EME 150A

Design Project 1 Bryce Yee & Thomas Yang

4/25/13

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Table of Contents

Abstract.................................................................................................................................. pg 3

Introduction........................................................................................................................... pg 3

1) Hole Puncher (TLY)

2) Planetary Gear Set (BNY)

3) Nerf N-strike Maverick Rev-6 (BNY)

4) Vertical Window Blinds Turning Mechanism (TLY)

5) Snowboard Binding Ratchet (TLY)

6) Promax Disc Brake Caliper (BNY)

7) 5 Speed Manual Transmission (BNY)

8) OXO Can Opener (BNY)

9) Stapler (TLY)

10) Trash Claw (TLY)

Detailed Design Descriptions................................................................................................... pg 9

1) Nerf N-strike Maverick Rev-6 (BNY)

2) Snowboard Binding Ratchet (TLY)

3) Promax Disc Brake Caliper (BNY)

4) Trash Claw (TLY)

Conclusion............................................................................................................................. pg 22

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Abstract

The purpose of this project was to explore and analyze the design of 10 devices so as to

better understand how and why things work. With this knowledge, we would be able to come up

with better designs as engineers in the future. From those 10 devices, 4 were chosen for further

detailed analysis. Several elements of each device are catalogued, such as stress analysis,

material properties, functionality, and steps of operation. During the exploration phase, several

devices were available to disassemble to further investigate the contribution of every piece to the

overall mechanical system. Documentation also includes hand sketched illustration(s) with a

significant level of detail. The sketches are used to visually convey device operation, location of

parts, and load distributions alongside device descriptions that may otherwise be difficult to

express solely through writing. In the end, it was concluded that many different factors, not just

function, influenced the decision process involved in making the product a reality.

Introduction

In this design project we explore and analyze the design of 10 everyday devices. The

details obtained entail not only how the device works but an analysis of their application of basic

engineering principles. The first section includes all 10 devices paired with a brief but concise

description of each device and its critical components. The second section includes 4

descriptions of selectively chosen devices from the first 10, each offering a deeper analysis of

their engineered designs including stress analysis, material properties, and expanded operational

descriptions. The 10 objects chosen are as follows:

1) HOLE PUNCHER (TLY)

Figure A

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The hole-puncher is a device that does exactly what it sounds like – it neatly perforates

holes in paper at a standard distance apart. In America, the most common hole-puncher is the 3-

hole-puncher, so as to match with the 3-ringed binders usually sold. In other parts of the world,

two-hole punchers are used. The one sketched above is one-third of a 3-hole-puncher. To operate

the mechanism, the user simply needs to insert the paper under the bottom of the Metal Puncher

and push down on the Lever. The Lever makes contact with and pushes down on the rounded top

of the Metal Puncher, which in turn bores into the paper with its sharpened edges. Finally, the

spring returns the Metal Puncher to its starting position.

2) PLANETARY GEAR (BNY)

Figure B Figure C

Planetary gearing, also called Epicyclical gearing, utilize compound gearing to create 3

separate gear ratios in a thin, compact size. A typical planetary gear set includes an outer ring

gear, 2 or more planet gears attached to a planet gear carrier, and a central sun gear (Figure B).

Each piece is connected to an individual input/output shaft as pictured in Figure C, with the

planet gear carrier shaft rotating around the sun gear input/output shaft. Gear sizes can range

anywhere between micro sizes of a few millimeters in toys to macro sizes of a foot or more for

automatic automobile transmissions. The gears are typically machined from hardened metallic

materials such as steel or titanium but can be made of any rigid material dependent on the

desired operation. Transmission gears in particular are also manufactured with a unique helical

teeth shape to reduce noise and frictional wear. The highest forces to pay attention to include the

torsion of the input/output shafts and the shear force on the teeth around the perimeter of the

gears.

To operate, two of the three shafts must be in motion while the third one remains

stationary. Each of the 3 types of gears (ring, planet, and sun) contain different numbers of teeth.

Gear ratios are determined by the ratio of the number of teeth on the input gear against the

number of teeth on the driven gear. Depending on the two types of gears engaged, different gear

ratio can be established to vary output torque and rotation speed. In many applications, the

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switching between gear ratios is controlled hydraulically by a computer which tracks the

necessary power output desired for optimal power delivery.

