Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: GLUED LEDGER HEAD
FIELD OF THE INVENTION
The present invention relates to components for
modular scaffolding systems and in particular relates to
components made of two different materials.
BACKGROUND OF THE INVENTION
Modular scaffolding systems are extensively used
in the building and repair industries and provide safe
and efficient access to elevated areas. Modular
scaffolding systems have advantages over the conventional
tube and clamp systems in that the components connect to
one another at predetermined positions and the systems
are more cost effective to erect and tear down.
Typically modular scaffolding systems are made of
galvanized steel components. There are a number of well
known proprietary arrangements that allow the components
to be securely fixed to each other. In most cases the
systems are not cross compatible.
Galvanized steel is the material of choice as it
is robust and able to withstand shock impacts and provide
good structural strength required to support the loads
that are applied to the scaffolding from wind, materials
and personnel.
In some industries there is an increasing tendency
to favour the use of lighter weight scaffolding systems
in general and in particular to use aluminum scaffold
products. For example, aerospace applications and
computer/IT applications often use aluminum scaffolding
products as such components are lighter in weight and if
accidentally dropped are less likely to cause extensive
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damage to the surroundings compared to the heavier
galvanized steel product.
All aluminum modular scaffolds have not been
considered acceptable for industrial applications (such
as refineries) as the end connections (ledger heads etc.)
of these components are prone to damage or need to be of
a significantly larger size to resist abuse and shock
loads that are common in the work place. In Canada the
use of aluminum scaffold products in the industrial
market has generally been restricted to aluminum tube and
clamp systems that use aluminum tube and steel tube
couplers.
Although aluminum scaffolding systems have not
been generally accepted, such systems are considered
desirable due to the lighter weight of the components.
Workers who are erecting the system or tearing it down
certainly prefer the lighter weight of the components.
Therefore in general the assemblers would prefer to use
lighter weight components for ease of assembly however
the actual labour component for erecting aluminum tube
and clamp systems is much higher.
A hybrid system that proposed using steel
connecting heads mechanically pinned or fastened to
aluminum tubes was not successful or accepted as the
softer aluminum tube deforms at the pin or fastening
point compromising the mechanical connection and
rendering the system prone to damage.
The present invention seeks to overcome a number
of deficiencies with respect to the prior art and
advantageously use a combination of steel elements
secured to aluminum tubes to provide a system which is
both effective and convenient to install due to the
reduce weight thereof.
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SUMMARY OF THE INVENTION
A modular scaffold component according to the
present invention comprises first and second steel ledger
heads with each ledger head including a fastening element
to mechanically connect the ledger head to spaced
scaffold support members. An elongate tube extrusion of
an aluminum or aluminum alloy is adhesively secured to
each ledger head and forms a structural element
therebetween. The elongate tube extrusion in each ledger
head has an overlapping sleeve type relationship with
opposed overlapping surfaces secured by an adhesive
structurally connecting the ledger heads and providing a
secure mechanical connection of each ledger head to the
tube extrusion for use in a scaffold system.
According to an aspect of the invention the
modular scaffold component includes a steel tube
connecting stub sized for receipt in the elongate tube
extrusion with the adhesive securement therebetween.
According to a further aspect of the invention the
steel tube connecting stub overlaps with the elongate
tube extrusion over at least one and a half inches in the
length of the elongate tube extrusion.
In a further aspect of the invention the steel
tube connecting stub is welded to a rear surface of the
ledger head.
In a modular scaffold component according to the
present invention the component includes at least a
ledger head and a connected elongate tube. The
improvement comprises an adhesive securement of the
ledger head to the elongate tube and each ledger head
includes a connecting portion having an overlapping
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sleeve type relationship with an end of the elongate
tube. Preferably an adhesive component is provided
within a gap of the overlapping sleeve type relationship
and forms a distributed mechanical connection of the
elongate tube to the at least one ledger head. The
elongate tube is of an aluminum or an aluminum alloy
material and the ledger head is of a steel material.
In a preferred aspect of the modular scaffold
component the adhesive is a two part adhesive forming a
permanent rigid connection of the ledger head to the
elongate tube.
With the present invention it is preferable that
the elongate tube includes interior reinforcing at the
top and bottom thereof to improve the characteristics of
the elongate tube with respect to bending. Reinforcing
on the interior is preferred as the outer diameter of the
aluminum tube can be the same as an equivalent steel
component. This allows the composite modular component
to be used throughout the system and in particular allows
clamping arrangements used with most modular systems to
also engage the aluminum tube component.
