E xa m en sa rbe te DEVELOPMENT OF A NEW CONCEPT FOR A V-STAY IN A HEAVY VEHICLE USING DYNAMIC ANALYSES
Bachelor Degree Project in Mechanical Engineering G2E, 30 credits
Spring term 2021 Lisa Hansson Mikaela Johansson
Supervisor: Kent Salomonsson Examiner: Tobias Andersson
Abstract
Society of today is struggling with both large amounts of emissions as well as congestion on the roads.
For this reason, AFRY in collaboration with Volvo GTT is working on develop and implement longer and heavier transports in traffic network. These combinations are called high capacity transport and have high performance-based demands. Dynamic stability is one demand that can be improved for the DUO- CAT, which is a high capacity transport combination. The hypothesis is that a displacement backward in the direction of travel of the v-stay can improve the dynamic stability. The v-stay is a component of the rear wheel suspension and has an important function regarding dynamic stability by absorbing lateral forces. To achieve better dynamic stability, the goal is to create counter steering on the rear axle of the DUO-CAT through small design changes on the v-stay. The suggestionfrom Volvo is to move the v-stay backward in the direction of travel, which in this thesis has become the focus in both concept generation and design work.
The thesis includes development of new concepts of the v-stay. An extensive evaluation process consisting of dynamic analysis was carried out in PTC Creo Parametric, which made it possible to compare the new concepts with the current v-stay. An important part of the thesis is to obtain a simplified model that simulate the physical conditions. The delimitations are to examine lateral acceleration with load on the axle and friction between asphalt and wheels. The maneuver in the analyses emulates a quick lane change at 80 km/h.
This has resulted in a new concept that includes the current v-stay where only the position on the frame and axle is changed with the help of new fastening components. The new concept provides an increased counter steering of 6%.
The conclusion is that a displacement backwards in the direction of travel of the vehicle gives an increased counter steering. Future work is required to achieve the desired improved steering and safety requirements.
Keywords: heavy vehicle, high capacity transport, dynamic stability, wheel suspension, v-stay, dynamic analysis.
Certification
This thesis has been submitted by Lisa Hansson and Mikaela Johansson to the University of Skövde as a requirement for the degree of Bachelor of Science in Production/Mechanical Engineering.
The undersigned certifies that all the material in this thesis that is not my own has been properly acknowledged using accepted referencing practices and, further, that the thesis includes no material for which I have previously received academic credit.
Lisa Hansson Mikaela Johansson Skövde 2021-05-17
School of Engineering Science
Acknowledgements
We would like to thank AFRY and Volvo GTT for making this thesis possible. A special thanks to the following who contributed to this thesis and provided us with knowledge, information and support along this thesis:
Emil Olsson - Supervisor, AFRY Lena Larsson - Volvo Trucks Emil Pettersson - AFRY Jan Hendriks - Volvo Trucks Anders Olsson - Volvo Trucks Niklas Fröjd - Volvo Trucks Stefan Preijert - Volvo Trucks Per-Axel Ohlsson - AFRY
Kent Salomonsson - Supervisor, Högskolan i Skövde Assar Jarlsson - Kinnarps
Table of Contents
1 Introduction ... 1-7 1.1 Project background ... 1-7 1.1.1 AFRY ... 1-8 1.1.2 Volvo Trucks ... 1-8 1.1.3 Kinnarps ... 1-8 1.2 Problem Statement ... 1-8 1.3 Purpose ... 1-8 1.4 Goal ... 1-9 1.5 Delimitations ... 1-9 1.6 Overview ... 1-9 2 Theoretical Frame of Reference ... 2-10 2.1 DUO-CAT ... 2-10 2.2 V-stay ... 2-11 2.3 Bearing capacity class and safety requirements ... 2-11 2.4 Vehicle dynamics ... 2-12 2.4.1 Roll over stability... 2-13 2.4.2 Steering ... 2-13 2.5 Product specification ... 2-13 3 Method ... 3-15 3.1 Current situation analysis... 3-15 3.2 Concept generation ... 3-19 3.2.1 Brainstorming ... 3-20 3.2.2 Morphological matrix ... 3-20 3.3 New concepts ... 3-20 3.3.1 Concept 1 ... 3-21 3.3.2 Concept 2 ... 3-21 3.3.3 Concept 3 ... 3-22 4 Concept selection and Results ... 4-22 5 Discussion ... 5-25 6 Conclusion ... 6-26 6.1 Future work ... 6-26
References ... 27 Appendix 1 Work Breakdown and Time Plan ...
Appendix 2 Density ...
Appendix 3 Stiffness ...
Appendix 4 Concept generation ...
Appendix 5 Contact pressure ...
Appendix 6 Concept 1 ...
