Single-shaft eccentric screw pump

The present invention relates to the structure for connecting the flexible rod, which is mainly metallic, to the drive shaft of the drive unit or the rotor in a single-shaft eccentric screw pump. Specifically, the invention relates to a single-shaft eccentric screw pump having a pump body and a flexible rod. The pump body includes an externally double-threaded rotor elliptic in section and a stator having an internally triple-threaded hole in the shape of a substantially equilateral triangle in aperture section. The pump rotor engages with the triple-threaded hole. The flexible rod couples the pump rotor and the drive shaft of a drive unit together.

In general, a single-shaft eccentric screw pump has a pump body including an internally double-threaded stator and an externally single-threaded rotor. The pump stator has a longitudinally tapped hole elliptic in section. The pump rotor is circular in section, and its thread pitch is ½ of the thread pitch of the tapped hole. The pump rotor is in slidably rotatable engagement with the tapped hole and creates a pumping action by revolving eccentrically around the axis of revolution in the pump stator, rotating on its own axis while revolving in the opposite direction around the axis of revolution. The pump rotor and the drive shaft of a drive unit are coupled by a coupling rod. In general, a means is adopted for allowing the pump rotor to revolve eccentrically with a universal joint interposed between the rotor and the coupling rod, or with a flexible and relatively long coupling rod used between the rotor and the drive shaft.

If the coupling rod is a metallic flexible rod, it is proposed or adopted that the rod be or is connected to the pump rotor or the drive shaft by means of flanges (refer to the connection between the flexible rod 6 and drive shaft 42 in FIG. 4). One end of the flexible rod is tapered and has a hole tapped in it. A connecting casing has an outward flange formed around it and a tapered bore formed in it, into which the tapered end of the flexible rod is fitted. A bolt extends through the end of the connecting casing, is screwed into the tapped hole of the flexible rod, and is tightened to fix the rod and the casing together. A joint case has an inward flange formed at its front end and a tightening ring formed at its rear end. The inward flange engages with the outward flange of the connecting casing. The pump rotor or the drive shaft has a step formed at one of its ends. The tightening ring engages with the end step, supports it, and is tightened to fix the connecting casing and the pump rotor or the drive shaft together.

A single-shaft eccentric screw pump disclosed as a prior art has a rotor and a drive shaft, which are coupled together by a flexible rod made of engineering plastic. One end of the flexible rod may be tapered. One end of the drive shaft may have a tapered bore. The tapered end may be fitted into the tapered bore and bonded to it with an adhesive. Otherwise, one end of the flexible rod and one end of the drive shaft may have a key and a key groove, which connect the ends together. For this art, reference may be made to JP H9-264264 A (paragraphs 0013, 0016, and 0018, and FIGS. 8 and 9).

The conventional single-shaft eccentric screw pump in which the rotor or the drive shaft is connected to the flexible rod by means of flanges has the following problems.

Because the pump is fitted with a flanged connecting casing and a flanged joint case, the number of parts of the pump is large, and the pump is complicated in structure.

In order to inspect and replace the mechanical seal etc. of the pump, it is necessary to remove the bearing in the bearing casing of the pump. This results in complicated disassembly and assembly.

The formation of the flanges lengthens the pump.

The rotor might have a tapered bore formed at one of its ends. The other end of the flexible rod might be tapered and fitted into the tapered bore of the rotor. A tension bolt might extend through the rotor from the other end of the rotor. The tension bolt has a front threaded end, which might be screwed into the tapered end of the flexible rod so as to connect the rod and the rotor. In this case, if the pump is a general single/double-thread single-shaft eccentric screw pump as described above, the rotor would be long, and its eccentricity would be great. The rotor of this pump could have a through bore for the tension bolt. The diameter of the through bore would be limited to a small value. This would make it impossible to secure sufficient thread strength for connection with the flexible rod.

The connecting structure disclosed in the foregoing Japanese publication is not sufficient in connecting strength if a metallic flexible rod is connected to the drive shaft by means of bonding or keying.

