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

Image:Assembly10_SteeringRack.JPG

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 Gaurd264Protects Pinch PointsExtruded, Welded, and MilledAluminum
002 Clevis233Connects steering rack to tie rodsMilled6061 Aluminum
003 5/16-24 Bolt
(Standard Purchase)
213Bolts clevis to steering rackForgedSteel
004 Pinion199Convert rotational motion to translational motionMachinedSteel
005 Rack1167Works with pinion to convert motion. Connects left and right toe linksMachinedAluminum
006 Extension2~71Extend rack to reach tie rodsExtruded and LathedAluminum
007 Set Screw
(Standard Purchase)
1<1Retains primary bushingTappedSteel
Sub-Assemblies*
001 Retaining Block/Mounting Block1189Retains primary bushingCNCedAluminum
002 Pinion Bushing121Smooths pinion rotationMachined, Extruded, and AssembledAluminum, 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

Design for Manufacturing

The steering rack is composed of relatively few materials. Barring any specialized alloys, we agree that most of the materials are aluminum, steel, and bronze. After inspection, we were able to differentiate which component was made of each metal based on mass properties and component functionality.

It is evident that the construction of the steering rack is a shop job. Because of the low demand, production volume is hopefully extremely low, thus eliminating the need for costly molds and setup costs from other production systems.

There is no doubt that each component is manufactured using a subtractive process, then the whole assembly is obviously a joining of all those components. The most elaborate of the parts is clearly the pinion, which can be fashioned out of bar stock that has been extruded in the same pattern as the teeth on the pinion and then turned down, or the teeth could be cut out separately. It is not clear which process was used but it is evident that the process is subtractive in nature. The mounting block would be an extremely difficult component to machine by hand, this is why we believe it to be CNCed, which is also a subtractive process. All other components are turned, tapped, died, and/or milled to specifications.

There were no exposed sharp edges to flag as saftey concerns. The pinch gaurds protect the manufacturer as well as the end user from incurring any injuries as a result of the teeth in the rack sliding through the mounting block. The corners on the clevis, however, are exposed and could pose a saftey hazard. Although, it seems unlikely that during the manufacturing process anyone could sustain a serious injury from bolting on the Clevis, this could be improved during a redesign.

There are few parts to machine and only two sets of fasteners. The only two that can be eliminated are those connecting the extensions to the rack, and that is accomplished by integrating the rack and extensions into a single, longer, rack.

The rack is comprised of the same comonents on each side, that is, one side mirrors the other. This allows for as much standardization in the manufacturing process as possible. The tolerances do not appear to be extremely tight, there is a significant amount of slop when turning the pinion before it engages the first tooth in the rack. However, we were unable to remove the bushing fit into the mounting block, this could be a result of their tolerance or perhaps they were adhered in. Ultimately, it is a simple design, a circular gear engages a linear gear to transform rotational energy into translational energy.

Design for Assembly

Many components are press fit together, we noticed this when trying to take them apart. The pinion had a hole through the exposed portion which could function as a slot through which to slide a rod to pull it and the pinion bushing out of the mounting block, which was convienent. Also, the threadings in the assembly were a simple way to attach the different components to one another.

The extensions could be called unnecessary except that they are required to modifiy the store bought components to fit the desired vehicle. They can, however, be eliminated from a redesign should manufacturing of the components be redesigned to specifically meet the end-users specifications. They also utilize threads, which always have the danger of cross-threading when assembling the components.

There is only one way in which to assemble the components (so that they accomplish the desired function). The rack needs to be inserted into the mounting block, then the pinion such that its teeth line up with those of the rack. This is followed by the pinion bushing which is secured by the set-screw. The extensions and clevises can be added as soon as the rack is in place, but only after the pinch gaurds have been inserted. All these components can be assembled in one orientation of the mounting block, however the assembly is small and light enough that it would not be difficult to jump back and forth between multiple orientations.

The pinion bushing is a sub-assembly which can be designed and manufactured elsewhere.

DFMA Summary

Keeping in mind that this rack is a modified design of a rack intended for a different vehicle, it is well suited for both manufacturing and assembly. The parts are similarly manufactured at a volume that allows for extreme specialization. Materials are chosen such that they are well suited for machining as well as simplification of the assembly and keeping the overall weight of the system down, which is important to the end-user. If the product were specially designed for the size vehicle that the end-user intends, then there are simplifications that can be made to aide in both the manufacturing and the assembly of the product. Materials can be re-evaluated to further lighten the system and a few of the components (namely the extensions and the gaurds) can be eliminated or integrated into existing components.

FMEA

Image:SteeringRack_FMEA.jpg


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. Overall, the impact of the steering rack on the environment is minimal, but can be easily decreased to an even lower amount by changing to secondary aluminum. At this point, there should be enough aluminum being recycled that we can assume a continuous supply of secondary aluminum.

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, DFMA

Andrew Simmons: Executive Summary, User Study, FMEA


References

Tilley, Alvin R. The Measure of Man: Human Factors in Design. Wiley. 2002.

Essma, Skip. Steering Effort Analysis of a Oval Racing Track Setup Champ Car. 2000 International ADAMS User Conference. 2000.

Economic Input-Output Life Cycle Assessment. Green Design Institute. Carnegie Mellon University. 20 Sept. 2009. <http://www.eiolca.net/cgi-bin/multimatrix/use.pl>.

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