Preferably the reinforcing on the interior of the
aluminum tube is also designed to allow a particular
registration with the ledger head. The ledger head
preferably includes a portion that is inserted into the
elongate tube and cooperates therewith to provide a
predetermined registration. In this way the orientation
of the ledger head with respect to the elongate tube is
simplified and the manufacture of the component is also
simplified.
Reinforcing of the elongate tube also has the
desirable characteristic that the strength of the
component will be the same or approximately the same as
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the corresponding steel component. The composite
component is lighter weight and easier to use, however
there is no requirement to retrain the labour or to have
a composite component which is of reduced strength
relative to a steel one. Providing a lighter component
that is a replacement for the heavier steel component
while still providing the same functionality is a highly
desirable feature of the present invention.
Adhesive securement of the ledger heads to the
elongate tube or other structural connecting component of
a reduced weight is the preferred arrangement. It is
possible to use other arrangements including the
mechanical securement of these components while providing
the particular advantages. If an alternate arrangement
is to be considered a filler type material is preferable
between the connecting portion of the ledger head and the
tube to avoid play or movement between these components.
For example a bonded type filler arrangement can reduce
play and can rely on mechanical securement of a pin type
connection or dimple type connections to maintain the
rigidity of the system.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown
in the drawings, wherein:
Figure 1 is a schematic view of a ledger as used
in scaffolding systems;
Figure 2 is a partial sectional view of a scaffold
ledger showing the connection of a ledger head to an
aluminum tube;
Figure 3 is a front view of a guard rail component
made of generally aluminum tubes with steel ledger heads
for releasable connection to the scaffolding structure;
Figure 4 is a further view of a scaffold component
made of generally aluminum tubes and having steel ledger
head connectors;
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Figure 5 is a side view showing an aluminum tube
connected to a ledger head having a downwardly extending
drop pin;
Figure 6 is a side view of a scaffold component
having an aluminum tube and an inner steel tube for
connection to a scaffold connecting member;
Figure 7 is a partial perspective view showing the
connection of a ledger head to an aluminum tube;
Figure 8 is a perspective view similar to Figure 7
without a mechanical pin connection between the ledger
head and the aluminum tube;
Figure 9 is a partial perspective view of an
alternate arrangement where the aluminum tube includes
thickened top and bottom sections and the ledger head
includes an extending spigot designed to have a
particular registration with the aluminum tube;
Figure 10 is a view similar to Figure 9 without a
connecting pin;
Figure 11 is a perspective view of a 5 X 5 foot
scaffold tower with raised work platform; and
Figure 12 is a perspective view of a preferred
cast ledger head.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 generally illustrates a ledger 2 that is
a modular component that is commonly used in scaffolding
systems and is sometimes used as a brace member in
shoring systems. There are a number of scaffolding
systems commonly used and typically these proprietary
systems use their own arrangement for connecting of a
modular bracing component to a scaffold leg or to a
scaffold frame. One common connecting system as shown in
Figure 3 uses a circular connecting rosette 104 that is
welded to the uprights 103 of a scaffolding frame 102 at
predetermined points. These rosettes 104 allow
convenient connection with a ledger head 4 or 4a as
generally shown in Figures 2 and 3. A drop wedge 5
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passes through the ledger head and through the rosette
and positively secures the ledger head 4a to the upright
103 of the scaffold frame 102. Typically the drop wedge
is captured or retained in the ledger head. These
5 rosettes can also be used on the uprights of scaffold
frame members to simplify connection. Other common
connection arrangements include a cup and prong type
arrangement where the prong is inserted into an upwardly
extending cup and a top member moves downwardly to retain
the prong in the cup. As can be appreciated, the present
invention as subsequently described can be used with
different scaffolding connecting arrangements and is not
limited to the particular ledger head design shown in the
drawings.
Ledgers of the general structure as shown in
Figure 1 are commonly used in modular scaffolding
systems. One of the desirable features of a modular
scaffolding system is the convenience and labour saving
in erecting the system. Modular systems are based on
fixed positions of various components that interconnect
in a simplified manner. In contrast, tube and clamp
scaffolding arrangements are custom designed and require
more skilled labour. In modular scaffolding systems it
is cost effective to use relatively large spacing between
the upright components. Many systems use a 5 foot, 7
foot or 10 foot bay spacing between frames. Conventional
ledgers made of steel tube with a cast or formed steel
ledger head either end thereof can easily be carried by a
labourer and steel frames can be carried by two
labourers.
