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To clarify from the start, We are talking about grease dispensing on parts in a production process. There are others who do an adequate job of dispensing grease into bearings on machine tools and other equipment so that topic will not be considered. This is a discussion of the production greasing process.
To specify a grease dispensing system we need to have the answers to at least four specific questions that can be simplified as how much?, how often?, how (method)?, and how to verify. Each of these will be covered in depth.
Grease Volume (How Much?)
1. The Spec calls for 2.5 grams of Nye NYOGEL® 760G with a density of 0.88
Volume = Weight / Density Volume=2.5 / 0.88 Volume = 2.84 cc or 0.173 CuIn
2. The Spec calls for 2.5 grams of Nye UNIFLORTM 8917 with a density of 1.90
Volume = Weight / Density Volume=2.5 / 1.90 Volume = 1.32 cc or 0.081 CuIn In
Looking at these two examples from the same manufacturer we can see that more than twice as much of one product will be needed to equal the very same weight of the other product!
Sometimes it is necessary to work with a visual spec - "the size of a "BB",
"a Hershey's Kiss", or "it should look like this". It's amazing how often a
description like this is ALL you have to work with. To help find the volume in
these situations, we provide a volume calculator.
Just find the nearest geometrical shape that the grease most resembles and enter a
few measurements to find the volume.
Production Rate (How Often?)
Once the volume has been determined, the production rate should be considered. Many air-operated grease dispensers can eject grease quite rapidly, but may take longer to re-fill for the next cycle. Some dispensers are operated by the grease pressure itself, and are slower to dispense and re-fill. It is a good practice to select components that can keep up with at least 1.5 x desired production rate. That way a minor increase in the rate or slowdown in the grease flow will not cause faults or defective parts. If the grease to be used has a high apparent viscosity, then a dispenser may operate at a slower rate. This should be considered in the selection process also. When in doubt, see if the dispensing equipment supplier will test your sample grease in the dispenser before making the final purchase decision. Some suppliers are very dedicated to your success and will be glad to prove out an application in the lab. Learn more about Viscosity.
Dispensing Method (How?)
There are two basic methods of automated grease dispensing, Time and Displacement. Which method is chosen depends entirely on the desired quality of the end product, it's that simple. Here is a description of the two methods:
Time Dispense - Time is the cheapest method for grease dispensing, and also the least predictable and least reliable. Grease is pumped through a pressure regulator, or often two regulators and the regulated pressure is fed to a dispense valve. The valve is controlled by a timer, to keep it open for a precise amount of time. During that time, an amount of grease will be forced out through the dispense valve, nozzle, and on to the product. The theory is that with constant pressure and time, the same amount of grease will be dispensed during each cycle. But here is where theory meets reality: The grease viscosity can vary significantly, thus changing the flow rate! Temperature is one major factor that affects viscosity, and anyone who has spent time in a manufacturing plant knows the wide range of temperature in that environment. There can also be variances in the manufacturing of the grease itself, and there can be separation (of the base oils from the solids in the grease) that causes variation within a single container. There can also be clogging of the nozzle orifice.
Since changes in viscosity and nozzle condition can cause changes in the flow rate, the volume dispensed will also show variation. The time and pressure can be adjusted to compensate, but this is unlikely in a production setting. The conclusion must be that time dispense should only be used where 1. The price of the equipment is a major concern, 2. Quality is a minor concern, and 3. where some external device is used to confirm that at least SOME grease was dispensed.
Since the output volume is determined by pysical displacement, changes in
viscosity and nozzle condition are irrelevant. There should be no need to constantly
re-adjust the process. Thus the conclusion shouyld be that displacement dispense
should only be used where 1. Quality is a major concern, and quality will be
enhanced by consistent grease volume on each part. 2. Price is a lesser concern,
and 3. where some external device is used to confirm that the dispenser did cycle.
Confirmation Method (How to Verify)
An extremely important and often overlooked part of a dispensing system is confirmation. It is common to see factories that boast of zero defect and continuous improvement, and still have no way confirming that a part was greased. Lack of grease on a production part that is engineered to be lubed is clearly a major defect. If the grease is in a visible location, operator inspection can be used, but is subject to human error. When the grease is applied in a hidden area, mechanical inspection is essential.
Grease Flow Sensors - A flow sensor is a switch that can detect the motion of grease passing through it. When located close enough to the dispense point (nozzle), this type of sensor provides a very high degree of confidence that grease did get to the part. When the grease dispenser is activated, a corresponding signal should come from the flow sensor. If there is no corresponding signal, or the signal is too short, then production must be stopped until the problem is corrected, and the lines are bled (free of any air that may have been introduced).
Flow Meters - A flow meter used for grease is usually a gear-type device that rotates
when grease is forced through the area between the gears. This rotation generates a chain
of pulses back to the machine PLC. The arrival of pulses indicates flow. The rate of the
pulses indicate flow velocity. This device should also be placed as near as possible
to the nozzle. To use a flow meter requires more complicated programming than other methods.
Most of the available flow meters were not designed specifically for grease, and have close
internal tolerances, requiring much finer filtration that is usually used in grease systems.
This can be a real problem when certain greases are used that normally include additives
with rather large particles in suspension. Like the flow sensors, a loss of signal or too short
of signal indicates a failed dispense, and production must be halted until the problem is corrected.
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