The Cylinder That Started Leaking
We used a cylinder (article 85) on a fast cycle (article 171). After 6 months, the rod started leaking (oil dripped from the rod seal). The problem: the rod seal (the seal around the rod) wore. The cylinder cycles 1000×/day (300,000 cycles in 6 months). The rod seal (a standard NBR) wore (the oil film broke, the seal dried). We either upsized the seal (a wiper + seal combo) or used a better seal material (PU, longer life). We switched to a PU seal (longer life). The leak stopped. The mistake was using a standard NBR seal on a high-cycle cylinder (the seal wore fast).
Cylinder rod seal life depends on the cycle rate and the seal material. This article covers the selection.
The Rod Seal
The cylinder’s rod seal (an O-ring, or a U-cup) keeps the air (and the oil mist, article 119) in. The rod slides through the seal. Over cycles, the seal wears (the rod rubs the seal). The seal leaks (air/oil escapes).
Step 1: Seal Material
- NBR (nitrile): Standard. Cheap. Life: ~1 million cycles (general). For low-cycle (100/day), it lasts years.
- PU (polyurethane): Wear-resistant. Life: ~5–10 million cycles. For high-cycle (1000+/day). More expensive, but lasts.
- FKM (Viton): High temperature/chemical. For hot/oil environments. Expensive.
The seal life rule: For high-cycle (1000+/day), use a PU seal (5–10 million cycles). The cylinder that leaked had an NBR seal (1 million cycles) on a 300,000-cycle/6-month duty. Switch to PU. For low-cycle (100/day), NBR is fine.
Step 2: Lubrication (Article 119)
The cylinder needs lubrication (oil mist, article 119). A lubricated cylinder lasts (the oil film protects the seal). A non-lubricated cylinder (dry air) wears the seal fast. If the FRL (article 119) has a lubricator, set it (a few drops per minute). If the cylinder is “lubrication-free” (a special seal), it doesn’t need oil (but check the spec).
Step 3: Rod Condition (Article 145)
The rod’s surface (chrome) must be smooth (no scratches). A scratched rod cuts the seal (it leaks). Protect the rod (a wiper, a boot). For a dirty environment (chips, dust), add a wiper (a scraper) on the rod (it wipes the dirt off the rod). The wiper keeps the rod (and the seal) clean.
Step 4: Speed (Article 171)
A fast cylinder (article 171) wears the seal faster (the rod rubs at high speed). For a fast cylinder, use a better seal (PU) and more lubrication. For a slow cylinder (0.1 m/s), the seal lasts (NBR is fine).
| Cycle Rate | Seal Material |
|---|---|
| Low (100/day) | NBR (standard) |
| Medium (500/day) | NBR (or PU) |
| High (1000+/day) | PU (wear-resistant) |
| Hot/oil environment | FKM (Viton) |
A Seal Life Checklist
- What is the cycle rate? (Per day?)
- Is it high (1000+/day)? Use PU?
- Is the FRL lubricating? (Article 119?)
- Is the rod smooth? (No scratches?)
- Is there a wiper? (Dirty environment?)
- Is the cylinder fast? (Article 171?)
- Does the rod leak? (Oil?)
- Are spare seals in stock? (Article 100?)
- Is the seal replaced? (Scheduled?)
- Is the cylinder life estimated? (Cycles?)
The Bottom Line
Cylinder rod seal life matches the seal to the cycle rate. The cylinder that leaked had an NBR seal (1M cycles) on a high-cycle duty. Use PU (5–10M) for high-cycle. Lubricate (article 119). Add a wiper (dirty). The cylinder that didn’t leak wasn’t the cheapest one — it had a PU seal.
