Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 27~91-1
This inve.ntion relates to llmited slip differentials for
motor vehicles or use in a driving axle or between two driving
axles.
Limited slip differentials incorporating reversible worm
wheel drives are known, as described in United States Patent
Specifications ~los. 2,559,gl6; 2,631,475; 2,859,641 and 4,191,071.
Such differentials are of ~he 'icross-axis planetaryl' type having
co-a~ial output shafts each carrying a sun gear in the form of a
worm wheel meshing with an associated worm or set of worms. The
axis of each worm is thus transverse to and displaced radially
from the axis of the sun g~ears. These differentials have spur
pinions which increase the radial bulk of the differential and
also the correct positioning of the kransverse worm assemblies
presents manufacturing difficulties.
In GB-A-124 563 there is disclosed a differential gear
assembly which does not specify worms and worm wheels and in which
a double pair of differential gear wheels for meshing with a set
o~ locking gears so as to form a continuous chain of gears around
the driving axle. The double pair of dif~erential gear wheels
secure a balanced thrus~ on themselves and balance the radial
thrust loads on the entire mechanisms.
However, in the present invention each oi a first pair
of diametrically opposed pinions is individually coupled to a
single one of a second and similar pair of diametrically opposed
pinions by means of an idler worm wheel so that two distinct
parallel gear trains are formed between output sun gears. Thus,
part of the torque difference between two output gears is provided
hy the thrust loads present in the assembly which therefore are
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.; ~
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thus used to induce a part of the locking ac~ion of ~he
differen~ial.
GB-A-512 770 discloses a differential gear mechanism
which does not have a con~inuous helical formation for both the
pinion portion and the worm portion of a worm member as in the
differential gear mechanism of the present invention. Indeed the
equivalent geariny part of the mechanism shown and described in
GB-A-512 770 provides separate helical gears of op~osite hand
which interconnect the output shaft and sun gears and moresver ~he
helix angle of the worm and wheel gears do not permit the worm and
wheel action to be reverslble as in the present invention.
Accordlng to the invention, there is provided a limited
slip differential for a vehicle comprising: a year carrier
mountable for rotation about an axis of the carrier; a pair of
dlametrically opposed first worm members mounted for rotatlon
relative to the carrier with their axes parallel to the carrier
axis and a similar pair of diametrically opposed second worm
members mounted ~or rotation relative to the carrier with their
axes parallel to the carrier axis; a first sun gear wheel coaxial
with the carrier axis and meshing with the first worm members, a
second sun gear wheel coaxial with the carrier and meshing with
the second worm members, each worm member comprising a worm
portion having a helical thread which meshes with an idler worm
wheel and having a continuous tooth form extension forming a
helical pinion portion meshing with a respective one of the first
and second sun gear wheels, wherein the helix angle of the worm
portions of each worm member and the worm wheels is such that the
worm drive is reversible and wherein each of the first worm
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members is individually coupled tv a single one o the second worm
members by a respective one of the idler wor~ wheels.
Aeeording to a feature of the invention, the idler worm
wheels may be rota$ably mounted on a common transverse pin with a
spacer bloek therebetween.
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An embodlment of the invention will now be de6cribed, by way of
example, with reference to the accompanylng drawlngs, ln which:
Flgure 1 16 8 chordal 6ectlonal view of a dlfferential accordlng
to the invention on ~he line I-I of Figure 2;
Figure 2 16 a cross 6ectional view along the line~ II of F~gure
3; and
Flgure 3 i5 a a~ial ~ectlonal view along the line III-III of
Flgure 2.
The differential shown in ~he drawings transmits drlve fro~
transfer gearlng (not shown) to half-6hafts 2 and h of a driven
axle, for example of a motor car.
The dlfferential has a gear carrier 6 formed by a bell-6haped
hou6ing 7 and a cover member 8 having a flange 8A and a hub portion
9 snugly engaged in the mouth of the housing 7. The flange6 8A may
be formed with a rlng of gear teeth or, as ~hown here, wlth a ring
of holcs 8B for bolts securing a bevel crown wheel or a transfer
gear (not shown). The houslng 7 and cover Member 8 are gecured
together by suitable bolts or screw6 (not shown) to form the gear
carrier 6.
Four worm members 10~-lOD are rotatably mounted on hollow pin6
11 6upported in the gear carrier 6 at each of their ends. The worm
members 10 and their plns 11 are parallel and equally spaced from
the axis of the shafts ~ and 4. Each pin 11 is formed with a
6eating llA for a grub screw llB. Two of the worm members, lOA and
loc, mesh with a worm wheel 12 at diametrlcally oppo6ed po61tion6.