List of Figures
Figure 1.1: Waterfall model of the report. ... 1-9 Figure 2.1: The DUO-CAT combination (Kinnarps, 2018). ... 2-10 Figure 2.2: Model in Creo over components in rear wheel suspension of the DUO-CAT. ... 2-11 Figure 2.4: A visual explanation of planes and rotations in vehicle dynamics. ... 2-12 Figure 2.5: Visual explanation of desired mechanical counter steering. ... 2-13 Figure 3.1: Subassembly one. Figure 3.2: Subassembly two. ... 3-16 Figure 3.3: Complete simplified wheel suspension in Creo... 3-17 Figure 3.4: Locations for each mechanism used in the model. ... 3-18 Figure 3.5: Shows the trajectory of the wheel suspension and placement of mechanisms. ... 3-19 Figure 3.6: Concept 1. ... 3-21 Figure 3.7: Concept 2. ... 3-21 Figure 3.8: Concept 3. ... 3-22 Figure 4.1: Frame roll. ... 4-23 Figure 4.2: Axle yaw. ... 4-24
List of Tables
Table 2.1: Product specification for v-stay, safety requirement is not addressed in the report. ... 2-14 Table 3.1: Clarify used constraints in the simplified model. ... 3-15 Table 3.2: Mechanism used and description. ... 3-18
List of Symbols
a Acceleration, m/s"
D Damping, Nmm/s
d 150 Nmm/s
F Force, Newton
g Gravitaion, 9.82 m/s"
k Stiffness, N/mm
𝑘$ 0.6k N/mm
𝑘" 0.4k N/mm
𝑘% Anti-roll bars, Nmm/rad
m Mass, unit kg
u Unstretched length of spring, mm
x Change in length of spring, mm
1 Introduction
Society of today is struggling with both congestion on the roads and large amounts of emissions. One solution to the problems is to enable transport of more goods per vehicle. High Capacity Transport (HCT) are longer and heavier vehicles that enable efficiency and decrease the environmental impact (Trafikverket, 2019). With these favorable developments, the economic aspect will be positively affected. The positive economic aspect is a result of each vehicle on the road holding more goods.
The Swedish Transport Administration is responsible for the Swedish road network. Five challenges facing the administration are: energy use, climate effects, lack of capacity and safety for people, infrastructure, goods and vehicles. Some problems that are experienced by the road transport sector are noise, emissions and queuing, therefore traffic reduction and resource efficiency measures such as HCT are highly topical. The HCT project started in 2007 as a sustainability project and the actors behind it saw the possibility to reduce fuel consumption per ton-kilometer (tonkm). Other benefits were quickly demonstrated, such as reduced transport cost, reduced number of vehicles on the roads and thereby reduced the number of accidents without increased road wear calculated in tonkm (Asp et al., 2019). The purpose of HCT is to reduce carbon dioxide emissions by loading the trucks heavier.
Higher transport capacity reduces climate impact and reduces fuel consumption up to 30%. A goal with HCT is also that the traffic rhythm should be maintained, and an increased transport efficiency should be created (Volvo Trucks, 2021-a). Another goal is that by 2030, 80% of all freight transport work will be carried out by HCT vehicles (Asp et al., 2019).
The possibilities and benefits of HCT can lead to the consequence that traffic which is currently transported on railways transfers to roads. In both Sweden and Finland there is an extensive rail network and where both countries allow high loads on roads, however, there is no evidence that the shift has taken place (Meers et al., 2016).
With higher and increasing weight of vehicles it is important that the safety of the trucks is maintained despite heavier loads. Therefore, certain safety measures are required including static stability, dynamic stability and track deviation on straight roads (TSFS 2018:40).
It means a great challenge to meet the demands and requirements of today. The thesis focuses on increasing dynamic stability for an HCT-vehicle.
1.1 Project background
AFRY in collaboration with Volvo GTT develops transport solutions which should allow for increased transport efficiency and sustainability. Both AFRY and Volvo GTT see the positive environmental aspects of longer and heavier vehicles and the trend towards climate neutral transports. A project that AFRY and Volvo GTT is a part of is the Duo2-project. Duo2 includes test driving of two different vehicle combinations, the DUO-trailer and the DUO-CAT. The DUO-CAT consists of a truck and two following center axle trailers and is the main focus in this thesis (Duo2, 2019).
Dynamic stability can be improved in the DUO-CAT. The v-stay is a part of the suspension which transmits acceleration- and braking forces to the frame and holds the axle in place laterally (Volvo Trucks Parts AB, 1991). The hypothesis is that a displacement backward in the direction of travel of the v-stay can help the dynamic stability of the vehicle.
1.1.1 AFRY
AFRY is an international company that helps their customers by working with digitalization, sustainable development and has expertise in, among other things, industry, energy and infrastructure. AFRY focuses on technology, design and consulting with a vision to provide leading solutions where the goal is to create sustainable technology and design solutions. The company's business concept is to create a more sustainable society through their work and expertise (Afry, n.d.).
1.1.2 Volvo Trucks
Volvo Trucks is one of the world’s largest brands within medium to heavy trucks, where 95% of the trucks built weigh over 16 tons. The organization’s core values are primarily quality, safety and sustainable development. Volvo Group Trucks Technology (Volvo GTT) is the Volvo AB company, which is responsible for technology research, engine development and product design within the truck business (Volvo Trucks, 2021-b). The thesis is carried out as a collaboration with Volvo GTT.
1.1.3 Kinnarps
In October 2018 Volvo GTT and Kinnarps received permission from the Swedish Transport Administration to test drive the DUO-CAT over a five-years period (Volvo Trucks, 2018). Kinnarps is a company that takes responsibility for the entire distribution chain, from production to delivery. 24 fully loaded containers leave Kinnarps factories every day (Kinnarps, n.d.-a). With Kinnarps' own logistics system that plans how to pack the truck in the best way, they reduce the transport's environmental impact from carbon dioxide emission and reduce consumption of packaging (Kinnarps, n.d.-b). Instead of traditional packaging, they use blankets and cardboard sheets as protection for the furniture, in this way 270 kg of packaging will be saved per container (Kinnarps, n.d.-a). Kinnarps' collaboration with Volvo and their DUO-CAT is also a step to further reduce the environmental impact (Volvo Trucks, 2018).
1.2 Problem Statement
For HCT combinations there are high performance-based demands, one of the demands is dynamic stability. The dynamic stability is affected by the vehicle combination layout, the tire specification and the suspension characteristics. At a quick lane change, lateral forces are generated. The thesis is about investigating how a suspension can use lateral forces to steer the axle in the vehicle's direction and thereby limit the trailer's movement.
1.3 Purpose
The thesis purpose is to, with small design changes on the v-stay, improve dynamic stability on a long and heavy combination, i.e. HCT DUO-CAT.
1.4 Goal
The goal is to evaluate and develop a suspension that can use the lateral forces generated during a quick lane change to steer the axle in the vehicle’s direction. As a result, limiting the movement of the trailer and stabilizing the combination.
• Evaluate whether design changes on v-stay provide counter steering.
• Develop a new concept for the v-stay.
• Design a concept using Creo Parametric that can be tested in the DUO-CAT combination.
1.5 Delimitations
The thesis focuses on how design changes on the v-stay can be made to achieve beneficial results for dynamic stability of the DUO-CAT, in this case on the rear axle of the last trailer. Design changes may only be made on the v-stay and cannot under any circumstances affect other surrounding parts. If the re-design of the v-stay needs new fastening solutions or similar, these parts must be designed and work with other surroundings. Volvo's suggestion is to create counter steering by displacing the v-stay backwards in the direction of travel. This meant that the thesis focus in the concept generation were on the displacement of the v-stay. In addition, the current situation analysis is performed on a simplified model with only the lateral acceleration, weight on the axle and friction between wheels and dry asphalt considered.
1.6 Overview
. Figure 1.1: Waterfall model of the report.
Theoretical frame of reference
Product specification
Current situation analysis
Conclusion Concept generation
Concept selection
& Results
2 Theoretical Frame of Reference
A feasibility study has been carried out to create a basis for the thesis. Collecting empirical data can be done in different ways, where interviewing people with knowledge in the subject and using documents of various kinds are two of them. These are included under the overall concept of qualitative research methods, which means that the methods are based on science, interpretation and analysis (Ahrne &
Svensson, 2011).
To gather information, observations were made on trucks where the v-stay is currently installed, to create an idea of position, size and function. In combination, recorded films of field tests were used, where the DUO-CAT makes a quick maneuver at 80 km / h. An in-depth work started to create a larger and broader understanding using interviews.
The main advantage of interviewing as a method is that it is flexible. During interviews, follow-up questions can be asked and in this way the answers can both be developed and deepened. The disadvantages are that it can be time consuming to both formulate good questions and to get interviews, which leads to a delay in a project. When formulating questions, it is important that they are not leading and both unspoken presumptions and valuing questions should be avoided (Bell &
Waters, 2016). The first interviews included two active vehicle dynamics at Volvo Trucks and the focus was on gaining clarity on what vehicle dynamics are and what parameters that are important for the thesis. An interview with the patent holder for the current v-stay clarified how it was designed from the beginning and its purpose.
The content within the theoretical frame of reference is based on what emerged during the interviews and different types of documents. According to Bell and Waters (2016), the choice of documents is the reliability of the sources and it is important to be critical.
2.1 DUO-CAT
The DUO-CAT combination consists of a three-axle truck followed by two center axle trailers, as is seen in figure 2.1. The combination has a length of 27.35 meters and a maximum technical gross weight of 66 ton. In addition to this the width is 2.55 meters and the total height is 4 meters, of which the swap bodies have a total height of 3.2 meters (Kinnarps, 2018). Figure 2.2 shows the rear wheel suspension in the DUO-CAT and its components with the exception of the anti-roll bars.
Figure 2.1: The DUO-CAT combination (Kinnarps, 2018).
Figure 2.2: Model in Creo over components in rear wheel suspension of the DUO-CAT.
2.2 V-stay
In 1993, Jarlsson, A, conceded a patent for the v-stay that he invented while working for Kinnarps AB, see figure 2.3. The function of the v-stay is to absorb acceleration/braking and lateral force. It is also to keep the rear axle in line without lateral forces affecting the suspension. The mounting of the center of the v-stay is placed on the rear axle and its outer attachments are placed on the frame. The attachment points enable the v-stay to follow the vertical movements of the rear axle. No maintenance is required on the two outer knots on the frame as they are supported by rubber bushings. The inventions made it possible to construct long vehicles without having the last pair of wheels swaying back and forth over the road (Jarlsson, 1993).
Figure 2.3: Shows the position of the v-stay on the DUO-CAT´s axle and frame.
2.3 Bearing capacity class and safety requirements
Every road and bridge in the public road network have a specific bearing capacity class. The classes specify the maximum weight for vehicles that are permitted for driving on roads and bridges. In Sweden there are currently four different classes of bearing capacity, where the fourth (BK4) allows vehicles of a maximum gross weight of 74 tons to be transported. The maximum gross weight also takes the wheelbase into account, which may lead to a lower allowed weight (Trafikverket, 2020). The external shape and appearance of the vehicle affects how the load is distributed on the road and hence how much wear the road is exposed to. To meet today’s heavier transports the Swedish Parliament decided to implement the newest and highest bearing capacity class, BK4 (Natanaelsson & Eriksson, 2020).
Safety requirements for vehicle combinations over 64 tons must withstand a turnable angle of at least 19.6 degrees in both directions. Also, the vehicle must be able to withstand a lateral acceleration of at least 3.5 m/s". For the dynamic stability the vehicle combination shall, at a speed of 80 km/h, have a maximum yaw velocity gain that not exceed a factor of 2.4 and the damping shall be at least 0.15.
Vehicle combinations are allowed to have a maximum track deviation of 0.4 meter when driving on a straight road (TSFS 2018:40).
2.4 Vehicle dynamics
Dynamics describes bodies motions under the influence of forces and vehicle dynamics refers to the forces, movements and changes in movement that affect vehicles. It is an engineering subject that uses terms, theories and methods from mechanical engineering, but also uses control/signal engineering and human behavioral science (Jacobson, 2020).
The degrees of freedom (DoF) for a vehicle is defined in a coordinate system and it consists of three main planes, as seen in figure 2.4. The in-road-plane (IRP) for ground vehicles is the ground plane and have the degrees of freedom longitudinal, lateral and yaw. Aside from IRP there are two out-of-road- planes (OORP), the transversal plane and the symmetry plane. Forces and moments in the IRP are longitudinal forces, lateral forces and yaw moments. The forces and moments that act in OORP are vertical forces, roll moments and pitch moments (Jacobson, 2020).
Figure 2.4: A visual explanation of planes and rotations in vehicle dynamics.
2.4.1 Roll over stability
Roll over accidents on the road are often violent and can in the worst-case cause death. According to analysis of roll over accidents, this occurs mainly for heavy vehicles. Factors such as the driver and the vehicle itself play a major role in the truck’s roll over control. It is important for the driver to be aware of the truck’s limitations and stay within them. The vehicle must be loaded correctly, and the suspension and tires must be in good condition (Winkler & Ervin, 1999). Static overturn limit is a measurement when the vehicle overturns. According to the BK4-requirement, the combination must be able to withstand a minimum level of lateral acceleration at 3.5m/s² (Fröjd et al., 2021).
2.4.2 Steering
Yaw is a rotation about the vertical axis to the right or left of its direction of motion. Analysis of the angle are performed on rigid bodies (Jazar, 2017). A truck driving forward and turning right is desired to create a mechanical counter steering on the rear axle to increase the dynamic stability of the truck and reduce oscillations, see figure 2.5.
Figure 2.5: Visual explanation of desired mechanical counter steering.
2.5 Product specification
The customer's primary needs are listed in product specifications and must contain a metric and a value of the metric. The specification provides a clear agreement on what requirements the product must achieve to satisfy the customer´s needs (Ulrich et al., 2020). It is important that the specification focuses on what is to be solved, not how. Adjustments and specifications of the requirements in the product specification may occur throughout the process, the product specification for the v-stay can be seen in table 2.1 (Österlin, 2010).
Table 2.1: Product specification for v-stay. Safety requirement is not addressed in the thesis, denoted by grayed table sections.
Product Specification v-
stay
Field Specification Requirement Desire Unit Control
method Description
General
New design Not affecting
surrounding parts Yes X Yes/No By measuring
and analysis
Design changes must only affect the v-stay, surrounding part will be maintained the same
Functions
Counter steering Axle steering 1 Deg By measuring
and analysis Axle counter-steering in the vehicle´s direction.
Movements
Vertical movement
V-stay´s vertical movement upwards
from axle 413 X mm By measuring According to drawing for v-stay
V-stay´s vertical
movement downwards
from axle 71 X mm By measuring According to drawing for v-stay
Measurements
Mounting on axle Mounting holes x4 d=19.2±0.7 X mm By measuring According to drawing for v-stay Distance between holes 95x155 X mm By measuring According to drawing for v-stay
Maximum installation
height on the axle <71 X mm By measuring According to drawing for v-stay Mounting on
frame Distance between
mounting brackets 757 X mm By measuring According to drawing for v-stay
Distance between
mounting holes 10 X mm By measuring According to drawing for v-stay
Saftey
Stiffness Lateral stiffness 15 X kN/mm By analysis According to technical
requirements for v-stay
Vertical stiffness 250 X N/deg By analysis According to technical
requirements for v-stay Stiffness end
bushings Radial stiffness (KSr) 50 X kN/mm By analysis According to technical requirements for v-stay
Axial stiffness (KSa) 2 X kN/mm By analysis According to technical
requirements for v-stay
Torsional stiffness (KSt) 61 X Nm/deg By analysis According to technical
requirements for v-stay
Cardanic stiffness (KSc) 140 X Nm/deg By analysis According to technical
requirements for v-stay Stiffness center
bushing Longitudinal stiffness
(KSlong) 50 X kN/mm By analysis According to technical
requirements for v-stay
Lateral stiffness (KSlat) 50 X kN/mm By analysis According to technical
requirements for v-stay
Leveling stiffness (KSlev) 60 X Nm/deg By analysis According to technical
requirements for v-stay
Roll stiffness (KSroll) 210 X Nm/deg By analysis According to technical
requirements for v-stay
3 Method
The chapter describes how the model for the wheel suspension is assembled, how its dynamic movement is analyzed and how the concept generation process is performed.
3.1 Current situation analysis
To see how the wheel suspension and the v-stay behave today, a current position analysis is made in the program PTC Creo Parametric.
The thesis intentions and goals are to investigate how lateral acceleration affects the v-stay and how to improve the function. Rigid body analyses are used in Creo Mechanism to analyze how the current v-stay affects the wheel suspension. In today's society the creation of a product is expected to be cost- effective and more efficient without sacrificing innovation and quality. PTC Creo Parametrics is a Computer- Aided Design (CAD) 3D modeling software in product development and is tool for achieving the demands of modern society. In Creo, parts are created, designed, and put together so that the movements correspond to reality (PTC, 2021-b).
Placement constraint and predefined constraint sets are available in Creo to easily model assemblies and enable realistic movements. Used constraints, DoF´s and description of these can be seen in table 3.1.
Table 3.1: Clarification of used constraints in the simplified model.
Placement
constraints DoF´s Description
Centered 0 Center two components in place (PTC, n.d.-h).
Coincident 0 Positions two components directly to each other (PTC, n.d.-h).
Default 0 Aligns the components coordinate system to the coordinate system of the assembly, making it locked in its origin (PTC, n.d.-h).
Distance 0 Positions two components directly to each other by a distance (PTC, n.d.-h).
Predefined
constraint sets
6DOF 6 Aligns the component coordinate system and the assembly coordinate system, which allows both rotation and translation in all axis (PTC, n.d.-e).
Ball 3 Placed through points as references. Ball includes the cone axis function, which limits the movement of the ball in two rotations (PTC, n.d.-e).
Pin 1 Aligns two axes and enables one rotational degree of freedom (PTC, n.d.-e).
Slot 4 A point that follows a non-linear trajectory in three directions (PTC, n.d.-e).
The procedure below is a description on how a simplified model of the wheel suspension is assembled.
Simplification has been done in order to focus on the v-stay and components that are not significantly affected by lateral forces has been removed. Components used are taken from Volvo's assortment in Creo and are the ones used in the DUO-CAT combination. One exception is the wheels, which are made throughout a simplification. The model is built through rigid subassemblies that only contain placement constraints.
Included parts in subassembly 1 (SA1) and the parts relative placement can be seen in the figure 3.1.
Subassembly 2 (SA2) consists of the components in figure 3.2, where relative placement also is clarified. The wheels are attached to the axle with a pin to allow rotation.
Figure 3.1: Subassembly one. Figure 3.2: Subassembly two.
The reaction rod and the v-stay combine the SA1 and SA2 with ball constraints into the simplified wheel suspension used in analysis, see figure 3.3. Since the reaction rod have the possibility of a lateral movement, a cone is used, and is determined to be 3 degrees according to the actual movements. In addition to this, a 6DOF is used in the same points, which provides attachment options for bushing loads.
Figure 3.3: Complete simplified wheel suspension in Creo.
A mechanical device that transmits motion and force from a source to an output is called a mechanism.
In Creo there are multiple functions where the capacity of the mechanisms can be tested and refined to get optimal properties (PTC, 2021-a). Used mechanisms and descriptions of these can be seen in table 3.2.
Table 3.2: Mechanism used and description.
Mechanism Description
3D contact Enable two surfaces to maintain contact and allow friction settings between two parts in different bodies (PTC, n.d.-a).
Bushing load
Bushing loads simulate movements between two bodies. The function allows to apply stiffnesses for each translation axis through the bushing (PTC, n.d-b).
Damper Simulate real forces that remove energy from a moving mechanism and dampens its motion. The damper acts in the opposite direction to the movement (PTC, n.d.-c).
Servo Motors
Creates a specific movement with one degree of freedom between two bodies (PTC, n.d.-f).
Spring Placed between two points in the model in and a stiffness is stated. The stiffness of the spring can be translational or torsional (PTC, n.d.-g).
To perform the analyses, a second assembly is created with a road set by default and the wheel suspension is placed using the constraint slot and 6DOF. In order for the wheels to have contact with the road 3D contact is applied. To imitate the trailers swap body, a box with the correct dimensions is created and placed on the frame. The load is defined by setting the density of the swap body so that its total mass corresponds to 20 ton. Further, it is important to accurately determine the density of each component in order for weight and inertia to be correct, which creates the conditions for the analysis to correspond to reality, see appendix 2.
In the DUO-CAT, the suspension consists of air bellows which are replaced with springs in Creo. The springs and the dampers have the function of enabling the frame to move up and down vertically in a realistic way. They also provide the wheel suspension and frame a chance to roll realistically. Values for the stiffness on the springs and for the bushing loads, corresponds to the DUO-CAT, see appendix 3. Values for dampers have been estimated to achieve steady state mode. The locations of the mechanisms can be seen in the figure 3.4
Figure 3.4: Locations for each mechanism used in the model.
Based on given stiffness from the air bellows, the springs change in length is calculated by the general equation 3.1. The unstretched length (u) is calculated by adding x to the tensioned length of the spring when the wheel suspension´s frame is in a horizontal position.
𝑥 =
()
=
*+) (eq. 3.1)
Initially, to create the right conditions, an analysis is made where the tires encounter the road and where the wheel suspension settles into steady state. Something that is generated by activating gravity. In Sweden, that value is estimated to be approximately 9.82m/s" (Fysikguiden, n.d). Friction always occurs when two surfaces are in contact with each other. The coefficient of friction is a value that depends on how rough different surfaces are towards each other. A variety of factors affect the friction between two materials, such as wet surfaces reducing the friction. The assumption that the surfaces are dry is made, it means that the coefficient of friction between the road and the wheel is 0.8 (Nordling & Österman, 2006).
To analyze how the v-stay behaves at lateral force a servo motor causes movement in a slot that represents a quick lane change. To counteract the roll over for the wheel suspension, there are one anti-roll bar for each of the axles. In the model, these are simulated by one torsional spring placed according to figure 3.5.
Figure 3.5: Shows the trajectory of the wheel suspension and placement of mechanisms.
HCT trucks may drive at a maximum of 80 km/h on a motorway (Transportstyrelsen, 2013), which means that the speed of the servo motor is set to 80 km/h in order to generate the actual condition of the vehicle when the maneuver takes place. The analysis lasts for 10s where the file change lasts for a time period of 4s.
3.2 Concept generation
Creating ideas and solutions requires a certain amount of creativity. Creativity is about the production of ideas, but not only in terms of quantity but also quality. To achieve a purpose and goal with creativity work, creative methods are needed (Wikberg et al., 2015).
3.2.1 Brainstorming
The brainstorming process creates and develops a large number of solutions in a short time. It is important that the rules are followed so the participants feel safe in coming up with new crazy ideas.
The rules are that the participants are not allowed to criticize themselves or others, all thoughts are okay to express, combining and improving proposals is not criticism, it is positive for further development and finally aims for quantity, not quality. Brainstorming starts with defining a theme and then all ideas are written down on a piece of paper. All pieces of paper are placed on a wall then the process is done again with a new theme or with the ideas that require further development. When all themes are finished, the proposals are evaluated and those that are not good enough are placed in an idea bank. The ideas that are considered feasible and innovative are further developed (Wikberg et al., 2015).
The session started with defining a theme. The theme for the session was a complete solution where the participants had the opportunity to come up with ideas for solutions for the design of the v-stay and its attachment points. The ideas were written down on separate post-it notes, which at the end of the session were placed on a large board. The various solutions were evaluated, and a new session was started with the aim of developing the ideas further, see appendix 4.
3.2.2 Morphological matrix
A morphological matrix is a method used to develop partial solutions based on certain criteria. The matrix consists of criteria that the solution must meet and are based on, in this case, data collection and the requirements specification. The criteria are written down one by one and then participants can write ideas for solutions for each criterion. Here are many ideas of importance. Then it is discussed how different solutions for each criterion can be combined to finally achieve different concepts. Each of these concepts is then sketched out and evaluated (Wikberg et al., 2015).
Defined criteria for the morphological matrix were attachment for the frame, attachments for the axle, v-stay geometry by form and v-stay geometry by section. Once these were defined, the participants were equipped with post-it notes where solutions for the criteria were to be noted, see appendix 4.
When the ideas began to diminish for the participants, the session ended and the post-it notes were placed in the matrix. Solutions for the different criteria were combined with each other to create several concepts.
3.3 New concepts
After the idea generation, three different concepts were further developed to be tested in the dynamic analysis. The concepts shown in the chapter are rough models, but which still enable further analytical work.
3.3.1 Concept 1
Concept 1 retains the design of the v-stay and involves minor adjustments to the side mounting brackets and a new component for the center knot at the axle. The basic idea with concept 1 is to move all components for the v-stay backward in the direction of travel and maintain as much possible of the already existing design, see figure 3.6.
Figure 3.6: Concept 1.
3.3.2 Concept 2
The idea with concept 2 is that the location of the side mounting brackets is in the same original place, but the v-stay is made longer to get a displacement backwards in the travel direction. The base of the side mounting brackets is the same, but small adjustments are made so that the angle of an extended v-stay is correct. The same component for the center knot on the axle as in concept 1 is used and enables a displacement of 95mm, see figure 3.7.
Figure 3.7: Concept 2.
3.3.3 Concept 3
Concept 3 requires major design changes and achieves a larger displacement backwards than the previously mentioned concepts. The v-stay is divided into two separate rods, which also entails design changes to all components, see figure 3.8. The concept contains four bushings, one at each end of the rods.
Figure 3.8: Concept 3.
4 Concept selection and Results
Concept selection is probably the most important choice in the whole process as the concept to be further developed is selected. In this process, 80-90% of the efficiency, performance and cost of the changed solutions are determined. Only optimizations are made after the concept selection (Wikberg et al., 2015).
To investigate how the three new concepts affect the wheel suspension, the same analysis is performed for all new designs. It is of great importance that no other parameters are changed for the analysis except the v-stay, which is a prerequisite for evaluating the generated results.
The analyses are performed dynamically and are analyzed through Creo mechanism measure results.
To understand and analyze how mechanisms move in dynamic analysis, the tool measures is used. In measures, graphs for position, velocity, acceleration, force and angles can be created (PTC, n.d-d).
Due to the predetermined maneuvering of the wheel suspension, which was manipulated through a slot, the highest lateral acceleration of the wheel axle for every analysis was achieved at 4 seconds to:
𝑎 = 5 m/s"
The simulations also showed credible results for contact pressure between wheels and road, frictional force both in the direction of travel and laterally regarding the load of 20 tons. Which is a contact pressure of 50 kN per wheel, see appendix 5.
Since the main purpose of the thesis was to create higher dynamic stability, the roll of the frame and the steering on the axle is considered to be of the utmost importance. In figure 4.1 the roll of the frame can be read out for all new concepts as well as the current v-stay. In terms of roll, no major difference is seen between the different v-stays. There is a slight reduction in the roll for concept 3, where the displacement backwards in the direction of travel is greatest.
Figure 4.1: Frame roll.
-2,5 -2 -1,5 -1 -0,5 0 0,5 1 1,5 2
0 1 2 3 4 5 6 7 8 9 10
De gr ee s
Time (s) Frame Roll
Current Concept 1 Concept 2 Concept 3
For all concepts, including the current, minimal steering is demonstrated. Figure 4.2 shows the steering for the different concepts and in its upper right corner the relationship between the desired steering (1 degree) and generated steering is emphasized. The current v-stay demonstrates a certain counter steering, which also concepts 1 and 2 demonstrates. Concept 1 shows an increased counter steering by 6% compared to the current v-stay, while concept 2 shows a reduced counter steering. Concept 3 has a comparatively higher yaw angle but generates positive steering.
