Compact full-traction differential

Details Compact full-traction differential Compact full-traction differential

2003-04-29

2004-08-31

Marmor, Charles A. (Department: 3681)

Planetary gear transmission systems or components

Differential planetary gearing

Planet pinion is worm gear

C475S344000, C074S458000

Reexamination Certificate

active

06783476

ABSTRACT:

TECHNICAL FIELD
This invention relates to all-gear differentials of the type commonly referred to as “limited-slip” designed primarily for automotive use and, more particularly, to such differentials designed for use in vehicles where efficiency, space, cost, and weight are critical.
BACKGROUND
The general format and operation of the type of differential being improved by the present invention is well known, and this prior art type of differential has enjoyed fairly widespread use and publicity throughout the world under the trademark “TORSEN®”.
As is shown in
FIG. 1
, a prior art limited-slip differential of the type just mentioned above includes a rotatable gear housing
10
, and a pair of drive axles
11
,
12
are received in bores formed in the sides of housing
10
. A flange
13
is formed at one end of housing
10
for mounting a ring gear (not shown) for providing rotational power from an external power source, e.g., from a vehicle's engine. The prior art gear arrangement within housing
10
is often called a “crossed-axis compound planetary gear complex” and includes (a) a pair of side-gear worms
14
,
15
fixed, respectively, to the inner ends of axles
11
,
12
, and (b) several sets of combination gears
16
organized in pairs, each combination gear having outer ends formed with integral spur gear portions
17
spaced apart from a “worm-wheel” portion
18
. [NOTE: While standard gear nomenclature uses the term “worm-gear” to describe the mate to a “worm,” this often becomes confusing when describing the various gearing of an all-gear differential. Therefore, as used herein, the mate to a worm is called a “worm-wheel”.]
Each pair of combination gears
16
is mounted within slots or bores formed in the main body of housing
10
so that each combination gear rotates on an axis that is substantially perpendicular to the axis of rotation of side-gear worms
14
,
15
. In order to facilitate assembly, each combination gear
16
usually has a full-length axial hole through which is received a respective mounting shaft
19
for rotational support within journals formed in housing
10
. [NOTE A few known prior art combination gears were formed with integral hubs that are received into the journals of housing
10
; but to facilitate design of the housing and assembly, the combination gears of most presently used limited-slip differentials of this type are shaft mounted.] The spur gear portions
17
of the combination gears
16
of each pair are in mesh with each other, while the worm-wheel portions
18
are, respectively, in mesh with one of the side-gear worms
14
,
15
for transferring and dividing torque between axle ends
11
,
12
. In order to carry most automotive loads, prior art differentials of this type usually include three sets of paired combination gears positioned at approximately 120° intervals about the periphery of each side-gear worm
14
,
15
.
While the conventional full-length axial hole through each combination gear
16
(for receiving mounting shaft
19
) facilitates assembly, it creates definite limitations and problems regarding the manufacture of combination gear
16
. First, and most obvious, combination gear
16
is often measurably weakened by the full-length axial hole. Also, the hole places a limitation on the depth of the gear teeth formed on the cylindrical surfaces of the gear, particularly on the depth of the teeth on worm-wheel portion
18
. Further, during necessary heat treatment for hardening the teeth formed on the cylindrical surfaces of combination gear
16
, the full-length axial hole for receiving mounting shaft
19
must often be “masked” to reduce potential heat distortions of the teeth caused by the relatively thin wall that is created at the bottom lands of the worm-wheel teeth.
The tooth contact patterns of most known worm/worm-wheel combinations are conventional “single-enveloping” patterns. [NOTE: While not appropriate for this use, certain machine tool worm/worm-wheel combinations have “double-enveloping” tooth contact patterns. However, these require the use of hourglass worms mated with fully conjugate worm-wheels of greater diameter, while the full-traction differentials of the invention, as will be explained below, are designed with cylindrical worms mated with hourglass worm-wheels of smaller diameter.] Single-enveloping worm/worm-wheel combinations are known to mesh with a “line contact” in a relative “screw-like” motion (as different from the rolling engagement of spur or helical teeth), and the same line contact pattern is shared by the side-gear worms and the worm-wheel portions of the combination gears in prior art crossed-axis limited-slip differentials. Line contact, particularly when experienced in the relative “screwing” motion of worm gearing, tends to squeeze lubricant off tooth surfaces and, therefore, necessitates the use of fairly viscous lubricants to prevent galling.
This type of prior art limited-slip differential also has another disadvantage. An undesirable torque imbalance occurs whenever the vehicle's engine is applying torque to the differential. This results from the fact that the helical side-gear worms have the same helix angle and are, in effect, in sliding contact with each other at all times so that, when under load, both are subject to thrust forces in the same direction (e.g., both are simultaneously thrust to the left when the vehicle is being driven forward or simultaneously thrust to the right when the vehicle is driven in reverse). For instance, when subjected with thrust force X directed to the left, the right worm presses against the left worm with the force of X, while the left worm is pressed against its respective thrust bearing with a force of 2X. Since the differential divides the torque between the two axles, and since the torque distribution is affected by the friction overcome in each leg of the division, the just-described imbalance affects the torque bias of the differential. The following exaggeration may help to explain the effects of this imbalance: If the torque bias of the differential is designed for 8-to-1 (i.e., eight times more torque being directed to the slower moving wheel), when turning in one direction, the just-described imbalance might result in a torque bias of 10-to-1; and while turning in the opposite direction, the just-described imbalance might result in a torque bias of 6-to-1. While the prior art uses washers and bearings with low-friction surfaces to reduce the imbalance, it would be preferable for vehicle handling and maneuverability to totally avoid such thrust duplication between the side-gear worms.
Nonetheless, in actual practice, this type of prior art differential does a remarkable job of preventing undesirable wheel slip under most conditions. In fact, one or more of these limited-slip differentials are either standard or optional on vehicles presently being sold by at least eight major automobile companies throughout the world, and there are two of these TORSEN® limited-slip differentials in every U.S. Army HMMWV (“Hummer”) vehicle (one differentiating between the front wheels and the other between the rear wheels). Nonetheless, these differentials are relatively large and heavy, taking up valuable space in the mechanically crowded modern vehicle; and they add fuel-consuming extra pounds to the vehicle's weight.
The invention herein is a significantly more compact “full-traction” differential that avoids the thrust duplication between the side-gear worms, is smaller in both size and weight, and is less costly to manufacture while meeting similar load-carrying specifications. [Prior art TORSEN differentials are presently commonly referred to as “limited-slip”, and almost all of the TORSEN differentials presently being manufactured and sold are designed with relatively low torque bias ratios, no greater than 5-to-1. While the invention disclosed herein can be designed with torque bias in that same range, it is preferably designed for torque bias ratios greater than 5-to-1. Therefore, the te

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