Squeeze Film Bearing

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SQUEEZE FILM BEARINGS

G SURESH KRISHNA



INTRODUCTION

A hydrodynamic bearing develops load carrying capacity by virtue of

relative tangential motion of two surfaces separated by a fluid film

and it requires a bearing geometry so that a convergent fluid film in

the direction of motion is produced. But a positive can be pressure

generated in a fluid film placed between two parallel surfaces if the

surfaces move towards each other in normal direction. The load

carrying phenomenon arises from the fact that a viscous fluid cannot

be instantaneously squeezed out from the interface with two

surfaces that are moving towards each other and this action providesa cushioning effect in bearings. When the load is taken off or two

surfaces move away each other, the fluid is sucked in and the film

can often recover its thickness in time for the next application. The

squeeze film action is seen in many cases.

1. During approach of faces of disk clutch under lubricated

conditions.

2. During walking with rubber soles on wet pavements.

3. Squeeze film damper.

4. Rolling of automobile tyres on wet roads.

Squeeze film damper

In simplest form, the squeeze film damper consists of an inner

bearing and an outer bearing. The inner bearing OD is permitted tomove radially, but is prevented from spinning, typically by using a

loose fitting anti-rotation pin. The inner bearing OD is the bearing

journal of the squeeze film damper and it operates against the

bearing housing bore, which acts as the damper bearing. The gap

between the squeeze film damper journal and the damper bearing is

filled with a lubricant.

During operation, the journal moves due to the rotor dynamic forcesand the fluid is displaced to accommodate this motion. As a result,



hydrodynamic forces are generated in the oil film that is developed

between the damper journal and the damper bearing. This helps

dissipate vibration energy and lower the forces transmitted to the

support structure.

REYNOLDS EQUATION FOR SQUEEZE FILMS

Squeeze film is a term denoting a hydrodynamic film that sustains a

negative ∂h/∂t, i.e. when the opposing surfaces are being squeezed

together. An extremely useful characteristic of squeeze films is that

they provide increased load capacity (although temporary) when a

bearing is suddenly subjected to an abnormally high load. This

feature is essential to the reliability of crankcase bearings which

must withstand transient combustion forces. A further aspect of

squeeze films is that the squeeze film force is always opposite in

direction to the motion of either bearing surface. Squeeze film forces

contribute to the vibration stability of a bearing. To analyse squeeze

film forces, the term ∂h/∂t is kept in the Reynolds equation and is

given precedence over the film geometry term ∂h/∂x. The Reynolds

equation with the squeeze term is in the form.

Assuming an isoviscous lubricant and zero entraining velocity this

equation becomes:

This equation defines the hydrodynamic pressure field when the

wedge effect is absent, e.g. when the load vector rotates as

mentioned previously. It can be integrated in terms of a specified

bearing geometry to provide load capacity, maximum pressure or

any other required bearing characteristic in terms of ∂h/∂t. The

‘squeeze time’ which means the time required for film thickness to



decline to some critical minimum value can also be determined by

integrating ∂h/∂t with respect to time.

To illustrate the principles involved, an example consisting of two

long parallel plates squeezed is considered.

PRESSURE DISTRIBUTION

For two parallel and infinitely long plates: h≠ f(x) and ∂p/∂y = 0

Squeeze film between two parallel plates.

Pressure gradient:

Load Capacity:

The load that the plates can support, or more exactly the force

separating the plates, can be obtained by integrating the pressure

distribution over the bearing area



Squeeze Time:

The time necessary for the film thickness between parallel plates to

change between specified limits

Similarly for finite parallel plates, the squeeze time is given by

For flat circular plates

Where

Δt is the time required for the film thickness to decline from ‘h1’ to ‘h2’ [s];

R is the radius of a circular plate [m];

h1 is the initial film thickness [m];

h2 is the final film thickness [m];

W is the load [N].

CAVITATION AND SQUEEZE FILMSIn a system where positive and negative squeeze occurs in the

presence of a liquid lubricant, cavitation is almost inevitable.

Cavitation affects squeeze film forces by the formation of

compressible bubbles in an otherwise incompressible lubricant.

Bubbles can also persist or grow in size by coalescence until the

squeeze characteristics of the system are fundamentally changed.

The persistence of bubbles even under temporarily positive lubricant

pressure is due to the much slower rate of bubble dissolution as

compared to the rate of bubble formation.



Mechanism of bubble accretion under oscillating squeeze

APPLICATIONS:-

Squeeze film dampers have traditionally have been used to

overcome the stability and vibration problems that are not

adequately handled with conventional bearings. This translates to

lower transmitted forces and long bearing life particularly formachinery that operates at super critical speeds. Some of the

examples of squeeze film dampers are

1. Squeeze film damper with a centred spring.

2. O-ring supported dampers.

3. Squirrel cage supported damper.( used in aircrafts engines)

4. Integral centering spring damper.

SUMMARY:-A positive pressure is developed in a fluid place between two

surfaces when the surfaces move towards each other. The finite time

required to squeeze the fluid out of the gap provides the squeezing

action that act as cushioning effect in bearings. In this mechanism, a

parallel film produces extremely large load carrying capacity.

The squeeze film lubrication phenomenon is observed in several

applications such as gears, bearings, machine tools, rolling elements

and automotive engines.

Squeeze Film Bearing

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