GB2203974A - Apparatus for manufacturing a crown wheel cam - Google Patents

Abstract

A tapered former 42 is provided around which an abrasive belt 41 is driven so as to grind the correct cam surface on a swash plate (60) for use in a crown wheel cam gearbox. After each cam surface is produced on the periphery of the swash plate the plate is rotated (62) to a new position and the process repeated until the swash plate is complete. The belt may be a continuous loop and may also be tapered for engagement over the tapered former. Alternatively the belt may be in the form of a spool arrangement (81, 82 Fig. 8 not shown) such that abrasion is continuously carried out by fresh belt to ensure accurate forming. In a further arrangement the belt (111, Fig 11 not shown) is oscillated by means of an electromagnetic vibrator (115) with fresh abrasive belt being supplied by indexing the belt as required. <IMAGE>

Description

For EQL The e invention relates to crown wheel cam reduction gearboxes having an eccentrically mounted crown wheel with a cam profile in at least one face thereof and in particular to an apparatus for producing the cam profile.

UK Patent GB2117474A describes a crown wheel cam gearbox having a crown wheel eccentrically mounted on an input drive shaft. A first set of cam profiles on one side of the cam wheel is engaged by a set of roller cam followers located in the gearbox housing and a second set of cam profiles on the other side of the crown wheel is engaged by a second set of roller cam followers located around the periphery of the output shaft of the gearbox.

The e reduction gearing is determined by he numbers of cam profiles on the two sides of the crown wheel and the nutters of cam followers.

UK Patent Application No. 8625047 describes a method for producing the cam profiles on the crown wheel rotor. The e method involves the use of a geared drive acting through a constant velocity joint to reproduce the rotating swash plate motion of the crown wheel rotor while a milling wheel is brought into engagement to grind the cams. This arrangement however places a great demand on the stiffness of the gearing and constant velocity joint since accurate forming of the cam profiles on the crown wheel id desirable for efficient operation. In this known arrangement wear on the milling machine cutter will also have deleterious effects on the cam profiles.

The object of the present invention is to provide an improved arrangement for accurately forming cam profiles.

The e invention provides apparatus for manufacturing a cam member comprising a cylindrical body with a plurality of identical cam profiles around the periphery of one end face thereof, the apparatus comprising; a) means to support the cam matter; b) a former having a surface profile complementary to the desired cam profiles; c) an abrasive coated belt located such that a portion of the belt engages the former surface profile; d) drive means connected to the abrasive belt such that; the belt can b driven lengthwise over the former; and e) means to bring the cam matter and abrasive belt into contact such that the correct cam profile can be ground on the cam matter. In a preferred arrangement the cam member support means is rotatable and an indexing means is provided such that successive cam profiles can be ground on the face of the cam matter. The belt may be a continuous loop or a single length stored in cartridge form. In addition the belt may be driven in one direction only or it may be oscillated. In the latter case fresh abrasive may be provided by locating the belt with releasable clamps such that the belt can be indexed periodically as required.

The invention ensures that the cam profiles are produced to an accuracy limited by: i) the accuracy of manufacture of the former; ii) the accuracy of the former's ultimate location in relation to the cam rotor; and iii) the constancy of the depth of cut produced by the abrasive belt.

Concerning iii), the use of abrasive belt in a single length or in a large continuous loop provides a fresh or a relatively fresh abrasive surface, respectively, thereby ensuring constancy of depth of cut. In addition to grinding pre-machined crown wheel rotors the invention may also be used to finish a forged cam matter with pre-shaped cam profiles, thereby avoiding machining operations.

The invention will now be described by way of example only with reference to the accompanying Drawings of which: Figure 1 shows a perspective view of a prior art crown wheel cam gearbox, partly cut away; Figure 2 shows a cross-section through the gearbox of Figure 1; Figure 3 shows a knawn cutting apparatus for producing the cam profiles crown wheel used in a crown wheel cam gearbox; Figure 4 is a schematic perspective view of cam grinding apparatus according to the invention; Figure 5 is a part sectional view perpendicular to the axis of the cam rotor showing an abrasive belt guided over a profiled former; Figure 6 is a perspective view of the apparatus assembled to a milling machine; Figure 7 is a cross-section through the crown wheel rotor during grinding;; Figure 8 illustrates a spooling arrangement for a straight abrasive belt; Figure 9 is a part perspective view of the use of a tapered abrasive belt; Figure 10 is a development of the cam surface ABCD of Figure 9 on the tapered belt; and Figure 11 shows a mechanism for oscillating an abrasive belt.

