Ratcheting screwdriver

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Revision as of 09:56, 24 September 2007

Executive Summary

Two rather simple improvements on the ordinary screwdriver include the ratcheting screwdriver and the electric screwdriver. The advantage of a ratcheting screwdriver is that the user does not have to constantly change their hand position. The portable electric screwdriver is useful because it requires no rotational user input, the user only needs to keep pressure applied to the screw-head, by pushing on the handle

The primary goals of our product analysis are to break the product down and find any possible areas for improvement. In this analysis we have decided to include Design for Manufacture and Assembly (DFMA), Failure Mode Effects and Analysis (FMEA), and Design for Environment (DFE). Also, each product was dissected piece by piece and a mechanical analysis was done on the important mechanisms.

The DFMA showed that in the ratcheting screwdriver there were several areas where parts could be put in incorrectly. Consideration should be put into making these parts asymmetrical. Also in both tools, several pieces were held together with screws. In both cases, these can be replaced with snap fasteners to cut down the number of components.

The FMEA for the ractheting screwdriver mainly consists of any part of the mechanism shearing. Causing it to rotate freely. However the Mechanical Analysis shows that all parts have been designed to high standards, and the likelihood is small that this will occur. The largest possible problem in the electric screwdriver is that the motor could get burned out. We recommend that a warning light be installed for when this starts to happen.

The DFE showed that consideration should be taken into trying to use more environmentally efficient power sources. Most of the power used to supply the manufacturing of both products come from coal and natural gas power plants.

Customer Needs

Easy to Use
-ratcheting/electric
-portable
-interchangeable bits

Practicality
-screwing/unscrewing
-affordable
-comfortable
-durable

System Functionality

Ratcheting Screwdriver
On the interior of the head piece of the screwdriver, there is an internal ring gear that connects with 2 pawls on the base piece of the screwdriver. The pawls are located at the front of the base piece, each consisting of a flat side and a curved side. When both are engaged, the ratcheting motion is locked, and the device acts as a regular screwdriver. A switch depresses either of the 2 pawls, pushing one into a recessed area and leaving the other engaged. With one pawl depressed, the gear is free to ratchet in one direction. A switch on the handle of the screwdriver allows the user to easily switch from depressing either of the pawls, or leave both engaged

Portable Electric Screwdriver/Drill
Power for the Portable Electric Screwdriver/Drill starts with a simple rechargable battery pack. This leads directly to the switch box and trigger mechanism assembly. When the trigger is depressed, current is allowed to flow through the wire, turning the motor. When the trigger is depressed the other way, current is allowed to flow in the other direction. This in turn allows the motor to move in reverse. Attached to the other end of the motor is a planetary gear set. Gearing down allows a small motor to run at high rpm and achieve high output torque. To the end of gear set the drill shaft is attached.

Also attached to the battery is a chip and a series of lights that identify how much power is left in the battery. A female adapter is also attached to allow the power cord to recharge the battery.

Customer Uses

A ratcheting screwdriver is used for loosening and tightening screws. The advantage of a ratcheting screwdriver over a regular screwdriver is rather than having to change your hand position after every half turn or so, the user just keeps their hand in the same position on the handle and rotates the handle clockwise and then counter-clockwise, or vice versa, repeatedly until the screw is tightened or loosened to the user's desire. The ratcheting screwdriver can also be used as a regular screwdriver if desired.

A portable electric screwdriver is used for the same purposes as a regular screwdriver or a ratcheting screwdriver. The portable electric screwdriver is useful because it requires no rotational user input, the user only needs to keep pressure applied to the screw-head, by pushing on the handle, just like a regular or ratcheting screwdriver.

Design For Manufacture and Assembly (DFMA)

• Minimize component count

Problem: The springs paired with each pawl add to the part count and can be difficult to handle.
Solution: Manufacture pawls with springs already attached.
Feasibility: Manufacturing such a part would be too difficult and expensive to make it worthwhile.

Problem: Each pawl used is also accompanied by a spring, and thus increases the part count by 2.
Solution: Eliminate one pawl and thusly one spring.
Feasibility: Redesigning the pawl to allow it to lock in both directions should be possible and effective in reducing the parts.

• Minimize use of separate fasteners

Problem: Screw used to fasten the top of the screwdriver to the body.
Solution: Replace screw with a snap-on design.
Feasibility: Snap-on parts are commonly used to replace screws in manufacturing parts.

• Use a base component for locating other components

Base of the screwdriver is the main body of the part and can be used as a centerpiece in assembly.

• Avoid base repositioning during assembly

With the base in place, little or no translation or rotation is required to insert the pawls and assemble the rest of the pieces.

• Make the assembly sequence efficient

With relatively few parts and an effective base that does not require repositioning, little time and movement are wasted in assembly.

• Avoid features that complicate retrieval

• Design components for handling and mating

The springs and pawls fit easily into their recessed areas. The handle is press fit onto the shaft of the screwdriver.

• Design components for symmetry (end to end)

The shaft, handle, screw, ball bearing, button, gears and springs are either symmetrical from end to end or clear in the direction in which they should be inserted. That leaves only the pawls as parts that are not symmetrical and can possibly be assembled incorrectly.

• Design components for symmetry about axis of insertion

The same symmetry problems are present here as in the end to end symmetry (above). All parts except for the pawls are either symmetrical or clear in the intended direction. The pawls could be assembled backwards and would result in malfunction of the screwdriver.

• Design nonsymmetric components to be clearly asymmetric

Problem: All parts that are nonsymmetric are easily distinguished with the exception of the button spring and the pawl springs.
Solution: Either clearly different colored/length springs to allow for clear distinction or interchangeable springs.
Feasibility: If the circumstances are such that there is no way to make the springs interchangeable, changing them to allow for clear visual distinction should not be a problem.

• Design components for straight-line assembly

Once again, the low number of parts should allow for an efficient assembly in an effective order.

• Design in location and alignment features

Problem: The pawls fit into the recessed areas in a number of ways, but will only work properly if assembled correctly.
Solution: The pawls are asymmetrical end to end or on each side, so an informed worker assembling the screwdriver should not have a problem.
Feasibility: While the pawls fit in multiple ways, they are not symmetrical in a confusing manner. There should be no problems with assembly.

• Maximize component accessibility

There are no constricted spaces that require component assembly. The recessed areas for the springs and pawls are in open space and are easily visible.

Design For Environment (DFE)

Ratcheting Screwdriver:

The power generation and supply for building 1 million dollars worth of ratcheting screwdrivers is 197 millions tons of CO2 and 2.4 million tons of CFCs, which are both considered greenhouse gases (resource 1). The power used to create the ratcheting screwdriver is mainly provided by coal power plants (1.8 terajoules (TJs)) and natural gas power plants (.420 TJ) (resource 2). The use of nuclear power plants would drastically decrease the greenhouse gases produced from power production. Nuclear power plants produce little to no greenhouse gases. Although, the drawback of nuclear power plants is that they produce nuclear waste that needs to be buried. However, nuclear waste can be reused for energy production because the uranium that initially goes into energy production gets converted into plutonium, which can then be recycled and used as an energy source. The only problem with this solution there are not many nuclear power plants built in the US and they would have to be built. Nuclear power plants take about 5-10 years to be built. The other problem is that people are afraid of nuclear power and the transportation of radioactive elements.

Small Battery-Powered Drill:

The power generation and supply for building and running the battery-powered screwdrivers is another issue that could be corrected through the use of nuclear power. The use of nuclear power would also reduce the greenhouse emissions from running coal plants. The small battery powered drill requires energy not only to manufacture the product, but also to power the use of the product. 165 million tons of CO2 and 2 million tons of CFCs are released into the atmosphere when 1 million dollars worth of battery-powered drills are manufactured. Then there is also more CO2 and CFCs released when the product is being used (resource 3). 1.57 TJs are produced by coal plants and .351 TJ are produced by natural gas power plants. If nuclear power plants replaced the coal and natural gas plants then the greenhouse gases produced by manufacturing this product would be eliminated.

Failure Modes Effects Analysis (FMEA)

Ratcheting Screwdriver Parts List

Part #	Part name	QTY	Function	Wt.(kg.)	Material*	Process*	Photo
001	Screwdriver Shaft	1	Holds the screwdriver bits in its tip, while the base is connected to the handle.	NA	Aluminum	extruding milling
002	Screwdriver Handle	1	Allows the user to grasp the screwdriver for use.	NA	Rubber/Plastic	injection molding
003	Ring Gear/Ratchet Housing	1	Rotates opposite the engaged pawls, and allows for the ratcheting of the screwdriver.	NA	Aluminum	turning milling
004	Pawl	2	Engages the ring gear to stop rotation in one direction, allowing rotation in the opposite direction, thereby allowing ratcheting. both pawls can also be engaged to sllow no ratcheting such that the screwdriver can be used as a regular/static screwdriver.	NA	Steel	forging grinding
005	Pawl Spring	2	The spring pushed the back end of the pawl, forcing it to engage the ring gear.	NA	Aluminum	drawing bending
006	Ball Bearing	1	Allows free left and right motion of the button.	NA	Aluminum	hot rolling cold rolling	[]
007	Body Screw	1	Attaches the screwdriver shaft to the screwdriver handle.	NA	Steel	threading
008	Button	1	Can be switched left, right, or middle, allowing left or right ratcheting or stopping motion.	NA	Aluminum	injection molding
009	Button spring	1	Pushes the ball bearing against the rear of the button.	NA	Aluminum	drawing bending	[]

Portable Electric Screwdriver/Drill Parts List

Part #	Part name	QTY	Function	Wt.(kg.)	Material*	Process*	Photo
001	Screwdriver/Drill Housing	1	Houses the internal parts of the drill	NA	Plastic	injection molding
002	Drill Cover	1	Covers the nose of the drill	NA	Plastic	injection molding
003	Drill Motor	1	Rotates the sun gear, draws current through the wires at the base of the motor to determine the direction and speed of rotation	NA	Plastic/Metals	stamping drawing turning
004	Rechargeable Battery Pack	1	The power source for the drill, draws power from the battery chargin power input and stores the voltage.	NA	Metal, Plastic, and Chemicals	injection molding
005	Lock Ring	1	Clamps the drill housing to the drill shaft	NA	Metal	stamping
006	Gear Housing	1	Acts as a cylindrical ring gear that houses the planetary gear system.	NA	Copper	extruding reaming
007	Reset Button	1	A button on the external part of the body that will reset the drill when depressed.	NA	Plastic	injection molding
008	Battery Charging Power Input	1	Draws voltage from the power cord and send it to the rechargeable battery pack where it is stored.	NA	Plastic/Metal	injection molding stamping
009	Housing Screws	2x3	Holds the plastic housing together	NA	Steel	threading
010	Trigger	1	Switches the direction of the drills rotating output.	NA	Plastic	injection molding
011	Washer	1	Keeps the drill motor and gear housing tight together.	NA	Copper	stamping
012	Planetary Gears	1	Gears within the gear housing that rotate around the sun gear, they increase the torque by reducing the speed.	NA	Copper	casting milling
013	Sun Gear	1	Transfers the rotational output of the motor to the planetary gears.	NA	Copper	casting milling
014	Intermediate Gears	1	Keeps smooth rotation between planetary gears and the gear housing.	NA	Copper	casting milling
015	Drill Shaft	1	Attached to the output of the gear housing and extends through the drill cover. Holds the drill bit in its external end of the shaft.	NA	Steel	extruding milling
016	Lights	1	Red, yellow, and green lights that tell when the drill is charged, charging, and needs charging.	NA	Plastic/Metal	stamping injection molding
017	Wiring	1	Basic red and black wires running current between electrical parts of the drill	NA	Plastic/Copper	drawing
018	Switch Box	1	Switches the direction of voltage to the motor depending on the direction of triggers depression.	NA	Plastic/Copper	injection molding stamping

Mechanical Analysis of the Gear Ratio for the Electric Screwdriver

The electric motor within the electric screwdriver produces a very high rotational speed, but very little torque. However, the customer needs a device capable of producing a relatively high amount of torque, but not necessarily at such a high speed. Thus, the electric motor is geared down before it is attached to the bit. After the dissection of the drill, we found that a planetary gear system was used to accomplish this. In this gear train, the annulus was held stationary within a housing. The sun gear was used for the input, and the output was therefore the planet carrier. This planet carrier in turn was connected to the sun gear for the next level. In total there were three levels of gears, each of which reduced the torque by a factor of 1/(1+A/S), where A is the number of teeth on the annulus, and S is the number of teeth on the sun gear. With three levels, the total gear ratio is 1/(1+A/S)^3. With S being 12 teeth, and A being 42 teeth, this brings the gear ratio to 0.0110. Effectively, this slows the rotational speed of the bit to just over 1 percent of the speed of the motor, while increasing the torque by about 91.

Mechanical Analysis of the Structural Strength of the Pawl

The pawl used within the manual ratcheting screwdriver is subject to a significant amount of stress. The outer notches apply a force to the pawl while a torque is being applied to the screwdriver. This causes both a shear load and a corresponding moment to be applied to the pawl. The analysis of the pawl determines the maximum torque that the screwdriver can apply without causing damage to the pawl. The torque applied to the screwdriver is resisted by the pawl at a radius of 1.1 centimeters. This radius is used to relate the force acting on the pawl and the torque applied to the screwdriver. Stress analysis calculated the maximum shear force acting on the pawl was the torque divided by 0.16 square centimeters times this radius. The maximum bending moment was calculated to be the torque times 0.4 centimeters divided by the radius of 1.1 centimeters. The maximum normal stress was then calculated to be 32.09 per centimeters cubed times the torque, while the shear stress is 5.68 per centimers cubed times the torque. Solving for the principle stress per unit torque, the result is 35.01 per centimeters cubed. Taking the failure stress to be ultimate strength of steel, it was determined that the pawl could withstand 22.85 Newton-meters of torque before failing.

References

1.http://www.eiolca.net/cgi-bin/multimatrix/display.pl?hybrid=no&key=9232578260&value=3425720201&selectsect=332212&selectvect=gwp&incdemand=1&demandmult=1&top=10&DefSort=1&newmatrix=US491IDOC1997 2.http://www.eiolca.net/cgi-bin/multimatrix/display.pl?hybrid=no&key=9232578260&value=3425720201&selectsect=332212&selectvect=fuels&incdemand=1&demandmult=1&top=10&DefSort=1&newmatrix=US491IDOC1997 3.http://www.eiolca.net/cgi-bin/multimatrix/display.pl?hybrid=no&key=5962855190&value=4772278266&selectsect=333991&selectvect=gwp&incdemand=1&demandmult=1&top=5&DefSort=1&newmatrix=US491IDOC1997 4.http://www.eiolca.net/cgi-bin/multimatrix/display.pl?hybrid=no&key=5962855190&value=4772278266&selectsect=333991&selectvect=fuels&incdemand=1&demandmult=1&top=5&DefSort=1&newmatrix=US491IDOC1997 5.http://en.wikipedia.org/wiki/Epicyclic_gearing