GB2136918A - Pump impeller arrangement - Google Patents

Abstract

In order to minimise obstruction of the eye and to enable assembly and dismantling by hand, there is provided a centrifugal pump impeller 5 which is threaded directly on to a stub shaft rather than being secured by nuts. It is desired to provide a stop for the motion of the impeller and this is obtained by a taper 8 matching a taper on the stub shaft. Cleanability of the thread form 7 is enhanced by providing a restricted return path for cleaning fluid. Figure 5 shows an axial slot 11 or bore 12 leading to the junction of the thread and taper for this purpose. Other, alternative return paths are shown in other Figures. <IMAGE>

Description

SPECIFICATION Pump impeller arrangement This invention relates to arrangements for the retention of pump impellers on drive shafts.

In designing pumps and other equipment for use in the food processing industry, great care has to be taken to ensure that crevices and dead areas where food particles may lodge, and serve as centres of infection, are kept to their minimum. It is also important to ensure thorough cleanability of all components so that any centres of infection so formed do not survive a cleaning cycle.

In many countries strict standards on materials and mechanical details, such as thread forms, are laid down to ensure hygienic operation.

In a centrifugal pump, an impeller is normally mounted on drive shafts and retained thereon by means of an arrangement of nuts. The presence of the nuts may interfere with the performance of the impeller by obstructing the "eye" or axial region of the impeller. Also, tools are needed to attach or remove the impeller from the shaft in view of the need to apply a substanial tightening or loosening torque to the nut over its restricted diameter.

If an impeller is threaded directly on to a shaft, there is normally some sort of taper to limit the movement of the impeller along the shaft and it has been found in practice that the thread forms in such an arrangment are not fully cleanable.

It has now been appreciated that this lack of cleanability is due to the fact that the thread form is a dead end passage so that cleaning fluid cannot flow through it, so that particles and deposits dislodged in the cleaning cycle are not easily removed.

In accordance with the present invention, there is provided a pump impeller arrangement comprising a drive shaft having a tapered zone leading to a hygienic thread form, an impeller having a matching female tapered form and a thread to receive the shaft thread form whereby the impeller may be directly threaded onto the shaft until the tapers lock, and a restricted passageway formed to vent the inner end of the mating threads whereby a flow of cleaning fluid penetrating the thread may be maintained during a cleaning operation.

By providing for a limited exit path from the bottom end of the thread, the cleaning fluids are provided with an exit from the bottom end of the thread and therefore are free to flow through and carry away any dislodged material.

The invention will be further described with reference to the accompanying drawings, in which: Figure 1 is an elevation of a stub shaft to receive an impeller for a centrifugal pump; Figure 2 is an enlarged section view of the free end of the stub shaft shown in Figure 1; Figure 3 is an elevation of an impeller; Figure 4 is an end elevation of the impeller of Figure 3 showing two alternative forms of venting; and Figures 5, 6, 7 and 8 are each sections on the line A-A of Figure 4 showing alternative forms of venting the thread form.

Turning first to Figure 1 and 2, a stub shaft is shown as having a hollow body portion 1 for attachment to the output shaft of the drive motor, and a projecting shaft 2 having a tapered zone 3, a threaded zone 4 a pair of chordal slots 5 and a diametral hole 6. The thread is shown as being of a coarse acme-type which is specified in certain standards so as to provide an adequate, although restricted, flow for cleaning purposes.

Turning now to Figures 3 and 4, an impeller is shown as having four blades 5, the form of which forms no part of the present invention, and a central hub or boss portion 6 which has an axial aperture 7 provided with a thread 4 (shown diagramatically only) matching the thread form 4.

As may be seen in Figures 5 to 8, the hub or boss portion 6 is provided, as an extension of the aperture 7, with a tapered zone 8 to match the tapered zone 3 on the stub shaft to limit the movement of the impeller along the stub shaft.

It will be appreciated that once the impeller is threaded onto the stub shaft, and the tapers are brought into tight mating, the restricted flow passage formed by the matching thread forms is effectively "dead ended" and in order to provide for in place cleanability of the thread form it is proposed, in accordance with the present invention, to provide venting of the thread form to allow through flow.

Figures 4 and 5 show one form of such venting as being provided by an axially extending slot 11 formed similarly to a key way and cut in either the hub of the impeller, the end of the stub shaft, or both. As actually illustrated, the slot 11 is cut first of all by drilling a hole axially in the hub portion 6 so as to break into the major diameter of the thread, and then extending this hole laterally by using a suitable burr.

An alternative arrangement is shown in Figure 6, and dotted in Figure 4, and consists of a drilled passage 1 2 drilled from the outside of the boss 6 into the thread form at the junction with the taper.

Figure 7 shows a still further alternative, which is not visible in Figure 4. This consists of a helical groove 14 cut into either or both of the tapered faces and starting adjacent the bottom of the thread.

Figure 8 shows a further alternative consisting of one or more straight grooves 1 5 extending generally parallel to the stub shaft, and again formed in the taper 3 and/or 8.

It will be seen that by adopting any one or more of these arrangements, a restricted passageway is provided to ensure that there is a complete, rather than dead ended flow path through the threads, and this has been found greatly to facilitiate cleaning without the necessity to dismantle the impeller from the stub shaft.

It will be appreciated that the thread forms are so handed that the impeller tends to tighten in the direction of rotation when under drive.

For safety's sake, a security spring clip is housed in the slots 5 and/or hole 6 to prevent the impeller from slackening too far upon inadvertent reversal.

It will also be appreciated that since the impeller constitutes its own nut, the obstruction at the eye of the impeller is kept to a minimum, and also the full diameter of the impeller may be used to apply torque for removal or assembly. This normally means that the impeller can be rernoved or attached by hand and without the need for additional tools.

Various modifications may be made within the scope of the invention.

Claims (5)

1. A pump impeller arrangement comprising a drive shaft having a tapered zone leading to a hygienic thread form, an impeller having a matching female tapered form and a thread to receive the shaft thread form whereby the impeller may be directly threaded onto the shaft until the tapers lock, and a restricted passageway formed to vent the inner end of the mating threads whereby a flow of cleaning fluid penetrating the thread may be maintained during a cleaning operation.

2. A pump impeller arrangement as claimed in claim 1, in which the restricted passageway is in the form of a generally axial slot adjacent to and communicating with the thread forms and formed in the shaft and/or the impeller.

3. A pump impeller arrangement as claimed in claim 1, in which the restricted passageway is in the form of a drilling extending through a hub of the impeller from the end of the taper.

4. A pump impeller arrangement as claimed in claim 1, in which the restricted passageway is in the form of a leakage path through the taper.

5. A pump impeller arrangement substantially as hereinbefore described with reference to the accompanying drawings.