VACUUM PUMP

This invention relates to the field of vacuum pumps and in particular to the field of liquid lubricant distribution within a gear box of a vacuum pump.

Vacuum pumps typically comprise a pair of contra-rotating shafts each having a rotor mounted thereon. These rotors act in cooperation with a housing component of the vacuum pump to convey fluid from an inlet of the vacuum pump to an outlet of the vacuum pump. To ensure that the shafts rotate in a synchronous fashion, a pair of accurate timing gears is used to drive one shaft in synchronicity with the other. The shafts are mounted within the vacuum pump via bearings. These timing gears and bearings are located within a gear box of the vacuum pump and need to be lubricated during operation. This may be achieved by providing an internal pump that can deliver lubricant directly to the gears and bearings through one or more nozzles. A device of this type is typically complex, may be inclined to block and may have high power consumption. Consequently it can be expensive and unreliable.

An alternative approach is to raise a liquid lubricant level in a reservoir located within the gear box so that part of a component to be lubricated is immersed within the liquid lubricant during rotation thereof. However, this means that significant quantities of liquid lubricant need to be stored in the gear box. This can lead to excessive wetting of the components, causing a significant increase in the power requirement for the vacuum pump. In addition, excessive liquid lubricant presents a challenge to sealing components of the device, which components therefore need to be of increased complexity in order to prevent transmission of the lubricant into a swept volume of the vacuum pump.

In order to overcome these disadvantages an oil thrower may be used to distribute the lubricant to the components that require lubrication. An oil thrower generally comprises a disc mounted on a common shaft of these components and of larger diameter than these components so that the disc is partially immersed within a liquid lubricant contained within a reservoir located below the shaft, with the components requiring lubrication being located above the level of the lubricant. As the disc rotates it collects lubricant from the reservoir and throws it upwards towards the components.

The quantity of lubricant delivered by an oil thrower of this type is highly dependent on the extent to which the disc is immersed within the lubricant. If the lubricant level is too high, the resistance experienced by the thrower and therefore the power consumption to rotate the thrower is increased, excessive wetting occurs and the aforementioned disadvantages arise. If the lubricant level is too low then inadequate wetting occurs and the components may seize, leading to reliability problems for the pump. In practice, the operational extremes of the lubricant level are close to each other, requiring the lubricant level to be monitored and controlled very closely. This results in short service intervals and an excessive amount of human intervention.

In order to extend the service life of a vacuum pump it is desirable to increase the quantity of liquid lubricant that is included in the gear box prior to delivery of the vacuum pump to the user so that the lubricant level does not become too low during operation of the vacuum pump. It is also desirable to minimise the impact of this increased quantity of liquid lubricant on power consumption and the wetting of the components to be lubricated.

The present invention provides a vacuum pump comprising a shaft, a gear box through which the shaft passes, the gear box comprising a primary lubricant reservoir, a lubricant thrower extending into the primary reservoir

and mounted on the shaft for rotation therewith, and a secondary lubricant reservoir for receiving lubricant thrown from the primary reservoir by the thrower, the secondary reservoir comprising an outlet from which lubricant is returned to the primary reservoir.

By providing such a secondary reservoir an increased quantity of lubricant can be included within the primary lubricant reservoir of the gear box prior to delivery of the vacuum pump to the user. Upon starting the pump, the additional lubricant is thrown from the primary reservoir to the secondary reservoir.

During subsequent operation of the vacuum pump, this additional lubricant is gradually returned to the primary reservoir so that a sufficient level of lubricant is maintained therein for extended operation of the vacuum pump. As the additional lubricant is returned to the primary reservoir continued action of the thrower rapidly disperses this additional liquid about the gear box, with some of the thrown lubricant being received and stored by the secondary reservoir. Thus the service life of the pump can be extended, as the level of lubricant in the primary reservoir effectively remains constant over the service life of the pump. Any increased power consumption required during the initial operation of the pump, due to the increased drag experienced by the interaction between the lubricant thrower and the additional lubricant, is masked by the normal high power consumption experienced by the vacuum pump upon initial operation.

