TECHNICAL FIELD

The present invention relates to a lubrication system for pumping grease from a grease reservoir to one or more lubrication points.

BACKGROUND

Many types of machine installations exist which have several components requiring grease lubrication. One method of supplying grease to the various components is by means of a centralized lubrication system. In a paper mill, for example, the bearings which support the rolls may be supplied with grease in this manner. The lubrication system is designed to deliver grease at the required flow rate to the relevant number of lubrication points over the distances concerned. Often, this requires the use of high pressure gradients. Furthermore, a grease must be selected which can be pumped at the required flow rate, at the applicable temperature range.

The pumpability of commercial greases varies considerably. Grease is a semisolid substance that typically comprises base oil held in a thickener matrix such as a metal soap. Pumpability of the grease depends on several factors, such as the viscosity of the base oil, the temperature of the grease, the structure of the thickener material, the ratio of oil to thickener and the presence of additives. However, pumpability varies even for greases which have a very similar composition and consistency, meaning that pumpability must be tested before a grease can be used in a centralized lubrication system.

It can be the case, therefore, that a grease which would optimally lubricate a particular component is not selected because, for example, it exhibits unacceptable pumping behaviour at low temperature. Therefore, either a different grease must be selected, or the lubrication system must be adapted to enable the optimum grease to be used. For example, the system could be provided with heating means so that the grease becomes easier to pump. An automatic greasing system of this kind is disclosed in EP 0595097.

Needless to say, heating the grease greatly increases the energy requirement. Furthermore, if a particular grease is not normally selectable because it is prone to bleeding under pressure, heating the grease does not solve this problem. "Bleeding" is understood to be the separation of base oil from the thickener matrix under the influence of system pressure at a given pressure gradient. In effect, oil is pumped out of the grease, leaving behind solid matter that can lead to clogging.

Consequently, there is room for improvement in terms of providing a centralized lubrication system that is adapted to allow the use of a wide variety of greases, including greases which are prone to bleeding. DISCLOSURE OF THE INVENTION

The present invention resides in a lubrication system comprising a grease reservoir, a pump and a lubrication pipe for supplying grease from the reservoir to one or more lubrication points. According to the invention, the system further comprises an oil supply device for supplying oil from an oil reservoir to a pipe bore of the lubrication pipe, such that downstream of the device, grease is pumped through the lubrication pipe together with an oil film which forms on the pipe bore.

The oil film on the pipe bore has a significantly lower viscosity than bulk grease, meaning that the oil film acts as a wall slip layer. The slip effect is most dominant when the grease moves in an undeformed state, which is referred to as plug flow. During plug-flow, which occurs at low flow rates and low pressure gradients, the presence of a slip layer increases the flow rate of grease that can be pumped at a given pressure gradient in the pipe. In a grease lubrication system, however, the grease is generally pumped at much higher flow rates and at relatively high pressure gradients, meaning that the grease is sheared and moves in a deformed state. Grease is a shear-thinning substance and its viscosity decreases with increasing shear stress. The difference between the viscosity of the oil film and that of the deformed grease therefore becomes relatively smaller as flow rate (and shear stress) increases. In other words, the slip effect may play a negligible role in terms of improving pumpability at a given pressure gradient. The presence of a slip layer, nevertheless, has a beneficial effect on the lubrication system, especially when pumping stops.

The cessation of pumping causes the pressure gradient in the pipe to fall, but the gradient does not drop to zero instantaneously. On the contrary, grease can retain pressure very well and it may take several minutes per metre of pipe length before the grease relaxes. During this relaxation time, the grease is still under pressure, but is unable to move, or may only creep, when the pressure gradient falls below a minimum threshold for pumping. The risk, therefore, is that the remaining pressure gradient causes oil to move (bleed) out of the thickener matrix, leaving behind solid matter. As explained in the introduction, this can cause clogging.

In a lubrication system according to the invention, in which the pipe bore of the lubrication pipe has an oil film that acts as a slip layer, the risk of bleeding and clogging is significantly reduced. As explained, the slip layer enhances pumpability at low flow rates of grease, meaning that the grease is better able to keep moving when pump operation stops. As a result, the relaxation time is considerably shorter.

