Gauge for checking linear dimensions of mechanical pieces

A gauge for checking linear dimensions of mechanical pieces, comprising a differential transformer transducer including a first member, substantially cylindrical, which supports a core, a second member, substantially cylindrical, which houses electric windings, and a sealing bellows having its ends fixed to the first and the second member, respectively. The two members are mutually movable and the electric windings provide a measurement signal representative of the position of the core with respect to the second member. The ends of the bellows are coupled to annular seats defined in the first and the second members, with a coupling which generates substantially compressive stresses at said ends.

The present invention relates to a gauge for checking linear dimensions of 
mechanical pieces, with a first member and a second member, these members 
being mutually movable; an element fixed to the first member; electric 
circuits associated with the second member, the electric circuits 
providing a signal representative of said linear dimensions depending on 
the position of said element; and a substantially tubular resilient seal, 
having its ends resiliently fixed to the first member and the second 
member, respectively, at least one of the ends including an annular 
portion; at least one of said members having an external surface defining 
a seat for housing said annular portion. 
Conventional gauges are known which comprise an inductive position 
transducer, particularly a differential transformer transducer, a 
cylindrical member housing electric windings and another member supporting 
a magnetic core movable within the windings along a substantially axial 
direction. It is also known, in these gauges, the use of a sealing bellows 
having its ends respectively fixed to the two members, by interference 
fitting generating substantially tensile stresses at the ends of the 
bellows. 
These conventional gauges present some drawbacks due to the effect of the 
tensile stresses on resilient materials, such as natural or synthetic 
rubbers and plastics, of which the sealing bellows is made. 
As a consequence of the aging of these materials, the desired coupling 
between the ends of the sealing bellows and the members carrying the 
windings and the core, may become loose, so that sealing of the transducer 
is lost. 
The object of the present invention is to provide a gauge having a seal 
which does not present the drawbacks of the conventional gauges. 
This problem is solved by a gauge of the kind set forth in which, according 
to the present invention, said seat is annular and between this annular 
seat and said annular portion there is a coupling generating substantially 
compressive stresses in the annular portion.

With reference to FIGS. 1-3, the manual plug gauge comprises a handle 1, 
preferably made of plastics, with an upper base 2--perpendicular to the 
longitudinal geometrical axis of the gauge--, defining a central opening 
or hole, not visible, for the passage of a cable 3. Handle 1, which is 
hollow, has substantially cylindrical internal and external surfaces, 
coaxial with the longitudinal axis of the gauge. 
The lower part of handle 1 ends with a flange 4, which serves for coupling 
the handle to a support element, essentially consisting of a bored plate 
5. 
The coupling between flange 4 and plate 5 is achieved by two screws 6, 7, 
passing through bores of flange 5 and screwed into threaded bores obtained 
in plate 5, in such a way as to clamp the lower face of flange 4 against 
the upper face of plate 5. Moreover, plate 5 supports a protection and 
centering nose-piece 8, by four screws, not visible, passing through bores 
of plate 5 and screwed into threaded bores, not visible, obtained in a 
flange 9 of nose-piece 8, in such a way that the upper face of flange 9 is 
clamped against the lower face of plate 5. The four screws for clamping 
flange 9 are accessible through openings, not visible, obtained in flange 
4. 
Plate 5 also acts as support element for the arm-set of the gauge, because 
to the upper face of plate 5 there are coupled the lower faces of two 
flanges 10, 11, which constitute, together with the arm-set, a single body 
or member 12, as can be seen in FIGS. 2 and 3; the coupling is obtained by 
two screws, one of which, 13, is visible in FIG. 1; these two screws pass 
through bores 14,15 of flanges 10,11 and are screwed into threaded bores 
of plate 5; seats, not visible, are obtained in flange 4 for housing 
flanges 10,11 and screws 13. 
Flanges 10,11 are oppositely arranged with respect to the vertical plane 
passing through the gauge axis and the axes of screws 6, 7. 
