WO2014121848A1 - A housing of a planetary gear and a method for making the same - Google Patents

Abstract

A method for manufacturing a housing (3) for a planetary gear, where thin gear elements (9) and possibly one or more spacer elements (12) are stacked, and a shell (11) of metal or plastic is overmoulded on the stack, thus forming the housing (3) of the planetary gear. A planetary gear obtained with the method is also claimed; a backlash-free embodiment provides that the housing includes two internally geared rings, and a pre-stressed elastic shell (110) which tends to rotate said two rings relative one to another until they abut against the planet wheels.

Description

A housing of a planetary gear and a method for making the same

DESCRIPTION

Field of the invention

The invention relates to the field of planetary gears and hybrid gears including at least a planetary gear stage. The invention relates more in detail to a method for making the housing of a planetary gear, a housing of a planetary gear and a planetary gear comprising said housing. Prior art

A planetary gear comprises basically a housing and one or more planetary gear stages. Each stage comprises a sun gear and a plurality of planet gears mounted on a carrier and revolving around the sun gear. The housing is configured as a hollow cylinder with an internal gear, projecting from the inner surface of the cylinder. The planet gears mesh with the sun gear and with the internal gear of the housing.

The planetary gear is a known kind of epicycloidal gear and has several advantageous features, including a notable transmission ratio in a compact package. Sometimes a reducer or multiplier may include planetary gears and conventional gears as well. The term of hybrid gear is used to denote a device comprising one or more planetary stages and also one or more conventional stages, for example spur stages.

The making of the internally-geared housing poses a number of technical issues. As a matter of fact, an internal gear is generally more difficult to obtain than an external one. This is even more true for housing of small multi-stage reducers which require a small and elongated housing. - -

Planetary or hybrid reducers for example are used in precision gearheads for electric micro-motors, reaching a reduction ration beyond 1000:1 . These gearheads typically have a small diameter, less than 50 mm and possibly less than 10 mm, and are required to provide long-term and low- noise operation at a high rpm. Hence they require the highest quality in terms of precision of size, profile and smooth contact surfaces of the teeth.

The conventional methods for making the housing of planetary gears include broaching, die casting, injection moulding. These methods however may be unable to provide the very high precision required by planetary gearheads. Broaching is a machining process that uses a tool to remove material and, hence, is subject to wear of said tool. Additionally, broaching may be unpractical when working on small parts, such as housings of small gearheads having a diameter of 10 mm or even less. Die casting and injection molding are sensitive to various parameters of the injection process (e.g. pressure or temperature) which may influence the quality of the teeth.

Another limitation of the known methods is that they provide an internal gear with substantially uniform features. The various stages of a multistage planetary gear, however, may have quite different requirements in terms of speed or torque. It follows that the most demanding stage dictates the material of the housing and design of the internal teeth, but this may increase costs.

Furthermore, making the internal gear through the entire housing is sometimes unnecessary. For example a hybrid reducer does not need the internal gear in the portion which hosts the spur stages. The current techniques however do not allow, at a reasonable cost, to locate the internal gear in a selected portion of the housing.

The above problems have not been solved yet, despite a growing demand for precision gearheads in hi-tech fields including medical equipments, robotics, aerospace and aviation industries, audio and video equipments. - -

This demand creates a strong incentive to ameliorate the performance of planetary gears and related manufacturing methods. Furthermore, there is a continuous request for customization in order to meet strict requirements, e.g. in terms of speed, transmission ratio, etc. Summary of the invention

The technical problem underlying the invention is to overcome the above drawbacks of the prior art. Accordingly, the invention provides a novel method for the manufacturing of the housing, which is reliable and highly customizable. The above problem is solved with a method for manufacturing a housing with an internal gear, for use in a planetary gearhead, comprising the steps of:

- stacking a plurality of ring elements, at least one of said ring elements having internal teeth projecting from their inner circumference, the stacked ring elements forming a substantially cylindrical hollow body with an internal gear;

- forming a tubular outer shell in such a way that said ring elements are made integral with said shell.

The individual ring elements, prior to stacking, can be made by cutting or die-casting or another known technique.

