In business and in life, it's the squeaky wheel that gets the oil. On the assembly line, it's also the bearings, gearboxes, cams, linear actuators and anything else that moves side to side, up and down, or around an axis.
Without lubrication, even the fastest, most accurate, most elegantly designed assembly system will literally grind to a halt. Lubrication does more than just reduce friction between moving parts. It also prevents corrosion, dissipates heat, and keeps mechanisms free of contamination.
Fortunately, keeping assembly machinery well-lubricated isn't too onerous a task. In fact, many motion control components, such as gearboxes and ball bearings, are available as sealed models that don't require any lubrication over their working lives. Other machines and components are designed to need relatively little attention from the grease gun.
A SCARA robot is a good example. Engineers should inspect the robot's Z axis weekly and lubricate it monthly. All other axes should be lubricated every 48 months. The Z axis requires more attention because it is usually uncovered and therefore more exposed to the factory environment. The other axes are sealed motors and are more protected. In between these scheduled maintenance breaks, engineers should always be looking and listening for sights and sounds of metal-on-metal grinding in the mechanisms.
"There is one simple caveat to these recommendations: Lubrication frequency is greatly determined by the application," warns John C. Clark, national sales manager for EPSON Robots (Carson, CA). "Robots in dusty environments or very high-speed applications will require more frequent lubrication."
A high-speed, multistation automated assembly system will have dozens of lubrication points. Some parts of the system, such as pneumatic grippers, take care of themselves, drawing lubrication from their air supply. Other parts must be checked and lubricated regularly. Chief among these are journal bearings or rolling contact bearings.
By some estimates, 50 percent of bearing failures are caused by inadequate lubrication. Bearings can be lubricated with either grease or oil. At normal temperatures and low speeds, grease is satisfactory. Grease is also good for heavy loads, especially if vibration or shock loads are likely. At high speeds and high temperatures, oil is necessary. Oil is also better for applications with low starting torque.
Greases for bearings are a mixture of oil and a thickening agent, usually a clay or a soap, such as lithium or barium. Oils are usually clean, stable mineral oils. Additives can be mixed with the lubricant to improve how well it adheres to parts, resists oxidation, or prevents corrosion.
When choosing a lubricant, engineers should make sure it can handle the operating temperature of the application, and that it is compatible with the structural materials of the machine, especially plastics and elastomers. The viscosity and film strength of the lubricant should match the speed, torque and load requirements of the application.
Even though lubricating assembly equipment is fairly easy, machine builders and component suppliers often struggle to get assemblers to perform even that bit of routine preventive maintenance. There are several reasons for this. "Engineers are under intense pressure to meet grueling production deadlines that don't allow time for routine scheduled maintenance. After all, no money is earned during downtime," says Clark. "The ability of robots to run nonstop gives the impression that they will run forever. And finally, the ‘if it ain't broke, don't fix it' mentality is still very much alive."