Microphone stand

From DDWiki

This page was compiled as a project for the 24-441 Engineering Design Course at Carnegie Mellon University.

1 Executive summary
2 Customer needs
3 System function
4 Product use
5 Product dissection
6 Design for 'X'
7 Failure mode and effects analysis
8 Mechanical analysis
9 Design/Analysis Team

Executive summary

In the following report, the design, use, and function of a boom-style microphone stand for use in the radio/broadcast industry has been studied. This was done to better understand the current state of the product in preparation for making recommendations to the client about modifications to the product or expansion into new product lines. The analysis consisted of a study of product use and function, structure, and mechanical function, as well as studying failure modes and the effects of design on manufacturability, assembly, use, safety, and the environment.

From the analysis, it is clear that the product generally functions as intended to position a microphone in the appropriate position for an on-air radio host. Testing of the device showed several equilibrium positions for the microphone, a conclusion that was supported by quantitative mechanical analysis of the system. While there is relatively minimal impact of the device on the environment, analysis of the design with respect to manufacturability, assembly, and safety indicated that possible improvements could be made in the interests of decreasing costs, improving quality of assembly and service, and improving the safety and sustained functionality of the device.

The detailed analysis of the device follows, and follow up questions and comments are welcome. With the analysis completed, we look forward to working to develop improvements to this product or expand the technology to a new line of products.

Customer needs

This device is essentially a positioning mechanism that can move in several directions. It is designed such that it has multiple degrees of freedom to increase the usability for the user, who may use the microphone either sitting or standing, or may be passing it between people during an interview setting. This microphone stand was designed to be a hands-free device and would eliminate the need for the user to have to physically hold the microphone in a particular location. While disc jockeys/on-air personnel are the primary day-to-day users of this device, there are other stakeholders. For radio station management, it is important that the device be cost-effective, and provide seamless performance at low cost. For engineering personnel, ease of installation/maintenance is important in an industry with little acceptance of downtime. Safety is another concern that has been taken into account in the design of the product. There is one potential pinch point in the middle of the lower and upper segments, and two others between the separate links of each segment. For general use, the microphone stand would not be folded into the closed position and therefore the central pinch point can be avoided. The other two could be resolved through a redesign of the microphone stand that uses only one linkage for each segment, or that has an enclosure around the separate segments to protect the user.

With a tightening market, radio stations are getting smaller and are being required to serve more functions. A device that can keep a microphone off the table while placing it wherever is needed is useful in keeping up with the demands of the station. This device is also easily moved to a different studio as needed in the era of consolidation/relocation.

System function

Figure 1. Force Diagram of the Microphone Stand.

The stand is fixed vertically to a surface by a clamp which allows for rotational motion in the z axis. The 2” rod that protrudes out from the bottom of the stand attaches to the clamp. The springs attached to the upper section of the stand create an opposing force to the microphone itself. The spring force creates a moment around point B as shown in Figure 1. This moment counteracts the moment caused by the weight of the microphone at point A.

There are two adjustments for the position of the microphone: at point B and at point C. Adjustments about point B are made by considering the lower section of the stand ("segment BC") as being fixed, and thus, the adjustments about B correspond primarily to adjustments in microphone height. Adjustments about C, taken irrespective of the position of segment AB, primarily act to change the distance from the base to the microphone. By adjusting the stand about both B and C, a full range of positions is achieved. The four bar linkage at AB consists of two bar links attached by triangular plates at the ends. The linkages move parallel with respect to each other, and provide stability for the microphone attachment while allowing the height of the stand to be adjusted. Two bars are used in place of one linkage for durability and for the spring attachments.

The microphone stand acts to resist the force of gravity acting on the microphone through a combination of springs and bolts. The bolts primarily maintain force equilibrium by resisting the weight of the microphone (significant) and of the parts themselves (relatively insignificant). Additionally, bolts at the joints B and C provide some resistance to the moments generated by the distance of the microphone from the base. This frictional resistance from the bolts is minor in terms of resisting moments when compared to that generated by the springs on each of the segments. The springs act to resist the moment created by the microphone by exerting forces on the upper bar relative to the lower bar (at point B) or on the bars relative to the base (at point C).

