7.2 - Project Prototype - Bottle Opener

Kinematic Analysis

System Overview

Our Bottle Opener is a fourbar linkage mechanism with a ground link (L1), a crank link connecting to a rotating black disc driven by a handle (L2), a coupler connecting our crank to our rocker (L3), and a rocker which contacts our bottle cap and arcs to pry it open (L4).

Link Lengths

The link lengths are as follows:

L1 (Ground link) = 125 mm.

L2 (Rotating disk) = 26.25 mm.

L3 (Crank Link) = 178.28 mm.

L4 (Rocker Link) = 118.695 mm.

These were chosen starting backwards from what we expected our bottle opener to do. We wanted a mechanism capable of handling rotation that could typically be supplied a motor and gear system and convert it into a high force arc at the output of our system capable of prying a bottle cap. We new the output motion would constrain the rocker and that a standard bottle cap requires around 3 mm of vertical lift to pop off. from this we could use a simple arc relationship such as arc ~ L4 * theta4 in order to calculate the size of L4. We then used equivalent ratios to size our system up to account for a comfortable physical handle for gripping which we deemed to be in the 100-120 mm. range.

Degree of freedom analysis

M = 3(n-1) - 2j1 - j2

M = 3(4-1)-2(4)-0

M = 1 DOF

Grashof Condition

S = 26.25 mm., L = 178.28 mm., P = 118.65 mm., Q = 125 mm.

S +L < P + Q → This is a Grashof mechanism. This means the crank is able to make a full rotation while our rocker oscillates. It is important to note that our bottle opener only uses a small portion (~45 degrees) of the crank’s range for prying open the bottle cap.

Position/Velocity/Acceleration Analysis

Some basic written math from class that related to our system can be found here:

Open

Figure 1: Position, Velocity, and Acceleration Calculations

Open

Figure 2: Position Analysis for the System

From our calculations we can clearly see that the coupler ranges from approximately 30-60 degrees in its motion in which its peak displacement occurs at roughly 250 degrees. The rocker, on the other hand appears to oscillate between 60 and 100 degrees and. This means if we were to take the total movement in the x-y plane we would see the path of point B (the contact between our mechanism and the bottle cap) we would get approximately 40 mm. of movement in the x and 30 mm. of movement in the y.

Open

Figure 3: Velocity Analysis for the System

These calculations show that our velocity from hand operation which we took as 1 rad/sec would result in approximately 0.4 to -0.2 rad/s for our rocker. It is important to note that this is not a velocity based mechanism as emphasis is more on force applied to pry the cap off.

Open

Figure 4: Acceleration Analysis for the System

Our acceleration analysis follows a similar thought process in which it appears to range from ~0.1 to 0.2 rad/s^2. As one might expect, our acceleration magnitudes appear to be the highest near points in which our motion reverses.

Open

Figure 5: Animation of Full 360 Degree Bottle Opener Motion

Force Analysis

Although there are no definitive sources on the force required to lift a crown cap bottle open, estimates online suggest that it would take 50-100N. When finding the specifications for the motor, it was important for the team to determine the mechanical advantage of the system.

Our current design uses a four bar mechanism and a lever. The lever has a mechanical advantage of 6.25, and the four bar mechanism’s mechanical advantage is dependent on the orientation.

Open

Figure 6: Mechanical Advantage Analysis

As can be seen from the simulation above, the max mechanical advantage of the system on the downstroke is roughly 0.25.

The total mechanical advantage of the system is the product of the mechanical advantage of the downstroke and the lever, which is 1.56.

When opening the bottle, a torque of 1.6 N*m is applied on the end of the lever. If we divide the desired output torque by the mechanical advantage of the system we can find the input torque, which is roughly 1.02 N*m.

This corresponds to roughly 0.75 foot pounds of torque.

Physical Prototype & Iteration Documentation

The functional prototype of the mechanism serves as a manually powered proof of concept for the bottle opener’s path of motion. Force transmission and materials selection will be addressed in the final build based on our kinematic analysis. The manufacturing methods available to us at TIW are laser cutting on the Trotec Speedy 300s and 3D printing on the Bambu P1Ps. While other equipment was available, we felt these to be the best due to their low cost and ability to rapidly iterate.

First we created a preliminary design purely in CAD to ensure we could simulate full rotation and have a visual representation of the project.

Open

Figure 7: Digital Prototype- Isometric View

 

Open

Figure 8: Digital Prototype- Side View

 

The structure of the digital prototype consists of one sheet metal ground link as well as a custom surfaced connector from ground to the output link. The sheet metal link was chosen for speed of design and ability to hold it upright as a handle and the custom connector was designed so as not to interfere with the bottle cap itself (as seen in the side view). The path of motion itself is limited since a crowned bottle cap only requires around 2mm of displacement to be removed, but to accpount for error we added extra range of motion. The outplink itself does not exceed going above parallel as that motion is typically only used for securing the hook underneath the bottle cap. For our final design we plan to simply slide the bottle cap into the groove where the hook will be.

The next iteration we did was with cardboard, popsicle sticks, and wooden skewers.

Open

Figure 9: Iteration using Cardboard, Popsicle Sticks, and Skewers

This iteration was constructed using cardboard, popsicle sticks and skewers to create a low cost prototype. The purpose of this was to ensure we could validate the kinematic behavior that we sought in our CAD. Physical testing ensured our system would be viable and exposed possible improvements such as increasing motion profile and exposing the need to ensure the geometry of the bottle opener properly interfaced with the bottle.

Open

Video 1: Motion of the Cardboard, Popsicle Sticks, and Skewers Iteration

Lastly, we created our physical prototype for submission. We first detailed out a full assembly in OnShape.

Open

Figure 10:Physical Prototype CAD- Isometric View

Open

Figure 12: Physical Prototype CAD- Side View

 

Open

Figure 11: Physical Prototype CAD- Exploded View

Open

Figure 13: Physical Prototype CAD- Transparent View

 

The design remained similar to the digital prototype but changes were made to account for manufacturing methods/tolerancing as well as forcing being applied during manual actuation. Firstly, a base plate was added for ease of testing and without having to account for manual actuation tipping over an upright structure. The second major change was 3D printing a conical input link as opposed to laser cutting a flat disk. A flat disc ran the risk of wobbling due to bending forces applied when the user grabs the handle (as we saw with our first build assignment). Swapping over to a conical structure allows for some of the downward force being transferred into compressive forces (instead of pure bending). Having a greater surface area of contact between the shaft and the input link also made 3D printing more advantageous than laser cutting as it is the point force is being applied. One other notable change was thickening the output link. Previously in the digital prototype it was around 15mm thick but in the physical prototype we doubled it to around 30mm in order to anticipate force transmission that it will be experiencing in the final design. Lastly, we made sure all of the lasercut bearing holes were a interference fit with a diameter of 21.75mm to account for kerf and 3D printed shaft holes were left at 8mm because thermal expansion and nozzle diameter caused it to be a interference fit as well.

Open

Figure 14: Physical Prototype- Full Assembly

 

Open

Video 2: Physical Prototype- Path of Motion

 

On the whole the manufacturing and assembly was a success and we were even able to recycle parts from Build Assignment 1 which helped cut down on volume of parts needed. One thing we noticed when assembling was that while the 3D printed connector is good for maintaining link 1 length and avoiding the bottle cap, it leaves a small surface area of contact between the connector and the shaft. This plus the geometry of the offset ball bearing made it very susceptible to bending which resultingly can cause wobble and slipping. The other point of concern to address in the future would be to watch out for slip when the mechanism is mounted upright, especially at the joint between the output link and coupler.

Draft BOM