Liquid Pocket Slider: a compliant hydrodynamic bearing for high-speed applications

Ref-Nr: TDO0158

Technology abstract

The Liquid Pocket Sliders are air-lubricated hydrodynamic bearings, outperforming any state-of-the-art devices of their class: No lubricants required, No need for coolants, Friction-free and wear-free in steady operational conditions, Maintenance-free, Failure tolerant, Embedded prognostic capability of wear detection, Overload indication and Adjustable stiffness. The last four characteristics of these novel bearings are peculiar of these devices making them unique.

Technology Description

An Italian SME is presently developing a new bearing technology (patent pending), conventionally referred to as the Liquid Pocket Slider (LPS), which is able to work at very high speeds, is failure-tolerant, mechanical-shock-resistant and experiences wear only at the start-up and shut-off phases. LPS bearings can find numerous industrial applications where high speeds and moderate loads are involved.

Details of one liquid pocket slider used in a laboratory test. Several similar elements compose the complete LPS bearing. In the first photo the slider is detached from the runner. In normal working conditions the sliders are loaded against the rotor surface.Unlike common ball bearings, LPS bearings have no rolling parts: instead of spheres, they have blisters filled with a liquid. The surface of these blisters - or pockets - is a flexible membrane, which naturally takes on the best possible shape to the passage of the unpressurized ambient air dragged by a running surface upon which the blister is loaded. Several sliding pockets can support a rotating shaft, or a rotor, forming a Liquid Pocket Slider (LPS) bearing.

In principle, a LPS bearing could work indefinitely at very high speeds, without undergoing wear or overheating. In fact its active surface never touches the opposite running surface except during the start-up and shut-down phases: above a certain critical speed, which depends on the working conditions, it literally lifts off, floating above the opposite surface.

LPS bearings possess a unique (optional) feature: the wear that accumulates in transient phases can be monitored, in order to avoid sudden malfunctions. Indeed, the idea includes the concept of ?sentinel cells?, which are special LPS cells working in parallel with the other ordinary elements of the bearing. The membrane of sentinel cells is purposely made to last less than the other membranes: for example 20, 30 or 50% less. The rupture of a sentinel cell is perfectly tolerable by the system, with a negligible affect on the total load capacity of the bearing; however, it flags the fact that a well-defined level of wear has been reached. Consequently, the user is informed about the remaining operative life of the device. Since sentinel cells are equipped with a pressure sensor, they can also be used to flag possible overload conditions.

Innovations & Advantages

Thanks to their peculiar features, LPS bearing would completely outclass their competitors. In fact, besides of all the advantages that also characterize other air bearings, LPS bearings display a number of additional benefits that make them unique:

TYPICAL ADVANTAGES OF AIR BEARINGS ADDITIONAL ADVANTAGES OF LPS BEARINGS
  • No need of liquid lubricants/lubricators
  • Low-noise
  • Low wear
  • No need for coolants
  • Shock tolerance
  • Failure tolerance
  • Proactive detection of wear levels
  • Improved self-centering action
  • Increased load capacity
  • Dynamic stiffness control in operation
  • Overload alert function
  • Standard models are possible

 

Further Information

The closest competitors of LPS bearings are foil bearings, which are state-of-the-art bearings working on a similar principle. They are currently used in air cycling machines to pressurize aircraft cabins with oil-free air, in small turbines for energy transformation (Capstone) and in industrial air blowers (Atlas Copco). However the active surface of foil bearings is not perfectly compliant because its elastohydrodynamic deformation is strongly affected by their own construction geometry. This means that foil bearings have to be tailored for each specific application. Furthermore, since foil bearings are, all-in-all, elastic suspensions, their ability to accommodate elastohydrodynamic deformations is strictly related to the overall rigidity of the system. Finally, even though they can undergo thousands of start-stop cycles thanks to proper anti-wear coatings, it is not possible to determine when their active surface will degrade, together with their performance. For these reasons foil bearings are still limited to a niche market.LPS bearings, instead, are fully compliant because their active surface is backed by a liquid rather than by springs. Furthermore their rigidity depends on the liquid compressibility and on the elastic coefficient of their membrane while their surface compliance depends on the membrane flexibility. Consequently, in LPS bearings surface compliance is largely uncoupled from the system stiffness. Finally, thanks to the possibility to include sentinel cells, LPS bearings can be far more reliable than any state-of-the-art bearing.Since LPS bearings are really compliant, they can be machined with normal tolerances and do not need to be tailored for a specific application. Consequently they can be produced as cheap, standard models.

Current and Potential Domains of Application

LPS bearings can be applied in numerous applications where high speeds and moderate loads are involved, including:

  • Space applications (Control Moment Gyros, Inertia Wheels)
  • Micro power generators
  • Pumps and compressors
  • Air cycling machines
  • Turbines
  • Household appliances (fans, air dryers, garden blowers, vacuum cleaners, etc.)