The present inventor has found that the reduction in relaxation time, relative to grease pumped with no slip layer, is proportional to oil film thickness and inversely proportional to the viscosity of the oil that forms the slip layer. Therefore, when a large reduction in relaxation time is desirable for a particular grease, an oil with a low viscosity is suitably selected and/or a relatively large oil film thickness is provided on the pipe bore. The presence of a suitable slip layer can reduce grease relaxation time to a fraction of a second per metre of pipe length, meaning that the grease does not remain under pressure long enough for bleeding to occur. Therefore, a grease which would not normally be selected because of the risk of clogging when pumping stops can be freely selected in a lubrication system according to the invention.

It should be noted that a wall slip layer is a phenomenon that may occur naturally when grease is pumped through a pipe. However, the mechanisms that lead to the formation of such a layer are not, as yet, understood and it is impossible to predict for a given type of grease the extent to which a slip layer will be present. In a lubrication system according to the invention, the presence of a slip layer on the pipe bore of the lubrication pipe is guaranteed.

Typically, the pipe bore has a diameter of between 5 and 30 mm and the oil supply device is adapted to provide an oil film in a thickness of between 0.01 and 100 microns. More typically, the oil film is provided in a thickness of between 0.1 and 20 microns. As mentioned, however, film thickness may be selected depending on the desired reduction in grease relaxation time.

The oil supply device may be any apparatus that is suitable for forming an oil film on the internal surface of a conduit through which a second substance flows. In one embodiment the lubrication pipe has an inlet section and an outlet section, whereby the inlet section is connected to the grease reservoir and the outlet section is connected to the oil supply device. Suitably, the inlet section extends into the outlet section over a certain overlapping length, whereby a small radial gap exists between the outside diameter of the inlet section and the inside diameter (bore) of the outlet section. At a closed end of the outlet section that surrounds the inlet section, an oil inlet is provided, which is in connection with an oil supply line. Thus, oil can be pumped from an oil reservoir to the inside diameter of the outlet section. The oil then flows through the radial gap, along the overlapping length, thereby creating an oil film on the bore of the outlet section. An arrangement of this kind is described in US 3826279. Several other arrangements have been disclosed for forming a film of a first liquid on the internal surface of a conduit through which a second liquid flows, as discussed for example in US 5361797, US 2821205, US 3993097 and US 3502103. The arrangements in question are intended for the lubrication of transportation pipelines, which carry viscous substances like crude oil or slurry in a plug-flow regime. The same basic principles may be applied to a pipe of much smaller diameter that carries grease.

Thus, a grease lubrication system according to the invention may comprise an oil supply device which has a dedicated pump for supplying oil from an oil reservoir to the bore of the lubrication pipe. In a preferred embodiment, the oil is pumped using the same pump which pumps the grease.

Suitably, the lubrication pipe comprises an auxiliary channel that branches off from the lubrication pipe at a first location and rejoins the lubrication pipe at a second location, downstream from the first location. The auxiliary channel contains oil and acts as the oil reservoir. Therefore, when grease is pumped from the grease reservoir, a quantity of grease enters the auxiliary channel at one end, causing oil to be forced out at the other end. Suitably, the oil inlet is provided at the second location where the auxiliary channel rejoins the lubrication pipe.

A variety of oils may be used for forming the oil film. In one embodiment, the "added" oil is the same as the base oil of the grease, which guarantees compatibility. In an advantageous further development, a different oil is selected in order to provide a desired functionality. For example, the added oil may have a considerably lower viscosity at low temperature than that of the grease base oil. As mentioned, the use of a low-viscosity oil leads to a shorter grease relaxation time. An additional advantage of this further development is explained in the following.