Body 12 which is stamped by bending a shaped sheet made of spring steel, 
also comprises an upper base 16, having a substantially rectangular 
perimeter and defining a hole 17 for the passage of cable 3, two 
connecting parts 18, 19 for connecting flanges 10, 11 to base 16, and two 
moving arms 20, 21. Arms 20, 21, which are obtained by bending two wings 
of the above-mentioned spring steel sheet, are connected to upper base 16 
through two sections 22, 23,--lightened by relevant chamfers, as shown in 
FIGS. 1 and 3--, which define two fulcrums for pivotal movement of arms 
20, 21 about relevant axes perpendicular to the arms. 
Arms 20, 21 have a substantially similar structure, with channel 
cross-section, for providing high resistance to bending stress, and 
present in their central sides two substantially cylindrical parts 24, 
25--slightly protruding toward the interior--obtained by a partial 
blanking operation. 
The two cylindrical parts 24, 25 constitute seats for housing and abutting 
the ends of a compression spring 26, which tends to move arms 20, 21 apart 
from each other. The mutual position of arms 20, 21 is detected by a 
differential transformer position transducer 28. 
Transducer 28 comprises a first member 29, substantially cylindrical, 
passing through a hole 30 of arm 21 and fixed to the latter through an 
adhesive. Member 29 shall be more detailedly described hereinafter. 
A stem 31 having an end fixed to member 29 carries at the other end a 
magnetic core 32. Transducer 28 comprises a second member 33, consisting 
of a substantially cylindrical shell terminating with a flange 34. Shell 
33 passes through a hole 35 of arm 20 and is fixed to the latter through 
an adhesive. 
Shell 33, which shall be more detailedly described hereinafter, houses the 
electric windings 36 of the differential transformer, wherethrough 
magnetic core 32 is movable along a substantially axial direction, due to 
the mutual displacements of arms 20, 21. 
Cable 3, which is connected to windings 36, gets out of shell 33 and passes 
through an opening 37 of arm 20, this opening communicating with hole 35. 
A sealing bellows 38, which shall be detailedly described hereinafter, has 
its ends fixed to member 29 and shell 33, for sealing transducer 28. 
Arms 20, 21 have, as above mentioned, channel cross-sections and thus 
present bent wings 39,40,41,42; arms 20,21 house at their free 
ends--within relevant holes 43,44 passing through further bent wings 45, 
46--small cylinders 47, 48, which carry relevant feelers 49, 50. 
Cylinders 47,48 are clamped by socket head screws, not visible, coupled to 
threaded rivets, also not visible, fixed to arms 20, 21 and passing 
through a hole defined by wing 39 and a hole 51 defined by wing 42. By 
loosening these screws it is possible to adjust the position of the feeler 
elements, i.e. cylinders 47,48 with feelers 49,50, along the measurement 
radial direction. 
Wings 39, 40 of arm 20 have two protruding sections 52, 53, the first of 
which ends with a substantially vertical flat side, while the second ends 
with a narrow horizontal edge. Similarly, wings 41, 42 have two protruding 
sections 54, 55, the first ending with a narrow horizontal edge and the 
second with a substantially vertical flat side. Protruding sections 52, 
53, 54, 55, which form integral parts of member 12, constitute reference 
elements for limiting the approaching movements of arms 20, 21. In fact, 
the free ends of protruding sections 52, 53 are arranged opposite the free 
ends of protruding sections 54, 55, respectively, with which they get into 
contact when arms 20, 21 take a determined mutual position such as to 
prevent any danger of damages in sections 22, 23 and/or in transducer 28. 
The edge shape of protruding sections 53, 54 reduces the inconveniences 
deriving from the possible deposit of foreign matters, like dust, grease, 
chips, etc., and guarantees a substantially constant position of maximum 
mutual approach of arms 20, 21. 
Nose-piece 8 presents openings 56, 57 for the passage of feelers 49, 50 and 
is closed at its lower end by a base or circular plate 58, perpendicular 
to the geometric axis of the gauge. Plate 58, suitably fixed to nose-piece 
8, presents a central opening 59. 