Figure 11 is an example of a 5 foot X 5 foot tower
200 commonly used in oil refinery maintenance
applications. The tower includes a raised work platform
202 having guard rails 204 thereabout. The tower
includes elongate legs or uprights 206 connected via
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rosettes to the guard rails 204, the ledgers 208 and the
diagonal braces 210. The ledgers 208 preferably use the
higher load carrying capability of the tubes shown in
Figures 9 and 10. With this arrangement it is not
necessary to distinguish between steel and steel/aluminum
components as the load carrying capacities are designed
to be similar to allow free substitution. In the case of
a 7 foot load carrying ledger a double tube aluminum
ledger will be used. These components all use the steel
ledger heads or connectors in combination with adhesively
secured aluminum or aluminum alloy tubes.
In essentially all applications, the workmen used
to erect these systems prefer a small bay spacing as the
components are lighter and therefore easier to carry and
assemble. In applications or industries where the labour
component has a significant influence in determining an
acceptable system, the smaller separation distances such
as 7 foot bay separation is often used where it would be
more cost effective to use the larger 10 foot bay
spacing. For higher load applications a 5 foot bay
spacing is used.
In order to overcome this bias while still being
able to achieve the desired labour savings in erecting of
a modular scaffolding system, the present invention uses
aluminum tubes that have a specific connection with a
steel ledger head or similar steel connecting component.
The ledger head 4 shown in Figure 2 is of a steel
material and can be formed from plate or may be a cast
component. The steel of the ledger head is desirable due
to its high strength and also its tolerance with respect
to potential damage on the work site. These steel ledger
heads are very durable and are not prone to damage.
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The ledger head 4 as shown in Figure 2 includes a
system connecting end 6 which will have the particular
shape for engaging the intended modular scaffolding
system. In this case there is a central slot 7 which
receives part of the flange of a connecting rosette 104.
A wedge member will pass through the center passage 9 to
effectively lock the ledger head to a scaffold component
as shown in Figure 3.
Directly opposite the system connecting end 6 is
the tube connecting end 8. This end of the ledger head 4
is designed to cooperate and engage the aluminum tube 14.
The aluminum tube 14 replaces the conventional steel tube
that would be used for such a ledger. In the structure
shown in Figure 2, the tube connecting end 8 includes an
inner stub collar 20 which is received interior to the
steel tube connecting stub 10. The length of the steel
tube connecting stub 10 is relatively short and is
typically in the range of 2 to 6 inches. The length of
this steel tube connecting stub provides a large adhesive
securement area with the aluminum tube 14. A suitable
adhesive 22 is shown that connects the steel tube
connecting stub 10 to the end of the aluminum tube 14.
The adhesive 22 is preferably a two part epoxy type
adhesive.
The steel tube connecting stub 10 in this case is
shown secured to the ledger head 4 by a weld 12. It can
also be seen that the inner stub collar 20 projects into
the steel tube connecting stub 10 a sufficient distance
to overlap with the end of the aluminum tube 14. The
inner stub collar 20 provides support for both the
connecting stub 10 and the end of the aluminum tube 14.
This overlap arrangement provides an effective
cooperation between the elements and opposes shear. The
adhesive 22 is selected to provide strong securement of
the aluminum tube 14 to the connecting stub 10. The
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adhesive is also preferably selected to electrically
isolate the steel tube connecting stub 10 from the
aluminum or aluminum alloy tube 14. This reduces the
possibility of corrosion due to the dissimilar metal
materials.
It has been found that the adhesive 22 can provide
a durable fixed securement of the aluminum tube to the
ledger head that is not prone to damage. The length of
the steel tube connecting stub 10 in a preferred
embodiment is of a length of about 2 to 3 inches and
provides sufficient area for adhesive securement of the
aluminum tube to the ledger head. This adhesive
securement provides effective connection for the design
loads for ledgers and other scaffold connecting
components. The adhesive is selected to comply with the
appropriate temperature ranges. For example, scaffolding
systems used in large refineries in northern Canada can
encounter winter conditions of -45 F whereas scaffolding
systems in the southern United States encounter summer
temperatures in excess of 100 F. The adhesive is
selected to be effective for the temperature ranges and
satisfy typical loads caused by dropping or otherwise
onsite job conditions that the components experience at
all temperatures within the predetermined ranges.
As can be appreciated from a review of Figure 2,
the steel tube connecting stub 10 provides an effective
arrangement for distributing forces exerted on the ledger
head to the aluminum tube without damage thereof.
It has been found desirable to increase the wall
thickness of the aluminum tube that would otherwise
replace the commonly used steel tube. For example, it is
common to use a steel tube with a 3.2 mm thickness and to
provide additional strength the aluminum tube has a
thickness of approximately 3.7 mm. As can be appreciated
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there will be more aluminum material however the overall
weight of the ledger will be significantly reduced. This
lighter system component (i.e. the conventional type
steel material for the ledger head in combination with an
aluminum tube type member connected thereto) acts as a
equivalent component with the desirable reduced weight.
