Images > Сергей (phoenix) > Crossbow limb manufacturing technology

Process inspired by Serый > Making limbs for a crossbow

Thanks for sharing your technique Ivo.
Those are powerful looking limbs you've made there - what draw weight?
I have a couple of questions;
How much resin do you squeeze out during press up?
Do you have any experience with carbon fibre?

Regards

James

Hi,

Actually this is not my technology ...it was initially introduced to the Russian forum by Serый and then reproduced by Сергей (phoenix) who was kind enough to take all those pictures. There are links at the top of the page you can check out(it's in Russian, but pictures speak for them selves)

As the authors described the process...The form is insulated by simpy covering everything with clear scotch tape, then either a fiberglass tow is used or a fiberglass cloth is dismantled...then a bunch of fiberglass strands is submerged in resin and excess resin is lightly squeezed through to alow better impregnation with resin (gloves are used). Then these buches are laid into the form and pressed(as illustrated above...by tightening the clamps)...form is set to produce a blank of even thickness through out full length of the limb, but I believe that by tightening the bolts (serving as stops) differently a slight taped can be produced. Once the blank is pressed and fully cured it is cut in half and limbs are given their final shape and taper by wet sanding (to avoid FG dust) or sanded dry(using sufficient evacuation, ventilation, respirator, eye-protection, etc...the list can go on and on depending on equipment)

The two sets of limbs in the picture are 100kg and 40kg in draw weight...100kg had a total of about 800 FG strands laid up in the form and 40kg used about 500. The dimensions of these particular limbs have not yet been made available, but what I do know is that they are 40cm in legth and the crossbow on which these limbs were tested (if I understood the author correctly) has a power stroke of 230мм...

As for carbon fiber...the stuff is better in a laminated limbs(which is probably what most of US bowyers/crossbowmen will make). If you know a little about bows..."while it is drawn - the back stretches while belly compresses". Carbon is very good in stretching(so it goes on the back of the bow well), but terrible in compression(so a fiberglass or some other synthetic material needs to be used on the belly)...as one my fellow crossbow builder gunsmith111 demonstrated in his attempt to make an all-carbon limb for a twinbow like crossbow he is building... the limb did not make it...it snapped in half not even making it past half way of what all-fiberglass limb could easily handle. An unfortunate, but very informative moment...

One for the future...Kevlar ...I thought some might ask about it later so I'll tell you what I know now - as opposed to carbon it is great in compression, but what makes it terrible for crossbow limbs is it decomposes over time loosing it's good qualities...there are more bad things, but you can read more about them on any bulletproof vest

Hi Ivo,
Thank you for your thorough and detailed reply. You have definately filled in all the gaps.
I am very familiar with carbon fibre; and its short comings . I used to use it in laminated longbows, but now do not think it is worth the effort.
The reason I asked about it is because I think it it could be more useful in a pressed fibreglass limb. You could mix carbon tow through the glass at a ratio of, say, 1 part carbon to 3 parts glass. The carbonfibres could have a higher density on the limb back, where they are more useful. The resulting limb would be stiffer, slightly lighter and maybe a bit faster than it's all glass counterpart. Also the carbon component would help dampen shock.
Well thats my theory. Now I need to find the time to test it!

Regards

James

I've thought about it too. It may be possible to achieve good results by mixing strands of different material, but the setup would be a little on the complex side. A little uneven distribution of forces in a limb and it will most likely twist wen stressed. It would be easier to introduce carbon in form of uni-directional carbon tape to keep things even...kinda like this:

Uni-D carbon tape is laid into the form(assuming the bottom of the form is the future limbs <Back> ) and saturated with resin(or saturated in resin and then laid into the form using a roller)...more layers are added(if needed)...then the fiberglass bundles are saturated in resin and laid over that...and the composite is pressed.

