Walker Adams manufactured off road buggy for off road carting events in Birmingham. These buggies are driven in gravelled uneven rally terrains. It's a fun event for various people from different groups. As this buggy is driven by various novice drivers on this uneven gravelled terrain small accidents occur in which the frontal tire get a side impact which in turn has the effect on the steering mechanism. Expensive parts of the steering mechanism such as the rack, rack housing and tie rod get damaged during the collision.
The aim of the project is to study the problems in the steering mechanism of an off road cart and to redesign the steering mechanism.
To study the steering mechanism used in the off road cart.
To identify the problem in the steering mechanism.
Make CAD model and do the analysis using CAD software.
Find the possible solution.
Redesign the model and test it on CAD software's.
Bending of steering inner tie rod is observed in buggies manufactured by Walker Adams. This occurs due to accidents in which there is a side impact on the buggies. As the event is focussed mostly on recreation for group of people, most of the drivers are novice and so the probability of accident increases. Due to which steering system get damaged and they have to be replaced which is not economical.
Even a small damage in the steering mechanism cannot be ignored because as there is damage in the steering mechanism the steering parts don't move through the same arc as the wheel moves vertically up and down. This phenomenon results into a problem called 'bump steer'. Bump steer causes the wheels to steer themselves without input from the steering wheels (Butcher, 2000). Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads.
In order to overcome this problem a redesign of the steering mechanism is to be carried out so that steering mechanism would be able to withstand the axial impact loads during the accident by absorbing the energy. This will cause fewer replacements of the steering parts and more comfortable handling by the customers.
The thesis is based on working with various CAD/CAM software like Catia which was taught in the academic modules. It will help in practical application of such software in realistic problem like proposed above.
It is very important for a driver of any vehicle to have control over the vehicle. It is mostly achieved by two primary types of control system: the steering system and the braking system. For the project understanding of the steering system was taken into consideration.
The aim of the steering mechanism is to make an arc such that all the wheels should travel about the same centre point (Richard Stone, 2004). Generally two types of steering mechanisms are used, parallelogram steering linkage and rack and pinion steering linkage.
Parallelogram steering linkage
Source: Â www.carbibles.com/steering_bible.html
Parallelogram steering linkage reduces the steering effort drastically so it is generally used in heavy vehicles. On the other hand rack and pinion steering is used on smaller, lighter and sports cars as it provides quick steering responsiveness to the driver.
Rack and pinion type steering linkage
Source: Â www.carbibles.com/steering_bible.html
Rack and pinion type steering linkage
Source: Â www.carbibles.com/steering_bible.html
The parts of a rack and pinion system include:
Pinion gear - The pinion gear is connected to the steering shaft/wheel. Whenever the steering wheel is turned, the pinion gear also turns.
Rack - The pinion gear is meshed with the rack housing. The rack is a steel bar mounted within the rack housing. The rack has teeth machined into it which mesh with the pinion gear. Thus, the rotary motion of the steering wheel is changed to side-to-side motion for steering control.
Tie rod - Tie rods are connected at each end of the rack. These rods transfer the rack movement to the steering arms. The rods include a feature for length adjustment during wheel alignment.
Steering arms - Tie rod connects to the steering knuckle through a steering arm, usually formed as a part of the steering knuckle. A steering arm transmits force to pivot the steering knuckle and turn a front wheel.
Damage to the steering mechanism causes various problems such as
Vehicle wanders side to side.
Excessive play in the steering.
Abnormal tire wear.
Vehicle pulls to one side.
Hard steering when the car is moving or still.
Steering wheel pulls while braking.
Shear loads acting on the shear pin can be found out either by keeping the shafts in tension or compression. By keeping the shafts in tension or compression the relationship between shear load and deformation can be figured out. The shear test results conducted on A325 and A490 bolts are shown in the figure below.( (Kulak G, 2001)
Source: Guide to Design Criteria for Bolted and Riveted Joints2001
It can be observed that as the tensile strength of the A490 bolt is higher than the A325 bolt the shear strength of the A490 bolt is also greater. As the strength of the shear pin increases the deformation capacity of the pin decreases (Kulak G, 2001).
