The Engineering Design Process

Before beginning , is important to keep in mind the engineering design process . Although not all of it will be used,is important to keep in mind and reiterate the design in order to produce a working/successful prototype.

 
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Problem Description

There could be many things to take into consideration such as ergonomics, minimizing material used and different manufacturing processes that can be used.

Since we want to keep this design simple, we can focus on common manufacturing techniques that can be used at home with basic metal working techniques.

The design will focus only on the main points that will only produced a working prototype that can be optimized later using FEA.

Thus the following specifications must be met in the design:

  • The riveter should allow a normal person to perform a rivet joint with moderate effort.

  • The riveter should be self-opening, meaning there must be some sort of spring that keeps it open when you release the handles,without adding significant effort to the riveting process.

  • The two handles have to pivot in a central piece, which can be thought of as a shaft. This shaft is subjected to forces, so it must be designed in a way that it does not break.

  • You would want the riveter to retain its functionality for a long time. (Fatigue considerations)

  • The force exerted by the riveter should be enough to buckle and compress the rivet. Then it should be enough to break the mandrel.

The end product of this project is to be able to design a hand held riveter that should be easy to use, practical , durable, and must be able to successfully install the aluminum rivet below.  

 

Theory and Background

The main methods of failure occurring during the usage of the pop rivet gun are the buckling of the rivet (dark grey) and the controlled fracture of the mandrel (light grey).

rivetgif.gif

We can relate to this buckling similar to the examples used in Euler’s and Johnson’s formulas. However, the rivet is a thin-walled tube and so the equations that could be used would not be able to properly characterize the critical load required for the axially loaded rivet. For the pop rivet gun to function correctly, the rivet undergoes compressive buckling prior to the mandrel snapping so the limiting force in the process is the tensile force needed to induce crack propagation over notched sections of the mandrel.

From manufacturer specifications for steel mandrels coupled with aluminum rivets, the required tensile force to result in the snapping of the mandrel was found to be 1400 lbs as determined in a specification sheet for our prescribed dimensions[2]. This value was then compared with our analytically determined breaking force which was reported as 1325 lbs. Since our results were based on many simplifications, as will be discussed later on, we decided to use the larger breaking force as the basis for the force analysis on our simplified pop rivet gun.

While a force study on the buckling of the rivet was not conducted, the following equation would allow for a good approximation as to the critical load by quantifying the critical buckling stress:

 
minimum critical buckling load

minimum critical buckling load

 

where t: thickness, r: outer-radius of cylindrical pipe, E: modulus of longitudinal elasticity, ν: Poisson’s ratio

Image 1: Failure modes for axial compression of cylindrical tube (from left to right: global, local, fracture, and axial crushing)

Image 1: Failure modes for axial compression of cylindrical tube (from left to right: global, local, fracture, and axial crushing)

For the study of the rivet, the axial crushing best characterizes the mode of failure due to a compressive load as the buckling primarily occurs on the non-domed section of the rivet as the bulbous end of the mandrel is pulled through the rivet

Image 2: Failure modes for fracture: (from left to right: tensile, shearing, and tearing)

Image 2: Failure modes for fracture: (from left to right: tensile, shearing, and tearing)

For the study of the notches along the radius of the mandrel that allows for consistent fracture sites, the following mode of fracture is assumed: opening or tensile mode. That is, the mandrel is in tension within the pop rivet gun and that the notches on the mandrel itself are located perpendicular and along the outer radius of the mandrel.

A stress intensity factor could then be obtained for the mandrel such that if the stress intensity factor is greater than the critical factor of the material makeup of the mandrel, then the crack would propagate along the plane of the crack front and result in a sudden failure once the crack reached critical length. The mandrel we considered was composed of 3430 Steel and so consulting a table, the critical factor would be obtained (K1C). From there, the critical factor could then be compared with the stress intensity factor unique to the geometrical properties and cross section type. The following set of equations could then quantify the critical load:

 
 
 

Description of System

As mentioned in the beginning rivets are used to permanently connect two parts together, without the use of a screw or a bolt . There are different methods of permanently attaching a part such as welding, or using bolts, however using a rivet is the most useful method since it does not require a welding skill and won’t come off when subjected to vibrations like bolts.  Rivets are widely used in different industries from airplanes, cars, structural components in architecture and manufacturing.

Due to its extensive use, rivets come in different sizes and materials. One of the most used rivets are pop rivets, which require a simple hand tool for its use.

Figure 3 Types of rivets used REF [Fundamentals of manufacturing: M.P. Groover]

Figure 3 Types of rivets used REF [Fundamentals of manufacturing: M.P. Groover]

 
 
Figure 4 How a pop rivet works REF [RIVCO Products]

Figure 4 How a pop rivet works REF [RIVCO Products]

As we will be designing a pop rivet gun is important to know that there are a lot of different designs, and different types are used for different rivet sizes. Our design is similar to the common designs in the market and requires an easy to use tool, with the implementation of moderate effort and a longer lifespan of 3 years.     

