UWE Bristol Engineering showcase 2015

Samuel Wort
BEng (Hons) Mechanical Engineering
Project Supervisor
Dr Ben Drew
Vibration Fixture Design for ‘Bookend’ Style Fixtures
Vibration Fixture
The purpose of a vibration fixture is to act to as an interface between a shaker
and a sample being tested. An ideal vibration fixture would have no resonant
frequencies, within the frequency spectrum of the test being conducted.
However this rarely achievable with large fixtures or tests carried to 2000Hz.
Existing Fixture
Using modal analysis, FEA and theoretical calculations the existing bookend
fixture was calculated as having a fundamental resonant frequency between
121Hz and 127Hz.
New Design of Fixture
The data obtained from analysing the existing fixture was used as a
benchmark to design a new fixture. The proposed final design would be of a
cast and machined magnesium construction and have an estimated overall
mass of 271kg. Which would allow test levels up to 19g n, to be carried out
with a sample mass of 50kg, using PARC’s 964 shaker system.
By carrying out FEA and theoretical calculations, it is envisaged that this
fixture design would have a resonant frequency in the 450Hz to 500Hz region
and a resonance in the 400Hz to 450Hz region with a 50kg sample attached.
Analysis Original Fixture Proposed Design
Modal 127.03Hz -
Calculated 123.47Hz 492.12Hz
FEA 121.77Hz 475.85Hz
Project summary
Based on current user experience, Product
Assessment and Reliability Centre’s large bookend
fixture has a fundamental (first) resonant frequency
of 100 – 150Hz, even as low as 30Hz under certain
conditions. Using theoretical calculations, simulation
and analysis of the existing fixture. A new fixture has
been designed which increases the fundamental
frequency above 450Hz.
Project Objectives
As no analysis of the existing fixture had been made
one of the first aims was to understand the dynamic
behaviour of the existing fixture. This was carried out
using calculations, model analysis and FEA. The FEA
carried out has allowed a judgment to be made as to
the viability of using FEA in fixture design.
The 2 nd aim was to re-design the existing fixture
which is referred to as bookend fixture or L-type
fixture, without affecting the overall size of the face
plate (0.815m x 0.77m). The outcome of this aim is a
concept design that could be put into manufacture
when the need arises, that will have a significantly
higher fundamental resonant frequency. The ideal
scenario would be to have a fundamental frequency
in excess of 500Hz.
Project Conclusion
It was determined that the existing fixture has a
resonance below 150Hz with FEA and theoretical
calculations reinforcing this. By designing a fixture
which is made from cast magnesium it is anticipated
that a bookend fixture of this size and style would
have resonance frequency in 450Hz to 500Hz range.
Yet the ideal approach to vibration fixture design is to
design an unique fixture for every sample tested. As
this can take into account sample mass, test levels
and frequency range. However this approach is often
cost prohibitive.