WO2013017897A1

WO2013017897A1 - Simple and low cost limited slip differential

Info

Publication number: WO2013017897A1
Application number: PCT/GR2011/000028
Authority: WO
Inventors: Theodoros DESSOS
Original assignee: Dessos Theodoros
Priority date: 2011-07-29
Filing date: 2011-07-29
Publication date: 2013-02-07

Abstract

Simple and low cost limited slip differential that operates by using the gears (3, 4, 5 and 6) of the differential. These gears are enclosed in a casing 10, whose sides are shaped according to the outer surface of the teeth of each one of said gears. Said casing encloses said gears in a way that the tolerance 20 between the inner surface of said casing and the outer surface of the teeth of each gear is very small. All areas between the inner surface of said casing 10 and the teeth of above mentioned gears 3, 4, 5 and 6 are filled with oil. When the two output shafts of the differential do not rotate with the same speed, the spider gears 3 and 4 of the differential rotate around their axles in respect to the side gears 5 and 6. During this rotation, each pair of co¬ operating gears operates as a hydraulic pump, pumping the oil out from the pressurization area 11 to the suction area 17, via slots / holes 15 that - for this reason - exist in the casing 10. By determining / adjusting suitably the size of the said slots / holes 15 and the said tolerance 20, the hydraulic resistance of the system is designated. Pressure of oil is remarkably increased due to said hydraulic resistance and therefore, restraining toque is developed to said differential gears, limiting their free rotation. In this way, the two output shafts of the differential operate as been partially engaged, meaning that when one of them is allowed to rotate without transferring any torque, the torque that is transferred to the other shaft is not almost zero, but remarkably high.

Description

Simple and low cost limited slip differential

The invention refers to a limited slip differential. Various types of differentials are generally used to vehicles as well as to several industrial applications. Differentials transfer power from one input shaft to two output shafts, each one of which rotates developing always the same torque with the other, even when the two output shafts are not rotating with the same speed. Some differentials are of limited slip type, which means that when their two output shafts are not rotating with the same speed, they proceed to some kind of limited connection / engagement between the two output shafts, achieving the transfer of torque from one output shaft to the other and / or developing a limited torque to one output shaft even in case that the other output shaft rotates free, without developing any torque. The limited connection between the two output shafts of a limited slip differential is usually achieved in one the following ways:

• Using some kind of clutch between the two output shafts, which is activated either mechanically or electrically, hydraulically, pneumatically, etc.

• Using several disks attached alternatively to the one or the other output shafts and a viscous oil among these disks. When the shafts (and the disks) rotate with different speeds, torque is transferred from the disks attached to one shaft to the disks attached to the other, by the shear developed to the viscous oil.

• Using some extra gears which in a pure mechanical way engage partially the two output shafts.

Despite their remarkable advantages, limited slip differentials are not widely used to vehicles and other applications, because they are rather complicated, sometimes they have big volume and, most important, their cost is quite high. The advantage of the limited slip differential of the present invention is that it is a very simple and low cost application which can be introduced almost in any differential. It does not generally comprise any additional moving parts and does not require other additional devices such as clutches, gears, sensors, electronic equipment etc. It just uses the existing gears of the differential.

A typical differential is shown on simplified Figure 1. Parts that can be seen in this Figure 1 are the output shafts 1 and 2 respectively, the input shaft 9, the two spider gears 3 and 4, the two side gears 5 and 6, the ring gear 7 and the pinion gear 8.

The present invention is shown on Figure 2 and Figure 3. These Figures are very simplified and their purpose is only to show the operation principle of the current invention. For this reason many details or components have been omitted, mainly for clarity reasons. Figure 3 shows the section A - B - C - D - E - F seen on Figure 2. According to the present invention, the two spider gears 3 and 4 and the two side gears 5 and 6 of the differential are enclosed in a can / casing 10, whose sides are shaped according to the outer surface of the teeth of each one of the four above mentioned gears and enclose said differential gears in a way that the tolerance 20 between the inner surface of the casing 10 and the outer surface of the teeth of each gear is very small, all along the part of the perimetric side of each gear, that its teeth are not engaged with the teeth of another gear. Said can / casing 10 may be divided to two or more parts that are assembled together, be fixed to suitably configured extensions 47 and 48 of the ring gear 7 and rotate together with it, enclosing totally the above mentioned gears 3, 4, 5 and 6, but not been in tough contact with their outer teeth surface, due to above mentioned tolerance 20. Number of parts of said casing 10 as well as their configuration and their other features may vary, according to the special design and operating characteristics of the differential that they are installed to. All areas (20, 11 , 17, etc) between the inner surface of the casing 10 and the teeth of above mentioned gears 3, 4, 5 and 6 are filled with oil or other suitable fluid. Said oil, is restricted / trapped among the inner surface of the casing 10 and the space between the teeth of each gear. Other areas (15, 16, 18, etc) within said casing 10, are also filled with oil or other suitable liquid.