In a grease lubrication system according to the invention, a small amount of oil is delivered to the one or more lubrication points, simultaneously with the grease. The oil may therefore be selected to provide auxiliary lubrication in conditions where the grease is not optimally able to form a lubricant film between tribological surfaces of components. Consider, for example, a truck with several bearings that are lubricated with the aid of a lubrication system according to the invention. Let us assume that the grease is a standard bearing grease with a metal soap thickener and a mineral base oil, that the oil in the oil reservoir is an ester oil and that the system must operate at a start-up temperature of -25 degrees Celsius. At this temperature, the viscosity of the grease base oil may be too high to form a satisfactory oil film. The ester oil, by contrast, has an optimal viscosity at this temperature and is able to form a good oil film. Although only a very small amount of ester oil is delivered in relation to the amount of grease, the ester oil film can provide the bearing lubrication for a short period time that is sufficient for the bearing operating temperature to rise to a level at which the grease base oil can form a satisfactory oil film. As a result, the bearings undergo less wear and friction during start-up, ultimately leading to improved bearing life.

Thus, a grease lubrication system according to invention has several advantages. These and other advantages will become apparent from the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 a is a schematic diagram of a lubrication system according to the invention comprising a grease pump and an oil supply device.

Fig. 1 b shows a detail of the oil supply device from Fig. 1 a

Fig. 2 is a flow curve showing the rheology of an example of a grease that may be pumped by the lubrication system;

Fig. 3 is a graph of flow rate against pressure gradient for the grease of Fig.

2, based on an oil film with four different thicknesses, the oil being the same as the base oil of the grease;

Fig. 4 is a graph of flow rate against pressure gradient for the grease of Fig.

2, based on an oil film with four different thicknesses, the oil being an oil with a lower viscosity than the base oil of the grease. DETAILED DESCRIPTION

A block diagram of a grease lubrication system according to the invention is depicted in Figure 1. The system 100 comprises a grease reservoir 1 10, which houses a grease lubricant (not shown) and a pump unit 1 15, connected via a main lubrication pipe 120 to a plurality of series-connected lubricant distributors. The lubricant distributors 130, 140 perform the distribution of metred quantities of the grease lubricant to a number of lubrication points 131 , 132, 133, 141 , 142, 143. In the system depicted in Figure 1 , the lubrication points deliver grease to rolling element bearings which support a row of axles on a rail vehicle. Alternatively, the system may comprise progressive feeders for supplying metred quantities of grease to the lubrications points 131 , 132, 133, 141 , 142, 143. The design of centralized lubrication systems for delivering grease lubricant is well known in the art and is not further discussed here.

In a grease lubrication system according to the invention, the system further comprises an oil supply device 150. Suitably, the main lubrication pipe 120 has an inlet section 122 that is connected to the grease reservoir 1 10. An outlet section 125 of the main pipe is connected to the oil supply device 150. The function of the oil supply device is to provide an oil film on a pipe bore of the outlet section 125, such that downstream of the device 150, both grease and a layer of oil are pumped simultaneously.

An example of a preferred oil supply device is shown in more detail in Figure 1 b. The device 150 comprises an oil reservoir 152 that is connected to an oil inlet 155 on the outlet section 125 of the main pipe. The oil reservoir is formed by an oil 153 contained in a channel 157 that branches off from the inlet section 122 of the main pipe. Therefore, when grease is pumped from the grease reservoir 1 10, some grease enters the channel 157 at one end, which forces oil out of the other end, the other end being connected to the oil inlet 155. Preferably, the channel 157 is detachable from the inlet and outlet sections of the lubrication pipe, so that it can be easily refilled with oil. In the depicted example, the outlet section 125 of the main pipe has a collar part 127 that fits around an end part 123 of the inlet section 122, such that the collar part 127 and the end part 123 overlap each other by a certain length. The oil inlet 155 is provided on the collar part 127, preferably towards a closed end of the collar part. Further, between the outside diameter of the end part 123 and the inside diameter of the collar part 127 a slight radial gap exists. When oil is pumped from the oil reservoir 152 into the collar part 127 via the oil inlet 155, the oil will flow through the radial gap, along most of the overlapping length, such that the bore of the outlet section 125 is provided with an annular oil film. To promote the formation of an annular film, the bore of the collar part 127 may be provided with a spiral groove.

