WO2001041961A1

WO2001041961A1 - Method for producing a crowned toothing with involute properties and shafts with such toothings

Info

Publication number: WO2001041961A1
Application number: PCT/DK2000/000688
Authority: WO
Inventors: Tom Tychsen; Hans Christian Petersen
Original assignee: Sauer-Danfoss Holding A/S
Priority date: 1999-12-10
Filing date: 2000-12-08
Publication date: 2001-06-14
Also published as: DE19959836A1; AU1995801A

Abstract

The invention concerns a method for producing a crowned toothing (2) with involute properties on a shaft (1), a working area of a tool with a movement path being movable relative to the shaft (1), which has one component parallel to the rotational axis of the shaft (1) and a second component radial to the rotational axis. In this connection it is desired to make the shaft more stable. The movement path has a third component perpendicular to the two other components, the working area making a reciprocating movement across the axial length of a tooth (4) in the direction of the third component.

Description

Method for producing a crowned toothing with involute properties and shafts with such toothings

The invention concerns a method for producing a crowned toothing with involute properties on a shaft, a working area of a tool with a movement path being movable relative to the shaft, which has one component parallel to the rotational axis of the shaft and a second component radial to the rotational axis. Further, the invention concerns a shaft with a crowned toothing in at least one end, which has involute properties and whose teeth and tooth gaps are inclined relative to the rotational axis.

A method and a shaft of this kind are known from DE 28 18 332 C2. A preferred application field for such shafts are hydraulic machines, in which a gear wheel orbits and rotates in a toothed ring. The shaft serves the purpose of leading the rotational movement of the gear wheel to the outside, or vice versa, to transfer a drive from the outside to the gear wheel, when the machine works as a pump. As the gear wheel is supported eccentrically in relation to the rest of the machine, the shaft must have a cardan connection with the gear wheel. This is provided by the crowned toothing, which permits an inclination of the shaft in relation to the gear wheel. In the following, this inclination of the shaft in relation to the gear wheel is also called "inclination angle".

The toothing is often arranged on an area of the shaft, which has an increased diameter. Thus, it should be ensured that also at the axial ends of the individual toothings, the shaft maintains a certain minimum diameter, as the load capacity of the shaft changes with the third exponent of the radius. On the other hand, the life of the toothing is prolonged, the longer the individual teeth of the toothing are. The longer the teeth, the more is the diameter of the shaft reduced at the axial end of the teeth. Until now this has required that the teeth were inclined by the double inclination angle, to make sure that the shaft could in- cline accordingly in relation to the gear wheel. The more the teeth and the tooth gaps, or, more precisely, their base, is inclined, the more the shaft is weakened by a corresponding diameter reduction.

A usual method for producing such a crowned toothing consists of a so-called peripheral milling. Here, the shaft is rotated around its rotational axis. A milling tool is likewise rotated around a rotational axis in such a way that in relation to the shaft its cutting device moves axis paral- lei to the shaft. For example, the cutting device can be led in a spiral shape round the milling tool. The milling tool itself and thus also the cutting device will proceed in the axial direction in relation to the shaft, that is, across the axial length of the toothing. Simultaneously with this movement, a lifting movement of the milling tool takes place radially outwards until reaching the centre of the toothing and subsequently a lifting movement radially inwards until reaching the axial end of the toothing. Normally, the movements are coordinated so as to ensure that a tip does not occur in the axial centre of the toothing, but that the transition is somewhat rounded.

The invention is based on the task of making the shaft more stable.

With a method as mentioned in the introduction, this task is solved in that the movement path has a third component perpendicular to the other two components, the working area being moved once in the direction of the third component and once back again across the axial length of a tooth. With this embodiment, a toothing is produced, which, all other things being equal, causes a reduced weakening of the diameter of the shaft. The reason for this is that the working area, for example, the cutting device of the ill- ing tool no longer has to penetrate so deeply into the shaft at the axial ends of the toothing. In fact, the cutting device is only lowered to the bottom of the groove, which is required anyway. The remaining space, which must be available for the corresponding counter-teeth is pro- duced in that the teeth of the toothing taper more strongly at their axial ends. The working area is thus additionally moved "laterally", that is in the circumferential or tangential direction. Compared with a conventional toothing, in which the tooth cross sections have in fact remained the same across the total axial length, the teeth only being inclined, the invention now provides that the tooth gaps expand from the axial centre of the toothing towards the axial ends. Accordingly, the inclination of the teeth and the resulting lowering of the base or the bottom of the tooth gaps does no longer have to be effected with the large angle used until now. The angle can practically be halved. As it has been the case until now, a milling tool can still be used as tool, the working area being formed by the cutting device. In fact, however, any other tool can also be used, whose working area is able to produce the desired shape of the teeth and the tooth gaps by means of material cutting or reshaping.

