A linear axis is judged by two things under cut: no stick-slip at low speed and stiffness at the frequency the cut actually happens at. The liquid hydrostatic guideway — especially the cylindrical form with internal-feedback restriction — solves both by carrying the slide on an oil film. But its dynamic behavior is governed by three parameters that do not rank the same way at all frequencies: film thickness, oil viscosity and supply pressure. Design guides that give one static number miss the point; the ranking flips with frequency.

1. Why the oil film wins the guideway job

A sliding way drags (stick-slip at low feed); a rolling way transmits ball-passing vibration and has little inherent damping. An oil film does neither: there is no dry friction to break loose, no rolling elements to print their geometry into the motion — the same error-averaging logic that makes hydrostatic spindles accurate — and the film is a viscous damper by nature. That is why hydrostatic slides crawl-free at low speed and add damping the structure cannot get elsewhere (see our oil-film vibration damping analysis).

2. Internal-feedback restriction, and a model you can actually use

In the internally-restricted cylindrical guideway, each pocket is fed through a restriction formed inside the bearing itself, so the film self-compensates as load shifts. The dynamic behavior is captured with classical tools: force balance on the slide plus flow continuity through the restrictor and lands, linearized about the operating point, then converted to a Laplace-domain transfer function between load and film thickness. That single transfer function yields both the time-domain step response and the frequency-dependent dynamic stiffness — no black box required.

3. Time domain: three laws from a 3,500 N step

Apply a step load (on the order of 15% of supply-pressure × effective area) and the linearized model gives clean, testable laws:

• Larger initial film thickness → larger steady-state displacement, but shorter transient. More clearance leaks faster, so the pressure re-balances quickly — at the cost of a softer static film.
• Higher oil viscosity → smaller steady displacement, but a longer transient. Thicker oil throttles the escape flow, holding position better while slowing the recovery.
• Higher supply pressure → both the transient time and the steady displacement shrink substantially. Pressure is the one lever that improves everything at once — the same conclusion the capillary-restricted bearing dynamics work reaches for spindles.

4. Frequency domain: the ranking flips — tune for your band

Sweep the excitation frequency and the parameter rankings reverse:

• At low frequency (quasi-static cutting, heavy slow passes): a large film thickness lowers dynamic stiffness, while higher viscosity and higher supply pressure raise dynamic stiffness and stabilize the response — the static intuition holds.
• At high frequency (fine finishing, interrupted cuts, higher harmonics): a large film thickness increases dynamic stiffness — the opposite of the static case — because the squeezed-film effect dominates and the extra clearance becomes a damping volume.

So "thin film for stiffness" is only half true. The honest design rule: identify the frequency band your process lives in, then choose film gap, viscosity grade and supply pressure for that band — not for a static load case that never occurs.

5. A worked parameter set

A representative build: 200 kg slide, 3 MPa supply, 220 mm bearing diameter, 30 μm film, ISO VG 32 guideway oil (≈2.76×10⁻² Pa·s). This is the scale where the transfer-function model and measured response agree well, and where the low-frequency/high-frequency split above is clearly visible in test. It is also the scale of most precision hydrostatic slides for machine tools.

Marusaku Seiko hydrostatic guideways and slides

Marusaku Seiko engineers hydrostatic guideways and linear axes around the frequency band the customer's process actually runs — film gap, restrictor design (internal-feedback and other restriction types) and oil specification chosen together, with the transfer function verified against measurement before the machine ships. Pairing a hydrostatic guideway with a hydrostatic spindle gives axes and spindle all oil-film supported — no stick-slip, no wear, damping rolling ways cannot provide.