WO1998035820A1

WO1998035820A1 - Moulded parts for transmissions

Info

Publication number: WO1998035820A1
Application number: PCT/GB1998/000336
Authority: WO
Inventors: Frederick Jeremy Dugard; Brian Robert Duke; Douglas Walton
Original assignee: CTP DAVALL Ltd; University of Birmingham
Current assignee: CTP DAVALL Ltd; University of Birmingham
Priority date: 1997-02-13
Filing date: 1998-02-13
Publication date: 1998-08-20
Anticipated expiration: 1999-08-13
Also published as: GB9803155D0; GB2322095A; GB9702918D0

Abstract

A two-part moulding for use as a gear, cam or pulley in a transmission system comprises a core made of a reinforced polymer, e.g. nylon or acetal, for strength, and an outer skin of a polymeric plastics material which has good tribological properties but is also a mechanical load-bearing material. The skin may be unalloyed nylon or acetal, or may include PTFE and/or silicone and/or molybdenum disulphide.

Description

MOULDED PARTS FOR TRANSMISSIONS

This invention relates to moulded parts for use in mechanical power and motion transmissions, and also to the manufacture of such parts. The fact that moulded polymer parts can run under dry conditions is a major factor in choosing them for mechanical power and motion transmissions. To obtain high performance, plastics components such as gears, cams and pulleys are frequently moulded from polymers containing strengthening materials and compounds. The strengthening materials and compounds, typically glass fibre or carbon fibre, add to both the strength and the stiffness of components. Injection moulded gears, cams, tooth belt and vee belt pulleys and both male and female bearing journals and shafts are often further modified by the addition of internal lubricants. These in turn increase the lubricity of the components, leading to lower friction and less wear. However, the use of polymer composites in this way can lead to a compromise in performance as: • internal lubricants can weaken the bulk strength of the polymer

• internal reinforcements can lead to surfaces which have poor tribological properties because the fibres are often left on or protrude from the surface, interfere with sliding and so result in high wear.

In WO93/21450 there is described a sleeve-shaped friction bearing in which a thin lubrication film of a suitable polymer is applied to a mandrel, and the film is covered by a support layer of hard or flexible polymer. US-A-5219499 describes a helical screw rotor comprising a central shaft, a rotor body, and an outer layer of polymer- based material . The aim is to avoid problems of shrinkage in manufacture and to achieve high precision. The outer layer consists solely of PTFE plastics material . Although PTFE has good frictional properties under pure sliding conditions, it has poor mechanical properties and would not be able to withstand the Hertzian contact stresses which occur in gear transmission systems for example.

It is an object of the present invention to provide moulded parts, especially for gear and cam transmission systems, which overcome these deficiencies.

In accordance with the invention there is provided a moulded power and/or motion transmission component comprising: a) a core portion of a polymeric plastics first material, and b) a skin portion of a polymeric plastics second material bonded to the core portion, wherein the skin portion extends over at least the part of the component which in use makes contact with another component, the first material is stronger than the second material, and the second material has a lower coefficient of friction than the first material but is a mechanical load-bearing material .

The first and second materials must be chosen to give sufficient bond strength between the core and the skin portions . The core of the component is preferably made from a polymer containing strengthening reinforcement whilst the skin is made from a material chosen to give low friction, support Hertzian stresses, and have good wear properties. In the case of components which mesh together, such as gears, cams, tooth belt and vee belt pulleys and both male and female bearing journals, the materials of the respective meshing surfaces need not be of the same or a similar compound in order to give optimum tribological properties at the point of contact .

A two-part moulding in accordance with the invention has a number of advantages. Inner core materials can be chosen for maximum strength or other properties, while surface materials can be chosen for optimum surface properties, e.g. tribological properties.

Certain embodiments of moulded parts in accordance with the invention have the following characteristics, either singly or in combination.

1. The outer layer is of a thickness which will withstand the Hertzian contact stresses which range in depth according to the application but are typically up to 2.0 millimetres deep, and obey the following formula, or its derivatives : -

Hertzian contact stress z_c= (l/π x( ( (i-vi**2) /ED + ( (ι-v2**2) /E2) ) where : - the values of Poissons ratio vl and v2 are typically between 0.3 and 0.4 the values of elastic modulus El and E2 are above a level of 0.35 x 10^"3 N/mm².

2. The outer layer has a coefficient of friction, μ, of below 0.4.

3. The outer layer is able to withstand a combination of Hertzian pressure x slide velocity PV, represented by the formula or its derivative: PV=Z_CV where : -

Z_c may be derived as above . V may be derived from the slide/roll ratio.

PV is limited by the allowable wear rate at a given stress and temperature, and is a balance of heat -in to heat-out (heat-in being friction/load dependent and heat -out being largely convection and conduction dependent) .