3) NER N-STRIKE MAVERICK REV-6 (BNY)

This is described in greater detail in a later section.

4) VERTICAL WINDOW BLINDS TURNING MECHANISM (TLY)

Figure F

This little mechanism is what’s responsible for controlling a set of vertical blinds, which

in turn controls the flow of sunlight into a room. A full set of blinds would consist of a single

Spur Gear Rod, but with several more of the above Spur Gears and 90 Degree Gear Connectors

connected to it in a row. A single window would have as many of these blinds (and just as many

of these mechanisms) as are needed to counteract the transparency of the window, as well as one

mechanism with a cylindrical control rod taking the place of where a vertical window blind

would be. Rotating the control rod would rotate the spur gear attached to it, which then slides the

90 Degree Gear Connector in a certain direction. The sliding of this part then in turn rotates the

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Spur Gear Rod, which then rotates the rest of the mechanisms (and hence, the blinds as well) to

follow the rod’s rotation.

5) SNOWBOARD BINDING RATCHET (TLY)

This is described in greater detail in a later section.

6) PROMAX DISC BRAKE CALIPER (BNY)

This is described in greater detail in a later section.

7) 5 SPEED MANUAL TRANSMISSION (BNY)

Figure E

The 5 speed manual transmission features 5 forward speeds and 1 reverse through 6

individual gear ratios selected by the user though a shift gate. Power is transmitted from the

engine flywheel to the clutch plate, rotating all gears along the lay shaft and all connected gears

free spinning around the main shaft at all times. Gears are engaged through the use of dog

clutches or collars which are splined to the output shaft. The dog clutches are operated from the

gear selector by shift arms which slide the dog clutches for and aft along the main shaft. Tapered

syncronizer gears aid in the engagement of the dog clutches with the rotating gears by matching

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the speeds of the dog cluch gear teeth with those on the side of the rotating gear with friction to

prevent teeth grinding. Once a gear is engaged, the power is transmitted from the layshaft to the

main shaft at a ratio determined by the size and number of teeth on the gears engaged. Lower

gears consist of ratios greater than 1, and higher more economical ratios are less than 1. Reverse

is achieved with an additional idle gear between the layshaft and main shaft gear. The gear

selector uses an H pattern gate and a tilting shift stick to knock specific shift arms back and forth

between gears dependent on the angle of tilt. All components discussed above are displayed and

labled in Figure E.

Shafts, gears, and splines are maufactured from steel alloys with high resistivity to heat

and large stresses. The rods, gear housing, and shift stick meanwhile can be maunfactured from

lower strength materials such as aluminum or compsites to save weight and machining costs.

8) OXO CAN OPENER (BNY)

Figure G Figure H

The OXO can opener design is modeled after the rotating cutting wheel design. The two

pliers halves are made of stainless steel for minimal deflection during operation and easy

cleaning. The handles are rubber coated for easy gripping and the key handle has large flat sides

for easy grip and maximum application of torque to the cutting blade assembly. To operate, the

cutting wheel must first be mounted on the inside of the can lip with the handles open as picture

in Figure G. Applying a significant amount of pressure on the handles brings the cutting blade

and serrated feed wheel ("lip grip gear") together. This also allows the cutting blade to pierce the

top of the can and engage the gears between them as picture in Figure H. Turning the key handle

clockwise rotates both the feed wheel and the cutting blade around the top edge of the can until

the top pops off. To prevent any deflection of the cutting blade arm, there is a small hook bent

out of the feed wheel arm to secure and stabilize the cutting blade arm inside while cutting. As

an added feature, the front of the can open is shaped as a bottle cap opener.

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9) STAPLER (TLY)

Figure D

An essential gadget for every office, the stapler is used to fasten sheets of paper together

to aid in organization. To work a stapler, the user simply needs to push down on the top handle.

Under the handle is a piece of metal named the hammer, just thick enough so as to push down on

a single staple and send it penetrating through the paper to the other side, where it is bent into a

ribbon. What the user doesn’t see in this process, however, is that also on the underside of the

top handle, is a compression spring attached at the tip. On the other end of the spring, it’s

connected to a carriage in the magazine cartridge. When the stapler is closed, the spring’s

tendency to compress pulls the carriage, which in turn pulls the load of staples to the front of the

cartridge, aligning a staple for the next time the hammer comes down.