The stack may comprise, in some embodiments, one or more spacer elements without internal teeth. Said spacer elements may be adapted for performing additional functions that will be explained below.

A proper alignment between the ring elements and, if provided, any spacer elements or other additional element, can be maintained with the help of fixation means. An example of suitable fixation means is a pin or similar. Said fixation means may be removable; in this case, they can be removed or not prior to the formation of the external shell. Said outer shell is preferably overmoulded around the ring elements. In this case, the ring elements are positioned in a mould, optionally with the above mentioned fixation means, and said shell is overmoulded around the stack, thus forming an integral piece.

According to various embodiments, the shell may form the body of the housing or a further outer sleeve member may be provided.

Said shell is made preferably of metal or plastic. In a preferred embodiment, the shell is made of an elastically deformable material. A preferred embodiment for this purpose provides that:

- said ring elements include at least a first ring element and a second ring element having internal teeth and forming at least a portion of an internal gear of said housing;

- prior to formation of said shell, said first and second ring elements are positioned with a rotational offset, in such a way that the teeth of the first ring element are misaligned relative to the teeth of the second ring element.

For example, two internally geared rings are loaded in a suitable mold, and are arranged in a position where the teeth of the first and second ring are slightly misaligned. Then, an elastic shell is provided (e.g. overmoulded) around said two rings, obtaining a single body where a rotation between the ring elements is opposed by elastic deformation of the outer shell, and where a rest position of the elastic shell corresponds to misalignment of the teeth of the two rings.

After the formation of said elastically deformable shell, said first ring element and second ring element are rotated relative to one another, to reach a position where the teeth of the two ring elements are substantially aligned. This happens at the expense of an elastic deformation of said shell, which is then pre-stressed and stores some elastic energy. When the teeth are aligned, the ring elements are advantageously connected by - - a removable fixation means to allow assembling of the gearing parts such as sun gear and planet wheels.

The above mentioned alignment or misalignment is referred to a width direction of the teeth. A particularly preferred embodiment provides a planet gear, or each planet gear, comprising two coaxial wheels; namely a first wheel meshing with the teeth of the first ring element, and a second wheel meshing with the teeth of the second ring element of the housing.

When the internal gearing parts are mounted, said fixation means are released; the elastic shell tends to recover its original shape and, as a consequence, the geared rings find an abutment against the planet wheels, thus compensating for any backlash of the device. Furthermore, a backlash which arises during the life of the device is automatically recovered by the deformable shell. In other words, the elasticity of the shell provides a backlash recovery capability, leading to substantially backlash-free operation. This noticeable advantage of the invention will be explained in further detail with the help of the examples.

The preferred materials for said elastically deformable shell are an elastomer or a thermoplastic elastomer. All the embodiments of the invention may optionally include one or more spacer elements in the stack. Said spacer elements may be realized in different forms: a spacer element can be similar to the ring elements, apart from lack of teeth, or may be different in shape or material. In some embodiments, one or more of the spacer elements may perform additional technical functions, such as providing a support for internal sensor means. For example, a spacer element may host sensor means for detection of relevant parameters, such as temperature, pressure, speed (rpm), position, vibration, etc. A related advantage is a better monitoring of the gearhead and possible prevention of failures. - -

A spacer element may be placed in a position of the housing corresponding to an interspace between two adjoining planetary stages; in a hybrid device, a spacer element can be located corresponding to conventional, non-epicycloidal stages. The ring elements forming the toothed parts of the housing need not be all identical. According to some embodiments, ring elements with different features may be stacked in order to realize regions of the internal gear with customized features. Different features between the ring elements may include for example: thickness, material, hardness, surface treatment, surface coating, working tolerance, as well as design of the teeth. Said features can be chosen for meeting particular demands such as less noise, reduced backlash, input rpm and others. In particular, regions of the internal gear can be customized taking into account the requirements of the gear stage they will engage during operation. It can be appreciated that the invention allows a full customization of the housing, taking into account the duty and performance of each stage.

Another object of the invention is a planetary gear assembly, for example a gear reducer or gear multiplier, according to the enclosed claims.

