Gate 4 - Product Explanation and Reassembly - Group 6 2012
Click here to return to the main page for Group 6.
This gate has several parts. The first part is project management. This section discusses the roles of each member and challenges faced. The next section is the reassembly, where videos of the wrench being reassembled are shown and the process is described. There is a section that describes the mechanisms used in the impact wrench and finally a section with three design revisions on the system level.
This section is a table of what changes group members had to make to have the gate progress.
Critical Project Review
|Management Roles||Goals||Successes||Challenges and Resolution|
|Project Manager and Intra-Group Communications Coordinator||
|Technical Expert: Communications Technology||
|Technical Experts: Dis-assembly Technicians||
This section contains the steps and tools necessary to reassemble the impact wrench. It also contains links to videos showing how to perform each step.
Re-assembly Guide for a Kobalt Impact Wrench:
- B.Rubber Mallet
- D.5/32” Allen Wrench
- E.13/16” Wrench
- Requires very little physical effort and/or very little mental comprehension
- Requires small amount of physical effort and/or a small amount of mental comprehension
- Requires some Physical effort and/or some mental Comprehension
- Requires a decent amount of Physical effort and/or mental comprehension
- Requires a lot of Physical exertion and/or mental exertion
Step 1: Anvil Assembly
- Hammer cage
- First you take the hammers and place them on top of each other with the internal patterns opposite of on another. 
- Slide the hammers into hammer cage with the outside indents lined up with the holes for the pins 
- Slide the pins into their holes 
- The Anvil now slides into the cage; make sure the protruded feature on the anvil is matched up with the wider section of the receptive hammer 
- Hold the Housing with the drive end upward and slide the assembly into the wrench from the bottom to prevent the pins from falling out 
- Flip the housing with anvil assembly over and hold with back end upward 
Challenges: The only Challenge faced in this step is coordinating the Anvil assembly into the housing but if you are careful it is no problem.
Step 2: Rotor Assembly
- Front Bearing/Bearing Carrier
- Rear Bearing/Bearing Carrier
- Outer Cylinder
- Place the most front bearing (with no through cuts) and Bearing carrier in first with the bearing facing the anvil. Place it so the small cut is upward. 
- The Outer Cylinder now goes in with the smaller pin sliding into the small cut 
- Insert the Rotor with the splined end going into the hammer cage and bearing carrier 
- Next spin the rotor around and while doing so slide the fins into the slots of the Rotor. The Fins should be slid into the rotor with the curved side toward the middle of the rotor 
- Take the rear Bearing and Bearing Carrier and place it over the end of the rotor with the bearing facing outward 
Challenges: The front Bearing and the Outer Cylinder go into the housing a little tight and might require some finesse get them into place.
Step 3: Back Plate Assembly
- Rubber washer
- Cork gasket
- Small spring
- Small set pin
- Set pin
- Adjustment Lever
- Place rubber washer over the smaller end of the plunger 
- Place cork gasket over inner side of back plate then slide the smaller end of the plunger into the hole of the gasket and back plate so that it sticks out of the finished side of the back plate 
- Put the small set pin on top of the spring and place in the smaller hole in the adjustment lever 
- Slide the adjustment lever onto the plunger that is on the finished side of the back plate 
- Line up the small side hole in both the adjustment lever and the plunger then drive the set pin in with a Rubber Mallet 
Challenges: The pin that holds the Adjustment Lever to the Plunger goes in very hard and takes a lot of finesse to set up. It greatly helps if you have someone else to lend a hand to hold pieces together.
Step 4: Attaching the Back Plate
- Rubber Ring
- Back Cover assembly
- Housing including assembled Rotor and Anvil
- Take the Back Cover and put the Rubber Ring on the lip of the Back Cover next to the Cork Gasket 
- Slide the plunger into the brass sleeve in the housing and push the Back Cover Assembly flush with the housing 
- Take the screws and screw the Back Plate Assembly to the housing with a 5/32” Allen Wrench 
Step 5: Muffler Installation
- Muffler Cover
- Insert the Muffler into the non-threaded hole in the bottom of the handle 
- Then place the Muffler Cover over the hole and screw it in with a screwdriver 
Step 6: Trigger and Tipper valve Installation
- Trigger Spring
- Tipper Valve and Spring
- Air Inlet Bushing
- Set Pin
- Put the Tipper Valve and Spring in the threaded hole in the bottom of the handle 
- Slide the spring on the trigger and then slide the trigger into its slot 
- Make sure that when the trigger is depressed the spring on the Tipper Valve moves 
- Take the Air Inlet Bushing and screw it into the threaded hole on the bottom of the handle. Tighten with a 13/16” Wrench 
- Depress the trigger a little so that the black plastic enters the housing slightly and then take the set pin and drive it into the pin hole of the housing with a hammer 
Challenges: The set pin that holds the trigger does take some finesse to navigate holding the pin and trigger somewhat depressed. This part also goes smoothly if another person is involved.
Frequently Asked Questions:
Q: Is the assembly the same as the disassembly?
A: Yes, The assembly is the exact opposite of how someone dismantles this product
Q: How was this product originally assembled?