The advantage of providing an oil film on the pipe bore of the outlet section 125 will now be explained. Let us consider an example in which the grease in the grease reservoir 1 10 is a lithium soap grease with a mineral base oil having a consistency of NLGI grade II. The grease has been prescribed by the bearing manufacturer as the optimum grease lubricant for the axle bearings. In this example, the oil 153 in the oil reservoir is the same as the grease base oil and has a viscosity of 12.7 Pa.s at -10 ^° C. Further, let us assume that the outlet section 125 of the lubrication pipe 120 has a bore diameter of 8 mm, that the lubrication system is at a temperature of -10 ^° C and that the grease is being pumped at a pressure gradient of 13 bar/m. When pumping stops, the system retains a pressure gradient of approximately 5 bar/m (500000 Pa/m), which slowly dissipates due to the high viscosity of the grease at low shear stress. The retained pressure gradient, Δρ/Δχ, is estimated from the grease yield stress, xyieid, and the pipe diameter, d, according to the following relationship: Δρ/Δχ = xyieid * 4 / d

The yield stress, xyieid, is obtainable from a flow curve for the grease, which is shown in Figure 2. The y-axis 210 represents shear viscosity (Pa.s) and the x-axis 220 represents shear stress (Pa). The grease has a low-shear viscosity limit, η _0, of approximately 1 .5.10 ⁵ Pa.s. As shear stress increases, the shear viscosity gradually decreases until the yield stress, xyieid, is reached. The yield stress occurs in a region where reversible deformation gives way to irreversible deformation and flow starts. As may be seen from Figure 2, the grease has a yield stress of approximately 1000 Pa.

When no oil film is present on the bore of the lubrication pipe, the time taken to dissipate a retained pressure gradient of 5 bar/m may be in the order of a few hours at a temperature of -10 ^° C and is proportional to the flow rate of the grease when pumping stops. The time to dissipate the pressure gradient is also known as the grease relaxation time. During this time, the grease moves very slowly, but there is a risk that the base oil will move faster. In other words, oil may bleed from the grease, thereby increasing the concentration of soap fibres. Over time, the fibres will lead to clogging, especially of the grease distribution units 130, 140 which are connected to the main lubrication pipe 120. For this reason, the grease in the grease reservoir 1 10 is deemed unsuitable for use in conventional centralized lubrication systems.

Now let us assume that the oil supply device 150 in the system of Figures 1 a and 1 b is configured to provide an oil film on the bore of the lubrication pipe, the oil film having a thickness of 10 pm. The present inventor has derived a formula for calculating an upper limit of an effective viscosity for grease, n _eff , which describes the grease flow at low pressure gradients, i.e. assuming that only the oil layer is sheared and that the grease moves as a plug. The effective viscosity, may be expressed as follows:

Heff = (HON * r) / (4 * h _oi i) [Equation 1 ] where

H _O N is the viscosity of the oil 153 at the system temperature (Pa.s)

r is the radius of the pipe bore of the lubrication pipe (m)

hoN is the thickness of the oil film formed on the pipe bore (m).

Therefore, when pumping has stopped, the effective viscosity of the grease in the outlet section 125 of the lubrication device is (12.7 * 0.004)/ (4*1 ^"5 ) = 1 .3- 10 ³ Pa.s. As a result, the system relaxation time is reduced by a factor of approximately 100. The hundredfold reduction in relaxation time means that there is practically no risk of bleeding, and the grease may be safely pumped in the lubrication system according to the invention.

The reduction factor is given by the ratio of the low-shear viscosity limit of the grease (see Figure 2) to the effective viscosity, i.e. η ₀ / n, _e ff (1 .5· 10 ⁵ /1 .3- 10 ³ ). Based on Equation 1 , the reduction factor, R, may also be expressed as:

R = Ho (4 * hoi,)/ (η _οΗ * r)

Therefore, if a greater reduction in relaxation time is needed, film thickness may be increased and/or an oil with a lower viscosity can be used.

