EP1052368A1

EP1052368A1 - Assembly of a shaft and a boss connectable to it

Info

Publication number: EP1052368A1
Application number: EP00201554A
Authority: EP
Inventors: Roelof Breken
Original assignee: IHC Holland NV
Current assignee: IHC Holland NV
Priority date: 1999-05-12
Filing date: 2000-05-02
Publication date: 2000-11-15
Also published as: NL1012048C2

Abstract

Assembly of an engine-driven shaft (1) and a boss (5) connectable with it, of a part (4), in which, for establishing said connection, said shaft (1) and said boss (5) are provided with mutually adapted screw threads arranged on an outer and inner conical part, respectively, said screw thread having a trapezoidal cross-section. For connecting a cutter shaft (1) and the boss (5) of a cutter (4), the screw threads (2, 7) of both parts and the parts themselves are designed such, that screwing said parts together can take place until one flank (9) and top (13) of one screw thread (2) tightly abut a flank (12) and a valley (14) of the screw thread (7) of said other part (4), while both trapezoidal screw threads (2, 7) have been designed such, that there is tolerance between the other flanks (10, 11).

Description

The invention relates to an assembly of an engine-driven shaft and a boss connectable with it, of a part mountable on said shaft, in which, for establishing said connection, said shaft and said boss are provided with mutually adapted screw threads arranged on an outer and inner conical part, respectively, said screw thread having a trapezoidal cross-section.
Such an assembly is known from US-3.933.210. This assembly is employed in particular for connecting parts used in drilling for oil, gas, and the like.
There, the end position of engaging parts is determined in that the boss almost abuts a shoulder on the shaft. The remaining small tolerance is eliminated by exerting a moment on the drilling rod, as a result of which the screw thread will be slightly deformed, so that the boss will come to lie against the shoulder of said shaft.
In using the assembly for connecting the boss of a cutter to the cutter shaft and for like purposes, this may cause difficulties. When replacing the cutter must take place e.g. during operating conditions, dirt may be present between the boss and the shoulder, as a result of which, after mounting, one cannot be absolutely sure, that both screw threads have completely engaged.
Further, said known assembly is particularly suited for accomodating impact forces exerted on the shaft in its longitudinal direction. There, the shaft will be subject to tensile forces and pressure forces so that certain threads of both screw threads will be loaded more strongly than other threads. This will adversely affect the strength of the connection. In the connection of a cutter shaft and the boss of a cutter, such impact forces will occur to a lesser degree than radial and tangential loads.
According to the invention it is provided for, that for connecting a cutter shaft and the boss of a cutter, the screw threads of both parts and the parts themselves are designed such, that screwing said parts together can take place until one flank and top of one screw thread tightly abut a flank and a valley of the screw thread of said other part, in which both screw
threads, having a cross-section substantially in the shape of an isosceles trapezium, have been designed such, that there is tolerance between the other flanks.
By maintaining a certain tolerance between the flanks of both screw threads, the possible presence of dirt particles will give less difficulties when mounting the cutter onto the cutter shaft.
Further, it should be stated that with the known suction dredger vessels, the boss of the cutter and the cutter shaft are provided with screw threads mounted on and in cylindrical parts, respectively. Therein, it is a difficulty, that when mounting the cutter on the shaft concerned, the shaft must perform a number of rotations and that the cutter must be brought centrically and square to said shaft in order to enable a proper mounting. Given the large dimensions and weight a cutter can have, said mounting can not be performed with ease.
When applying the invention, on mounting the cutter, it can be slid across part of the conical end of the shaft so that it is centered in relation to said shaft, before it should be rotated. Thus, in sliding on the shaft, the cutter will almost automatically end up in the correct position in relation to the shaft.
Further, the number of rotations to be performed by the cutter during mounting can be much smaller than in case of cylindrical screw thread.
When fixing the cutter on the shaft, all flanks of the threads of the screw thread will be under almost the same load. As a result of this, the occurrence of tension concentrations and plastic deformation will be considerably reduced.
Due to the smaller tension concentrations, it is possible to employ a smaller structure while the tension remains the same, or larger loads compared with cylindrical screw thread can be allowed while the size of the structure remains the same.
Further, it should be stated, that naturally it should be possible to remove the boss from the shaft. To that end, it is desirable, that the loosening moment is smaller than the moment that can be produced by the engine with a directio of rotation opposite to the normal direction of rotation. Then, the boss can be held and the engine can be rotated in said direction of rotation.