When the two output shafts 1 and 2 of the differential rotate with the same speed, the same torque is transferred to each one of these two shafts. The casing 10, together with the above mentioned gears 3, 4, 5 and 6 of the differential, rotates around the axle of the output shafts 1 and 2 with the same speed with them. The spider gears 3 and 4 do not rotate around their axles, in respect to the side gears 5 and 6.

When the two output shafts 1 and 2 respectively do not rotate with the same speed, the casing 10 together with the above mentioned gears 3, 4, 5 and 6 of the differential, rotates around the axle of the output shafts 1 and 2 of the differential, with the average speed of the two output shafts 1 and 2 and the spider gears 3 and 4 rotate around their axles, in respect to the side gears 5 and 6. During this rotation, at the areas 11 of every pair of co-operating gears where the teeth are engaged and the teeth of one gear (for example of gear 3) move approaching the teeth of the other gear (for example of gear 5), the oil that was trapped in the space between the casing 10 and two neighbor teeth 12 and 13 of engaged gear 5 is pressed by the relevant tooth 14 of the other engaged gear 3 and its pressure is increased, as it is forced to be displaced but it is trapped among the inner surface of the casing and the teeth of said co-operating gears. Pressurized oil finds its way out through outlet slots / holes 15 that - for this reason - exist in the casing 10, at the area where the above mentioned gears are engaged. As long as the gears 3 and 5 rotate against each other, oil is continuously displaced out of the gaps between the teeth of these gears as they are co-operating. The same happens with the oil between the teeth of co-operating gears 3 and 6, with the oil between the teeth of co-operating gears 4 and 5, and with the oil between the teeth of co-operating gears 4 and 6. In other words, the co-operating gears of the differential operate like hydraulic gear pumps, pumping continuously oil out, through said slots / holes 15. Obviously, a small quantity of the oil finds its way out, not through the slots / holes 15, but around the teeth of the gear, due to said tolerance 20 between the inner surface of the casing 10 and the outer surface of the teeth of each gear. Oil been displaced through said slots / holes 15 (or around the teeth of the gears, due to said tolerance 20), is guided through area 16 of the casing 10 and slots / holes 18 to the suction area 17 of co-operating gears, which is the area where the teeth of said co-operating gears are engaged and the teeth of one gear move apart from the teeth of the other gear. In this way, oil fills continuously the space between the teeth of each gear at the suction areas 17 and, due to the gear rotation, oil is transferred back to the pressurization areas 11.

Obviously, depending on which output shaft rotates faster than the other, areas of oil pressurization can be either areas 11 , or areas 17 and areas of oil suction be either areas 17 or areas 11 respectively.

As it comes from the above mentioned, by determining / adjusting suitably the size of said slots / holes 15 and said tolerance 20, the hydraulic resistance of the system is designated, according to the flow-rate of the oil, or in other words according to the rotation speed of a gear against its co-operating gear. Pressure of oil is remarkably increased in area 11 due to said hydraulic resistance, approximately according to the square power of the oil flow-rate. Since the pressure of the oil is applied to one side only of the gear tooth that is engaged, a relevant force is developed to the tooth and therefore a torque is applied to the gear. Said force is applied to all four points of gear teeth engagement, of the differential. (Two points per spider gear). Due to the above, a limited engagement is achieved between the gears of the differential, depending on the difference between the rotation speeds of the two output shafts. In other words, as it happens to a gear hydraulic pump, a resisting torque is developed to the shafts of the engaged gears due to the oil pressurization. As a result, and in spite of what happens to a conventional differential, if one output shaft of the differential is allowed to rotate without transferring any torque, the torque that is transferred to the other output shaft is not zero, but remarkably higher, depending upon the difference between the rotation speeds of the two output shafts of the differential and the design details of the differential gears 3, 4, 5, 6 and said casing 10.