10) TRASH CLAW (TLY)

This is described in greater detail in a later section.

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Detailed Design Descriptions

1) NERF N-STRIKE MAVERICK REV-6 (BNY)

Nerf Blasters are popular children's toys that use motorized and or mechanical air pumps

to fire foam dart ammunition. The blasters are primarily made of plastic with the exception of

internal components such as shafts, springs, screws, and sealants. The Maverick Rev-6 features a

6 dart barrel which resembles that of a revolver and is operated with one or two hands.

The dimensions of the blaster are 11.75" x 6.75" x 2.5" (length, height, width). Max

firing range is 30 feet with good accuracy dependent on the style of dart used.

The operations for firing the Nerf N-Strike Maverick Rev-6 are as follows:

1. Push in the barrel release button located on the left middle side of the gun and swing the dart

barrel out to its maximum of 30 degrees.

2. Load any Nerf branded dart into the front of the barrel with firm pressure one by one, rotating

the barrel as you load.

3. Reposition the barrel back to its locked position with force or by holding the barrel release

button until the barrel is positioned correctly. Rotate the barrel until at least one audible click is

heard.

4. Pull back the slide until an audible click is heard then release the slide to return it back to its

original position.

5. Aim the blaster at target with the front sight or any other sight attachment on the tactical rail.

Squeeze the trigger to fire.

Figure 1 Figure 2

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6. Reload the same as step 4. When all darts have been fired, return to step 1.

The Maverick Rev-6 utilizes a mold injected, hardened, and glossy coated type plastic.

This durable colored plastic is the material of choice for all Nerf Blasters. Few other materials

offer the affordability, rigidity, and safety of plastics especially in the toy industry. The low price

of plastics was a definite contributor to the $9.99 MSRP of the Maverick Rev-6 but at the same

time offered flexibility for design and light weight operation. Many of the Maverick Rev-6's

parts including internal gears, air chamber, and external casing are all fabricated from plastic.

The loads bared by these parts are modest but acceptable for plastic material. Heavy abuse by the

user or improper use may exceed the stresses for which the parts are designed for and reduce the

reliability of the blaster but are necessary sacrifices. The Maverick Rev-6 is after all a children's

toy made for pure entertainment purposes. The child demographic also has on average a short

attention span, allowing for the design of a very finite product life of a few years or less.

The external design of the Maverick Rev-6 is similar to the rest of the line of N-strike

Nerf blasters sold at the time. Most, if not all, imprinted lines into the outside casing are made to

convince the user the blaster is made from many metal components held together by large flat

head screws. The four colors used are golden yellow, black, metallic grey, and safety orange.

The outer casing consists of two main halves molded of hardened and glossy plastic, thick

enough to contain any damage due to rupture of internal components from the user.

Figure 3

A detailed description of the mechanical operation and its parts follow.

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The air pump mechanism is variable-volume air chamber (labeled "air chamber" in

Figure 3) sealed with a rubber o-ring between the two halves. The slide of the blaster expands the

air chamber and locks the firing spring once the slide is pulled all the way back. When the trigger

is pulled, the spring lock is released allowing the spring to uncompress and shrink the air

chamber volume forcing air out into the barrel. The air is directed into an air restrictor and

continues to the top dart in the barrel. The air chamber can be identified as the most stressed

element of the blasters mechanisms. Several components interact with it including the spring

lock, external slide, and the firing spring. The violent movement from the uncompressing firing

spring and the oscillating pressures during rapid firing require of the air chamber to be over

engineered or risk the blaster becoming dysfunctional. Large ribs of plastic are used throughout

the rear for the slide hook, firing spring mount, and along the sides of the air chamber for added

stability. The o-ring is also thick in diameter and greased to ensure an air tight seal between the

two halves of the air chamber.