The wor~ wheel 12 is located between the worm members lOA and lOC
and is rotatably fiupported on a hollow support pin 14 mounted ln the
housing 7. The other two worm members, loB and lOD, mesh with a
further worm wheel 16 located between them, and also mounted on the
6upport pin 14 but spaced alon~ the support pln 14 by means of a
spacer block 17. The pin 14 has a seating 14A for a securlng grub
screw lS ln the spacer block 17.
The worm wheels 12 and 16, the spacer block 17 and the ~upport
pln 14 are aligned on an axis YY whlch intersects the axls XX at
right angles.
Half-shaft sun-gear6 18 and 20 are re6pectlvely 6plined to (or
formed lntegrally with) the inner ends of the hal~-shafts 2 and 4
and are aligned on the axis XX. Each half-shaft sun gear is within
the gear carrier 6 and meshes wlth an extension one of the worm
members of each pair of worm members meshing with a respective worm
whe~l. Thus, the half-shaft sun gear 18 meshes with worm members
lOB and lOC which mesh respectlvely with worm wheels 16 and 12. The
half-shaft sun gear 20 meshes with worm members lOA and lQD which
mesh respectively with worm wheels 12 and 16.
Each worm member thus has a worm portion 22, having a h~lical
thread and meshing with a worm wheel, and an extension forming a
pinion portion 24 meshing with a half-shaft sun gear. The heli~
angle of the worm portions 22 and worm wheels 12 and 16 is such that
the worm drive is reversible tin this case 43.1 to the axisJ for
the worms) i.e~ operates bi-directionally. In this embodiment the
half-shaft sun gears 18 and 20 and the pinion portions 24 of the
worm members are helical, the pinion portion 24 being formed by
continuations of the worm formation of the worm portions 22.
Each worm member also has a reduced-diameter untoothed portion
25 ad~oining the worm portion to avoid engagement with the other
half-shaft sun gear.
In operation, when the vehicle is travelling straight ahead
with the road wheels turning at the same speed the gear carrier 6 is
rotated about the axis XX. The rotary motion is transmitted to the
half-shafts 2 and 4. The half-shafts, the gear carrier 6 and the
gearing within the gear carrier rotate as a unit. There is no
movement of the worms 10 or worm wheels 12, 16 about their axes.
When the vehicle corners, one half-shaft must rotate faster
than the gear carrier 6 and the other half-shaft slower than the
gear carrlerO If the half-shaft 2 rotates faster than the gear
carrier 6 it, through the half-shaft sun gear 18, causes the worms
lOB and loc to rotate about their axes in the gear carrier 6. The
worms lOB and lOC in turn cause the worm wheels 16 and 12 to turn
zbout the support pin 14. Because the worm drive ls reversible,
the worm wheels 16 and 12 cause the worms lOB and lOA to rotate in
the opposite sense to worms lOB and lOC. Although the worm drive is
reversible, drive ls not transmitted with 100% efficiency from the
worm wheels to the worm members. In this embodiment the efficiency
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is about 70%. As a result, there is frictional resistance to
operation of the device and it functions as a limiked slip
differential. The rotation of the worm members lODZ and lOA
causes the sun gear 20 and -~he half-shaft 4 turn backwards
relative to the carrier 6.
The steel worm members, worm wheels and spacer block
must be robust and wear resistant and in this embodiment the worm
wheels (and advantageously the worms and the spacer block
especially where engine oil is the only available lubricant) can
be case hardened and then receive anti-scuffing treatment such as
'No Scuff' available from British i!eat Treatments Limited of
Lowestoft.
The housing 7 is formed wlth large openings 26 both to
reduce mass and to assist the circulation of lubricating oil.
Preferably, the lubricating oil ~s hypoid gear oil. Lubrication
is ensured by oil passages L drilled in the various components.
The limited slip differential descri~ed above may be
used ~o transmit drive to two fur~her 6uch differentials in the
two driving axl.es (e.g~ front and rear) of a four-wheel drive
vehicle.
In a modification (not shown in the drawings), the grub
screws llB and seatings llA are omitted and the untoothed portions
25 of the worms are replaced by flanges of corresponding diameter
and axial length on the pins to provide axial location of the
latter while allowing them to rotate.
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If desired, further friction in the differential can be
generated by axially biassing the worm members, for example by
means of Belleville washers located on the pins between the worms
and either the casiny or the pin flanges (if used).
B