The stand is functional only in two dimensions; that is, the existence of matching bars and springs does not directly change the function of the stand, but rather provides redundancy, stability, and aesthetics.

Product use

The purpose of this product is to position a microphone in a fixed position in space. To achieve this, the microphone stand that we studied is first mounted to a fixed surface, such as a table top. This mounting device, while not explicitly studied here, allows for full 360 degree rotation of the stand, allowing the microphone to be pointed wherever needed. In typical use, the user will first rotate the stand such that the microphone faces him/her. The user will then position the microphone's height by adjusting the upper portion of the stand, and the distance from the microphone to them by adjusting the lower portion of the stand.

The stand is designed so that once it is positioned, it will stay fixed in that position. In the event that the stand does not stay, the set knobs at the upper and lower joints can be tightened to fix the stand in a position. The stand that was dissected was an old model and the set knobs may have been worn down. As a result, the set knobs did not tighten fully and the microphone stand was not able to stay in a fixed position. There were two equilibrium points where the stand was able to stay in place without additional human support. In a new stand, the set knobs would provide enough friction so the stand could stay in place, but would not render the stand completely immovable. The set knobs serve to provide extra friction at the joints in the case that the springs alone cannot support the desired position. Additionally, there may be a knob on the mount that connects the stand to the table that would fix the rotation of the stand.

Our analysis and research indicates that other possible uses for this product may include:

Lamp Stand
Magnifying Lamp Stand
Computer Monitor Stand
Music Stand
Medical Lighting equipment
Heavy Document Holders

Product dissection

The first step in designing an improved product is to fully understand the current product. This includes the purpose, function and structure of the product as a whole, as well as the properties of each of its components. In order to examine the complete functionality of the microphone stand, and to prepare for further design analysis, each component was individually removed and documented in the following table.

**Bill of Materials**
Part #	Part Name	Qty.	Function	Weight (g)	Material	Manufacturing Process
01	Knurled knob (sub-assembly)	1	Friction knob to hold microphone in place	6	Plastic	Injection molding
01	Knurled knob (sub-assembly)	1	Friction knob to hold microphone in place	6	Steel	Extrusion, threading
02	Lock washer (small)	2	Spacer between bolt and surface	0.15	Steel	Stamping
03	Headless bolt	1	Holds microphone attachment in place	2	Steel	Extrusion, threading
04	Microphone attachment	1	Holds microphone by threaded rod	47	Steel	Turning
05	Grasping clamp	2	Clamps microphone attachment in place	13	Steel	Casting
06	Grasping pin	1	Aligns grasping clamps	0.5	Brass	Extrusion
07	Lock washer (large)	1	Spacer between bolt and surface	0.2	Steel	Stamping
08	Bolt	4	Holds upper angled plate sub-assembly	2.5	Steel	Extrusion
09	Angled plate - bent	2	Secures frame to attachment	11	Sheet metal	Stamping, bending
10	Upper spring	2	Counteracts weight of microphone	53	Steel	Wrapped wire extrusion
11	Upper spring attachment	4	Holds spring in place	2	Steel	Turning
12	Threaded rod	2	Holds attachments together	4	Steel	Extrusion, threading
13	Bar linkage	5	Linkage, houses XLR cable	94	Sheet metal	Extrusion
14	End fitting cap	4	Upper linkage cap, guides XLR cable	(n/a)	Plastic	Injection Molding
15	Lock washer (medium)	2	Spacer between bolt and surface	0.2	Steel	Stamping
16	Nut	3	Tightens fixture	0.5	Steel	Purchase
17	Headless 1/2-threaded rod	1	Connects lower and upper sections	4	Steel	Extrusion, threading
18	Spacer (medium)	2	Spacer for central sub-assembly	0.5	Steel	Extrusion
19	Spacer washer	1	Holds central sub-assembly	0.2	Steel	Stamping
20	Washer (medium)	1	Holds central sub-assembly	0.2	Steel	Stamping
21	Tightening cap	1	Tightens upper to lower section connection, secures joint angle	3	Plastic	Injection molding
22	Angled plate - flat	2	Base for upper to lower connection	10	Sheet metal	Stamping
23	End fitting cap (flat)	2	Upper linkage cap	(n/a)	Plastic	Injection molding
24	Spacer (large)	3	Spacer for lower sub-assembly	2	Steel	Extrusion
25	Long 1/4-threaded rod	1	Secures lower sub-assembly	5	Steel	Extrusion, threading
26	Spacing bolt/rod	1	Spaces and holds lower springs	2	Steel	Turning
27	Lower spring	2	Counteracts weight of microphone	35	Steel	Wrapped wire extrusion
28	Lower spring attachment	2	Holds spring in place	2	Steel	Turning
29	Base bracket	1	Connects frame to mounting section	47	Sheet metal	Stamping, bending
30	White bushing	1	Protects and guides XLR cable	0.5	Plastic	Injection molding
31	XLR cable	1	Carries signal to microphone	50	Various	Extrusion

