Transcript Final Report Presentation

Final Report
Finite Element Analysis
of a Toggle Mechanism:
Sensitivity to Link Sizes and Compliance Material
By: Joseph Hughes
MANE 6980
Toggle Mechanism Finite Element Analysis
was Divided into Three Evaluations
 Analysis of a baseline mechanism was establish to
provide a basis for the evaluation of any changes
 Sensitivity to Linkage Length
 Use varying length of linkages to model potential
machining tolerances and their effect on the stresses
within the linkage
 Sensitivity to Compliant Material
 Vary the stiffness of the compliant material element
within the linkage to determine their impact on the
stresses within the linkage
The Toggle Mechanism was Analyzed in
ABAQUS
 The toggle linkage was modeled in ABAQUS with two
links, a slide, and a stop.
 To simulate the motion of a toggle mechanism through
the toggle point the center pin was displaced downward.
Mesh Concentrated in Areas of
Suspected
 Mesh was concentrated in areas of suspected high stress
 Linear brick elements with incompatible modes utilized
for low computational cost and due to use of bubble
shape functions, have similar accuracy of higher order
elements
Force Within Over-Toggle Mechanism Is Sensitive
To Link Length
 The pins of the linkage were not explicitly modeled, but using
the “hinge connector” built into the ABAQUS program one was
able to still evaluate the forces in the pins
 A space was initially set between the slide and stop to allow
some travel of the mechanism prior to contact
0.012” Initial Gap
Displacement of the Center Pin Causes
Movement of Linkage
 The motion of the linkage is shown in the figure below
 Stress in the links within the linkage are reported along
with the force within the center pin
Results of Evaluation of the Baseline
Mechanism
 Do to use of connector, unrealistic stress concentration
around pin holes, but provides realistic pin forces to
evaluate stress in pin and links
 Force plots from the center pin were evaluated in vertical
and horizontal directions (see figure below for location of
forces)
Vertical
Horizontal
Forces within the Pin Illustrate Theory of
Toggle Mechanism
 As the pin is displaced the vertical force increases due to
slide contacting the stop, but decreases and goes to 0 lbf
as the links align due to infinite mechanical advantage
afforded by linkage
 The horizontal force in the pin increases to a maximum at
the toggle point of the mechanism
Vertical Force
Horizontal Force
Baseline Analysis Tested for Verification
 A mesh density study was performed
 Roughly tripling the number of elements resulted in
approximately 1.5% decrease in the horizontal force in the
pin and max stress 0.3% increase
 An analysis was performed with quadratic reduced
integration elements
 Horizontal pin force was reduced by 1.8%, but max stress
was increase by 3.8%
 Decreasing the time step by a factor of two had no effect
on the force within the pin and a decrease of 0.05% in
max stress
Small Increases in Link Length Creates Large
Variations in Pin Force
 Table below provides results of evaluation
 Increase of only 0.002” to the links increases force by
78%.
Large Decreases in Stiffness of Compliant
Material have Little Effect on Pin Force
 Table below provides results of evaluation
 Cutting the Elastic Modulus from the baseline by 4
resulted in a 10% decrease in pin force
Conclusion
 The pins is linkages like this are very highly loaded
 The increase in length of the links a small significantly
increases the forces within the mechanism
 This is likely why most toggle mechanism (such as a pair of visegrip pliers) come with an adjustment feature
 A significant reduction in stiffness (50-75%) of the compliant
material reduced the forces within the links, but only by a
small percentage
 The forces and stresses within the linkage are driven by
compression and while the reduction in stiffness of the
material reduces the amount that the links need to compress it
is not as significant as the increase from the linkage length.
References Were Used Throughout The Analysis

[1] Tso, Pei-Lum. “The Kinematic Synthesis of Toggle Clamps.” Journal of Manufacturing
Science and Engineering, Vol. 120 August 1998, pages 648-655

[2] Zhang, Yi. “Introduction to Mechanisms”, Chapter 4.0. Carnegie Mellon University
accessed 7/26/12, http://www.cs.cmu.edu/~rapidproto/mechanisms/chpt4.html

[3] Matweb.com AISI Type 304 Stainless Steel, Accessed on 7/26/12,
http://matweb.com/search/DataSheet.aspx?MatGUID=e2147b8f727343b0b0d51efe02a61
27e&ckck=1

[4] Standard Specification for Stainless Steel Bars and Shapes, dated 1 May 2010,
Specification Number ASTM-A276

[5] SAE J467b, October 68. Society of Automotive Engineers, Inc

[6] Standard Specification for Precipitation-Hardening Stainless and Heat-Resisting Steel
Plate, Sheet, and Strip, dated 1 March 2006, Specification Number ASTM-A693

[7] ABAQUS/CAE 6.11. “Abaqus User Manual.” Dassault Systemes, Providence, RI, 2011