Locking Pin Project, Part II

Abstract

This Master Thesis project is performed at Tetra Pak in Modena, Italy. The overall objective
of the overall Locking Pin project as this thesis project is a part of, is to understand the root
cause of a failing locking pin. The locking pin is located in a filling machine and failures due
to fatigue. The function of the locking pin is to bind up the filling pipe to a pressure flange.
The locking pin failures have caused Tetra Pak a lot of problem for the last ten years. The
pins have failed approximately 100 times over the last ten years. The worst scenario is that
the broken pin goes into a package, while the machine is still running, and the entire
production needs to be inspected.
The goals set out for this project, the Master Thesis project, is to focus on the following to
parts of the overall project:
• Material characterization: Inspect and complete, when necessary, the data on the
material currently utilized in the locking pin production.
• Manufacturing process: Investigate the locking pin production, focusing on
revealing possible failures or imperfections during the locking pin manufacturing
process.
Based on the goals accounted for above, the project was divided into the following five steps:
1. Setting the goals of the thesis project
2. Obtain and analyze information
3. Material characterization
4. Investigate the manufacturing process
5. Formulate the conclusions of the project and, if needed, suggest further studies
The first part of the project was to retrieve mechanical properties of the actual pin material.
An important and common test to decide the fundamental properties of a material is to run a
tensile test. The aim of the tensile test was to establish yield strength, tensile strength,
Young’s Modulus and reduction of area and compare them with a material data sheet from
Tetra Pak. Only small variations were noticed.
As to the validity of the data, the machine failed after only two tests. But the purpose of this
test was just to control and compare the result with an already existing test made by TP, and
critical variations could be excluded.
A fatigue test was also performed to obtain the fatigue properties of the current used locking
pin material. The most commonly used fatigue-test, rotating bending, was performed. After
the test was preformed, new fatigue data was available for comparison to the analysis data of
the actual locking pin stresses. However, this data is insufficient due to the fact that a more
complex load case, mixed bending and torsion, also need to be carried out. Furthermore, the
probability considerations have not been considered – only one test has been carried out on
each stress level.
A chemical composition analysis was performed and the test showed no critical variations
with Tetra Pak’s requirements of the material. Some of the intended tests were canceled
because of information from test performed by our collaboration partners during the project.
The second part of the project was to investigate the manufacturing process. The different
steps in the process were inspected and followed by a detailed review of every step. The most
critical steps that could have impact on the pin failure, Bending, Electropolishing and Shoot
peening, were investigated further by performing different tests.
The electropolishing process was checked so it didn’t have any negative impacts on the pin.
No visual damages were found on either the surfaces or the investigated pins, which means
that the electropolishing process was correctly done. A crack detection test was performed at
new manufactured pins and no discontinuities were found. Possible entrapped cracks can’t
be excluded though and because of this issue, we can’t assure that the pins don’t break
because of the manufacturing process.
The grain size reveals a lot of information regarding the mechanical properties of the material
and according to the grain size measuring, the grain size was acceptable for the current
material. Vickers microhardness test, the last investigation, showed that there is a correlation
between hardness and fatigue limit; the harder the materials surface the higher fatigue-limit.
The manufacturing performance at the supplier seemed to be according to standard. The
random samples that were tested corresponded to the expectations. However, it’s always a
statistical problem with random samples, since just a limited number of pins could be
investigated by us. Our suggestion is to remove the electropolishing from the manufacturing
process, though this step seems to be unnecessary because the particles from the shot peening
penetrate deeper than the electropolishing.
The report ends with conclusions regarding improvements of the fatigue-life and a proposal
for a changed manufacturing process.

• author: Kuralic, Ismar and Miraglia, Susanna
• supervisor: Per-Erik Andersson
• organization: Innovation
• course: MMK820
• year: 2009
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Locking pin, material testing, manufacturing process, fatigue
• language: English
• id: 1510885
• date added to LUP: 2009-11-25 15:40:49
• date last changed: 2010-02-01 14:40:05