I chose this item because this project was my greatest undertaking this term, and I am very proud of what my group was able to achieve. The task of the design project was to build a device capable of converting and storing gravitational energy, and using it to complete the task of twisting a bolt. The project was ultimately a success, but we experienced many setbacks and problems en route to a viable solution. This is why I think our prototype report represents my best recent work, and I was certainly forced out of my comfort zone somewhat as the group leader.
“Jamaican Bacon”
Detailed Design and Prototype Report
MECH 3401: Laboratory 6 Group 2
Professor Roger Kempers
March 29, 2019
Jacob Ham 212977385,
David Bruni 214993984,
Gurnek Tak 214326714
Table of Contents
List of Figures 2
Prototype CAD Model #
Prototype Images #
Additional Design Calculations and Expected Performance #
Simulations #
Initial Prototype Test Results #
Improvements #
References #
Appendix #
Statement of Team Participation #
List of Figures:
Figure 1 #
Figure 2 #
Figure 3 #
Figure 4 #
Figure 5 #
Figure 6 #
Figure 7 #
Figure 8 #
Figure 9 #
Prototype CAD Model
Figure 1: Labelled Top View of Twisting Device
Figure 2: Labelled Side View of Twisting Device
Figure 3: Front view of Twisting Device
Figure 4: Isometric View of Twisting Device
The operation of our design is quite simple. The screwdriver-like device is slid into the frame. The spool slides onto the coupling nut that is attached to the drill bit. 2m of rope is then wrapped around the spool to allow for the 2m drop height. The other end of the rope is attached to a bag of potatoes weighing 10 pounds. The rope is fed through our pulley system that is attached to the frame to allow for a horizontal pulling force to be applied to the rope attached to the spool. The bag of potatoes is dropped which causes the spool to unravel and the whole shaft to rotate as a result. The shaft rotating causes the elastic to twist which creates elastic potential energy. Once the spool is fully unraveled it is removed from the coupling nut and the locking mechanism, which is a wrench, is placed on the coupling nut to lock the shaft in place. The screwdriver like the device is then removed from the frame and transported to wherever it needs to be used. Once, the user wants to use the screwdriver they place the drill bit onto the screw that they desire to screw into the hole and then they release the locking mechanism. The release of the locking mechanism will cause the bands to unwind and create a burst of rotational energy which will screw the screw into the hole.
Some key details of our design are the spool, the bands, and the locking mechanism. The spool is 3D printed and has a tight fit around the coupling nut. The spool allows our design to be portable because it is detachable from the coupling nut. The bands are how we are storing our energy. Therefore, the stronger the resistance in our bands the more torque we can achieve. Finally, the locking mechanism is the most important part of our design. The locking mechanism allows us to store energy for an hour. The locking mechanism for our device is a wrench which goes around the coupling nut. It allows us to easily use the rotational energy whenever we want.
Our prototype CAD model changed drastically from our last report due to the fact that a scissor lift design would not function properly for our desired needs. Our group failed to realize that a scissor lift is good at holding objects up, but poor at using the weight of an object to turn a shaft. Our old design can be seen in figure 1 and figure 2.
Figure 1: Scissor lift assembly Figure 2: Elastic Potential Energy Storage
We decided to get rid of the scissor lift idea due to the complexity to assemble it and the fact that it would be unable to generate any energy. Our new design features a simple pulley design that is attached to a shaft that is free to rotate.
Prototype Images
Figure 3: Labelled Top View of Prototype
Figure 4: Labelled Isometric view of Prototype
Figure 5: Side view of Prototype
We were unable to build the frame for the prototype design so we just pulled on the rope attached to the spool to make the elastic turn. Two key components not pictured in the photos above are the rope wrapped around the spool and the pulleys attached to the frame for the pulley system.
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Additional Design Calculations and Expected Performance
The following assumptions can be made to simplify calculations:
- Bearings are frictionless and all elastic energy is conserved
- The effective diameter of spool does not increase as the string is wound
Bending moment on machine shaft:
M = F*r = (4.54 N)*(0.14 m) = 0.54 Nm
Therefore the rod-coupling nut assembly is more than strong enough to withstand the bending moment from a ten-pound mass.
Number of elastic twists:
#twists = (2 m)/(2*π*0.0127 m) = 25 twists
The mechanism will be capable of twisting the elastic a maximum of 25 times from a single drop. No simple formula exists for calculating the released torque, but from manually testing the device, we are confident that it can provide enough force to turn a bolt into a pre-tapped hole.
Simulations
Solidworks was utilized to simulate the force of the bag of potatoes on the frame for our pulley system. A force of 45N was applied to the top of the frame and the following test results can be found below. The base of the frame was also fixed to the ground.
A fine mesh was applied to the frame in order to get accurate displacement, stress and strain calculations.
Figure 6: Mesh of the Pulley Frame Figure 7: Mesh Details of Pulley Frame
Below you can see the von Mises stresses acting on our frame. The maximum von Mises stress was 3.995*10^5 N/m^2 located right where the load was applied
Figure 8: Von Mises Stresses on Pulley Figure 9:Von Mises Stresses on Pulley
Frame (Side View) Frame (Isometric View)
Our frame experienced little to no displacement over its whole area with the highest displacement value being 7.443*10^-1 mm.
