Device for fastening a first part such as an accessory to a second part such as a support with a bush for eliminating the assembling backlash between the two parts and bush used for the fastening of these parts

The present invention relates to a device for fastening a first part to a second part, the two parts having an assembling backlash between them, this device comprising a bush with inner and outer threads with opposite pitches, interposed between the threads of a fastening screw and an inside thread of the second part matching with the outer thread of the bush.

The present invention relates to a device for fastening a mechanical part 
to another mechanical part with an assembling backlash between them. 
Such a device can be used in particular for the fastening of accessories 
such as refrigeration compressors, alternators, hydraulic pumps or 
crankcases for thermal engines of automotive vehicles. 
FIG. 1 shows a known embodiment for fastening a refrigeration compressor 1 
of a thermal engine to a crankcase of same. The known fastening device 
comprises two parallel fastening flanges or lugs 1a solid with compressor 
1 and comprising respectively two parallel inner bearing faces 1b facing 
one another by a distance or dimension A. Moreover, the fastening device 
comprises a support 2 assembled by welding, solid with the crankcase (not 
shown) by fastening screws 3 and comprising two parallel rigid support 
plates 2a presenting two substantially parallel opposite outer bearing 
faces 2a1 separated from each other by a distance or dimension B. For 
reasons of assembling the support plates 2a between flanges 1a, the 
dimension A between the flanges 1a of compressor 1 is necessarily greater 
than the dimension B between the outer faces 2a1 respectively of the two 
support plates 2a. The two flanges 1a are fastened respectively to the two 
support plates 2a with the aid of coaxial fastening bolts 4 ensuring a 
strong clamping of the inner bearing faces 1b respectively against the 
outer faces 2a1 of the support plates 2a. However, inasmuch as the 
dimension A shall be greater than the dimension B, the clamping force of 
the fastening bolts 4 deforms the dimensions A and B to the contact of the 
clamping flanges 1a with the fastening lugs 2a in order to obtain a 
positive clamping. Thus, this force stresses the flanges of the compressor 
and the fastening plates 2a with a risk of fatigue rupture of these parts. 
The present invention has for an object to eliminate the above drawback of 
the known device by providing a device for fastening a first part 
comprising at least two substantially parallel fastening lugs, 
respectively with inner bearing faces opposite to each other, to another 
part with two opposite parallel outer faces disposed between both bearing 
faces of the fastening lugs with an assembling backlash, and comprising at 
least two screws for the fastening respectively of the two fastening lugs 
to the second part and characterized in that it comprises a bush with 
inner and outer threads with opposite pitches interposed between the 
threads of one of the fastening screws and an inside thread of the second 
part matching with the outer thread of the bush, the inner thread of the 
bush matching with the thread of the screw with a reduced backlash in 
order that the bush can move axially in the inside thread during the 
operation of tightening of the fastening screw so as to abut one of its 
two free ends against the inner bearing face of the fastening lug through 
which the said fastening screw passes. 
According to a feature of the invention, the bush comprises a drive 
configuration, such as a slot, for the positioning thereof, before 
assembling the first part to the second part, in the inside thread of the 
second part so that the aforesaid free end of the bush be substantially 
level with the aperture of the inside thread opening onto the outer face 
of the second part opposite to the aforesaid bearing face of the fastening 
lug. 
According to still another feature of the invention, the second fastening 
screw is in coaxial relationship with the first fastening screw and holds 
firmly the other outer face of the second part against the bearing face of 
the second fastening lug during its tightening. 
The invention relates also to a bush used in particular for the fastening 
of a first part, such as an accessory, to a second part, such as a 
support, the two parts having between them an assembling backlash and 
characterized in that it comprises inner and outer threads with opposite 
pitches, the inner thread of the bush being machined so as to have a 
reduced backlash with the corresponding thread of a screw for fastening 
the first part to the second part. 
According to a feature of the invention, the bush comprises, at one of its 
two free ends, a drive configuration, such as a slot, for its positioning 
in an inside thread of the second part.

