BPA Spacer Cart
MECHANICAL/STRUCTURAL ISSUE ANALYSIS AND REDESIGN
ME 493 Final Report - Year 2016
June 3rd
Sponsor company:
Bonneville Power Administration
Group members:
Contact Engineer:
Kevin Machtelinckx
Academic Advisor:
Huafen Hu, Ph.D.
- Joshua Ponder
-Carlos Jiménez
- Robert Lawrence
- Austin Ferrante
-Mackenzie LarsonWeber
-Sam Levin
- Bao Phan
- Stephen Randall
Executive Summary:
The maintenance of high voltage transmission lines is a critical operation that ensures that the
power grid upon which our cities rely on is always working. In order to perform this
maintenance, linemen must use Spacer Carts; hanging carts that provide a stable platform from
which they can perform electrical line repairs. The condition of these carts is critical to the work
of the linemen, as well as their safety. The current Spacer Cart utilized by BPA was designed inhouse to address the unique challenges of performing repairs on transmission lines of the
Pacific Northwest; however, the current design has defects in regards to structural integrity and
ergonomics. BPA would like to have the design issues addressed through an in-depth
engineering re-design. The work commissioned by BPA through PSU’s Mechanical Engineering
Capstone includes a re-design of the cart focusing on three main areas: structural support for
the frame, ergonomic re-design of the crossbar linking the arms, and ergonomic re-design of the
pinch wheel mechanism that maintains the cart attached to the electrical lines.
Table of Contents:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction …………………...…………………………………………….Page 1
Mission Statement
…...…………………………………………………….Page 2
Project Timeline
…………………………………………………………Page 2
Main Design Requirement
...……………………………………………….Page 4
Top Level Design Alternatives
...……………………………………….Page 7
Final Design ...……………………………………………………………….Page 10
Product Design Specification Evaluation
...……………………………….Page 14
Conclusion
…………………...…………………………………………….Page 16
Appendix
...……………………………………………………………….Page 17
Introduction and Background Information:
The Bonneville power administration is a federal nonprofit agency located in the Pacific
Northwest; responsible for maintaining 75 percent of the high voltage transmission lines in
Oregon, Washington, western Montana, and part of eastern Montana. Given the importance and
scale of such an operation, maintenance must regularly be performed on the transmission lines.
Spacer carts provide an effective means for linemen to travel along the power lines while
performing the necessary maintenance required to keep the lines in working order. While the
current cart design is capable of performing its duties, several details regarding ease of use and
safety were not addressed in its initial design. Under normal operating conditions, impacts to the
support arms cause stresses in the frame. If left alone, these stress impacted locations
eventually fail. Another separate issue with the current design is the lack of an efficient
engagement method for the pinch-wheels. Due to the effort required to engage the current
pinch-wheel assembly, linemen sometimes choose not to use them. This causes the cart to be
more prone to slipping and derailment.
Figure 1 - Marty Lyons demonstrating the use of a Spacer Cart at 35 degrees @ BPA facility
Mission Statement:
The purpose of the project is the re-design of the spacer cart currently in use by BPA.
Overall project scope is limited to addressing safety/functionality limitations with the current
design. The goal of the new design is to reduce stresses in the frame caused by impacts
sustained by the arms during normal operation, and to address access and ease of use issues
experienced with the arms, cross-bars, and pinch wheel assemblies. These new designs must
all pass design requirements/envelopes specified by BPA for line clearance. BPA would like a
prototype available to them for testing by June 1, 2016.
Project Timeline:
A project timeline was created and submitted to BPA in order to track the project
milestones, utilizing the seven steps outlined in the design process taught in the Capstone
sequence. A Gantt chart showing the milestones and delivery dates is shown in the graph
below.
Figure 2: Spacer Cart project timeline
The main goal of the project was to complete a prototype of the different cart modifications
which were scheduled to be fabricated and tested by June first. Several iterations of design
review were needed for the modified components. This caused the final design iterations to be
by delayed until early May. One of the components to be manufactured required special tooling
delaying its delivery for 6 weeks. As such, instead of providing a finished prototype for testing by
June 1, we modified the final goal to provide a complete print packet to BPA, alongside FEA
analysis, by June 2. BPA will be reviewing the prints and finite element analysis. They will also
be fabricating and testing the modified designs during summer 2016.
Main Design Requirements:
The design requirements include performance, safety, environment and ergonomics,
maintenance and parts, installation, and cost. The following tables present the Product Design
Specifications (PDS) as discussed with BPA.
