The development purpose of the centralized lubrication system is to deliver the right amount of lubricant to multiple lubrication points at the right time and at the right place. However, it can be challenging to confirm that the lubricant reaches the lubrication point reliably. Currently, most centralized lubrication systems on the market do not provide a method to confirm the flow rate to the lubrication point.

Due to lack of confirmation, manual lubrication is sometimes chosen. This in turn reduces the reliability of mechanical lubrication. The good news is that there are some solutions to improve the reliability of centralized lubrication systems through lubricant flow confirmation.

Over the years, centralized lubrication systems have been continuously developed with different distribution devices. There are three main types of automatic lubrication systems suitable for the distribution of grease and oil: series progressive, two-line and single-line parallel systems. Because the lubricants in these three systems are distributed to lubrication points and lost in the application, they are sometimes referred to as total loss systems.

Other centralized lubrication systems include oil circulation systems and oil mist lubrication systems. Because of the differences in the lubricant distribution units of these systems, the lubricant flow confirmation must be performed in different ways.

A typical series of progressive systems distribute lubricant through distribution valves. Larger systems may have a primary diverter valve feed and multiple secondary diverter valves. The working principle of the progressive system helps to lock the entire system in the event of a blockage at any lubrication point.

Examples of series progressive systems (provided by Lincoln Industrial)

Most series of progressive systems are equipped with an indicator pin at one of the ports to provide a visual indication of the correct system function. Some progressive systems are equipped with proximity switches or cycle counting devices to prevent the system from locking or jamming.

Common practices for monitoring indicator pins may include proximity switches (top left), proximity switches with cycle counting (bottom left), and cycle counters with data logging (top right). (Provided by Lincoln Industries)

Although the monitoring indicator pin improves the reliability of the tandem progressive system, it does not cover the failure of the feed line or the leakage of grease fittings at the lubrication point. In order to provide 100% lubrication guarantee at the lubrication point, it is necessary to confirm the lubricant flow rate.

The dual-line lubricant distribution valve can also be equipped with an indicator pin. The movement of the indicator pin can confirm the flow rate from the valve. Unlike progressive distribution valves, two-line distribution valves can be arranged in parallel, making them independent of each other. Nevertheless, the indicator pin in the two-line distribution valve does not cover the feed line.

A typical two-wire metering valve has a piston pin indicator. Its movement can be inspected visually through a transparent cap or monitored with a proximity switch. (Provided by Lincoln Industries)

The single-line parallel system dispenses lubricant independently through syringes. Blocked lubrication points or fuel injector failure will not affect the entire system. One or several blocked lubrication points are not easy to detect at the system level.

Most single-line injectors are also equipped with an indicator pin, but this pin also cannot confirm the lubricant flow at the lubrication point. Although the proximity switch can be used to monitor the function of the injector, it is still one step away from the flow confirmation of the lubrication point.

Example of a single-line parallel lubrication system (provided by Lincoln Industrial)

Centralized oil circulation system (provided by SKF)

When the application needs to extract heat from the bearing while providing lubrication, an oil circulation system is usually chosen. In these systems, large or high flows of oil are pumped through the system. The excess oil is directed to the return tank and filtered before re-entering the lubrication system.

If necessary, the system can be expanded with a progressive distribution valve or equipped with a flow restrictor to properly distribute the oil flow. Most oil circulation systems are equipped with flow meters for flow confirmation. An optical laser flow sensor can also be used to monitor the flow on the transparent pipe section.

Oil mist lubrication systems are widely used in large rotating equipment with relatively small temperature fluctuations. The oil mist is generated by compressed air through a venturi or vortex, and is transported through a pipeline to the classifier fittings at the lubrication point, where it condenses into oil droplets to lubricate the bearings.

Because the oil mist system has pressure in the pipeline and the oil mist flow is relatively difficult to determine, monitoring such systems is challenging.

The latest development in high-speed bearing lubrication is the air/oil system. In this type of system, liquid oil is injected directly into the air stream from a positive displacement pump at specific time intervals. The compressed air stream then pushes the oil as droplets into the feed line to lubricate the bearings.

A flow confirmation sensor has been developed for this type of air/oil flow. One such device is called an oil trace sensor, which can be used to monitor the continuity of oil flow in an air/oil system.

