Elimination of vibration instabilities in hydrodynamic bearings

Tilting pad radial bearings provide stability at any load angle. Lemon bore (elliptical) or offset sleeve (to achieve an elliptical arrangement) bearings do not eliminate vibration instabilities if the load angle lies in the major axis of the ellipse, since the oil film stiffness in this region may not be sufficient to prevent vibration instabilities.

If bearing instabilities are experienced, a possible modification is to rotate the major axis of the ellipse to provide sufficient oil film stiffness at the load angle. This procedure takes time and must be repeated in the field each time the machine is disassembled. If the above procedure is not successful, installation of multi-lobe or tilting pad bearings will be required. This procedure will be time consuming and can delay tests for months.

Hydrodynamic bearings support the rotor using a liquid wedge formed by the motion of the shaft. Oil enters the bearing at supply pressure values of typically 103-128 kPa (15-20 psig). The shaft acts like a pump which increases the support pressure to form a wedge. The pressure of the support liquid (usually mineral oil) is determined by the area of the bearing. The maximum desired design wedge pressure for oil is approximately 3,450 kPa (500 psi). However, it has been common practice to limit hydrodynamic bearing loads to approximately 1,725kPa (250 psi) in compressor applications.

In a simple hydrodynamic bearing, the primary force is the load, which acts in the vertical direction for horizontal bearings. However, the fluid tangential force can become large at high shaft speeds. The bearing load vector then is the resultant of the load force and fluid tangential force. The fluid radial force opposes the load vector and thus supports the shaft.

It has been demonstrated that the average velocity of the oil flow is approximately 47-52 percent of the shaft velocity. The fluid tangential force is proportional to the journal oil flow velocity. If the fluid tangential force exceeds the load force, the shaft will become unstable and will be moved around the bearing shell. This phenomenon is known as oil whirl.

Types of bearings

Regardless of the type of hydrodynamic bearing, all its surfaces are lined with a soft, surface material made of a composition of tin and lead. This material is known as Babbitt. Its melting temperature is above 204 degrees Celsius (400 degrees Fahrenheit), but under load will begin to deform at approximately 160 degrees Celsius (320 degrees Fahrenheit). Typical thickness of a Babbitt over steel is 1.5 mm. Bearing-embedded temperature probes are a most effective means of measuring bearing load point temperature and are inserted just below the Babbitt surface. RTDs or thermocouples can be used.

There are many modifications available to increase the load effectiveness of hydrodynamic bearings. Among the methods available are:

• Copper backed Babbitt or ‘Trimetal’ – to aid in heat removal
• Back pad cooling – used on tilt pad bearings to remove heat
• Direct cooling – directing cool oil to maximum load points

Straight sleeve bearings are used for low shaft speeds (less than 5,000 rpm) or for older turbo-compressor designs. Frequently, they are modified to incorporate a pressure dam, in the direction of rotation. The pressure dam must be positioned in the top half of the bearing to increase load vector. This ensures that the tangential force vector will be small relative to the load vector, thus preventing shaft instability.

It should be noted that incorrectly assembling the pressure dam in the lower half of the bearing would render this type of bearing unstable. When shaft speed is high, other alternatives prevent rotor instabilities are:

• Elliptical or lemon bore bearing
• Offset half bearing
• Three lobe bearing
• Four lobe bearing

The most common types of anti-whirl bearings are the three and four lobe design. Elliptical and offset bearing designs do prevent instabilities but tend to increase shaft vibration if the load vector passes through major axis of the bearing. These types of bearings may have to be rotated in the bearing brackets to prevent this occurrence.

The most common hydrodynamic bearing for higher speed applications is the tilt pad journal bearing which offers the advantage of increased contact area since the individual pads conform to the shaft orbit. In addition, this type is also a highly effective anti-whirl bearing since the spaces between the pads prevent oil whirl. Most end users specify tilt pad radial and thrust bearings for turbo-compressor applications.

Lemon bore or offset sleeve bearings have caused extended FAT periods necessary to modify bearing split line orientation or changes to three or four lobe or tilt pad bearings. To positively eliminate vibration instabilities, use tilting pad radial bearings and not ‘lemon or offset sleeve anti-whirl type bearings’ to positively eliminate vibration instabilities.

This best practice has been used since the early 1980s when offset bearings were required to be changed to multi-lobe bearings during the FAT. Delivery delay was three months. It should be noted that the vendor made the modified multi-lobe bearing standard on all subsequent turbines. Use of this best practice has resulted in trouble-free turbine operation and reliabilities 99.5 percent and higher.