Table 1: PDS - Performance
Performance
Requirements Primary
Custome
r
Metric
Target
Cart must
climb/descen
d a line at an
angle.
degrees
35
Custom
er
Defined
Site Testing
Weight Rating BPA
Working Load
Limit
lbs
550
Custom
er
Defined
Site Testing
Pinch Wheels
Must traverse
on wire and
increase
contact
friction
½
Custom
er
Defined
Site Testing
Angle of
Incline
BPA
BPA
Metrics &
Targets
inches
Target
Basics
Verification
Table 2: PDS - Safety
Safety
Requirements Primary
Custome
r
Structural
Frame
Safety Factor
Metrics &
Targets
Metric
BPA
Frame to
withstand
vibrations
and impact
load under
working
weight limit
(Impact
Loads to be
provided by
BPA)
Physical
Testing
BPA
Establish
safety factor
based on
industry
standards
(applicability
to be defined
by BPA)
Specified
applicabl
e design
criteria
Table 3: PDS - Environment and Ergonomics
Target
Target
Basics
Verificatio
n
550
Withstand /
support all
loads
(Static /
Dynamic)
associated
with normal
cart
operation
and WLL
Lab
Testing
N/A
Customer/
Project
Team
Decision
Customer
Interview
Environment and Ergonomics
Requirement Primary
Metrics &
s
Custome Targets
r
Crossbar
Redesign
Wind, Rain
and Cold
Environment
Operational
Design
Envelope
Metric
Target
Target
Basics
Verificatio
n
BPA
Crossbars
should be
redesigned
to be easier
for
technicians
to remove
Operatio
n
Efficienc
y
BPA
Foreman
Approval
Ease of
Use
Site
Testing
BPA
Cart must
withstand
use in wind,
rain and
cold
environment
s
Duration
Years
5
Corrosio
n
Resistan
ce
Field Use
BPA
All physical
modification
s must fall
within
operational
envelope
restrictions
Design
envelope
standard
as
provided
by BPA
All
modification
s fall within
design
envelope
restriction
Specified
Design
Physical
Envelop Inspection
e
Table 4: PDS - Maintenance and Parts
Maintenance and Parts
Requirement
s
Frame
Reinforceme
nt
Primary
Customer
Metrics &
Targets
BPA
Years in
service
without
frame
repair
Table 5: PDS - Installation
Installation
Metric
Target
Years
5 years
Target
Basics
Custome
r Defined
Verification
Site Testing,
FEA
analysis
Requirement
s
Pinch Wheel
Assembly
Cross-Arm
Bar
Assembly
Primary
Metrics &
Customer Targets
Metric
Target
Target
Basics
Verification
BPA
Pinch Wheel
mechanism
to be
installed
easily
Operatio BPA
n
Foreman
Efficiency Approval
Ease of
Operatio
n
Site Testing
BPA
Cross-arm
bar assembly
to be easy to
use while on
the line
Operatio BPA
n
Foreman
Efficiency Approval
Ease of
Operatio
n
Site Testing
Table 6: PDS - Cost
Cost
Requirements
Primary
Metrics
Customer &
Targets
Cost must not
BPA
exceed original
spacer cart
production cost by
a significant
amount
N/A
Metric
Target
Target
Basics
Verification
N/A
A
relativ
e
value
BPA will
decide what
cost(s) are
acceptable.
Record
keeping of
budget
expenditures and
production
costs
Top Level Design Alternatives:
The redesign of the pinch wheels went through several iterations, each had its own
advantages and disadvantages.
The first squeeze wheel design, design A, consisted of a small housing attached to the
frame which utilized an ACME screw and nut to vertically adjust the height of the wheel. A pin
was to be inserted into one of two holes at different heights to secure the housing to the main
frame. This provided a secondary means of vertical adjustment.
The primary advantages of this design were that it was light, and the assembly could be
removed without much effort. The disadvantage of this design was that tools were required for
adjustment.
The second squeeze wheel iteration (design B), consisted of two contact wheels as
opposed to just one. The purpose of these wheels is to provide contact with the conductor when
it was at a significant angle. The two wheeled design would have pivoted with the line, ensuring
constant contact.
The height of the pinch wheels would have been adjusted by turning a handle, which in
turn drives a power screw, raising or lowering the wheels. Once in the desired position, a
serrated locking nut would have been lowered onto the adjustable nut controlled by the handle
to prevent any further rotation. The mounting assembly to the frame could also have been
adjusted by a lever which would have locked the pins into holes set in the frame at different
heights. The purpose of this was to provide further height adjustment.
The advantage of this design was that tools were not needed for adjustment. This design
also allowed for the possibility of a braking mechanism to be implemented at a later date. The
primary disadvantage of this design was that it would have added significant weight to the cart.