This has significant advantages to the workforce
responsible for erecting and dismantling of such modular
systems. Typically ledgers are divided into load bearing
ledgers and non load bearing ledgers. A 10 foot ledger
is non load bearing and the thicker aluminum type is
satisfactory for this application. For load bearing
applications common in 5 foot ledgers a different
structure of the aluminum tube and ledger is used.
From the above, it might be considered that an all
aluminum system would be preferable. Unfortunately an
all aluminum system with aluminum ledger heads is prone
to damage or the ledger heads must be greatly oversized
to provide the required durability. This results in a
component that is less desirable and/or more expensive to
make. The steel/aluminum hybrid system as described
herein having steel ledger heads in combination with an
elongate aluminum tube extrusion is both cost effective
to manufacture and light weight.
Figures 3, 4, 5, 6, 7 and 9 show a ledger
connected to a ledger head that also includes securing
ports for a pin connector to provide additional
mechanical securement of the ledger head to the tube.
This pin type connection is normally not necessary as the
adhesive securement of the components is sufficient,
however it does provide a further well recognized
connection. There may be some reluctance to initially
accept the high structural integrity of the adhesive
securement and therefore a mechanical pin connection is
also provided. Furthermore the adhesive and mechanical
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connections complement one another particular during
assembly. For example, the pin connection maintains the
position of the components as the adhesive cures or
otherwise sets. With the pin connection the core after
initial gluing is reduced. Figures 8 and 10 show a ledger
head and aluminum tube connection that relies solely on
the adhesive securement of the component.
The ledger head as shown in Figure 2 has been
described with respect to a separate steel tube
connecting stub 10. This arrangement is desirable for
existing ledger head designs that can be used directly or
require only a small modification to receive the steel
tube connecting stub. As the system becomes more widely
accepted it is anticipated that the ledger head and the
connecting stub 10 will be an integral component of the
ledger head. In this case the ledger head would not
include the stub collar 20 as basically the stub collar
and the steel tube connecting stub 10 are an integral
20 component of the ledger head 4.
The particular connection of the ledger head 4 to
the aluminum tube 14 can also be used for connecting of
other modular dual material scaffolding components.
Figure 3 shows a guard rail frame 30 connected between
two uprights 100 and 102 shown with rosette connections
104. This is one particular type of scaffolding
connection system however it can be appreciated that
other arrangements are also possible. In this case the
upper rail of the guardrail frame 30 includes steel
ledger heads 4a including drop wedges 5. The steel
connectors 4b associated with the lower rail of the guard
rail frame 30 are drop pin connectors having a fixed
extending pin 9 which is received in a slot of the
rosette 104. The upper rail 32, the lower rail 34 and
the connecting tubes 36, 37 and 38 are of an aluminum
tube construction. The connection between the upper rail
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32 and its connecting components are similar to the
structure as described in Figure 2. This is also true
for the lower rail 34 and the connectors 4b.
As can be appreciated from a review of Figure 3,
the ends of the guard rail frame 30 (i.e. the connecting
components 4a and 4b) are of a conventional steel
configuration and are typically of galvanized steel.
These connections provide a robust structure that is not
prone to damage. The intermediate components connecting
these connectors to the uprights 100 and 102 are of a
lighter weight aluminum tubing and as such the guard rail
frame 30 will be of significantly reduced weight relative
to a similar guard rail frame that uses steel tubing.
Figure 5 shows further details of the connector 4b
shown in Figure 3. This connector includes an inner
steel tube connecting stub 10, an outer aluminum tube 14
and a suitable adhesive provided between these two
components. A connecting pin 15 has been shown.
An aluminum frame 40 is shown in Figure 4 attached
to steel vertical members of a scaffold frame 100 and
102. The tubular members of the frame 40 as well as the
center connecting member are of an aluminum material and
thus provide a significant weight advantage. The
downwardly angled diagonal tubes 42 and 44 are also of
aluminum and connected to the short aluminum tube 46 and
48 associated with the steel leg engaging members 47 and
49. This aluminum frame provides the weight advantages
while still providing good load bearing capabilities.
Designing the combination steel/aluminum components to
have equivalent strength as the existing steel components
allows mixing of components without regard to type.
The connector 110 shown in Figure 6 includes the
outer aluminum tube 14 in combination with the inner
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steel tube connecting stub 10. These components are
secured by an adhesive. The end 112 of the steel tube
connecting stub 10 has been adapted for welded securement
to a similar steel tube. As can be appreciated the steel
tube connecting stub 10 can take different configurations
appropriate for connection to a steel component.
Additional details of the hybrid system can be
appreciated from a review of Figures 7, 8, 9 and 10.
