Because a lip seal has a finite life by design, and an isolator does not.

A lip seal works by rubbing on the shaft. That contact wears the lip and, over time, wears a groove in the shaft. As the lip hardens with heat and age, its sealing force falls. Eventually it stops excluding contamination, and once water or dust reaches the lubricant the bearing's life collapses.

Because contamination is the leading cause of premature bearing failure, the seal frequently determines when the bearing fails.

An isolator has a running clearance instead of contact. Nothing rubs, so there is no wear mechanism, no heat, no power loss and no shaft grooving. It continues to work indefinitely.

The trade-offs are a higher purchase price, a larger axial space requirement, and the fact that a clearance seal cannot retain pressure - so it is a protection device rather than a pressure seal.

By making the route in long and convoluted, and the route out short and assisted.

The rotor and stator interlock so the clearance between them is not a straight gap but a path that changes direction repeatedly. Liquid or particulate trying to travel inward must negotiate each of those turns, and at each one inertia and surface tension work against it.

Meanwhile the design provides an easy escape. Drain passages at the bottom let anything that penetrates fall back out under gravity, and expulsion ports around the rotor throw material outward by centrifugal action whenever the shaft turns.

So the seal does not rely on being tight; it relies on being asymmetric. Contamination that enters is continuously being returned to atmosphere rather than accumulating and working inward.

That is also why orientation matters: the drain must be at the bottom, and a seal fitted rotated will not drain.

The centrifugal expulsion stops working, which is why better designs add a positive closure for the stationary condition.

A purely clearance-based labyrinth relies partly on rotation to expel contamination and to maintain the air movement that resists ingress. Stopped, it is simply a gap - and that is exactly when a machine cools, draws moist air in as the housing contracts, and condenses water inside the bearing.

That cooling-and-breathing cycle is a major source of water contamination in equipment that runs intermittently, and it happens while everything looks perfectly dry.

Better isolators therefore include a shut-off element: an O-ring or a vapour block that is displaced outward by centrifugal force when running, opening the clearance, and returns to close the path when the shaft stops.

If the machine spends significant time stopped, particularly outdoors or in a washdown area, specify a design with positive standstill sealing rather than a plain labyrinth.

On equipment where seal failure keeps causing bearing failure - typically wet, dirty or washdown environments.

Process pumps are the classic case, and isolators are widely retrofitted to them. Electric motors, particularly outdoors or in wet areas, are another. Fans and blowers, gearboxes, conveyor pulleys, and food and beverage equipment subject to high-pressure washdown all benefit.

The economic case is straightforward where a bearing failure is expensive in downtime rather than in parts. An isolator costs more than a lip seal but eliminates a recurring failure mode, and on critical rotating equipment the payback is usually rapid.

They are less compelling on clean, dry, indoor equipment with no contamination problem, where a lip seal will run for years anyway - and on very small shafts or in very tight axial spaces, where an isolator may simply not fit.

Bronze, stainless steel and engineering polymers, with the choice driven by the environment and by any hazardous-area requirement.

Bronze is the traditional material and remains common for general industrial and pump duty. Stainless steel is used in washdown, food, chemical and marine environments where corrosion would otherwise seize the clearance or degrade the drain passages. Engineering polymers such as PTFE-based compounds and PEEK are used where chemical resistance or electrical isolation matters, and they are lighter and cheaper in some sizes.

Hazardous areas impose an additional requirement: the seal must not be able to generate a spark if the rotor contacts the stator, so non-sparking material combinations are specified and certified for the purpose.

Electric motor applications may also call for a design that provides a path to earth for shaft currents, or specifically avoids one, depending on whether the bearing is insulated - worth checking on inverter-driven motors.

Very often yes, and retrofit is one of the largest uses - but the axial space and the housing bore have to suit.

Many isolators are made to fit the standard lip seal envelope for common pump and motor frames, so they drop into the existing seal bore during a routine overhaul. Split designs exist that can be fitted without dismantling the shaft assembly, which suits large machines and in-situ work.

What to check before ordering is the housing bore diameter and depth, the shaft diameter, and whether there is a shoulder or feature the seal must clear. An isolator is generally axially longer than the lip seal it replaces, and if the housing was machined to the exact depth of a lip seal there may not be room.

Also confirm the shaft surface condition. A shaft already grooved by an old lip seal does not prevent an isolator working - one of its advantages - but the sealing element must be positioned clear of the groove.

Essentially none in service, but the drain must stay clear and the orientation must be right.

Because nothing contacts, there is no wear to monitor and no adjustment to make. The seal is generally expected to last the life of the equipment and is often reused through bearing changes.

The one thing that will defeat it is a blocked drain. The design depends on contamination that enters being able to leave, so if the drain passage packs with dust, fibre or product residue, material accumulates inside instead. In dirty process environments the drain is worth checking at routine inspections.

Orientation follows from that - the drain must be at the lowest point, so a seal must be installed the right way up. It is an easy thing to get wrong on a machine assembled in an unusual position.

Otherwise, the correct maintenance is to leave it alone and change the bearing on condition rather than on seal failure.