The invention has the following advantages. The manufacturing process is improved, especially with regard to quality control. The teeth of the ring elements can be made separately with a suitable process having a high precision, and the making of the housing - by stacking the ring elements and overmoulding the outer shell - does not affect their quality. Hence the invention has an advantage in terms of quality and repeatability, over the less predictable processes of broaching the inner side of a hollow piece, or die-casting of the whole housing.

Furthermore, the inventive method allows for a modular production since a shorter or longer housing can be easily made by varying the number of ring elements. The method will deliver a constant quality and precision no - - matter of the L/D ratio (length/diameter) of the housing.

Another advantage is that the process is particularly suitable for making housings for small reducers, with a diameter of 10 mm or less, where the conventional methods are unpractical and/or expensive. Another advantage is that the internal gear can be positioned only when it is actually required, i.e. where revolving planet gear need to mesh with the housing. This is an advantage especially in the hybrid devices where the internal gear can be provided only in the region of the housing intended to receive the planetary stages. This leaves room for additional items such as sensors of temperature, pressure, etc. which may be associated to spacer elements, as mentioned above. Another advantage is the option to vary the material or other features of the ring elements, leading to a high degree of customization. The latter is a notable advantage especially for demanding fields, where planetary gearheads are becoming more and more common but a customized design is often required.

The backlash-free embodiments have the significant advantage of backlash-recovery feature located in the housing, thus avoiding a complicated design of smaller parts such as the planet wheels or planet carrier, which can be conventional. This is an advantage over prior-art systems which, for example, provides a planet carrier made in two parts and an elastic means inserted in between, see e.g. JP-3-048045.

These and other features and advantages of the invention will now be elucidated with the following detailed description of some embodiments.

Description of the figures Fig. 1 shows the main parts of a planetary gear coupled with a motor.

Fig. 2 is a sectional view of the housing of a planetary gear, according to an embodiment of the invention.

Figs. 3 and 4 are a perspective view and a sectional view of the housing of - - a planetary gear, according to another embodiment of the invention.

Fig. 5 is a perspective view of a housing for a planetary gear stage according to a further embodiment of the invention, featuring a recovery of backlash, Fig. 6 is a detail of Fig. 5,

Fig. 7 shows a planetary gear stage including the housing of Fig. 5.

Fig. 8 is a plane view of the housing of Fig. 5, and Fig. 9 shows the detail "X" of Fig. 8.

Fig. 10 shows the housing of Fig. 8 while the internal gears are assembled, and Fig. 1 1 relates to detail Ύ".

Fig. 12 show the stage during operation, and Fig. 13 is the detail "Z". Detailed description of preferred embodiments

Fig. 1 shows a common embodiment of a planetary gear assembly 1 including a first stage 2a and second stage 2b received in a housing 3 with an internal gear.

Each stage comprises a sun gear and planet gears on a carrier. In Fig. 1 , the sun 5 and planet 6 of stage 2a are shown. In use, the planet gears engage the internal gear of the housing 3, while revolving around their sun gear. Fig. 1 shows also a motor 7, which may be coupled to the planetary gear assembly 1 .

An embodiment of the housing 3, in accordance with the invention, is shown in Fig. 2. The housing includes a stack 8 of several ring elements 9. Said ring elements 9 are preferably in the form of laminated gears and can be made of metal or plastics. At least some of the ring elements 9 have teeth 10 projecting from the inner circumference, so that the plurality of elements 9, when packed one against the other and suitably aligned, form - - the internal gear 4 of the housing 3.

A shell 1 1 is overmoulded around the stack 8, in such a way the ring elements 9 remain partly embedded in said shell . As apparent from Fig. 2, a portion of the outer circumference of the ring elements 9 remains embedded in the material of the shell 1 1 .

The material of the shell 1 1 is preferably a plastic material and more preferably a thermoplastic material. However, the shell 1 1 can also be made of metal.

According to embodiments of the invention, the internal gear 4 may extend substantially all along the housing 3, or only in some regions of the housing. Non-toothed regions of the inner surface of housing 3 can be defined by suitable spacer elements between the ring elements 9. A spacer element may be similar to ring elements 9, or have a different shape. Hence the stack may comprise several groups of toothed elements, which are preferably spaced apart by spacer elements.

