Fluid bearing

The hydrodynamic sliding bearing is a lubricating oil ( often also with a gas ) working machine element.

When with him related hydrostatic slide bearings, the oil film is set using an external oil pump under pressure, which do not touch the two against each other moving parts directly. There is a low-loss fluid friction.

The hydrodynamic slide bearing, the pressure in the oil film produced by the pumping action of the two part surfaces themselves to form a wedge in which the oil is transported into and thereby undergoes pressure increase. During start and expiry this is not sufficient to prevent solid contact throughout. In these phases, there is mixed friction.

Description

Explanation on the example of the vertically loaded hydrodynamic journal bearing oil:

Lubricating the wedge is formed by the eccentric position of the shaft in the bearing bush. By the rotation of the shaft an oil flow is produced in the wedge inside of shear stresses between the shaft surface and the oil. Due to the cross-sectional narrowing increases the pressure in the wedge bar to sometimes over 100. The self-adjusting position also includes a small horizontal offset to the right ( see figure). The red marked narrowest point is off-center, soon after the oil pressure disappears. The load force (hatched with radial strokes surface ) in the gap resulting force P is vertical and passes through the shaft axis. It is in equilibrium with the weight force F of the shaft.

The micro-roughness on the surfaces of the shaft and bushing store the lubricant. Experiments have shown that too smooth or even polished surfaces reduce the capacity and lifetime of the bearing. The special features of this friction conditions are examined and described by the tribology.

Design measures

A radial bearing is limited in the axial direction, it has the width B. The oil gap must be filled completely. The oil chamber is closed by means of sealing rings on both sides. Because oil is pressed out with considerable pressure on the edges of the lubricating wedge, the sealing washers must not be placed directly on the edge of the bush. The current flowing through the seals leak oil is returned in large warehouses with a pressure feed pump. The power is in the unpressurized portion of the annular gap (center left or above it ).

Speed ​​ranges

In the hydrodynamic sliding bearing, the friction coefficient is a function of the rotational speed ( relative speed between the sliding surfaces ), which is (as shown) illustrated by means of the Stribeck curve. There are areas with different types of friction:

After overcoming the static friction sliding friction with the shaft begins. The sliding friction is first mixed friction, which decreases until the speed is large enough for fluid friction. On increasing the speed of the coefficient of friction (increased flow resistance in the lubricant film ) increases again. The rated speed is chosen for safety above the reversal point. When excessive speed in the radial bearing the eccentric position of the shaft for unique lubricating gap geometry is too small. A gap is successively briefly at any point on the circumference materialize, leading to oscillations in the camp and its destruction. High speed radial bearings can stabilize itself by a jack with two ( " Lemon Game Camp", see figure) or more arcuate wedge faces will be painted instead of a round socket.

Sommerfeld number

The theory of friction in flows derived mainly by Osborne Reynolds. Arnold Sommerfeld she turned on the processes in the hydrodynamic plain bearings. From him comes the Sommerfeld number, a dimensionless measure of the relationship between load, geometry, speed and viscosity of the oil used. It is defined as follows:

Where:

• : On the projected bearing surface ( diameter x width ) related bearing force
• : The relative skill ( difference in diameter / diameter)
• : Is the dynamic viscosity of the lubricant at temperature in the lubricating gap
• : Angular velocity of the shaft

It is used for example as a parameter in the Gumbel curve ( see Figure; named after Ludwig Gumbel ), in which the relative position of the shaft center is shown for female middle.

The following figures apply for corresponding applications:

Applications

Hydrodynamic plain bearings are characterized by

• Wear-free continuous operation (with hydrostatic support of starting and stopping )
• Suitability for high bearing loads and speeds (for the bearings are no longer applicable )
• Suitability for very large bearing diameter
• Easy installation with a split bearing shells
• Absorbing oil film ( silent operation )

Disadvantage is caused by the addition, for example, compared to bearings created oil system overhead in production and maintenance (eg oil change).

The advantages make it an ideal storage type in wholesale and heavy engineering, wherever heavy and thick waves are to be stored. An example is the storage of long waves of a turbine-generator machine group in electric power plants. Such is not stopped for months, so the times of start and stop setting (if any wear, but mostly hydrostatically supported) are negligible.

Internal combustion engines usually have a hydrodynamic sliding bearing for the crankshaft. When the motor is supportive of the added pressure of the oil pump. For large motors (eg marine engines ), the oil pump system is anticipated to start, so that the lubrication is improved even at the start. At start-up of motor vehicle engines is initially only available mixed friction. Your camp must make do with the use from previous volume still adhering oil.

Hydrodynamic bearings are eg also with advantage for very rapidly rotating rotor used in turbomachinery and turbochargers. The spindle motor of hard disk drives, they are both as hydrodynamic as well as aerodynamic friction bearings ( air bearings, " lubricant " Air is ) used. The read heads float in the aerodynamic friction bearing on a cushion of air over the hard drives.

• Storage (technology)