GB2363391A - Friction clutch with nickel-chromium alloy spring means and processes associated with making such spring means - Google Patents

Abstract

A process of coating a nickel-chromium alloy includes providing a boron powder covering on the alloy surface and heating the coated alloy for 3 to 7 hours at temperatures of 800 to 1000{C in a nitrogen atmosphere. The process may involve cooling the heated alloy and repeating the heating/cooling cycle. Successive heating/cooling cycles may involve lower peak temperatures. The process is useful for providing a low-friction, wear-resistant surface coating to friction clutch spring means of alloys of nickel-chromium.

Description

I i t-' 2363391 I Friction clutch with nickel-chromium alloy spring means

and processes associated with making such spring means The present invention relates to friction clutches, in particular for motor vehicles with internal combustion engines and more particularly to springs and spring means usable in such clutches aswell as processes of making such springs and spring means.

Internal combustion engines deliver useful driving torque only within the speed range between no-load speed and full-ioad speed. Therefore, motor vehicles generally require a transmission system and a device for separating the engine from the power train. With manually shifted gears, a friction clutch is typically used for this purpose and additionally also allows the starting of an initially stationary vehicle. Known friction clutches consist of a clutch housing in which a pressure plate is guided non-rotatably and loaded by spring means to clamp at least one clutch disc which, in the rest state, i.e. the clutch engaged state, is held between the pressure plate and a torque input part owing to the spring loading. The clutch disc which is connected to a torque output part, for example the gear shaft, is therefore dragged with the pressure plate and the input part owing to friction and therefore performs substantially the same rotating movement as the input and output part. Therefore, the force generated by the spring means must not fall below a minimum value during operation, in order to transmit the engine torque reliably. Furthermore, the elasticity of the spring material is important because the friction clutch is released by elastic deformation of the spring means. As the disengagement mechanism is affected by friction, at least partial coating of the spring means with low-friction, hard materials is often worthwhile. Conventional friction clutches usually have spring means made of quenched and drawn steel such as, for example, 50CrV4. Coating of the spring means with hard material such as chromium is known, for example, from P 35 42 87.3. However, it is found that the -pring means, in particular during operation with high-performance engines, are heated to temperatures at which these conventional spring means materials can lose their elastic properties and their strength in the high temperature phase and sometimes even irreversibly. Such reversible or irreversible setting losses of the spring means can lead to a reduction in the pressing force. Relatively strong spring means are therefore required which ensure high torque transmission even when subjected to high temperatures. However, they also increase the inertial masses to an undesirable extent.

It is accordingly an object of the invention to provide a friction clutch, of which the spring means are substantially independent of temperature, frOma process for producing spring means of this type. A further object of the invention is to adopt a suitable coating material for the spring material from a process for coating the spring material with the coating material.

Disclosed is a friction clutch, in particular for use in the power train of a motor vehicle, comprising a clutch housing, a pressure plate guided non- rotatably with respect to the clutch housing, at least one clutch disc or lining and spring means for urging the pressure plate against the disc or lining wherein at least a part of the spring means is an alloy which consists of 40 to 65% of nickel.

A nickel content of 50 to 55%, of the type used in a preferred embodiment, has proven particularly favourable. In a further advantageous configuration, 10 to 30%, or preferably 15 to 25%, of chromium is provided in the spring means.

Also present may be at least one of the elements iron, niobium and molybdenum. The iron content can optionally be 10 to 30%, the niobium content 2 to 8% and the molybdenum content 1 to 6%. In particular if the spring me,ns s to undergo a heat treatment, it should additionally contain titanium and aluminium, preferably 0.5 to 5% and 0.1 to 5% respectively.