Design for 'X'

As part of our study of the product, we examined its design as it related to manufacturability, assembly, usability and the environment.

Design for manufacture

To determine the effect of design on manufacturing costs and time, we examined the design of the microphone stand as it relates to manufacturing processes.

The bars of the stand (5 total) are made from identical sections of extruded steel. This aids in decreasing costs and simplifying the design. Two lower bars are then bent to improve aesthetics and to fit the geometry of the stand. These bars are clearly bent to distinguish from the upper bars, and are identical in terms of left vs. right.
There are two sets of angled plates: one set is bent while one is flat. Using identical geometry for these two separate plates allows for one common stamping process and then a second bending process only when needed. Additionally, the design of the parts fits together well to minimize waste on sheet stock.
Many of the cylindrical parts that either held the springs in place or were spacers seemed to be manufactured by turning. This manufacturing process is relatively expensive; the use of standard premanufactured parts might have been used to replace these parts.
There are some plastic pieces such as the turning knob and the bushing that appear to be made through injection molding. Injection molding is fairly cost effective, with relatively low waste.
The top springs were of thicker material than that of the lower springs. This was to counterbalance the weight of the microphone at a harder angle. These springs are just normal industrial springs with no special features. There were differences between the upper and lower springs, but not the left and right springs.
Other pieces such as washers, bolts, nuts, and clamps were steel and cast into their respective shapes. Many of these parts were of different sizes and proportions, which is disadvantageous from a manufacturing perspective; if part totals had been reduced, it would have decreased costs, inventory needs, etc.

Design for assembly

In considering the characteristics of the design as it relates to the ability to assemble the stand, there were areas of improvement:

Although the assembly looked very similar in terms of the links, there were bolts and nuts of different sizes all throughout the assembly. As this affects the price of manufacture, it also makes assembly a bit more complex than it needs to be. In addition, this means stocking a larger inventory at the assembly facility, which adds costs and increases the chance of error.
In reassembling the microphone stand, there were many different configurations that would have worked. Pictures had to be used to aid in the reconstruction. Had there been geometric features to force assembly in a particular way, it would have improved maintenance and initial assembly.
Assembly at certain points needed three hands to manage. This also could have been a design consideration as this microphone needs to be taken apart for maintenance.

Design for usability

In considering the usability of the device, we noted a few things:

There are two positions where the microphone stays in equilibrium. One angle is about 45 degrees from the vertical and another at about 135 degrees. This is very limited as the users of the microphone stand are diverse in height and seating positions. An alternative design could improve the number of equilibrium positions.
The bottom of the stand has a simple insert-in-slot swivel mechanism so the microphone can be rotated to any degree in that plane with a lock, providing excellent usability.

The channel provided for the microphone cable prevents the cable from getting tangled or restricted.

Design for safety

Several safety concerns and points of interest were found:

One main issue with the stand is that once a microphone is removed from the top, the springs snap the arms with such violent force that it could injure the user of the stand while maintenance or installation is under way. This could be prevented with some sort of locking mechanism, or a hydraulic damping unit to prevent sudden movements.