Figure 7 shows the described connection arrangement with
two ports for receiving a connecting pin that is used in
addition to the adhesive securement. Figure 8 is a
prospective view essentially the same as Figure 7 but
without a connecting pin securement. Adhesive securement
can be supplemented with a mechanical securement to meet
any design or safety requirements. As a steel stud or
tube is stronger than an equivalent sized aluminum tube,
it is generally preferred to have the aluminum tube
sleeve the steel connecting stud or tube. This
relationship can be reversed as the more important issue
is the adhesive securement therebetween.
The structures of Figures 9 and 10 are similar to
Figures 7 and 8 however the cross section of the aluminum
tube has been modified. Figure 9 again includes ports
for a connecting pin whereas Figure 10 relies solely on
the adhesive securement. Other securing arrangements can
be used in combination with the adhesive if desired,
however the adhesive on its can provide the required
strength as the adhesive securement area can be increased
when necessary.
In Figures 9 and 10 both the steel tube connecting
stub l0a and the aluminum tube 14a have been modified to
provide several advantages. Basically the ledger head
with the steel tube connecting stub secured thereto can
only be received in the modified aluminum tube 14a in one
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of two orientations. With this arrangement it is easy to
assemble the ledger heads to the aluminum tube and
provide accurate alignment of the two ledger heads.
The modified cross section of the aluminum tube
14a provides additional strength with respect to bending.
A thickened portion is provided at the top surface of the
modified aluminum tube 14a and the bottom surface of the
tube has also been increased. This structure provides a
reinforced or higher strength top and bottom flange
joined by the side members. Similarly the connection to
the ledger head is also improved. This modified tube
also cooperates with the ledger head to provide
registration and thus simplifies assembly of the
components. The steel tube connecting stub 10a includes
a top and bottom surface that closely corresponds to the
modified tube 14a to provide the registration function.
A modified ledger head 304 is shown in Figure 12
and is a one piece cast steel material with the
connecting stub 310 including a flattened top surface 330
and a corresponding flattened bottom surface 332.
The sides 340 and 342 of the connecting stub 310
each include a pair of shallow grooves 336 and 338 that
preferably have a depth of approximately 2 mm. The
grooves 336 and 338 are positioned either side of the
connecting port 350. Each of the sides 340 and 342
include a center flattened portion 354.
The purpose of the flattened portions 354 and the
shallow grooves 336 and 338 are to assist in the
distribution of the adhesive (preferably an epoxy
adhesive and most preferably an acrylic epoxy adhesive).
These flattened portions 354 and the grooves 336 and 338
provide channels that allow the epoxy to flow and provide
a large adhesive securing surface about the connecting
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stub 310. As can be appreciated from a review of Figure
9, the modified aluminum tube extrusion 14a includes a
flattened top surface 360 and bottom surface 362 interior
to the pipe for closely engaging surfaces 330 and 332 of
the cast ledger head. These grooves and flattened area
provide additional space to receive excess epoxy while
still providing efficient coating about the connecting
stub. In addition these grooves will act to further
improve the attachment of the tube to the ledger head.
As can be appreciated, the ledger head of Figures 9, 10
and 12 with the modified aluminum pipe extrusion as shown
in Figures 9 and 10 provide a simple arrangement where
the ledger heads are registered with the tube and thus
improve the assembly of the components. The grooves 336
and 338 provided either side of the connecting stub in
combination with the flattened portions 354 also simplify
securement of the ledger head to the tube and also assist
in providing excellent locking of the ledger head to the
tube extrusion.
The modified tube extrusion as shown in Figures 9
and 10 include the thickened top area 380 and the
thickened bottom area 382 that provides additional
strength for the tube extrusion to oppose bending of the
tube in the vertical plane assuming the tube was
horizontal. These act in a way similar to the top and
bottom flanges of an I-beam type shape. The size of the
areas 380 and 382 can be adjusted to provide the desired
structural characteristics of the tube while also
simplifying the registration of the ledger head with the
tube. The opposed curved side walls 384 and 386 of the
tube 14a closely correspond to the side walls 340 and 342
of the ledger head. These sidewalls are somewhat
thinner, preferably about 3.20 mm with an outside tube
diameter of 48.30 mm.
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It is highly desirable to maintain the common
outside steel tube diameter of 48.30 mm to allow
compatibility with a host of existing clamps and other
components designed for this tube diameter.
