3 Lobe Bearings

Introduction

3 lobe bearings are hydrodynamic bearings whose characteristic high level of performance in high speed rotating equipment has been made possible by the optimisation of manufacturing quality. Precision processing spindles equipped with these bearings are characterised by their accurate and vibration-free operation, minimum operating bearing clearance, and high stiffness and damping coefficients. Such spindles are capable of producing a part with an accuracy of 0.001 mm. The characteristic of the multilobe bearing, is the noncylindrical bearing bore. This deviates from conventional, cylindrical, hydrodynamic bearings by having two or more lobes.

Distributed across the entire shaft diameter, there are as many individual hydrodynamic carrying forces directed at the center of the shaft as there are lobes. The vector total of all the individual carrying forces represents the effective load capacity of the MGF bearing towards the outside. This results in a strong centering effect being applied to the shaft which produces good concentricity and generates a defined shaft position. By matching the lubricant viscosity to the shaft´speripheral speed and the wedge gap shape, the degree of the hydrodynamic carrying force and the bearing friction can be varied to meet individual requirements.

Advantages of Three lobe bearings

■ Virtually no metal to metal contact between shaft and bearing while the machine is operating.

■ Dampened, low-oscillation, noise and wear-free shaft operation. If the oil supply is operating properly, virtually unlimited bearing service life.

■ Several supportive lubrication films distributed around the shaft circumference guarantee that the shaft is generally centered, thus significantly improving concentricity.

■ Permits high continuous loading.

■ Shock loads of several times the level of the continuous load are acceptable.

■ Low friction losses.

■ Good lubricant flushing and cooling effects.

■ Structural adaptability to every existing machine construction.

3 lobe bearings are being successfully used in:

Turbochargers

Boiler feed pumps

Large electric motors

Refrigeration turbines

Water turbines

Centrifugal test rigs

Turbine test rigs

Noise test rigs

Precision drills and lathes

Transfer machines

Precision borers

Grinders

Bearing Assembly and Lubrication

To ensure proper oil supply to the MGF bearing, it must be remembered that the lubricant not only serves as the medium to create the required hydrodynamic force in the wedge gaps, but must also ensure that the heat generated is removed from the bearing zone. The oil supply can take the form of immersion, oil spray, and external lubrication. Before assembly, all contaminants must be removed from the bearing, either by cleaning it out with compressed air or immersing it in gasoline. The bearing should be secured by a pin, key, or by clamping to prevent rotation. Before installing the shaft, the bearing lobes and the bearing points on the shaft should be lubricated with the oil that will be subsequently used.

Bearing Geometry

Up to a bore diameter of 20 mm, journal bearings designed to operate in bidirectional rotation are equipped with 3 lobes. For bore diameters above 20 mm, the bearings are equipped with 4 lobes. The ratio between the lobe width and the bore diameter is 1:2, 3:4, or 1:1.

Bearing Materials Used for 3 lobe Bearings

Since there is no metallic contact between the shaft and the bearing where hydrodynamic lubrication is used, the bearing material is of secondary importance. However, the bearing and the shaft are in contact during starting and stopping phases. For this reason, the bearing uses sliding surfaces with the best possible emergency running properties. The materials used in bearing are C15/Tegostar738 (white metal alloy), CuSn7Pb7Zn3 (composition of copper, tin, lead, zinc)

Lubrication GroovesIf necessary, the calculated oil flow through the bearing can be increased by opening the sides of the lubricant grooves while maintaining a specified feed pressure. As long as all lubricant grooves are completely filled with oil during operation, and a minimum volume of oil flows through the bearing, correct operation of the hydrodynamic lubrication is guaranteed. These conditions are met if, when the lubricant grooves are opened, an adequate amount of oil can still be retained and prevent negative pressure in the bearing. The oil flow at the thrust bearing is throttled by an annular ring groove, and can gently run out via a short ring land. With this installation method, the thrust lubrication grooves are fully or partly open.

Lubricants (Cooling and Cleanliness)During operation, bearing system looses a certain part of its efficiency which is converted to thermal energy. The heat generated is carried off by the lubricant and returns to the oil reservoir. This constant addition of heat increases the temperature in the entire lubrication system, including the bearings, to a point where the heat dissipated into the environment (the air, the floor, coolant) equals the heat generated by friction. If the bearing operating temperature is to be kept as low as possible, provisions must be made for adequate oil cooling in the reservoir. It may be necessary to use oil/water or oil/air heat exchangers. The cleanliness of the oil is very

important because, at hydrodynamic oil film thicknesses of 0.001 to 0.01 mm, even microscopic particles can cause wear as they pass through the lubricant gaps. Experience has shown that this occurs most often when the machine is first put into service. Despite thoroughly cleaning all machine components, oil reservoirs, and tubing during assembly, tiny particles often remain. These are only dislodged by the oil flow during operation, causing the oil to become contaminated. For this reason, it is necessary to carefully filter the lubricant at the inlet to the bearing.

Effect of Load Orientation on the Stability of a Three-Lobe Bearing

Multi lobe bearings are found to be more stable than circular bearings. A three lobe bearing also possesses good stability characteristics. Sometimes the line of action of the load does not passthrough the axis of a bearing and is shifted on either side by a few degrees. Load orientation is one of the factors that affect the stability of a three lobe bearing. The effect of load orientation on the stability of a three-lobe has been discussed in this paper. The results show that stability of a three-lobe bearing supporting either rigid or flexible rotor is increased for the positive values of load orientation when the load line is shifted in the opposite direction of rotation. The analysis of multi-lobe bearings was first published by Pinkus .It was followed by Lund and Thomson and Malik, who gave some design data which included both static and dynamic characteristics for laminar, as well as turbulent flow regimes. The experimental stability analysis of such types of bearings showed that the analytical stability analysis reflects the general trends in experimental data. The factors affecting the bearing stability are load, L/D ratio, viscosity of the fluid, clearance between the journal and the bearing, type of fluid (compressible or incompressible), rotor unbalance, flow regime (laminar or turbulent), , load orientation, friction, shaft and liner flexibility, groove angle etc. Load orientation is one of the parameters that affects the stability of a bearing. The present study is undertaken to investigate the effect of load orientation on the performance of a three-lobe bearing supporting rigid and flexible rotors.