1. The Oldest Bearing and the Quietest Machine
Long before the rolling element, the ball and the roller, there was the plain bearing: the journal that rides on a film, the shaft sliding in its bush, the oldest machine element and, in its quiet competence, still one of the most important in the designer’s kit. The plain bearing, the sliding contact bearing, carries the shaft on the surface of its bush, and its genius is that it need not touch the surface at all when it runs right, floating on the film of oil that the motion itself creates. The machine that uses plain bearings well runs with a smoothness and a damping that the rolling bearing cannot match, absorbs the shock, forgives the misalignment, and fits into the space that a ball bearing would need to be forced into.
This article is the foundation for sliding contact bearing design: the regimes of lubrication, the arithmetic of the journal bearing, the materials of the Bush, the selection between the plain and the rolling element, and the lubrication and the maintenance that let the film do its work.
2. How the Film Forms: The Regimes of Lubrication
2.1 Boundary, Mixed and Hydrodynamic: The Three Lives of the Bearing
The plain bearing lives its life in three regimes, and the boundary between them is the thickness of the oil film. In boundary lubrication the film is too thin to separate the surfaces completely, and the shaft and the bush touch through the lubrication additives, the extreme-pressure and the anti-wear packs that coat the metal and carry the load; the boundary regime is the world of the slow, the oscillating, the heavily loaded and the stopping-and-starting bearing, where the design must supply the protection by material and by additive rather than by film. In mixed lubrication, part of the load is carried by the film and part by the touching asperities, the regime of the variable-speed and the marginal-load machine, and the wear that it produces is the wearing-in that the designer budgets. In hydrodynamic lubrication the film is fully developed, thick enough to separate the surfaces completely, and the load is carried entirely by the pressure that the motion pumps into the wedge of oil; this is the goal regime of the high-speed, constantly-rotating journal bearing, the regime in which the bearing does not wear because it does not touch.
The design’s first question is therefore the duty: how long the bearing runs at what speed, how much of its life is start-stop versus continuous, and which regime each phase occupies. The bearing that spends its life in boundary lubrication cannot be designed as if it floated on a film, and the bearing that could run hydrodynamically but is starved or misloaded never reaches its regime; the designer names the regime first and then selects for it.
2.2 The Sommerfeld Number and the Eccentric Shaft
The hydrodynamic journal bearing is described by a dimensionless number, the Sommerfeld number, that gathers the operating conditions into one figure of merit. The number is computed from the speed, the viscosity, the load, the journal diameter and the clearance, and its value predicts the bearing’s most important behaviours: the eccentricity of the shaft within the bush, how far the shaft centre is displaced from the bush centre by the load, and the resulting minimum film thickness, the smallest gap between the shaft and the bush, the number that all the physical wisdom of the design protects. The larger the Sommerfeld number, the higher the speed or the viscosity or the lighter the load, the more centred the shaft and the thicker the film; the smaller the number, the heavier the effective load and the thinner the film, and the design’s job is to keep the minimum film thickness safely above the combined roughness of the two surfaces, so that the asperities never touch.
The eccentricity explains the bearing’s quiet paradox: the shaft in a hydrodynamic bearing does not run centred, it runs offset towards the loaded side, pumping oil through the wedge between itself and the bush, and the pressure of that wedge is what carries the load. An understanding of the eccentric shaft is the understanding of why the clearance matters, why the viscosity matters, and why the bearing that is perfectly aligned and generously fed runs its film without wear.
3. The Journal Bearing Design Process
3.1 The Load and the Pressure Check
The journal bearing design begins with the loads: the radial force the bearing must carry, applied through the shaft, and the projected area it has to carry it with, the product of the diameter and the bearing length. The design closes the arithmetic with the bearing pressure, the load divided by the projected area, and the check that this pressure falls within the limit the material can sustain, typically one and a half to ten megapascals for the common bearing metals at their rated speeds. The pressure number is the designer’s screen: too high a pressure for too low a speed pushes the bearing towards boundary lubrication, and too low a pressure, surprisingly, is its own risk, because the lightly loaded bearing can run with an unstable film that whips and hammers at speed.
The length-to-diameter ratio is the shape decision: the long bearing, the L/D of one and above, carries the load with a lower pressure, but it is sensitive to the misalignment and the deflection of the shaft, which concentrate the load at the edges; the short bearing, around half to one, tolerates the bend and the misalignment better but raises the pressure. The designer chooses the ratio by the shaft’s rigidity and the alignment the machine can guarantee, and the bearing whose proportions match the shaft’s true behaviour outlives the bearing whose proportions are copied.
3.2 Clearance and Viscosity: The Film’s Two Masters
The clearance, the radial gap between the journal and the bush, is the first master of the film. The clearance is set by the standard fit, the diametral clearance of one to two thousandths of the shaft diameter for the common industrial bearings, tight enough that the film is not starved and open enough that the shaft can float and the oil can enter; the clearance that is too tight seizes the bearing as the temperature closes the gap, and the clearance that is too wide lets the film collapse and the shaft hammer. The designer also accounts for the thermal expansion, the different materials of the shaft and the bush growing at different rates, because the bearing that is snug in the drawing office is tight in the running machine.