The trigger and barrel rotating system are connected together to perform in sync and

minimize the reload time of the Maverick Rev-6 (see Figure 3). The trigger is made of ribbed

plastic and slides about 3/4 inches in a horizontal motion towards the rear of the blaster. The

return spring for the trigger (not pictured) is mounted on the end of the connecting arm. The

connecting arm is attached to the top of the trigger with a small screw and slides past two plastic

gears locked to the barrel shaft. The connecting arm need only be of sufficient size to perform

the operation and bares not credible forces. The gears are used to rotate the barrel at different

stages of firing as the trigger moves back and forth. The gear located towards the trigger rotates

the barrel into firing position once the trigger is half way depressed. The other gear located

towards the barrel rotates the barrel to the next loaded dart as the trigger returns to its initial

position after a half way depression or full depression. The gears also need not be of incredible

size and strength but sufficient enough to rotate the barrel.

The trigger also influences the release of the spring lock or "hammer" as labeled in Figure

3 of the blaster by bumping a triangular lever arm upwards at the triggers most reward position.

The lever arm causes the spring lock to move downwards, releasing its latch on the rear of the air

chamber housing, and allowing the spring to uncompress.

The slide of blaster is located at the top rear of the gun (See Figure 2) and is used to ready

the blaster for firing. Its operations include compressing the firing spring and engaging the

hammer. The slide moves about 2 inches rearward, expanding the air chamber via an internal

hook, until an audible click can be heard. This secures the rear of the air chamber housing in full

extended position as the spring lock moves up into position. Once the blaster has been loaded,

the slide can be released and will return to its initial position via an internal return spring along

the slide's guide rails. The slide also contains two notable external features, the tactical rail and

the loop hole. The tactical rail is used to mount accessories from the Nerf N-Strike series such as

a red dot sight or flash light. The accessories are slid on from the front and locked into position

by a small orange spring-loaded tab (can be seen in Figure 2) once positioned far enough on the

rail. The loop hole is an advanced feature and can be utilized in the case the user wants operate

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and reload the Maverick Rev-6 with one hand, as is the case when dual wielding the blaster with

another. To do so, the user must supply string or rope which has enough length to wrap around

the users shoulder and through the blasters loop hole, but short enough to pull the slide all the

way back when the users arm is extended.

2) SNOWBOARD BINDING RATCHET MECHANISM (TLY)

Figure 1: Traditional Snowboard Bindings Figure 2: Scale of Snowboard Bindings

Snowboarding is an alternate means to the thrill and fun of skiing down a mountain, and

has exponentially grown in popularity over the last few decades. Riders go down the mountain

on special minimal-friction boards, twisting and turning their bodies so as to use the board’s

sharp edges in controlling their movement. All the forces caused by the twisting and turning are

transmitted to the board through their feet, and foot bindings are what ensure that this

transmission is completed.

The cheapest and most reliable designs of foot bindings are actually still the traditional

snowboard bindings, as can be seen in Figure 1 above. The straps need to be long enough to

accommodate the extreme range of boot sizes, but the Ratchet Mechanism itself isn’t very big

(Figure 2). The working principle behind these bindings is that of a ratchet, allowing continuous

linear motion in one direction and inhibiting motion in the other.

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The Ratchet Mechanism is connected to a strap, which is attached to the snowboard on

one side of the rider’s boot. On the other side, the Ladder Strap is attached. The Ladder Strap

meets the Ratchet Mechanism over the rider’s boot in the middle, slotting through the Ratchet

Mechanism until the Pawl clicks and hooks on, as illustrated in Figure 3 on the next page. This

securely binds the rider’s boot to the snowboard, allowing the rider to maneuver the snowboard.

Figure 3: Ratchet Mechanism with Ladder Strap. Figure 4: Ratchet Mechanism.

In order to tighten and more securely fasten the boot to the snowboard, the user would

simply lift the Ratchet Tightening Lever. This causes the Tightening Hooks at the end of the

Lever to grab onto the Ladder strap, pushing the strap up through the Ratchet Mechanism. At the

same time, the Pawl slides up the inclines of the ladder strap, locking with a click sound every

few millimeters. The Pawl is forced down securely with the Pawl Spring that runs under the

Pawl’s center.

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To loosen, the user would pull up on the Ratchet Loosening Lever, making contact with

the ridge on the Pawl behind it. The Pawl is subsequently lifted high enough to disengage the

ladder strap’s teeth, which allows the strap to be loosened or come off the Ratchet Mechanism

entirely. Figure 5 below demonstrates what was just described.

