LIMITED SLIP DIFFERENTIAL

Abstract: In order to provide pulsating thrust on vehicle half-shafts during excess differential actions such as occur on spin-outs, skids on ice etc, the side gear teeth are modified to exhibit two different eg involutes in the single tooth engagement zone (P₁-P₂). Their junction line is a convex discontinuity (Q), the modified teeth becoming thicker. The pinions are typically unmodified. The pulsating increases friction, which reduces spin-out tendencies. The friction can be increased more by inserting friction rings between the side gears and their bearings.

Description: The present invention relates to a limited slip differential for transmitting drive differentially to two wheels on a vehicle, which may be driven round corners or on skidding surfaces, on-road or off-road. It is desirable to allow the two wheels to rotate at somewhat different speeds without losing drive torque to either, for instance while cornering or having different tyre pressures or tyre loadings. Differentials serve this purpose well, but if one wheel loses adhesion e.g. on soft sand or ice, spin-outs can occur and effective traction can be lost by the driving torque becoming applied only to the skidding wheel. The differential housing is driven about the axis of the two driven ground wheel shafts, the housing rotates the axles of one or more freely rotating bevel pinions and these engage two side bevel gears rotating with the respective wheels. An arrangement to allow some but not excessive slip would be thus most desirable. Our parent company Eaton Corporation (fluid Power Operations) makes a differential for cars and small trucks which senses excessive differential action by responding at low speeds independently (with a fly weight) to a threshold speed difference and bypassing differential action by clutching either or both output axles to the differential housing. This can be unsuited to heavy goods vehicles (HGV's) with high rotating inertias to control. Other attempts propose that driving tooth contractings be not simply root-to-tip but partly transversely across the tooth faces, in the bevel gearing between the pinions and side gears. The resulting variable radius of the contact point is claimed to increase friction between the housing bearing carrying the side-gears and wheel shafts, by increasing the axial thrust always exerted on the driven member in a bevel drive. Even a modest increase in friction (not total locking) is usually quite adequate to regain enough traction to exit the bad ground surface. These operate automatically. Other proposed differentials can be locked by the driver, which leads to damage if the locking is applied impatiently during spin-out or if not disengaged after use. The invention proposes the use of a rotational irregularity, since we find that the desired pulsating action which increases bearing or clutch friction sufficiently during spin-outs) can be made to interfere very little with low speed differencing during cornering, and operates satisfactorily on HGV's. GENERAL DESCRIPTION All bevel gears and pinions exert an axial thrust load on the driven members by virtue of the geometry. The higher the pitch angle the higher the axial thrust load. This means that side gears with greater numbers of teeth and therefore a higher pitch angle tend to produce higher axial thrusts. The invention takes advantage of the axial thrust to apply selective but automaticaly increased pressures between the backface of the side gear and the differential case to produce a braking action. The differential gears according to the preferred embodiments of the invention are designed conventionally except that the face apex passes through the pitch apex to ensure a constant profile contact ratio from toe to heel. The number of teeth in the side gear should also be divisible by the number of differential pinions, otherwise the desired pulsating action becomes more or less dissipated. One member has teeth cut with a normal profile to the designed pressure angle whilst the other member is cut with a modified profile from normal in order to produce a change in the pressure angle of mesh during each tooth engagement. This profile modification can be accomplished in many different ways using non involute or involute profiles and could equally be applied to either or both members; the present preference is to use two involute curves meeting at a convex portion or ridge transverse to the progression of tooth contact; the two involutes are applied to each flank of each tooth of each side gear member, and not the pinion of pinions, because it is easier to cut complicated profiles on the side gear teeth. Also application of the invention to the pinions tends to undercut the teeth, weakening them at the roots. At the start and finish of single tooth contact the modified profile preferably coincides with the normal tooth profile in embodiments to ensure smooth transfer of motion to the next tooth. In Fig. 1, P1 and P2 are the points of start and finish of single tooth contact. The object of the profile modification is to bring about a change during each tooth engagement in the axial thrust force exerted through the side gear, normally accompanied by a change in velocity, which changes in turn each produce a pulsation during differential action to resist wheel spin by bearing friction between the side gear and the differential case. Friction can be increased by the use of multiple clutch plates for better performance if desired. Although the pulsation action will be present during any differential action, at low differential speeds such as would be encountered under normal cornering the effect would be marginal. However, when wheel spin occurs the effect will be to transfer motion to the stationary wheel to regain traction. The main advantages of the proposed invention are:- 1. It can be used in existing differential carriers without the need of new designs to cases. The modified side gears can be substituted for the normal ones, in new or old differentials. 2. As only one member requires modification, existing gears can be used. 3. The design utilises the full facewidth of the gears throughout each tooth engagement, therefore no loss of strength and no requirement to increase facewidth. 