VISCOUS COUPLING

Abstract: A viscous coupling has a chamber (32) containing a high viscosity fluid. The chamber is bounded by surface portions of rotatable members (10,20,28) to be coupled by the coupling, and the rotatable members have disc-like elements (40) which are interleaved with each other. There are at least three of these rotatable members (10,20,28), and the torque is transmitted between all of the members. Each wall section bounding the chamber is arranged so that no net axial force is exerted on the wall section by pressure arising inside the chamber.

Description: VISCOUS COUPLING This invention relates to a viscous coupling, for example of the type used in a four-wheel drive transmission in a motor vehicle. A viscous coupling is known from GB-A-1 252 753. This coupling has an enclosed chamber containing interleaved disc-like elements. One set of the elements is mounted on a driving member and the other set of elements is mounted on a driven member. The chamber is filled with a high viscosity fluid which penetrates between the interleaved elements. When one set of elements is turned, the viscosity of the fluid drags the other set of elements along, thus allowing torque to be transmitted between the driving and driven members. Viscous couplings are also known from EP-A- 68 309 and US-A-2 883 884, in which three rotatable members are interconnected through one coupling. A first set of disc-like elements is mounted on one member, and other sets of elements are mounted on the other members, with both other sets being interleaved with first set. With such couplings, a considerable pressure is built up internally in the viscous fluid when relative movement between the components takes place. The chamber which contains the high viscosity fluid has to be able to contain this pressure and prevent any of the fluid leaking out through the seals. According to the present invention, there is provided a viscous coupling for providing a limited-slip coupling between three or more coaxial rotatable members, the coupling comprising an enclosed chamber bounded by wall sections each of which is connected to one of the rotatable members, three or more sets of elements Cin the chamber, each set being in driving connection with one of the rotatable members and all the sets being mounted for rotation about a common coupling axis, each set of elements being interleaved with another set of elements, a substance in the chamber in contact with the elements and which transmits torque from the elements of one set to the elements of the set with which it is interleaved, the torque transmitted increasing with increase in the mutual speed difference between the sets, wherein the chamber is sealed by seals arranged between the wall sections, and wherein each of the wall sections which has a surface lying within the chamber and extending other parallel to the couplng axis also has an oppositely facing surface within the chamber, of corresponding area, so that there is no net axial force exerted on any of the wall sections by pressure arising in the chamber. With this construction, there is no tendency for the different wall sections to move relative to one another under the pressure which arises internally, and the seals are therefore not unduly stressed. This makes the coupling more reliable than is the case in prior art couplings where there are net forces acting on the chmaber walls which inevitably affect the life of the chamber seals. Since the forces acting on the components are low, construction can be sinplified and the cost thereof may be reduced. Although a preferred form of the invention provides coupling between three rotatable members, the invention is not limited to this particular number of members and the coupling may couple more than three rotatable members. The chamber is preferably -annular in shape, and a first set of elements may be mounted on the outer periphery of the chamber with second and third sets being mounted on the internal periphery, at axially spaced positions and both being interleaved with the first set. There may be a gap between the second and third sets of elements, and the first set may be split into two subsets, one subset interleaved with the second set and the other subset interleaved with the third set. The coupling can be arranged so that the coupling conditions between the first and second sets of elements are different from the coupling conditions between the first and third sets. This can be achieved by mounting the second and third sets on sleeves of different diameter, or by varying the dimensions of the elements so that the area of overlap is different. This facility is particularly desirable in the case of a coupling for use as a centre differential in a four wheel drive system. The seals between the chamber wall sections are are preferably all arranged at the same radius from the coupling axis, to avoid any net forces acting on the wall sections. The invention will now be described further, by way of example, with reference to the accompanying drawing which shows a section through a viscous coupling in accordance with the invention. A first rotatable member 10 is driven in rotation by a pinion 12 meshing with a spur gear ring 14 on the periphery of the member 10. Fast with the member 10 is a chamber housing 16 and the member 10, the gear ring 14 and the housing 16 are all secured together by fastenings, one of which is indicated at 18. A second rotatable member 20 has an input shaft 22 splined to a coupling sleeve 24, with an output shaft 26 splined to the other end of the coupling sleeve. A third rotatable member 28 is tubular and surrounds the output shaft 26 of the second rotatable member. This third member 28 is splined to a sleeve 30. An enclosed chamber 32 is bounded by three components, namely the housing 16 and portions of the coupling sleeve 24 and of the sleeve 30. The chamber is sealed by three seals 34, 36 and 38 each of which seals one of the chamber-bounding components to an adjacent component.The components each have surfaces parallel to the coupling axis and other surfaces which are not parallel. The housing 16 has surfaces A which are parallel to the axis, and surfaces B1, B2 and B3 which are not so parallel. Note that the chamber internal pressure will act on the surface B3, as this is within the seals and the viscous fluid in the chamber will be in contact with this surface. The coupling sleeve 24 has only surface C facing into the chamber, and this surface is parallel to the axis. The sleeve 30 has a surface D which is parallel to the axis, and surfaces E1 and E2 which are not parallel. During operation, an internal pressure is produced in the chamber when there is relative movement between the elements. This pressure is exerted on all the walls of the chamber. The pressure on surface B1 is counteracted by the pressure on surfaces B2 and B3, and since the combined surface area normal to the coupling axis of surfaces B2 and B3 is equal to the surface area of surface B1, there will be no net axial force on the housing 16 and therefore no tendency for the seals 34 and 38 to be displaced by relative movement between the components. Similarly, for the sleeve 30, the surfaces El and E2 are equal in projected area normal to the coupling axis. Each of the components supports a set of thin, disc-like elements. The elements are indicated generally at 40 in the drawing. The elements on the sleeve 30 are interleaved with one half of the elements on the housing 16 and the elements on the coupling sleeve 24 are interleaved with the other half of the elements on the housing 16. The chamber 32 is filled with a high viscosity fluid. Examples of suitable fluids are (a) silicone fluid or uncured rubber in mineral oil, and (b) distilled water, a solution of zinc nitrate, calcium carbonate and a water softener (sodium hexametaphosphate) known as Calgon. The coupling described can be used as a centre differential in a four wheel drive vehicle transmission to control the relative slip between the front and rear axle differentials. For example, the housing 16 can be driven from the engine via the gearbox, the coupling sleeve 24 can be coupled to one of the front wheels and, via a front axle differential, to the other front wheel and the sleeve 30 can be coupled to the rear wheels drive shaft which-feeds into a rear axle differential. The coupling is compact and allows three coaxial members to be driven together. To determine the-ratio of torque transmission between the two transmission paths (first member to second member and first member to third member) the spacing between the interleaved elements could be changed, or the area of overlap-could be adjusted either by changing the sizes of the elements 40, or by changing the distance between the chamber walls in one or the other part of the chamber. Other possible ways of changing the ratio of torque transmission are to change the viscosity of the fluid or to change the form of the plate elements, perhaps by including slots or holes in them.