 
 

Figure 5 Different types of rivet guns  REF [EZVID]

 

Working Assumption

The sample being analyzed is the aluminum blind rivet with steel mandrel found in McMaster-Carr [1]. Rivets composed of the same materials with the same dimensions require a force of 1400 lbs to fracture the mandrel of the aluminum rivet [2]. Because of inconsistencies when manufacturing the aluminum rivet, the 1400 lbs force should be the minimum force that is exerted by the riveter head mechanism since there may be rivets that are slightly thicker and require a greater force. However, we will assume that every rivet will have the same dimensions with strict tolerances so the exact force for fracture is 1400 lbs. The maximum amount of force being applied by the riveter onto the rivet is when the handle is completely horizontal so we will assume that the 1400 lbs are applied onto the rivet when the handle is horizontal. With stress analysis of the rivet mandrel, we calculated the force for fracture to be approximately 1325 lbs assuming the ultimate strength of steel to be 108000 psi [5]. Our analytical approach was very similar to the 1400 lbs fracture force found in experimental tables [2]. Because experimental data is more accurate for our application, we will round our fracture force to be 1400 lbs. This value will make for simpler calculations and provide safer, more accurate results.

According to medical studies, the average female grip strength is about 70 lbs while the males’ is about 100 lbs [3]. We decided to use the female average since this would allow for more dynamic use and reduce the required force to operate the riveter. This measurement is close to the maximum grip strength for a female, but because we want the user to exert only a moderate effort, we will assume that the grip strength being output will be 75% of that maximum (52.5 lbs).

When designing a handheld tool, the geometry of the model should be as ergonomic as possible with different materials to accommodate for user comfort and structural properties. However, our design is based on the assumption of uniform aluminum properties and constant cross sections. This design will be based on fully prismatic and circular cross-sections to prevent situations that cannot be analyzed by common formulas and will require the implementation of finite element analysis.

Assuming the design is to remain competitive with other mid-priced riveters on the market with three year warranties, the riveter should be able to support the use of approximately three years of moderate use. If the riveter is used approximately twenty times every day, it should be able to withstand about 21900 uses. As for our placement of the applied force, most users will have a hand size of about 3.123 inches when considering a four finger grip on a handle [4]. Since the force being applied onto the handle is not on applied at a single point, we considered a distributed load. To accommodate for larger hand sizes and positioning on the rivet handle, we rounded our applied force length to be 4 inches along the handle.

 

Further Discussion

As we researched the rivet, we discovered that the buckle of the steel mandrel requires 1400 lbs of force in order to fracture that piece. This obtained information will be used as a reference as we calculate our proposed design rivet tool. We will be calculating the rivet tool optimum handle length given our assumed loads and distances.

First we divide the rivet tool into separate parts.  Part one will be the cylinder that is subjected to the 1400 lb of force, which will be applied to part two by a pin. (FBD1)

Part two is the upper handle, this has two holes on the right side that acts as pivot point for the tool. (FBD2) Here we measure the piece for our unknown forces and lengths. First, we applied the forces at critical locations. Starting from the leftmost edge we have the load applied by the moderate 75% two handed woman’s grip. The load that we are using is 105 lbs since we researched that the average women's grip strength is 70lbs and 75% of the grip is 52.5 lbs. According to medical studies the average hand length is 3.5 inches, however, we averaged the length to 4 inches then set the distance of the total force of the distributed load to be concentrated at 2 inches, which is the midpoint of the grip size.  Then next key distance will be the first pin. This position will be our main point of reference. Here we have our unknown length (L) to the right of our initial 2 inches. From this point we locate our spring, this spring will be one 1 inch to the left of the pin. We chose the length simply to avoid interfering with the joints. The spring that we are using has a spring force of 4 lbs which is relatively small and will not add too much weight to the rivet tool. Furthermore, it will not significantly affect the force needed to be applied since the forces exerted are much larger. Here we have our principle moment, which is our main point of reference where we find our unknown lengths. The next key location is another pin that holds the cylinder that is subjected to the 1400 lb load. This position will be held at a distance (D1) to the right of the first pin, in addition to being half an inch from the rightmost edge. These four locations starting from the leftmost edge are 2 inches to the applied load, length (L) to the pivot pin, 9/16 inches (D1) to the second pin, and half inch to the rightmost edge. This gives us the total length (LT) of the optimum handle length that was created by the principle moment. Our total length was calculated to be 10.5 inches.

The third piece is the bottom handle, here we find the resistance force by taking the sum of the forces in the y-direction. (FBD 3).

 

Material Consideration

Rivet is a standardize size comprised of two different materials at two different sections. The head is made of Aluminum 5020 and the Mandrel is steel AISI 4340. Rivet tool will also be steel AISI 4340, since the it is much stronger than the aluminum. [5]

Steel AISI 4340

Tensile strength, Ultimate: 108,000 psi

Tensile strength, Yield: 68,200 psi

Modulus of Elasticity:27,557-30,458 ksi


Aluminum 5052

Tensile Strength, Yield 28,000 psi           

Tensile Strength, Ultimate: 33,000 psi           

Modulus of Elasticity: 10,200 ksi