The following is a comparison of a ten foot
aluminum ledger with the modified tube extrusion shown in
Figures 9 and 10 to an all steel ten foot ledger:
Attribute Regular Aluminum Limiting
Steel Bearer Factor
Ledger w/Glued
Ledger
Heads
Weight of Aluminum loft 11.1 5.98 -
Ledger c/w ledger heads
and wedges (kg)
Calculated Safe Bending 0.50 0.52 Allowable
Moment (kN.m) Bending
Stress in
Tube
End Vertical Load 8.9 8.9 Rosette
Capacity (kN) Capacity
Safe tensile strength 14.19 14.19 Rosette/Wedge
of ledger head to Capacity
rosette connection (kN)
Ten and seven foot ledgers are not designed for
substantial load carrying capability. In contrast a five
foot steel ledger is often referred to as a bearer for
significant load carrying capacity. The modified
aluminum tube ledger is designed to have properties
generally the same as the conventional steel tube and
significantly stronger than conventional or regular
aluminum scaffold tube.
The following tables provide a comparison of the
modified aluminum tube to a regular aluminum tube, an
equivalent high strength aluminum tube having a thickened
wall to provide properties similar to a conventional
steel tube and the conventional steel tube section.
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REGULAR ALUMINUM SCAFFOLD TUBE OPTIMIZED ALUMINUM LEDGER TUBE
3.66 3.20
M M
am'
0 o
SECTION PROPERTIES SECTION PROPERTIES COMPARISON TO REGULAR
ALUMINUM SCAFFOLD TUBE
SECTION MODULUS: 5330 mm"3 SECTION MODULUS: 7746 mm"3 45% Stronger
MOMENT OF INERTIA: 128730 mm"4 MOMENT OF INERTIA: 187085 mm"4 45% Stiffer
WEIGHT: 1.392 kgtm WEIGHT: 1.872 kgim 34% Heavier
The table above shows the section properties of
aluminum scaffold tube, commonly used in Canada, compared
to the optimized version referred to in the present
patent application. It can be seen that, although the
optimized version is 34% heavier, it is 45% stronger and
45% stiffer. The optimized section is as strong as the
regular steel scaffold ledger tubes when subjected to
bending stresses.
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EQUIVALENT STRENGTH ALUMINUM TUBE OPTIMIZED ALUMINUM LEDGER TUBE
6.28 3.20
O CDC,
M C
dam'
--00
SECTION PROPERTIES SECTION PROPERTIES COMPARISON TO REGULAR
ALUMINUM SCAFFOLD TUBE
SECTION MODULUS: 7746 mm'3 SECTION MODULUS: 7746 mm"3 Identical strength
MOMENT OF INERTIA: 187085 mm-4 MOMENT OF INERTIA: 187085 mm"4 Identical
Stiffness
WEIGHT: 2249 kgtm WEIGHT: 1.872 kgtm 17% lighter
The table above shows the section properties of an
aluminum scaffold tube with a constant wall thickness and
with the same strength and stiffness as the optimized
aluminum ledger tube. It can be seen that, because the
aluminum is not distributed in the most favourable
manner, the regular tube is 17% heavier compared to the
optimized section.
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STEEL SCAFFOLD LEDGER TUBE OPTIMIZED ALUMINUM LEDGER TUBE
3.20
0 3.20
CD CD
le
0
0 T,
SECTION PROPERTIES SECTION PROPERTIES COMPARISON TO REGULAR
ALUMINUM SCAFFOLD TUBE
SECTION MODULUS: 4774 mm"3 SECTION MODULUS: 7746 mmA3 62% Higher
MOMENT OF INERTIA: 115292 mm"4 MOMENT OF INERTIA: 187085 mm"4 62% Higher
WEIGHT: 3.539 kgtm WEIGHT: 1.872 kgtm 47% Lighter
The table above shows the section properties of a
typical steel scaffold ledger tube with the same bending
strength as the optimized aluminum ledger tube. It can be
seen that the optimized aluminum ledger tube is 47%
lighter compared to the steel tube.
As can be seen the modified tube is lighter and
structurally stronger by providing more material on the
interior of the top and bottom areas of the tube
(preferably of equal size). The side walls have been
slightly reduced (relative to conventional aluminum tube)
to a thickness of 3.20 mm.
From the above, it can be seen that the safe load
capacities of ledgers is limited by the strength of the
rosette connection. This means that the performance of
the aluminum ledger is equivalent to steel ledgers. The
aluminum ledger will deflect more under load, but there
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is no regulatory limit imposed upon the maximum allowable
deflection of scaffold ledgers. As can be appreciated as
the length of the ledger decreases the strength will
increase as it is preferred to use the same tube
extrusion. With this arrangement the aluminum bearer
with glued ledger head can be freely used as a
replacement for the conventional all steel bearer.
Two part acrylic adhesives are normally available
in different grades having different working times. The
adhesive is preferably selected to be operative once
cured in a temperature range from -40 C to 149 C.
