Steering rack
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Contents |
Executive Summary
Post-Production Stakeholders
Usability Study
During normal operation of a motorsport-style steering mechanism, the user grasps the wheel at positions horizontally opposed across the wheel center at its straight-ahead (neutral steer) orientation. This hand position on the wheel does not change during operation. Pulling down with one hand and pushing upward with the other rotates the wheel; this rotational motion is converted to rectilinear by the steering rack and used to exert a moment around the kingpin. This alters the angle of the front tires relative to vehicle centerline and thus exerts a moment on the entire vehicle.
Two key pieces of information are also relayed to the driver through the steering: surface quality and approaching grip limit. Surface quality is usually relayed through textural feedback; surface roughness and bumps exert forces on the tires. These carry through the steering mechanism and are perceived by the driver through steering wheel vibration.
The mechanism by which grip is relayed is more complicated. As lateral g-loading increases, the tires produce a self-centering moment which first increases and then decreases. Peak grip occurs just after the maximum centering moment. The driving perceives this moment as a force acting counter to his inputs at the steering wheel.
Two installations of the existing product were studied: one which had been in service four years, and one that had been in service two. They were otherwise mechanically identical. Overall, user satisfaction with the product was low; a number of key issues were noted by most or all users.
Regardless of age, complaints common to both racks centered on high effort, a lack of steering feel, and a relatively slow steering ratio. This is indicative of high friction levels within the mechanism when under load, as a slow ratio will usually result in lower effort and friction damps the small motions in the wheel which usually communicate surface condition and grip level to the driver.
Both racks also demonstrated inappropriate levels of free play; however, the older mechanism was drastically more compromised. This indicates excessive wear in the mechanism, possibly due to the steel pinion abrading the aluminum spur rack. Although not immediately hazardous, this wear does severely compromise performance.
Some broader ergonomic issues were also noted. Since more than ninety degrees of steering wheel turn are available in each direction and the wheel passes very close to the user’s legs, some drivers needed to remove one hand from the wheel to fully rotate the wheel. Additionally, the position of the complete assembly in the car can result in impacts with the driver’s lower legs, which causes minor bruising.
Functionality
The steering rack produces linear motion from an applied torque to the pinion. The pinion is supported by two bushings housed in the Mounting Block. The teeth on the pinion engage those on the rack which translates linearly between two bushings separately housed in the Mounting Block. The rack has extensions and clevises to reach the tie rods on the chassis which in turn rotate the wheel, thus steering the car. The pinion is designed to attached to a steering wheel so that the driver need not grip and turn the pinion directly, as they would be able to produce little torque.
Bill Of Materials
Figure 1: Steering Rack Sub Assembly
This assembly was originally a stock Prowerks Aluminum Steering Rack altered by Carnegie Mellon's Formula Society of Automotive Engineers to better fit an FSAE race car. We then disassembled their altered steering rack into its individual components and sub-assembilies. The components, and some basic information about each one, are listed below.
| Part No. | Part | Qty. | Wt. (g) | Function | Manuf. Proc. | Material | Picture |
|---|---|---|---|---|---|---|---|
| 001 | Pinch Gaurd | 2 | 64 | Protects Pinch Points | Extruded, Welded, and Milled | Aluminum | 
|
| 002 | Clevis | 2 | 33 | Connects steering rack to tie rods | Milled | 6061 Aluminum | 
|
| 003 | 5/16-24 Bolt (Standard Purchase) | 2 | 13 | Bolts clevis to steering rack | Forged | Steel | 
|
| 004 | Pinion | 1 | 99 | Convert rotational motion to translational motion | Machined | Steel | 
|
| 005 | Rack | 1 | 167 | Works with pinion to convert motion. Connects left and right toe links | Machined | Aluminum | 
|
| 006 | Extension | 2 | ~71 | Extend rack to reach tie rods | Extruded and Lathed | Aluminum | 
|
| 007 | Set Screw (Standard Purchase) | 1 | <1 | Retains primary bushing | Tapped | Steel | 
|
| Sub-Assemblies* | |||||||
| 001 | Retaining Block/Mounting Block | 1 | 189 | Retains primary bushing | CNCed | Aluminum | 
|
| 002 | Pinion Bushing | 1 | 21 | Smooths pinion rotation | Machined, Extruded, and Assembled | Aluminum, Bronze, Rubber, and Steel | 
|
` *Sub-Assembilies were not deconstructed further. It was decided that doing so would result in the destruction of some components with little or no knowledge gained.
Figure 1: Retaining Block sub-assembly with components labeled.
Figure 2: Pinion Bushing sub-assembly with components labeled.
DFMA
FMEA
Design for Environment (DFE)
Designing a product for minimal environmental impact is a vital component a professional engineer must consider. The ASME Code of ethics says the following in Canon #8 regarding environmental impact:
"Engineers shall consider environmental impact and sustainable development in the performance of their professional duties."
To understand the full impact on the environment our product will create, an Economic Input-Output Life Cycle Assessment (EIO-LCA) was completed. The EIO-LCA takes into account Manufacturing, Transportation, Use and End-Of-Life. eiolca.net was utilized to create out EIO-LCA for the steering rack. The tool took information for the 1997 Industry Benchmark US Department of Commerce EIO Model and calculated many different parameters of interest.