It was determined by BPA that pinch wheel design option A would handle powerline
angles of up to 35 degrees without the use of design B. In light of this, the implementation of
design B would be unnecessary. Nonetheless, an issue that remained with design A was that
tools would still be necessary for adjusting the height of the pinch wheel while the cart was in
operation. To resolve this issue, a new iteration, (Design C) was created. The new design
incorporated the same handle and screw mechanism for raising and lowering the pinch wheels
as design B, but at a smaller scale. This solved the issue of having to use tools. It was also
inverted with the adjustment onto the top as opposed to the bottom in order to simplify
adjustment. A secondary height adjustment attached to the frame is also implemented. This
would allow the height to be adjusted by inserting a pin into one of three holes. However, when
design C was presented to BPA, it received negative reviews. The primary issue with design B
was that it incorporated a double cantilever beam which connected the pinch wheel assembly to
the frame. Due to the pinch wheel-conductor interactions, it was noted that the double cantilever
beam would cause issues in the cart. Both design B and C were ultimately rejected.
Design D, the new and finalized design for the pinch wheels meets the requirements and was
approved by BPA. It is light, easily adjustable and removable without tools being required.
The initial redesign of the frame and cross arms presented by the PSU capstone team
were satisfactory and no further iterations were necessary.
Design A: Same mounting arrangement - Removable - Lightweight - No significant structural
change -
Design B: New mounting arrangement - New Operation - Assures traction at any line angle Adds significant weight to cart - Requires conductor guides be easily removable
Design C: Same handle/screw adjustment as design B but smaller and lighter - No tools
required for height adjustment
Final Design:
Support Arms:
This is the most important component of the project. The frame arms are subjected to high
stress levels while the cart is secured on the conductor and in motion, this is especially true
when they travel over line components such as spacers, dampers and armor rods. After being
subjected to many hours of operation, cracks resulting from fatigue and plastic deformation
begin to form in the arms, frame, and pivot points.
For the final design, it was decided that a truss would be added to the original arm design. This
was approved by BPA as the addition of struts to the arms distributed the impact loads
experienced by the frame more effectively than the original design. The trusses are pinned
together near the center of the cart with a removable ball lock pin, simplifying the alignment of
the cross-bars. The purpose of the support arm redesign is to absorb and distribute the stresses
in the frame caused by the impact moments generated when the cart passes over obstacles
during regular operation. The addition of the struts also helps to prevent bending and plastic
deformation from occurring in the arms. The current support arm design is shown below.
Cross Bar:
The main purpose of the cross-bar is to provide support to the two vertical “candy cane”
arms, keeping them parallel to one another. This is especially important when going over
obstacles along the conductor. Without the cross-bars installed, the rear idler wheels tend to get
stuck on obstacles causing the rear arms to experience impact loads. The impact loads transfer
through the arms to the pin connections, and impart stresses on the frame.
A cotter key and hole is used to secure the cross-bar to the support arms. When the holes don’t
line up exactly, installation of the cross-bar becomes difficult and time consuming. Because of
this, the cross-bars often go unused.
It was decided that the final design would consist of a bolt action method for removing
and locking the cross arms into place. The design itself consists of a bolt action assembly in
which a knob is protruding from the inner cross-bar shaft and can be locked in place by sliding
the knob into a locking groove cut in the outer support sleeve affixed to the drive arm candy
canes. When this action is complete, the cross bar will be aligned with the idler arm’s outer
support sleeve retaining pin holes. Last the ball lock retaining pin would be inserted into the idler
arm support sleeve. The cross bar can be easily removed by simply removing the ball lock pin
and sliding the knob out of the groove. A spring is also utilized to provide the necessary tension
on the knob against the groove to prevent the protruding knob from sliding out of the locking
groove.
Pinch Wheels:
The Pinch wheels secure the spacer cart to the conductor, which is important when the
conductor is at a steep angle. When the cart is operating on a steep incline the rear idler wheels
often lift off of the conductor, requiring the linemen to realign the wheels so they can regain
contact with the line. The pinch wheels were incorporated to keep all four cart wheels in contact
with the line at all times during operation. They were designed to be moved out of the way when
not needed. The main issue with the original design was that it was not user friendly and
required tools to engage and disengage. For this reason the pinch wheels were rarely used.
