Load capacity comes from the oil film, not metal contact
A liquid hydrostatic spindle builds a pressurized oil film between the journal and the bearing through an external oil supply. The journal floats on that film with, in theory, zero metal contact and zero wear. The stiffness — the ability to resist deformation under radial load — is set by the oil-film pressure field. That pressure field reacts sharply to clearance. This is exactly why clearance is the linchpin, not the motor. The motor supplies speed and torque; the precisely controlled film carries the cutting force.
Quantified manufacturing errors: a small clearance shift collapses stiffness
A study on manufacturing errors in hydrostatic bearing spindles for precision machine tools produced clear, quantified conclusions:
| Manufacturing error | Variation | Effect on oil film stiffness |
|---|---|---|
| Radius clearance | +10 um positive deviation | Stiffness drops to 29.9% (about 70% loss) |
| Throttle orifice diameter | +/-0.05 mm | Stiffness varies up to 1.34x |
The meaning is direct: no matter how large the motor torque, if bearing clearance and orifice machining deviation are not controlled, the whole spindle's rigidity collapses. This is why a hydrostatic spindle demands more from ultra-precision machining and assembly calibration than a standard motorized spindle — what you buy is a tamed oil film, not a motor.
Throttle ratio and restrictor: the two levers of clearance control
Optimal throttle ratio. Hydrostatic design has a theoretical optimal throttle ratio; deviating degrades both stiffness and flow.
Capillary vs orifice throttling. Capillary throttling gives higher stiffness and lower temperature rise; orifice throttling carries more load but is extremely sensitive to bore diameter (+-0.05 mm already shifts stiffness significantly).
JB/T 11767 context. A supply pressure of 1.6–3 MPa and a temperature rise of 35 deg C or less mark the normal operating boundary for the throttle system. Exceeding them signals that clearance or throttling has drifted outside the design window.
What buyers should ask: how is clearance controlled?
During selection, do not ask only about motor parameters. Ask three things:
- What is the clearance machining tolerance, and is there a paired grinding or matching process?
- Is the restrictor made in-house or sourced? How is bore consistency guaranteed?
- Is each unit measured and calibrated for oil-film stiffness, rather than given only a theoretical calculation?
MRDY's clearance control approach
The MRDY series is built on a hydrostatic structure and controls clearance matching, restrictor consistency, and per-unit oil-film stiffness calibration so that dynamic rigidity stays inside the design window — instead of stacking motor parameters. For the buyer this means acceptance should deliver measured stiffness plus rotation accuracy, not just a polished motor spec sheet.
Related reading: hydrostatic spindle principle and selection, and hydrostatic vs rolling bearing comparison. See the full MRDY hydrostatic spindle line.
Application industries
Hydrostatic spindles with tightly controlled oil-film stiffness serve precision turning and grinding, aerospace components, semiconductor equipment, medical device machining, mold and die, and heavy-duty CNC machine tools where thermal stability and sub-micron accuracy matter.
Key parameters at a glance
| Parameter | Typical value or boundary |
|---|---|
| Rotation accuracy | 0.5 um class |
| Radial runout | 0.5 um or less |
| Oil-film supply pressure | 1.6–3 MPa (JB/T 11767) |
| Temperature rise | 35 deg C or less |
| Oil filtration | 10 um or less |
| Oil temperature control | +/-0.5 to 1 deg C |
Request a stiffness datasheet
If you are specifying a hydrostatic spindle and need measured oil-film stiffness and rotation-accuracy data per unit, contact Marusaku Seiko. Share your load, speed, and accuracy targets and we will match a hydrostatic or hydrostatic direct-drive spindle configuration to your process.