The model can take into account the different impacts on Manufacturing, Transportation, Use and End-Of-Life. For our application of the Steering Rack, we only considered the environmental impact of the manufacturing phase because the vast majority of impact occurs then. In terms of transportation, the steering rack is relatively lightweight object that only gets transported once between sectors in its lifetime. From the aluminum and steel manufacturer to the machine shop to the end user. This product does not require heavy transportation nor continuous transportation. Therefore, we decided to neglect the transportation sector. In terms of use, there is no complimentary products or energy to maintain and keep the steering rack functional besides negligible amounts of oil and grease. Finally, End-Of-Life was neglected due to the fact that after the steering rack and ultimately, the car, have served their purpose, they either get disassembled and reused or dissassembled and used as a teaching tool for younger members. Alternatively, the car may be sold complete to an interested alumni. Therefor, End-Of-Life considerations are minimal.
Part 1: Motor Vehicle Parts Manufacturing
In the EIO-LCA, Steering racks fall under the category "Motor Vehicle Parts Manufacturing." More specifically, a steering rack in sector 336330: Motor Vehicle Steering and Suspension Components Manufacturing. Note: all of the following figures and numbers are based on a $1 million worth of products.
Figure #: Economic Activity in the Motor Vehicle Parts Manufacturing Category, based on $1 Million
Figure #: Conventional Air Pollutants in the Motor Vehicle Parts Manufacturing Category, based on $1 Million
Figure #: Greenhouse Gases in the Motor Vehicle Parts Manufacturing Category, based on $1 Million
On all of these figures, Iron and Steel Mills makes up a considerable percentage of all emissions, much more than aluminum. If you look at the bill of materials, you notice that only 1 major component and 2 minor components are made up of steel, while the majority of the remaining components are aluminum. Because of this, and the fact sector 336330: Motor Vehicle steering and suspension components only make up 5% of the Motor Vehicle Parts Manufacturing category. This is shown in figure #:
Figure #: Composition of Motor Vehicle Parts Manufacturing Category, by percentage
Part 2: Motor Vehicle Parts Manufacturing - Part by Part
Because of the high amount of aluminum in our steering rack compared to most steering racks and the rest of the category, we decided the break down the product into more detailed impact sectors. This is how it was broken down:
1. Aluminum Extruded Manufacturing: This category takes into account raw materials and extruded aluminum pieces that contribute to the Pinch Guard, Rack, and Extension. Money spent in this sector: $100
2. Iron and Steel Forging: This category takes into account the raw materials for the pinion, set screw, and bolts. Money spent in this sector: $60
3. Machine Shop: This category takes into account the final machining that must be completed for the product to function. This includes work to the extensions, clevis, rack, extension, retaining block, and pinion. Money spent in this sector: $131.61
Note: The raw material used to create the retaining block is not reflected in this analysis due to its minor differences from the extruded aluminum category.
To properly estimate the environmental effect of each sector, estimations were made as to how much capital was spent in each sector to make the final product, which sells for $291.61. The estimations and final calculations are shown in figure #, at the bottom of the section.
Figures #, #, # show the Greenhouse Gas for each of the three categorys and figure # shows the a graphical represention and each categories contribution to the whole effect.
Figure #: Greenhouse Gases in the Extruded Aluminum Category, based on $1 Million
Figure #: Greenhouse Gases in the Iron and Steel Forging Category, based on $1 Million
Figure #: Greenhouse Gases in the Machine Shop Category, based on $1 Million
Figure #: Percentage of Contribution to Greenhouse Gases
Part 3: Primary vs. Secondary Aluminum Production
In this part, we will compare the greenhouse gas emissions of the use of primary aluminum production and secondary aluminum production. Assuming that all the money used for the aluminum extrusions and other aluminum parts in the steering rack, which was estimated at $100, in 2009, is used for aluminum production. The following two categories were compared:
1. Primary Aluminum Production - Sector 331312 2. Secondary Smelting and Alloying of Aluminum - Sector 331314
For the same amount of capital product produced, the primary aluminum sector produces more than six times the amount of implied Greenhouse Gases than secondary production. With this outstanding difference, if we replace Aluminum Extruded Manufacturing with Secondary Smelting and Alloying of Aluminum, the amount of greenhouse gases produced would be reduced by 32%.
Summary
The following table is a summary of all the data for the implied environmental impact of the Steering Rack.
Mechanical Analysis
Two types of mechanical analysis were performed on the steering rack. The first type looked at the motion of the steering system, particularly the change in toe angle as the steering wheel is rotated. Secondly we looked at the forces exerted in the steering system as a result of the driver steering the car.
Motion Analysis
For this portion of the mechanical analysis we used OptimumK, a program specially created to analyze the kinematic motion of automotive suspension systems. This program requires the user to input the location of each suspension point in order to define all kinematic members such as the a-arms, steering tie rods, and the steering rack itself. The program then uses an iterative solver to solve the equations for this extremely complex geometric problem.
In this case the steering motion was modeled as the steering wheel is turned from 0 degrees to 135 degrees, approximately the motion on the existing steering setup. The results from this study show that at the full steering lock the front wheel will be steered 23 degrees from center. Additionally, the steer curve indicates that the rate of steering increases slightly as the system is moved through its motion. This can be beneficial to the driver as the car will be more stable when traveling in a straight line but still have enough maximum steer angle to make the tightest corners.
Force Analysis
Team Member Roles
Eric Blood: DFE
Andrew Charters: Mechanical Analysis
Graydon Loar: Bill of Materials, DFA, DFM
Andrew Simmons: Executive Summary, User Study, FMEA
===References===