4. The differential side gears and pinions can easily be produced by precision forgoings which makes for economical manufacture except for cutting the master modified gear (usually the side gears). For systems allegedly relying on variable leverage in the prior art, US-A-3 703 108 reviews other such systems and proposes arranging that each tooth-to-tooth contact between pinion and side gears move along the side gear tooth (as well as from tip to root) whenever any differential action occurs. This proposal claims to result in time-varying leverages and torques in the drive to each wheel (although the present inventor doubts whether the leverage can vary). Moreover the geometry is chosen so that at nearly all instants the torques are claimed to be different to each wheel. For the present invention, it was desired to avoid contact progressing other than direct from tip to root, for ease of manufacture. The proposal of UA-A-3 703 108 brings about this assymmetry and longitudinal movement of the tooth contact point by dimensioning one or more of the cones which define all bevel gear geometry, to have its apex non-coincident with the centre point of the differential. Another limited slip effect is produced by the so-called "tight" differential in use by Eaton Corporation wherein axial thrust is increased by use of oversized teeth (and/or reduced end-play in the side-gear bearings). However, wear reduces the limited slip action and friction is increased during cornering which is felt undesirable. As with most gear meshing, the present invention requires in each meshing pair of gear wheel, a degree of overlapping of drive between tooth pairs. Therefore, for part of the time only one tooth pair participates, and for the rest of the time two pairs share the drive. Accordingly it is essential to the invention that the number of engaged tooth pairs lie between one and two. During the active driving part of each tooth flank, there will thus be a single tooth drive portion and, each side of the latter, drive portions that overlap in time with driving by another tooth pair. According to presently preferred embodiments of the invention, the pinion teeth flanks follow an involute curve, of conventional form and pressure angle. Many parameters can be varied as is well known, but for optimum performance a nominal pressure angle of 25° has been chosen, and the following description will allude to "normal" side gear teeth as a reference to clarify the modifications of the invention. To the normally skilled designer, their (unmodified) flanks would be "normally" conformed specifically to cooperate with the involute pinion teeth. Embodiments modify the side gear teeth, and the following description will reference the modified teeth with normal teeth, so that the skilled man will readily appreciate the modification taught by the invention. In practice, modified and unmodified side-wheels will usually be interchangeable, it is predicted. The modified side gear tooth in one inventive aspect comprises a thickening of the tooth at its single-tooth drive portion and a thinning of the tooth in its two overlapping drive portions, in relation to a normal tooth, the modification being preferably such that the two limits of the single tooth drive portion (hereinafter called "P₁" and "P₂") are crossover points, in that they are coincident with those limits on a normally cooperating side tooth. The contour in the preferred tooth crosses over so that the thickening becomes thinning immediately outside the single tooth drive portion P₁P₂ (see Fig. 1). Therefore the pressure angle of drive and the transmitted rotational speed to the side gear, and most significantly the axial thrust through the side-gear, will change once during each tooth-pair engagement, which causes pulsating. This pulsating only occurs during differential action, i.e. assymmetry in the drive (or suspension, tyres etc) of the vehicle. The pulsating axial thrust will increase friction at the differential housing bearing which the output or side gear shafts, and slip will thus be limited even if one ground wheel encounters a zero friction surface (eg wet ice). Further objects, features and advantages, of the present invention will appear from the following description of embodiments thereof, given in conjunction with the drawings, in which:- Figure 1 shows a side view of a side gear tooth, illustrating the engaging flanks and the modifications compared with a normal single involute-curved tooth. Figure 2 shows a spur gear equivalent to the bevel gear, in a plane as viewed from the bevel cone apex. Figure 3 is a gear teeth interaction diagram illustrating the inventive modifications. Figures 4, 5 and 6 show respectively a pinion tooth, a standard side gear tooth unmodified by the invention, and the tooth as so modified, and successive positions of the cutter of the blank for forging. In general, gear teeth engagement surfaces are involute curves, and tooth-to-tooth engagement commences and finishes gradually, between which there is a rolling together motion between surfaces, the tangent to which, at one radius i.e. at the "pitch circle", is inclined at a constant "pressure angle" to the radius of the tooth. This pressure angle is often 22.½°, presently preferred herein is 25°. Every bevel gear wheel has a base circle or cone (from which the involute curve is derived), a root cone (roots of teeth), a top or outside cone (i.e. tips of teeth), and a pitch cone, where the two engaging gears move identically at the engagement area, i.e. as though rolling together. All the cone apices typically coincide at the centre of the differential, but not in the invention of US-A-3 703 108 (McCaw) referred to above. The presently proposed modified teeth each has two component involute curves, two pressure angles averging to say 25° (and two base cones) relating to each flank. For acceptable noise and wear, one mating tooth pair must always be in mesh but never more than two. Preferred embodiments of the invention select he contact ratio at 1.3. Referring to Figure 1, the two side gears of a differential have modified teeth each as shown in full line form. "Normal" or reference teeth would have the dashed line form, and have one involute curve from