I assume that way there is less chance of uneven distribution of carbon strands through out the composite...perhaps a little more expensive due to use of uni-d tape, but success is worth it. I have the carbon tape, a crap load of fiberglass, and a few different forms ready for testing various designs and techniques, so I'm also half way into testing phase...All that's left is to get my shop cleaned up for "THE DAY"

Hi Ivo,
I'm waiting for the same day!
The way I was thinking of doing it was to use a two post system, let me explain....
The glass and carbon tow would be wrapped around two adjustable posts- just like making an endless string. The posts would be ridged to allow the tow to be distributed evenly up and down the posts with no bunching. Once all the strands are laid up, the jig would be laid on its side on a clean surface and the epoxy would be worked into the strands, prehaps even with a comb. The tow would be wound firmly enough to keep it in place, but not so firmly as too hinder glue saturation. When to all the strands are saturated, the posts would be pulled apart, imparting as much tension as possible to the tow.
This bundle of of pretensioned and glue saturated fibre (still on the jig) would then be turned on its side again, pressed in the form and left to cure.
The action of pressing the bundle will inpart even more tension to the strands in the same way a bridle draws the prod to the tiller.
Does that make sense? I hope I am not just rambling on here!
To incorporate carbon, I would use two jigs.
The first jig would be firstly wound in several layers of carbon tow, then the outer layer of the bundle would be a very fine layer of glass tow. The fine layer of glass means that there is no soft carbon fibre exposed on the back of the limb.
The second jig would be wound thickly with glass tow. This will be the belly.
The two bundles, still on their jigs are placed one on top of the other in the form, pressed together and left to cure
A strip of scrim would be placed in the centre of each bundle to provide lateral stability and resist splitting.
Well thats my theory everyone! It may work, or it may be a dog. I can't comment until it is tested! What I need is a 48 hour day, to get all my projects out of the way

Regards

James

So I've been thinking and actually loved the idea...we talked about it once with the guys at arbalet.info forum...the reason tow was not used is because guys couldn't get their hands on it easily when fiberglass cloth kits are preactically on every corner so in the end they settled with dismantling the cloth.

I liked your idea a lot...and I'd like to add one little detail



http://sketchup.google.com/3dwarehouse/details?mid=5ab8ca270822a30bd90c105376b9c5d5

I think using little clips(forgot to draw the lock ) to lock in place the tow and keep it from separating while introducing the resin...I'm sure it will also help hold everything together much better while combining with other loops like it and carrying them to the form.

PS: I know the wooden things in my drawing are huge...don't panic....it's an exaggeration.

Hi Ivo,
I'm pleased you understood my ramble! and thankyou for simplifying all my writing with your well executed 3D diagram. I've just down loaded Google Sketch It.
Lets see if I can get my head around it
I like your contribution of the clips. It will make the lay up much easier, and when they are closed, will provide a certain ammount of tension to the fibres.

Regards

James

Some picts. how did I make fiberglass prodd using uni-d glass tape and fine modelling plywood as a middle layer.

Excellent presentation Regerald!

I see your measurements varied slightly from the ones I took from the chinese made 150lb all-fiberglass prod...as well as the technology...you are using wood veneer laminations and uni-d fiberglass tape here. By how much do your measurements vary from the ones here? ...and what draw weight were you able to achieve?



Lastly... Can you please shoe us the full draw picture ...and if it's possible the way you are planing to mount the bow.

Regereald...Просто Супер!

Как я понимаю размеры твоего лука немного отличящтса от тех что я намерял с китайца...и технология тоже немного другая...насколько сильно отличятся твои размеры от моих?...и какая сила натяжения достигаетса при полном натяге?

***3Д***

На последок... если не трудно можно посмотреть как лук выглядит при полной натяжке...ну и конечно же хотелось бы увидеть крепеж.

kiwijim wrote:...I've just down loaded Google Sketch It.
Lets see if I can get my head around it ...

Sure play with it...go here "Google SketchUp - Not your average CAD" and follow a few link around...download a few models from 3D Warehouse...it's fun yet great tool. Enjoy!

Here is full-draw image.. It seems like edges of a prod have to be little thinner, next time I will make better shape =)
This prod is 58cm (23'') cross-section in the middle is 38/10mm (1.52''/0.4'') and 25/5.5mm (1''/0.22'') at the edges. Draw weight is about 130 lbs with a 7'' draw distance.
Current string is little bit too loose, it wasn't designed for this limbs. Here it's just for testing..

Hi Regerald,
I really like the profile of this prod . Also, I like how you've used a wood core to reduce limb mass. It must be fast!

Regards

James

Yes, overall weight is only 200g, (0.44 lbs), so I predict it must be a fast one.. But on other side, this wood veneer seems to be a "weak point". So I would advice to use some hard wood laminates (for example maple) or fiberglass laminates, it will give a better reliability.