Figure 5: Disengagement of Pawl Figure 6: Moments on a Ratchet Mechanism

Except for the two springs and the Ratchet Tightening Lever, every other major part

mentioned in the paragraph above is made of plastic. Plastic is cheap and durable enough, and

really could have been used for the Ratchet Tightening Lever as well. In this specific ratchet

mechanism, the manufacturers used cast aluminum primarily for aesthetic reasons. The Pawl and

Tightening Lever springs, however, are made of steel because of steel’s high yield strength and

ability to return to its original form.

As long as the user takes decent care of the part, this type of snowboard bindings should

last a long time. With the help of Figure 6 above , one can see that all the force applied by a

reasonable person (denoted by F1) is transferred through the rivet (“P”), into the Tightening

Hooks, and then instantly into moving the strap. There is a reaction force F2 as the Tightening

Hook makes contact with the strap that is also transferred through the rivet, making this the point

of maximum stress. So, if the part were to fail, it would fail here, at the rivet labeled “P”.

Nevertheless, only after lots of use will the rivet start to wear and come loose, as most of the

time the force will transmit through the pivot seamlessly.

When the Ratchet Mechanism fails, it’s usually because the user had dropped his

bindings on the gravel road one time too many when taking the snowboard out of his car, causing

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one of the many pins, nuts, and bolts holding the mechanism together to come loose. It is very

rare that a specific part of the mechanism would fracture; partial disintegration is the main worry.

If the user doesn’t realize that the pin attaching the pawl to the housing is loose when he

straps himself in and continues to go down the slope, the consequences could be disastrous.

Without his feet secured, a snowboarder would have a difficult time controlling the snowboard,

meaning that he could crash into other people, trees, rocks, or go flying off a cliff. However,

there are two ratchet mechanisms per boot, one that straps the rider’s ankles and one that straps

the rider’s toes, bringing the grand total to four ratchet mechanisms per pair of feet. So, if only

one ratchet mechanism breaks, the rider still has a reasonable amount of control over his board,

what with one foot still completely strapped in. If all four of them break, the rider would just

tumble off his snowboard, possibly crashing into the various obstacles on the slopes, but

probably won’t go flying off a cliff. The worst scenario would be if three of the mechanisms

break, as the rider would then have very little control over the snowboard but still be bound to

the snowboard and its will.

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3) PROMAX DISC BRAKE CALIPER (BNY)

Disc brakes are a popular alternative method of stopping a bicycle using friction to the

more common v-brake or u-brake methods. Disk brakes are more commonly equipped on

automobiles because they offer the most powerful and efficient stopping power for their size and

are easy to maintain. The disk brake system requires a disc brake equipped wheel hub and

properly mounted tabs near the wheel hub(s) of the bicycle, a setup very different from the edge

mounted v-brake or u-brake. The caliper is made of steel and plastic but other metal alloys can

be substituted for steel if a lighter weight is desired.

The dimensions of the caliper are 3.375" x 2.5" x 2.25"(length, height, width). The brake

pad size are 1" circular disks which slide against a 160 mm rotor. The caliper is painted black

with the exception of the steel bolts, steel washers, and aluminum piston access plate which

remain their natural material color.

Figure 1

The Promax mechanical disc brake is entry level disc brake intended to provide the

advantage of increased stopping power while keeping the cost to a minimum. The mechanical

disc brake utilizes the same steel brake lines used with v and u-brakes that are featured on the

majority of bicycles sold in the past half century. This means they are compatible with the

majority of hand brake levers and easy to install with few changes made to the bicycle frame and

wheels. Similarly, the only required maintenance are routine checks of the condition of the brake

pads to ensure sufficient material is present and checks to prevent other mechanical rubbing that

may lead to brake noise or even failure. The brake disc rotors are manufactured in many sizes

varying between 140 mm - 225 mm in 20 mm increments. Stopping power increases with rotor

size as there is more leverage the further away the frictional force from the caliper is applied

from the wheel axis of rotation.

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The operation of the Promax disc brake caliper is as follows:

1. The initial position of the brake lever arm is the furthest from the brake line mounting

arm. This widens the slot between the brake pads in the disc slot (Figure 10) so the rotor

can rotate without any contact with the caliper brake pads.

