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?