Thanks for the answer.

The profile is great even with the recurved tips sticking out so much...makes me think of "static recurves" ...thou may be a "perversion" and not something I would recommend , but adding a "string bridge" like on old Mongolian horse bows would be an interesting experience.



I also agree on using maple...many bowyers recommend it for well performing bows.

Also....making a laminate from maple(using epoxy and hot box) will make it into what is called "action wood" ...something I'm dreaming of pressing in my little shop.

Very good read...enjoy

Yanked from Grodon Composites website...link


Imagine
doing more than a million deep knee bends without getting tired.

That may be out of the question but the common thread that runs through
all of our products is their ability to withstand deep deflection over
numerous cycles without fatigue or loss of performance.



FEA Deflection demonstration




The raw materials for G-Flex™ high-performance springs are epoxy resin
and advanced fibers such as E-glass, S-glass or carbon.

Epoxy resin is a thermosetting polymer or plastic that becomes
permanently hard after “curing.” Epoxies are classified as “structural
adhesives” or “engineering adhesives.” These high-performance
adhesives are used in the construction of aircraft, automobiles, bicycles,
golf clubs, skis, snow boards, and other applications where high-strength
bonds are required.

Pound for pound, glass fibers are stronger than steel. They
have a high ratio of surface area to weight and an amorphous structure
that gives them the same properties both along the fiber and across the
fiber. Key performance attributes include tensile strength, modulus or
stiffness, and fatigue resistance. While other fibers may have one attribute
that is better than glass fibers, glass often provides the best combination
of properties.

Carbon fibers are known for their exceptional strength and
light weight. Gordon Composites knows how to produce springs in volume
with precise control for consistent quality and performance at competitive
costs. Our customers include leading companies in vibratory conveyors and
commercial furniture. They count on Gordon Composites to produce thousands
of parts - all having the same dependable performance they need.

For more about the science behind G-Flex™ high-performance
composite springs, click on the following link:




A material is said to be elastic if it deforms under stress but returns
to its original shape when the stress is removed. Springs are common examples
of flexible elastic objects used to store and release mechanical energy.
Another example is the limbs of a modern archery bow. Although bow limbs
don’t seem very elastic or springy when handled off the bow, that’s
exactly what they are – springs.

It is well known that the purpose of the bow limb is to store
and release energy. Bows function by converting elastic potential energy
stored in the limbs into kinetic energy and movement of an arrow. Energy
stored in the limbs of the bow as they are deflected is transformed into
rapid motion when the string is released, transferring the motion to the
arrow.

In forming the composite for a spring or bow limb, the trick (both art
and science) is to get the fibers perfectly aligned and thoroughly saturated
with resin. Voids or misaligned fibers negatively affect the composite
properties.


Alignment is achieved by having all of the fibers under a precise amount
of tension as they go into the forming area; the saturation is achieved
at Gordon Composites with a proprietary roll-forming process developed
through decades of experience. The goal is a composite in which each fiber
is surrounded and bonded with the matrix at a specific ratio so it will
work efficiently as it stores and releases energy.



Historians tell us the first composite bows were probably developed several
millennia ago in Asia. The materials used were wood, horn and sinew. Even
then, archers understood that different properties are needed on the two
sides of a bow limb – bone for compressive strength on the belly or
face, and stretchable sinew for tensile strength on the back. The wooden
core provided a flexible gluing surface and generally didn’t receive
a lot of mechanical stress.

The advantage of the original composite bows over bows
made from a single piece of wood was their combination of smaller size and
high power. They also had disadvantages. Their construction required much
more time and a greater variety of materials, and the animal glue traditionally
used could lose strength in humid conditions and be ruined by submersion.

Despite those drawbacks, composite bows had significant advantages and
were quickly adopted by others, touching off a competitive quest for improvement
that continues today. The nomads who developed the first composite bows
would hardly recognize today’s compound bow with fiber-reinforced
composite limbs.

University-trained engineers now use the latest in
computer-driven design technology to develop bow systems that are light,
draw smoothly and efficiently release energy when sending an arrow on its
way. Among the most critical components of a modern bow are the limbs that
store and release that energy. And while those limbs may not look very sophisticated,
they are actually complex material systems within the bow assembly. Limbs
can be as intricate as the compound bow itself, and while making them is
still an art, it has also become a science.

