2RS vs ZZ: sealed, shielded, or open?
A 6204, a 6204-ZZ and a 6204-2RS are the same 20 × 47 × 14 mm bearing with three different closures. They all fit the same shaft and housing — the choice between them is not about size, it is about the trade the closure makes: more protection costs more friction. Getting it wrong means a bearing that either runs hot or lets grit in, even though the number is right.
The three closures
| Type | Code (typical) | Protection | Friction & speed |
|---|---|---|---|
| Open | (none) | None — relies on external sealing/lube | Lowest friction, highest speed |
| Shielded | ZZ, 2Z | Keeps out larger debris; retains grease | Low friction, high speed |
| Contact sealed | 2RS, 2RS1, DDU, LLU | Best — resists dust and moisture | Higher friction, lower speed |
Open bearings
No shield, no seal. An open bearing has the lowest friction and the highest speed capability, but it has no protection of its own — it depends on the machine to keep dirt out and lubricant in. That is exactly right in a sealed, oil-bath environment like a gearbox, where an internal seal would only add drag and block the oil the bearing is meant to run in. Out in the open it will not last.
Shields (ZZ)
A shield is a metal disc fixed to the outer ring with a small non-contact gap to the inner ring. Because nothing rubs, a shielded bearing keeps almost the low friction and high speed of an open one, while holding its grease in and keeping larger particles out. What it will not do is stop fine dust or water — the gap is a barrier, not a seal. ZZ is the sensible default for clean-ish indoor machinery running at speed: electric motors, fans, general rotating equipment.
Contact seals (2RS)
A contact seal is a rubber lip that presses against the inner ring, closing the gap completely. That gives the best ingress protection — dust, splash, damp — and makes the bearing effectively greased for life. The cost is friction: the lip rubbing generates drag and a little heat, which lowers the speed limit and raises running temperature compared with a shield. 2RS is the right choice for dirty, wet or outdoor duty — pumps, agricultural and construction equipment, anything washed down — where keeping contamination out matters more than the last few percent of speed or efficiency.
How to choose
- Environment. Clean oil bath → open. Clean-ish and dry → shielded. Dirty, wet or washed down → contact sealed.
- Speed. Near the bearing's speed limit, a contact seal's drag matters — prefer a shield or open bearing, or a low-friction seal.
- Lubrication. Sealed and shielded bearings are packed and closed at the factory; you cannot relubricate a 2RS. If the machine has a grease line and a relubrication schedule, that points to an open bearing.
- Efficiency and heat. Seal drag is lost energy and added temperature. In high-speed or energy-sensitive designs it is not trivial.
One side, not both
You do not have to choose the same closure on both faces. A single shield or seal is coded with the singular suffix (Z or RS instead of ZZ or 2RS) — useful when one side faces the dirty world and the other faces a clean, oil-fed housing that needs to breathe or be relubricated.
The closure does not change the fit
Open, ZZ and 2RS versions of the same bearing share identical boundary dimensions, so all three drop into the same seat — see the 6204 cross-reference. What changes is the letters, and every brand writes them differently: SKF 2RS1, NSK DDU, FAG 2RSR, NTN LLU all mean "two contact seals". Decoding that map across brands is the job of the seal & shield suffix guide; this guide is about which closure you actually want in the first place.
General engineering reference only. Speed limits, temperature ranges and seal friction figures are specific to each bearing and seal design — confirm them in the manufacturer's catalogue for your application.