M uch of today’s large steel mill equipment was installed 40 to 50 years ago when maintenance labor was plentiful, oil inexpensive, and used lubricants easily disposed of. Now owners are upgrading this equipment by boosting power and adding sophisticated controls. Yet, they often leave 40-year-old lubrication systems in place.

On the other hand, manufacturers of high-volume products are installing transfer lines that require sophisticated lubrication systems with predictive maintenance capabilities.

For both types of machines — upgraded mill equipment and new transfer lines — modern central lubrication systems offer many advantages.

Replacing outdated systems

Lubrication methods have evolved over the years from manual to sophisticated central systems. Generally unsuitable for large machines, manual lubrication is limited to a small number of lubrication points — usually on equipment that operates intermittently and at low speeds. Here, the amount of lubricant alternates between too much (right after it is applied) and too little (just before it is re-applied).

Other drawbacks include forgotten lubricant points, sporadic application, lubricant mixups, safety and health concerns, and process contamination.

Central, continuously circulating, oil lubrication systems are the next step up. Long used on large mill gear drives, these systems, Figure 1, avoid the problems of forgotten lubrication points, sporadic application, and lubricant mixups. But they may be susceptible to heat build-up caused by lubricant churning, plus leakage, oil contamination, lubricant replacement and disposal difficulties, and minimal monitoring. Also, they tend to apply too much lubricant on machine components if improperly adjusted.

Today, designers are replacing both of these methods with automated central lubrication systems that apply small amounts of lubricant at frequent intervals, thereby maintaining a consistent amount of lubricant on the drive components. Also, these systems monitor and control their own performance.

Sorting out systems

All central lubrication systems contain a pump, reservoir, and distribution network. But they differ in their delivery methods. There are two basic systems, parallel and series-progressive, plus hybrid systems that combine elements from both types.

Parallel systems include circulating oil, dual and single-line, orifice, and oil mist versions. The first type, circulating oil, supplies a continuous flow of oil to all lubrication points simultaneously, Figure 1. A gravity return loop directs oil back to the sump or reservoir.

Dual and single-line parallel systems pump lubricant simultaneously to dispensing devices, each of which dispenses a measured amount of lubricant. Dualline systems use double-acting feeders, Figure 2, to dispense lubricant. Here, reversing the flow discharges one side of the feeder while priming the other for the next cycle. Single-line systems use spring-loaded injectors. Removing pressure in the system lets each injector retract and recharge for the next cycle.

Orifice systems deliver oil simultaneously to all lubricated points. At each point, an orifice with an elastomeric check valve controls the amount of oil delivered.

Oil mist systems generate and distribute finely atomized oil to lubrication points.

Series-progressive systems include grease, oil, air-oil, and air-grease spray types. Grease or oil systems pump lubricant to multi-piston, divider valves that operate in sequence. Each piston completes its stroke and discharges a measured amount of lubricant into its delivery line before the next piston is activated, Figure 3. Blockages or broken lines generate pressure feedback, which can be monitored to detect abnormal operation.

Air-oil systems mix measured amounts of oil from divider valves with a flow of compressed air that carries the oil to the lubrication point, Figure 4.

Air-grease spray systems use compressed air to atomize measured amounts of grease from a divider valve.

Mill drives get air-oil

Series-progressive airoil systems can be used to lubricate large mill drives. This approach borrows metering and delivery concepts developed for high-speed machine tool spindles. Though mill gears and bearings run at much slower surface speeds, the same techniques — supplying only the needed amount of oil and conveying droplets of lubricant with compressed air — work well for these components.

Because the system delivers just enough oil to gears and bearings, the absence of excess oil reduces heat buildup from lubricant churning and viscous shear. Further, the air flow helps to cool both lubricant and lubricated components. And power consumption by both machinery and lubrication system is reduced.

Compared to older lubrication methods, air-oil systems eliminate the need to handle, condition, and dispose of thousands of gallons of lubricant. Reduced oil consumption stretches lubricant dollars as much as 75%.

Bearings. In a typical arrangement for lubricating bearings (right side in Figure 4), individual shots of oil from a divider valve travel through a tube to a mixing tee where they combine with a constant flow of air. The air and oil mixture then enters a tube leading to the lubrication point, and the air flow continually delivers minute quantities of oil through the tube to the bearing. In addition to delivering oil, the air flow can also pressurize a sealed drive housing to keep out contaminants.

Gears. To lubricate gears (left side in Figure 4), a divider valve sends a measured amount of oil (or grease) to either a spray valve, which incorporates both a nozzle and shut-off control, or to a spray nozzle. The nozzle directs an intermittent oil spray at the pressure (loaded) side of each gear tooth.

Monitoring and control

Programmable controllers have simplified control of even the largest multi-zone central lubrication systems. Based upon sensor feedback or elapsed time, these controllers determine lubricant volumes, plus lubrication intervals and sequences, and they monitor lubrication system performance. Controls for individual machine zones permit adjusting lubrication to match sporadic machine operation.

Monitoring components typically include cycling indicators, pressure sensors, and controller I/O. Bearing temperature, vibration, filter condition, and metallic-debris sensors can be added to help users predict when maintenance is required. Alarms, data logging, and communication with plant controllers further increase the monitoring and control capabilities.

Specifying a system

Whether you design the system or farm it out to a lubrication specialist, considering these checkpoints will help you get the most from central lubrication.

1. Operating requirements of each lubrication point (speed, load, duty cycle).
2. Gear or bearing mounting configuration at each point.
3. Type of lubricant required at each point.
4. Environmental factors (temperature, contaminants).
5. Plant standards (plumbing, air, electrical).
6. Lubrication and maintenance schedules.
7. Type of monitoring (current status or ongoing retrospective analysis).
8. Faults to be detected (pressure, flow, temperature, vibration).
9. Operator and network interface requirements.
10. System programming responsibility (inhouse or vendor).

George Nemes is an application engineer for Lubriquip Inc., Cleveland.