"The lower shear strength of a bolt observed in a tension type shear test as compared with a compression type test"
A typical shear load versus deformation curves are shown in the figure below which was tested on A440 bolts.
Source: Guide to Design Criteria for Bolted and Riveted Joints2001
Misalignment of the joints is dependent upon the geometry of the holes as well as the tolerance provided during the fabrication process. According to the test conducted by D Vasarhelyi and W Chang misalignment formed because of the hole clearance does not have huge impact on the slip resistance or the ultimate strength of the joint (D D. Vasarhelyi, 1965).
It is observed that the effective life of the mechanical spring is ended by yielding; fracture is very rare mode of spring failure. Elastic limit of the spring material has a impact on the properties of the mechanical spring. For material selection and manufacturing of the mechanical spring a general formula Ïƒf/E is used, in which Ïƒf stands for the stress in fracture and E is the young's modulus of the spring material. The value of Ïƒf/E should be generally higher for the material used for manufacturing.
In material selection of the spring it is very essential to follow proper guidelines and design consideration when designing springs. Common steps used for material selection in spring design are as follows,
Shock loads, high endurance limits and high strength.
Walsh, R. (2001). Spring calculations die and standard types. In:Â Handbook of machining and metalworking calculations. New York: McGraw-Hill. 10.1.
A model having a imaginary centreline which shows the coiled spring is generally used for the analysis of a helical die spring, this model was given by Haringx. However certain aspects like the pitch and the wire curvature were not taken into consideration. In 1939 further investigation was carried out by Biezeno and Grammel which resulted in local stiffness of a point along the centreline.
Carl Fenstermacher suggests using friction springs for applications involving shock absorbing impacts. He argues that friction springs are better than conventional springs as in friction springs the tensile stresses in outer rings and compressive stresses in inner rings are almost constant while in conventional springs maximum stresses are developed at outer edges while the core of the spring is unstressed. (Fenstermacher, 2008)
Die springs are helical compression springs that are made from rectangular wire. For the same deflection, they can carry roughly 30% more load than compression springs that use round wire. Die springs are used mainly in die machines, but are suitable for a variety of high-static applications where maximum cycle-life is important. Applications include brakes, clutches, agricultural equipment, and aircraft components. Most die springs are made of oil-tempered and chrome-alloy metals. Because surface defects can significantly reduce a spring's fatigue life, the spring wire may be specially-treated and cost more than commercial-grade materials. Chrome-silicon wire has a lower tensile strength than commercial-grade music wire, but features a higher maximum service temperature.Â
Die springs carry specifications for inner diameter (ID), outer diameter (OD), and wire diameter, as well as free length, solid height, spring rate, and end configuration. Free length is the overall length of a die spring in the unloaded position. Solid height is the length of a die spring when under sufficient load to bring all coils into contact with adjacent coils. Spring rate is the change in load per unit deflection. Usually, this specification is expressed in pounds per inch. There are two basic end configurations for die springs: closed and open. With closed die springs, the space between the coils is reduced to the point where the wire at the tip makes contact with the next coil. With open die springs, there is no reduction in pitch at the end of the coils. Sometimes, the ends of die springs are ground to create a flat surface that is perpendicular to the spring axis.Â Â Â
Selecting die springs requires a careful analysis of application requirements. Service conditions, operating temperature, and physical stops are all important considerations. Parameters for die springs used under cyclic conditions also include minimum and maximum operating loads, deflections, and heights. In some cases, a die spring that has an infinite life under low-deflection conditions can take a set if compressed to solid height. Die springs that are designed for static applications should provide stable spring force output over time. Die springs that are designed for cyclic conditions should also be able to survive the intended life without breaking.Â (http://mechanical-components.globalspec.com/LearnMore/Mechanical_Components/Springs/Die_Springs)
Spring Terminology -
Compressed LengthÂ -
The length of the spring after operating force has been applied. The compressed length is computed by subtracting the initial compression and the operating travel from the free length.Â
Amount of change in spring length when force is applied.Â
Elastic Limit -Â
The maximum compression stress that a die spring can endure without taking permanent set.Â
Free LengthÂ -
The length of a die spring before it is subject to any operating force or load.Â
This identifies the outside diameter of the die spring. Product of springs are available in eight different hole sizes matched to standard drill sizes. Each spring is made to fit in the hole so the O.D. of the spring is actually less than the hole diameter.Â
This is force built up by compressing the spring. Load is also directly related to stress. As the spring is compressed, load is generated and stress on the coils increases.Â
Operating TravelÂ -
The distance which is subtracted from the spring length after operating force has been applied.