Furthermore, due to the fact that the limited slip differential of the present invention does not transfer torque via shear of viscous oil, possible oil temperature rise does not significantly reduce its efficiency, as happens to conventional multiple disks limited slip differentials.

The differential of the present invention can also be introduced in place of the central differential in a 4-wheel drive vehicle.

According to the above mentioned and referring to Figure 3, oil is moving from pressurization area 11 to suction area 17, through slots / holes 15, area 16 and slots / holes 18. However, due to the high hydraulic resistance of slots / holes 18, vacuum gaps may appear to the suction area 17, reducing in this way the efficiency of the limited slippage of the differential. This problem is solved with the introduction of check valves of any type (swing, ball, plate, disc etc). According to Figure 4, swing check valves 19 and 50 are used. When the two output shafts rotate with the same speed, these valves 19 and 50 are in the closed position by any means, for example by the use of a spring, or (in case of an alternative suitable configuration), by the centrifugal forces developed due to the rotation of the gears 3, 4, 5 and 6 around the differential output shafts axis. When the output shafts do not rotate with the same speed and therefore, gears 3 and 4 rotate against gears 5 and 6, oil finds its way out from the pressurization area 11 , through the opening 12, through area 23 and through the slots / holes 15 which exist in the closed swing valve 19. Then, oil passes through area 16 and forcing the swing valve 50 to the open position reaches the suction area 17, through areas 49 and 21. In this way, areas of oil pressurization are only the areas 11 , 12 and 23. Areas 16, 49, 21 and 17 are low oil pressure areas and due to their relatively large cross - section size, negligible pressure drop is developed to the oil moving through them, preventing the formation of vacuum gaps between the teeth of the gears, at the oil suction area 17.

Seals 30 and 31 of any type, are placed to the output shafts to keep the oil within the casing 10. Sealing of any type, (preferably labyrinth type), is also provided to the inner 32, 33, 34, 35 and to the outer 36, 37, 38, 39 front side of each gear, to prevent oil outlet and oil pressure drop. Areas 40, 41 , 42 and 43, at the outer front side of each gear (as well as the relevant areas at the inner front side of each gear) beneath above mentioned seals, are areas of low oil pressure and possible oil leaks towards these areas are guided back to low pressure areas 16 of the casing 10 by means of special casing 10 modification or piping / tubing (not shown on Figures 2, 3 and 4 for clarity purposes).

Preferably, the spider gears 3 and 4 of the differential are not fixed to their shafts 45 and 46 respectively, by rotate around them, while the shafts 45, 46 are fixed to ring gear extensions 47 and 48. Since the casing 10 is also fixed to said ring gear extensions 47 and 48, oil can not actually leak out of the casing 10.

Temperature rise may be observed to the oil in said casing 10, due to oil throttle through said slots / holes 15 during the operation of the limited slip differential. Cooling of oil may be achieved in various ways, such as by additional coolant /oil that exists in the outer casing of the differential and soaks the outside surface of said casing 10, as it rotates inside the (non - rotating) outside casing of the differential. Cooling blades attached to the outside surface of said can / casing 10 provide more efficient cooling. In a more sophisticated - though expensive and complicated- option, oil inside said casing 10 may be cooled via recirculation to external cooling device.

