Grease life plots can help engineers decide when it's time to relubricate bearings. They also show why it's important — based on life — to keep greases cool.

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Grease both lubricates and keeps contaminants out of bearings. But its job is often made difficult by back-and-forth, or oscillatory, motion. Printing presses are a classic example: Frequent returns to home coupled with short advances can wreak havoc on lubricants as well as the seals that protect them.

Oscillatory motion also causes another problem: It concentrates and magnifies fatigue on bearing rolling elements and raceways. Under normal operation, stressed contact areas overlap along bearing surfaces. But oscillating motion isolates, rather than distributes, stress, allowing it to build up in localized areas. The cyclic contact accelerates not only bearing fatigue, but also seal wear, especially where shaft finishes are rough, marred, or otherwise compromised. It also tends to trap lubricants, which leads to shearing within local volumes of grease.

Even in these cases, however, long-term lubrication is possible. In fact, there are now systems that allow oscillating machines,even printing equipment, to run for years before relubrication.

How they do it

One solution that's been particularly successful is based on a centralized single-pipe lubrication system. Unlike progressive lubrication, single-pipe systems can be adapted in a modular fashion because they don't rely on a main distributor. More direct lube runs also mean that assembly is easier and most of the overall conduit lengths are shorter. Typical application areas include roller bearings and curve controls, primarily on the operator side.

With cartridges of lubricant optimized for operating conditions, bearing type, and other variables, the right grease can be continuously supplied to the lubrication point. A standard cartridge can deliver measured doses for up to five years. When the lubricant reservoir runs low, another cartridge can be easily swapped in.

Sometimes, fresh grease is pressed directly through relubrication lines to displace used grease, which bleeds out through specially designed repositories. The required amount of lubricant here depends on the geometry and design of the bearing units to be lubricated.

Self-contained lubrication systems can accommodate just about any number and arrangement of bearing points. They also simplify assembly and reduce maintenance work.

Laying the groundwork

To fully leverage the benefits of integrated oscillating roller bearing systems, it is necessary to safeguard long-term grease life.

Main cylinder bearings in most new printing machines are lubricated with oil or grease. This makes it possible to accurately predict lifetime (per application or loading case) for various solutions.

Depending on bearing dimensions and operating conditions (speed, load, temperature) original grease life can be estimated from standard charts. If relubrication is necessary, the L01 interval can be derived from diagrams, using the approximation L01 = L10/2.7, which is valid for selected greases in properly sealed and clean systems.