FLEXIBLE BOOT FOR UNIVERSAL JOINTS

Abstract: A flexible boot for universal joints including a joint body (12) having an inner ring (10) fixed to one end of a first shaft (6) and an outer ring (5) provided on one end of a second shaft (9) comprises both ends fixed to the outer ring and to the first shaft, respectively, and a bellows portion (4) positioned between the both ends and having at least one valley (4a). There is provided at least one elastic holder ring (7) closely contacted with an outer peripheral surface of the at least one valley to prevent acute folding of the valley of the flexible boot.

Description: The present invention relates to a flexible boot for universal joints such as a constant velocity joint used with a drive shaft of an automobile and the like. DESCRIPTION OF THE PRIOR ART A conventional constant velocity joint includes, as shown in Fig. 9, an inner ring 10 fixed to one end of a first shaft 6, an outer ring 5 formed on one end of a second shaft 9, and a flexible boot 1 having a bellows portion 4 and fixed at its both ends to the outer ring 5 and the first shaft, respectively. The flexible boot 1 serves to sealingly hold therein grease used as lubricant for the constant velocity joint and to prevent foreign matters such as dust and water from entering into the joint. The bellows portion 4 of the flexible boot 1 can follow the movement of the joint by expanding and contracting peaks and valleys of the bellows portion 4 even when the first shaft is inclined at a certain operating angle with respect to the second shaft. To this end, the peaks and valleys of the bellows portion are so configurated that they have a smoothly curved bent portions for dispersing the stress applied thereon, and the bellows portion 4 is made of material having high elasticity and good resistance to fatigue due to the expansion and contraction thereof, such as rubber and resin. In the above-mentioned conventional constant velocity joint, however, there was a problem that the valleys 4a of the bellows portion 4 were damaged in a short time when the joint often operated at the large operating angle between the first and second shafts, even if the bellows portion of the flexible boot was made of the material having the good resistance to fatigue due to the expansion and contraction thereof. More specifically, as shown in Figure 9, when the constant velocity joint is operated at the large operating angle (i.e. when the first shaft 6 is largely inclined with respect to the second shaft 9 as shown), the valleys 4a of the bellows portion on the highly inclined side of the shaft 6 are compressed between an end surface 5a of the outer ring 5 of the second shaft 9 and an outer surface of the first shaft 6, with the result that some of the valleys 4a cannot maintain its smoothly curved configuration, thus creating acute folds at the positions B in the valleys 4a. Due to such folded portion B, the condition which exceeds the elastic limit of the material of the bellows is generated in the folded portion (i.e. the condition which changes molecular construction in the material), as in the case where, when a rubber plate is acutely folded, a folded mark is created in the folded portion. When the folded portions B are suddenly expanded due to the change in the operating angle (i.e. the change in the inclination of the first shaft with respect to the second shaft), the molecular construction in the material of the bellows portion will be destroyed. Thus, by repeated folding and expansion of the bellows portion, the valleys of the bellows portion of the conventional flexible boot will be prematurely destroyed. In summary, in the conventional flexible boot, when the valleys of the bellows portion of the flexible boot are acutely folded, the service life of the flexible boot is remarkably or extremely reduced. An object of the present invention is to provide a flexible boot for universal joints such as a constant velocity joint which can solve the above-mentioned problem of the conventional flexible boot. GB-A-1 336 129, upon which the prior art portion of claim 1 is based, discloses a bellows in which a constricting wire is provided in the valleys between successive convolutions of the bellows so as to prevent undue radial expansion of the bellows due to centrifugal forces acting radially outwardly as a result of the weight of lubricant carried within the bellows. The present invention, instead of having such a constricting wire, utilises an elastic ring which has no greater resistance to radial deformation than the material of the bellows itself so as closely to adapt to changes in cross-sectional shape of the bellows and maintain its position in the bottom of the valley between successive convolutions so as to act as a block preventing any acute folding or fold line occurring in the valley on the compressed side of the bellows when the bellows is being used on a universal joint having a substantial difference in angularity between the rotational axes of the two portions of the joint. In the present invention, since the flexible boot is provided with the elastic holder ring closely contacted with the outer surface of the valley, even when the flexible boot is operated at the large operating angle, the valley can be smoothly bent about the elastic holder ring. Further, even if the valley of the flexible boot is compressed between the end surface of the outer ring of the second shaft and the surface of the first