Figs. 3 and 4 show an embodiment of the invention where the housing 3 comprises three individual groups 8a, 8b and 8c of toothed elements 9. The internal gear 4 then comprises separate portions 4a, 4b, 4c. Spacer elements 12 may be provided between the aforesaid groups.

Said portions 4a, 4b, 4c may engage different stages of the planetary gear, when the assembly is mounted. For example the portion 4a may engage the planets of a first stage 2a. The ring elements forming the groups 8a, 8b, 8c may be identical or may have different features, e.g. taking into account the different stress due to speed of the respective stages.

One or more of the spacer elements 12, according to another optional feature, may include sensor means for detection of temperature, pressure, speed, vibrations or other parameters. For example, a sensor area 12a is - - illustrated in the Figures.

The invention can provide backlash-free embodiments when the shell is made of a deformable material. One of such embodiments is shown in Figs. 5 to 13. Referring to Figs. 5 to 7, the housing 3 comprises a first internally geared ring 9A and a second internally geared ring 9B, which form together the internal gear of the housing 3, or at least a part thereof. The rings 9A and 9B have the same number of teeth.

A deformable shell 1 10, which is made for example of a thermoplastic elastomer, is overmoulded around the outer surface of rings 9A and 9B. The outer surface of said rings 9A, 9B has preferably a pattern suitable to facilitate the grip of the outside shell 1 10, said pattern being for example in the form of external teeth 13 which remain trapped in the thermoplastic material, during the overmoulding step. More preferably, the housing 3 comprises an outer body 21 made for example of metal, around the elastically deformable shell 1 10.

Before the making of the shell 1 10, the rings 9A and 9B are set in a position where the teeth of one ring are misaligned relative to the teeth of the other ring. The offset can be appreciated in the enlarged view of Fig. 6 when looking at the teeth 14, 15, and is denoted with e in Fig. 9. More in detail, the teeth 14 of the ring 9A are offset relative to the teeth 15 of the ring 9B below, in the width direction of the teeth, that is the axial direction W of the housing 3. The misalignment of the teeth corresponds to shell 1 10 being unstressed (rest position). Prior to mounting the internal members of the planetary stage or stages (sun gear, planet wheels, etc.), the alignment of the teeth is restored by rotating the ring 9A and/or 9B against the opposition of the elastic shell - -

1 10 which, as a consequence, stores a certain amount of elastic energy. A releasable fixation means is provided to keep the rings 9A, 9B in the aligned position. For example, said fixation means include holes 16A and 16B passing through the rings 9A and 9B and possibly through the shell 1 10. When said holes are axially aligned, a pin can be inserted through, to block the rings. At this stage, the sun and the planet gears can be mounted.

The fixation means is then released, e.g. the above mentioned pin is removed. Under the force exerted by the elastic shell 1 10, the rings 9A and 9B tend to return to the originally misaligned position; doing so, the internal gears of the rings will find an abutment against the teeth of the planet wheels, thus eliminating any backlash (play) in the device.

As long as the shell 1 10 has an elastic strain, any backlash originated in the device (e.g. due to wear) will result in the rings 9A and 9B finding a new abutment position. Hence, the device is able to automatically compensate for the backlash.

Fig. 7 shows a preferred embodiment where planet gears 17 include a first wheel 18 meshing with the first ring 9A, and a second wheel 19 meshing with the ring 9B. The sun gear (pinion) is denoted with 20. Accordingly, once the fixation means is released, the first ring 9A finds an abutment against the first wheel 18 and the second ring 9B does the same against the second wheel 19.

The assembly procedure and the backlash recovery ability are even better illustrated in the Figures 8 to 13. Figs. 8 and 9 show the housing 3 in a position where the deformable shell 1 10 is unstressed and hence the offset e exists between the rings 9A and 9B. It can be noted that holes 16A, 16B are misaligned. Figs. 10 and 1 1 show the forced alignment of the rings 9A and 9B, by means of a fixation pin 22 inserted in the holes 16A and 16B, now in axially aligned position, for assembling the internal - - planet wheels 17. Figs. 12 and 13 show the arrangement of the geared rings when in use. After removal of the fixation pin 22, the rings 9A and 9B rotate relative to one another, until they find an abutment against the teeth of the wheels 18, 19. This result in an offset e1 which is less than the free offset e (see Figs. 13 and 9). Accordingly a backlash in the device is eliminated.