The viscosity is the second master, and the choice of the oil is the choice of the film’s thickness. The oil must be viscous enough that the film stays thick under the load and thick enough that it does not fail at the lowest operating speed, and fluid enough that it pumps into the wedge and does not drag the motor; the oil temperature is the mediator, because the viscosity falls rapidly as the oil heats, and the designer selects the ISO viscosity grade from the predicted running temperature, the speed and the load, budgeting the hot running condition that thins the film dangerously.
3.3 Material Selection: Tin, Lead, Bronze and the Polymers
The bearing material is chosen for its ability to live with the dirt and the vibration the film cannot fully control, and the classic bearing metals are a study in the trade-offs. The tin-based and the lead-based babbitts, the soft white metals, have the excellent embeddability and conformability that let the bearing swallow the hard particle and locally deform to the misaligned shaft, protecting the precious journal, but they are soft and fatigue-limited, and they need the strong steel or bronze backing to carry the load. The bronze and the copper-lead materials are harder and stronger, carrying the higher pressures and the better fatigue, but they yield less, and they need the smoother and the cleaner running conditions to protect them from the abrasive wear. The aluminium-tin, the modern compromise, combines the strength with a self-lubricating character, and the polymers, the PTFE and the acetal, run dry or lightly lubricated, the perfect choice for the oscillating and the occasional-duty applications where the oil supply is the nuisance.
The selection is made by listing the demands: the bearing pressure, the speed, the shaft hardness, the duty cycle, the potential for the misalignment and the contamination, and the cost of the maintenance. The soft bearing protects a soft or valuable shaft; the hard bearing carries a hard duty on a hardened shaft; and the polymer runs where the oil would wash away. The designer who matches the bearing material to the true operating environment, not to the catalogue’s default, is choosing the life of the machine element it protects.
4. Lubrication and the Practical Bearings
4.1 Getting the Oil In: The Feeding and the Grooves
A hydrodynamic bearing is only as good as its oil supply, and the feeding is the detail that decides whether the film forms. The simplest feeding is the drip or the ring, the oil dropped or carried by the rotating ring into the top of the bearing; the next is the pressure-fed circuit, the pump that delivers the oil through the drilled passages and the grooves to the loaded zone, the choice for the high-speed and the heavily loaded bearings that cannot trust the passive supply; and the groove, the channel cut into the surface to spread the oil, is cut with care, because a groove that crosses the loaded zone destroys the wedge pressure it should help create. The classic layout feeds the oil to the unloaded side and lets the shaft carry it into the wedge, and the groove is placed in the unloaded quadrant or arranged in the axial or the circumferential pattern that the duty needs.
The designers of the critical machinery also monitor the supply: the oil pressure, the flow and the temperature are the bearing’s vital signs, and the instrumentation that watches them is the early warning that catches the starving bearing before the metal seizes. The bearing that is fed by design, not by habit, forms its film every time it starts.
4.2 Hydrostatic and the Dry Running Cousins
Not every plain bearing can wait for the motion to build its film. The hydrostatic bearing pumps pressurised oil into the recesses of the bush from an external pump, floating the shaft on the oil pressure even when it is stationary, the design of the heavy rotating tables, the telescopes and the precision spindles that must move freely from the standstill and position without the stick-slip; the hydrostatic bearing gives the highest stiffness and the lowest friction at zero speed, at the price of the continuous pump and the clean oil. At the opposite end of the cost spectrum, the self-lubricating and the dry-running bearings, the porous sintered bronzes and the polymers, carry the modest loads with no oil system at all, the bearings of the door hinges, the household appliances and the maintenance-free mechanisms, and their design is the PV limit, the product of the pressure and the surface velocity that the material can sustain, the same arithmetic of the pressure and the speed that the hydrodynamic bearing handles with its film.
4.3 The Selection Decision Table
| Duty | Recommended bearing | Why it wins |
|---|---|---|
| High speed, continuous, heavy | Hydrodynamic, babbitt on steel | Full film, no wear, high capacity |
| Zero-speed precision positioning | Hydrostatic | Film before motion, high stiffness |
| Start-stop, oscillating, dirty | Bronze or polymer, groove-fed | Embeddability, tolerance of the debris |
| Maintenance-free mechanism | Sintered bronze, self-lubricating | No feed system, PV-limit design |
| Misaligned, shaft deflection | Short L/D, conformable material | Edges follow the shaft without loading |
Sliding contact bearing design is the discipline of the film. The duty is named, the regime is identified, the pressure and the clearance and the viscosity are computed, the material is matched to its environment, and the oil is delivered to the wedge with the respect the film deserves. The plain bearing that is designed this way is the quietest element in the machine, carrying the heavy shaft day after day on a film too thin to see and too strong to doubt, and the designer who masters it masters one of the oldest and most elegant arts of mechanical engineering.
