"Oil is softer than steel" sounds like the end of the argument, and it is exactly backwards. Stiffness in a bearing is not the material's modulus — it is the slope of force versus displacement, and a properly compensated hydrostatic film delivers five to ten times the usable stiffness of an equivalent rolling bearing, with measurements in the 500 N/μm class on production spindles. Here is why the intuition fails and where the numbers come from.
Where the stiffness actually comes from
A hydrostatic pocket fed through a restrictor sits at an equilibrium film thickness. Push the journal toward the pocket and the film thins; escaping oil meets more resistance, so pocket pressure rises, pushing back. Pull away and pressure falls, letting supply re-fill the gap. The bearing is therefore an active feedback element: displacement generates its own restoring force, limited only by how hard the restrictor lets the pressure respond. Rolling bearings have no such mechanism — their stiffness is the Hertzian contact slope, which is high locally but nonlinear, load-dependent, and degraded by the clearance and the small number of rolling elements actually carrying load at any instant.
Linearity is the underrated part
The rolling bearing's stiffness curve steepens with load, which sounds good until you notice the shaft rides on two or three balls at rest and the effective stiffness fluctuates as elements pass through the load zone — a micro-nonlinearity that shows up as roundness error and vibration. The hydrostatic film's force–displacement curve is nearly linear across its working range, which is why roundness figures like 0.1 μm radial runout are routine on film-supported spindles and exceptional on rolling ones. Predictability under varying load is worth as much as the peak number.
The knobs behind the number
Film stiffness is set by four quantities a designer actually controls: supply pressure, film thickness, effective pad area, and restrictor characteristic. Raise pressure and stiffness climbs roughly proportionally; halve the film thickness and stiffness roughly doubles — until manufacturing tolerances and thermal growth eat the margin, which is why the film-thickness-versus-pressure trade is a design optimization rather than a knob to slam. This is also where the restrictor choice from our selection guide re-enters: compensation quality sets how much of the theoretical stiffness survives contact with a real load case.
And the part balls cannot offer at any stiffness
Damping. An oil film dissipates vibration by viscous shear — no metallic contact, no ball-pass excitation, no wear debris — which is why film-supported spindles produce better surface finishes and why chatter resistance improves even at equal stiffness. Stiffness positions the tool; damping keeps the cut quiet.
The summary we give customers
A rolling bearing is a shape that carries load. A hydrostatic film is a controlled system that carries load. That difference is why Marusaku Seiko specifies film-supported spindles for work where stiffness must be high and repeatable: see the direct stiffness comparison and the full hydrostatic spindle line.
