Felt Wicking and Lubrication
The subject of felt and lubrication embraces the application of felt to wick feed lubricating systems for bearings and other mechanical movements, where oil or other coolants are required to be fed in a controlled manner without failure or interruption. Lubricating systems of this type may be classified as: (1) bottom wicks; (2) siphon wicks; (3) absorbent or pad feed and (4) top feed.
Siphon and pad feed lubricating or wick systems are the most widely used although bottom wick systems are generally considered the most efficient. They are entirely automatic, require no attention other than occasional cleaning, and allow 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 wick functions as a semi-controlled obstruction. The system to be used for any application is that which satisfies design and operating conditions. In all cases, a first consideration is the selection of proper materials to transport oil-based lubricants.
WHY FELT IS GOOD FOR OIL WICKING AND LUBRICATION
Felt is made up of a large number of capillaries which are formed between the fibers. 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 on the density of the felt and on the diameter of the fibers in the felt. High density felts 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 holding capacity 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 from compression. Accordingly, they have the ability to maintain their capillary structure under compression and other stresses.
Felt also has ideal surfaces for transferring oil to axles or other moving metal parts. It does not ravel; it can give up its oil either slowly or quickly depending on the type of felt and the application. Wool felt has particularly good resistance to heat, and will not glaze when properly applied. Lubrication by felt, or any other material, is stopped when its capillaries are blocked, either by dirt or by compression or by pinching, etc. In fact, this is one of the disadvantages of woven fabrics for lubrication. Besides their tendency to unravel easily, where the warp and filling yarns interlace the capillaries are pinched off and the flow of the liquid in the yarns is blocked or slowed down. Some felts are much better than others for the various types of lubrications; the performance of a felt depends on the type 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 lubrication purposes. Thus, as much care should be exercised in the selection of felt wick materials, as is required in the selection of suitable lubricants.
FACTORS TO CONSIDER IN SELECTING A FELT FOR LUBRICATION
Considerable laboratory test data is available which compares various grades of commercial felts with respect to wicking and liquid holding capacity using different oils and for various temperatures. In this article typical examples are presented with Spinesso 38 a light weight spindle oil at 20 degrees C or with an SAE-10 grade oil.
The wicking property of the felt is governed by the height to which the liquid is sucked up a vertically-held sample. This depends on the time Interval allowed for the liquid to creep up the sample, but generally the equilibrium (maximum height) occurs after say 2 to 4 hours and thereafter the wicking height remains at about that level. The maximum wicking height (value after 24 hours) was found to increase linearly with increasing felt density (see Figure 4). The dense felts of the types F-l, F-2, gave wicking heights approaching 20 cm (8") which could be attributed to the presence of very fine capillaries in the material. The actual wicking height was shown to increase by a further 5 cm (2") when the temperature was increased from 20 degrees C to 82 degrees C (68 degrees F to 180 degrees F). This is to be expected since the viscosity and the surface tension of the oil decreases with the increase in temperature. A similar change in wicking height, but in this case a decrease, was also observed when the light weight spindle 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 the height of rise of oil. A 1/4 felt and a 1" thick felt of the same style and density will lift 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 the amount of oil expressed as ml. of oil per cubic inch of felt, in each 1" section along the 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 of the wicks. The lower density felts, except the poorer quality, have a large volume of oil at the 5 cm (2") level, but the volume of oil decreases at this level. Thus, if a bottom wick felt only has to wick oil to a height of about 5 cm (2") and if a lot of oil is needed at that level, then the F-6 or F-7 felts, the F-10, 11 or 12, would be very 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 a small piece of felt is saturated with oil and installed in a sealed machine to lubricate a part for very long periods of time, or where the felt has to feed a steady film of oil some distance from a reservoir, the high-density, high quality felts such as F-l, F-2, or F-3 would seem advantageous. Therefore, in selecting felts for wicking, one would consider the height to which the oil must be lifted, the rate it must be wicked, and the amount of oil carried by the wick to a specified height.
In tests where the oil- filled felts were compressed under the same external pressure up to 5 Ibs sq/in. (0.35 kg sq/cm) the oil was easily squeezed out of the low density felts whereas the high density felts retained practically all of their original 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 of their oil after three hours of tumbling.