Machine element or mechanism – Gearing – Nonplanetary gearing differential type
Patent
1988-12-08
1991-07-09
Herrmann, Allan D.
Machine element or mechanism
Gearing
Nonplanetary gearing differential type
F16H 3504
Patent
active
050294911
DESCRIPTION:
BRIEF SUMMARY
This invention relates to differential mechanisms particularly, but not exclusively, for use as automative differentials in motor land vehicles.
Conventional automotive differentials have a serious operational defect in that power is automatically distributed broadly pro-rata to speed distribution. As a crude generalisation, this can be said to ensure that, when a differential is working and wheels driven via that differential are turning at different speeds, the power will always tend to be distributed in favour of the wheel least able to use it.
Thus, in a condition where one wheel is on gravel or ice and the other wheel is on a surface capable of providing good traction, the wheel on the surface with zero traction will receive 100% of the power being transmitted via the differential and, in consequence, will spin to no avail, while the wheel on the good surface will receive no torque and will not move at all. The end result, of course, is that the vehicle in which the differential is mounted will not move at all.
This problem has been widely recognised for a very long time and many major engineering companies and numerous others have attempted and continue to attempt to solve it.
On a very wide range of both 2- and 4-wheel drive vehicles from high-performance sports and racing cars to heavy-duty, off-highway vehicles, the stated defect in conventional differentials has been regarded as so severe that modified differentials have been, and still are, widely used in an attempt to minimise the adverse effects. The most common form of modified differential is known as a "limited-slip" differential.
In these the ability of one output shaft to turn faster than the other is restricted to a figure, typically in the range of 20-40%. These limited-slip differentials are very much more complex and expensive than conventional differentials and by no means eliminate the stated problem, they simply mitigate its worst excesses. Nevertheless, improved limited-slip differentials continue to be developed and launched by leading automotive engineering companies such as Zahnradfabrik Friedrichshafen AG (ZF AG).
A particular form of "limited-slip" differential which is capable of providing a torque as a function of the speed is based on viscous couplings and is marketed by as the Viscous Control Unit.
In a situation where the two wheels on a driven axle have unequal traction these viscous couplings limit the amount of torque that can be wasted by the wheel with the least traction in a particularly smooth manner and are being adopted widely throughout the automative industry at the present time, particularly on 4-wheel drive vehicles. They do not, however eliminate the basic defect, again they simply mitigate its effects.
An alternative to a limited-slip differential has been the lockable differential. In this the differential capability can be completely blocked when required and thus both wheels will turn at exactly the same speed. Clearly, this approach can provide a significant degree of traction in adverse circumstances but, if maintained for any period of time, will lead to massive tire wear and excessive stresses throughout the transmission system incorporating the differential.
The most refined approach to lockable differentials is perhaps the "ASD" System announced by Daimler-Benz AG at the beginning of 1986. This very sophisticated system relies on multiple sensors to identify wheel-spin and a computerized hydraulic differential locking system. It is, however, by virtue of the sophistication, complex and expensive.
The basic problem in known ordinary differentials stems from the inter dependence in such differentials of speed and torque distribution. It has been recognised that the ideal solution to the basic defect would be completely to split the functions of speed variation and torque distribution. A number of attempts have been made to achieve this. Some of these are described in patents granted to Vernon E. Gleasman, for example U.S. Pat. No. 2,859,641. The technology disclosed in this Patent has been used for
REFERENCES:
patent: 1283283 (1918-10-01), Patch
patent: 1568358 (1926-01-01), Welsh
patent: 2034318 (1936-03-01), Wales
patent: 2220432 (1940-11-01), Wales
patent: 2850919 (1958-09-01), Wildhaber
Eureka Jan. 1988-Published by Innopress Limited "Shuffling Racks Control Unique diff", pp. 38-43.
Axial Wave Drive BV
Herrmann Allan D.
Ta Khoi Q.
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