The final design presented by the capstone team to BPA resolved many of the issues
that the linemen were facing during operation. The design itself consists of an easy-to-use
height adjustment method in which a nut is turned on a threaded screw by a ratchet. As the nut
turns, the pinch wheel is raised or lowered to the desired height. The design also consists of a
simple method of moving the pinch wheel out of the way of the conductor when not in use. A
spring actuated pin lock handle located on the arm mounting assembly is retracted and the
upper portion of the squeeze wheel can be pulled to disengage the pinch wheel from the frame.
A cam lever can then be pulled to release tension and enabling the pinch wheel assembly to
pivot 180 degrees out of the way of the conductor when it is not in use. For repairs or routine
maintenance, the entire assembly can be removed entirely by pulling both the upper and lower
portion of the pin lock handles.
Design D (Final Design): Design is light - easily adjustable and removable without tools being
required - minimal unwanted stresses during operation.
Product Design Specification Evaluation:
Each item listed in the PDS has been evaluated, and the results compiled in the tables below.
Table 7: Product Design Evaluations
Performance
Requirements
Angle of
Incline
Metrics & Targets
Cart must
climb/descend a line
at an angle.
Target
Result
35 degrees
Site testing will be conducted
with the modifications once BPA
completes fabrication. The
design modifications will allow
the cart to function as well as
the original design in regards to
its climb and descent, therefore
this target should be easily met.
Weight Rating
Working Load Limit
550 lbs
The modifications to the cart
have increased the cart weight
by 23 lbs. Current cart weight is
app. 350 lbs, therefore
expected weight is < 400 lbs.
Pinch Wheels
Must traverse on wire
and increase contact
½ inches
The final design has a height
adjustment of 7/16”
friction. Adjustment of
wheel height when in
contact with wire to
be at least ½ inch.
Safety
Structural
Frame
Frame to withstand
vibrations and impact
load under working
weight limit
Safety Factor
Establish safety
factor based on
industry standards
550 lbs
FEA analysis was performed
using 2500 lbs as detailed in
Appendix A1. The modifications
on the cart arm frame shows
that it is able to withstand the
static force without yielding.
Lab testing of the new design
will be performed by BPA in
summer of 2016.
Lower than
8:1
After performing research, the
team recommends the use of a
5:1 safety factor based on 29
CFR 1926, Subpart L. See
Appendix A2.
Table 8: Product Design Evaluation (continued)
Environment and Ergonomics
Requirements
Metrics & Targets
Target
Result
Crossbar
Redesign
Crossbars should
be redesigned to
be easier for
technicians to
remove
BPA
Foreman
Approval
Crossbar has been redesigned
to use a bolt-action lock with a
ball-lock pin instead of a cotter
pin.
Wind, Rain and
Cold Environment
Cart must
withstand use in
wind, rain and cold
environments
5 years
Metric will be determined based
on field use.
Operational
Design Envelope
All physical
modifications must
fall within
operational
envelope
restrictions listed
in Appendix A4.
Modifications
meet design
envelope
All design modifications
performed by the Capstone
team meet the design envelope
established by BPA.
Maintenance and Parts
Frame
Reinforcement
Years in service
without frame
repair
5 years
Metric will be determined based
on field use. FEA analysis of
dynamic loads has not yielded
conclusive data. FEA analysis
of static loads meets current
requirements.
Installation
Pinch Wheel
Assembly
Pinch Wheel
mechanism to be
installed easily
BPA
Foreman
Approval
Improved design uses a springactuated pin lock handle that
allows the entire pinch wheel
assembly to mount/unmount
from the cart arms. It eliminates
the need for tools for
installation.
Cross-Arm Bar
Assembly
Cross-arm bar
assembly to be
easy to use while
on the line
BPA
Foreman
Approval
Bolt-action lock and ball-lock pin
provided easier interface than
current cotter-pin design.
A relative
value
All of the modifications to the
cart can be done in a per-piece
basis, therefore the only costs
incurred will be in fabrication of
new components and labor cost
for assembly. The modified
parts can then be installed on
the existing carts.
Cost
Cost must not
exceed original
spacer cart
N/A
production cost by
a significant
amount
Conclusions:
The PSU capstone team has succeeded in meeting essentially all possible PDS specifications
pertaining to overall design, however true evaluation of the design’s overall performance, safety,
ergonomics, maintenance, installation and cost remain to be seen after prototyping and
destructive testing. FEA analysis of dynamic loads has not yielded conclusive data pertaining to
whether or not fatigue will affect the cart in 5 years’ time. Furthermore, it is still unknown as to
whether or not the modified components will withstand long term environmental exposure.
Regardless, both BPA and the PSU capstone team are very satisfied with the outcome of the
project and the team has full confidence that the redesigned spacer cart will prove to be reliable,
structurally sound, and ergonomically pleasing.