Bow limb fabrication begins with the selection of raw materials. The most
commonly used are fiber glass reinforcements and epoxy resin. That sounds
simple enough until you find that there are hundreds of types of glass fibers,
hundreds of epoxy resins and dozens of ways to put them together.





Glass fibers are made by pulling molten glass through devices called bushings
with hundreds of tiny holes. Because the glass is pulled and stretched or
elongated just before it cools, the fibers are even smaller than the tiny
holes the glass flows through. Fibers typically range from 10 to 22 microns
in diameter, which is thinner than most human head hair (17 to 100 microns).

The raw materials used for the glass will determine a variety of factors
such as the temperature needed to melt the glass and the strength of the resulting
fibers. Without getting too technical, glass compositions with higher silica
content and metallic oxides will require a higher melting temperature and
be harder to make into fibers, and thus more expensive, but they will also
impart higher performance characteristics to the fibers.

As soon as each filament of glass emerges from the bushing and cools, a chemical
sizing is applied to the surface of the glass. The sizing has two functions:
keep the fibers from abrading and breaking, and help the fibers bond securely
with the resin.

If the glass is melted at a higher temperature and is less viscous, and/or
the glass is pulled faster by the forming winder, the glass fibers will be
thinner. Lower temperatures and/or slower pull will produce thicker glass
fibers. Fiber glass producers, like textile makers, have for centuries referred
to fiber diameter with terms like yield or tex. For example, a product with
a 450 yield will have 450 yards of fiber in a pound.

The strands of glass are wound onto a temporary core to form a package that
can be shipped to a composite fabricator.

Pound for pound, glass fibers are stronger than steel. They have a high ratio
of surface area to weight and an amorphous structure that gives them the same
properties both along the fiber and across the fiber. Key performance attributes
include tensile strength, stiffness, modulus and fatigue resistance. While
other fibers may have one attribute that is better than glass, glass often
provides the best combination of properties. For example, carbon is known
for being light and strong, but it can also be brittle in some applications.
Compared to carbon, glass can undergo more elongation before it fails.





Most bow limbs are made with epoxy resin, which is a thermosetting polymer
or plastic that becomes permanently hard after “curing.”

Epoxy or polyepoxide resin polymerizes and crosslinks
when mixed with a catalyzing agent or “hardener.” In general,
epoxies are known for their excellent adhesion, chemical and heat resistance,
and good to excellent mechanical properties.

When epoxy is mixed with the appropriate catalyst, the
resulting reaction is exothermic – meaning that it generates heat – and the oxygen
molecule on the epoxy monomers is “flipped.” This occurs throughout
the epoxy and a matrix with a high stress tolerance is formed and “glues” the
materials together.

Epoxies are classified as “structural adhesives” or “engineering
adhesives.” These high performance adhesives are used in the construction
of aircraft, automobiles, bicycles, golf clubs, skis, snow boards, and
other applications where high strength bonds are required.

Epoxies are exceptional adhesives for wood, metal, glass, stone, and some
plastics. They can be made flexible or rigid, transparent or opaque/colored,
fast setting or extremely slow. Epoxy adhesives are almost unmatched in heat
and chemical resistance among common adhesives.

There are more than 100 commodity epoxy manufacturers
in the world and countless “formulators” who
blend or customize epoxies for specialty applications. The resin used in
the manufacture of bow limbs comes from the second category, the custom
epoxies designed to achieve specific performance requirements.





As the name implies, making a composite brings two or more constituent materials
together that remain separate and distinct on a macroscopic level while forming
a single component.

In the case of a bow limb, the process begins when glass fiber roving doffs – the
packages of glass fibers – are arranged in a creel, which is simply
a holding device with racks of shelves. The ribbons or “ends” of
the glass fibers are pulled from the creel into the area where the composite
is formed.

At Gordon Composites, the creel for making bow limbs
is one of the largest in the world with up to 3,000 ends pulled for a single
sheet of composite material. When you consider that each end may be made
up of 800 or more individual glass filaments, a bow limb could contain more
than a million glass fibers.

As stated above, the trick in forming the composite for a bow limb is to
get the fibers perfectly aligned and thoroughly saturated with resin. Alignment
is achieved by having all of the fibers under a precise amount of tension
as they go into the forming area; saturation is achieved with a proprietary
roll-forming process. The result is a composite in which each fiber is surrounded
and bonded with the matrix at a specific ratio so it will work efficiently
as it stores and releases energy.