The fastening device according to the invention will be described when used 
for the fastening of a refrigeration compressor to the crankcase of a 
thermal engine of an automotive vehicle, although this device can of 
course be used for the fastening of other various accessories such as for 
example alternators, hydraulic pumps of thermal engines. 
As shown in FIGS. 2 to 4, the reference sign 5 designates a refrigeration 
compressor of a thermal engine rigidly fastened to the crankcase 6 of a 
thermal engine of an automotive vehicle. 
The compressor 5 comprises two pairs of fastening lugs 5a, 5b solid with 
the same. Both fastening lugs of each pair are substantially parallel and 
comprise respectively two parallel inner bearing faces 5a1, 5b1 opposite 
to each other, one of the bearing faces comprising a bearing plate under 
the shape of a washer 7 which is solid for example through welding with 
the bearing face. The bearing face 5a1; 5b1 of each pair of fastening lugs 
is separated from the inner bearing face of the bearing plate 7 by a 
dimension A. The two fastening lugs of each pair comprise respectively two 
coaxial through holes 5a2; 5b2 which permit screws 8 for fastening 
compressor 5 to crankcase 6 to pass therethrough, the heads of the 
fastening screws 8, when tightened, resting respectively on the outer 
faces of the fastening lugs 5 opposite to the bearing faces 5a1; 5b1. 
The fastening lugs 5 are fastened to the crankcase 6 through the medium of 
a common support 9 which is itself solid with crankcase 6 through the 
medium of fastening screws 10. The support 9 comprises two pairs of 
flanges 11a, 11b; 12a, 12b for the fastening respectively of the two pairs 
of fastening lugs 5a, 5b to the support 9. The two flanges 11a, 11b; 12a, 
12b of each pair are substantially parallel and their opposite parallel 
outer faces 11a1, 11b1; 12a1, 12b1 are separated from each other by a 
dimension B smaller than the dimension A. The outer faces 11a1 and 12a1 of 
flanges 11a and 12a respectively are machined whereas the opposite outer 
faces 11b1 and 12b1 of flanges 11b and 12 respectively are cast in one 
piece. The two flanges 11a, 11b; 12a, 12b comprise respectively two 
coaxial threaded holes, the inside thread of the flange 11a or 12a 
matching with the thread of the associated fastening screw 8. 
The fastening device according to the invention comprises a bush 13 shown 
in FIG. 5 and comprising an outer thread 13a matching with the inside 
thread of the flange 11b or 12b and an inner thread 13b, made in the 
through hole of the bush, and matching with the thread of the associated 
fastening screw 8 with a reduced backlash. The direction of the pitch of 
the spirals of the inner thread is opposite to that of the pitch of the 
spiral of the outer thread. For example, the bush 13 has an outer thread 
13a of the left-handed type whereas the inner thread 13b is of the 
right-handed type. Moreover, the bush 13 comprises a drive configuration 
13c, such as a slot, for the positioning of the bush in the inside thread 
of the flange 11b or 12b. 
The operation of the fastening device according to the invention follows 
partly from the above description and will now be described in detail. 
First, the bush 13 is positionned, with the aid of a screwdriver inserted 
in slot 13c, in the inside thread of flange 11b or 12b until the free end 
of the bush comprising the slot 13c is substantially level with the 
aperture of the threaded hole of flange 11b or 12b. The fastening lugs 5a 
or 5b are put in assembling position so that the bores 5a2 or 5b2 and the 
inside threads of flanges 11a, 11b or 12a, 12b respectively be 
substantially coaxial. Then, a fastening screw 8 is introduced into the 
bore 5a2 or 5b2 and is screwed in the inside thread of flange 11a or 12a 
and tightened until the inner face 11a1 or 12a1 is strongly held in 
contact against the machined outer face of flange 11a or 12a. In this 
mounting position, a space corresponding to the assembling backlash equal 
to the difference between the dimension A and the dimension B remains 
between the inner face of the bearing plate 7 and the outer face 11b1 or 
12b1. Then, another screw 8 is introduced into the bores 5a2 or 5b2 and 
screwed in the inside thread of the bush 13. The reduced backlash between 
the inner thread 13b of bush 13 and the thread of this screw 8 ensures a 
friction of the bush against this screw 8 which permits both the rotation 
of the bush 13 and the rotation of the screw. The bush 13, which is 
rotated, moves axially in the inside thread of flange 11b or 12b in the 
direction of arrow C and abuts at its free end comprising the slot 13c 
against the inner face of the bearing plate 7 during the definitive 
tightening of the fastening screw 8. Thus, the axial motion of bush 13 
permits suppressing the backlash between the dimensions A and B which 
renders the dimensions of the faces to be assembled equal without 
stresses. 
Therefore, the bush used in the fastening device of the invention has for 
an advantage that it lowers the fastening stresses of the fastening lugs 
of the accessory to be assembled on an engine, due to the interval 
existing between the bearing faces of the support and the bearing faces of 
the accessory and resulting from the manufacturing tolerances both of the 
support and of the accessory. Thus, there is obtained a rigid fastening of 
the accessory on the support, which suppresses automatically the aforesaid 
backlash without adjusting wedges and without stresses. Moreover, it is 
not necessary to machine the face 11b1 or 12b1 located opposite to the 
bearing face of plate 7.