The inner core is able adequately to support the bending stresses imposed upon it which are usually up to and above 50MPa and which broadly obey the following formula or its derivatives : - Bending stress ρ=F_t/(b.mn) where: the value of tangential force F_t varies according to application and size the value of face width b varies according to application and size the value of normal module mn varies according to application between the limits of 0.4 and 10. If a material is too weak in tensile stress it will fail due to root bending stress overload and Hertzian stress fatigue. If a material has poor lubricity it will fail due to wear. Thus this invention brings together a combination of properties which push the power throughput properties of gears, pulleys and cams to higher^' limits than a monolithic construction can achieve.

Examples of materials which can be used are the following:

• Cost-reduced supporting core material with enhanced property surface material such as when solid lubricants (PTFE powder and fibres, silicone spheres, aramid fibres, graphite and MoS2) are introduced at the surface to enrich it, along also with silicone oil, special mineral and synthetic oil .

• An absorbent core material may be used with a specialised permeable surface layer to enhance lubricity. • Conductive surface coatings may be used particularly for statically sensitive applications. None of the above preclude the possibility of operating these moulded components in external lubricants such as oil, grease, water or aqueous based substances . Manufacturing methods by which the moulded parts may be made include :

• the Battenfeld GmbH 2K process employing a single shot twin barrel injection technique.

• the Admix Ltd process employing a single shot side chamber injection technique, and

• by over-moulding a specialised surface layer upon a supporting core.

Other methods such as powder coating could alternatively be employed. The first two of these manufacturing methods employ simultaneous injection moulding of the two materials. Size of moulding is not seen as a limitation to this technique; the normal size limitations of injection moulding apply.

Gear mouldings using these two techniques have been successfully produced. As a result it has been possible to demonstrate that

• complex shapes such as gears with many teeth and with normally accepted flange and rim geometries can be successfully moulded

• superior wear (as compared to plain polymer and reinforced polymer composites) resulting from dual- moulding a PTFE enriched nylon or acetal surface layer onto a core of glass fibre reinforced nylon under a high torque and speed. • the surface bond between the surface and core materials is able to withstand the Hertzian sub- surface stresses as well as the bending stresses resulting from normal gear action without degradation. Various materials have been found acceptable for both the core and the skin. The core is preferably a reinforced polyamide (nylon) material or a reinforced acetal material. Alternatively, one can use polybutyleneterephthalate (PBT) , polyestersulphone (PES) , aliphatic polyketone ("Carilon"), liquid crystal polymer (LCP) or polyetherketone (peek) , suitably reinforced. The reinforcement for any of the aforesaid materials can be glass fibres, carbon fibres, steel fibres, ceramic fibres or aramid fibres. The reinforcement is preferably in the range of 10 to 70% by weight, more preferably 30 to 45% by weight.

The skin can be unalloyed polyamide (nylon) material, e.g. nylon 66. Alternatively, one can use an acetal material,

PBT, PES, "Carilon", LCP or PEEK, as above. Each of these materials, if not used in unalloyed, form, can be alloyed with one or more of PTFE, silicone oil, molybdenum disulphide, graphite and mineral lubricants. If PTFE is used, it is preferably in an amount up to 30% by weight, more preferably between 15 and 20% by weight.

The following Table 1 shows test results for various materials. Gears comprising a driven gear and a driving gear were moulded and tested for weight loss over 24 hours under load. The results show clearly that the monolithic gears without fibre reinforcement performed poorly. The monolithic polyamide nylon gears with fibre reinforcement performed better, but the two-part moulded gears in accordance with the invention all showed very little weight loss.

TABLE 1

Note: Cl and C2 are controls. Pi to P5 are of two part construction. GF - Glass Fibre. CF - Carbon Fibre

Claims

1. A moulded power and/or motion transmission component comprising : a) a core portion of a polymeric plastics first material, and b) a skin portion of a polymeric plastics second material bonded to the core portion, wherein the skin portion extends over at least the part of the component which in use makes contact with another component, the first material is stronger than the second material, and the second material has a lower coefficient of friction than the first material but is a mechanical load-bearing material .

2. A component according to claim 1, in which the core portion is a reinforced polymeric plastics material.

3. A component according to claim 2, in which the core portion comprises reinforced polyamide or reinforced acetal material .

4. A component according to claim 1, in which the core portion is made of polybutyleneterephthalate , polyestersulphone or aliphatic polyketone .

5. A component according to any preceding claim, in which the core portion is reinforced with one or more of glass, carbon, steel, ceramic and aramid fibres.

6. A component according to claim 5, in which the fibre reinforcement is present in an amount of 10 to 70% by weight, preferably 30 to 45% by weight.

7. A component according to any preceding claim, in which the skin portion comprises a polyamide material .

8. A component according to any of claims 1 to 6, in which the skin portion comprises an acetal material .

9. A component according to any of claims 1 to 6 , in which the skin portion comprises polybutyleneterephthalate, polyestersulphone or aliphatic polyketone .

10. A component according to any of claims 7 to 9 , in which the skin portion includes polytetrafluoroethylene .

11. A component according to claim 10, in which the polytetrafluoroethylene is present in an amount up to 30% by weight, preferably 15 to 20% by weight.

12. A component according to any of claims 7 to 11, in which the skin portion comprises silicone oil and/or molybdenum disulphide.