All the possible sharp edges on the stand were either filleted or rounded.

Design for environment

Another important factor to consider when designing a product is its impact on the environment. It is a good idea to start analyzing environmental issues during the design process, since this is the time when the most environment-related changes can be made. The environmental impact of the product can be evaluated by running a Life Cycle Assessment (LCA), which is split into four main categories: manufacturing, transportation, use and end of life. To simplify the process for this analysis, an Economic Input-Output LCA (EIO-LCA) was used, which looks at industry sector averages in order to account for the entire supply chain.

In terms of the environmental impact of a microphone stand, most of the LCA categories can be disregarded. The use of the product does not have any inputs or outputs that affect the environment - specifically, there are no energy (or other) inputs necessary to operate the device. The end of life for most products does not typically have much of an impact as well, and transportation costs can be factored into the EIO-LCA. Therefore, this DFE analysis focuses on the manufacturing processes that are involved in the production of the stand.

After careful review of the various industry sectors that are comparable with this particular device, it was decided that since the microphone stand is mainly assembled from individual steel parts, the closest industry sector approximation would be "iron and steel mills". Although the exact manufacturing processes are unknown, typical and seemingly similar steel manufacturing processes account for more than half of the economic activity in this sector, making it a fairly comparable model to the actual product. However, one should keep in mind that since this sector represents the manufacturing of the individual components of the stand, there may also be additional environmental effects related to the assembly of the overall product, which are not included for this basic analysis.

Through an examination of the results of this model, based on $1 million worth of microphone stand products, we find that the main contributor of environmental impact in the form of economic activity in the production of this device is the iron and steel mills themselves. This contributor also tops the charts in the areas of energy and the most common toxic releases, air pollutants and greenhouse gas production. Somewhat surprisingly, power generation and supply creates only 30 percent of the amount of environmental impact produced by the iron and steel mills, which translates to only 20 percent overall. For economic activity, the same top contributer is followed by wholesale trade, truck transportation and power generation and supply, but these three factors make up only 10 percent of the total economical environmental impact, while the iron and steel mills supply almost 50 percent.

At the suggestion of the client, a deeper look was taken into more detailed manufacturing processes involved in the production of the microphone stand. Although a representative sector for stamping could not be found, the sector for "turned product and screw, nut and bolt manufacturing" was determined to be a relatively accurate model, as it consists of approximately 50 percent activity related to turned products and just under 50 percent of activity related to small fasteners, which are both largely present in this product. In this sector, the largest contributors of economic activity were the sector itself and the iron and steel mills, while the areas of energy, air pollution and contaminant gases were dominated by power generation and supply. Power generation and supply accounts for 35 percent of energy activity, while the steel mills and turned product sectors account for 20 percent and 13 percent, respectively. This breakdown of sector activity is closer to the predicted outcome, as opposed to the data from the iron and steel mills sector.

Overall, the microphone stand has relatively low environmental impact effects besides the simple manufacturing of the parts. Some additional areas of interest could include the nature of the paint or other coating on some of the individual components, and the inputs and outputs of the product assembly process. A more detailed EIO-LCA sector breakdown would be necessary in order to sufficiently investigate these effects.

Failure mode and effects analysis

Failure Mode and Effects Analysis (FMEA) is a design method used to identify and analyze modes, causes and effects of possible failures within a product or system. When used during the design process, FMEA can help to determine corrective actions that can be taken to improve the overall product, reduce downstream product development and manufacturing costs, and protect against liabilities. The true value of FMEA lies in going through the analysis process and then making the appropriate decisions.

Close documentation of the microphone stand and its components resulted in the following FMEA table. This analysis focuses on the most significant parts of the stand and the most severe potential failures, and is less concerned with failures involving small parts such as bolts and washers. Component failure modes are ranked based on their severity, probability of occurrence and detectability, and the product of these rankings is known as the Risk Priority Number, or RPN. For failure modes with a high RPN, additional methods are prescribed that may be necessary to improve end product quality, and to reduce the risk of occurrence and increase detectability controls. Assuming that these actions are carried out, a revised RPN is estimated in order to asses the level of improvement.