Desirably the adhesive resists dilute acids, alkalies,
solvents, greases, oils, moisture, salt spray and
weathering. Furthermore it is preferable that the
adhesive is nonconductive and provides an electrical
insulating property separating the steel ledger head from
the aluminum tube and thus reduces possible galvanic
action therebetween.
Due to the scaffold application the adhesive must
be tolerant to vibration and a more flexible adhesive is
preferred. More brittle adhesives can crack under impact
load that can occur by dropping or striking with a hammer
that may occur during connection to a scaffold leg.
Two suitable adhesives for the glued ledger head
and pin structure are as follows:
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Adhesive 1:
Manufacturer: Extreme Adhesives Inc.
Type: Methacrylate 2-part Adhesive System
Product Name: Extreme 310
Shear strength 3,300 psi
as rec'd aluminum):
Tensile Elongation: 600
Thermal Service -65F to +260F
Range:
Mix Ratio: 1:1
Working Time 13 to 17 minutes
at 75 degrees F):
Colour: Straw or Black
Clean-up Solvents: MEK, Acetone
Adhesive 2:
Manufacturer: Parsons Adhesives Inc.
Type: Methacrylate 2-part Adhesive System
Product Name: 7120
Shear strength 3,000 to 3,300 psi
as rec'd aluminum):
Tensile Elongation: 45o to 500
Thermal Service -40F to +260F
Range:
Mix Ratio: 1:1
Working Time 120 to 130 minutes
at 75 degrees F):
Colour: Cream or White
Clean-up Solvents: MEK, Acetone
It is believed an appropriate amount of tensile
elongation is desired to avoid brittle fracture of the
adhesive by vibration or shock load during use of the
components. A tensile elongation of the adhesive of at
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least about 20% is considered appropriate. The preferred
adhesives above have much higher elongation and still
provide sufficient tensile strength in excess of 20,000
lbs.
The following is an example of the assembly
procedure:
Surface Preparation:
IMPORTANT: In order to maximize joint strength,
preparation of the joint mating surfaces must take place
not more than two hours before assembly.
STEP 1: Clean the ends and interior surfaces of the
aluminum ledger tube and the mating surfaces of the
ledger head castings with MEK or Acetone for a length of
2". Allow to dry.
STEP 2: Abrade the internal mating surfaces of the
aluminum bearer with a flapper wheel or by hand sanding
using 100 grit abrasive paper. Carefully remove all grit
and dust from inside the ledger tube.
STEP 3: Carefully abrade the galvanized mating surfaces
of the ledger head by hand using 100 grit abrasive paper,
taking care not to break through the zinc galvanized
coating.
Application of Adhesive:
Note: To be undertaken in a working temperature of 70
degrees F to 75 degrees F.
STEP 4: Mix a sufficient amount of adhesive in accordance
with the manufacturer's recommendations.
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STEP 5: To achieve an even layer of adhesive, trowel the
material onto both ledger heads and inside the aluminum
extrusion using a 24 tpi hacksaw blade, or similar, as
the trowel.
STEP 6: Apply adhesive to all mating surfaces inside both
ends of the aluminum profile, for a length of at least
2", making sure that there is a build-up at the curved
inside portions on the inside of the aluminum profile
(where there is a wall thickness 3.2mm). Adhesive must
also be applied to end surfaces of the aluminum profile
to provide a sealed joint against the rear of the ledger
head casting.
STEP 7: Apply the adhesive to the mating surfaces of both
ledger heads, making sure that all surfaces are covered
and that there is a build-up of adhesive on the curved
sides of the ledger head mating surfaces. Adhesive must
also be applied to the flat mating surface where the end
of aluminum ledger tube will butt against the rear of the
ledger head.
Assembly of Ledgers:
Note: Assembly must be carried-out within the working
time of the selected adhesive, after mixing and in
temperatures that comply with the adhesive manufacturer's
recommendations.
STEP 8: Push the ledger head fully into the end of the
aluminum ledger tube until it will go no further. Ensure
that the end of the aluminum ledger tube is fully in
contact with the flat surface at the rear of the ledger
head.
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STEP 9: Immediately secure the joint with the specified
zinc plated steel coil pin to hold the ledger head in
place while the adhesive cures.
STEP 10: Repeat steps 8 though 10 for the second ledger
head.
STEP 11: Clean off any excess adhesive that has been
squeezed out the end of the aluminum ledger tube during
assembly with acetone or MEK.
STEP 12: Secure Wedges to both ledger heads.
STEP 13: Store the assembled ledger assembly
horizontally, in a warm area, during the curing period.