Figures 1 and 2 show a known reduction gearbox described in UK patent number GB2117474A connecting an input drive shaft 2 to an output shaft 3. A crown wheel 4 is mounted on bearings on the input shaft 2 such that its axis 7 is at an angle to the rotational axis of the input shaft 2. Saw tooth cam profiles 5 and 6 are provided respectively on the input and output sides of the crown wheel 4. The cam profiles 5 are engaged by a set of tapered input rollers 13 located in the casing 18 of the gearbox and the cam profiles 6 are engaged by an output set of tapered tollers 14 located at an end 24 of the output shaft 3. As the input shaft 2 is rotated the cam wheel rotates relative to the input shaft and executes a swash plate motion.Movement of the cam wheel through the interaction of cam profiles and rollers causes the output shaft 3 to rotate with a reduction gearing dependent on the numbers of rollers 13,14 and cam profiles 5,6.

Figure 3 shows an apparatus described in GB Patent Application No.

8625047 for producing the cam profiles 30 on the crown wheel rotor 31. The crown wheel rotor 31 is simultaneously rotated and swashed relative to a milling machine cutting tool 32. The movement is controlled by operation of an appropriate gear train 33 and constant velocity (CV) joint 34 which forces the axle 35 attached to the crown wheel rotor 33 to rotate about its own axis while the CV joint rotates about the axis 36 of the gear matter 37.

This prior art arrangement has two disadvantages leading to inaccurate cam profile finish: 1) grinding is done by a small cutting tool and this promotes wear on the surface of the cutting tool; and 2) any play in the gear train 73 or CV joint 72 will cause inaccurate movement of the crown wheel rotor relative to the milling machine.

Figure 4-6 illustrate the technique adopted in the present invention. A thin abrasive coated belt 41 is pulled over a suitably profiled former 42 by a drive pulley 45 rotated about axis 46. The belt 41 is flexible and controlled such that it follows the exact contour of the profiled former 42. The shape of the former 42 is such that the correct cam surface is ground in the cam rotor 60 when the belt and former assembly reach a predefined stop position 61 in relation to the cam rotor. To enable all of the cam surfaces to be ground, the cam rotor 60 is mounted upon an indexing table 62 which rotates at intervals equal to the cam pitch. The desired cam profile is tapered and conical and thus the former 42 is part conical. To ensure that the belt is correctly positioned with no slack two pulley guides 43 are provided, freely rotatable about angled axes 44.The belt 41, former 42 and pulley guides 43 are assembled to a back plate 63 and the assembly 64 is attached to a milling machine 65. The indexing table 62 supporting the cam wheel rotor 60 is mounted on a platform 66 moveable vertically between a lowered position as shown and an upper position when the platform engages the stop.

As can be seen with reference to Figure 7 the shape of the former 42 is not exactly casnplementary to the cam profile since allowance is made for the finite thickness "t" of the belt 41. Thus the former is shaped such that on ccnrrpletion of the grinding operation, as shown in the Figure, the lines connecting the loci 70 of the points midway between the teeth of the crown wheel rotor intersect at the centre 71 of the crown wheel rotor.

There are several possible arrangements for the abrasive belt, these are: 1) a straight continuous belt - this has cylindrical geometry as though cut from a uniform hollow cylinder; 2) a tapered belt - this is conical in geanetry as though cut from a hollow cone; 3) a straight strip - this involves feeding the belt fran asupply spool to a take-up spool; and 4) a curved strip this would require a carglex spooling arrangement.

Figure 8 shows an arrangement in which a straight abrasive strip 80 is driven over the profiled former 42 to a take-up spool 81 from a feed spool 82.

This enables grinding to be done throughout the operation by fresh abrasive.

An alternate continuous belt arrangement is shown in Figure 9. A tapered belt 90 is driven by a tapered pulley 91 over the tapered profiled former 42. In this arrangement the abrasion is carried out scircumferentially of the crown wheel. rotor 60 as can be seen with further reference to Figure 10. Development of the cam profile ABCD is shown on a developed portion of the abrasive belt 90. The lines CB and DA have respective radii of curvature "r" and "R" corresponding to the inner and outer radii of the crown wheel rotor 60. This arrangement allows the grinding to be done with a belt only marginally wider than the cam teeth.Furthermore, since the grinding is done along the length of the cam face this method ensures the smoothest motion of the rollers which engage the cam surfaces. One disadvantage of this method is that the belt is more difficult to manufacture than a plain belt. Secondly, it will probably be necessary to provide flanges to prevent sideways slipping of the tapered belt. Such flanges will require careful design to prevent damage to the edge of the belt.

