deliberately not publishing a hard rpm boundary between them: the limit depends on bearing size, preload, lubrication and the maker's design, and the same nominal bearing is rated differently by different builders. Use the spindle maker's maximum speed rating, and treat the bearing type as an explanation of why that rating differs between models.

Run-in and warm-up are not folklore. A cold spindle taken straight to maximum speed puts thermal shock into bearings that were machined to tolerances of a few microns. Stepping up through speeds over a few minutes — and after long idle periods — is cheap insurance on the most expensive part of the machine.

Duty rating: continuous or intermittent

A spindle's power rating is meaningless without the duty it was measured under. Standard duty classifications (from the IEC rotating-machines series) include continuous running at constant load and intermittent periodic duty, and a maker quoting the second will print a bigger number for the same hardware.

What to do with this:

If the listing gives one power figure and no duty, assume it is the flattering one.
If your work is long and continuous — production runs, long 3D toolpaths — you want a continuous figure, and it may be well below the headline.
If your work is intermittent, the headline figure is closer to usable, and the limit becomes thermal soak over the whole job rather than peak load.

What the power figure is measured at

This is the trap: a spindle sold as “2.2 kW” is 2.2 kW at or near its rated speed. At lower speeds it makes less power, and at the very top of its range it makes full power but very little torque. Both facts fall out of the same two-region behaviour explained on the speed and torque page.

Before comparing two spindles by kilowatts, convert to torque at the rpm you actually cut at. The comparison usually changes.

The nose: ER and beyond

The tool interface determines what cutters you can hold:

ER collet noses (ER11 up to ER32 on larger spindles) cover most router spindles. One holder takes a range of shank sizes, which is why it dominates.
ISO / BT / HSK tapers appear on automatic tool-change spindles, where the tool is gripped by a machine rather than by hand.

Pick the nose for the largest shank you will ever hold, not the one you use most. Sizing to today's cutters is a decision you will pay to undo.

Collet and tool-holder dimensions are covered in detail on a separate reference site and are deliberately not duplicated here.

A short buying sequence

Decide the speed range you cut in, not the spindle's maximum.
Convert your required cutting to a torque at that speed, and check the spindle can deliver it.
Match the voltage to the supply you actually have — before you order the drive.
Choose cooling for your duty, not for the spec sheet.
Then, and only then, compare prices.

The full version is on the how-to-choose page.

Parts

Spindles

Questions

Air-cooled or water-cooled?

Air-cooled is simpler, cheaper and fine for intermittent work. Water-cooled runs quieter and holds temperature under continuous duty — see cooling loops for what the loop involves.

What does the duty rating actually mean?

How long the spindle can hold rated load before it overheats. A spindle sold as intermittent duty is not a production spindle, whatever its power figure says.

How do I compare power figures between two spindles?

Compare rated power together with the speed it is rated at, then work out torque at your cutting speed. Two spindles with the same kW can be very different machines.

Sources and method

Three kinds of statement appear on this page, and each is labelled where it occurs:

Derived — the comparison is by construction and duty; the speed and power limits quoted are nameplate classes, not measurements.
Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.

Standards and documents

ISO 1940-1 — balance quality requirements for rotors (G grades)
ISO 230-1 — test code for machine tools: geometric accuracy
IEC 60034-1 — rotating electrical machines
The manual for your own drive and spindle — the only valid source for maker-specific values