Permanent SetÂ -
This happens when the elastic limits are exceed and the spring does not return to its original length when the load is released.
The distance the free length of the die spring is reduced by the pressure of the assembled tool.Â
The manufacturing process of closing a compression spring to solid to eliminate load loss in operation.
This is a nominal identification of the inside diameter of the die spring. Producto die springs are available in eight rod sizes matched to standard stripper bolts. Each spring is designed to fit over the rod so the I.D. of the spring is greater than the rod diameter.
The length of a spring under sufficient load to bring all coils into contact with each other.
In a spring this describes the internal force that resists deflection under load. This force is equal to, and in opposite direction to the external load. Stress is expressed in thousands of pounds per square inch of sectional area.
There are many causes of spring failure few of them are mentioned as follows -
Over stressed (load loss)
Insufficient or no pre-load
Operating frequency is high
Changing the material of the existing tie rod can also be looked as an option. According to Michael Ashby of Cambridge University a good material for light and stiff tie rod will be having higher 'specific stiffness', E/Ï, where E is Young's modulus and Ï the density. (Ashby, 2000)
For impact loading the stress developed in the rod is a function of its volume irrespective of whether this volume is made up of a long rod of small area or a short rod of larger area is stated buy Robert Juvinall and Kurt Marshek (Robert C. Juvinall, 2005)
Today designers can perform FEA analysis of the design to check the effects on the design strength performing various design iterations using advanced FEA packages like ANSYS and I-DEAS. FEA analysis for buckling and geometric nonlinear analysis of a tie rod was conducted by George Campbell, Wen Ting of Ford Motor Company and Peyman Aghssa, Claus Hoff of MacNeal-Schwendler Corporation the result showed that both nonlinear static and nonlinear buckling analysis gives a buckling load which is close to the experimental buckling load. (George Cambell)
According to Lakshmininarayana developing an appropriate model for finite element analysis is very important as numerical solutions are provided for the models in a finite element method, so it is very important to assess the accuracy of the solutions. If this given criteria is not satisfied then the finite element model has to be corrected until sufficient accuracy is gained. (Laxshmininarayana, 2004)
In design engineering and manufacturing engineering CAD/CAM increases the productivity, minimise the cost and improves the quality of the product. According to John Stark use of CAD/CAM may be a strategic tool for the company in producing a new type of product (Stark, 1986).
Finite Element Analysis
Comparison and Evaluation
Evaluation of Design
Shear Pin Design
This section gives the details of different steps carried out for the design, analysis.
The main study was to evaluate the design made by Steve Adams. For doing so, the failed parts of the steering mechanism like the rack, rack housing and the tie rods were studied. The reason and trends for the failures were observed. Previously made CAD design models were also studied and any changes required were taken into consideration.
To initiate the design process CAD models of the rack and pinion type steering mechanism were made using Catia V5. The design was made of the actual steering mechanism used in the off road buggies. Various aspects like direction of the loads, bending moments, impact loads were also studied.
Mechanism of the first type of design model which was shear pin type was taken into consideration. Materials used for the pins were studied and also additional research for the different materials was carried out. These materials were compared to get the optimum material useful for the design. After the material was chosen theoretical calculation were made to get the shear load capacity of the pin for the chosen material. A CAD model of the design was made using Catia V5. The designed model was tested with finite element analysis software to see the stress distribution in the model. Any changes required in the CAD model were made after studying the FEA results. A final model was selected after the process.
