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Transcript of web.mit.eduweb.mit.edu/millersa/www/2.70/rev.docx · Web viewFR DP A R R C Functional Requirements...
1. Create a preliminary FRDPARRC table for a pair of pliers you would like to have.
FR DP A R R C
Functional Requirements
Design Parameters Analysis References Risks Countermeasures
Ergonomic Grips part
well High
mechanical advantage
Rubberized handle
Textured jaws
Long handle Spring to
make jaws default open
FBD (see below)
Material selection for grip, linkages, spring, and pin
Machinery’s Handbook
MIT OCW (2.001) FUNdaMENTALS
Tolerances in pin joint
2. Make a sketch of the pliers you would like to have as a stick figure.
3. Sketch each element as a very simple line sketch, and label the forces on each element.
4. For each element, write the basic equations (and the words describing them) that you would use to do the detailed design of each element.
The mechanical advantage of the pliers is a result of the ratio of the length of the jaws and the length of the handles. The force exerted on the part is:
F part=Fhandle∗L
l
Where F_handle is the force exerted by the user on the handles, L is the length of the handles, and l is the length of the jaws.
The shear stress on the pin is
τ= FA
Where F is the force and A is the cross sectional area parallel to the force. So, increasing the area will decrease the stress.
The optimal material can be chosen based on the shear stress predicted by the max force that could conceivably be applied, along with a safety factor.
5. For a sliding interface between two elements (such as the pin join), what are the forces on this interface and how would you design it to last a long time?
The longevity of the pin is going to depend on the shear forces on it as well as its material. The shear forces are determined by the geometry of the pliers. The links in a pin joint are offset from each other and loads are offset by the half-thickness of the links, so there is a moment (Ch 4, FUNdaMENTALs). To eliminate the moment, I would make a clevis joint, so there is no moment and the pin can only fail in shear.
6. What are errors in components that would make the tips not exactly line up? What are the parasitic errors in the system? Load-induced?
The tips could potentially not line up if there is a lot of clearance between the linkages and the pin which holds them together. Some pairs of pliers have a sliding joint between them to accommodate a wider range of part sizes. The tips on these pliers will not line up if the joint is oriented such that the jaws are not parallel when they are clamping the part. If the jaws are not parallel, the part will not be gripped as well, and may slip out. Additionally, under great load, the material will deflect and can cause the tips to not line up. If the joint is a sliding joint, it will have 2 degrees of freedom and thus can have parasitic motions when it is being opened and closed.
7. Update the FRDPARRC table
FR DP A R R C
Functional Requirements
Design Parameters Analysis References Risks Countermeasures
Ergonomic Grips part
well High
mechanical advantage
Rubberized handle
Textured jaws
Long handle Spring to
make jaws default open
FBD Shear
stresses in pin
Linkage deformation
Material selection for grip, linkages, spring, and pin
Machinery’s Handbook
MIT OCW (2.001)
FUNdaMENTALS
Tolerances in pin joint
Shear forces in pin joint
Part slipping
Somehow eliminate pin joint?
Could 2 linkages nest inside each other?