A rotary table is judged by one number under cut: stiffness. For ultra-precision indexing and turning, the liquid hydrostatic rotary table is the default where large load, high stiffness and micron-level runout must coexist. The two parameters that decide its stiffness are deceptively simple — restrictor orifice diameter and bearing clearance — and they interact in a way that punishes one-parameter tuning.
1. Why conical beats cylindrical
A conventional cylindrical hydrostatic table must stack a separate radial and thrust bearing to support two axes, and once machined the radial gap cannot be adjusted. The conical hydrostatic table (see how it compares with mechanical and rolling rotary tables) replaces them with one pair of conical bearing surfaces that supply radial and axial support simultaneously: more compact, larger angular stiffness, and — critically — the bearing clearance is adjustable at assembly by shimming. That adjustment is not a convenience; it is the knob that sets stiffness.
2. Restrictor forms and the throttle ratio
Hydrostatic tables are fed through restrictors — orifice, membrane, capillary, self-compensating. The orifice-type conical table is the practical favorite: simple to machine and assemble, with better damping and stability than self-compensating designs. The physics is captured by the throttle ratio β = supply pressure P₀ / working pressure P_w and the stiffness relation J = F / e (load over displacement). Self-compensating conical tables already reach axial and radial runout below 0.05 μm; a variable-recess compensation shows more than 3× the radial stiffness of a fixed-recess design, and a larger half-cone angle improves axial load capacity, axial stiffness and damping.
3. Clearance: stiffness rises then falls, and the peak is parameter-dependent
In a study sweeping bearing clearance from 14 to 22 μm (1.5 MPa supply, 0.4 mm orifice), the oil-film pressure inside the cavities falls as clearance grows — a larger exit area means smaller land resistance, more leakage, smaller ΔP against atmosphere. Stiffness, which is set by the pressure difference across the four cavity pairs, therefore rises then falls, peaking near 16 μm for that particular parameter set. The peak location is not universal: it is set jointly by orifice diameter, land width and oil viscosity.
4. Orifice diameter: the cross-over that breaks naive rules
Sweeping orifice diameter at 0.3 / 0.4 / 0.5 mm (clearance held at 18 μm) shows larger orifice → higher cavity pressure (smaller throttle resistance, smaller ΔP drop). But the orifice that wins depends on clearance: below ~15 μm the smaller orifice is stiffer; between ~15 and 19 μm the 0.4 mm orifice is best; above ~19 μm the larger orifice wins. The reason is that cavity pressure's sensitivity to orifice change flips as clearance shifts. Conclusion: orifice and clearance must be co-optimized, not tuned one at a time.
5. Simulation vs experiment — and why the numbers diverge
Fluid simulation (Fluent, pressure-inlet 1.5 MPa, convergence <1×10⁻⁴ MPa) and measurement agree in trend across clearance and orifice: each orifice reaches its stiffness maximum as clearance grows, and the orifice-size ranking reverses past a clearance threshold. But the measured stiffness is only about half the simulated value. The gap is explained by two omissions in the model: it ignored the table's own weight, and — confirmed by an ANSYS fluid–structure coupling pass — the bearing deforms under high-pressure oil, with rotor deformation up to 4.06 μm and stator 1.18 μm (>5 μm combined). At small clearances the cavity pressure is high and the deformation error is largest — which is why real tables are specified to a 0.001 mm accuracy grade only after the deflection correction is in. A real table design must carry the structural deflection into the film-gap correction, or the stiffness promise will not survive the shop floor.
Marusaku Seiko hydrostatic rotary tables
Marusaku Seiko applies this conical, orifice-fed hydrostatic architecture to rotary tables for vertical turning, indexing and ultra-precision dividing — clearance set at assembly, orifice sized for the duty, and the structural deflection accounted for so the stiffness you specify is the stiffness you get. The MRDY-630 hydrostatic rotary table is a worked example of the clearance-and-orifice co-optimization above.