In view of the foregoing points, the object of the present invention is to provide a double/triple-thread single-shaft eccentric screw pump in place of a general single-shaft eccentric screw pump as described above. The double/triple-thread single-shaft eccentric screw pump has an internally triple-threaded stator and an externally double-threaded rotor. Because this pump is high in pressure resistance, the rotor can be short. Because the eccentricity of the rotor is small, the through bore which can be formed through it for a tension bolt can be large in diameter so that the bolt can fasten the rotor with sufficient thread strength. In comparison with the flange type connecting structure, the rotor overhangs little. The whole length of this pump is short. The pump casing of this pump is short and small in diameter. The mechanical seal of this pump is simple to disassemble and assemble. It is easy to make this pump small in size and light in weight. At least one end of the flexible rod of this pump has a taper-stop tension bolt type connecting structure, which is mechanically stronger than a bonding or keying type connecting structure.

In order to achieve the foregoing object, a single-shaft eccentric screw pump according to the present invention comprises a pump body, a pump casing, and a flexible rod, the pump body including an externally double-threaded rotor elliptic in section and a stator having an internally triple-threaded bore in the shape of a substantially equilateral triangle in aperture section, the rotor being in engagement with the stator bore, the flexible rod coupling the rotor and a drive shaft together, the drive shaft being connected to a drive unit, the single-shaft eccentric screw pump being characterized by: the flexible rod being tapered at least one of its ends, the tapered end having a tapped hole formed at its axis; the rotor or the drive shaft having a tapered bore formed at its axis, the tapered end being fitted into the tapered bore; the rotor or the drive shaft further having a through bore formed at its axis; a tension bolt extending through the through bore from the other end of the rotor; and the tension bolt having a front threaded end, the threaded end being screwed into and fastened to the tapped hole in the tapered end of the flexible rod so as to connect the rod and the rotor or the drive shaft together.

As described above, the pump body includes an externally double-threaded rotor and an internally triple-threaded stator. This enables the pump to be shorter than a conventional single-shaft eccentric screw pump having an externally single-threaded rotor and the same discharge rate. This also enables the through bore of the rotor to be large in diameter, so that the engaging parts of the rotor and tension bolt can have sufficient thread strength. Accordingly, not only the drive shaft but also the rotor can be connected by a taper-stop tension bolt. In comparison with the conventional flange type, the rotor or the drive shaft does not need to have a protrusion formed at an end of it, to which a flange would be fixed. Accordingly, the rotor or the drive shaft overhangs little, so that it is possible to minimize the bending moment exerted from the flexible rod particularly on the rotor. This improves the discharging performance of the pump. Because the rotor or the drive shaft has no flange and needs to have no protrusion or the like for engaging with a flange, the pump is simple in structure. This makes it easy to disassemble and inspect the mechanical seal in the pump. This also makes it easy for the pump to be small in size and light in weight.

As described in claim 2, the flexible rod may be tapered at both its ends. Each of the tapered ends has a tapped hole formed at its axis. Each of the rotor and the drive shaft may have a tapered bore formed at its axis. One of the tapered ends of the flexible rod is fitted into the tapered bore. Each of the rotor or the drive shaft may further have a through bore formed at its axis. A first tension bolt may extend through the through bore of the rotor from the other end of the rotor. A second tension bolt may extend through the through bore of the drive shaft from the other end of the rotor. Each of the tension bolts has a front threaded end, which is screwed into and fastened to the tapped hole in one of the tapered ends of the flexible rod so as to connect the rod to the rotor and the drive shaft.

In the single-shaft eccentric screw pump described in claim 2, both ends of the flexible rod are connected to the drive shaft and the rotor by taper-stop tension bolts. Accordingly, the drive shaft and the rotor need to have no flange. This makes the pump simple in structure and enables the pump casing to be smaller in diameter, making it easy for the pump to be small in size and light in weight.

As described in claim 3, the first tension bolt may further have a rear threaded end, which engages with a nut. It is preferable that a cover for covering the nut be fitted to the end of the rotor which is away from the flexible rod.

In the single-shaft eccentric screw pump described in claim 3, the cover covers the nut and the rear threaded end of the first tension bolt. The cover keeps the nut and this bolt end out of contact with the liquid being transferred. This prevents the nut and the parts adjoining it from corroding. Accordingly, the pump can be used stably for a long period of time.

As described in claim 4, one end of the flexible rod may be connected to a central portion of the rotor or a central portion of the drive shaft at any point between both ends of the rotor or any point between both ends of the drive shaft.