The secondary reservoir may comprise means for controlling the rate of return of lubricant to the primary reservoir. This means may be located within the secondary reservoir, preferably over or within the outlet of the secondary reservoir. The means for controlling the rate of return of lubricant may be a plug type device which is inserted into the outlet or, a cover that is located over the outlet. Alternatively, or additionally, the means for controlling

the rate of lubricant return may be located in a flow path extending from the outlet of the second reservoir to the primary reservoir. The flow path may be represented by a conduit and the means for controlling the rate of lubricant return when located in this conduit may be a plug type device as already mentioned above or may take the form of convolutions in the conduit and/or variations in the internal dimension of the conduit so that the flow therethrough is restricted.

Said means may be formed from porous material, for example, a sintered metal, a sintered plastic and/or a felt pad.

The gear box may comprise a rolling bearing for supporting the shaft, and the outlet of the second reservoir may be in fluid communication with a race of the bearing. In so doing the bearing lubricant is filtered to remove any debris prior to lubrication of the bearing, thus reducing wear experienced by the bearing.

The present invention is described below in greater detail, by way of example only, with reference to the accompanying figures in which:

Figure 1 illustrates a vertical partial cross section through a gear box of a vacuum pump; and

Figure 2 illustrates an isometric view of part of the gear box of Figure 1.

Some components of a gear box 10 of a vacuum pump are illustrated in Figures 1 and 2. A cavity which acts as a primary reservoir 20 for a liquid lubricant, preferably oil, is formed by a housing component 30 of the vacuum pump. Two shafts 40, 45 protrude into the gear box 10 and extend into a swept volume (not shown) of the vacuum pump. Bearings are used to support the shafts within the gear box 10. One bearing 48 is illustrated in Figure 1 supporting one of the shafts 45.

lntermeshing rotor components are mounted on the shafts within the swept volume of the vacuum pump. Upon rotation of the shafts, fluid is conveyed through the vacuum pump from an inlet to an outlet thereof by virtue of the interaction between the rotor components and a stator component which is also formed within the housing 30 of the vacuum pump. A timing gear 50, 55 is mounted upon each respective shaft for transmitting torque from one shaft to another whilst maintaining synchronicity between the rotor components. These timing gears 50, 55 are also located within the gear box 10.

An oil thrower 60 for distributing oil about the gear box 10 during operation of the vacuum pump is mounted upon one of the shafts 45. The oil thrower 60, preferably a disc, is sized such that its radial extreme dips into liquid lubricant, typically oil, contained within the primary reservoir 20 during normal operation of the vacuum pump. The optimum level of oil is determined by the position of components, such as gears and bearings present in the gear box 10. The maximum level is governed by gear, shaft seal and bearing diameters, and should be lower than these components so that no part of the components runs through the oil. The lower level is governed by the diameter of the thrower 60, the tip of which must run through the oil whilst not clashing with the other shaft 40. As the shaft 45 rotates the oil thrower 60 also rotates and the radial extreme of the thrower is drawn through the surface of the oil located in the primary reservoir 20. The oil is collected by the thrower and is thrown towards the internal surfaces of the gear box 10, thereby distributing oil about the gear box 10.

The extent of the immersion of the oil thrower 60 into the oil affects the power consumption of the vacuum pump. If the extent is too great then the drag experienced by the oil thrower 60 contributes significantly to an elevated power consumption requirement. It is, therefore, desirable to keep the oil in the primary reservoir 20 of the vacuum pump low, i.e. in contact with only the

radial extent of the thrower 60, during normal operation of the vacuum pump in order to keep the power consumption down.

However, as discussed above, the quantity of oil that is present within the vacuum pump as supplied to the user is a limiting factor on the service life of the vacuum pump, as the vacuum pump needs to be taken out of use once there is insufficient oil remaining within the gear box 10 to provide a sufficient level of lubrication to the bearings 48. There is, therefore, a conflicting desire to supply the vacuum pump with a significant quantity of oil so that the service life of the vacuum pump can be extended.