shaft, since the elastic holder ring is maintained in close contact with the outer surface of the valley, the valley is smoothly bent around the elastic holder ring, thus preventing the acute folding of the valley of the flexible boot. Fig. 1 is a partial sectional view of a constant velocity joint having a flexible boot according to an embodiment of the invention; Fig. 2 is a partial sectional view of the constant velocity joint of Fig. 1, showing the joint in a large operating angle; Fig. 3 is a sectional view of the flexible boot taken along the line III-III of Fig. 2; Figs. 4 to 8 show examples of elastic holder rings applicable to the invention; and Fig. 9 is a partial sectional view of a conventional constant velocity joint. The invention will now be described with reference to the accompanying drawings. Fig. 1 is a partial sectional view of a constant velocity joint having a flexible boot according to an embodiment of the invention, and Fig. 2 shows the joint of Fig. 1 wherein the joint is in a large operating angle. The constant velocity joint includes a joint body 12 having an inner ring 10 fixed to one end of a first shaft 6 and an outer ring 5 provided on one end of a second shaft 9, and a flexible boot 1 having a bellows portion 4 including valleys 4a. The valleys are defined by bight portions connecting successive convolutions of the bellows and curved when viewed in section longitudinally of the bellows. The flexible boot 1 has both ends 2 and 3 fixed to the outer ring 5 and the first 6 by means of annular fastening bands 11 and 11a, respectively. Between the inner ring 10 and the outer ring 5, there is arranged a ball 8 for transmitting rotation from one of the shaft to the other. Further, a sealing or closed space 13 for accommodating grease for lubricating the joint body 12 is defined inside the flexible boot 1. At least one (two in the embodiment shown in Figure 1) of the valleys 4a of the bellows portion 4 of the flexible boot 1 is provided with a corresponding elastic holder ring 7 closely or tightly contacted with an outer peripheral surface of the valley to prevent acute folding of the valley. In the condition shown in Figure 2, the valleys 4a of the bellows portion 4 are compressed between an end surface 5a of the outer ring 5 and the surface of the first shaft 6. However, since the elastic holder rings 7 are closely arranged on the outer peripheral surfaces of the corresponding valleys 4a of the bellows portion, the valleys 4a are smoothly bent around the corresponding elastic holder rings 7. When the constant velocity joint is in the large operating angle as shown in Figure 2, expanded side of the flexible boot 1 (refer to the side 4b of the bellows in Figure 1) is subjected to tensile stress. Thus, in the condition shown in Figure 2, a cross-sectional configuration of each valley 4a cannot be maintained in true circular shape but presents an oval shape as shown in Figure 3 which is a sectional view taken along the line III-III of Figure 2. Accordingly, it is preferable that as well as the stiffness or rigidity to radial deformation of the elastic holder ring 7 being equal to or less than the rigidity to radial deformation of the bellows portion 4, that the inner diameter of the elastic holder ring 7 when relaxed is smaller than an outer diameter of the corresponding valley 4a of the bellows portion. With this construction, as the sectional configuration of the valley 4a of the bellows is changed by radial deformation of successive circumferential portions of the bellows, the corresponding elastic holder ring 7 can follow the changed configuration of the valley so that the elastic holder ring is always closely contacted with the outer peripheral surface of the corresponding valley 4a of the bellows portion about the entire periphery of the bellows. Further, since the elastic holder ring 7 closely engages the corresponding valley and is not more rigid than the bellows, there is no relative movement between the elastic holder ring and the corresponding valley; thus, there is no abrasion or wear of the elastic holder ring and the valley of the bellows portion. Referring to Figure 4, an example of the elastic holder ring 7 is illustrated in a partial cross-sectional view. It is preferable that elastic holder ring 7 has a smoothly curved inner surface which engages the outer peripheral surface of the corresponding valley 4a of the bellows portion. In Fig. 4, a dimension of a curvature R of the smoothly curved inner surface of the elastic holder ring 7 may vary in accordance with the whole configuration of the bellows portion 4, radius of the valley 4a in free condition, thickness of the valley and the like; however, in general, it has been found that preferable result is achieved when the curvature R is larger than about one half of the thickness of the valley 4a. The inventor of the present invention manufactured the flexible boot having the elastic holder rings made of the same material as that of the flexible boot and mounted thereon. According to this flexible boot, in the condition that the constant velocity joint is in the position shown in Fig. 1, the dimensions of the flexible boot and the elastic holder rings are as shown in the following table 1: Flexible Boot E.H. Ring O.D. of Valley R.C. of Valley I.D. (D) (R) Valley & E.H. Ring nearest to O.R 5 73.7 mm 2.8 mm 73 mm 1.5 mm Valley & E.H. Ring nearest but one to O.R 5 62.6 mm 2.3 mm 62 mm 1.5 mm (In the above table 