Application Note 24
Improved oil injection procedure for bearing retention
The purpose of this development is to extend the practical usefulness of a method invented in the late 1940's at SKF Sweden for injecting high pressure oil between press-fit bearing races and shafts to hydraulically enlarge the races and facilitate their easy positioning or removal. The SKF technique is known as 'oil-injection' and is extensively used today for mounting and dismounting large mechanical interfering friction fits. The improvement described herein raises the level of friction within a coupling in which the parts are first positioned by the traditional oil-injection method employing a friction reducing medium followed by injecting a friction enhancing medium.
The key benefit provided by the improved procedure is it provides a stronger friction coupling. The increase in friction extends the potential design tradeoffs between contact pressure, the size and weight of components and the material type and condition, which results in smaller, lighter, more easy to handled assemblies.
A further benefit is the friction enhancer reduces the difference between the static and dynamic coefficients of friction. In fact, under favourable conditions if the friction enhancing medium has displaced all traces of the oil-injection medium, then in the event of an overload and slip, residual friction enhancing medium will cause the coefficient of friction to rise and arrest the slip. Therefore, couplings can potentially be designed to arrest slip automatically without the need to reduce load - which is a valuable fail-safe feature. The number of overload cycles that a given coupling can withstand will depend upon the ability of the materials to withstand the accelerated wear, thus the harder/tougher the surfaces the longer they last! Where it is impractical to use heat-treated parts, work-hardened surfaces are a useful substitute. Patents are pending.
To inject an anti-lubricant a further reservoir is added to the pump and a switch is provided to switch between the lubricating and anti-lubricating sources, this is illustrated in the diagrams below. The pump may need to be modified to ensure it does not seize up due to the anti-lubricating medium.
Figs.1 to 4 below are schematic diagrams that illustrates the principle. Fig. 1 shows the a cross section end view of a shrink fit or press-fit friction coupling between a flange and shaft. The flange is held held gripped in place by friction due to elastic contraction of the flange, leaving residual tensile stress in the flange balanced by residual compressive stress in the shaft.
An oil way is provided through the flange through which high pressure oil is pumped at a pressure sufficient to hydraulically expand the flange and maintain a thin film of oil flowing between the flange and shaft.
A hand pump is shown with two fluid reservoirs that are coupled to the pump via a change over valve. The left hand reservoir holds a lubricating medium that is used for positioning and the right hand holds an anti-lubricating medium that after positioning displaces the lubricating medium and thereby provides enhanced friction grip. .
Figs. 2 to 4 are schematic diagrams showing a magnified view of an idealised asperity contact between the flange and shaft. The space around the contact in Fig 2 is shown filled with lubricating oil that remains held within the surface roughness after positioning with the oil injection method. The lubricant is represented with minus signs to indicate it has friction reducing properties.
After positioning the flange, the reservoirs are switched and anti-lubricant shown as plus signs implying an increase in friction is pumped in as shown in Fig. 3, the anti-lubricant progressively displaces the lubricant by flushing it out and hydraulically separating the touching asperities. When flushing is complete the anti-lubricant pressure is relaxed and the parts come into contact again trapping some anti-lubricant at the interface. As the film thickness of the trapped anti-lubricant thins it is believed that the material becomes chemically unstable, causing it to release single atoms of hydrogen. According to wear theory these would diffuse into the asperity causing localised lowering of the yield strength of the deforming asperities, causing them to deform more and thereby forming larger cold pressure welds as shown in Fig. 4. Upon cessation of plastic deformation within the asperities the hydrogen atoms are said to rapidly diffuse out and the materials recover their previous strength, but the resultant static friction is increased due to the enlarged cold pressure welds.
Fig. 5 shows a conventional tapered coupling being made with a lubricant (from left side reservoir) and Fig. 6 shows the tapered coupling being strengthen by introducing anti-lubricant (from the right side reservoir)
Fig. 7 shows a double pump arrangement for feeding an interfering-fit flange onto a parallel shaft. A tapered ring is placed between the flange and a tapered section at the end of the shaft, the tapered ring is progressively expanded by operating the upper pump injecting lubricating medium under the tapered collar as it is externally forced up the shaft lead in taper until the outer diameter of the tapered ring carrying the flange slightly exceeds the diameter of the parallel shaft. The lower pump is now activated pumping lubricating medium from the left side reservoir and the flange with its expanded bore is slid onto the parallel section of the shaft. Optionally the feed on ring can be removed before stopping the upper pump. .
Fig. 8 shows the bore in the flange being maintained in its expanded condition by pumping as it slides along the parallel shaft to its desired position, when the ring is positioned the hydraulic medium is switch from the left hand reservoir to the left hand reservoir and anti-lubricant is pumped in displace all the lubricant from the interface between the flange and shaft prior to ceasing pumping. Upon ceasing pumping the flange bore contracts (partially elastically relaxes) to bring the overlapping surfaces into enhanced friction contact.
Known friction enhancement materials work for most combinations of common metals but they work less well on copper and its alloys, such as bronze ships propellers, which is a common application for oil-injection.
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