Claims

1 . A method for manufacturing a housing (3) with an internal gear (4), for use in a planetary gearhead, the method comprising the steps of:

- stacking a plurality of ring elements (9), at least one of said ring elements having internal teeth (10) projecting from their inner circumference, the stacked ring elements forming a substantially cylindrical hollow body with an internal gear (4);

- forming a tubular outer shell (1 1 , 1 10) so that said ring elements (9) are made integral with said shell (1 1 ).

2. A method according to claim 1 , where said ring elements are kept in position with fixation means, prior to formation of said shell.

3. A method according to claim 1 or 2, said shell being overmoulded around the stacked ring elements.

4. A method according to any of claims 1 to 3 wherein: - said ring elements include at least a first ring element (9A) and a second ring element (9B) having internal teeth and forming at least a portion of an internal gear of said housing,

- prior to formation of said shell, said first and second ring element are positioned with a rotational offset (e), the teeth (14) of the first ring element being misaligned relative to the teeth (15) of the second ring element, and

- the shell (1 10) formed around said first and second element is made of an elastically deformable material.

5. A method according to claim 4 wherein: - after the formation of said elastically deformable shell (1 10), said first ring element and second ring element are rotated relative to one another, to reach a position where the teeth of the two ring elements are substantially aligned, while inducing an elastic deformation of said shell, and

- said ring elements are connected by a removable fixation means (22) suitable to keep said elements in said position against the elastic reaction of the shell.

6. A method according to claim 4 or 5, said elastically deformable shell (1 10) being made of an elastomer or a thermoplastic elastomer.

7. A method according to any of claims 4 to 6, the outer surface of said ring elements having a pattern (13) suitable to facilitate a grip of the shell on said surface.

8. A method according to any of the previous claims, where the ring elements forming the stack comprise at least one non-toothed spacer element (12).

9. A method according to claim 8, said spacer element comprising sensor means (12A).

10. A method according to any of the preceding claims, wherein the stacked ring elements have different shape or different mechanical features.

1 1 . A method according to claim 10, the ring elements differing by any of thickness, material, hardness, surface treatment, surface coating, working tolerance, design of the teeth (10).

12. A method according to any of the previous claims, wherein the shell (1 1 ) forms the body of the housing (3).

13. A planetary gear assembly (1 ) comprising at least one planetary stage (2a, 2b) and a housing (3), characterized in that the housing comprises a plurality of stacked ring elements (9), at least one of said ring elements (9) having internal teeth (10) projecting from their inner circumference, and a tubular outer shell (1 1 , 1 10) which is integral with said stacked ring elements.

14. A gear assembly according to claim 13, said shell being overmoulded around said ring elements.

15. A gear assembly according to claim 13 or 14, characterized in that:

- the housing (3) includes at least a first ring element (9A) and a second ring element (9B), both having internal teeth (14, 15);

- said shell (1 10) is made of an elastically deformable material, and - said shell (1 10) is pre-stressed in such a way that when the teeth

(14) of said first ring element and teeth (15) of said second ring element are aligned, said shell undergoes an elastic deformation and tends to rotate said first element and second element relative to one another, thus providing a recovery of backlash.

16. A gear assembly according to claim 15, characterized in that it comprises at least one planet gear (17) and said planet gear comprising a first wheel (18) meshing with said first ring element (9A) of the housing, and a second wheel (19) meshing with said second ring element (9B) of the housing.

17. A gear assembly according to any of claims 13 to 16, said stack (8) comprising ring elements (9) with different mechanical properties.

18. A gear assembly according to claim 17, said stack (8) comprising at least a first group (8a) of ring element and at least a second group (8b, 8c) of ring elements, the elements of the second group differing from the elements of the first group, to form customized portions of the housing (3).

19. A gear assembly according to any of claims 13 to 18, said housing comprising one or more non-toothed regions (12).

20. A gear assembly according to claim 19, said housing comprising sensor means located in a non-toothed region.

21 . A gear assembly according to claim 20, said sensor means (12A) being associated to a spacer element (12).