Expedient spring constructions for the spring means made of the nickelchromium alloy will be described hereinafter. If there is only a little space available, it may be expedient to design the spring means as a helical spring. This may be advantageous owing to the compactness of helical springs, in particular perpendicularly to the direction of action. On the other hand, if the space is restricted in the direction of action, it is advisable to design the spring means as a spring washer. Spring washers require only little space in the direction of action. Furthermore, their simple shape simplifies the machining of their surface, for example the production of a surface layer by fusion or galvanisation. It is also advantageous to use a diaphragm spring as the spring means made of nickel-chromium alloy. Diaphragm springs are spring washers with integrated actuating tongues or levers and a further advantage of using such spring means is that the production and assembly of the corresponding actuating mechanisms need not be changed.

During the engagement and disengagement of the clutch, in particular in the case of integrated actuating levers of the diaphragm springs but also in the case of other spring means, friction will occur. Therefore, a lowfriction, wear-resistant surface of the spring means is desirable. In one embodiment, the surface of the spring means, at least in part, contains boride. A design in which the surface of the spring means consists, at least in part, of boride is preferred. Depending on the application, boride can be applied to the surface in the molten state, for example by powder coating processes or also by electroplating, higher temperatures being applied to the spring means in the first case. This may be more advantageous in the case of heat- hardened nickel-chromium alloys. In other cases, fo- example if heating is undesirable, electroplating S advantageous.

A process for coating a nickel-chromium. material which is to be used, in particular, for coating a nickel-based spring means for friction clutches is particularly preferred and is characterised i-ri that the surface of the spring means made of nickel-chromium alloy is covered with boron powder and is then heated for 3 to 7 hours to 800 to 1000'C, while the material is located in a protective gas atmosphere.

In another aspect the present invention provides a process for producing spring means from nickel-chromium alloy, which includes a heat treatment of the nickel-chromium alloy. owing to the specific thermal properties of nickel-chromium alloys, it may be expedient if the heat treatment includes the shaping of the spring means- In one advantageous embodiment of the process, the spring means is tensioned in a device during the heat treatment so that it receives the desired shape. It may be expedient to carry out the heat treatment of the process at least intermittently under a protective gas atmosphere. It may be advantageous here to use nitrogen as protective gas. Moreover, the process can comprise, in the context of the heat treatment, at least one cooling phase which is expediently carried out using a gaseous coolant. An advantageous way of carrying out the process may involve using nitrogen for this coolant. A preferred process proposes the following steps:

a) fixing a spring blank for forming the spring means in a holding device b) heating the fixed spring blank to 900 to 10OTC for 0.5 to 1.5 hours c) cooling of the spring blank to room temperature in a nitrogen atmosphere d) heating of the spring blank to 700 to 75TC for 5 to 11 hours e) cooling of the spring blank to 600 to 6500C at a cooling rate of above 500C per hour and maintaining the temperature for 5 to 11 hours and f) cooling of the spring blank to room temperature in a nitrogen atmosphere.

A friction clutch with the spring means with 40 to 60% of nickel has the particular advantage over conventional friction clutches that the spring means suffers no or only insignificant setting losses during operation. Nickel-chromium alloys of this type can have properties of strength and elasticity comparable to those of steel, and they have substantially smaller reversible and no irreversible losses of strength under the influence of heat. Therefore, the spring means set only slightly during operation and invariably return to its original shape. The setting losses of the spring means to be allowed for when tuning the clutch are therefore considerably smaller than with state of the art clutches. Therefore, tolerances can be closer and the clutch will operate more precisely over its entire service life and at all operating temperatures. As losses of pressing force are not encountered, the spring means made of the nickel-chromium alloy can be adapted very precisely to the requirements. The friction clutch can therefore have a very compact and light construction so losses of inertia in the power train can be reduced. A longer service life of the nickel-chromium alloy spring means can also be expected. Therefore, repair costs can also be reduced.

The invention may be understood more readily, and various other aspects and features of the invention may become apparent, from consideration of the following description.

