Operating principles and reliability of hydrodynamic bearings

The reliability of rotating machines

Since the invention of the wheel, and even, according to some experts, the potter’s wheel that preceded it the reliability of the machines has been a prime concern for operators.

The reliability of rotating machines is a major challenge for the industry. A process machine provides the expected production and therefore generates invoicing and customer satisfaction.

These positive results come from the adequacy of the machine, correct installation, proper operation, monitoring and maintenance. The majority of those factors put plenty of pressure on the maintenance managers of industrial sites.

One of the major components that provide proper functionality is the journal bearing. The component’s origins date back o few millennia before Christian era when the potter’s wheel and wheel became popular. It was necessary to control the friction and wear of an axle in its housing. The choice of materials, their sizing, the addition of a lubricant, are all problems that have appeared and accompanied the development of mechanics.

The journal bearing, which is a constant and essential component in high-power industrial machines.

Although it is the ancestor of bearings, the journal bearing is the most advanced mechanical component since the industrial revolution, thanks to the material used and the calculation capabilities made possible by computer technology. More precisely, this family is divided into two branches. The first is the friction bearing where only the friction coefficient controls the efficiency. The second is the hydrodynamic bearing. It is here that we enter an extraordinary, unknown and mysterious world.

The hydrodynamic bearing

The hydrodynamic bearing, as its name does not indicate, is a pump. A pump which, provided that a viscous fluid is made available to it, will create a pressure field between the shaft and its housing. It is the function of the hydrodynamic bearing that avoids solid-solid contact, limits heating, prohibits seizure, causes the coefficient of friction to drop, tolerates contaminants, dissipates vibration, and carries the load. Additionally, it is self-regulating. The power demand increases, the temperature increases, the viscosity decreases, the flow rate increases, the calories are evacuated in proportion and a new balance is established. All of this occurs without human intervention or control.

Hydrodynamic bearing technology is not widely taught and remains the domain of a few rate experts. On-site in the middle of the machines, it is rarely talked about and easily forgotten. Under ideal conditions, it does not wear out and does not require any special maintenance, aside from lubricant filtration and regular draining. In the best case, we follow its temperature, the position of the shaft and its accelerations, but as soon as we leave the instructions, it is urgent stop everything.

All too often, a machine that has always run smoothly will one day burn a bearing. The damage is read on the spare part, the journal bearing in the bearing. The first reflex is to change it thinking the bearing has served its time.

But a pad rarely has an intrinsic reason to fail. There is almost always an extrinsic motive. If it is not treated with the belief that replacing the bearing alone will be sufficient, then there is a high probability the reliability of the machine will deteriorate and the bearing will be grilled again under the same conditions. When that happens, you have to unravel the mystery of its failure by checking all aspects of the machine.

Despite its discretion, the hydrodynamic bearing experiences all the power of the machine. Its successive improvements have led to the interposition of an intermediate part: the journal bearing (or the bush). Its a critical piece, which, in the event of a problem, will sacrifice itself to preserve the noble organ that is the shaft. In nominal operation, its lifetime is infinite, there i no solid/solid contact but lamination of layers of molecules between them in the hydrodynamic film.

Moreover, if the bearing is guaranteed lubrication (the « gavage »), whether by splash, floating ring, gravity or pressure supply, the bearing itself pumps the fluid its needs to operate. A steady-state regime is therefore established in which the pressure is balanced with the loads at a given temperature and regulated by a lubricant flow rate and the thermal conductivity of the bearing. In this ideal state, our bearing – which allows energy, pressure and linear velocity levels to be reached that are beyond what would be possible with the best ball bearings – has excellent tolerance to external stresses. Its film of lubricant will behave like a shock absorber, with a stiffness and a damping coefficient very favorable to the attenuation of vibrations.

Any particle contamination in the lubricant can be prevented by embedding it in the surface layer of the bearing. Other particles, compatible with the thickness of the hydrodynamic film, will simply be transported to the next filter.

A caloric input will decrease the viscosity, increase the flow rate and regulate the temperature at the bearing. The clearances inherent in the mechanism will facilitate tolerance to misalignment. The accommodating power of the anti-friction material might, in some cases, help position the shaft and optimize its load-bearing capacity.

The absence of metallurgical compatibility between the metals present will allow « Touchette » without leading to seizure and its terrible consequences.