A further disadvantage of supplying the vacuum pump with a significant quantity of oil is that the gears 50, 55 can become engulfed in oil such that an increased quantity of oil is inclined to pass along the shafts 40, 45 towards the swept volume of the vacuum pump. A sealing mechanism 75 is provided between the gear box 10 and the swept volume for inhibiting passage of oil into the swept volume. This sealing mechanism 75 is designed to inhibit passage of oil in vapour form. If, however, an increased level of oil is passed along the respective shafts 40, 45 the sealing mechanism will experience excessive wetting which can affect detrimentally the performance of the sealing mechanism 75, resulting in some oil being passed into the swept volume of the vacuum pump. This, in turn, can affect detrimentally the pumping performance of the vacuum pump, due to contamination of the fluid passing therethrough by the oil.

In order to accommodate an increased quantity of oil within the gear box 10 without experiencing the disadvantages discussed above, a secondary reservoir 70 is provided. The secondary reservoir 70 is situated within the gear box 10 at a location elevated above the primary reservoir 20 as illustrated in Figures 1 and 2.

In operation, the shafts 40, 45 rotate thus causing rotation of the oil thrower 60 which, in turn, causes oil to be thrown about the gear box 10. Some of the oil is transferred directly from the primary reservoir 20 to the secondary reservoir 70, and some is brought into contact with an internal surface of the gear box 10, from where it drains into the secondary reservoir 70.

Oil levels within the primary reservoir 20 are illustrated in Figure 1. The dashed line A indicates an appropriate level for normal operation of the pump which represents a level at which the impact on power consumption is minimised. The solid line B represents a greater quantity of oil with which it is desirable to include in the vacuum pump as supplied to the user. At initial start up of the vacuum pump, there are many factors which raise the power consumption of the vacuum pump and so the increased power consumption caused by the increased immersion of the oil thrower 60 within the oil is less significant. The action of the oil thrower 60 causes the secondary reservoir 70 to be filled (represented by dashed line A') thus reducing the level of oil in the primary reservoir 20 (to the dashed line A).

Operation of the vacuum pump continues with this lower level of oil in the primary reservoir 20 so that the power consumption associated with the oil thrower is kept to a minimum. The gears 50, 55 are less engulfed in oil so that the performance of the sealing system 75 is maximised and transfer of oil into the swept volume is minimised if not entirely prevented.

The secondary reservoir 70 comprises an outlet connected to a conduit 77 which ultimately feeds oil back into the primary reservoir 20. In a preferred embodiment, a restrictor or other means is provided to control the flow rate of oil returning form the secondary reservoir 70 to the primary reservoir 20. The restrictor may be provided by a convoluted and/or a small bore configuration of the conduit 77. Alternatively, as in the example

illustrated in Figures 1 and 2, a plug device 80 may be located over or within the outlet of secondary reservoir 70. The plug device 80 is preferably made of a porous material so that the flow rate of oil that passes therethrough is reduced. This effectively controls the quantity of oil that returns to the primary reservoir 20 and hence the rate of increase of the level of oil in the primary reservoir 20 which, in turn, governs how much oi! is distributed to the components of the gear box 10 by the oil thrower 60. The porous material may be a sintered metal or a sintered plastics material or, alternatively it may be provided by one or more felt pads. The use of such a plug device 80 has the added benefit of acting as an oil filter, retaining any debris contained within the oil passing therethrough.

The flow path of the oil from the secondary reservoir 70 to the primary reservoir 20 may be routed via one or more components that require lubrication, such as the bearing 48. This is especially advantageous where a plug device 80 is implemented, as the oil supplied to the bearing 48 is cleaner than it would be if it were supplied directly from the primary reservoir 20. Consequently, the life of the bearing 48 may be extended as wear caused by such debris is minimised.