2. The brake line is pulled into tension with a brake lever towards the left as pictured in

Figure 1. This causes the lever arm to rotate the internal piston housed within the caliper

in a counterclockwise motion.

3. The underside of the piston head rolls three steel ball bearings along inclined slopes

around the shaft of the piston (not pictured) causing the piston to move to the right as

pictured in Figure 2.

4. While the piston moves axially right, a spring is compressed and twisted which is located

just below the lever arm and just above the internal ball bearings as pictured in Figure 3.

5. The rightward movement of the piston pushes the piston-side brake pad towards the

caliper mounted brake pad, sandwiching the rotor in between, and creating a friction

force against the rotation of the rotor.

6. As the brake line is relieved of tension, the spring "springs back" to its original shape and

reverses the movement of all the components to return them to their initial positions.

Figure 2

Figure 3

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The design of the Promax brake caliper is formed around only the necessary components

need for operation. The central barrel is used for housing all the mechanisms that move the

caliper piston back and forth. The caliper disk slot is very rigid to ensure minimal deflection of

the caliper under extreme brake forces. The lever arm and brake line mounting arm are shaped

for both form and function with tapered designs.

The tapered design of the lever arm from its base at the piston shaft cover end to the

brake line clamping bolt at the top is representative of the bending forces experienced by the

lever arm. The moment induced by the tension of the brake line is greatest near its mate with the

piston shaft near the bottom meaning the base must be made thicker than the top. This will

ensure the lever arm is stable for the lifetime of the caliper. The brake line mounting arm on the

other hand does not have to endure such forces but still made very stable with a similar tapered

shape to ensure the brake line stays in one spot. The stable positioning simplifies the direction of

the brake line force when under tension.

The caliper mounting bolts are 5 mm hardened steel hex bolts that provide the connection

of the caliper with the mounting bracket and resist a strong shear force induced when braking.

The force origin starts with the frictional force from the brake pads. The brake pads are secured

to the caliper causing the caliper to move to the left (referencing Figure 1) creating a shearing

force on the caliper mounting bolts. The bolts are both large in diameter and lengthy enough to

add spacers in case the caliper needs to be position further from the wheel hub. They were

chosen from a range of metric standard bike hex bolts to withstand the shear force with ease. The

fork mounting bolts that attach the fork mounting bracket to the bicycle front fork are the same 5

mm hex bolts that must withstand the same shear force as the caliper mounting bolts.

Aside from the lever arm and the fork mounting bracket, the entire caliper is foraged

from a single piece of steel, bored out to house the piston and cut to slot onto a rotor. The ball

bearings (not shown), piston, lever arm, piston spring, and hex bolts are also made of steel. The

only components made of different materials are the aluminum piston access plate (Figure 3) and

the rubber spring-piston housing cover (Figure 3). The brake pads are 1" round steel discs with a

small handle for easy placement and slotting to keep the pads from rotating. The pads are also

held in place via magnets on the piston surface and the inside of the piston access plate. The use

of magnets eliminates the use of any glue, screws, or thin spring elements that may have to be

replaced. The pads use an organic or ceramic compound which creates the friction necessary for

stopping when forced against the rotor.

The most stressed element of the caliper is the piston spring as experienced firsthand. The

spring is about 1/8" in diameter and must endure both axial compression and torsion along with

the added stresses from an already bent neutral shape. The combination of these stresses in

constant oscillation from thousands of braking demands slowly alters the effectiveness of the

spring. At some point the shear force at a 45 degree angle is enough break the spring in half.

Without the spring, the caliper becomes difficult to use since the lever arm will not return back to

its original position once the brake lever is released. Instead, the user must manually retract the

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brake to prevent rubbing of the brake pads on the rotor. The spring itself is not sold individually

which means the entire caliper must be replaced in the case of a spring failure.

Another element that could fail near the same time as the spring is the brake line itself.

The brake line clamp at the top of the lever arm applies constant pressure to secure the brake line

through static friction while also bending the brake line at the edge of the clamp. Through the

same oscillating movement from thousands of braking demands, the brake line will have been

bent back and forth enough to cause necking of the steel threads. Weather erosion or a stronger

than usual force could eventually lead to failure.