The pins were tested on a manual universal testing machine to get approximate value of the load bearing capacity. After having satisfactory results the pins were again tested on a testometric which is a computerised universal testing machine, the results of the tests were evaluated to get the final result.
The results obtained from the hand calculations, FEA and experimental testing of the pins were compared with each other.
Second model comprising of the spring mechanism was studied. To initiate the design procedure of the spring various types of springs which can be used in the design were compared with the spring used in the design. After the spring was selected various spring manufacturers catalogue were observed. List of materials used for the spring were studied to find the ideal material required for the design. Hand calculations were made in order to find the axial load carrying capacity the selected spring. A CAD model was made using Catia V5 CAD software. The designed model was tested with finite element analysis software to see the stress distribution in the model. Any changes required in the CAD model were made after studying the FEA results. A final model was selected after the process. The two outcomes viz. Hand calculations and the finite element analysis were compared with each other.
Finally a comparison was made between the two design models and conclusion was stated. Any further modification or recommendations were also given.
Tools used -
This section gives the brief description of the various tools used in this thesis.
Catia V5 -
Catia V5 is a generally referred as 3D PLM software as it supports various stages of product development from conceptualization, CAD, CAM and CAE. Use of Catia V5 as a FEA software is also gaining popularity. This software is very popular with various industries like aerospace, automotive, shipbuilding etc. Notable companies like Boieng, BMW, Porche, GD electric boat etc to name few uses the package for design development stage to manufacturing stage. Catia produces a 3 dimensional model which is incorporated with many manufacturing machines like the modern CNC's to produce the final product.
Manual Universal testing machine -
Avery Denison machine for tensile and compressive test having the capacity of 25kN was used. The apparatus was setup as to test the shear force acting on the pins. This process gave an approximation of the shear load carrying capacity of the pins for various diameters.
Computer controlled universal testing machine -
Testometric computer controlled universal testing machine was used for the testing of the shear on the pins. The machine has many advantages over the manual Avery Denison universal testing machine which was used previously, which includes the precision, computerised generation of graphs etc.
+/- 0.5% of reading down to 1/1000th of load cell capacity.
Vertical space mm
Crosshead travel / resolution mm
980 by 0.001
0.001 to 1000mm/min
+/- 0.1% under stable conditions.
Spigot Size mm
Universal input. (de-rate max speed at 115V)
First design model-
In the first design model the tie rod was modified to a hollow shaft and a solid shaft mechanism as shown in the figure below. In both the shafts a two holes were drilled in order to accommodate the shear pins. Shear pins were designed to absorb the shock and to break on excessive loading. The shear pins breaks to preserve the rest of the assembly which is the main advantage of the pins. It is very economical and easy to change the shear pins than the major parts of the assembly. The figure below shows the actual model designed by Steve Adams for shear pin mechanism.
Second design model -
In the second design model spring was introduced between the tie rods on both sides. The spring chosen was a helical compression spring. To avoid the buckling phenomenon on the springs during axial load a rod was introduced inside the inner diameter of the spring. A spring system has an advantage that it regains its position after the load is released, therefore it is not needed to change the spring after each incident. The figure below shows the actual model designed by Steve Adams for spring mechanism.
The basic function of any mechanical spring is to store energy elastically. Therefore the selection of spring material that displays the property of elasticity (the ability to return to the original shape and dimensions when applied forces are removed) must be considered for use when manufacturing a spring. Die springs are produced by cold forming processes. It can be produced with or without subsequent heat treatment.
Chromium Vanadium Alloy Spring wire.
This is used for conditions involving shock and fatigue and where temperatures are somewhat higher than normal (up to 220 C). These steel contains carbon (0.45 to 0.55), manganese (0.6 to 0.9), chromium (0.8 to1.1) and vanadium (0.15 to 0.25 %) with small percentage of silicon. (A Text book of machine design, Rajendra Karwa. Laxmi publications (p) ltd isbn - 81-7008-833-X)
Chromium Silicon Alloy Spring wire.