Further, DE-659.079 can be referred to, from which it is known per se to provide conical parts of a shaft and a boss with screw thread, in which the shaft is not provided with a shoulder to be abutted by said boss. There, however, a screw thread having a sawtooth cross-section is used, its planes square to the longitudinal axis serving for absorbing the impact forces exerted on the shaft. It will be clear, that the sharp angles of the teeth are susceptible to damage. This would be a disadvantage with employment with a cutter and cutter shaft particularly when the cutter is to be replaced under operating conditions.
From EP-0703396 it is known to connect two pipes by means of a threaded bush. There, the pipes are provided with conical ends upon which trapezoidal screw thread is provided. There, the aim is to obtain a best possible sealing between both pipes since they must be suitable for transporting a medium at a high pressure. Therefore, the ends of both pipes abut against a shoulder provided in the threaded bush. In order to guarantee said abutment, there will have to be a certain dimensional tolerance. Due to this, it is not always guaranteed that the flanks of the screw threads abut each other in de desired way.
EP-0273016 shows a connection between two pipes almost corresponding to the one disclosed in EP-0703396. US Patents 4508375, 5498035, 4828295, 5360239 and DE Patent DE-207183 also relate to sealingly connecting pipes.
With the invention described above, the sealing between the parts plays no role whatsoever, but the only aim is to obtain a best possible connection between said parts, as a consequence of which a large torque can be transferred.
The invention is further explained by way of an embodiment, illustrated in the drawing, in which:
Fig. 1 shows schematically a longitudinal section across a cutter shaft and cutter mountable on it;
Fig. 2 shows a side view of a cutter shaft on an enlarged scale;
Fig. 3 shows a cross-section across some threads of the cutter shaft and the cutter after a cutter has been mounted on said shaft; and
Fig. 4 shows a cross-section according to Fig. 3, but of a somewhat modified embodiment of the connecting according to the invention.
Fig. 1 shows a cross-section across a part of the shaft and the boss before it is screwed onto said shaft.
In the drawing, the shaft is indicated by 1 and the threaded 2 conical part by 3. The cutter 4 is provided with a boss 5 having a conical bore 6, in which screw thread 7 has been provided.
Obviously, the screw thread 7 of the boss 5 will have to be adapted to the screw thread 2 of the shaft 1. This should be taken into account when replacing a cutter 4. Thus, the invention relates both to the design of the shaft 1 and of the boss 5, which can be separate parts.
Fig. 2 shows the end of the cutter shaft 3 on which the screw thread 2 is provided. Seen in an axial plane, the apex α of the conical part 3 and of the outer plane 8 of the screw thread will be between 10° and 50°.
The cross-section of the screw thread is trapezoidal and the angle β enclosed by the flanks 9 and 10 of the scrw thread, seen in an axial plane, will be between 10° and 50°.
As appears in particular from Fig. 3, after the cutter 4 has been screwed on the cutter shaft 1, the flank 10 of the screw thread 2 of the cutter shaft 1 will be at a small distance from the flank 11 of the screw thread 7 of the cutter 4, while the other flanks 9 and 12 fixedly abut each other. The top 13 of the screw thread 2 will also fixedly abut the valley 14 of the screw thread 7.
With the embodiment according to Figure 4, the top 13 of the screw thread 2 does not abut the valley 14 of the screw thread 7, but the top of the screw thread 7 abuts the valley of the screw thread 2.
It will be clear, that only one possible embodiment of a way of connection according to the invention has been illustrated in the drawing and described above. Naturally, it is also possible to connect other parts, such as e.g. a pump impeller instead of a cutter, in the way indicated and to further make changes without leaving the inventive idea, as it has been indicated in the accompanying claims.

Claims

Assembly of an engine-driven shaft (1) and a boss (5) connectable with it, of a part (4) mountable on said shaft, in which, for establishing said connection, said shaft (1) and said boss (5) are provided with mutually adapted screw threads arranged on an outer and inner conical part, respectively, said screw thread having a trapezoidal cross-section, characterized in that for connecting a cutter shaft (1) and the boss (5) of a cutter (4), the screw threads (2, 7) of both parts and the parts themselves are designed such, that screwing said parts together can take place until one flank (9) and top (13) of one screw thread (2) tightly abut a flank (12) and a valley (14) of the screw thread (7) of said other part (4), in which both screw threads (2, 7), having a cross-section substantially in the shape of an isosceles trapezium, have been designed such, that there is tolerance between the other flanks (10, 11).
Cutter shaft (1), characterized by a conical end portion (3) being provided with trapezoidal screw thread (2).
Cutter (4) provided with a boss (5), characterized by a conical bore (6) having a trapezoidal inner screw thread (7) therein.