Every hydrostatic bearing has two halves: the pad that shapes the oil film, and the restrictor that decides how the film behaves under load. The pad gets the attention; the restrictor decides whether the bearing is merely adequate or genuinely stiff. Choose it wrong and no amount of pad geometry will save the stiffness curve.
The throttle ratio is the contract
A restrictor sits upstream of the pocket and drops supply pressure P₀ down to the working film pressure P_w. Their ratio — the throttle ratio β = P₀ / P_w — is the single number that links restrictor and pad. Tune pad and restrictor together and the bearing sits at its stiffness optimum for the design load; treat them separately and the "optimum" exists only on paper. This is why a restrictor selection guide has to start from the load case, not from the component catalog.
Fixed restrictors: capillary and orifice
A capillary is a long, thin tube; flow through it is laminar and proportional to pressure differential. It is forgiving, cheap, and insensitive to small burrs or viscosity swings in the moderate range — the workhorse for steadily loaded bearings. Its weakness is that stiffness is modest: compensation only comes from the pressure drop itself. An orifice is a short, sharp hole; flow scales with the square root of pressure, which gives slightly better stiffness in a more compact part, but the small bore is less tolerant of contaminated oil and the characteristic shifts with temperature as viscosity changes. Both are fixed devices: once built, their throttle ratio is locked, so they suit applications where load direction and magnitude are predictable.
Variable and self-compensating: membrane and internal feedback
When load varies widely or stiffness targets are aggressive, fixed restriction stops being enough. A membrane-feedback restrictor uses a flexible diaphragm that deflects under pressure imbalance, opening and closing the gap to the pocket in real time — it is the highest-stiffness realization of variable restriction and the reason precision grinders with demanding surface-finish targets use it. An internal-feedback design builds the compensation into the bearing land geometry itself: no external diaphragm to fatigue, fewer parts, and compensation distributed around the pad. The trade is machining precision — the oil-cavity flatness work shows why sub-micron flatness is where this design earns or loses its stiffness.
Selection logic in practice
A working selection sequence looks like this: define the load envelope (magnitude, direction, dynamic content) → choose the film thickness the accuracy budget allows → size the restrictor for the throttle ratio at mid-load → then check the extremes. If stiffness collapses at the extremes, move up: capillary → orifice → membrane. If contamination or maintainability dominates, internal feedback earns its machining cost. And whatever the choice, verify it: the theory-versus-measurement gap in real hydrostatic builds is too large to skip bench validation.
Where we stand
Marusaku Seiko builds hydrostatic spindles and rotary tables with the restrictor chosen per load case — capillary, orifice and membrane types across the MRDY range — and stiffness validated against measurement, not simulation alone. The oil film's 5–10× stiffness advantage over rolling elements is only real if the restriction in front of it is engineered for the job: see the film stiffness data.
