Automatic lubrication systems are generally considered very reliable. But, how reliable are they really? This question should be of interest to those people working with or intending to install an automatic lubrication system.
The Properties Of Grease
Greases, because they are Non-Newtonian (where the viscosity is not constant over a whole range of shear stresses and rates), do not have a viscosity, but instead have an apparent viscosity classified by an ASTM worked-penetration number corresponding to an NLGI consistency number (National Lubrication Grease Institute), as shown in Figure 1.
Figure 1. Apparent Viscosity Chart for Greases
The grease has to be carefully selected to deal with the operating condition of the plant to which it is being applied and its ability to work properly within a centralized lubrication system.
Greases have two special properties that must be considered when used in an automatic lubrication system: separation and internal flow resistance.
Grease consists of a physical matrix containing an oil, thickener and additive package. During the operation of a centralized system, when grease is passed through a restrictor or dispensing valve, the oil may, if not selected carefully, separate from the thickening system and form a hard, solid mass that prevents the spools from moving correctly.
Grease exposed to high pressure also has a tendency to separate. Closed cavities, where grease is entrapped and never replaced, are therefore potential separation areas. Pressure gauges based on a Bourdon tube or any other trapped cavity should be inspected or replaced on a regular basis. In order to prevent separation in the clearances between a spool and dispenser housing, the clearance must be small enough to stop the oil from separating. For a 6-millimeter-diameter spool, the clearance must be around 2 microns. This clearance is smaller than used for most hydraulic valves.
Another important property is the internal resistance to flow of grease. After each lubricant injection cycle, the dispenser has to be recharged. The pressure in the distribution line must descend to a level lower than the discharge pressure so the spool can return to its initial position and thus be ready for the next greasing cycle. Therefore, the grease pressure in the distribution line between pump and dispenser must relax sufficiently for this to take place.
The stiffer the grease and the longer the line, the more difficult it will be.
Single-line Injector Systems
The most important factor limiting the reliability of single-line injector systems is the spring force needed to reload (prime) the injector. The strength of a spring has practical limitations. Therefore, single-line grease injector systems are not often used in harsh applications with low temperatures and with standard greases pumped over long distances.
Figure 2. Single-line Injector System
The control of a single-line injector system is a pressure switch that is normally placed at the very end of the system. The pressure switch is adjusted to react at a pressure when all injectors (plus a safety factor) have operated. A signal from the switch is then interpreted by the control unit as confirmation that a completed cycle has occurred. As a matter of fact, the only thing we know is that the set pressure has been reached at the end of the main line – nothing else! For instance, we aren’t certain that each of the injectors has actually dispensed grease.
The system consists of six essential components:
1. Manual or automatic volumetric pump and lubricant reservoir
2. Reversing valve which enables lubricant to be dispensed alternately down each distribution main, allowing the pressure remaining in the unpressurized line to properly decay
3. Twin distribution main lines to convey the lubricant to the feeder blocks
4. Feeder blocks which dispense lubricant in precise amounts as the bearings require
5. Tail pipes for connecting the feeder blocks to the bearings
6. End-of-line pressure switch or transducer to control the completed lubrication cycle
Both upper and lower main lines are connected to a pump and feeder blocks. Due to the position of the reversing valve, the upper main line is under pressure when the lower main line pressure is in decay, and thus the lubricant in this line returns to the reservoir.
The pressure in the upper main line activates the rightward movement of a slide valve piston in the distributors. A quantity of lubricant equal to the one which displaced the slide valve piston is returned back to the reservoir through the lower main line. The slide valve’s position now allows the lubricant under pressure to activate the feed distributor piston, displacing a quantity of lubricant to the bearing which is on the right side of the distributor piston (Figure 3). The pressure switch or transducer should be preset to a pressure that ensures that all the dispensers have operated successfully.
Figure 3. Dual-line Injector System
Recharging the dispensing valve is made with full pump pressure (compared to a spring pressure in a single-line system). During the operation, no internal restrictions occur in the grease flow in the dispensers (see progressive systems). This makes the dual-line system one of the most reliable and versatile systems.
As with a single-line system, a successfully completed lubrication cycle is indicated by the pressure switch or transducer informing that the pre-set pressure has been achieved. This does not prove that all the spools have performed correctly within the distributor nor that the bearings have received the correct amount of grease.
Some dual-line system providers offer devices that check the movements of the spool. This solution, however, does not guarantee that the bearings have been lubricated.
Single-line Progressive Systems
The heart of these systems is the progressive divider. It has at least three dispensing elements, each with a hydraulically driven spool feeding a fixed amount of grease during the stroke from one end to the other. The volume is defined by the diameter and the length of the stroke of the piston that cannot be adjusted. The spools are internally connected through an ingenious cross-porting arrangement that forces them to work in sequence, one after the other. If one spool is not able to fulfill its stroke, the divider will totally stop working.
By monitoring the movement of one single spool in a divider, the entire divider is monitored.
Systems are often designed with one primary divider and a number of secondary dividers. Monitoring one spool in any of the dividers will give full control of the whole system, provided that there is no leakage in the tubing from the divider to the wear surface.
Figure 4. Single-line Progressive System
Figure 5. The figure illustrates a system with one primary and three secondary dividers.