This is an idyllic picture that accurately describes what has been point on silently in most machine bearing in the industry for centuries. However, the person in charge of rotating machines at such a site is not so serene. This description goes hand in hand with the hazards associated with the operation of rotating machines.

Although the hydrodynamic bearing is generally auto stable, there are several situations that lead to its failure.

Hydrodynamic bearing technology

Its reliability, its tolerance to degraded operating modes and its safety in case of breakage make the hydrodynamic bearing an essential technology found in all high-power machines (compressors, turbines, motors, alternators, gearboxes, pumps, rolling mills, crushers and so forth). Still, sometimes it fails.

Only three conditions govern the establishment of a hydrodynamic regime. It requires a viscous fluid at disposal, surface convergence and relative motion. It is the hydroplaning of the tire on the puddle or the skier’s spatula on the water. These are examples of hydrodynamic regimes, which carry the load with little friction. But behind this simplicity are a multitude of a parameters that guarantee correct operation or, on the contrary, can lead to breakdowns.

A hydrodynamic bearing is dimensioned and designed for power; symbolized by the PV factor when P represent the load (more precisely the pressure) and V the relative linear speed. Under these conditions, the operating temperature, the position of the shaft in its housing, the fluid flow rate, the stiffness, and the damping of the lubricant cushion are determined and are compatible with proper operation.

These nominal operating conditions are not always met. Let’s start with the viscous fluid. It must be available in sufficient quantities in the desired viscosity range; not contaminated lubricant is a major cause of loss of the machine’s reliability.

Next comes surface convergence. In a radial bearing, it results from the difference in diameter of the shaft and its housing. It can also be improved by specific lobes and/or bore geometries, such as lemon bore or offset bore. These geometries are guaranteed by the bearing manufacturers with levels of precision that are among the finest in industrial mechanics. The surface convergence, the clearance and the thickness of the oil film can be measured in hundredths of a millimeter. They are the result of a dimensional chain integrating at least three components, which are the body, the shaft and the bearing. The stacking of these dimensions are geometries, with their respective tolerances, will result in good surface convergence and operating clearances. Components taken one by one must remain in conformity under clamping, after assembly and in operation under kinematic effects, thermal expansion, elastic deformation and misalignment.

Finally, the relative speeds is dictated by how the machine is driven. Stops, starts or reversals are critical phases, which cause the speed to drop to zero and deprive the bearing of its hydrodynamic speed with its oil cushion interposed between the solids. Special attention must be paid to temperature changes, cold starts, after a long stop or after disassembly/reassembly. These characteristics specific to hydrodynamic bearings mean that disassembly and reassembly cannot be treated as a standard operation where it would be sufficient to take new parts and reassemble the assembly for it to work. Numerous checks are necessary and are carried out by specialists trained and experienced in this technology.

Dismantling

This phase is fundamental to the expertise of the bearing and must be entrusted to specialists. The parts must ne marked and any stacks of shims correctly noted. Specialists will look for traces of molecular attrition, overheating, pollution or degradation of the lubricant, electric arcs, fatigue, cavitation, buttering, shocks, scratches, and wear. This way includes incorrect assembly, incorrect tightening or contact, foreign bodies, excessive loads, cold starts or reversal of direction, poor lubrication, or oil film breakage. The clearances will be measured and compared to the nominal ones.

Reassembly

Specialists will also ensure that the bearing has been crafted by a professional able to guarantee the quality of the metals used and their suitability – from the shell to the electrolytic coating, including the anti-friction alloy (white metal or red metal).

Dimensional and geometrical qualities must be guaranteed. The same applies to the housing, which also must be evacuated. Specialists will check and adjust the lift, the clearances and the lineage. Tightness and the absence of particles that could interfere with the load-bearing capacity will be guaranteed. Lubricant supplies should be checked, as well as pinning and direction of rotation in the case of ramps or complex geometries (multi-center bores) or variable geometries. The quality of the lubricant will be controlled.

Starting the machine

This is a critical phase that reveals quickly whether an error has been made during the previous stages. A bearing instrumented with temperature sensors will check that the temperature stabilizes asymptotically at its nominal level. Lubricant pressure and /or flow rate are also good indicators. Noise and especially vibration measurements can reveal problems with alignment or balancing or moving parts. Depending on the machines, a starting protocol will be used scrupulously followed (lubricant temperature setting, hydrostatic lift, progressive power loading). This means that in this field, more than any other, prior training and more than one evaluation is anything but superfluous and quickly pays for itself.