Lubrication of Roller Chains
Roller chains are a broad classification of chains used for power transmission purposes. They all have in common a sleeve roller that rotates around the pin joints or axles of the chain; this roller reduces friction and wear between the chain and a sprocket.
Of these, drive chains transmit torque between two sprockets; an example of this is the chain on a bicycle or motorcycle. Drag chains are designed to pull a load; an example would be the chain used to pull a roller coaster to the top of the hill. Timing chains accurately and smoothly transmit rotational position.
All of these transmit power through chain tension. The Rigid Chain is a unique product that, unlike the above, can also push. SERAPID Rigid Chain becomes rigid when straight and can then be used, open-ended, to push or lift a load.
Corrosion, abrasion, and the right materials
Because of their application and geometry, lubricating these chains has some specific requirements and difficulties. The operating conditions may present excess wear opportunities: the chains may be outdoors, in direct contact with rain and dirt, and infrequently cleaned or lubricated. The chain rollers might be rolling over sprockets that are dirty or gritty, or the chain plates that slide past each other and pivot on the chain axle may have contamination between the plates causing abrasive wear. If the rollers are buried in the chain, it can be difficult to get lubrication between the roller and the axle. And, of course, any oxidation will add to corrosive and abrasive wear. Only the automotive timing chain operates in a condition where lubrication is well controlled, clean, and continuously supplied.
Ideally, the chain material should have high tensile (yield) strength and a hard surface to minimize friction and wear. It should also be corrosion resistant. Unfortunately, many materials with good corrosion resistance are not compatible with the high strength and hardness requirements., to get past this, some components can be hardened (rollers and axles), and surface treatments can be used to increase corrosion resistance and lubricity. There may even be sacrificial plating on the chain plates that create a galvanic pair to protect the unplated components.
All of this requires that we can get lubrication where it is needed. Because of the chain's speed and load, it typically operates in a boundary lubrication regime where there is not enough lubrication or speed present to start a hydrodynamic layer. The chain components slide and roll on each other with only a boundary layer of lubrication. The thickness of this layer is measured on the molecular scale.
Effective lubrication has some important requirements. First, it must penetrate. It must pull itself between the chainplates and between the roller and the axle. This penetration is done by capillary action, so the lubricant must have high surface tension - the lubricant molecules must have a high attractive force and a low contact angle with the metal to draw the lubricant in where it is needed.
The lubricant must also have EP/AW (Extreme Pressure / Anti Wear) additives. EP additives keep their lubricity and are not extruded from between the surfaces, even in molecular thicknesses. Often MoS2, Graphite, or PTFE are used as EP additives. AW additives chemically bond or react with the surface, assuring that the lubricant polymer chain becomes one with the chain's surface. This bonding separates the metal molecules of the chain components and prevents adhesion and galling.
Chain lubricant comes in several forms; the most common is a wet lubricant. It goes on wet and stays wet. Dry lubricants go on via a wet carrier that quickly evaporates. These often will not have the AW additives that chemically interact with the metal to ensure it remains in place. Frequent relubrication may be necessary. Its greatest benefit is that dirt will not stick to it.
Similarly, wax-based lubricants go on wet but quickly dry, leaving a waxy film; the parts will feel reminiscent of a paste waxed car, very smooth and slippery. Grease is sometimes used where water is a problem because it excludes water very well. Unfortunately, grease is an oil with a thickener; this thickener defeats the penetration capability that is so critical for the chain.
Planning a maintenance schedule
Applying lubrication can be periodic and manual or automatic. For a manual system, the lubrication can be dripped or sprayed. The advantage is that the lubrication can be put exactly where it is wanted. Unfortunately, it is easy to forget, and it may never get lubrication.
For automatic lubrication, it can be dripped, sprayed, brushed, or even applied with a sprocket applicator. Wet lubrication is the easiest method of lubricant for this.
Both dry and wax-based lubricants rely on a very volatile carrier which generally does not lend itself to unsealed storage, as the carrier can evaporate in the dispenser. Solids can settle, so these lubricants should be checked for that likelihood. If settling is likely, the dispenser should have an agitator in the reservoir. A brush as a dispenser is not suggested for either dry or wax lubricants.
There are many grease distribution systems available. A brush style is generally easiest for grease because it will spread the lubricant as much as possible. The grease will not migrate into areas not covered by the brush.
Whatever automatic lubrication system is used; it should not be operated continuously. It should only operate while the chain is in motion, and it does not need to operate every time. It is beyond this article's scope to recommend lubrication volume but simply put; the reservoir should be sufficient to supply lubricant between maintenance periods, but not so much that it causes puddling below the chain drive.
By Robert C. Adams, PhD, PE
Global Director of Research and Development