design for a machine used for superfinishing the internal...
TRANSCRIPT
Design for a Machine used for Superfinishing the Internal Surfaces of
Gears that are Components of a Gearbox of a Truck
BADEA LEPADATESCU 1)
VALENTIN DITU 2)
ANISOR NEDELCU3)
1), 2), 3)
Transilvania University of Brasov, Faculty of Technological Engineering and Industrial
Management, B-dul Eroilor nr.29, Brasov
ROMANIA
[email protected], [email protected], [email protected]
Abstract:- In the paper is presented a machine tool that is used to accomplish the surface quality that is required for
the internal surfaces of gears that are components of a gearbox of a truck. As is known in a gearbox of a truck are many gears that are mounted on a needle bearings. For this reason, the internal surfaces of these gears need to have
a very high surface quality to work properly. The machine that is presented in the paper was designed to realize the entire requirement regarding the surface quality for all the types and sizes of gears that are in the truck gearbox.
Key-Words: - surface finish, reliability, gear, high production, load bearing capacity
1 Introduction In many instances, superior workpiece quality in regard to load bearing capacity, efficiency,
smoothness of running, operational safety, and last but not least, longevity, requires superior surface finishing of these workpieces. By logical extension,
this means continuous adaptation and optimization of
existing production processes. The superfinishing
process gives to workpieces as called “final touch”, being an economical machining operation and tipically the last and therefore the quality-
determining process, that makes it more important.
Some of the advantages of using superfinishing
process are presented below: - High flexibility;
- Low noise volume; - Low energy costs;
- Low initial investment;
- Low tool costs; - Less friction;
- Short processing time. The superfinishing process allows the targeted
influencing of the following parameters:
- Reduction of surface peak-to-valleys and increase of material contacta rea to over 90%, due to
the plateau like surface structures; - Improvment of roundness, creation of microgeometries with particular applications;
- Optimization of tribological properties through criss-crossinf finishing marks.
Superfinishing removes the amorphous structure of the material, or ‘soft skin”. This layer is usually 2 to
8 µm thick and is created by high temperature
generates by the grinding wheel. It enables the user
to achieve virtually any surface texture parameter, because only the roughness peaks are removed. The
geometry of the workpiece remains unchanged. The main advantage of this method is consistent
finish over the entire surface. Superfinish improves
the bearing ratio resulting in greatly improved wear resistance. Stock removal is typically in the order of
a few microns. The achievable surface finish quality is comparable to levels reached only by honing or
lapping. Superfinishing process is a very clean
process, because coolant or grinding emulsion is used of honing oils like Stone grinding. Recycling and
waste disposal are simplified. This process is suitable for any material that could
also be finished with geometrically undefined blades.
In addition to the most diverse metal alloys and grades, other materials can also be finished, such as
ceramics, plastics, non-ferrous metals as well as coatings, such as tungsten carbide , chromium, or
copper. After the process of superfinishing the workpiece
surface has a tightly controlled cross-hatch patter,
obtained by the interaction of three interrelated motions. These motion are the oscillation of the
stone, rotary movement of the workpiece and pressure of the abrasive tool on the workpiece.
During the superfinishing process, parts pass through
several distinc phases. When the abrasive tool makes initial contact with the part, dull grains fracture or
Mathematical Methods and Optimization Techniques in Engineering
ISBN: 978-960-474-339-1 171
pull away from the matrix to produce a new cutting surface. As the tool “self dresses”, relatively large
amounts of stock removal phase, abrasive grains begin to dull, while surface irregularities and geometry continue to improve. This results in a
cross-hatced surface free of irregularities and amorphous material.
2 Machine to superfinish the internal
surfaces of the gears In this paper is presented a machine to superfinish the internal surfaces of the gears which are components
of a gearbox of the truck. It is known that in the
gearbox of the truck are a lot of gears with different diameters that are used as the intermédiate gears,
without transmitting the torque, only to transmit the rotary motion to other shafts. For this reason, these gears have the internal surfaces smoothly and these
surfaces need to have a very high surface finish in the values of roughness between 0,4-0,2 µm Ra. The
previous process is grinding and the last machining process is superfinishing that gives the final
dimension and roughness for these internal surfaces. These gears from the gearbox have an internal diameters between Ø 35-110 mm and the width
between 20-50 mm. One of these gears is shown in Fig.1.
Fig.1 Gear with internal surface obtained after the process of superfinishing.
To can machining all these types of gears was
designed a machine that is presented in the sketch of Fig.2. In this figure the workpiece (4) is held in jaws (3) and has a rotational motion through the machine
gearbox (1). The superfinishing head has a oscillation movement with the amplitude of 1-3 mm and a
frequency of 1500cd/min. These oscillation movement is realized by the motor (8) and the mechanism (7).
It is used a plunge process when the workpiece is held and rotated, while the superfinishing multistone
(the superfinishing stones are between 3-6, according
with the internal diameter of the workpiece) makes
contact with the internal surfaces of the part. During the superfinishing process were used abrasive stones with silicone carbide, which is a combination of pure
white quartz, petroleum coke, sawdust and salt in an electric furnace.