4) TRASH CLAW (TLY)

Figure 7: The Picker

The claw is one of the human race’s many inventions in our quest to mitigate the natural

shortcomings of being a homosapien. With this instrument, we’re able to either grab things

normally beyond our reach or that we wouldn’t want to touch with our bare hands. As such,

claws can be used for many purposes such as groundskeeping, physical aid, or poultry processing

besides purely handling trash, which is what the claw that I’m about to describe is supposed to

do. Deemed “The Picker” by its manufacturers, this specific claw is about 30 inches in total

length, as can be seen in Figure 8 on the bottom left.

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Figure 8: Scale of Claw Figure 9: Close-up of Pivot, Rod Housing, and Metal Rod

The way that this claw operates is simply by transmitting a force through a number of

connections until it is applied where it needs to be, as can be traced in Figure 10 below. A

distributed load caused by gripping the handles of the claw (labeled as F1) pulls back on the

Metal Rod that stretches the length of the claw. When the Metal Rod is pulled back, the Rod

Housing is also dragged back along with it, which in turn rotates the Upper Jaw around the Pivot.

This rotation clamps the Upper Jaw down against the fixed Lower Jaw with a force labeled as F2

in Figure 10, allowing the user to grab onto an object. To let go of the object, the user would

merely release the pressure applied at the handle. The Tension Spring then does its job by

returning the handle to its original position, re-opening the jaws in preparation to grab something

else.

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Figure 10: Trash Claw Free-body Diagram and Magnifications on the side

The Handle, Jaws, Rod Housing, and the rest of the claw’s body is made of hardened

plastic. Under intended use, the parts mentioned above wouldn’t be subject to any major forces

that plastic couldn’t withstand, and since plastic is cheap, the manufacturers naturally went with

a material that would allow them to set a competitive selling price. For the Tension Spring,

Pivot, and Metal Rod, however, steel is used.

The primary reason the manufacturers used a steel Pivot is probably just because that’s

what was most easily obtainable. Steel is what these Allen-wrench screws are standardly made

of, since it has a lower friction coefficient than that of plastic. Steel is again preferred for the

Metal Rod and Tension Spring, but this time due to its elasticity and higher yield strength.

As the Metal Rod is drawn back, it undergoes a deformation as the distance between the

Rod Housing and the Pivot changes. The Rod Housing and Pivot are at its greatest separation

when the Metal Rod is either at its maximum withdrawal or extension, and is at its smallest

separation when the Rod Housing is directly under the Pivot (see Figure 9 or 10 to aid in

visualization). Since one of the pieces in that section of the claw must be made of a material able

to contract and extend appropriately to accommodate this, the manufacturers chose steel because

of its greater Young’s Modulus. Similarly, a Tension Spring needs to be made of a material that

won’t plastically deform or break under a decently strong grip of the handles, but needs to be

able to spring back to its former shape without fracturing.

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The point where a fracture is most likely to take place, however, would be in the joint

between the Metal Rod and the handle. If the distributed load was to be concentrated into a

single load, the distance between its point of application and the joint is greater than the distance

between the force exerted in the Rod Housing and the Pivot, meaning that a greater moment

would occur in the first joint. Still, especially with a sturdy metal screw that secures that joint,

the chances of failure are not very high. The Picker should last a lifetime when being used

according to its function.

Even if the claw were to fail while in action, most of the time drastic consequences

wouldn’t result because of the nature of the job the claw is usually employed for. Hazards caused

by the device’s lack of performance is solely dependant on how the object being unintentionally

dropped fares when making contact with whatever is beneath it. If if a butcher working above

other people were to drop a piece of raw meat on a person below, that person should probably

rinse the bacteria off of himself. Or, if the claw was being used to maneuver a sensitive bomb,

the life of at least the operator hinges on the mechanism’s ability to work. On the other hand, if

an empty plastic bottle was dropped on the ground, the operator should still live.

Conclusion

We discovered that when designing a product, many different factors have to be taken

into consideration when making the product a reality. Function, material choice, economic

feasibility, aesthetics, failure point identification, and a host of other things influence design.

Sometimes, however, others in order to optimize the final product overall, development of

certain factors have to be sacrificed in exchange for the improvement of others. As engineers, it

is our job to determine the best path in fulfilling the company's goals for the product, and this

project helped us understand some of the things to think about when making such decisions.