As can be appreciated, particularly with the
designs shown in Figures 9, 10 and 12, the connecting
stubs of the ledger heads may be coated with a mixed two
part adhesive and then subsequently inserted in an
appropriate length of the aluminum tube. Registration of
the ledger head with the aluminum tube is achieved due to
the particular cooperation therebetween. Once the stub
is inserted, a suitable pin connector can be inserted if
the ports are provided or the components may merely be
maintained in the assembled orientation until such time
as the adhesive has cured. The connecting stub overlaps
with the end of the tube 14a and includes a large area
for adhesive securement therebetween. Basically this
area determines the mechanical strength of the
connection. Furthermore the required skill to make the
connection is low and reliability is high.
In the preferred arrangement the entire surface of
the connecting stub is coated with adhesive whereby a
strong mechanical connection of the components is
achieved. The particular level of this mechanical
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connection is a function of the surface area of the
adhesive securement between the aluminum tube and the
ledger head. A connecting stub of approximately one and
a half inches in length coated thereabout and cooperating
with an appropriate sized aluminum tube will provide a
mechanical securement having a shear strength of 3000 to
4000 psi. As can be appreciated there is some tolerance
between the connecting stub and the interior of the
aluminum tube which is essentially filled by the two part
adhesive. The two part adhesive will also assist in the
insertion of the connecting stub into the tube. It is
anticipated that if the epoxy has a working time of about
10 to 20 minutes that the epoxy can be dispensed in an
automated manner such that the two parts mix upon
application to the connecting stub and this will provide
more than sufficient time to allow the ledger head to be
properly positioned either end of the tube. Typically
the assembled parts will be placed on a rack or other
suitable structure to avoid inadvertent forces until such
time as the epoxy is cured or partially cured.
It is also possible to use the modified aluminum
or aluminum alloy tube section and the cooperating steel
ledger heads in a mechanically secured configuration.
The close fit of the ledger heads in or about the
aluminum tube provides a larger area for distributing
loads therebetween. It is preferable to fill any gap or
tolerance between the overlapping surfaces of the tube
and ledger head with a filler material that need not
provide the adhesive securement earlier specified. A pin
or other mechanical securement can be used. With this
arrangement a ledger or other scaffold component that
previously used a steel connecting arrangement can be
modified using the modified tube to provide weight
advantages while be generally equivalent or allowing
substitution for the conventional heavier steel
component.
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A vertical pin securement through the thickened
portions may be preferred for both the mechanical and the
mechanical adhesive securement.
With this arrangement it is desirable to provide a
mechanical securement that distributes the load and
avoids high stress areas or a loose connection. It is
also desirable to electrically separate the steel ledger
from the aluminum tube.
In some all steel scaffold systems, a ledger is
fabricated as a single piece component with the ledger
heads formed by cutting and deforming the ends of the
tube. Basically the ends are slotted and flattened to
form ledger heads. For this type of system an
aluminum/steel system would use short steel tubes with
integral ledgers adhesively secured to an aluminum tube.
In this way the same type of ledger head connection is
maintained.
The present arrangement allows effective
integration of steel scaffolding posts or frames that use
conventional mechanical connection of components with the
hybrid dual material scaffold components. The connecting
components such as guard rails, guard rail frames, ledger
members, diagonal bracing members, and other components
which connect to such uprights, advantageously use a
steel head or connecting portion to form the mechanical
connection. The steel connecting components have been
modified to include an adhesive securement of an aluminum
tube to one side of the component. Thus a significant
portion of each of these connecting members are made of
aluminum or aluminum alloy and are of reduced weight
relative to the same component fabricated using steel
tubing. Advantageously the system is lighter weight and
as such it is possible to use greater separation between
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the upright members (more cost effective) or more
favorable conditions for the labour erecting the system
if they choose to use a smaller separation distance
between uprights.
In refineries in northern Canada modular
scaffolding systems are used extensively and 10 foot base
spacings could be used for many applications. However in
most cases smaller bay separation is used as the work
force assembling the systems prefers the smaller and
lighter weight components. The hybrid system as
disclosed herein can be used to allow a larger spacing to
be used while also providing a system which is
lightweight and thus more acceptable to the installing
workmen. The modified system does not need to replace
all of the existing steel components as the upright
members continue to be of steel and only the connecting
components are replaced or added. Furthermore it can be
appreciated that the system is fully compatible with a
mixture of steel and aluminum hybrid components
particularly the reinforced aluminum tubes that are
designed to have the same load carrying capacities as the
equivalent all steel component. This allows mixing of
the system components without concern regarding different
load ratings. This provides a gradual turnover of the
equipment to the new hybrid system components.
Although various preferred embodiments of the
present invention have been described herein in detail,
it will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
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