Appendix:
A1 - Finite Element Analysis Studies:
Support Arms Study
The purpose of the study is to see of how much of an improvement the new cart arm design is
over the original. Because static load testing has been done on the original design that will be
the focus of the report. In the original test, one of the idler arms failed at a load of 9310 lb. This
will be the approximate load applied to both the original and new cart arm designs.
Result Summary
Although the model of the original support arm did not yield the same results as the
physical test, the similarity in application of loads and boundary conditions between the modified
and original FEA models still give a reasonable approximation of how much better the new
design performs. Based off of these results, it is reasonable to say that the new design will meet
the newly specified safety factor requirements for the spacer cart.
Detailed Analysis:
Model
The study was conducted with ABAQUS FEA software. The parts were modeled under
static loading conditions using shell and beam elements. A unit system of inch-pound-seconds
was used. The locations of the boundary and loading conditions for both models can be seen
below.
Original Support Arm
Modified Support Arm
The axles in both assemblies were assigned pinned boundary conditions (U1=U2=U3=0), while
the bottom of the vertical bars were restrained in the X and Z directions (U1=U3=0). Each
assembly was assigned a total load of 2500 lb in the negative Y direction (U2). The shells were
joined to the beams with tie constraints.
Material Properties
The Parts in the assembly are made up of 4130 chromoly and ASTM A36 Steel. The properties
assigned to them are as follows:
Elastic Modulus (Psi)
Yield Stress (Psi)
ASTM A36 Steel
21,072,000
53,700
4130 Chromoly
28,300,000
63,100
Results
The results for both models indicate that neither model yields at the failure load acquired
through physical testing. The locations and magnitudes of the largest stresses experienced in
both models can be seen below.
The modified support arm experiences approximately half the maximum stress of the original.
Conclusion
While the model of the original cart does not fail at the same loading as the physical test,
useable information can still be gathered from this study. Because both models share the same
boundary and loading condition schemes, it can be assumed that the differences in maximum
stress values are similar to the difference that would be seen had the original model behaved
identically to the physically tested support arm. Additionally, it should be noted that the
maximum stress located in the original support arm model occurred in the same location as the
fracture in the physical test run by BPA.
Crossbar Study:
The original crossbar for the BPA spacer cart primarily serves as an anchor point for the
cart to be carried to the destination by helicopter. However, the redesign for the cross-bar
includes modifying the existing tube collar with a slot for the bolt handle; this slot weakens the
structural integrity of the collar. The purpose of this analysis is to determine whether or not the
slot will cause yielding failure of the collar.
Result Summary
The analysis indicated that the slotted collar does not yield from the loading that would
be experienced while being transported by helicopter, with the collar experiencing a maximum
stress value of 76% yield stress (48,000 psi stress versus the 63,100 psi yield strength). This
means that the cross-bar redesign offers a factor of safety of approximately 1.3. While not ideal,
it indicates that the redesign shouldn’t fail under normal conditions, and manufacturing and
testing can proceed.
Detailed Analysis:
Model
The analysis was performed with Abaqus FEA software. The model was defined using
the inch-pound-second system of units and tested under static conditions. The model used was
composed of full 3D models of the cross-bar and the bolt collar, arranged together as shown.
The collar and bar were partitioned as necessary in order to allow the majority of each
part to be meshed with hex elements. The collar is held fixed by two 0.25 in2 face partitions
positioned at either end of the collar. The bar is pinned at the far end, to simulate the support of
the second collar tube. Force is applied as a vertical traction load along a 4.25 in long area of
the lower surface of the cross-bar with a total magnitude of 400 lbs to simulate the cart being
carried by helicopter. Contact conditions are described between the bolt and the bolt slot edges,
and the inner surface of the bolt collar and the outer surface of the bolt end of the cross bar.
Material Properties
Contact Properties
Results
The results of the static analysis indicate that the entire crossbar assembly yields in two regions,
as shown.
Yield strength for 1080 carbon steel is approximately 53,700 psi. This stress is exceeded
in the regions that are colored orange red; this means that the crossbar has supposedly yielded
at the pinned end, and in the center where the load is applied. However, the focus of the study
is the slotted collar, and so the indicated yielding of the crossbar will be ignored.
An enhanced view of the slotted collar is shown on the next page. There are two notable
regions of high stress, which are labeled with the highest stress value in those regions.
However, neither of these regions exceeds the 63,100 psi yield strength of 4130 Chromoly,
meaning that the slotted collar does not yield. The highest stress experienced is roughly 48,000
psi, or 76% of the yield strength, offering a factor of safety against yielding of 1.31. It is notable,
however, that the highest stress is in a location that indicates the tab is more likely to deform
lengthwise instead of bending upwards and uncurling.
