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Felt Wicking and Lubrication

The subject of felt and lubrication embraces the application of felt to wick feedlubricating systems for bearings and other mechanical movements, where oil orother coolants are required to be fed in a controlled manner without failure orinterruption. Lubricating systems of this type may be classified as: (1) bottomwicks; (2) siphon wicks; (3) absorbent or pad feed and (4) top feed.

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Siphon and pad feed lubricating or wick systems are the most widely usedalthough bottom wick systems are generally considered the most efficient. They

are entirely automatic, require no attention other than occasional cleaning, andallow a return of the unused liquid to the reservoir. In top feed applications,where there is a reservoir with a wick extending from a bottom outlet, the wickfunctions as a semi-controlled obstruction. The system to be used for anyapplication is that which satisfies design and operating conditions. In all cases, afirst consideration is the selection of proper materials to transport oil-basedlubricants.

WHY FELT IS GOOD FOR OIL WICKING AND LUBRICATION

Felt is made up of a large number of capillaries which are formed between thefibers. The capillaries hold the oil, and oil is wicked through them to a metal part.The number and size of the capillaries in a felt are dependent almost entirely onthe density of the felt and on the diameter of the fibers in the felt. High densityfelts have more fibers and capillaries per cubic unit and the finer the capillaries.

Finer capillaries not only transport or wick liquid along longer distances and

higher heights, they also hold the liquid more tenaciously. This liquid holdingcapacity makes felt materials both effective and efficient as liquid reservoirs.Some felts can absorb some four to five times their own weight of oil.

In addition, certain felts have good resistance to and good recovery fromcompression. Accordingly, they have the ability to maintain their capillarystructure under compression and other stresses.

Felt also has ideal surfaces for transferring oil to axles or other moving metalparts. It does not ravel; it can give up its oil either slowly or quickly depending onthe type of felt and the application. Wool felt has particularly good resistance toheat, and will not glaze when properly applied. Lubrication by felt, or any othermaterial, is stopped when its capillaries are blocked, either by dirt or bycompression or by pinching, etc. In fact, this is one of the disadvantages of wovenfabrics for lubrication. Besides their tendency to unravel easily, where the warpand filling yarns interlace the capillaries are pinched off and the flow of the liquidin the yarns is blocked or slowed down. Some felts are much better than othersfor the various types of lubrications; the performance of a felt depends on thetype of oil lubricant used, the temperature of the oil and on the actual lubrication

job the felt has to do. It can be said that no one felt is best for all lubricationpurposes. Thus, as much care should be exercised in the selection of felt wickmaterials, as is required in the selection of suitable lubricants.

Felt-Wool andSynthetic

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Heat-Resistant,Non-Asbestos,High-TemperatureSynthetics

Felt SpecialtyProducts

Wicking & Felt Lubrication - Lubricating Felt | Metric Felt Company http://metricfelt.com/wicking.html

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FACTORS TO CONSIDER IN SELECTING A FELT FOR LUBRICATION

Considerable laboratory test data is available which compares various grades ofcommercial felts with respect to wicking and liquid holding capacity using differentoils and for various temperatures. In this article typical examples are presentedwith Spinesso 38 a light weight spindle oil at 20 degrees C or with an SAE-10grade oil.

Wicking Felt

The wicking property of the felt is governed by the height to which the liquid issucked up a vertically-held sample. This depends on the time Interval allowed forthe liquid to creep up the sample, but generally the equilibrium (maximumheight) occurs after say 2 to 4 hours and thereafter the wicking height remains atabout that level. The maximum wicking height (value after 24 hours) was foundto increase linearly with increasing felt density (see Figure 4). The dense felts ofthe types F-l, F-2, gave wicking heights approaching 20 cm (8") which could beattributed to the presence of very fine capillaries in the material. The actualwicking height was shown to increase by a further 5 cm (2") when thetemperature was increased from 20 degrees C to 82 degrees C (68 degrees F to180 degrees F). This is to be expected since the viscosity and the surface tensionof the oil decreases with the increase in temperature. A similar change in wickingheight, but in this case a decrease, was also observed when the light weightspindle oil was replaced with a more viscous SAE 30 oil at the same temperature.There was no significant difference in wicking performance along the length and

across the width of the felt, as most felts tested had fibers laid in both directions.

It should be noted that felt thickness, or size of the felt wick is not a factor in theheight of rise of oil. A 1/4 felt and a 1" thick felt of the same style and density willlift oil to the same height in a given period of time. The thicker felt will, of course,carry a greater amount of oil in actual ml. or grams of oil.

DISTRIBUTION OF OIL IN FELT WICKS

Figure 5 shows not only the height of rise of the oil in the wicks, but also theamount of oil expressed as ml. of oil per cubic inch of felt, in each 1" section alongthe length of the felt strips after wicking 24 hours at 21 degrees F (70 degrees F).It is evident that high-density felts that wick oil fast and carry oil to high levels,also have a uniform distribution of oil throughout practically the whole length ofthe wicks. The lower density felts, except the poorer quality, have a large volumeof oil at the 5 cm (2") level, but the volume of oil decreases at this level. Thus, ifa bottom wick felt only has to wick oil to a height of about 5 cm (2") and if a lot ofoil is needed at that level, then the F-6 or F-7 felts, the F-10, 11 or 12, would bevery good. On the other hand, if the metal part to be lubricated is above 7.5 cm(3") from the oil reservoir, then these felts would not serve the purpose. Where asmall piece of felt is saturated with oil and installed in a sealed machine tolubricate a part for very long periods of time, or where the felt has to feed asteady film of oil some distance from a reservoir, the high-density, high qualityfelts such as F-l, F-2, or F-3 would seem advantageous. Therefore, in selectingfelts for wicking, one would consider the height to which the oil must be lifted, therate it must be wicked, and the amount of oil carried by the wick to a specifiedheight.

In tests where the oil- filled felts were compressed under the same externalpressure up to 5 Ibs sq/in. (0.35 kg sq/cm) the oil was easily squeezed out of thelow density felts whereas the high density felts retained practically all of theiroriginal oil content.

In addition, similar observations were made in vibration tests where high density

felts retained most of their initial oil content whereas low density felts lost most oftheir oil after three hours of tumbling.