In this connection, it is preferred that the tool is guided in a lifting movement parallel to the second component, whose lift is:

a = 1 ' tan α 1 being the axial length of the toothing and α the inclination angle of the shaft. Compared with traditional toothings, the lift a has thus practically been halved, as until now, this lift has been proportional to tan2α. Accordingly, with otherwise unchanged conditions, the diameter of the shaft can be kept larger.

Preferably, the lifting movement of the second component can be made concurrently with the reciprocating movement of the third component. This saves additional working procedures and at the same time produces a correspondence between the maximum tooth height and the maximum tooth width.

In a preferred embodiment, the movement of the third compo- nent is produced in that the tool is displaced. With the displacement of the tool, which, during the movement, must reciprocate once along the axial length of the teeth (in relation to the tangential or circumferential direction) , the corresponding movement of the working area, for exam- pie, the cutting device, can be effected in a relatively simple way. However, then a machine tool is required, which permits such a three-axle movement of the tool.

In an alternative embodiment the movement of the third component is produced in that the working area is arranged along a spiral on the tool and the tool and the shaft are rotated, a proportionality factor between the speeds of tool and shaft being changed during the movement of the first component. With the traditional milling, the speeds of shaft and milling tool had a fixed relation to each other, so that the cutting device could always work exactly axis parallel to the shaft. When now the speed of the milling tool is somewhat increased, the cutting device hastens. When the speed of the milling tool is reduced, the cutting device will get behind its desired position. With this procedure, it is required that each tooth, or rather, each tooth gap is practically milled twice, the speed on the first passage of the milling tool being increased towards the axial ends of the toothing, being reduced towards the centre and on the second passage being reduced towards the axial ends and increased towards the axial centre, to produce the opposite flanks of the toothing. A speed control of this kind is much easier than the movement of the milling tool in the direction of a third axis. A similar con- sideration also applies, when another tool is used instead of the milling tool.

Preferably, a groove forming the tooth gap is broached before guiding the working area along the movement path. This can contribute to a relief of the tool, particularly in the case of large toothings.

By means of a shaft as mentioned in the introduction, the task is also solved in that the base of the tooth gaps expands in the direction of the axial ends and the angle of the base curve corresponds to the inclination angle.

As described above in connection with the method, this can be reached in that the bases of the tooth gaps must no longer be led that deep into the material of the shaft, thus causing less weakening of the diameter of the shaft.

In this connection it is preferred that a radial distance at each tooth base between a first section line in the axial centre and a second section line in the area of an axial end of the toothing is maximum as large as at each tooth peak. With this embodiment it is possible that the tooth gaps expand from the axial centre of the toothing with the angle formed between the bases of the tooth gaps and the rotation axis of the shaft. Preferably, the tooth base decreases from the axial centre in a straight line radially inwards and the tooth flanks are defined by a curve, whose curvature is larger at the axial ends than in the axial centre. In connection with the shaft, the ideal state suggests that the tooth base decreases from the axial centre in a straight line radially inwards. In practice, however, this has not been possible until now, as the inclination angle α has always been subjected to certain tolerances, which were determined by the other components of a machine, in which the shaft was used. When now the tooth flanks are curved so that the curvature at the axial ends of the toothing is larger than in the axial centre, space can be procured for these tolerances, without requiring an additional lowering of the base curve of the tooth gaps. This again increases the stability of the shaft.

In the following, the invention is described in detail on the basis of a preferred embodiment as shown in the draw- ings:

Fig. 1 a perspective view of a shaft with a crowned toothing at both axial ends

Fig. 2 the shaft mounted in a gear wheel

Fig. 3 a longitudinal section through an axial end

Fig. 4 sectional views A-A and B-B according to Fig. 5

Fig. 5 a comparison between a traditional toothing and a new toothing

Fig. 6 a schematic view of a milling process Fig. 1 shows a shaft 1 with toothings 2 at both axial ends.

Between the two toothings 2, a shaft 3 is provided. The toothings 2 have teeth 4 and tooth gaps 5. The toothings 2 have a crowned shape, that is, the diameter in the axial centre of each toothing 2 is larger than at the axial ends of the toothing. Substantially, the toothings correspond to an involute toothing, that is, a rolling of this toothing in an internal toothing (not shown in detail) of a gear wheel 6 is possible, as it takes place, for example, in hydraulic machines, in which a gear wheel rotates and orbits in a gear ring. A device of this kind is shown in section in Fig. 2. The shaft 1 must be able to transmit a rotary movement of the gear wheel 6 to an axis 7, which, together with the rotating axis of the gear wheel 6 en- closes an eccentricity e. Accordingly, the shaft 1 is inclined by an angle α, this angle being called "inclination angle" in the following.