**Design FMEA**
Item & Function	Failure Mode	Effects of Failure	S	Causes of Failure	O	Design Controls	D	RPN	Recmd Actions	S*	O*	D*	RPN*
(03) Headless bolt Holds microphone attachment in place	Deformation or fracture	Operation impaired Microphone not rigidly secured Potentially dangerous to user	8	Fatigue; improper use of microphone attachment	5	Shear strength testing	3	120	Choose stronger material increase bolt diameter	8	2	2	32
(10) Upper spring and (27) Lower spring Counteracts weight of microphone Holds rigid positioning of frame	Fatigue deformation or fracture	Loss of positioning control Unstable and unsafe	9	Excessive use; fatigue and/or yield; stress on springs and linkages resulting in loss of stiffness and spring force	4	Shear strength testing to determine stress limits; spring stiffness and material control	6	216	Repetitive and extended use (to failure) testing Increase spring wire and coil diameters	9	2	2	36
	Springs can slip or become detached from mechanism	Stand cannot be rigidly positioned Erratic and generally inoperable	8	Incorrect or excessive usage	6	Testing spring connections	2	96	Secure connections to spring attachments and spacers	8	2	1	16
(13) Bar linkage Main frame linkage Houses XLR cable	Deformation	Difficulty with positioning Possible damage to XLR cable resulting in erratic microphone operation	7	Unintended stresses on structure	3	Strength testing; control of thickness, material and shape of bent metal	2	42	None	-	-	-	-
(17) Headless 1/2-threaded rod Connects lower and upper sections	Stress deformation or fracture	Unstable connection resulting in loss of positioning control Potentially dangerous to user	9	Excessive stress or fatigue; over-tightening; attempting continued repositioning without loosening connecting rod	5	Strength testing; material selection	4	180	Choose stronger material Increase diameter Add stop to prevent over-tightening	9	3	2	54
(25) Long 1/4-threaded rod Secures lower sub-assembly	Deformation or fracture	Instability of entire frame	7	Excessive use or stresses	5	Shear strength and load testing	3	105	Choose stronger material Increase diameter Balance out loads with small supplementary components	7	3	1	21
Metal parts	Oxidation/rust	Loss of smooth movement and control of joints and connections Decreased overall durability	6	Use under inappropriate conditions (temperature, weather)	2	Material selection; inform customers of recommended conditions for operation	2	24	None	-	-	-	-

$*$ Predicted values

As described in the FMEA table above, the most significant possible failures for this product involve the springs and the bolts that connect each major joint of the microphone stand. Since the stand is in general a basic linkage system, with the springs contributing tension and counter-moments, if any of the joints or springs fail the entire system could lose its rigidity and positioning ability. It may also become unstable, which could be potentially harmful to the user if the stand were to fall over. Based on this analysis, the major recommendations would be to focus the design controls on strength testing and re-design of each of the components in question, and to ensure that the customer has a proper source of information on the appropriate and intended methods of use of the product. Although a product such as a microphone stand is unlikely to be purchased by a customer who does not already have some knowledge about the product's manner of usage, it is important that the product and its documentation clearly communicate the design and manufacturing intent for the usage of the product in order to avoid unexpected failures.

Mechanical analysis

Figure 2. Simplified Free Body Diagram.

In order to better understand the function of the microphone stand and to determine equilibrium positions of the microphone, a simple mechanical analysis was performed. This analysis considered the simple linkages of the microphone stand in two dimensions (along the "plane" of the stand), and represents a simplified model of the stand. In addition, for the sake of analysis, it was assumed that the mass of the stand itself was negligible in comparsion to that of the microphone.

A simple free-body-diagram of the stand is shown in Figure 2. The mass of the microphone is represented by m, and is supported by two linkages of length L, with their position described by two angles, $θ$ and $φ$ . In order to appropriately analyze the stand, each of the three joints will be examined in some detail.