Embodiments of the invention will be described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal section of a multiple-disc clutch constructed in accordance with the invention, the upper half showing the engaged state and the lower the disengaged state and Figure2 shows a)aJongitudinal section of a single-disc clutch constructed in accordance with the invention andb)anend view of part of the spring means of the clutch.

A multiple-disc clutch constructed in accordance with the invention and shown in Figure 1 comprises a housing 5 containing a hub 1 with internal teeth 10 for mounting on a gear shaft in a non-rotatable and axially displaceable manner. The housing 5 is composed of three components namely a cylindrical part 51 adjoined at one end to a drive plate 5", which is connected with screws to a drive shaft, and adjoined at the other end to a cover plate 5"', opposite the drive plate 5". A first set of friction linings 2 is connected non-rotatably to the hub 1 via further external teeth 11. A further set of friction linings 3 is connected non-rotatably to the cylindrical part 5' of the clutch housing 5. As shown in the drawing, the friction linings 2 and the friction linings 3 are interspersed alternately in the axial direction so torque can be transmitted between the friction linings 2 and 3 and the housing 5 and the hub 1 then rotates. The friction between the linings 2 and 3 is determined inter alla by the pressing force of spring means in the form of a diaphragm spring 6 which loads a pressure plate 4 guided nonrotatably but axially displaceable in the clutch housing 5. The spring 6 is supported on the cover plate 5111 of the clutch housing 5 in such a way that the pressure plate 4 is released when radial ly-inwardly located diaphragm spring tongues of the spring 6 are pressed axially in the direction of the hub 1. On the other hand, if the diaphragm spring tongues are not pressed, the spring means 6 is biased to urge the friction linings 2 and 3 together via the pressure plate 4 so the clutch housing 5 and the hub 1 are connected non-rotatably to one another by frictional contact between the friction linings 2 and 3.

The elasticity and strength of the material of the spring 6 are decisive for the torque transmissible by the clutch. The bias of the spring means 6 also determines the force on the pressure plate 4.

The spring 6 is produced from a nickel-chromium alloy. This material has elasticity and strength comparable to that of spring steel, but is plastically deformed far less under high temperatures. In contrast to conventional steel spring means, the bias of the spring 6 made of nickelchromium alloy remains substantially constant. The friction clutch can therefore be produced more precisely and with smaller tolerances. As the strength of the spring 6 changes only insignificantly with the temperature, contact pressure and lateral forces can be adapted precisely to the other design parameters.

The nickel content of the spring 6 is generally 40 to 65% and more preferably 50 to 55%. The spring 6 may also contain chromium generally 10 to 30% and more preferably 15 to 25%. The spring 6 can also contain at least one of the elements iron niobium and molybdenum. Preferably if iron is present it is in the range 10 to 30% if niobium is present it is in the range 2 to 8% and if molybdenum is present it is in the range 1 to 6%.

The spring 6 may additionally or alternatively contain aluminium and/or titanium. If aluminium is present it is preferably in the range 0.1 to 5% and if titanium is present it is preferably in the range 0. 5 to 5%--Friction can occur at points of contact between the spring 6 and the pressure plate 4 or the clutch housing 5 owing to the disengagement process. Friction can also arise between the diaphragm spring tongues and the release or disengagement mechanism (not shown). Frictional effects are manifested in the form of wear, which is why a hard, smooth surface is desirable. The basic nickel-chromium alloy can then form the core of the spring 6 while the exterior surfaces at least partly consists of boride. The boride surface is preferably produced by the fusion of boron powder. Particularly preferred here is a process in which the nickel- chromium alloy is covered with boron powder and is then heated for 3 to 7 hours to 800 to 1000'C. A orotective gas atmosphere such as nitrogen preferably prevents undesirable reactions at the surface of the spring 6. Other processes such as, for example, galvanisation or plating can also be adopted.

