- Buyer's Guide
Centralized lubrication systems were developed to deliver lubricant to multiple lubrication points in the right amount, at the right place and at the right time. However, it can be challenging to confirm that the lubricant reliably reaches the lube point. Currently, most centralized lubrication systems on the market do not offer a means of confirming the flow to the lube point. Because of this lack of confirmation, manual lubrication sometimes is selected. This in turn lowers the reliability of machinery lubrication. The good news is that there are solutions available to improve the dependability of centralized lubrication systems with lubricant flow confirmation.
Centralized lubrication systems have evolved through the years with different dispensing devices. There are three major automatic lubrication systems that are suitable for dispensing both grease and oil: series progressive, dual-line and single-line parallel systems. Because the lubricant in these three systems is dispensed into a lube point and lost in application, they sometimes are called total-loss systems.
Other centralized lubrication systems include oil-circulation systems and oil-mist lubrication systems. Due to the differences in the lubricant-dispensing units of these systems, the lubricant flow confirmation must be implemented differently.
A typical series progressive system dispenses lubricant through divider valves. Larger systems may have primary divider valve feeds and multiple secondary divider valves. The operating principle of a progressive system facilitates a lock-down of the entire system when there is a blockage at any lube point.
An example of a series progressive system (courtesy Lincoln Industrial)
Most series progressive systems are fitted with an indicator pin at one of the ports to provide visual indication of proper system function. Some progressive systems are equipped with a proximity switch or cycle-counting device to prevent a lock-down or jammed system.
Common practices of monitoring the indicator pin may include a proximity switch (top left), a proximity switch with cycle counting (bottom left) and a cycle counter with data logging (top right). (courtesy Lincoln Industrial)
While monitoring the indicator pin improves the reliability of series progressive systems, it does not cover the fault of a feed line or leaking grease fitting at the lube point. In order to provide 100-percent assurance of the lubrication at the lube point, lubricant flow confirmation is needed.
Dual-line lubricant-dispensing valves can also be fitted with an indicator pin. The movement of this indicator pin can confirm the flow from the valve. Unlike the progressive divider valve, the dual-line dispensing valves can be arranged in parallel so that they are independent of each other. Nevertheless, the indicator pin in dual-line dispensing valves does not cover the feed line.
A typical dual-line metering valve comes with a piston pin indicator. Its movement can be visually checked through a transparent cover cap or monitored with a proximity switch. (courtesy Lincoln Industrial)
A single-line parallel system independently dispenses lubricant through injectors. Lube point blockage or a faulty injector would not affect the system as a whole. One or several blocked lube points would not be detected easily at the system level. Most single-line injectors are also fitted with an indicator pin, but again this pin does not provide confirmation of lubricant flow at the lube point. While it is possible to use a proximity switch to monitor the functioning of an injector, this is still one step away from flow confirmation at the lube point.
An example of a single-line parallel lubrication system (courtesy Lincoln Industrial)
A centralized oil-circulation
system (Courtesy SKF)
When applications require extracting heat from bearings while providing lubrication, an oil-circulation system generally is chosen. In these systems, a large volume or high flow of oil is pumped through the system. Excess oil is directed to a returning reservoir and filtered before re-entering the lubrication system. If necessary, the system can be expanded with progressive divider valves or fitted with flow restriction to properly distribute oil flow. Most oil-circulation systems are equipped with a flow meter for flow confirmation. The flow may also be monitored with an optical laser flow transducer over a transparent tube section.
Oil-mist lubrication systems are utilized extensively in large rotating equipment where temperature fluctuation is relatively small. Oil mist is generated through a venturi or vortex with passing compressed air and is carried through pipes to classifier fittings at lube points where it is condensed into oil droplets for lubricating bearings. Since an oil-mist system has pressure in the pipeline and the oil-mist flow is relatively difficult to qualify, monitoring this type of system is challenging.
A more recent development in high-speed bearing lubrication is the air/oil system. In this type of system, liquid oil is injected from a positive-displacement pump directly into the air stream at a specific time interval. The compressed air stream then propels oil as droplets through the feed line to lubricate the bearing. Flow confirmation sensors have been developed for this type of air/oil flow. One such device is called an oil-streak sensor, which can be used to monitor the continuity of the oil flow in air/oil systems.