To enable this inclination, it is required that the teeth 4 of the toothing 2 have the already described crowned shape. Until now, this shape has been produced in that a shaft blank was used, which has in the area of its axial ends a corresponding diameter expansion. As shown schematically in Fig. 6, this shaft 1 is then rotated around its rotation axis 8. At the same time, a milling tool 9 is rotated around its rotation axis 10. The milling tool 9 has a cutting device 11, which is guided in a spiral around the milling tool 9. In many cases, the milling tool is additionally inclined by an angle, which corresponds to the gradient angle of the spiral line. Thus, the representation in Fig. 6 is only to be understood schematically. When now the milling tool 9 is rotated with a speed, which is adapted to that of the shaft 1, an involute toothing is produced, when the milling tool is moved in the direction of a double arrow 12. The double arrow 12 shows the direc- tion of a first component of a movement path of the milling tool 9 in relation to the shaft 1. This movement path has a second component, which in the representation in accordance with Fig. β is directed vertically to the drawing level, that is, radially to the shaft 1. This gives a movement radially outwards until the axial centre of the tooth. Then the milling tool 9 is moved radially inwards again.

Until now, it has been required for this inward and outward movement to enclose an angle 2α to secure the inclination angle α. The cutting device 11 has penetrated accordingly deeply into the shaft 1, thus weakening its diameter.

In orbit machines, in which one end of the shaft moves in an eccentric path in relation to the other, the toothing, which transmits the torque, cannot be a normal shaft toothing, on the contrary, it must have a shape, which is basically conical. As the actual size of the eccentricity depends on tolerances, and is thus not constant, not in the different positions of the machine either, this conical shape must have a slight curve superimposed on itself, to prevent a loading of the extreme edges of the toothing. This would cause a too large reduction of the life.

Until now, as described above, this shape was obtained in that a peripheral milling machine, which produces the involute toothing, is moved further radially into the workpiece at increasing distance from the axial centre of the head. This movement follows a curve, called "bottom curve". In this way, two things are obtained. For the tooth sides, this means a profile displacement, which produces the desired shape of the tooth sides. For the tooth bottom it means that also here material is removed. To obtain the desired profile, the tool must penetrate twice as far into the shaft as desirable in relation to the bottom. The rea- son for this lies in the tool geometry. The involute of the toothing is produced by a tool with straight tooth sides, which have, for example, an angle of 30° in relation to the centre line. When the tool is moved into the shaft, the movement in the side direction is equal to sin 30° = 0.5 times the radial movement. In other words, twice as much material as necessary must be removed from the bottom. As the torsion stability of the shaft is determined by the resistance torque, which is a function of the smallest radius in the third exponent, this removal of material means a very heavy reduction of the strength of the shaft, which has a negative influence on the life. This condition can be slightly improved by an increase of the side angle of the tool, however not much, as the radius of the tooth peak of the tool will then always become smaller, and in the end too small.

To reduce this diameter weakening, it is now provided that in connection with the movement in the direction of the double arrow 12, the milling tool is not only moved radially outwards and radially inwards, but at the same time also in the direction of a double arrow 13, that is, tan- gentially to the shaft 1. Thus, the tooth gaps 5 are expanded in the direction of their axial ends. As appears from Fig. 3, this will reduce the lift to a value a, a being dependent on the axial length 1 of the toothing and the contact angle α, which can be described as follows:

a = 1 x tan α

The movement in the direction of the double arrow 13 can be replaced by a speed control of the milling tool 9. When, for example, in the area of an axial end the speed of the milling tool 9 is increased in relation to the speed of the shaft 1, the cutting device 11 hastens, thus causing a widening of the tooth gap 5 at this axial end of the toothing. On its way to the axial centre, the speed of the milling tool 9 is reduced again. In the axial centre it then corresponds to the speed of the shaft 1 (or has the origi- nally chosen proportionality factor to it) . When the milling tool 9 comes closer to the other axial end, its speed is increased again. Thus, a tooth side can be produced. The other tooth side is produced by a corresponding, reversed movement of the milling tool 9, the speed being reduced correspondingly at the axial ends.

With this procedure it is avoided that more material as necessary is removed. It is only required to add a third component to the tool movement. Part of the movement re- moves the material from the bottom. Thus, this is merely a straight line with a straight angle to the centre line, which is equal to the inclination angle α. 1 being the axial distance from the axial centre of the toothing 2 and a the depth of the "groove" or the tooth gap, results in the formula above. When the side angle of the tool is β, this amount will become sin (β) x the total or the desired contribution.