The spring 6 can be subjected to heat treatment. In a preferred process a spring blank made from the compositions set forth hereinbefore is fixed in a holding device the blank is then heated to 900 to 10OCC for 0.5 to 1. 5 hours. The blank is then cooled in a nitrogen atmosphere before reheating to 700 to -7500C for 5 to 11 hours. The blank is cooled gently to 600 to 6500C at a cooling rate of above 500C per hour and this temperature is maintained for 5 to 11 hours. Finally the blank is cooled at room temperature in a nitrogen atmosphere.

Figures 2a and 2bshow a further embodiment of a friction clutch with spring means in the form of a diaphragm spring 6 also made of nickelbased material. This so-called single-disc clutch comprises a flywheel 9, which is connected non-rotatably to the clutch casing 5 by screws 12, and a pressure plate 4, which is connected non- rotatably either to the flywheel 9 or the clutch housing 5. Between the flywheel 9 and the pressure plate 4 is a clutch disc 1. with friction linings 2, which is mounted nonrotatabl.)he hub 1 fitted on a gear shaft. As in the first embodiment, the spring means 6 is supported on the clutch housing 5 in such a way that the pressure plate 4 is released when the inner diaphragm spring tongues are pressed axially in the direction of the hub 1. In the rest position, on the other hand, the biased spring 6 presses the friction linings 2 and the flywheel 9 together via the pressure plate 4 so the flywheel 9 connected to the drive or crankshaft and the hub 1 are nonrotatably connected to one another by the frictional contact of the friction linings 2. To disengage the clutch, the diaphragm spring tongues are pressed with a release bearing 14 axially in the direction of the hub 1. As the spring 6 rotates at engine speed whereas the release bearing 14 is stationary, friction occurs between release bearing 14 and diaphragm spring tongues particularly if the spring 6 is not located exactly centrally in the clutch casing S. Therefore, the spring 6 is also provided with an external boride layer. The spring 6 can be produced by the same processes as the spring 6 of the first embodiment.

The coating process according to the invention and the production process according to the invention can just as well be used for other components made of nickel-based material which are to be coated and/or shaped.

Instead of the diaphragm springs 6 shown in the two embodiments, simple spring washers or helical springs can also be used as spring means. Also the spring means constructed in accordance with the invention need not necessarily engage the pressure plate or the like, but can be used in any other application where it is subjected to temperature variations.

Claims (11)

Claims

1. A process of coating a nickel-chromium alloy by covering a surface of the alloy with boron powder and then heating the alloy for 3 to 7 hours to 800'C to I OOO'C in a protective gas atmosphere.

2. A process according to claim 1, wherein nitrogen is used as the protective gas.

3. A process for producing spring means made of nickel-chromium allow which utilizes the coating treatment of claim 1.

4. A process according to claim 3, wherein the coating treatment includes the shaping of the spring means.

5. A process according to claim 3 or 4, wherein the spring means is tensioned in a device during the coating treatment.

6. A process according to claim 3, 4 or 5, wherein the coating treatment takes place at least in part under a protective gas atmosphere.

7. A process according to claim 6, wherein the protective gas consists at least substantially of nitrogen-

8. A process according to any one of claims 3 to 7, wherein at least one cooling phase is carried out in a gaseous coolant during the coating treatment.

9. A process according to claim 8, wherein the gaseous coolant is nitrogen.

10. A process according to any one of claims 2 to 9, ffirther comprising the steps of a) fixing a spring blank in a holding device b) heating of the fixed spnng blank to 900 to IOOOT for 0.5 to 1.5 hours i I C) cooling of the spring blank to room temperature in a nitrogen atmosphere d) heating of the spring blank to 700 to 750T for 5 to I I hours e) cooling of the spring blank to 600 to 650T at a cooling rate of above 50'C per hour and maintenance of the temperature for 6 to I I hours 0 cooling of the spring blank to room temperature in a nitrogen atmosphere

11. A process of producing spring means and/or coating a nickel-chromiurn alloy substantiaUy as described herein.