This material contains carbon (0.5 to 0.6), manganese (0.5 - 0.8), chromium (0.5 to 0.8) and silicon (1.2 to 1.6 %). This spring's are resistant to set when used moderately elevated temperatures (up to 250 c). (A Text book of machine design, Rajendra Karwa. Laxmi publications (p) ltd isbn - 81-7008-833-X)
For loads acting on the spring the factors taken into consideration are load and the deflection. For materials used for static and impact loads properties of materials like high elastic limit and high impact values are taken into account.
CAD Designs -
First design model -
Use of shear pins -
Figure below shows the modified tie rod for the first design model. In this model holes were drilled in order to accommodate the shear pin.
Assembly of the design -
Final assembly of the first design model -
Second design model -
Final assembly of the second design model -
Results and calculations:-
Shear strength of the shear pin for double shear joints under load.
Double shear failure
The first test was carried out on Aver-Denision machine which is manual universal testing machine. The result of it gave the approximation of the shear load capacity of a single pin which was around 8000 N.
Then the test was carried out on computerised universal testing machine testometric, Total three tests were carried out on the testometric for a single pin of diameter 3.5mm. The results obtained are shown below -
Test 1 -
Result of test 1 showed that peak load in shear for the single pin was 9837N.
Test 2 -
Test 3 -
The ultimate shear load on pin -
Fusp - Ultimate shear load on the pin (N)
Ïƒsup - Ultimate shear stress of the pin material (N/mm2)
Dp - Diameter of the pin (mm)
C - Spring Index
d - wire diameter (m)
D - Spring diameter (m)
DiÂ - Spring inside diameter (m)
DoÂ - Spring outside diameter(m)
E = Young's Modulus (N/m2)
F = Axial Force (N)
FiÂ = Initial Axial Force (N)Â
G = Modulus of Rigidity (N/m2)
KÂ WÂ = Wahl Factor
L = length (m)
LÂ 0Â = Free Length (m)
LÂ sÂ = Solid Length (m)
nÂ tÂ = Total number of coils
n = Number of active coils
p = pitch (m)
Ï„Â = shear stress (N/m2)
Ï„Â iÂ = initial spring stress (N/m2)
Ï„Â maxÂ = Max shear stress (N/m2)
Î¸Â = Deflection (radians)Â
D = Mean Diameter of spring(mm)
b = Largest section dimension(mm)
t = Smallest Section dimension(mm)
n = Number of Active turns
F = Axial Force on Spring
KÂ 1Â = Shape Factor (see table)
KÂ 2Â = Shape Factor (see table)
KÂ WÂ = Wahl Factor (see table)
C = Spring Index = D/(radial dimension = b or t)
Î´Â = linear deflection (mm)
Wahl factor corrects for the transverse shear effect and the curvature.
In which corrects for curvature and corrects for transverse shear (Allen Strickland Hall, 1961).
Strain Energy U = T2 I/2GJ + F2I/2AG
T = FD/2
I = ð›‘Dn
A = ð›‘d2/4
Applying Castiglianos theorem for finding the total strain energy
As spring index C = D/d;
Generally is approximated to 1, thus can be ignored.
Spring rate =force/ deflection
In practice generally approximates to 1, thus can be ignored.
Spring rate for spring with rectangular cross section
Stress for spring with rectangular cross section
Table for shape factor:-
Comparing both the designs shows that the design of the shear pin mechanism is stronger and can absorb higher load preventing damage to the steering parts like the rack and the rack housing. Also changing the shear pin is very economical.
On the other hand second design model of spring can take a lesser load and but this can be increased by using a spring of higher stiffness or load bearing capacity. As the spring does not fail in every incident of crash it is economical than the shear pin mechanism.
Having a look at both the design mechanisms it is concluded that both the models serve the purpose of absorbing the external loads and protecting the steering mechanism parts like the rack and the rack housing in an economical way. But the second design model of the spring mechanism proves to be advantageous as it does not have to be replaced after each and every incident of accident and the spring gets back to the original position after the external load is stopped.
Stress depends upon the volume of material in the springÂ within a given load and deflection. If you can increase the volume of material in your spring you will decrease the stress level. You can do this in several ways -
Increase the number of active coils
Increase the wire size
Increase the outside diameter
Use a rectangular wire or nested springs