By contrast, the straight belt and straight strip arrangements are easily configured with appropriate guide pulleys 33. The guide pulleys 33 are angled to ccsnpensate for the shorter length travelled by he edge of the belt which passes around the narrower end of the profiled former. A similar arrangement could also be used with the straight strip shown in Figure 8. The present invention provides the possibility of making cam profiles more accurately than before since the accuracy is determined principally by the shape of a profiled former about which an abrasive belt is driven.

Preferably the abrasive belt should be endless with no join since problems can occur as a join passes over the pulleys. Some stretching of the belts has been found to occur and some tensioning of the belt may be required to provide sufficient friction over the face of the driving pulley. Control of continuously rotating belts at high speed has been found to be difficult.

Some of the above difficulties are alleviated by using an abrasive belt that is oscillated over a profiled former. ane such arrangement is shown in Figure 11. A length of abrasive belt 111 is oscillated over a profiled former 112 by means of a reciprocating drive member 113 connected by means of a connecting rod 114 to an electranagnetic vibrator 115. The drive member 113 is formed of two similar sectored portions 116,117 and is rotatably mounted on a pivot bearing 118. Linnear movement of the end 119 of the connecting rod 114 causes the drive member 113 to make reciprocating arcuate movements about the pivot bearing 118. The abrasive belt 111 is attached to each sectord portion 116, 117 by clamps 1110.

The oscillating belt has a number of advantages compared with a continuously rotating belt: a) Better ctftting has been found to occur Seuse some abrasive particles that are blunt in one direction are sharp in the other and therefore more abrasive particles are available for cutting; b) The oscillating motion of the belt helps to prevent clogging of the belt by waste material; c) By providing a spool of abrasive belt which can be indexed through the machine on release of the belt clamps 1110 a fresh abrasive surface can easily be presented for the final grinding operation; d) Control of the belt is much easier than with a continuously rotating belt; e) No problem has been experienced with belt stretching.

There are also a number of disadvantages: a) Indexing and tensioning of the belt must be done quickly and easily and this presents an added canplexity; b) The maximum surface speed of the belt is lower than for a continuously rotating belt and thus the efficiency of the grinding action is likely to be lower in terms of material removal rate against power required to drive the belt.

Claims (10)

Claims

1. Apparatus for manufacturing a cam member comprising a cylindrical body with a plurality of identical cam profiles around the periphery of one end face thereof, the apparatus cawrising: a) means to support the cam matter: b) a former having a surface profile catlementary to the desired cam profile; c) an abrasive coated belt located such that a portion of the belt engages the former surface profile; d) drive means connected to the abrasive belt such that the belt can be driven lengthwise over the former; and e) means to bring the cam member and abrasive belt into contact such that the correct cam profile can be ground on the cam matter.

2. Apparatus as claimed in claim 1 wherein the cam matter support means is rotatable and an indexing means is provided such that successive cam profiles can be ground on the face of the cam matter.

3. Apparatus as claimed in claim 1 or 2 wherein the profile former is tapered.

4. Apparatus as claimed in any one preceding claim wherein the belt is a single length stored in cartridge form.

5. Apparatus as claimed in any one of claims 1 to 4 wherein the abrasive belt is oscillated over the profile former.

6. Apparatus as claimed in claim 5 wherein there is provided means to index the belt such that a fresh area of abasive belt is provided over the profile former.

7. Apparatus as claimed in any one of claims 1 to 4 wherein the belt is a continuous loop.

8. Apparatus as claimed in any one of claims 5 wherein the abrasive belt is guided by two pulleys, one on either side of the profile former, with their axes of rotation angled such that the belt is retained on the profiled former.

9. Apparatus as claimed in any one of claims 6 wherein the abrasive belt is tapered.

10. Apparatus substantially as claimed and described herein with reference to Figures 1,2,4-7 and 10 of the accompanying Drawings.

11. Apparatus substantially as claimed and described herein with reference to Figures 1,2, 5 and 8 of the acccaynying Drawings.

10. Apparatus substantially as claimed and described herein with reference to Figures 1,2, 5 and 11 of the accompanying Drawings.

10. Apparatus substantially as claimed and described herein with reference to Figures 1,2, 5 and 9 of the accompanying Drawings.