1, "O.D." is outer diameter; "R.C." is radius of curvature; "I.D." is inner diameter; "E.H. Ring" is elastic holder ring; and "O.R." is outer ring) Comparison test between the constant velocity joint of the invention including the flexible boot having dimensions shown in the table 1 and the conventional constant velocity joint was conducted to examine fatigue life of the valley of each joint. The flexible boot of the conventional joint was made of the same material as that of the joint of the present invention and had the same dimension as shown in the table 1. Further, in the comparison test, the respective joints were operated at a speed of 600 r.p.m. and at the operating angle of 23 to 42°. A cycle of variation of the operating angle was 30 c.p.m.. The result of the comparison test is shown in the following table 2: Fracture Time Fractured Portion Conventional F.B. 20 hrs. Valley nearest to O.R. F.B. having one E.H Ring on Valley nearest to O.R. 82 hrs. Valley nearest but one to O.R. F.B. having two E.H Rings on Valleys nearest and nearest but one to O.R. Not fractured up to 200 hrs. (close the test) None (In the table 2, "F.B." is flexible boot; "E.H. Ring" is elastic holder ring; and "O.R." is outer ring) As seen from the table 2, in the conventional flexible boot, the valley positioned nearest to the outer ring (referred to as "first valley" hereinafter) of the second shaft was fractured in about 20 hours. On the other hand, in the flexible boot of the invention including one elastic holder ring provided on the first valley, the first valley was not fractured in about 82 hours, but the valley positioned nearest but one to the outer ring (referred to as "second valley" hereinafter) was fractured at that time. This means that the fractured portion is shifted from the first valley (having the elastic holder ring) to the second valley (not having the elastic holder ring), and, therefore, that the effect of the invention is noticeable or remarkable. Further, in the flexible boot including two elastic holder rings provided on the first and second valleys, as shown in Fig. 1, none of the valleys were fractured up to 200 hours. This means that the service life of the flexible boot of Fig. 1 will be longer than that of the conventional flexible boot by ten times or more. Figs. 5 to 8 are partial sectional views of other examples of the elastic holder rings 7. In Fig. 5, the elastic holder ring is preferably made of the same material as that of the flexible boot. The elastic holder ring shown in Fig. 6 comprises a closely contacted metallic coil spring formed in a ring-shaped as garter spring used for oil seal. Although the material of this elastic holder ring is harder than that of the flexible boot, since this ring comprises the closely contacted coil spring, it is possible to make the rigidity (of the ring) to the radial deformation equal to or smaller than that of the bellows of the flexible boot. Further, in this elastic holder ring, since an inner surface which will be contacted with the outer peripheral surface of the corresponding valley has a smoothly curved or round surface, the technical effect similar to that of Fig. 4 can be achieved. The elastic holder rings shown in Figs. 7 and 8 have annular and U-shaped cross-sections, respectively, to reduce the rigidity of the elastic holder rings. Using the present invention, since the elastic holder ring is in close contact with the outer peripheral surface of the valley, the valley of the flexible boot can be prevented from acutely folding, whereby the service life of the flexible boot can be considerably increased even when the universal joint is operated at a large operating angle.

Claims: A flexible boot for universal joints including a joint body (12) which has an inner ring (10) fixed to one end of a first shaft (6) and an outer ring (5) provided on one end of a second shaft (9), said boot having opposite ends adapted to be fixed to said outer ring (5) and to said first shaft (6), respectively, a bellows portion (4) positioned between said body ends and having at least one valley (4a) defined by a bight portion connecting successive convolutions of the bellows and curved when viewed in section longitudinally of the bellows, and at least one holder ring (7) closely contacted with an outer peripheral surface of said at least one valley, characterised in that the holder ring (7) is elastic and has a rigidity to radial deformation that is equal to or smaller than the rigidity to radial deformation of the adjacent portion of said bellows portion (4) so as to avoid undue constriction of the bellows during flexing whilst maintaining contact with said valley to prevent acute folding of said valley of the flexible boot. A flexible boot according to claim 1, wherein the inner dimension of said elastic holder ring (7) when relaxed is smaller than the outer dimension of the valley (4a) in which it is received. A flexible boot according to claim 1 or 2, wherein that surface of the elastic holder ring (7) which is adapted to contact said outer peripheral surface of said valley (4a) is smoothly curved. A flexible boot according to claim 1, 2 or 3, wherein said at least one valley (4a) is adjacent the end of the bellows to be fixed to the outer ring (5). A universal joint including a flexible boot as claimed in any preceding claim.

Inventors: UCHIDA, SUSUMU
Applicants: KEEPER CO. LTD
Application year: 1988
Main classification: F16D3
IPC codes: F16D3, F16J3