Should testing indicate that the slotted collar requires reinforcement, the two locations of
stress concentration should be the first points to be reinforced.
Conclusion
This analysis found that the slotted collar does not yield from the loading that would be
experienced while being transported by helicopter, with the collar experiencing a stress value
equal to 76% of the yield stress. However, this analysis does indicate that the cross bar itself
yields under this load; actual usage of the current crossbars shows that this is not the case. This
discrepancy indicates it is likely that either the loading used in this model is larger than the
actual loading, or the cross bar has different material properties than those used in this study.
Either way, the results of this study are promising, and indicate that manufacturing and testing
can proceed.
A2 - Factor of Safety Research Documentation:
ANSI B77.1 Research:
Excerpts from ANSI B77.1 w possible relevancy:
Chapter 1: General Requirements
Chapter 2: Aerial Tramways
\
Chapter 3: Detachable Grip Aerial Lifts
Brake references:
Chapter 3: F.O.S. Reference for cable interface
- Testing of cable interface systems:
- Testing of the carriers:
Chapter 4: Fixed Grip Aerial Lifts
Stopping distances:
- Brake References:
- Factor of Safety References:
ANSI B77.1 Research Conclusion:
After reviewing chapters 1-3 of the ANSI Aerial Tramway safety standard, it was determined that
little or none of the document pertains directly to the spacer cart. The way in which factors of
safety are defined is based on the aerial tramway type which none of the described types match
BPA’s application directly.
The above notwithstanding, there are several references to factors of safety, friction coefficients,
and braking speeds which may be of use when deriving a safety standard for the Aerial Line
Carts. It was also noted that brakes are required on all types of these systems.
29 CFR Subpart L Part 1926 Research
1926.450 Covers scope, application, and definitions.
Scaffold means any temporary elevated platform (supported or suspended) and its supporting
structure (including points of anchorage), used for supporting employees or materials or both.
Mobile scaffold means a powered or unpowered, portable, caster or wheel-mounted supported
scaffold.
Catenary scaffold means a suspension scaffold consisting of a platform supported by two
essentially horizontal and parallel ropes attached to structural members of a building or other
structure. Additional support may be provided by vertical pickups.
Suspension scaffold means one or more platforms suspended by ropes or other non-rigid
means from an overhead structure(s).
Competent person means one who is capable of identifying existing and predictable hazards in
the surroundings or working conditions which are unsanitary, hazardous, or dangerous to
employees, and who has authorization to take prompt corrective measures to eliminate them.
Maximum intended load means the total load of all persons, equipment, tools, materials,
transmitted loads, and other loads reasonably anticipated to be applied to a scaffold or scaffold
component at any one time.
1926.451 (a)
(1) Except as provided in paragraphs (a)(2), (a)(3), (a)(4), (a)(5) and (g) of this section, each
scaffold and scaffold component shall be capable of supporting, without failure, its own weight
and at least 4 times the maximum intended load applied or transmitted to it.
(3) Each suspension rope, including connecting hardware, used on non-adjustable suspension
scaffolds shall be capable of supporting, without failure, at least 6 times the maximum intended
load applied or transmitted to that rope.
(6) Scaffolds shall be designed by a qualified person and shall be constructed and loaded in
accordance with that design. Non-mandatory Appendix A to this subpart contains examples of
criteria that will enable an employer to comply with paragraph (a) of this section.
1926.451(b)
(2) Except as provided in paragraphs (b)(2)(i) and (b)(2)(ii) of this section, each scaffold platform
and walkway shall be at least 18 inches (46 cm) wide.
(3) Except as provided in paragraphs (b)(3)(i) and (ii) of this section, the front edge of all
platforms shall not be more than 14 inches (36 cm) from the face of the work, unless guardrail
systems are erected along the front edge and/or personal fall arrest systems are used in
accordance with paragraph (g) of this section to protect employees from falling.
(10) Scaffold components manufactured by different manufacturers shall not be intermixed
unless the components fit together without force and the scaffold's structural integrity is
maintained by the user. Scaffold components manufactured by different manufacturers shall not
be modified in order to intermix them unless a competent person determines the resulting
scaffold is structurally sound.
1926.451(d)(14)
Gasoline-powered equipment and hoists shall not be used on suspension scaffolds.
1926.451(f)(6)
Exception to paragraph (f)(6): Scaffolds and materials may be closer to power lines than
specified above where such clearance is necessary for performance of work, and only after the
utility company, or electrical system operator, has been notified of the need to work closer and
the utility company, or electrical system operator, has de-energized the lines, relocated the
lines, or installed protective coverings to prevent accidental contact with the lines.