Furthermore, thermal relief device is provided regarding the oil in said casing 10, in order to prevent pressure rise of the oil due to its thermal expansion. This device may be of any kind and in its simple option, it comprises just one or more easily deformable part (s) of the outer area of the can / casing 10 at a certain point of it, where oil pressure is low, such as at areas 16. When the oil expands due to its temperature rise, said easily deformable part (s) of the outer area of the can / casing 10 are easily deformed outwards, so that the available volume for the oil increases, preventing oil pressure rising. In a more advanced option, said tolerance 20 between the inner surface of the casing 10 and the outer surface of the teeth of each one of the gears 3, 4, 5 and 6 is not constant, but it is adjustable during the operation of the differential, according to the current operating conditions. More particularly, said tolerance 20 is adjusted to be relatively low when the difference between the rotation speeds of the two output shafts is high (achieving in this way high oil pressurization and therefore high degree of engagement between the two output shafts 1 , 2 of the differential) or (said tolerance 20) is adjusted to be relatively high when the difference between the rotation speeds of the two output shafts is low (achieving in this way very low oil pressurization and therefore very low or even negligible degree of engagement between the two output shafts 1 , 2 of the differential). Furthermore, said tolerance adjustment can be carried out not only according to the difference between the rotation speeds of the two output shafts of the differential as mentioned above, but according other criterions, related to the vehicle driving conditions, the driver's preferences etc. All above mentioned can be realized in several ways. According to a simple application, the two halves of the casing 10, or just part of these two halves, (each one of which surrounds one side of said gears 3, 4, 5 and 6 but is not in tough contact with the outer surface of their teeth due to said tolerance 20), are slightly been moved in a direction that is vertical to both the spider gears (3 and 4) axis and the side gears (5 and 6) axis, either approaching to each other (so that said tolerance 20 is reduced), or moving apart from each other (so that said tolerance 20 is increased). Said slight movement of the two halves of the casing 10 can be each time activated either manually or automatically according to the current operating conditions of the differential.

Additional sets comprising spider gears, side gears and casings full of oil can be introduced for even better results. Shaft of each side gear of each said additional set, is connected to the relevant output shaft (1 or 2) of the differential. Said additional set may operate either independently, or together with the side gears (5 and 6), the spider gears (3 and 4) and the casing 10 of the differential. Regarding vehicle application of a conventional differential, when the vehicle is still and one of its power wheels is on ice or slippery road, the vehicle is very difficult to start moving, since the wheel on slippery road skids and spins and the torque delivered to the other power wheel is almost zero. However, using the limited slip differential of the present invention, the torque delivered to the power wheel that is not on slippery road is high and increases as the difference between the rotation speeds of the two output shafts of the differential increases. The degree of the output shafts engagement is not just linearly proportional to the output shafts rotation speeds difference, but approximately proportional to the square power of said output shafts rotation speeds difference. On the other hand, when the vehicle is moving on a turn, the difference between the rotation of the two output shafts is low, resulting to very low oil pressure rise in the differential and to very low or even negligible degree of output shafts engagement, allowing smooth an efficient operation of the differential, without any vehicle tyre skid.