Figure 10: Displacement of Pulley Frame Figure 11: Displacement of Pulley Frame
(Side View) (Isometric View)
Finally, the frame experienced basically no strain with the highest strain value being 4.258*10^-5 right where the load was applied.
Figure 12: Strain on Pulley Frame Figure 13: Strain on Pulley Frame
(Side View) (Isometric View)
We didn’t do any design calculations for our frame in our last report because we originally planned to use a scissor lift design. Overall, from our FEM analysis, we can see that our frame will not fail when the 10-pound bag of potatoes is dropped.
FEM analysis could not be implemented for the CAD assembly of the twisting device itself since most of the components were imported directly from McMaster-Carr’s website. Analysis failed because many of these parts were highly complicated and we were not able to mate them properly. We tried removing the threaded part because it was causing interference in the mating, but that ruined the equation of the part that we imported from McMaster-Carr. Due to the equation of the part being ruined, the FEM analysis failed when we tried to run it. Also, we had meshing issues due to the shaft collars and the coupling nuts being too complex. Overall, the parts imported from McMaster-Carr gave us issues when we tried to do a FEM. That is why there is no FEM on the twisting device even though it is an integral part of our design.
Initial Prototype Test Results
We were able to get 21.5 turns of our spool compared to the 25 predicted by our design calculations. This is because our rope had a thickness of ⅛ of an inch so it stacked on top of itself after a certain amount of revolutions. Due to this, we lost 3.5 revolutions.
The prototype was tested and was successfully able to generate enough torque to put a screw into a threaded hole. We were able to film a video of our device putting the screw into the hole. Also, we weighed our design and it only weighed 5 pounds.
The primary objective of this design project is to make a machine that can harness the gravitational potential of a raised 10 lb mass and store its energy for at least one hour, before using it to perform.
The secondary objectives are that the energy storage device is detachable and easily portable, such that the useful task can be performed at a separate location. The energy also should be easily controllable upon release. Finally, the cost should be kept to a minimum without compromising performance.
The design meets the primary objective, which was able to harness the gravitational potential energy of the 10 lb mass (=44N of force) successfully for over 1 hour and complete the task of turning a screw into a pre-threaded hole.
The design meets the secondary objectives of being detachable and easily portable. We have made our screwdriver design be able to easily slide in and out of our frame to make it detachable. Also, our design weighs 5 pounds which any person would be able to lift making it easily portable. We can easily perform our task in another location due to the weight of our design and the size of it. Also, we added a handle to our design to make carrying it and using it easier.
A bill of materials was made to summarize exactly how much our design costs so that we can determine if costs were kept to a minimum.
| Part Name | Quantity | Cost ($CAD) |
| Mounted Pulley for Rope-for Horizontal Pull | 3 | 27.17 (total) |
| Threaded-Shank Number 3 Phillips Bit, 10-32 Thread | 1 | 9.04 |
| 18-8 Stainless Steel Hex Nut | 1 pack | 4.98 |
| Clamping Shaft Collar | 2 | 7.10 (total) |
| Oil-Embedded Mounted Sleeve Bearing | 2 | 25.32 (total) |
| Connecting Rod | 2 | 17.93 (total) |
| 18-8 Stainless Steel Coupling Nut | 2 | 18.03 (total) |
| Stubby Combination Wrench | 1 | 15.58 (total) |
| Subtotal | 125.15 | |
| Tax | 16.27 | |
| Total | 141.42 |
As you can see in the bill of materials above we were at least $50 under budget therefore keeping our costs to a minimum.
Design Improvements
One improvement that we would like to implement to improve our design is to use bands with more resistance so that we could generate more torque with our design. To implement this change we would have to buy stronger bands.
Another improvement is we would like to properly tolerance our spool design to avoid slipping when we are twisting our bands. Not having a properly toleranced design is also dangerous because the energy of the band is not controlled. The way we would implement this change is by applying the proper tolerances in Solidworks and by trial and error until we create the spool that fits perfectly on our design.
We also decided that we would like to move the handle farther away from the elastic part of our design. When prototyping, we noticed that the bands will hit the user’s hand when the lock is released. This will potentially hurt the user and will create many losses in the rotational energy of the drill bit. The way we would implement this change is by moving the handle slightly more to the right to prevent the user’s hand from being hit.
We noticed when prototyping that having two bands causes a lot of losses because the two bands collide with each other when they unravel. To minimize these losses we were thinking of only using one band or twisting the bands so much that they are constantly in contact with each other.
Another improvement would be to increase the number of turns of the elastic we can get with 2m of rope. The way we would implement this change is by decreasing the diameter of the spool to allow the rope to wrap around the spool more times, therefore giving more turns.
Finally, we would like to control the rotational energy better when we release the locking mechanism. To implement this change we were thinking of creating a gearbox to allow better control of the rotational energy.
Statement of Team Participation
| Task | |
| Prototype CAD Model | Jacob, Tak, David |
| Prototype Images | Jacob, David, Tak |
| Additional Design Calculations and Expected Performance | Jacob |
| Simulations | Tak, David |
| Initial Prototype Test Results | David |
| Improvements | David |
Jacob's Portfolio 