The clearance between scaffolds and power lines shall be as follows: Scaffolds shall not be
erected, used, dismantled, altered, or moved such that they or any conductive material handled
on them might come closer to exposed and energized power lines than as follows:
*Insulated Lines
_____________________________________________________________________
|
|
Voltage
| Minimum distance
| Alternatives
____________________|________________________|_______________________
|
|
Less than 300 volts.| 3 feet (0.9 m)
|
300 volts to 50 kv.| 10 feet (3.1 m)
|
More than 50 kv.....| 10 feet (3.1 m) plus | 2 times the length
| 0.4 inches (1.0 cm) | of the line
| For each 1 kV over | insulator, but never
| 50 kV.
| Less than 10
|
| feet (3.1 m).
____________________|________________________|_______________________
*Uninsulated lines
_____________________________________________________________________
|
|
Voltage
| Minimum distance
| Alternatives
____________________|________________________|_______________________
|
|
Less than 50 kV.....| 10 feet (3.1 m).
|
More than 50 kV.....| 10 feet (3.1 m) plus | 2 times the length of
| 0.4 inches (1.0 cm) | the line insulator,
| For each 1 kV over | but never less than
| 50 kV.
| 10 feet (3.1 m).
____________________|________________________|_______________________
1926.451(f) (12)
Work on or from scaffolds is prohibited during storms or high winds unless a competent person
has determined that it is safe for employees to be on the scaffold and those employees are
protected by a personal fall arrest system or wind screens. Wind screens shall not be used
unless the scaffold is secured against the anticipated wind forces imposed.
1926.451(g) Fall Protection
(1)(i) Each employee on a boatswains' chair, catenary scaffold, float scaffold, needle beam
scaffold, or ladder jack scaffold shall be protected by a personal fall arrest system
(3) In addition to meeting the requirements of 1926.502(d), personal fall arrest systems used on
scaffolds shall be attached by lanyard to a vertical lifeline, horizontal lifeline, or scaffold
structural member. Vertical lifelines shall not be used when overhead components, such as
overhead protection or additional platform levels, are part of a single-point or two-point
adjustable suspension scaffold.
(3)(i) When vertical lifelines are used, they shall be fastened to a fixed safe point of anchorage,
shall be independent of the scaffold, and shall be protected from sharp edges and abrasion.
Safe points of anchorage include structural members of buildings, but do not include
standpipes, vents, other piping systems, electrical conduit, outrigger beams, or counterweights.
(3)(ii) When horizontal lifelines are used, they shall be secured to two or more structural
members of the scaffold, or they may be looped around both suspension and independent
suspension lines (on scaffolds so equipped) above the hoist and brake attached to the end of
the scaffold. Horizontal lifelines shall not be attached only to the suspension ropes.
(3)(iv) Vertical lifelines, independent support lines, and suspension ropes shall not be attached
to each other, nor shall they be attached to or use the same point of anchorage, nor shall they
be attached to the same point on the scaffold or personal fall arrest system.
(4) Guardrail systems installed to meet the requirements of this section shall comply with the
following provisions (guardrail systems built in accordance with Appendix A to this subpart will
be deemed to meet the requirements of paragraphs (g) (4) (vii), (viii), and (ix) of this section):
(4)(i) Guardrail systems shall be installed along all open sides and ends of platforms. Guardrail
systems shall be installed before the scaffold is released for use by employees other than
erection/dismantling crews.
(4)(ii) The top edge height of top rails or equivalent member on supported scaffolds
manufactured or placed in service after January 1, 2000 shall be installed between 38 inches
(0.97 m) and 45 inches (1.2 m) above the platform surface. The top edge height on supported
scaffolds manufactured and placed in service before January 1, 2000, and on all suspended
scaffolds where both a guardrail and a personal fall arrest system are required shall be between
36 inches (0.9 m) and 45 inches (1.2 m). When conditions warrant, the height of the top edge
may exceed the 45-inch height, provided the guardrail system meets all other criteria of
paragraph (g) (4).
(4)(iii) When midrails, screens, mesh, intermediate vertical members, solid panels, or equivalent
structural members are used, they shall be installed between the top edge of the guardrail
system and the scaffold platform.
(4)(iv) When midrails are used, they shall be installed at a height approximately midway
between the top edge of the guardrail system and the platform surface.
