Seals vs Shields: 2RS, ZZ, LLU and DDU

Every sealed bearing carries a two or three letter suffix, and it is the part of the designation most buyers never decode. A 6205-2RS, a 6205-2RSH, a 6205-2Z and a 6205-ZZ sit on the same shelf and behave in completely different ways: one carries a metal shield that never touches the inner ring, one carries a rubber lip that drags on a polished land, and the difference appears as friction torque and a reduced speed ceiling. The dimensional envelope is fixed by ISO 15; the closure is not, and the closure is what decides how long the bearing survives in a dirty or wet machine.

This guide decodes the closure suffixes used by SKF, FAG, NSK, NTN and INA, explains what each one does to friction, limiting speed, grease retention and ingress protection, and gives a selection sequence you can apply to a real position. The vocabulary is fixed by ISO 5593 and the boundary plans for radial bearings by DIN 625-1, so the terms used here are the terms you will meet in any catalogue.

What the suffix letters actually mean

A closure suffix describes two things at once: whether the closure touches the rotating ring, and what it is made from. Almost every cross-reference error comes from mixing those two questions.

The four families of closure

Every suffix you will meet falls into one of four families.

• Metal shield, non-contact (Z, ZZ, 2Z, Z2). A stamped steel washer with a small clearance to the inner ring. Low friction, wide temperature range, poor against fine dust and no protection against liquid.
• Rubber seal, contact (RS, RS1, 2RS, 2RSH, 2RSR, LLU, DDU). A synthetic rubber lip that rubs on a ground land on the inner ring. Best sealing, higher friction, lower speed ceiling.
• Rubber seal, low-friction or non-contact (RZ, 2RZ, LLB, VV, EE). A rubberised washer whose lip stands off the land. Between a shield and a full contact seal in both sealing and friction.
• Special (NR, N and combined designs). NR adds a snap ring for axial location in the housing; it says nothing about sealing and is often combined with a seal elsewhere in the number.

Contact seal versus non-contact seal

The single decision that matters most is contact or non-contact. A contact lip must be pressed against the inner ring, so it generates a running friction torque and heats the seal edge; that is why the limiting speed drops. A non-contact closure relies on a narrow gap, so it seals only as well as the gap is small, and the gap must be smaller than the particle you actually have to exclude.

Suffix to meaning: brand by brand

The same physical design is sold under different letters by different makers. The table below maps the common conventions so a buyer can decode any catalogue without guessing.

A 2RS from one factory and a 2RSH from another are both contact sealed, but the rubber hardness, the lip angle and the pre-load of the lip can differ enough to change measured friction torque by a noticeable margin on the same size. Treat the letters as a family, not as a specification, and confirm the construction in the glossary or on the manufacturer drawing.

Friction, speed and the ingress consequence

Closure choice is a trade between sealing and speed, and both sides of that trade are quantitative.

Why ZZ runs at full catalogue speed

A shield touches nothing, so the bearing limiting speed is essentially the open value. A contact seal adds drag at the lip, so the manufacturer publishes a reduced limiting speed. The reduction follows the seal type and the size; a workable planning figure is that a contact-sealed deep groove ball bearing runs at about 60-70% of the open limiting speed, and a low-friction seal somewhere between.

Ingress protection follows the same ranking. No bearing closure is formally IP-rated, but the practical behaviour maps closely: a shield behaves like a coarse mesh, a low-friction seal like a light gasket, and a contact seal like a lip seal on a rotating shaft. Only the last one is dependable under a directed water jet. If speed and contamination both matter, separate the two jobs - run an open or shielded bearing at full speed and seal the position externally.

Grease life versus seal choice

A seal retains grease as well as excluding dirt, and those are two halves of the same function. Grease loss through an open bearing degrades life long before contamination does, which is why a sealed bearing often outlives an open one on a low-speed duty even though its speed rating is lower. Where the duty is slow enough that the speed penalty is irrelevant, the re-lubrication interval becomes the governing factor and a sealed unit removes the task entirely.

Labyrinth, felt, V-ring and duo seals for housed bearings

Housed and split units use closures that a ball bearing suffix does not describe, because the seal runs on a shaft rather than on an inner ring. These are common on ball bearing units and pillow blocks.

Felt, V-ring and duo seals

Each addresses a different environment.

• Felt seals are cheap, run dry and suit clean, low-speed duties such as conveyor rolls; they fail quickly against water or abrasive dust.
• V-ring seals are axial lip seals that fling water and dust off a rotating shaft and tolerate a few millimetres of misalignment.
• Duo or double-lip seals combine a lip and a flinger and are the normal choice for washdown and marine positions.

Labyrinth seals sit alongside these: they use a long, non-contacting path and run at full speed, but they let fine dust through if the gaps open with wear. Combined designs pair a labyrinth with a lip so the labyrinth takes the speed and the lip takes the liquid.

The practical selection for a dirty, wet mounted unit is covered in the UCP and UCF guide, and the broader contamination strategy is set out in the harsh environment bearings reference.

The -2Z ambiguity and how to confirm with a drawing

The suffix -2Z usually means two metal shields, one each side. On some older or generic designations, however, Z can denote a single low-friction seal rather than a shield, and 2Z can appear on a bearing that is shielded on one side and open on the other. The letters alone are not proof. Before ordering, confirm the closure against the dimension drawing or the seal cross-section: a shield is a flat metal washer seated in a groove, a contact seal has a visible lip and a rubber body, and a non-contact seal sits between them. The basic designation rules are set out in the designation system guide.

Sealing choice by environment

Work from the environment, not from habit. The table below is the mapping used most often at the quotation desk.

Where two requirements conflict, speed usually wins the closure decision and a separate external seal handles the contamination. That is why high-speed fans often use shielded bearings inside a machined labyrinth rather than a contact-sealed bearing. Corrosion adds a third variable - see the corrosion protection notes before specifying a standard carbon-steel sealed part for a wet duty.

How to Select a Closure Suffix

Work through these steps before writing the part number.

1. Establish the dn value. Multiply bore in millimetres by speed in rpm. Compare it with the limiting speed rating for the closure you are considering, not for the open bearing.
2. Classify the contaminant. Decide whether the main threat is dry dust, liquid, or both. A shield handles the first, a contact seal is needed for the second.
3. Check the temperature. Standard nitrile lips are limited to roughly 100 degC and steel shields run higher, so a hot position may force a shield or a special compound.
4. Decide the lubrication route. If there is no way to re-grease, choose a sealed-for-life bearing and accept the lower speed.
5. Confirm the suffix against a drawing when cross-referencing between brands, since the same letters do not guarantee the same design, and verify the shoulder and chamfer so the seal lip runs on a clean land and is not damaged at mounting.

Common mistakes

• Assuming all 2RS seals have the same compound. Nitrile and fluorinated rubber have different temperature limits and chemical resistance; 2RS is not a material specification.
• Assuming all 2RS seals have the same friction. Lip pre-load and geometry differ, so the speed limit differs even when the letters match.
• Running a contact seal at catalogue speed. The open bearing rating does not apply; using it overheats the lip and cooks the grease.
• Forgetting the external seal. Where contamination is severe the bearing closure is the second line of defence, not the first.

Our deep groove ball bearing range is listed with the closure variant on every record, and the friction and efficiency consequences are worked through in the friction torque guide. For sealed equipment that must wash down, the food and beverage notes cover the hygiene side, and brand-level examples can be compared on any SKF listing in the directory.