the tip down to the point on the flank where the tip of the pinion ceases contact therewith. In Figure 1, this one involute curve extends radially to an extent W + X + Y + Z. This so called "normal" tooth assumes none of the well-known foreshortening from the theoretical maximum commonly practised in order to avoid noise due to contact at points or corners of the tip. With the preferred 1.3 ratio, the radial distances W and Z correspond to two-teeth meshing, and X and y to single-tooth meshing. For this embodiment, the single-tooth meshing boundary points P₁ and P₂ are chosen (by calculation for a "normal" tooth) since points P₁ and P₂ are to coincide on the modified and unmodified teeth. To design the side gear teeth profiles, points P₁ and P₂ common to "normal" teeth are determined to be conjugate with a first involute, that of the pinion, then the tooth profiles are cut away at W and Z according to second and third involutes. The result is that firstly over the radial extends W and X, the pressure angle is increased corresponding to the second, involute curve from the tip through P₁ to point Q, and secondly, over the radial extents Y and Z, the pressure angle is decreased compared with that of the normal tooth, corresponding to use of the third involute curve from Q through as far as Z (towards the root). Thus the invention provides two separate pressure angles and involute curves, and a discontinuity at point Q, which may be very sharp or somewhat sharp according to choice, noise, wear etc. During any differential action, when each tooth engagement passes through Q, the aforesaid velocity transmission, pressure angle and axial thrust pulsations occur in the driven side gear. As far as the invention is concerned, any tooth shape may be chosen outside P₁, P₂ to ensure thinning of the teeth, and over the single tooth meshing section between P₁, P₂ to ensure thickness of the teeth. The thickened or convex section causes (at least) two different pressure angles to be identifiable. The tooth strength, reproducibility, noise factors, smooth take-up of stress etc are engineering factors also needing care, but lie within the normal skilled man's abilities. It is not likely that non-involute curves would be engineeringly practical for the modified teeth, and so the choice to two involutes, namely the second and third involutes described above with a junction line at Q, will give the most economical satisfactory solution. It is repeated that during sections W and Z engagements, driven tooth velocities are reduced by the difference in the involute curve position from the normal involute position (dashed line) in Fig. 1. During section X and Y engagement, tooth mesh motion is increased corresponding to the difference in involute position i.e. the difference between the dashed and full line. The average, as taken between P₁, P₂, of the two involutes is conjugate with the pinion member produced with a standard involute curve of pressure angle Cp. The modified side gear involute curves, of pressure angle C¹ and C², coincide with a normal conjugate tooth at points P₁ and P₂ only. METHOD OF DESIGN The gear in question is usually of the straight bevel variety but could be "zerol" bevel (Gleason registered name), eg designed in accordance with normal bevel gear practice such as A.G.M.A. or Gleason recommendation. The face cone should desirably pass through the pitch apex to ensure a constant profile contact ratio from toe to heel, with the root cone of the mating part parallel to the face cone to enable maximum root radius and maximum strength. All the above are to standard bevel design except that the face angle is designed to pass through the pitch apex and not the root angle. All calculations are carried out in the equivalent spur gear plane at the mean cone distance In more detail, to design the start and finishing engagement points P₁, P₂, reference is made to Figure 3, AD being the "line (or path) of action" tangential to the "base circles" used to define the involutes of normal side gear teeth (before modification). To clarify the modification, BP₂ must be modified for shortening (the radial extent Z in Fig 1), likewise CP₁ (section W). On the line of action, these lengths are equal, and BP₁ or AP₂ equals the base pitch. P₁ and P₂ correspond with those points in Figure 1. The gears should be designed with a profile contact ratio of 1.2 to 1.4 as this will ensure correct operation and smooth transmission under normal operation. To establish the profile contact ratio and start and finish of single tooth contact. In order to avoid under cut with low tooth numbers, the side gear is chosen to be modified, but similar modification on the pinion instead would produce a similar pulsing axial thrust through the side gear. It is now necessary to calculate the diameter at start and finish of single tooth contact (P₁ & P₂) and hence the thickness at these points of the standard gear profile. The design chosen uses a standard pressure angle of 25° which is modified to 20° at the root and 30° at the tip to achieve the required motion. Reference is now made to Figures 2, 4, 5 and 6 (Figure 6 being the modified tooth), for design details of a preferred modified side gear tooth. Since differentials will almost invariably operate reversibly, the other flank of each side tooth will have the mirror image profile of the first modified flank (in the bisecting radius). a) VARIABLE PRESSURE THURST FORCE ON SIDE GEAR TF TF = FT X THAN Ø X SIN ( T ) WHERE FT = TANGENTIAL TOOTH LOAD Ø = PRESSURE ANGLE OF MESH T = PITCH ANGLE As the gears will be operating at varying pressure angle of mesh between Ø₁ and Ø₂ then the FLUCTUATION OF THRUST LOAD STF IS STF = FT X SIN T X [TAN Ø₁ - TAN Ø₂] b) VARIABLE SPEED Since the pinion member has a constant pressure angle Ø p , the variation in motion transmitted to the side gear as the gears pass through one tooth mesh can be calculated by comparing the "normal" side gear tooth form based on Ø p with the modified one. The preferred parameters are given by way of example. Even totally non-involute teeth meshing, between unity and two contact ratio, and having a convex discontinuity on each single-tooth contact face of one tooth set should give some of the inventive advantages.