To achieve the desired surface finish was used abrasive materials with fine grit sizes between 400 and 1200 on the FEPA scale.
The time of machining was between 30-40 seconds, depending on the size of part diameter.
Mathematical Methods and Optimization Techniques in Engineering
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Fig.2 Sketch of the superfinishing machine.
One of the important problem that must be solved was to design a device that can be hold and center the
workpiece in those three jaws. For this reason were realized for every size of workpiece a special device
as can be seen in Fig.3.
This device has as a body a ring (6) where are fixed by screws three small parts (4) with two balls (3) on
each. This device allows to center the part and to transmit the rotational movement that is needed
during the machining process.
Fig.3 Device to hold the workpiece in the superfinishing machine.
Mathematical Methods and Optimization Techniques in Engineering
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3 Factors that influence the workpiece
surface finish To obtain the surface finish for the workpiece is
needed to take into consideration the next main factors:
- the pressure of the abrasive stone over the worpiece
surface; - the grain size of the abrasive stone;
- the cutting speed during machining process. During the tests was used the superfinishing machine that is shown in Fig.4. This machine was build to
superfinish all the range of the gears from the gearbox of a truck and is used because we need to
know the optim process parameters for each of the workpiece type.
Fig.4 The superfinishing machine used in machining internal surfaces of the gears.
3.1 The influence of the abrasive stone
pressure on the part surface finish
In Fig.5 is shown the graph of the influence of the abrasive stone on the part surface finish when was
used a cutting speed of 15 m/min and an abrasive stone with grain size of 600. It can be seen that if is
used a pressure of the abrasive stone on the part
surface finish more than 25.10-4 Pa, is possible to obtain the better surface quality of the part in the
same processing time as with smaller presuure.
The dependence between part the surface finish and pressure of the abrasive stone on the part surface is
shown by equation(1): Ra = 0, 00404∗ t2 – 0,036634∗ t + 0,937818
(1)
with an error compared with the tests results of: Er = 1, 24441∗ 10-2 .
Mathematical Methods and Optimization Techniques in Engineering
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Fig.5 The influence of the abrasive stone pressure on the part surface finish.
3.2 The influence of the grain size on the part
surface finish
The dependence between the surface finish and the
abrasive grain size is shown in Fig.6. It was used two type of abrasive stones, with grain size of 600 and
1200, maintaining the cutting speed at a value of 15 m/min and a pressure of stone on the part surface of
25.10-4 Pa. It can be seen that in case of using abrsive
stone with grain size of 1200 the surface finish is with better quality for the same time of processing.
Usually, two types of grain size is often used in the superfinishing machines that can have
simultaneously semifinishing and finishing stations
during machining. Based on mathematical programs was obtained the
value of surface roughness Ra according with the garin size of abrasive stone:
Ra = 7,622178∗ (grain size)-0,558
(2)
with an error compared with the experimental values
of: Er = 7,8632∗ 10-3 .
Fig.6 The dependence between abrasive grain size and part surface finish.
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
10 200 30 40 50 60 70 80 90
gain size =400
v=20m/min
I-p=0,5x10 MPa3
Ra[ m]
t(s)
II-p=1,3x10 MPa3
III-p=2,5x10 MPa3
III
II
I
0 15 30 45 60 75
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
t(s)
RI-gain size 600II-gain size 600-1000
v=25m/min
p=2,5x10 MPa3
a[ m]
B
A
I
II
I: p=15.10-4 Pa
II: p=25.10-4 Pa
III: p=35.10-4
Pa
V=15 m/min
I: grain size 600
II: grain size 1200
V=15 m/min
P=25.10-4Pa
Mathematical Methods and Optimization Techniques in Engineering
ISBN: 978-960-474-339-1 175
3.3 The influence of the cutting sped on the
part surface finish
Cutting speed has an important influence on the part surface finish specially on the machine tools with
two stations of machining: semifinishing and finishing. For the finishing station the cutting speed
is bigger which ensure obtaining a high surface quality of the part after processing. It was used a
pressure of abrasive stone on the part surface during
machining of 25.10-4 Pa. Fig.7 shows this dependence between the cutting speed and part
surface finish.
Fig.7 The dependence between the part surface finish and cutting speed.
The value of surface roughness depending on the cutting speed is giving by the equation:
Ra = 0,00026∗2
sF - 0,006408∗Fs + 0,474286
(3)
with an error compared with test results of Er = 2,5463∗ 10-3 .
4 Conclusions
The superfinishing machine that was presented is
usefully to machining process of all types of gears that are component of a gearbox of a truck.
The device of holding worpieces ensures a good centering and its simple construction gives flexibility
when is changing the oart to be machined. This operation is the last in the operations plan and ensure
a good roughness for internal surfaces of the gears.
By this process is reduced wear, energy consumption and leads to savings in service and maintenance
costs. Using application of artificial neuron network for modeling and analysis, we can predict and obtain the
best process parameters of superfinishing technology for each type of workpiece material and size
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0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
gain size =600
I-F=2daN/cm2
a[ m]
R
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III-F=1daN/cm2s
III
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I
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