Spindle assembly (direct-drive spindle unit: bearing cartridge with its motor coupled coaxially behind it)

01

Bearing arrangement and preload in a coupled-drive cartridge

GMN describes spindle ball bearings as angular contact bearings that need "alignment with a second bearing", with nominal contact angles of 15° to 25° and precision classes ABEC-7, ABEC-9 and higher per ISO 492. It gives n·dm reference values of up to 1.7 · 10⁶ mm/min for its S series (15° and 25°), up to 2.0 · 10⁶ mm/min for SM (15°, 17°, 19°) and up to 2.1 · 10⁶ mm/min for KH (15°, 17°, 25°), and says spindle bearings offer higher axial rigidity than deep groove ball bearings "due to their steeper contact angle and targeted preload". [1]

NSK's sealed super precision angular contact bearings for machine tool main spindles come as universal arrangement (SU) bearings whose preload offset is controlled on both faces, so that "designated preload can be set for any combination of bearings" in DB, DF and DT arrangements; a "V" mark on the outer diameter shows the orientation. Its ROBUST series lists BNR types at 18° and BER types at 25° contact angle, with ceramic (Si3N4) balls in the H and X types, and its speed chart distinguishes grease with position preload from grease with constant-pressure preload. [5]

GMN offers all its series as hybrid bearings with ceramic balls, which it says reduce rolling element mass, lower friction torque, improve start-up dynamics and provide electrical insulation. [1]

Give the spindle maker the machine and process (milling, grinding, turning), tool interface, maximum and continuous speed, power and torque at the spindle, bearing series, contact angle and arrangement if specified, preload method, lubrication, housing form and mounting, drive (belt, coupling, attached motor), runout and balancing requirements, coolant through the spindle if needed and quantity.

02

Coupling, attached motor housing and lubrication

GMN names three drive concepts for milling spindles and says "direct drives are particularly responsive and require little maintenance", while "belt drives offer high flexibility at lower costs" and motor spindles combine drive and spindle in one unit. [4] For mounting it lists spindle housings with an attached motor housing (TK-M and TF-M, with or without flange) and a block housing with an attached motor (BH-M), next to a separate spindle housing for belt spindles (TK). [3]

The coupling between motor and spindle has to run true at spindle speed. Schmidt-Kupplung describes its Spinplus coupling as "structurally balanced for smooth running in drive trains at high speeds" and, because of its high concentricity, designed for speeds of up to 16,000 min⁻¹ depending on size. [6] Design News reported in 2016 that R+W showed its SP6 coupling for high-speed spindle motors in milling and grinding machines, promising greater shaft concentricity; R+W's sales manager said "the advantage is better balance". [7]

GMN states that its grease-lubricated spindle bearings run "maintenance-free" over the bearing life, while oil-air lubrication carries oil in a stream of air to the lubrication points and is "especially suitable for very high rotation speeds"; its standard grease-lubricated spindles have a barrier air seal. [2] Its +AG bearings take grease relubrication through the outer ring for n·dm values "up to 2.3 · 10⁶ mm/min". [1]

Editorial calculation from GMN's bearing table: the SM 605 C TA bearing has a pitch diameter dm of 8.6 mm and speed values of 235,000 min⁻¹ with oil and 175,000 min⁻¹ with grease, so n·dm is 235,000 × 8.6 ≈ 2.02 · 10⁶ mm/min with oil and 175,000 × 8.6 ≈ 1.51 · 10⁶ mm/min with grease, about a quarter lower; the oil figure is slightly above the 2.0 · 10⁶ mm/min GMN states for the SM series, so confirm with GMN which value applies. [1] Check that the coupling's speed rating is at or above the spindle's rated speed.

03

Shaft, bearing set, coupling, motor housing and lubrication

In a direct-drive unit the coupling and the attached motor housing replace the pulley and belt of an externally driven spindle.

04

Taper, speed, runout and balancing figures for a spindle unit

Speed here is limited three ways: by the bearing n·dm, by the lubrication and by the coupling's rated speed; the rated speed of the assembled unit comes from the spindle maker.

05

Send the machine, speed and bearing data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [type; process; tool interface]
Speed: [max; continuous; power; torque]
Bearings: [series; contact angle; arrangement; preload]
Lubrication: [grease; relubrication; oil-air]
Drive: [motor; coupling type and rated speed; attached housing]
Accuracy: [runout; balancing grade]
Coolant: [through-spindle; pressure]
Quantity: [units per order]

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