THE PROBLEM OF GREASE LUBRICATION.
Page 9
If you've noticed an error in this article please click here to report it so we can fix it.
Why Grease so Olten Fails to Lubricate Motor Vehicle Parts which Have a Very Limited Amount of Movement. A Suggested Solution to the Problem.
GREASE lubrication by means of screw-down cups was not common here in engineering until the Stauffer lubricator came into use. The lubricators were sold and a special grease known as Stauffer grease was sold for use with them. At that time few people used any other grdase than that sold for the purpose, but with the introduction of the motor gearbox and differential box came a demand for a large quantity of grease. The quantity of grease necessary to fill up such boxes was so large that prices began to rule the matter.
• A very low quality.of grease was found quite, good enough for gearboxes and differential eases. In such instances, an addition of foreign matter of certain kinds did not materially affact the lubricating qualitie"S.of the grease.
Any foreign substance siich as those mentioned would, however, be detrimental to a grease which is used for lubricating a bearing where a thin film is introduced between the runningsurfaces. Much of the cheaper motor grease sold to-day contains a large proportion of foreign matter, usually of the nature of dried clay kfuller's earth). Such substances have no lubricating qualities, although they may not be harmful for the purpose of keeping the teeth of gears from coming Into mettllic contact with each other.
The solidified grease which will be found in most shackle pins that have been in use for some time is not the result of pressure, atmospheric action, or, the introduction of foreign matter in the shape of dust from the air, as has been suggested by recent correspondents to the Opinions page of The Commercial Motor. What really happens is a filtration of the grease ; that is to say, the particles of solid matter are arrested where the-aperture becomes contracted and the more fluid grease is pushed past them. It is exactly what takes place in a filter press. Such a substance as cocoa nibs ground to a 'fine powder is put into a canvas bag which is placed inside a perforated cylinder. A piston is brought to bear on the bag with great pressure. The result is that, the more fluid (cocoa butter) is .forced through the interstices of the canvas ; While the more solid matter remains behind.
It would appear that two different -Classes of .grease are required by the motor mechanic, one for gear and differential boxes, which may contain a certain percentage of foreign matter, and should, therefore, be low in price, and another which should be feast grease and entirely composed of lubricating matter. This could be used for lubricating bearings of such parts as shackles, steering joints,. etc. and would of necessity be a more expensive 'material.
A simple test for earthy matter can be carried out by any driver as follows :—Put some of the grease to be tested in a basin, cover it well with petrol. Place the basin in a pan of hot, water so aS to assist the grease to dissolve in the petrol. Stir frequently, until the grease is all dissolved. Allow the mixture to stand for some time and see if a sediment forms. If so, pour off petrol and add a lot more and allow to settle again after stirring well. When the deposit is free from the grease it can be put in an iron spoon and placed over a fire. There will be a little grease left, which will soon boil away leaving any earthy matter behind it. This earthy matter will be distinguished easily from any greasy matter by its ability to withstand great. heat, which no grease can do..
In such bearings as those of shackle pins, uni
versal joints and similar parts, where there is great pressure and little movement; there will always be difficulty in lubricating the surfaces in contact. Lubricant may be in the neighbourhood of the joint', but is often not at the place required. In a commercial vehicle, the pressure on the bearings of the shackle pins is over 1,000 pounds per square inch of projected area. Now, it is not conceivable that a pressure anywhere near equal to this is to be got from an tirclinary screw-clOwn lubricator. The lubricant, therefore, must be introduced between the surfaces in contact by the movement betwen the pin and its hole, as in any ordinary revolving bearing.
In a revolving bearing, the oil is introduced at the point where there is the least pressure, as in Fig. 1. This causes a wedge-shaped film to be carried to the point of the greatest pressure at B. Fig. 2 shows a bearing which, instead of revolving:, merely oscillates between the points A and B. The are of movement is slightly greater than the arc of contact which is between C and D. In this case the bearing may, at each oscillation, be expected to bring sufficient lubricant to the part where it is required. Fig. 3, however, shows a bearing in which the are of movement, E to F, is less than the arc of contact, C to D, and in such a. bearing the oil supply would. be quite insufficient and a dry place would occur. This should make clear to all the difficulty of lubricating a shackle pin, as the movement is sufficient to force the lubricant away and not sufficient to bring a fresh supply.
Efforts ha,ve• been made to reduce the movement of both shackle pins and universal joints with the hope of reducing the difficulty of efficient lubrication. These changes have, however, only resulted in increasing the difficulty. Oil has the }Dower of creep
ing, which grease has not. Capillary actionwill help oil to get to the place where it is required, but not so with grease. A shackle pin designed as in Fig. 4, where there is a slight bead• of oil above the surfaces in contact, has been found to answer very well, as it allows the oil to gravitate and creep to where it is required. Any rebound of the springs sets up a pumping action which is found.to be so beneficial in other cases for introducing oil.