(4)(vii) Each top rail or equivalent member of a guardrail system shall be capable of
withstanding, without failure, a force applied in any downward or horizontal direction at any point
along its top edge of at least 100 pounds (445 n) for guardrail systems installed on single-point
adjustable suspension scaffolds or two-point adjustable suspension scaffolds, and at least 200
pounds (890 n) for guardrail systems installed on all other scaffolds.
(4)(ix) Midrails, screens, mesh, intermediate vertical members, solid panels, and equivalent
structural members of a guardrail system shall be capable of withstanding, without failure, a
force applied in any downward or horizontal direction at any point along the midrail or other
member of at least 75 pounds (333 n) for guardrail systems with a minimum 100 pound top rail
capacity, and at least 150 pounds (666 n) for guardrail systems with a minimum 200 pound top
rail capacity.
(4)(xi) Guardrails shall be surfaced to prevent injury to an employee from punctures or
lacerations, and to prevent snagging of clothing.
1926.452(r) Catenary Scaffolds
(1) No more than one platform shall be placed between consecutive vertical pickups, and no
more than two platforms shall be used on a catenary scaffold.
(2) Platforms supported by wire ropes shall have hook-shaped stops on each end of the
platforms to prevent them from slipping off the wire ropes. These hooks shall be so placed that
they will prevent the platform from falling if one of the horizontal wire ropes breaks.
1926.452(w) Mobile Scaffolds
(1) Scaffolds shall be braced by cross, horizontal, or diagonal braces, or combination thereof, to
prevent racking or collapse of the scaffold and to secure vertical members together laterally so
as to automatically square and align the vertical members. Scaffolds shall be plumb, level, and
squared. All brace connections shall be secured.
(1)(i) Scaffolds constructed of tube and coupler components shall also comply with the
requirements of paragraph (b) of this section;
(2) Scaffold casters and wheels shall be locked with positive wheel and/or wheel and swivel
locks, or equivalent means, to prevent movement of the scaffold while the scaffold is used in a
stationary manner.
(4) Power systems used to propel mobile scaffolds shall be designed for such use. Forklifts,
trucks, similar motor vehicles or add-on motors shall not be used to propel scaffolds unless the
scaffold is designed for such propulsion systems.
(5) Scaffolds shall be stabilized to prevent tipping during movement.
(6) Employees shall not be allowed to ride on scaffolds unless the following conditions exist:
(6)(i) The surface on which the scaffold is being moved is within 3 degrees of level, and free of
pits, holes, and obstructions;
(6)(ii) The height to base width ratio of the scaffold during movement is two to one or less,
unless the scaffold is designed and constructed to meet or exceed nationally recognized
stability test requirements such as those listed in paragraph (x) of Appendix A to this subpart
(ANSI/SIA A92.5 and A92.6);
(6)(iii) Outrigger frames, when used, are installed on both sides of the scaffold;
(6)(iv) When power systems are used, the propelling force is applied directly to the wheels, and
does not produce a speed in excess of 1 foot per second (.3 mps); and
(6)(v) No employee is on any part of the scaffold which extends outward beyond the wheels,
casters, or other supports.
(8) Where leveling of the scaffold is necessary, screw jacks or equivalent means shall be used.
(9) Caster stems and wheel stems shall be pinned or otherwise secured in scaffold legs or
adjustment screws.
(10) Before a scaffold is moved, each employee on the scaffold shall be made aware of the
move.
1926 Subpart L Appendix A (Non-mandatory guidelines)
(r) "Catenary scaffolds." (1) Maximum intended load -- 500 lbs.
(2) Not more than two employees shall be permitted on the scaffold at one time.
(3) Maximum capacity of come-along shall be 2,000 lbs.
(4) Vertical pickups shall be spaced not more than 50 feet apart.
(5) Ropes shall be equivalent in strength to at least 1/2 inch (1.3 cm) diameter improved plow
steel wire rope.
Conclusion:
From the definitions outlined in 29 CFR 1926.450 we believe the spacer cart qualifies as a
mobile, catenary-type, suspended scaffold. Specifically, it is a temporary, elevated platform,
used to support materials and employees. The spacer cart is also powered, portable, wheelmounted, and supported by horizontal and parallel ‘ropes’. Some of the requirements, such as
the distance from power lines and the tie off locations for personal arrest systems, are not
feasible given the location of work to be accomplished, and will likely require consultation with
an engineer accredited by the Scaffold & Access Industry Association (SAIA)
A3 - Drawings
Do to drawing native sheet size (ISO A2) - all drawings will be provided via separate
correspondence.
A4 - Design Envelope Restrictions
Figure 3: Design change limit envelope - Axial view
Figure 4: Design change limit envelope - Transverse view