Plate 5 has substantially an annular shape, with a central opening 60, and 
comprises two parts 61, 62 protruding towards the interior and having 
approximately the shape of triangular prisms, with chamfered edges. Parts 
61, 62,--which are diametrically opposite along a direction perpendicular 
to that according to which are arranged flanges 10, 11--form reference 
surfaces for limiting the opening displacements of arms 20, 21. In fact, 
as can be seen from FIG. 1, arms 20, 21 pass through opening 60 because 
base 16 and the free ends of arms 20, 21 are arranged at opposite sides 
with respect to plate 5; in proximity of plate 5, the central side of the 
channel cross-sections of arms 20, 21 is adjacent to protruding parts 61, 
62. 
Therefore, if piece 63 shown in FIG. 1 is taken away, the opening 
displacements caused by spring 26 are limited due to the contact of said 
central sides of the channel cross-sections of arms 20, 21 with the edges 
of protruding parts 61, 62. 
Conversely, when nose-piece 8 is inserted into the bore of a piece 63 to be 
checked, feelers 49, 50--which in rest conditions protrude of a determined 
amount with respect to the external surface of nose-piece 8--cause closing 
displacements of arms 20, 21, which take a measurement position; then it 
is possible to read, on an instrument of a supply and detection group 64, 
connected to a connector 65 arranged at the end of cable 3, the deviation 
of the bore diameter from the diameter of a master piece which, as it is 
usual, is used for zero-setting the gauge. 
The preceding description shows that a single element, namely plate 5, acts 
as both a central support--to which there are coupled handle 1, arms 20, 
21 (through the intermediate support element including flanges 10, 11, 
base 16 and lightened sections 22, 23) and nose-piece 8--and a limiting 
device for limiting the opening displacements of arms 20, 21. 
Limiting of the opening displacements is obtained, as described above, by 
two parts 61, 62 of plate or ring 5, which protrude towards the interior 
and which, having ends shaped substantially as sharp edges, define 
reference surfaces of limited areas, thus reducing the effects of foreign 
matters which may deposit on arms 20, 21 or on the reference surfaces and 
providing a limitation of the arms opening at a value substantially 
constant. 
Handle 1 comprises, on upper base 2, two lugs, one of which, 66, shown in 
FIG. 1, having a shape substantially prismatic but with two sides 
constituting parts of cylindrical surfaces having the same diameter as 
that of base 2. 
The gauge also comprises a cable clamping device including two elements 67, 
68 which are clamped, through screws not shown, against lugs 66. Elements 
67, 68 include semi-cylindrical threaded seats, not visible, which clamp 
the sheath of cable 3. To elements 67, 68 there is coupled an end of a 
rubber hose 69 which is provided for stiffening cable 3, for preventing 
possible breakages of its sheath in proximity of the cable clamping 
device. 
The cable clamping device prevents damages of the wires of cable 3 and 
transmission of mechanical forces acting on cable 3--at the exterior of 
the gauge--to the cable portion housed within handle 1 and consequently 
from this cable portion to moving arm 20. 
Holes, not visible, obtained in elements 67, 68, communicate with a hole, 
also not visible, obtained in upper base 2 of handle 1. These holes, as 
well as openings 59, 60 permit to clean the interior of the gauge without 
any necessity of disassembling any gauge element. In fact, by means of a 
nozzle inserted through opening 59, it is possible to blow compressed air 
or another suitable fluid in the support and protection shell consisting 
of nose-piece 8 with plate 58, plate 5 and handle 1. The compressed air, 
which flows within the plug gauge substantially from down to up passing 
through the interior of nose-piece 8, opening 60 and the interior of 
handle 1, removes any foreign material and flows out towards the exterior, 
together with the foreign material, through the holes obtained in base 2 
and elements 67, 68. Partially, the compressed air also flows out through 
the annular gaps delimited by cylinders 47, 48 and by the surfaces of 
openings 56, 57. 
Due to its simplicity, the cleaning operation may be frequently carried 
out, for surely preventing the deposit, on gauge elements critical for the 
proper gauge operation--such as cylinders 47, 48, protruding parts 61, 62, 
reference surfaces defined by sections 52, 53, 54, 55, arms 20, 21, spring 
26, and lightened sections 22, 23--, of foreign material in a quantity 
which might seriously affect the accuracy and repeatability of the 
measurements or cause some surfaces to oxidize. Transducer 28, since it is 
sealed by bellows 38, is not detrimentally influenced by the cleaning 
operation through compressed air. 