Normally, β = 30° and the contribution thus is 0.5. The rest (1 - sin(β)) is removed by a rotation of the workpiece around its own axis. In principle, this corresponds to a helical toothing with an angle α x (1 - sin(β)) . In relation to a helical toothing, however, a bevel must be available on both sides. Measured as the curve measure of a cylinder, whose diameter is the basic circle of the involute, the shaft must be rotated by a x tan (α) x (1 - sin(β) ) . However, in fact the inclination angle α will not be constant, but will change with the tolerances and positions of the motor, in which the shaft is mounted. To provide for this, α is calculated with a tolerance allowance. This gives room for a larger angle.

The small variations needing attention at α because of different tolerance sums in the motor positions are compensated in that in the longitudinal direction the tooth side gets a small curve. This curve should have its largest curvature at the ends of the tooth sides, so that the contact substantially appears at half distance of the tooth side from the axial centre of the head. A typical curve of this kind is an ellipse. With this additional curvature of the tooth sides it is in fact possible to produce the bottom of the tooth gap as a line with the inclination α.

To simplify the production of the tooth gaps 5, a groove can be made before producing the tooth sides, which then form the basis of the tooth gap 5.

The shape of the toothing produced with this method is shown in Figs. 4 and 5, showing, for a better explanation, a comparison of traditionally produced toothings and new toothings.

In Figs. 4 and 5, the partial view a shows a traditional toothing, and the partial view b shows a toothing produced according to the new method. In Fig. 4, the outer line A-A is the limitation line of the teeth 4 and the tooth gaps 5 in the axial centre 14 of the toothings 2, whereas the inner line B-B shows the limitation of the toothing in the area of an axial end. Accordingly, the outer line is the section line A-A according to Fig. 5, whereas the inner line is the section line B-B according to Fig. 5. Initially, it can be seen that in Fig. 5a the bottom of the tooth gaps is formed by a straight groove. This means that the cutting device 11 of the milling tool 9 is passed through here in a straight line. Thus, the tooth cross sections remain the same across the axial length of the toothing 2. They are merely inclined by the corresponding inclination angle. Accordingly, the distance between the lines A-A (axial centre) and B-B (end area) changes obvi- ously between tooth peak and groove bottom 15, that is, y> x, or rather, y = 2x.

For the teeth 4, produced in accordance with the new method, the circumstances are different. Here the distance x between the two lines A-A and B-B in the area of the tooth peaks is always at least as large as the corresponding distance y in the area of the bottom of the tooth gap

5, that is, x > y, in the ideal case x = y.

Claims

Patent Claims

1. Method for producing a crowned toothing with involute properties on a shaft, a working area of a tool with a movement path being movable relative to the shaft, which has one component parallel to the rotational axis of the shaft and a second component radial to the rotational axis, characterised in that the movement path has a third component perpendicular to the other two components, the working area being moved once in the direction of the third component and once back again across the axial length of a tooth.

2. Method according to claim 1, characterised in that the tool is guided in a lifting movement parallel to the second component, whose lift is:

a = 1 ' tan α

1 being the axial length of the toothing and α the inclination angle of the shaft.

3. Method according to claim 2, characterised in that the lifting movement of the second component can be made concurrently with the reciprocating movement of the third component.

4. Method according to one of the claims 1 to 3, charac- terised in that the movement of the third component is produced in that the tool is displaced.

5. Method according to one of the claims 1 to 3, characterised in that the movement of the third component is produced in that the working area is arranged along a spiral on the tool and the tool and the shaft are rotated, a proportionality factor between the speeds of tool and shaft being changed during the movement of the first component.

6. Method according to one of the claims 1 to 5, characterised in that a groove forming the tooth gap is broached before guiding the working area along the movement path.

7. Shaft with a crowned toothing in at least one end, which has involute properties and whose teeth and tooth gaps are inclined relative to the rotational axis, characterised in that the base of the tooth gaps ex- pands in the direction of the axial ends and the angle of the base curve corresponds to the inclination angle

(α) .

8. Shaft according to claim 7, characterised in that a radial distance at each tooth base (5) between a first section line (A-A) in the axial centre (14) and a second section line (B) in the area of an axial end of the toothing (2) is maximum as large as at each tooth peak.

9. Shaft according to claim 7, characterised in that the tooth base (5) decreases from the axial centre in a straight line radially inwards and the tooth flanks are defined by a curve, whose curvature is larger at the axial ends than in the axial centre.