The secondary divider is equipped with a micro or proximity switch that senses the movement of the spool. If any of the dividers (P, S1, S2 or S3) should stop, the whole system stops and an alarm would be initiated at the control unit. If, however, the pipe connection between P and S1 (or S2) is broken or has an external leakage, no alarm will be initiated. Also, if the divider S2 has a significant internal leakage, the alarm may fail to appear.
There has always been a desire to know the performance of a divider in an uncomplicated way. Assalub performs a simple diagnostic test:
a) The starting pressure of the divider is measured.
b) The external leakage when the outlet is plugged is measured. Inlet pressure (say, 1,450 psi [100 bars]) is known and controlled.
Knowing these values, you can obtain a good idea about the status of the divider.
The following figure illustrates these sample results:
Figure 6. Illustration of Sample Results
A divider that leaks too much might not give an alarm when one outlet is blocked. The ideal divider has not only a low leakage but also a low starting pressure. Number 5 is the best and numbers 3 and 4 should never have been manufactured!
Unfortunately, manufacturers of dividers very seldom give any information on these properties. It would be of great value if the manufacturers could agree on providing the above figures for their progressive dividers.
One disadvantage with progressive dividers is that every spool can expose the grease to restrictions. Assume that one spool feeds a bearing where a high pressure must be applied (Outlet 2a in the upper view in Figure 4). A high pressure is created behind the spool; and before the spool has reached its end-position, the spool opens to the next spool. Now, if the resistance of the bearing connected to the next spool is low, the first spool will remain in the restrictive position. The grease is forced through a narrow passage which can separate the oil from the soap. Eventually, the hard soap can create serious problems in the function of the distributor or in the bearing.
Originally, multi-line lubricators were designed for manual replenishment of the grease reservoir; unfortunately, both air and unwanted external particles are often suspended in the grease. With this problem in mind, the lubricators were designed accordingly.
Figure 7. Multi-outlet lubricator with grease vane (1) and pre-feed roller (2).
The roller forces the grease into the suction cavity.
This unique design can handle greases up to NLGI Class 4.
A sturdy design of the lubricators took care of external particles, and the introduction of a scraper vane (homogenizing the grease) overcame the introduction of air into the grease. In the illustrated unit, an arrangement was designed to force grease into the inlet cavity, preventing cavitation of the grease during the priming operation.
By utilizing a sealed reservoir arrangement, the risk of polluting the grease is almost eliminated. Air can still be a problem as many pumps used for replenishment suck air when the grease barrel is almost empty.
Multi-line lubricators are usually designed for high pressure, often 5,000 psi or higher. The high pressure enables longer pipes to be used. The high available pressures could help to purge clogged pipework.
The old compression grease cup has been modernized; instead of twisting the cap manually, various automatic solutions have been introduced. The first one used a spring to push the grease out to the bearing. Later on, an expanding gas was used for the same purpose. Finally, an electric motor was engaged either to squeeze the cap down or to activate a pumping device to dispense grease into the bearing.
Spring-loaded lubricators could only be used at a constant temperature, as fluctuating temperatures affect the apparent viscosity of the grease and, as a result, the volume of grease dispensed. Another limitation is that once it has been started, it will work continuously until the container is empty. The maximum available pressure is usually less than 15 psi.
A more modern type of single-point lubricator utilizes an expanding gas as a drawing force. The gas is derived through an electrochemical reaction in a bladder. The maximum available pressure is only 70 to 130 psi.
From a reliability point of view, both lubricator types have drawbacks. If the apparent viscosity was increased or the grease hardened, the output would decrease or might even stop. Gas-powered lubricators might also suffer from leakage of gas.
The latest type of single-point lubricators has a piston mechanism driven electrically. They are capable of creating a pressure of up to 1,000 psi, thus representing a higher degree of reliability.
The higher pressure has inspired designers to use the lubricator for multiple outlets by using progressive dividers. Unfortunately, this reduces the reliability of the lubrication system as longer lubrication pipes and the progressive divider require an adequate pressure to operate to overcome some back pressure at the bearing.
Reliability of Automatic Lubrication Systems – A General View
After what has been previously discussed, it is obvious that all systems have their Achilles’ heel. It is important that maintenance professionals are aware of the weaknesses of their systems in order to plan the control routines accordingly. Every system should be checked periodically. Consider the type of system and application when check intervals are stated.
For single-line and dual-line systems, the position of dosers’ indicator pins should be observed. Some systems (single-point, progressive and some single-line systems) lack function indicators and are difficult to check. If the system pressure allows, an overpressure indicator (Figure 8) could be used on each lubrication line. This is simply a relief valve with an atmospheric outlet. If a lube point clogs, the grease will extrude from the device, indicating a problem that can be rectified during the next routine maintenance sequence. There are also overpressure valves similar to the described which give an electric output when the pressure exceeds the set level. The lubrication pipes connecting the lube points to the dispenser or lubricator are a potential source of system malfunction. They should be checked visually on a periodical basis.
Figure 8. Overpressure Indicator Figure
Figure 9. Flow Transducer Unit Installed Directly in Grease Point
Automatically controlling the lubrication of a bearing can be performed either by a stationary installed vibration servo or by measuring the real amount of grease being fed into the bearing (Figure 9). The figure shows a flow transducer unit installed directly in the grease point. The unit closes/opens a switch which can be used to monitor the real grease flow. Each dosage from the automatic system will be measured by the transducer; and when the difference is larger than specified, an alarm will be energized. The unit can be monitored by a special electronic unit or by a PLC or computer.