A: This product was assembled mostly by hand except for the press fit parts which are not dissected in this guide
This section overviews the mechanisms that are present in the impact wrench.
Mechanism: Plunger/Back Plate and Switch
The plunger acts as a gate valve, connecting the user interface (back plate) with the control of the air flow to the rotor mechanism.
- The plunger is connected to a switch on the back plate which allows the user to rotate the plunger. It is an obstruction inside of the valve sleeve, and as it is rotated it becomes less obstructive to the air flow, allowing more air to pass through the valve. It can also be turned such that the motor would spin in reverse.
- The switch on the back plate uses a spring inside a pin to create compression so that once the pin enters one of the grooves designated for specific pressures, it locks into place until the user applies force to move it to another setting.
Ideal Gas Law:
where P is the pressure, V is the volume, n is the moles of gas, R is the gas constant and T is the temperature
where F is the normal force per unit area A, or the force applied perpendicular to the surface
After the trigger is engaged and air is allowed to flow through, the plunger alters the volume of the container through which the air may flow. Given all of the assumptions, PV = constant. Therefore, an increase in volume given by the plunger would yield a decrease in pressure proportional to the increase in volume and vice versa.
Back Plate and Switch
F= -kx (3)
where F is the force applied by the spring, k is the spring constant which is characteristic of the spring and x is the displacement of the spring from its rest state.
Forces of Friction:
f_s= μ_s(F) (4)
f_k= μ_k(F) (5)
where f_s is the maximum force of static friction, μ_s is the coefficient of static friction, f_k is the maximum force of kinetic friction, and μ_k is the coefficient of kinetic friction
The action of the spring pin locking into place in the grooves is defined by Hooke’s Law.
Mechanism: Rotor and Fins
The rotor and fins act together as an impulse turbine to alter the pneumatic, stored energy to rotational energy via impulse, which it uses to spin the hammer.
- The fins serve as a point of impact for the compressed air flowing into the housing. The direction of the flowing air is changed, thus creating an impulse, or change in momentum that is transferred to the fins, causing the rotor to spin.
- The rotor is connected to the hammer and spins due to the force exerted by the compressed air on the fins. The spinning completes the conversion of the potential energy of the compressed air into mechanical energy and finally the torque output.
J=∫Fdt = ∆p = m∆v (6a)
where J is an impulse, ∫_∆tFdt is the integral of the force F over an interval of time t, ∆p is the change in momentum, m is the mass, and ∆v is the change in velocity
The force exerted on the fins by the compressed air is a function of the pressure of the compressed air and the surface area of the fins. There is a direct relationship between the surface area of the fins and the force on the fins, and there is also a direct relationship between the pressure created by the air and the force exerted on the fins.
When air enters the rotor chamber it incurs a change in momentum caused by the contact with the initially stationary surface of the fins. This change in momentum is transferred to the fins as an impulse, which causes an increase in velocity of the fins and thus the rotation of the rotor.
Newton’s Second Law:
F=dp/dt=d(mv)/dt=m dv/dt=ma (6b)
where F is the force, and dp/dt is the rate of change of its linear momentum (p) with respect to time (t) and d(mv)/dt, the rate of change of the mass (m) times the velocity (v) with respect to time; m is brought out as a constant under the assumption of constant mass and dv/dt becomes acceleration, a
Torque as the Time Derivative of Angular Momentum:
where τ is torque, dL/dt is the rate of change of angular momentum with respect to time, dr/dt is the rate of change of the position vector (r) of the point relative to the center, and × indicates a cross product
The force of the fins against the surface of the grooves in the rotor causes the rotor to accelerate.
Due to the conservation of linear momentum, the linear momentum of the fins created by the impulse from the contact with the compressed air is transferred to the rotor at points of distance r from the rotor’s central axis. The change in linear momentum at these points on the rotor’s grooves, now a change in angular momentum given the cylindrical shape of the rotor, then creates torque. The torque spins the rotor which, in turn, spins the hammer.
The trigger is a central part of the interface between the user and the control mechanisms of the impact wrench. It is the control mechanism that serves as a signal to allow air to flow into the device.
- The trigger is engaged by the user when the user applies pressure perpendicular to the surface of the trigger.
- The force exerted by the user on the trigger overcomes a spring mechanism in order to cause the shaft attached to the trigger to push the tipper valve to a position which allows for air to flow into the wrench. When the user releases the trigger, the spring forces the trigger back into place, thus disengaging the shaft from the tipper valve and allowing the tipper valve to close, no longer allowing air to flow into the device.
F= -kx (3)
Hooke’s Law defines the amount of force that must be exerted by the user to overcome the force of the spring. The spring constant of the material used is an important factor in determining how much force must be exerted by the user. Design of the trigger is further limited by the parameters within which a reasonable distance of displacement can be applied as the trigger cannot logistically extend or be depressed a great distance. The utilization of the spring action in returning the trigger back to its original place and thus closing the valve allows the user to signal the mechanism to shut off without exerting another force.
This section contains three design revisions on a system level for the impact wrench.