Although many centralized lubrication systems have a built-in indicator to verify that the lubricant is being delivered correctly at the distribution valve, this does not guarantee that the lubricant flow reaches its target point. The most reliable way to ensure proper lubrication performance is to confirm the flow rate at the lubrication point.

The lubricant flow in the centralized lubrication system can be divided into two types: intermittent and continuous. For intermittent flow, the lubricant can be oil or grease, while continuous flow is usually used for oil, oil mist or air/oil.

A variety of sensors can be used to achieve flow confirmation, including flow meters, non-flow switches, flow monitoring AC switches, inductive flow switches, thermistor flow switches, and magnetic field flow switches.

Lubricant flow meters or flow sensors are very similar to fuel gauges at gas stations. A flow meter with a pair of elliptical gears equipped with magnets and reed switches is the most widely used in the industry. These types of flow meters require downstream evaluation units or pulse meters to monitor the flow. The evaluation unit has upper and lower limits that can be set and monitored.

For example, the volume of lubricant passing through the meter in 60 minutes should be within 10 to 12 cubic centimeters. If the amount of lubricant in the specified time interval exceeds the limit, a warning or alarm is displayed or communicated. Although the flowmeter can be used in intermittent flow systems, it is more suitable for oil circulation systems.

The no-flow switch is designed for continuous flow or semi-continuous flow lubrication systems. It is realized by an electric contact switch, which is activated by a plunger under the action of a spring force. Under normal conditions, a continuous flow of lubricant pushes the plunger away from the electrical switch, counteracting the spring force on the plunger. When the lubricant flow stops, the spring force pushes the plunger towards the electrical switch and finally pushes the switch to close.

The closed switch can either send out an alarm or directly shut down the protected machine. A typical no-flow switch does not require a controller or electrical equipment to protect the machine, and is popular on large gas compressors that require direct protection. The no-flow switch can be installed close to the lubrication point, but there will still be a short pipe connecting the outlet of the no-flow switch to the lubrication point inlet.

Most flow monitoring switches are used to check the flow in the oil circulation system. However, there are also certain models that can be used for intermittent oil flow. Keep in mind that different flow rates as the viscosity of the lubricant changes will require different models to suit the application.

The inductive flow switch is designed to detect intermittent flow from the distribution device. The flow detector uses an inductive sensor to monitor the movement of the non-return ball driven by the lubricant flow. The flow sensor can withstand pressures up to 3,000 psi.

At present, grease flow sensors with a flow sensitivity of 0.016 cc and above per injection can be used. However, since the force from the lubricant flow is not sufficient to overcome the bias spring force, the flow rate of 0.03 cc per cycle from the injector in the single-line parallel system cannot be recognized. The spring force can be adjusted to compensate for the viscosity of the lubricant.

Flow confirmation sensor and its application

The thermistor flow switch monitors the lubricant flow based on the temperature change of the sensing element. Some thermistor flow switches require a downstream controller to evaluate the signal from the lubricant flow. In an air/oil lubrication system, the flow rate is relatively low, so only a thin layer of oil passes through the pipe or feed line. Detecting the flow of this thin layer requires a sensitive device. Due to the faster heat exchange rate brought about by the mixed flow of air and oil, the thermistor flow switch is a viable option.

The operating mechanism of the magnetic field flow switch is very similar to that of the inductive flow switch. The magnetic field flow switch does not use an inductive sensor to monitor the movement of the non-return ball, but uses a magnetic field sensor to detect the movement of the magnetic non-return ball through non-ferrous metals such as stainless steel, brass, and aluminum.

This sensing technology provides better sensor body material strength, making high pressure applications (5,000 psi) possible. By adding a magnetic field to align the plunger, the sensitivity of the magnetic field flow switch is improved, which also provides more flow restrictions for better detection of small flow and low viscosity applications.

With so many changes in systems and sensors, choosing the correct flow confirmation sensor can be challenging. The following factors should be considered:

The table above summarizes the different sensors and their applications.

Remember, the reliability of centralized lubrication systems can be improved by confirming the lubricant flow rate. However, the implementation of such confirmations should follow certain guidelines, including: