CALTRANS DIVISION OF EQUIPMENT MOBILE CRANE STABILIZATION DESIGN
Gabriel Pineda
B.S., California State University, Sacramento, 2004
PROJECT
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
MECHANICAL ENGINEERING
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
SPRING
2011
CALTRANS DIVISION OF EQUIPMENT MOBILE CRANE STABILIZATION DESIGN
A Project
by
Gabriel Pineda
Approved by:
__________________________________, Committee Chair
Dr. Kenneth S. Sprott
____________________________
Date
ii
Student: Gabriel Pineda
I certify that this student has met the requirements for format contained in the University format
manual, and that this project is suitable for shelving in the Library and credit is to be awarded for
the Project.
__________________________, Department Chair
Susan L. Holl, Ph.D.
Department of Mechanical Engineering
iii
________________
Date
Abstract
of
CALTRANS DIVISION OF EQUIPMENT MOBILE CRANE STABILIZATION DESIGN
by
Gabriel Pineda
When designing mobile equipment, workers safety is a paramount concern for Engineers at
Caltrans Division of Equipment. A hazardous condition occurred to a crane mounted
maintenance truck when it attempted to lift a load, lost stability and nearly overturned. It was
determined that the stabilization system in place was not sufficient to the size of the crane in use,
thus at full capacity would cause a tipping condition. To correct this stabilization issue, a pull out
type hydraulic outrigger was designed and built. Design criteria for the outrigger was for it to be
rugged for everyday use, structurally sound, safe to use, simple to implement in the field,
lightweight, and cost effective.
The hydraulic outriggers were designed using NX 6 and Solidworks software for threedimensional CAD modeling and detail drawings. A finite element analysis of the outriggers
structure was created using Solidworks simulation software, testing for deflection, stress, and
loading. A vehicle-tipping analysis was also completed to ensure the equipment would be stable
in a lift. Budget savings took into consideration labor costs, materials costs, design costs, and
future maintenance costs.
The results of the analysis show that by keeping the design simple and modifying the
existing hydraulic stabilization system to include a pullout type outrigger, we were able to design
a stabilization system that effectively corrects the dangerous tipping condition and is structurally
sound in a cost effective, safe, and easy to employ package.
_______________________, Committee Chair
Dr. Kenneth S. Sprott
_______________________
Date
iv
ACKNOWLEDGMENTS
This project would not have been completed without the help and support of many people.
I would first like to thank Angela Wheeler from the California Department of Transportation,
Division of Equipment, for providing me guidance and support in the design of the stabilization
system. I would like to thank my advisor Dr. Kenneth S. Sprott for his guidance and
understanding on this project. Thanks are also extended to the many people at San Joaquin Delta
College for inspiring me to be sedulous in my studies with their many words of wisdom.
I would also like to give a special thanks to my friends and family for loving and
supporting me throughout my education. Especially my parents who without their endless
support emotionally and financially, I would have never been able to accomplish my goals. This
is ultimately for them; they have sacrificed many things in order to put four children through
college, a great accomplishment in its self.
v
TABLE OF CONTENTS
Page
Acknowledgments…………………………………………………………………………..…..v
List of Tables …………………………………………………………………………………..viii
List of Figures…………………………………………………………………………………..ix
Chapter
1. INTRODUCTION………………..…………………………………………………….…...1
Problem Description………………………………………………………………..…..1
Significance of Problem…...…………………………………………………………...5
Scope of Study………...…...……………………………………………………….….6
Organization of Project…...…………………………………………………………....6
2. LITERATURE SURVEY.…………………………………………..……………………...8
Overview………….……………………………………………………………………8
Injury Statistics…………………………………………………………………………8
Design and Safety Standards…………….……………………………………………..9
Mobile Crane Design…………….…………………………………………………….10
3. DESIGN OF OUTRIGGER STABILIZATION SYSTEM……..…………………………16
Engineering Requirements…………………………………………………………..…16
Static Stability Requirements………………………………………………………..…17
Outrigger Retrofit Design Considerations ………….…………………………………..19
Telescoping Pull-out Assembly ……….……………….………………………………20
Loads ……….……………….…………………………………………………………22
vi
Mounting Tube…………….………………………………………………………......23
Extension Tube…………….…………………………………………………………..25
Final Assembly…………….…………………………………………………………..27
4. TESTING……..……………………………………………………………………..……...30
Static Stability Calculation…………………………………………………….………30
Finite Element Analysis……………………………………..………………………...32
Prototype Trial………………………………………………………………………....35
5. RESULTS……..……………………………………………………………………..…..…37
Stabilization System safety………………………………………...…………………..37
Structural Integrity ……….…………………………………….……………………...37
Certification…………………………………………………………………………....38
6. CONCLUSIONS……..…………………………………………………………….………39
Significance of Results……………………………………………………………..….39
Goals Realized……………………………………………………………………...….39
Future Process and Design Considerations………..…………………………………...40
Appendix A. Three Dimensional CAD Models…………………………………………….....43
Appendix B. Failed Stabilization Test Pictures………………………………………………..47
Appendix C. Installation Figures………………………………………………………………49
Appendix D. Final Pictures…………………………………………….……………………...53
Work Cited ………………………………………………………………………………….....54
vii
LIST OF TABLES
Page
1.
Table 1 Safety Factor Comparison…………………………………………………..14
2.
Table 2 Soil Carrying Capacity………………………….…………...........................15
3.
Table 3 Maximum Capacity Chart – Venturo ET18KX Telescopic Crane ….……...51
viii
LIST OF FIGURES
Page
1.
Figure 1 Sign Truck……………………..………….……………………………….2
2.
Figure 2 Sign Truck Stability Diagram…………….……………………………….3
3.
Figure 3 Failed Stability Calculation , Stability Factor 1.12………………………..4
4.
Figure 4 Commercial Truck-Mounted Crane - Telescoping Boom …….…………11
5.
Figure 5 General Stability Area for Corner Mounted Crane………..……………...13
6.
Figure 6 Ideal Stability Calculation, Stability Factor 1.4…………………………..18
7.
Figure 7 Ineffective Outrigger Stabilization System ……………….……………. 19
8.
Figure 8 Allowed Design Envelope.. ………………………….…………………. 21
9.
Figure 9 Mount Tube …………………………………………...………………….23
10.
Figure 10 Mount Tube Retaining/Side Plates………………...……………………. 24
11.
Figure 11 Mounting Bracket…………………...……………..……………………. 25
12.
Figure 12 Tube with Jack Mount ………………….…………...…………….……. 27
13.
Figure 13 Extension Tube Assembly Retaining/Slide …….………………………. 27
14.
Figure 14 Complete Assembly Pull-Out Outrigger …..……………………………. 28
15.
Figure 15 Assembly……………………………..………………….………………. 28
16.
Figure 16 Front View Assembly …..………………………….……………………. 29
17.
Figure 17 Top View Assembly …..………………..…………………….…………. 29
18.
Figure 18 Final Design Stability Calculation, Stability Factor = 1.67 ……..………. 31
19.
Figure 19 Second Order Tetrahedral Mesh of Extension Tube ……….……………. 32
20.
Figure 20 Extension Tube von mises stress distribution…..……….…….…………. 33
21.
Figure 21 Second Order Tetrahedral mesh of Assembly …..………………………. 34
22.
Figure 22 Outrigger Assembly von Mises stress distribution ….………..…………. 35
ix
23.
Figure 23 Extension Tube Assembly…..………………….…………..……………. 43
24.
Figure 24 Sign Truck Body…………………..……………………..………………. 43
25.
Figure 25 Rear Profile.……………………………..……………….………………. 44
26.
Figure 26 Drivers Side Profile, Left Side Outrigger not shown ….………..………. 44
27.
Figure 27 Passenger Side Profile…………………..………………….……………. 45
28.
Figure 28 Max Tipping Angle of Crane………………..………...…………………. 45
29.
Figure 29 Underside Profile………………………………………………...………. 46
30.
Figure 30 Failed Stabilization Configuration ..……………..………………………. 47
31.
Figure 31 Tipping Condition Lifting 1691 lb load at 117.75” for center of crane.…. 47
32.
Figure 32 Driver Side Outrigger Lifting During Test………………………………. 48
33.
Figure 33 Venturo Flatbed Recommended reinforcement for ET18KX…....………. 50
34.
Figure 34 Caltrans Crane Rienforcement Drawing E2-D208-04..…………….……. 52
35.
Figure 35 Final Sign Truck Drivers Side……………………………………...……. 53
36.
Figure 36 Final Sign Truck Passenger Side………...………………………………. 53
x
1
Chapter 1
INTRODUCTION
California Department of Transportation also known as Caltrans is a government
department that has been serving the people of California for over 100 years. With their mission
statement “to improve mobility across California”, it is Caltrans task to build and maintain the
state highway system and various public transportation systems throughout California. One of
the many goals of Caltrans is to be the safest transportation system in the nation for users and
workers.
Among the many departments within Caltrans is the Division of Equipment, whose job it is
to purchase, design, fabricate, and repair over 13,000 pieces of mobile equipment within the
Caltrans fleet1. The equipment ranges from the specialized snow moving equipment, passenger
vehicle boat ferry’s, and lane barrier moving machines, to the numerous highway maintenance
vehicles; bridge trucks, cone trucks, litter pickups, 4-yard dump trucks, and highway marking
vehicles to name a few.
Problem Description
Caltrans, Division of Equipment designs and builds specialized trucks to meet the needs of
California on the roads and highways. One truck in particular is designed to be capable of
installing, replacing and removing road signs also known as the Sign Truck as shown in figure 1.
This is done by use of a truck mounted mobile crane capable of a 1125lb load at 16 feet from the
center of the crane.
2
Figure 1 – Sign Truck
While in the certification process, the Sign Truck began to tip, thus failing certification.
The cause of the instability was determined to be the failure of the stabilization system to counter
the force of the lifting load. Figure 2 below displays a stability diagram of the Sign Truck. In
Figure 2, the tipping line represents the fulcrum drawn from the front axle center of mass to the
rear outrigger. The risk of overturning is greatest when the crane boom is at a right angle to this
line.
3
Figure 2 – Sign Truck Stability Diagram
4
Figure 3 shows the failed geometry results based on the stability diagram of figure 2. The results
of the Stabilization Moment (MS) and Tilting moments (MT) are used to find the safety factor
against overturning (MS/MT). The factor of safety against overturning is required to be greater
than or equal to 1.17 as ASME standard8 or 1.4 for severe operation4.
Failed Stability Calculation for Rear Mounted Crane
Input Weight
(lbs)
PR =
6200
G1 =
197
G2 =
1050
PL =
1125
Rear Axle Weight
Crane Pedestal
Crane Weight
Crane Load
Stabilizing Moment (MS):
Ms =
19228.13
ft-lbs
Tilting Moment (MT):
Input Distance
(in)
Outrigger Distance
from Center Line of
Truck
X=
45.7
F.Axle to R.Axle
R.Axle to Outrigger
Outrigger to Crane
A=
B=
C=
165
23.875
11.25
Center line to Crane
Crane Maximum Reach
Center Gravity of Crane
F=
L=
H2 =
37
192
20
MT =
17157.49
ft-lbs
Safety Factor Against Overturning:
MS / M T =
1.12
=> 1.40
Rear Mounted Crane Standard
Extension outside of body
-2.3 inches
Calculated
Pedestal to Tipping
Line
Crane load to Tipping
Line
Rear Axle to Tipping
line
LR =
Tipping Line Angle
w° =
LC =
a=
8.99
183.01
38.80
13.60
Figure 3 – Failed Stability Calculation, Stability Factor 1.12
5
The results shown in figure 3 and indicate a stability factor of 1.12, which does not comply with
standards and confirms the mode of failure to be the stabilization system as there is no margin of
safety for shock conditions. Nine vehicles in total were outfitted with the failing outrigger/crane
configuration.
In order to correct, all nine vehicles will need to be retrofit with a new stabilization system.
The new system includes outriggers that extend further away from the body, thus moving the
tipping line out and stabilizing the unit. The new outrigger will be engineered to counter the
tipping reaction of the vehicle and the stresses involved during a lift, all within the space
available.
Significance of Problem
In the past, the Sign Truck may have been designed for a smaller crane and for this reason
the outriggers were set at a fixed position along the side of the trucks body. This was sufficient to
counteract the tipping force with a lower capacity crane. However, the current configuration of
the vehicles are outfitted with a larger 18,000 ft lbs moment rated crane, making it necessary to
correct the stabilization system to resist the new tipping force created. Tipping is a very serious
issue, causing a danger to workers safety. The Sign Trucks must be safe to operate and pass
certification before Caltrans workers can use them. There are many safety standards that must be
met in order for a crane to pass certification. The most critical standards for mobile cranes are
federal OSHA 1910.180 and ANSI B-30.5, which must be followed to insure workers safety.
6
In a National Institute for Occupational Safety and Health (NIOSH) Publication in 2006, it
was reported that one crane tips over every 10,000 hours of crane use in the United States. 80% of
these accidents are attributed to exceeding the crane capacity. More than half of these accidents
were a result of swinging the boom or not fully extending the outriggers2. Operator error is
difficult to correct because it is human nature to make mistakes, however engineering a system
that eliminates some of that possible error is a safer and more reliable way to get the job done.
Scope of Study
After examining the problem, it was determined that the truck was tipping because the
crane position relative to the outrigger on the passenger side of the vehicle was too short for the
capacity of the crane. This project is geared toward the research, design, and testing of a new
pull-out type outrigger that exceeds the current industry safety standards for tipping and structural
stability. The final product will be designed for maximum manufacturability and cost. The
project will be fully designed using three dimensional CAD, and FEA software packages.
Prototype outriggers will be built and tested in the Caltrans, Division of Equipment Headquarters
shop.
Organization of Project
The following chapter is composed of a literature survey of mobile crane design, injury statistics,
and industry standards for design. Chapter 3 deals with the design of the outrigger stabilization
system including the use of three dimensional CAD software. Design requirements and
7
considerations are also discussed. Testing for structural integrity, as well as prototype testing is
included in chapter 4. Finite Element Analysis software is also used to analyze the structure.
Chapter 5 shows the results found from the various tests performed. The significance of the
results along with goals realized and Caltrans future design considerations are expressed in the
final chapter.
8
Chapter 2
LITERATURE SURVEY
Overview
An incredible variety of cranes have been designed since the introduction of high-strength
steels in the 1950’s. The proliferation of cranes in construction is impressive, with approximately
125,000 cranes operating among all sectors of the United States construction industry alone6.
When it comes to cranes there are two basic forms, the mobile crane and the tower crane. The
mobile crane is the focus of this survey.
Injury Statistics
Mobile crane incidents can cause massive production delays, devastating property
damage, and loss of life. Estimates suggest that cranes are involved in up to one-third of all
construction and maintenance fatalities6. To add to this research by the Journal of Construction
and Management, it was found that mobile cranes represented over 88% of the fatal accidents
investigated. This is a significant amount for one crane type. The contributing factors leading to
fatalities caused by “crane tip over” were overload, loss of center of gravity control, outrigger
failure, high winds, side pull, and improper maintenance11. The research shows that most
accidents involving mobile cranes are due to carelessness or inattention of the operators or people
around them. By incorporating electronic safety measures and engineering out some factors that
attribute to this carelessness or inattention, many of these types of accidents may be reduced.
Many in the industry are attempting to improve conditions in the work force so that working and
operating around a crane becomes much safer. Currently legislation has targeted the employers
9
as well as operators of mobile cranes. Requiring certification and training, where in the past it
was not required for most small cranes. As well as increasing inspections of mobile cranes and
construction sites.
Design and Safety Standards
With the recent acceptance of very high-strength steels and the increasing needs at
construction sites, cranes have experienced a remarkable growth in lift capacity and boom
lengths. With this growth have come situations where the old stability margins might not provide
adequate reliability5. With new technology, cranes have evolved to be safer and more reliable
than they ever have been in the past. For instance, mobile crane technology now has the
capability to have electronic monitors to check load sensors, dynamometers, pressure sensors, and
a myriad of other electronic safety devices. However even with the new devices for safety it is
still up to the engineers and operators to be knowledgeable and cognizant of what they are doing.
Leading the push for safety standards is the American Society of Mechanical Engineers otherwise
known as ASME. ASME provides the standard for Mobile and Locomotive Cranes known as
B30.5. B30.5 contains provisions that apply to the construction, installation, operation,
inspection testing, maintenance, and use of cranes and other lifting and material handling related
equipment9. Other sets of standards that are applicable are the Occupation Safety and Health
Administration (OSHA), with Title 29, 1910.180 also, Local California Cal/OSHA, Title 8
standards. OSHA and Cal/OSHA standards mirror or reference many of the ASME B30.5
standards. The Society of Automotive Engineers (SAE) also has various standards for mobile
10
cranes as they relate to commercial vehicles. The Internationally Organization for
Standardization (ISO), also publishes a set of standards for the mobile carne industry.
Mobile Crane Design
Mobile cranes are comprised of hoisting machinery combined with both a base carrier and
a revolving superstructure. These cranes, can be either mounted directly onto wheels or truck
mounted, usually stand on four outriggers that are located at the corners of the lower base carrier.
The outriggers extend laterally and bear on the ground during hoisting operations to keep the
crane horizontal8. Mobile cranes exist in many forms, from locomotive cranes to crawler type
cranes; the emphasis of this report is on the Commercial Truck-Mounted Crane.
The American Society of Engineers defines a commercial truck-mounted crane as: A crane
consisting of a rotating superstructure (center post or turntable), boom, operating machinery, and
one or more operator’s stations mounted on a frame attached to a commercial truck chassis,
usually retaining a payload hauling capability whose power source usually powers the crane. Its
function is to lift, lower, and swing loads at various radii9, as shown in figure 4.
11
Figure 4 – Commercial Truck-Mounted Crane – Telescoping Boom9
For this study, we explore the mechanics of a Commercial Truck-Mounted Crane where the crane
is placed in the rear of the vehicle.
In most cases commercial truck mounted cranes use a telescopic type crane, which is the
case for this investigation. The telescopic crane gets its designation from the style of boom which
consists of several nested closed-tube sections that are extended or retracted by a hydraulic
cylinder; boom angles are controlled using one or more hydraulic cylinders located between the
boom base and the crane turntable6. Mobile boom cranes can vary in lifting capacity from 1 to 6
tons for commercial truck mounted cranes and up to 1000 tons for the very large specialized
crane mounted truck systems. Commercial truck mounted cranes usually have booms that can
extend up to 25 feet, however specialized crane mounted truck system can extend to lengths of up
to an amazing 600 ft.
12
The first aspect in the design of a truck mounted crane system is to determine if the body
will support the loads involved with lifting. Most manufactures will have a recommended
practice for reinforcement depending on the size or type of crane and body type. In this case, a
flatbed body is utilized. The manufactures recommended body reinforcement is shown in figure
33 of appendix C. The main support structure is made of two MC 4 x 13.8 lbs. /ft. ship channels
and a boxed ½” steel mounting plate for the crane. The truck body used for this survey employed
the recommended design as shown in figure 34 appendix C, which illustrates the build drawing
used for the sign truck body.
The next important feature of a truck-mounted crane is stability. It is necessary to check
the trucks stability against overturning to ensure the safety of the workers. Figure 5 shows the
general stability area of a rear corner mounted crane truck. The figure shows the areas at which
the crane is most adept and least likely to lose stability.
13
Figure 5 - General Stability Area for Corner Mounted Crane4
The stability system in a truck-mounted crane mainly consists of two or four outriggers.
There are many outrigger styles but the basic purpose is to level and move the tipping fulcrum of
14
the vehicle to the point at which there is a safe stability factor from overturning at maximum load.
Stability Factor is the ratio of tipping load to the rated load. Stability factors vary from one
standard to the next as shown in table 1.
Comparison of Safety Factors for Hook Load in the Stability of
Mobile Cranes Supported by outriggers
USAa
United Kingdomb Australiac
1.17(=1/0.85)
1.25
1.33(=1/0.75)
a
U.S. Department of Labor,OSHA, 2011
a
American Society Of Engineers, ASME B30.5-2007
ISOd
1.25
Japane
1.27
b
British Standards Institution, 1991.
c
Standards Association of Australia, 1995.
d
International Organization for Standardization, 1991.
e
Labour Standard Bureau, Ministry of Labour, Japan, 1978.
Table 1 – Safety Factor Comparison8
Table 1 shows the various stability factors that were established on the assumptions of a level
machine, on firm supports in calm air and in the absence of dynamic effects. For this reason, it is
not entirely correct to use these factors as a working stability factor. Dynamic effects of a
bouncing weight or swinging load can possibly cause an upset condition if the vehicle is already
on the ragged edge of the stability factor. For this study, the stability factor that is used is 1.4
based on the Utility Vehicle Design Hand Book released by the Society of Automotive Engineers
(SAE)4. The Handbook recommends a stability factor of 1.4 for severe operation conditions, 1.2
for normal operations and 1.1 for gentle operations. Severe operation takes into account shock
loads and other dynamic effects and increases the safety factor accordingly. In the interest of
15
safety for the Caltrans workers and operators, a stability factor of 1.4 or above will be the
standard for crane-mounted vehicles.
Another concern involving the stability of the crane is the ground bearing pressure of the
outrigger pads. The pad is the area underneath the outrigger leg that distributes the load of the
crane to the ground. Soil bearing pressure refers to the ability of soil to support load applied to
the ground. Table 2 below indicates estimated bearing capacities of different soil types for
buildings and cranes. Mobile crane operators need to be cognizant of the type of soil they are
operating on and apply cribbing when required underneath outrigger jacks to spread the load over
a larger surface area. Engineers must also be aware of soil conditions that the crane may
encounter when designing the outrigger pads and cribbing structure.
Table 2 – Soil Carrying Capacity5
16
Chapter 3
DESIGN OF OUTRIGGER STABILIZATION SYSTEM
Engineering Requirements
The basic requirements that the stabilization system must have are the following:
-
The system will need to be made of materials that can be easily purchased and machined
o
-
Size constraints limit design to be under the truck frame
o
-
Common Sizes of Structural Steel tube, angle and plate
Conceivably a telescoping or swing out outrigger extension.
Cost of materials and build should be minimal
o
Simple design that can be made in Caltrans HQ Shop with the equipment that is
available.
o
-
Durable
o
-
-
Will need to reuse as much of the existing system as possible.
Able to withstand the punishment of daily use and environment
Easy to use
o
The outrigger must be easily stored for driving
o
Not too heavy for a Caltrans worker to set up
o
Operation must be simple
Designed to current safety and industry standards
o
ASME B30.5, OSHA 1910.180, Cal/OSHA Title 29, and SAE.
o
Federal Motor Vehicle Safety Standards (FMVSS)
17
Static Stability Requirements
After analyzing these requirements, the first concern was to determine how far the
outriggers needed to extend in order to achieve ideal stability by current standards. In figure 6 the
cranes maximum capacity (PL), vehicles rear axle weight (PR), mounting pedestal (G1) and crane
weight (G2) are used for the stability calculation to determine the distance (X) the outrigger must
extend to meet current stability regulations with a safety factor of 1.4. Microsoft Excel was used
to create a calculation spreadsheet in order to input and process data. Results from Figure 6
indicate that the ideal tipping angle would be 15.81 degrees. From this angle, it was calculated
that an outrigger that extends 53.5 inches from the center of the truck body would accomplish a
safety factor of 1.4 for stability. The design goal was to design an outrigger extension that meets
the length of 53.5 inches from the center of the truck or extend beyond thus increasing the
stability factor.
18
Ideal Stability Calculation for Rear Mounted Crane
Input Weight
(lbs)
6200
PR =
197
G1 =
1050
G2 =
1125
PL =
Rear Axle Weight
Crane Pedestal
Crane Weight
Crane Load
Stabilizing Moment (MS):
If G2 lies inside of tipping line
Ms =
23143.97 ft-lbs
Tilting Moment (MT):
Input Distance
(in)
Outrigger Distance
from Center Line of
Truck
X=
16493.88
ft-lbs
53.5
F.Axle to R.Axle
R.Axle to Outrigger
Outrigger to Crane
A=
B=
C=
165
23.875
11.25
Center line to Crane
Crane Maximum Reach
Center Gravity of
Crane
F=
L=
37
192
H2 =
MT =
Safety Factor Against Overturning:
MS / MT =
1.40
=> 1.40
Rear Mounted Crane Standard
Extension outside of body
5.5 inches
20
Calculated
Pedestal to Tipping
Line
Crane load to Tipping
Line
Rear Axle to Tipping
line
LR =
Tipping Line Angle
w° =
LC =
a=
16.07
175.93
44.95
15.81
Figure 6 - Ideal Stability Calculation, Stability Factor 1.4
19
Outrigger Retrofit Design Considerations
The current failed stabilization system is made up of two solid mounted hydraulic jacks
with hydraulic drivelines, and safety limit switches. Two square tube members bolted to the
truck frame and body as shown in figure 7 are used to brace each jack.
Figure 7 – Ineffective Outrigger Stabilization System
After analyzing the current unsuccessful outrigger system, it was determined that several
components could be salvageable in the interest of saving expenses. Reusing the current
hydraulic jack assembly would save time and costs. Ordering a new jack would take anywhere
from four to five weeks and delay the manufacture as well as adding cost. The current jacklegs
were in good condition and would need minimal adjustments to retrofit to the new design. The
20
safety limit switch is a safety device that signals the operator that the outrigger is so the will not
drive off with an outrigger still deployed. This safety device was also in good condition and
could be easily transferred to the new design. The jack includes hydraulic hoses to extend down
and retract the outrigger pads. These hydraulic lines were not able to be reused because they
were not long enough to extend beyond the truck body. To determine if the jack pad should be
reused the ground bearing pressure would need to be calculated. The Jack pad on the assembly
had a surface area of 64 inches square. With an estimated maximum load of 7000 lbs force acting
on the outrigger, the bearing pressure on the ground would be 109 psi or 7.8 tone/ft2. According
to Table 2, this pad would be sufficient for compact gravel or road surface however blocking
would be needed for loose or firm gravel. This would not be a problem, as distributing the load
or blocking is a required practice for most outrigger pads. Blocking is an extra pad that
distributes the load over a larger area.
Telescoping Pull-out Assembly
One of the engineering requirements specifies a size limitation. This limitation was placed
because the current body would undertake a significant rebuild if a traditional style outrigger
cross tube were to be mounted across the rear of the truck or under the frame. A system that
would “bolt in” and utilize current parts would be far more desirable in saving cost, time and
labor. It is for this reason the stabilization needed to be contained within the truck frame rail and
outside of the body. Figure 8 shows the area of which the new stabilization needed to be
contained.
21
Figure 8 – Allowed Design Envelope
In order to fit within the design envelope a swing out type outrigger and telescopic type
outrigger system were investigated. An arm extension of 8 to 12 inches was required to pivot out
from the body in order for the swing out type outrigger system to work. Several objects that were
obstructive to this design were the tool circuit hydraulic lines, hydraulic valve, the hydraulic oil
filter and the rear wheel. In order to use the swing out style these object would need relocation.
Ultimately, the telescopic type outrigger system provided the best option because the
extension arm would fit within the given space without the need to modify the filter or tool circuit
location. The two MC 4 x 13.8 lbs. /ft. ship channels used for body reinforcement provided the
ideal location to mount the outrigger. The limiting length of the pull out was determined by the
distance allowed from the truck frame to the outside edge of the body. With this length, a pull
out of 12.5 inches to the center of the outrigger jack could be accomplished. Based on this
22
distance, it was calculated that the factor of safety against overturning would be 1.67 as shown in
figure 18 on page 31.
Loads
To begin the design of the outriggers the engineering stresses of the structure were
calculated based on the worst-case scenario of outriggers supporting the entire weight of the truck
and payload, plus the weight of the crane and cranes maximum load. As the leverage increases,
outrigger pressure increases nearest the load. This worst case scenario is calculated at the static
moment of least stability where the lifting load is furthest away. Using the stability diagram in
figure 2 of page 3, the maximum bearing load (BL) on the inside outrigger pad is calculated. The
moment equation is based point S, which is the driver’s side outrigger point of contact with the
ground.
(1)
(2)
Solving for BL where W is the distance of the driver side outrigger to the centerline of the truck
which is 48 inches, and TL is the vehicles maximum payload of 3500 lbs, we get BL = 8334 lbs.
This will be the load used in calculating structural integrity for all components of the stability
system. By SAE standard J1063 and B30.5 instruction for design of a prototype outrigger
23
structure the minimum strength margins for yielding in uniform rated loads will be 1.5 or above,
and for stress concentration areas surrounded by considerable low stress will be 1.1 or above.
Mounting Tube
To begin the design of the telescopic outrigger a mounting base was needed.
The
Mounting Tube would be made of standard 5” x 5” x ¼ x 20 inch long structural steel square
tubing. This was selected because of the availability in house and structural strength of the
member. The mounting tube would need to be able to withstand a moment load of up 18000 ft
lbs and the rear axle weight of the truck plus cargo payload.
24
Figure 9 – Mount Tube
Another key element was the reinforcement of the leading edge on the mount tube. This
was added because the concentrated load of the cantilevered extension tube was too great for the
tube alone. A 5 ½ x 5 ½ x ¼ x 2 inch long tube was welded to the end. This increased the cross
sectional area and reduced the stress. For the assembly four plates made of 4130 were attached
with ¼ -20 flat head screws to retain the extension tube and provide a hard slide plate for the
extension tube to slide against as shown in figure 10.
25
Figure 10 – Mount Tube Retaining/Slide Plates
The next feature of the assembly is the mounting angles. The angles (shown in figure 11)
were necessary because mounting tube could not be mounted underneath the body reinforcement
Channel. Each Angle mount was reinforced with two ¼ inch plates. A four bolt pattern was used
to attach four ½ - 13 hex head bolts to the body reinforcement channel.
Figure 11 – Mounting Bracket
26
Extension Tube
The design of the telescoping extension tube required a structural steel component to fit
within the dimensions of 5 x 5 x ¼ x 20 inch Mounting Tube. The member needed to be able to
withstand a force of approximately of 8334 lbs at the jack attachment. To begin, a structural steel
tube was assessed with varying thicknesses in order to choose the structure that was best
structurally and kept the weight manageable for a pull out. Treated as a simple cantilevered beam,
the applied bending moment was calculated to be 150,012 ft lbs. To calculate the bending stress
(σmax) the following equation was used:
(3)
Where M is the bending moment, Y is the distance from the neutral plane and I is the
moment of inertia. From the Ryerson Steel structural beam handbook the moment of inertia of a
4 x 4 steel beam of 3/8” wall has a moment of inertia of 10.7 and a distance of 2 from the neutral
plane. This gives a bending stress of 28039 psi. The structural steel was made to meet ASTM
A500 Grade B which has a yield stress of 46000 psi. This would give the extension beam a
safety factor of approximately 1.64 from yield. Another important factor in tube selection is the
deflection in the beam, if the beam deflects too much it is a sign that failure can occur. The
deflection was analysed using the deflection equation for cantilevered beams:
(4)
27
Where W is the bearing load, L is the length to the max bending stress, E is the elastic modulus of
the material (E = 29 x 106) and I is moment of inertia. The deflection for the extension beam is
0.0195 inches, which is minimal.
A mounting plate for the outrigger jack (as shown in figure 13) was welded on and six ½ 13 bolt clearance holes were used for attachment. To retain the extension tube so that it stops
before pulling out, four 4130 steel plates were attached to the end once assembled as shown in
figure 13. Figure 12 also shows a 13/32 diameter hole carefully positioned in the center of the
beam so that it would not affect the bending stress. The hole is used to allow a 3/8 inch diameter
pin to lock the outrigger in the out position so that it would not slip during a lift.
Figure 12 – Extension Tube with Jack Mount
28
Figure 13 - Extension Tube Assembly Retaining/Slide Plates
Final Assembly
Figure 15 shows the completed assembly of the pull-out style outrigger. The design is
simple and allows for easy bolt in underneath the truck body.
Figure 14 – Complete Assembly Pull-Out Outrigger
29
To accommodate the outrigger jack, the side of the truck frame is sectioned out as shown in
figure 15. Another feature added was a handle to the outrigger jack to make it easier to pull out.
Figure 15 - Assembly
Figure 16 – Front View Assembly
30
Figure 17 – Top View Assembly
31
Chapter 4
TESTING
Static Stability Calculation
Based on figure 2, the following equations were used to solve for the safety factor against
overturning:
Stabilization Moment:
MS = (PR x LR)+(G1 x LC)+(G2 x (LC-H2))
(5)
Tilting Moment:
MT = PL x a
(6)
Where,
LC = ((A+B+C)*(sin(wº)) – (F/cos(wº))
(7)
LR = (sin(wº))*A
(8)
a = L - LC
(9)
wº = Tan-1(X/(A+B))
(10)
These equations were entered into a spreadsheet to evaluate several conditions, including mode of
failure (figure 3), ideal stability dimensions (figure 6), and final stability verification (figure 18).
32
Final Design Stability Calculation for Rear Mounted Crane
Rear Axle Weight
Crane Pedestal
Crane Weight
Crane Load
Input Weight
(lbs)
PR =
6200
G1 =
197
G2 =
1050
PL =
1125
Stabilizing Moment (MS):
Ms =
26542.46
ft-lbs
Tilting Moment (MT):
Input Distance
(in)
Outrigger Distance
from Center Line of
Truck
X=
60.5
F.Axle to R.Axle
R.Axle to Outrigger
Outrigger to Crane
A=
B=
C=
165
23.875
11.25
Center line to Crane
Crane Maximum
Reach
Center Gravity of
Crane
F=
37
L=
H2 =
192
MT =
15922.38
ft-lbs
Safety Factor Against Overturning:
MS / MT =
1.67
=> 1.40
Rear Mounted Crane Standard
Extension outside of body
12.5 inches
20
Calculated
Pedestal to Tipping
Line
Crane load to Tipping
Line
Rear Axle to Tipping
line
LR =
Tipping Line Angle
w° =
LC =
a=
22.16
169.84
50.30
17.75
Figure 18 – Final Design Stability Calculation, Stability Factor = 1.67
33
Finite Element Analysis
Finite element analysis, also known as FEA, is a numerical method used for solving field
problems described by a set of partial differential equations12. In this case it will be used for
solving partial differential equations to predict the outcome of materials and structure in response
to force. The output of the partial differential equations will be the stress, strain, and deflection of
the given part. Because the number of simultaneous equations can get very large during an
analysis, computer software is used. FEA software is a very powerful analysis tool for design
engineers to realize the nature of their designs. For this study, Solidworks Simulation was used to
analyze and optimize the design.
The first step in a model is to apply the loading conditions and constrains. The extension
tube was given a cantilevered load of 8334 lbs to mimic the maximum loading condition, and
constrained 16 inches at the end of the tube. Next, a mesh is created using second order
tetrahedral elements because they offer more accurate results for solid meshing.
Figure 19 - Second Order Tetrahedral Mesh of Extension Tube
34
The results for the extension tube indicated a von Mises stress of 28,917 psi at the constrained
end that simulates the Mounting Tube face location. The maximum deflection was found to be
0.044 inches. The maximum stress was below the max yield of 46,000 psi for the structural steel
tube. The factor of safety for this load case was 1.6.
Figure 20 – Extension Tube von Mises stress distribution
The second component analyzed for stress distribution under loading was the mounting
tube. The mounting tube was given a simulated cantilevered load of 8334 lbs to mimic the
maximum loading condition and was constrained by the eight mounting bolt holes. The assembly
model was then meshed using second order tetrahedral elements, as shown in figure 21.
35
Figure 21 – Second Order Tetrahedral Mesh of Assembly
The results of the maximum von Mises stress analyss indicated a stress of of 43,272 psi and
a maximum deflection of .018 inches. The maximum stress of the assembly is located at the
connection point between the Angle bracket and the top of the mounting tube. These results
however were not used in the strength determination because the point load would increase no
matter how refined the mesh became. This is a common occurrence for FEA models and is
known as elasticity theory where sharp re-entrant corners are infinate. In some cases the stress
anomolies can be repaired with simplifying the model or adding fillets to edges, however in this
instance the geometry proved to be to beyond my experties and must be studied at a futer time in
more depth.
36
Figure 22 – Outrigger assembly von Mises stress distribution
Prototype Trial
In addition to a stabilization calculation of the failed outriggers shown in figure 3, a live
test was performed to verify the tipping condition. Figure 31 in appendix B, shows the load in a
tipping condition when attempting to lift 1691 lbs at 117.75 inches from the center of the crane.
The crane should have been capable of a load of 1800 lbs at 120 inches. This was below the
manufactures maximum capacity. Figure 32, in Appendix B, shows the left outrigger lifting
from the ground indicating the beginning of a tipping condition.
After manufacture, the prototype telescoping outrigger was tested for stability as well. The
test procedure consisted of a 1,691 lb. test load at various radius and angles of lift. The procedure
began with a test load of 1,691 lb. at 108 inches from the truck. This load was then pivoted
around the vehicle to check for any areas of instability. At the same time the structure was
37
visually examined to make sure there were no signs of possible failure. The consecutive lifts
were made at 6” intervals away from the truck and rotated around the full motion of the crane.
The full capacity of the crane was reached at 128.5 inches from the vehicle which is in line with
the rated capacity of 1,680 lbs at 128 inches from the center of the crane. The crane is designed
to shut all functions that extend the radius of the crane out if the working load exceeds the rated
load. This occurred at 128.5 inches from the center of the crane. The crane showed no signs of
instability or structural damage.
38
Chapter 5
RESULTS
Stabilization System Safety
A stability factor of 1.67 was achieved which was greater than the standards of crane
safety of 1.14. With a foot print of 64 in2 the outrigger pad has a bearing load of 130 psi or 9.37
ton/ft2, which was acceptable for most soil conditions, however a block pad or cribbing
underneath the outrigger pad of approximately 144 in2 or greater is recommended.
Structural Integrity
The vehicles may never see a maximum capacity lift or have the truck loaded to maximum
capacity, however the outrigger was designed based on the worst case possible. The worst-case
event delivered a structural safety factor of 1.6, for the extension tube and was verified with FEA
models. This was in agreement with the minimum strength margins set by SAE for prototype
outrigger testing. The mounting tube FEA model had mixed results with spikes near and above
the yield strength of the material. One possible reason for the divergent stress results could be the
theory of elasticity, were the stress in sharp re-entrant corners is infinite. The more refined the
mesh elements became, the larger the stress developed. Rounding sharp edges in the model, and
simplifying the geometry to alleviate this issue helped with some sharp edges, but for some there
was no remedy. After comparing several results it was determined, the max von Mises stress
could not be used for these models as they varied widely from 20,000 to 200,000 psi, however,
displacements of consecutive models did converge around to 0.018 inches, which was helpful in
understanding the rigidity of the structure. Based on the FEA analysis of the edge stress
39
concentrations it is safe to allow the irregularities in the edges of the parts because the
surrounding areas had high factors of safety relative to the inconsistencies.
Certification
A licensed mobile crane certifier does the certification of every truck-mounted crane that
leaves the Caltrans yard. A certification certificate is given if the vehicle passes a battery of tests
and visual checks. Once the new stabilization system was complete and tested, the certification
process was initiated.
During the final certification, it was noticed that the licensed mobile crane certifier was not
correctly testing the unit. While setting the outriggers up for the certification lift, the certifier
failed to extend the outrigger far enough down into the ground to properly unload the spring
weight of the rear end. This misstep could have potentially caused a tipping failure by not
distributing the weight into the outriggers and moving the tipping line of the truck in an
unpredictable manner. This demonstrates how easy it is for human error to occur. Once
corrected the test continued and the new stabilization system passed certification.
40
Chapter 6
CONCLUSIONS
Significance of Results
The new stabilization system will provide safety and stability to the Sign Truck and allow
the vehicles to be utilized in the field for many years to come. The system was designed to be
easily mounted and contained underneath the body of the vehicle whereas typical systems are
mounted to the rear bumper. This allowed for ease in manufacturing and time savings. Many of
the concepts and design elements will help in evaluating and designing future mobile truck
mounted cranes within Caltrans Division of Equipment.
Goals Realized
- Tipping condition was eliminated by use of a pull-out type outrigger allowing the Sign
Truck to be used safely in the field. The new system extends a total of 14.5 inches from
the side of the truck, and achieved a factor of safety of 1.67 against overturning.
- Stabilization system was structurally sound and rugged for everyday use.
- Compact unit that is contained underneath the body.
- Safety systems that were implemented were; an operator warning if the outriggers are not
stored correctly, an emergency stop switch, a lift warning rear mounted horn, also
signage was use to warn of possible pinch or crush points.
- Simple to implement outrigger, where the operator only needs to grab the pull handle to
extend the telescopic outrigger out, lock the outrigger in place with a pin, and use the
hydraulic lever to extend each of the outrigger pads to the floor.
41
- Overall, the telescopic outrigger assembly weighs 80 lbs. The pull out arm weighs 35 lbs,
which is within the realm for any operator to use.
- Because various parts were reused from the last stability system and stock build materials
were used for the structure, costs were able to stay within $300 for parts and $600 for
labor per vehicle.
Future Process and Design Considerations
To avoid similar problems in the future, stabilization studies must be made for every
mobile crane over 2000 lbs in capacity. This would allow design engineers to adjust stabilization
systems prior to a truck build. In addition, it would be advantages to create a pilot or test vehicle
prior to completing an entire run of vehicles. In this case it could have saved design and labor
time had the problem been caught early on.
Designing outriggers is a time consuming process. In the future where possible, I would
advise to use the crane manufacturer’s pre-engineered outriggers. On future sign trucks, the body
may need a redesign for use with manufacturer’s full stabilizer system. Current utility bodies
include a bumper step with outrigger included that mount directly to the rear frame and crane
support. The Sign Truck can benefit from this type of set up.
Even with proper training, human error still exists. As demonstrated by the incorrect use of
the outriggers by the crane certifier. Perhaps, prominent signage for proper crane use will be in
future builds. Another possibility could be a limit sensor that makes certain the weight of the
truck is unloaded from the rear leaf springs before the crane is allowed electrical power. Another
would be outrigger load sensors so the operator would know if the rear truck springs are unloaded
and if there is potential for soil overloading. Most cranes at present come from the manufactures
42
with many safety features like capacity overload sensors and load blocks, it may be possible to
have manufactures add in more safety features as costs of electronics become more affordable.
The next step is to have a field review of the stabilization setup with the end users and get
feedback based on the Sign Truck performance also to inspect for and damage or signs of
structural issues like chipping paint. Based on that feedback the stabilization system on Sign
Trucks and future mobile crane vehicle design will continuously improve.
43
APPENDICES
44
APPENDIX A
Three Dimensional CAD Models
Figure 23 – Extension Tube Assembly
Figure 24 – Sign Truck Body
45
Figure 25 - Rear Profile
Figure 26 – Drivers Side Profile, Left Side Outrigger not shown
46
Figure 27 – Passenger Side Profile
Figure 28 – Max Tipping Angle of Crane
47
Figure 29 - Underside Profile
48
APPENDIX B
Failed Stabilization Test Pictures
Figure 30 – Failed Stabilization Configuration
Figure 31 – Tipping Condition Lifting 1691 lb load at 117.75” for center of crane.
49
Figure 32 – Driver Side Outrigger Lifting During Test
50
APPENDIX C
Installation Figures
51
Figure 33 – Venturo Flatbed recommended reinforcement for ET18KX10
52
Table 3: Maximum Capacity Chart - Venturo ET18KX Telescopic Crane10
53
Figure 34 - Caltrans Crane Reinforcement Drawing E2-D208-04
54
APPENDIX D
Final Pictures
Figure 35 – Final Sign Truck Diver Side
Figure 36 – Final Sign Truck Passenger Side
55
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Venturo. 2011. 15 April 2011 <http://www.Venturo.com/et18kx.aspx>
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9.
American Society of Mechanical Engineers. ASME B30.5-2007 Mobile and
Locomotive Cranes . New York, March 7, 2008
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Venturo Crane Instruction Manual. ET18KX 21300, 7-29-03
56
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Beavers, J.E., Moore, J.R., Rinehar R., and Schriver R. Crane-Related Fatalities in the
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