A digital system could be used to replace the current mechanical system in the pneumatic impact wrench. This new system would come with advantages and disadvantages. It would require a lot of changes in the construction of the tool, especially at the user interface level. In fact, the digital system would only affect the user interface level of the tool.
In general, the hammers, anvil, and rotor will act in much the same way and should not be changed. It is at the user interface level that many changes will be made. The trigger mechanism can be left the same as well, as it used only to open and close the air inlet.
A digital control center will have to be added to the tool’s housing and it will require a power supply. This control center will be able to take input from the user as he/she selects the amount of torque they would like the tool to output. Currently, the amount of pressure allowed to enter the rotor is controlled by the plunger. The plunger is connected to a mechanical switch on the exterior of the tool which the user uses to rotate the plunger manually. What this system would do is control a small motor attached to a gear. A corresponding gear would be attached to the end of the plunger and these 2 gears would rotate together. This motor would be able to rotate and hold the plunger with much greater precision than the current mechanical mechanism being used.
The housing would need to be changed in order to accommodate these changes. Currently, the back plate is used to communicate to the user the setting of the impact wrench. This arrangement could stay; however, the engraved markings would be removed and replaced with a digital screen, and buttons to increase or decrease torque. It would look just like the face of a digital watch. Also, the motor which rotates the plunger will need a place to reside. Since it will interact with the plunger, it will need to be placed at the rear of tool. A small power supply will need to be added into the tool. At the top of the housing, a small battery compartment will have to be made. How much energy is needed will depend on the size of the control motor, but room for 3 AA or AAA batteries could be easily accommodated.
PROS AND CONS
The digital system will allow the tool be controlled with a higher degree of precision. This addresses societal concerns because many customers may want to be able to control these aspects. Digital systems are also appearing everywhere and many people like them. This could make the tool more attractive to younger users, another societal benefit. However, economically the cost would most likely increase. The market for pneumatic tools does allow some flexibility toward price increases: if someone has already invested in an air compressor, especially a high end one, they might not be as reluctant to spend more money for high end air tools. Environmental and global concerns are not affected negatively by this change as is makes the tool less recyclable and less durable.
An example of the proposed electronic control can be seen here:
A design change that could be made for the energy system is changing from pneumatic power to electrical power. In changing to this type of system many parts would have to be different along with a redesign as a whole.
First off the rotor and drivetrain would have to be changed to be an electric motor, which would receive energy from a power outlet or a battery. The hammers and hammer cage would be changed to a mechanism assembly due to the change in power. The anvil would be designed to be much shorter and less complex. This is due to the fact that it would no longer have a hammer system so it could be shorter and lighter. The inlet system would need a complete redesign since it no longer would have to receive compressed air. The air inlet would be replaced with an electric cord or battery attachment which would run electricity to the motor. The trigger system would have to be changed due to the fact that this one opens the air flow depending on how hard the trigger is pressed. It would have to be changed to go to an electric regulator. That being said the regulator that is on the pneumatic wrench would need to be changed so that it was in the same system as the electric regulator for the trigger.
The only other major redesign would be the casing. Due to the lack of compressed air running through the casing it no longer needs to be metal so plastics could be used.
The reasons for the change in this system can be seen in some of the GSEE factors. In the societal sense, this change would help because the wrench would be lighter and easier to use for an inexperienced person. Also, many people do not have an air compressor so this would be a viable option for these people and a battery powered one is easier to move around. In the economic sense, this change would lower the price a decent amount due to the fact that the parts would be cheaper as can be seen in the fact that the casing is plastic instead of metal. In making the change to an electric driven impact wrench the intended consumer changes. The consumers for a pneumatic impact would be people who need a high torque for tough jobs such as people who do work on cars, do construction, or do their own home repairs. An electric impact wrench is aimed more towards someone is does not need as high of power and just needs it for small around the house type jobs. In the end the electric powered system would have a lower torque however it is just as viable if not more so in everyday use.
A system level design that could be made would be to have the wrench use hydraulic power instead of pneumatic.
The design of the impact wrench would remain relatively the same. One major difference would be the need for an outlet to allow the liquid used in a hydraulic system back out of the wrench. The liquid would rotate the rotor and fins then fall back through a hole which would allow it to recirculate through the reservoir. There would also need to be a seal between the rotor subsystem and the anvil subsystem to maintain the pressure of the liquid and prevent leaking.
The only redesign to the housing would be the exit tube for the liquid and a tighter sealed system.
Pros and Cons
Hydraulic powered systems are capable of producing much higher pressures and power than pneumatic systems. The average pneumatic system can only produce pressures around 100 psi where as hydraulic systems range from 500 to 5000 psi making the the hydraulic wrench more powerful. Economically, the weight of the wrench would be about the same making the hydraulic impact wrench the best power-to-weight ratio, able to deliver more power in a smaller size which means the price of the product would remain the same or decrease. An societal disadvantage is the availability of a hydraulic power supply. Normally proper hydraulic power supplies are only available in large factories or construction sites which means the intended customer would change from everyone who has a pneumatic compressor to manufacturing companies or construction crews.