Although many centralized lubrication systems have a built-in indicator to verify that the lubricant is being delivered properly at the dispensing valve, this does not guarantee that the lubricant flow reaches its target point. The most reliable way of ensuring proper lubrication performance is to confirm the flow at the lube point.
The flow of lubricant in centralized lubrication systems can be divided into two types: intermittent and continuous. For intermittent flow, the lubricant can be either oil or grease, while continuous flow is usually for oil, oil mist or air/oil.
A number of sensors are available to achieve flow confirmation, including flow meters, no-flow switches, flow-monitoring AC switches, inductive flow switches, thermistor flow switches and magnetic-field flow switches.
A lubricant flow meter or flow transducer is very similar to a fuel meter at a gas station. Flow meters that have a pair of oval gears fitted with a magnet and a reed switch are the most widely used in industry. These types of flow meters require a downstream evaluation unit or pulse meter to monitor the flow. The evaluation unit has upper and lower limits that can be set and monitored. For example, the lubricant volume through the meter should be within 10 to 12 cubic centimeters in 60 minutes. If the lubricant volume in the specified time interval exceeds the limit, a warning or alarm will be displayed or communicated. Even though a flow meter can be used in intermittent-flow systems, it is more suitable for oil-circulation systems.
A no-flow switch is designed solely for continuous-flow or semi-continuous-flow lubrication systems. It is implemented by an electrical contact switch that is activated by a plunger under spring force. In normal conditions, the continuous lubricant flow pushes the plunger away from the electrical switch, countering the spring force on the plunger. When the lubricant flow stops, the spring force drives the plunger toward the electrical switch and eventually pushes the switch closed. The closed switch can either set off an alarm or directly shut off the machinery being protected. A typical no-flow switch does not need a controller or electrical device to protect the machinery and is popular on large gas compressors where direct protection is desired. A no-flow switch can be installed close to the lube point, but there still will be a short tube connecting the outlet from the no-flow switch to the lube point inlet.
Most flow-monitoring switches are intended for checking the flow in an oil-circulation system. However, there are also certain models available for intermittent oil flow. Keep in mind that different flow rates with lubricant viscosity variations will require different models to suit the application.
An inductive flow switch is designed to sense the intermittent flow from a dispensing device. The flow detector utilizes an inductive sensor to monitor the movement of a check ball driven by lubricant flow. The flow sensor can be subjected to pressure up to 3,000 psi. A grease-flow sensor with flow sensitivity of 0.016 cc and above per shot is currently available. However, the flow of 0.03 cc per cycle from an injector in a single-line parallel system will not be recognized because the force from the lubricant flow is not high enough to overcome the biasing spring force. The spring force is adjustable to compensate for the lubricant’s viscosity.
Flow confirmation sensors and their applications
A thermistor flow switch monitors lubricant flow based on the change of temperature at the sensing element. Certain thermistor flow switches require a controller downstream to evaluate the signal from the lubricant flow. In air/oil lubrication systems, the flow is relatively low, so there’s only a thin layer of oil passing through the pipe or feed-line tubing. Detecting the flow of such a thin layer requires a sensitive device. Thermistor flow switches are a viable option due to the faster heat-exchange rate brought by the mixed flow of air and oil.
The operation mechanism of a magnetic-field flow switch is quite similar to an inductive flow switch. Instead of monitoring check-ball movement with an inductive sensor, magnetic-field flow switches utilize a magnetic-field sensor to detect the movement of a magnetic check ball through non-ferrous materials such as stainless steel, brass and aluminum. This sensing technology provides better sensor body material strength, making high-pressure application (5,000 psi) possible. The sensitivity of magnetic-field flow switches has been improved by adding a magnetic-field aligning plunger, which also offers more flow restriction for better sensing of small flow volume and less viscous applications.
With so many variations in the systems and sensors, selecting the right flow confirmation sensor can be challenging. The following factors should be taken into consideration:
The table above summarizes the different sensors and their application.
Remember, the reliability of centralized lubrication systems can be improved by confirming the lubricant flow. However, implementation of such confirmation should follow certain guidelines, including: