Lubrication of roller chains

Roller chains are a broad classification of chains used for power transmission purposes. They all have one feature in common, which is a sleeve roller that rotates around the pin or shaft of the chain; this roller reduces friction and wear between the chain and sprocket.

Among them, a drive chain transmits torque between two sprockets; a bicycle or motorcycle chain is an example. Energy chains are used to pull loads, such as the chains used to pull a roller coaster to the top of a mountain. The timing chain transmits rotational position accurately and smoothly.

Due to their application and geometry, lubricating these roller chains presents some special requirements and difficulties. Operating conditions can present an opportunity for excessive wear: the chain may be outdoors, in direct contact with rain and dust, and not cleaned or lubricated frequently. Chain rollers can roll on dirty or gritty sprockets, or there can be contamination between the chain plates that slide against each other and pivot on the chain axle, causing wear. If the rollers are buried in the chain, it is difficult to get lubrication between the rollers and the sprocket. Of course, any oxidation will increase corrosion and abrasive wear. Only automotive timing chains work when lubrication is well controlled, clean and continuously supplied.

Ideally, the chain material should have high tensile (yield) strength and a hard surface to reduce friction and wear. It should also be corrosion resistant. Unfortunately, many materials with good corrosion resistance cannot meet the high strength and hardness requirements. To overcome this problem, some components can be hardened (rollers and shafts), and surface treatments can be used to improve corrosion resistance and lubrication sex. It is even possible to perform sacrificial plating on the link plates, creating a pair of platings to protect the unplated parts.

All of this requires us to be able to get lubrication where it needs to be. Due to the speed and load of the chain, it usually operates in a boundary lubrication system where not enough lubrication or speed exists to initiate the hydrodynamic layer. There is only one boundary lubrication layer when chain components slide and roll against each other. The thickness of this lubricating layer is measured on a molecular scale.

There are some important requirements for effective lubrication. First, it has to penetrate. It must pull itself between the chain plates and between the rollers and the shaft. This penetration is accomplished by capillary action, so the lubricating oil must have a high surface tension - the lubricating oil molecules must have a high attraction force and a low contact angle with the metal in order to draw the lubricating oil where it is needed.

The lubricant must also have EP/AW (extreme pressure/anti-wear) additives. EP additives maintain their lubricity without extruding between surfaces, even in molecular thickness. Usually MoS2, graphite or PTFE are used as EP additives. The AW additive chemically bonds or reacts with the surface to ensure that the lubricating oil polymer chain is integrated with the surface of the chain. This bonding separates the metal molecules of the chain components, preventing sticking and wear.

Chain lubricants come in many forms, the most common being wet lubricants. It can be used wet and stays moist. Dry lubricating oil is carried out through a rapidly evaporating wet carrier. These lubricants typically do not have AW additives, which chemically interact with the metal to ensure it remains intact. Frequent relubrication may be required. The biggest benefit is that dirt doesn't stick to it.

Likewise, wax-based lubricants come in wet but dry quickly, leaving a waxy film; the parts feel reminiscent of pasty waxed cars, very smooth and creamy. Grease is sometimes used where there is water because it repels water so well. Unfortunately, grease is an oil with a thickening agent; this thickening agent destroys the ability to penetrate, which is vital to the chain.