It rotates with the shaft and seals against a face, rather than being fixed in a housing and sealing against the shaft surface.

A radial lip seal is pressed into a housing bore and stays still. Its lip, usually spring-loaded, bears radially on the rotating shaft, and it needs a precisely finished shaft surface and a machined housing seat to work.

A V-ring is stretched onto the shaft and grips it by its own elasticity, turning with it. Its conical lip presses axially against a stationary counterface. No housing bore is required, and no special shaft finish, because the shaft surface is not the sealing interface.

The consequences are that a V-ring can be retrofitted where there is no seal housing at all, that it accommodates far more misalignment and end float, and that it is fitted and replaced from outside without dismantling the assembly.

Because centrifugal force acts on the rotating lip and lifts it away from the counterface.

The V-ring spins with the shaft. As speed rises, the flexible conical lip is thrown outward, and above a certain surface speed it lifts clear of the stationary face it was pressing against.

That sounds like a failure and is in fact the design intent. At those speeds the seal is no longer relying on lip contact: the rotating ring acts as a slinger, flinging water and dirt outward and away by centrifugal action, and the narrow gap that remains is an effective barrier to splash and spray.

The benefit is that friction and wear fall to almost nothing at high speed, so the seal lasts far longer than a contacting seal in the same position, and it consumes no measurable power.

At low speed and standstill the lip returns to contact and seals statically - so the seal works across the whole speed range by two different mechanisms.

Contamination coming in, far more often than lubricant going out.

The V-ring's strength is excluding water, dust, splash, mud and process debris from a bearing arrangement. It handles the harsh outside environment and does it while tolerating misalignment and flooding that would quickly destroy a lip seal.

It is much less suited to retaining pressurised oil. The axial lip contact is light and becomes non-contact at speed, so it cannot hold pressure and will not reliably retain a low-viscosity lubricant.

The classic arrangement therefore uses two seals in series: a V-ring on the outside taking the environmental abuse, and a conventional lip seal behind it retaining the lubricant. The V-ring keeps the contamination away from the lip seal, and the lip seal's life is transformed as a result - which is usually a far cheaper solution than upgrading the primary seal.

On the shaft, outside the bearing arrangement, bearing against any reasonably flat, smooth stationary surface.

The counterface can be the machined face of a bearing housing, a flange, an end cover, or a simple stainless washer added for the purpose. It does not need the fine finish a lip seal's shaft surface requires, though it should be flat, clean and reasonably smooth, and it should be corrosion-resistant in wet environments - a rusting counterface will abrade the lip.

Axial position matters: the seal must be fitted so the lip is compressed by the correct amount against the face, and manufacturers specify that compression. Too little and it does not seal at rest; too much and friction and wear rise sharply at low speed.

The seal grips the shaft by elasticity alone, so on high-speed or vertical applications a clamping band is sometimes used to make sure it cannot creep along the shaft.

Nitrile as the general-purpose default, with a range of elastomers for temperature and chemical duty.

Nitrile covers most applications - water, dust, general industrial conditions, mineral oils and moderate temperatures - and is the usual choice.

EPDM suits hot water, steam and many chemicals but must not be used with petroleum products, which destroy it. Fluoroelastomers handle high temperatures and aggressive chemicals at greater cost. Silicone gives a wide temperature range and is used where flexibility at low temperature matters.

Because the ring is a single homogeneous piece of elastomer with no spring and no metal case, material selection is the whole specification - there is nothing else to change.

Check the temperature at the seal rather than the ambient. A seal sitting next to a hot bearing housing runs considerably warmer than the surrounding air, and elastomer life falls steeply with temperature.

Usually yes, and it is one of the main reasons to choose them.

Most V-ring profiles can be stretched over a shaft end, and some can be opened and re-joined, so the seal can be installed without removing pulleys, couplings or bearings. That makes them the practical answer for retrofitting protection to machinery already in service.

It also makes replacement quick - a worn V-ring is changed from outside during a short stop rather than requiring a strip-down.

The corollary is that they can also creep along the shaft if not correctly sized or if the shaft is very smooth and the speed high, which is why a clamping band is used in demanding positions.

When retrofitting, check there is a suitable stationary face for the lip to run on. If there is not, adding a simple stainless washer or a machined collar is generally straightforward and much less work than fitting a conventional seal.

Considerably more than a lip seal, which is why it suits machinery with real-world tolerances.

Because the lip is flexible and presses axially, it can follow a counterface that is not perfectly perpendicular to the shaft, and it accommodates shaft run-out by flexing rather than by losing contact. Angular misalignment that would cause a lip seal to wear a groove on one side is absorbed.

End float is handled the same way: the lip compresses and extends as the shaft moves axially, within a stated range, and continues to seal throughout.

That tolerance makes V-rings well suited to agricultural and construction machinery, fans and blowers, electric motors, pumps and conveyor pulleys - applications where the shaft moves, the housings are not precisely machined, and the environment is dirty.

Manufacturers publish the permitted misalignment and axial movement for each profile and size; they are generous but not unlimited.