Joint 1

At joint 1, the mass of the microphone is directly below the joint in normal operation. Thus, there are no moments that must be resisted by this joint. A force equal to the microphone must be resisted by the clamping mechanism at the joint. That is:

$F_c = F_m = m_m \times g$

Joint 2

In order to maintain static equilibrium at joint 2, there is a required reaction force due to the weight of the microphone, as well as a required reaction moment due to the distance of the microphone from the joint.

A simple force equilibrium shows that the overall required reaction force transferred from the plate at joint 2 to the lower linkage is:

$F_2 = F_m = m_m \times g$

For the purpose of this analysis, it is assumed that this force is carried entirely in the bolt that connects the upper section of the stand to the lower section (neither bolt A nor B, but rather a third, not shown bolt). Furthermore, it is assumed these bolts only carry force, and no moments.

A moment equilibrium taken about bolt B while considering only the weight of the microphone shows that the required reaction moment around bolt B is:

$M_2 = F_m \times d_\perp = m_m \times g \times L \times \cos\theta$

Figure 3. Detail of Joint 2.

Under the assumption that the bolts at the joint create no moments, this restoring moment must be created through the springs that connect bolt B to the lower bar (Figure 3). This restoring moment is easily calculated by considering the component of the force perpendicular to the length of the upper linkage.

$F_s = k_1 \times x_1$ $F_\perp = k_1 \times x_1 \times \cos\psi$

where $ψ$ is the internal angle formed by the spring and the lower bar of this joint. Thus, the moment can be calculated by considering this component of the force and the distance from point B.

$M = k_1 \times x_1 \times \cos\psi \times \frac{L}{2}$

Please note that this relationship is only valid when the two bars are roughly parallel. Additionally, while $k 1$ is a constant, $x 1$ varies with $θ$ .

Substituting these expressions with the moment caused by the microphone to find an equilibrium expression yields:

$k_1 \times x_1 \times \cos\psi \times \frac{L}{2} = m_m \times g \times L \times \cos\theta$

This expression provides a relationship between equilibrium positions and the mass of the microphone, spring stiffness, and stand geometry. While there is insufficient information to compute a solution to this equation, it does imply a few things:

Equilibrium positions are highly dependent on the angle of the stand.
While the spring stiffness and length affects the difficulty in moving the stand, it does not appear to have as significant an effect on the actual equilibrium position as does the position of the spring attachment point along the length of the bar.
To truly gain a full range of equilibrium positions, a variable stiffness spring would be required, or there would have to be some variable moment caused by bolts in the joint. Alternately, an adjustable spring attachment point could be used.

Joint 3

As at joint 2, the connection from joint 3 to the base carries only forces and no moments. In the same way as joint 2 (ignoring the weight of the stand):

$F_3 = F_2 = m_m \times g$

The moment required to maintain equilibrium at joint 3 is found in a similar way as at joint 2:

$M_3 = m_m \times g \times L \times \left(\cos\theta + \cos\phi\right)$

Figure 4. Detail of Joint 3.

This is generated by the springs that attach the base to the lower bars (Figure 4), and specifically the perpendicular distance between the attachment on the bars and the attachment on the base. The moment generated can be determined from:

$M_3 = k_2 \times x_2 \times d_2$

As at joint 2, $x 2$ and $d 2$ are functions of $φ$ . However, as the stand rotates, the spring is initially taut, then slack, then taut again. Because of this frequent shift it is difficult to quantify variation throughout rotation. Thus, the equilibrium positions are best described as:

$k_2\left(x_2\left(\theta\right)\right)\left(d_2\left(\theta\right)\right) = m_m g L \left(\cos\theta + \cos\phi\right)$

Again, this relationship suggests a finite number of equilibrium positions, which suggests that to truly get a full range of stability for the microphone, the use of a variable spring or added moment resistance from bolt friction is required.

Design/Analysis Team

Sarah Marmalefsky, Noah Lorang, Bryan Springer, Brian Shyu

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Category: Design studies