The effect of oil film thickness and oil viscosity may be seen from the graphs in Figures 3 and 4 respectively. Equation 1 was used to calculate flow rate at increasing pressure gradient for the grease of Figure 2. The calculations were performed for varying oil film thickness, based on two different oils with different viscosities. Again, the pipe diameter is taken to be 8 mm and the system is assumed to be at a temperature of -10 degrees Celsius. In Figure 3, the calculations are based on the same oil as described above (having a viscosity of 12.7 Pa.s at -10 ^° ). In Figure 4, the calculations are based on an oil with a viscosity of 0.12 Pa.s at -10 ^° C. The y-axis 310 in Figure 3 represents grease flow rate (cm ³ /min) and the x-axis 320 represents pressure gradient in the lubrication pipe (bar/m). Four curves 331 , 332, 333, 334 are shown, whereby the first curve 331 gives the calculated flow rate based on a film thickness of 0 pm. The second curve 332, the third curve 333 and the fourth curve 334 give the calculated flow rate based on a film thickness of 1.0, 10 and 100 pm respectively.

The system retains a pressure gradient of 5 bar/m when pumping stops, as explained above. At this pressure gradient, the grease can be moved at a flow rate of approx 0.06 cm ³ /min when no oil film is present (first curve 331 ). When the oil film thickness is 1 .0 pm (second curve 332)), 10 pm (third curve) and 100 pm (fourth curve), the flow rate increases to approx. 0.2 cm ³ /min, 2 cm ³ /min and 20 cm ³ /min respectively. Thus, flow rate increases in proportion to oil film thickness at the given pressure gradient of 5 bar/m. The pressure gradient does not remain constant, of course, when pumping stops. The pressure gradient falls to zero during the relaxation time, as explained above. Therefore, as oil film thickness increases, the pressure gradient falls to zero more quickly (relaxation time shortens) because relatively, the grease can be moved more quickly. The curves in Figure 3 also show how the effect of the oil film for the given oil is most dominant at low pressure gradients. Let us again assume that before pumping stopped, the grease was being pumped with a pressure gradient of 13 bar/m. It can be seen that at this pressure gradient, an oil film with a thickness of 1 micron (second curve 332) does not increase the flow rate of grease that can be pumped, relative to zero film thickness (first curve 331 ). The increase is negligible when film thickness is 10 microns (third curve 233b). When the film thickness is 100 microns, the increase in flow rate is significant, but it may not always be desirable to provide the oil film in such a relatively large film thickness. An improvement in pumpability - i.e. an increase in the flow rate of grease that can be delivered at a given pressure gradient - can be achieved at relatively high pressure gradients, and at relatively low film thickness, when the oil that forms the oil film has a low viscosity. This may be seen from Figure 4, in which, as mentioned, the oil has a viscosity of 0.12 Pa.s at . -10 ^° C.

In Figure 4, the y-axis 410 represents grease flow rate (cm ³ /min) and the x-axis 420 represents pressure gradient in the lubrication pipe (bar/m). Again, four curves 431 , 432, 433, 434 are shown, whereby the first curve 431 gives the calculated flow rate based on a film thickness of 0 pm. The second curve 432, the third curve 433 and the fourth curve 434 give the calculated flow rate based on a film thickness of 1.0, 10 and 100 pm respectively.

At a pressure gradient of 13 bar/m, the grease can be pumped at a rate of around 20 cm ³ /min when oil film thickness is zero (first curve 431 ). The presence of an oil film with a thickness of only 1 micron (second curve 432) already produces a significant increase in flow rate at this pressure gradient. As may be seen further from the third and fourth curves 433, 434, the increase in flow rate is proportional to film thickness.

Comparing Figure 4 with Figure 3 also shows the effect of an oil film with relatively lower viscosity when pumping stops. At a retained pressure gradient of 5 bar/m and a film thickness of 10 microns (third curve 433), the flow rate of grease is approx. 100 cm ³ /min. For the oil of Figure 3, a film of the same thickness resulted in a flow rate of 2 cm ³ /min. Thus, the effect of the oil film becomes more dominant when a low viscous oil of 0.12 Pa.s at -10 ^° C is selected. In a lubrication system according to the invention, the selection of a low viscous oil for forming the oil film on the pipe bore of the lubrication pipe has several benefits. A relatively thin film (with a thickness in the order of 1 micron) can greatly reduce the grease relaxation time and improve pumpability at the system's operating pressure gradient. Suitably, the viscosity of the oil film at the system temperature is at least 10 times lower than that of the grease base oil. The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the subsequent patent claims.