Claims: CLAIMS 1. A viscous coupling for providing a limited-slip coupling between three or more coaxial rotatable members (10, 20,28), the coupling comprising an enclosed chamber (32) bounds by wall sections each of which is connected to one of the rotatable members, three or more sets of elements (40) in the chamber (32), each set being in driving connection with one of the rotatable members (10,20,28) and all the sets being mounted for rotation about a common coupling axis, each set of elements (40) being interleaved with another set of elements (40), and a substance in the chamber in contact with the elements (40) and which transmits torque from the elements of one set to the elements of the set with which it is interleaved, the torque transmitted increasing with increase in the mutual speed difference between the sets, wherein the chamber is sealed by seals (34,36,38) arranged between the wall sections and wherein each of the wall sections which has a surface (E1) lying within the chamber and extending other than parallel to the coupling axis also has an oppositely facing surface (E2) within the chamber (32) of corresponding area, so that there is no net axial force exerted on any of the wall sections by pressure arising in the chamber. 2. A coupling as claimed in Claim 1, wherein the chamber is annular in shape, and a first set of elements is mounted on the outer periphery of the chamber with second and third sets being mounted on the internal periphery, at axially spaced positions and both being interleaved with the first set. 3. A coupling as claimed in Claim 2, wherein a gap is present between the second and third sets of elements, and the first set is split into two subsets, one subset interleaved with the second set and the other subset interleaved with the third set. 4. A coupling as claimed in any one of the preceding claims, wherein the seals between the chamber wall sections are all arranged at the same radius from the coupling axis, to avoid any net forces acting on the wall sections. 5. A coupling as claimed in any preceding claim, arranged so that the coupling conditions between the first and second sets of elements are different from the coupling conditions between the first and third sets. 6. A coupling as claimed in Claim 5, wherein the elements of the second and third sets are mounted on sleeves of different diameter. 7. A motor vehicle having a four wheel drive transmission, wherein the centre differential is a viscous coupling as claimed in any preceding claim.

Inventors: DIGGINS, RAYMOND
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1985
Main classification: F16H1
IPC codes: B60K17, F16D35, F16H48