The hole of handle base 2 is diametrically dimensioned in such a way as to 
permit withdrawal of handle 1--after having disconnected connector 65 from 
group 64, disassembled the cable clamping device, and loosened screws 6, 
7--by letting cable 3 and connector 65 pass through the same hole. 
In this way assembling and disassembling of the gauge are rendered easy and 
it is possible to assemble the gauge by easily and quickly assembling a 
plurality of unfinished products, one of which constituted by the handle 
and another by an arm-set equipped with transducer, cable and connector. 
The transducer shell 33 has a through hole 70 wherein there are housed 
windings 36. The fixing of windings 36 and the sealing of the hole 70 end 
adjacent to flange 36 are carried out by a suitable resin. The fixing of 
shell 33 to arm 20 is also carried out by a resin, which can be applied in 
part within a slot 71 obtained in the external surface of the shell, near 
flange 34. Flange 34, which abuts against the central side of the channel 
cross-section of arm 20, presents a cut 72 and two chamfers 73, 74, which 
are provided for permitting to detach, by a suitable tool, shell 33 from 
arm 20. 
A hole 75 passing through shell 33, permits the passage of cable 3. 
The end of shell 33 opposite that including flange 34 presents an annular 
slot 76 having a cross-section with a substantially 
rectangular-trapezoidal shape, as can be better seen in the detail of FIG. 
7. 
Member 29 of transducer 28 defines, as shown in FIGS. 8-10, a through hole 
77 including two sections having different diameters which are connected 
by a frusto-conical section. Hole 77 has a threaded end to which there is 
screwed stem 31, for permitting axial adjustments of the latter. Stem 31 
is locked in a way not shown and hole 77 is sealed by a suitable resin. 
Member 29 has a flange 78 defining two side chamfers 79, 80, which abuts 
against the central side of the channel cross-section of arm 21. The end 
of member 29 opposite to that having flange 78 presents an annular slot 
81, having a cross-section with a substantially rectangular-trapezoidal 
shape, as shown in the detail of FIG. 10. 
As shown in FIG. 11, bellows 38 has substantially a tubular form and 
terminates at its ends with two annular portions 82, 83 protruding towards 
the interior. Annular portions 82, 83 have cross-sections with 
substantially rectangular-trapezoidal shapes. 
The coupling of bellows 38 to member 29 and shell 33 is carried out by 
fitting annular portion 82 in slot 76 and annular portion 83 in slot 81. 
The dimensions of slots 76, 81 and of annular portions 82, 83 are chosen 
to obtain a coupling by interference-fit owing to which annular portions 
82, 83 are subjected substantially to resilient compressive stresses, 
along the direction of the axis of bellows 38. 
Therefore, in rest condition, the thicknesses of annular portions 82, 
83--along the direction of the axis of bellows 38--are larger than the 
widths of the corresponding sections of slots 76, 81, respectively. 
The internal diameters of annular portions 82, 83 in correspondence with 
the larger bases of the trapezoidal figures defined by the cross-sections 
of the same annular portions, are substantially equal to the external 
diameters of shell 33 and member 29 in correspondence with the bottom of 
slots 76, 81. 
The above described shapes of bellows 38 and slots 76, 81 and the 
consequent coupling generating compressive stresses in annular portions 
82, 83 have been found to generate very safe coupling and sealing, even 
after a considerable aging of the material--such as natural or syntetic 
rubbers and plastics--of which bellows 38 is made. Another advantage 
resides in that the mutual positions of annular portions 82, 83 with 
respect to shell 33 and member 29--along the direction of the bellows 38 
axis--are strictly constant. 
Thus, a very good sealing of transducer 28 is achieved and this prevents 
any negative influence on the transducer of the above described cleaning 
operations. 
Obviously, the invention can undergo changes and variants equivalent from a 
functional and structural point of view without departing from its scope. 
For example, a seal or gasket similar to the described sealing bellows, 
possibly with different cross-sections of the annular portions, can be 
used for providing sealing between a feeler carrying stem and a bushing. 
Moreover, it is possible to provide a coupling subjected to compressive 
stresses on only one of the annular portions of the seal.