Hydrostatic Rotary Table Load Capacity and Overturning Moment
A hydrostatic rotary table earns its place on a five-axis or turning-milling center by holding a workpiece steady on a film of oil only a few microns thick. The first number most buyers ask for is also the one most often misread: how much can it carry? Load capacity here is not a peak breaking figure. It is the load the table supports while keeping its oil film intact, its stiffness up, and its runout within specification. This article explains where that capacity comes from, how overturning moment enters the picture, and how to read a real spec sheet such as the MRDY-630.
Where the capacity actually comes from
The load a hydrostatic table carries is generated by the pressurized oil field between the rotating table and its base. Each oil recess is fed through a restrictor that raises recess pressure as the load rises, so the film thickness stays nearly constant under load. At a rated supply pressure of 3.5 to 4 MPa, a φ630 mm table such as the MRDY-630 carries 1000 kg of workpiece while the oil film keeps the rotating member fully suspended and free of contact. Capacity, then, is a function of oil-film pressure and bearing area, not a mechanical safety factor added to the casting.
Overload flattens the film
Exceed the rated load and the film is squeezed thinner. Stiffness drops, runout grows, and at the extreme the surfaces can touch, damaging the bearing and erasing the precision the table was bought for. That is why a useful spec quotes load under guaranteed accuracy: the figure held while the stated runout is still met. For the MRDY-630 that runout is ±0.001 mm. A peak maximum-possible number that ignores accuracy is a trap, not a specification.
Overturning moment from off-center loads
Most real workpieces do not sit dead center. A heavy fixture hung out to one side creates an overturning moment the film must resist without losing parallelism or tilting the axis. The table handles this through the pressure distribution across several recesses: an eccentric load raises pressure on the loaded side and lowers it on the opposite side, and the restrictor network rebalances. When sizing a table, take the moment (mass × offset) and check it against the manufacturer rated overturning capacity, not only the vertical load.
Bearing diameter versus table size
Experienced buyers note a useful rule: the ratio of bearing diameter to table diameter matters more for life and accuracy than the headline load number. A large table on a small bearing runs the film at higher local pressure and ages faster; a generously sized bearing spreads the load and keeps the film thick and stable. Match the bearing envelope to the workpiece envelope, not just to the weight.
Reading the MRDY-630 specification
| Item | MRDY-630 reference |
|---|---|
| Table diameter | φ630 mm |
| Rated load | 1000 kg @ 3.5–4 MPa |
| Rotary accuracy | ±0.001 mm (1 µm) |
| Drive motor | 485SJM-1350BH torque motor, 1350/2400 N·m @ 90 r/min |
| Feedback | Heidenhain RCN2380 closed-loop rotary encoder |
| Seal | Air-curtain seal against chips and dust |
These figures describe a table sized for precision work where the load stays inside the film-pressure window. Treat them as the accuracy-rated operating point, not a limit to be pushed.
Margin and thermal reality
Leave margin. Guidance for similar direct-drive tables suggests roughly 30 percent torque headroom; the same prudence applies to load. Size for the heaviest realistic fixture, then add margin so the film never approaches its thin limit. Also remember that published accuracy is usually a cold-state value. The number that matters in production is the runout after several hours of continuous running, once oil temperature has settled. A constant-temperature oil supply is what keeps that number from drifting.
For the three metrics that decide a hydrostatic rotary table purchase, see our guide on selection criteria for hydrostatic rotary tables. If you are weighing hydrostatic against rolling tables, the comparison hydrostatic vs rolling rotary table covers the trade-offs. Our hydrostatic direct-drive rotary table product page lists the full MRDY series.
Talk to us about your workpiece
If you are matching a rotary table to a specific part, send the drawing and the heaviest fixture load. We will check the vertical load, the overturning moment, and the oil-film margin, then recommend a configuration that holds accuracy through a full shift.