Figure 4.2: Axle yaw.
Concept 1 is the most effective concept to meet the requirements of the product specification and the goal of the thesis. It does not affect any other components of the wheel suspension and utilizes the mounting requirements that exist. An exception is the maximum installation height on the axle where concept 1 has an added height of 10 mm, due to the new center mounting bracket. The requirement for vertical movement is met. As the current v-stay is used for concept 1, the safety requirements for this are also considered to have been achieved. The new fastening component for the centre knot and the adjusted side mounting brackets where the holes have been displaced 5mm forwards are shown in appendix 6.
-0,06 -0,04 -0,02 0 0,02 0,04 0,06
0 1 2 3 4 5 6 7 8 9 10
Degrees
Time (s) Axle Yaw
Current Concept 1 Concept 2 Concept 3
Counter steering Positive steering
-1 -0,5 0 0,5 1
0 1 2 3 4 5 6 7 8 9 10
5 Discussion
At the start of the thesis, interviews and meetings were conducted with knowledgeable people to get guidance on how the wheel suspension works and is affected while driving. Based on that knowledge, a deeper research was done in the significant areas. During this period, the thesis grew when engineers at Volvo explained how complex the v-stay´s function is. In order to limit the problem to a possible level for the period of the thesis, contact was made with the supervisor at Volvo. The purpose was to clarify the expectations from Volvo for our work.
A greater part of the time was spent creating a model in Creo that emulate reality as much as possible and then creating the analysis. Performing dynamic analyses in Creo mechanism generates rigid body analysis. This means that no components can be bent, rotated or deformed. For example, the frame of a truck moves slightly and is compliant when driving. This is something that is not included in our analysis.
When the analyses began to take shape, several meetings where held with engineers at Volvo. They gave great insight into the signification of all the components on the wheel suspension and what it would mean if some were removed. Simplifications are good but they must never be compromised so much that the result is unreliable. The engineers at Volvo become interested in the thesis and developed a simple model in Abaqus where they performed similar analyses. Those analyses confirmed that a displacement of the v-stay provides steering. It has been helpful and a basis for how much of a displacement we should implement to our concepts. The advantage of supplementing with different types of analyses is that outputs can be compared and the results increase in credibility.
The goal of increasing counter steering has been achieved. The increased steering is not sufficient to achieve the desired counter steering of 1 degree. This may be due to unreliability of the simplified model or simply that the rearward displacement in the direction of travel is not sufficient. Concept 3, which had a larger displacement, provided greater steering but, which was positive steering. The installation height for concept 3 was lower than both the current v-stay and concepts 1 and 2. Simple analyses have been performed to investigate whether the design height has an impact on whether it generates positive steering instead of counter steering, but these are not complete. This should be further investigated to confirm whether there are design problems with the concept, whether the problem lies in the model or whether an increased displacement gives positive steering. Additional something that should be examined is the fact that the v-stay in concept 3 is divided into two components where two bushes with stiffness is added at the attachment on the axle.
6 Conclusion
A displacement of the v-stay center provides a steering of the axle. More analyses and tests are needed to achieve the desired steering without affecting surrounding components.
6.1 Future work
Important for future work is to change only one parameter at a time to be able to interpret the effect of each parameter and create a deeper understanding of the wheel suspension's movements. The analyses are performed only on the last trailer on the DUO-CAT, for more accurate results and further work it is recommended that the entire truck is simulated and analyzed. In addition, the wheels and the road are simplified. The road created in Creo is therefore completely flat, which does not correspond to reality. The same applies to the component´s ability to bend, rotate and deform which is prevented in rigid body analyses.
The chosen concept (concept 1) requires more design work and to achieve all safety requirements.
Finite Element Method (FEM) should also be carried out to ensure this. FEM analyses were something that was not done during the time-limited thesis. No analyses have been performed on how surrounding components are affected by the new concept and the increased steering, something that should be implemented in future work. Also, to ensure whether the installation height is significant for the steering of the axle, more analyses should be performed on this area. If future work leads to the need for a divided v-stay to achieve a larger displacement backwards in the travel direction, more analyses are required on which bushings are to be used for the brackets on the axle. The final step should be to produce a prototype of the concept and run it on a test track to then find out the exact results.
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Appendix 1 Work Breakdown and Time Plan
In the start-up of the thesis, a time schedule was created for the work on how the time would be distributed between the various abortion tasks. Halfway into the thesis, the schedule was followed.
Figure 1.A: Pre-calculated schedule
Figure 1.B: Actual schedule
Appendix 2 Density
Appendix 3 Stiffness
Appendix 4 Concept generation
Brainstorming
Morphological matrix
Appendix 5 Contact pressure
0 10000000 20000000 30000000 40000000 50000000 60000000 70000000 80000000 90000000 100000000
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