Claims

1. Simple and low cost limited slip differential which uses the spider gears 3 and 4 and the side gears 5 and 6 of the differential in order to operate. Said gears are enclosed in a casing 10, whose sides are shaped according to the outer surface of the teeth of each one of above mentioned gears. Said casing 10 encloses said gears in a way that the tolerance 20 between the inner surface of the casing and the outer surface of the teeth of each gear is very small, all along the perimetric side part of each gear that its teeth are not engaged with the teeth of another gear. Said casing 10 may be divided to two or more parts that are assembled together, be fixed to the ring gear 7 and rotate together with it, enclosing totally the above mentioned gears 3, 4, 5 and 6, but not been in tough contact with their outer teeth surface, due to above mentioned tolerance 20. Number of parts of said casing 10 as well as their configuration and their other features may vary, according to the special design and operating characteristics of the differential that they are installed to. All areas between the inner surface of said casing 10 and the teeth of above mentioned gears 3, 4, 5 and 6 are filled with oil or other suitable fluid. Said oil is restricted / trapped among the inner surface of the casing 10 and the space between the teeth of each one of said gears 3, 4, 5 and 6; due to this, when the two output shafts 1 and 2 of the differential do not rotate at the same speeds (and because of this each spider gears 3 and 4 rotate against the side gears 5 and 6), the oil that is restricted in the space between the casing 10 and two neighbor teeth 12 and 13 of side gear 5 is pressed by the relevant tooth 14 of the spider gear 3 and its pressure is increased, as it is forced to be displaced but it is trapped among the inner surface of the casing and the teeth of said co-operating gears. Said pressurized oil finds its way out through outlet slots / holes 15 that - for this reason - exist in the casing 10, at the area where the above mentioned gears are engaged. The same happens with the oil between the teeth of co-operating gears 3 and 6, as well as with gears 4 and 5, and gears 4 and 6. As long as each spider gear rotates against relevant side gears, oil is continuously displaced out of the gaps between the teeth of said gears, at the areas of oil pressurization, which are the areas where the teeth of one gear move approaching the teeth of the other (co-operating) gear. In other words, the co-operating gears of the differential operate like hydraulic gear pumps, pumping continuously oil out, through said slots / holes 15. Obviously, part of the oil finds its way out, not through the slots / holes 15, but around the teeth of the gear, due to said tolerance 20 between the inner surface of the casing 10 and the outer surface of the teeth of each gear. Oil been displaced through said slots / holes 15 (or around the teeth of the gear, due to said tolerance 20), is guided through area 16 and slots / holes 18 to the suction area 17 of co-operating gears, which is the area where the teeth of said co-operating gears are engaged and the teeth of one gear move apart from the teeth of the other (cooperating) gear. In this way, oil fills continuously the space between the teeth of each gear at the suction area 17 and, due to the gear rotation, oil is transferred back to the oil pressurization area 11. By determining / adjusting suitably the size of said slots / holes 15 and said tolerance 20, the hydraulic resistance of the system is designated. Pressure of oil is remarkably increased in area 1 1 due to said hydraulic resistance, approximately according to the square power of the oil flow-rate, which means according to the rotation speeds of side gears 5 and 6 against spider gears 3 and 4, or, in other words, according to the difference between the rotation speeds of the two output shafts 1 and 2 of the differential. Since the pressure of the oil is applied to one side only of the gear tooth that is engaged, a relevant force is developed to the tooth and therefore a torque is applied to the relevant gear, restraining its rotation. Said force is applied to all four points of gear teeth engagement, of the differential. (Two point per spider gear). As a result if one output shaft of the differential is allowed to rotate without transferring any torque, the torque that is transferred to the other shaft is not zero, but remarkably higher, depending upon the difference between the rotation speeds of the two output shafts of the differential and the design details of the differential gears and the said casing. Seals of any type are placed to the output shafts to keep the oil within the casing 10. Sealing is also provided to the inner and the outer front side of the gears. Oil cooling and thermal relief devices are also provided.

2. Limited slip differential as described in claim 1 comprising check valves of any type (swing, ball, plate, disc etc) 19, 50, equipped with slots / holes 15. Check valve 19 at area of high oil pressure is closed forcing oil outlet through slots / holes 15, while check valve 50 at area of low oil pressure is open (preferably by been forced by the oil flow), allowing easy oil flow towards the suction area 17, avoiding in this way the development of high pressure drop of oil at the suction area 17 and preventing the formation of vacuum gaps between neighbor teeth of the gears.

3. Limited slip differential as described in claim 1 , characterized in that the tolerance 20 between the inner surface of the casing 10 of the differential and the outer surface of the teeth of each one of the gears 3, 4, 5 and 6 is not constant, but it is adjustable, by any means, during the operation of the differential, according to the operating conditions. More specifically, said tolerance 20 is adjusted to be relatively low when the difference between the rotation speeds of the two output shafts is high (achieving in this way high oil pressurization and therefore high degree of engagement between the two output shafts 1 , 2 of the differential) or (said tolerance 20) is adjusted to be relatively high when the difference between the rotation speeds of the two output shafts is low (achieving in this way very low oil pressurization and therefore very low or even negligible degree of engagement between the two output shafts 1 , 2 of the differential). Above mentioned can be realized in several ways. According to a simple application, the two halves of the casing 10, or just part of these two halves, (each one of which surrounds one side of said gears 3, 4, 5 and 6 but is not in tough contact with the outer surface of their teeth due to said tolerance 20), are slightly been moved in a direction that is vertical to both the spider gears (3 and 4) axis and the side gears (5 and 6) axis, either approaching to each other (so that said tolerance 20 is reduced), or moving apart from each other (so that said tolerance 20 is increased). Said slight movement of the two halves of the casing 10 can be each time activated either manually or automatically according to the current operating conditions.

4. Limited slip differential as described in claim 1 characterized in that one or more sets of spider / side gears and relevant casings full of oil are introduced, additionally to the spider gears 3, 4 side gears 5, 6 and casing 10 described in claiml . Shaft of each side gear of each said additional set, is connected to the relevant output shaft (1 or 2) of the differential.