Claims: 1. A limited slip differential comprising a housing rotatable by a prime mover; at least one driving pinion mounted to be freely rotatable about its own axis on the housing; two parallel driven side gears coupled to bearing-mounted output shafts axial of the side gears; first bevel teeth on the pinion for meshing with second bevel teeth on the side gears during differential action; and modifications on the first and/or second teeth, characterized in that the contact ratio of meshing of said teeth during differential action is selected between one and two; the modification comprises two-part profiling of that portion (P₁-P₂) of the face corresponding to single-tooth contact with teeth of the other set during meshing; first (X) and second (Y) parts of the modification profiling produce by bevel action first and second axial thrust forces on each side gear; whereby for any speed of differencing, the first and second axial forces are different; and whereby each tooth meshing causes an axial thrust force pulsation on the side gear bearings. 2. A limited slip differential according to claim 1, characterized in that the side bearings comprise friction-­ increasing members arranged to respond to axial thrusts. 3. A limited slip differential according to claim 1 or claim 2, characterized in that each modified tooth has a discontinuity line (Q) within its single-tooth engagement contact range (P₁-P₂); and two different profiles extend from the discontinuity line (Q) through the single-­ tooth contact boundary lines (P₁, P₂) and thence through the overlapping or double-tooth meshing contact zones (W, Z). 4. A limited slip differential according to claim 3, characterized in that the average of the two different profiles (eg 20° and 30° contact angles) corresponds to that profile (eg 25° contact angle) which would have been chosen for an unmodified or "normal" tooth to be conjugate with teeth of the other set; and the single-tooth meshing contact boundaries (P₁, P₂) coincide with the single-tooth boundaries had an unmodified (normal) tooth been adopted (dashed contour, Fig. 1). 5. A limited slip differential according to any of claims 1 to 4, characterized in that the pinion teeth are normal, and the side-gear teeth are modified, whereby undercutting at the root region is avoided. 6. A limited slip differential employing a bevel drive from a freely rotating pinion to vehicle mounted axial half-shafts through parallel side-gears, the bevelled teeth on the gears having involute profiles, either the pinion or the side-gear teeth being modified over a single-tooth contact zone, characterized in that the modified teeth comprise a first curve and the modified teeth are thickened (solid line) within the single-tooth contact zone (X-Y) compared with a fictitious "normal" or reference tooth (dashed line) which would give normal eg conjugate meshing with the unmodified teeth; thinned at zones (W, Z) outside the single-tooth contact zone compared with said reference fictitious tooth; and at both boundary lines (P₁, P₂) thereof, coincide with the reference tooth profile. 7. A limited slip differential according to claim 6, characterized in that the modified teeth (solid profiles) each exhibit second and third involutes extending from a junction discontinuity line (Q) through said concident single-tooth boundary lines (P₁, P₂) to the tip and root regions respectively. 8. A limited slip differential according to claim 7, characterized in that the first involute exhibits a 25° pressure angle, the second involute exhibits a 30° pressure angle, and the third involute exhibits a 20° pressure angle. 9. A limited slip differential according to any of claims 1 to 8, characterized by a tooth meshing contact ratio of 1.3.

Inventors: BARNETT, DAVID WILLIAM LESLIE
Applicants: EATON LIMITED
Application year: 1986
Main classification: F16H1
IPC codes: F16H48, F16H55