What
comes out the end of this roll-forming process is known as bar stock or
a homogenous block of material. Although we call them “laminates’ in
the composite industry, our use of the term comes from the fact that we
are fusing glass and resin. In the archery business, laminates generally
refer to bow limbs that are made by combining various layers of materials
with different properties to achieve a tailored or engineered structure
with specific performance properties.

Bar stock is generally made with standard E-glass and
shipped to the bow manufacturer as a solid block of material in a specific
geometric shape called a “billet.” Bow manufacturers generally
do their own fabrication, converting the billet to meet their specific
design criteria. Gordon Composites can support the manufacturer with engineering,
machining and construction.

Limbs made from billets are less expensive than laminated bow limbs and
are very easy to fabricate and build into prototypes. They are able to achieve
deep deflection and high-stress performance and are used very successfully
in the majority of bow-limb applications.




By fusing different layers of composite material together, bow designers are
able to tailor the physical properties of the limbs more precisely. For example,
a designer can move the neutral axis on a part so it is stiffer on one side.
Thinking back to the first bows with horn and sinew, today’s engineer
is able to optimize the tensile and compressive strengths – along with
sheer strength – and put them exactly where they are needed during
very deep deflection to maximize performance.

A typical laminate construction or lay-up generally starts with a composite
billet that is machined to a specific shape. Thin layers of composite material
are then laminated to the bar stock, giving this type of bow limb its name.
The added layers can contain high-performance glass or carbon materials, sometimes
arranged in a weft or cross direction. Reinforcements can also be embedded
as a scrim or fabric layer.

This blending of materials and properties takes the design and engineering
of bow limbs to the next level, by not only allowing high and lasting performance,
but also the complex shapes that make modern bows so smooth and accurate.



Although I don’t have much personal experience with compression-molded
bow limbs because Gordon Composites does not use the process, I will mention
it here so you have a more complete understanding of composite bow limbs.

This process begins by winding continuous glass fibers
that have been impregnated with epoxy around a set of pins to align the
fibers. The combination is placed in the cavity of a matched-metal tool-and-die
press. Heat and pressure are applied to shape the limb as it “cures.”

There is generally less finishing cost with compression-molded limbs because
the part emerges from the press in a near-net shape. The process is used very
successfully in many applications and is attractive to bow manufacturers who
want to produce their own limbs.

On the flip side of the equation are high tooling costs and a lack of flexibility
driven by the desire to offset those costs by using molds for a long time.




When
designing archery limbs with Gordon Composites materials there are some basic
guidelines to follow to ensure successful limb performance and durability:

1. As a starting point, develop a design based on minimum material
   strength properties. Gordon material strength minimums are based on
   a 3 sigma standard deviation referenced to the data average.
   
2. Once a proper
   strength minimum is set, safety factors should be determined for fatigue
   life, axle shear, mounting shear, limb twist, limb deflection, brace
   height and limb length.
   
3. It is always important to strive for a constant stress design
   along the length of the limb.
   
4. In general, average stiffness properties are typically used
   to determine the nominal stiffness of the limb, realizing that the
   stiffness will vary within a batch of material (normally 3 sigma) and
   average stiffness will vary from batch to batch.
   

After deciding what the key performance elements of a targeted
limb design are — e.g. high fatigue resistance or high speed — it
is imperative to stay within design guidelines.




Without a doubt, one of the toughest tests for composites is the deep deflection
and numerous cycles required by archery bows.

And while an archer might draw and release a bow several
thousand times, manufacturers often test bows up to a half-million cycles
to make sure there is no fatigue or “creep” in bow limb performance.

An archer might have the strength to move the ends of
the limbs several inches closer to each other but mechanical testing devices
apply enough force to have them nearly touch each other. Manufacturers want
to be sure the failure point is well beyond an individual’s ability
to draw the bow.

Archery bows function by converting elastic potential
energy – today
stored in parts of the modern bow frame called limbs – into kinetic
energy and movement of an arrow. Energy stored in the limbs of the bow
as they are deflected is transformed into rapid motion when the string is
released, transferring the motion to the arrow.

By fusing various layers of composite material together,
bow designers are able to precisely tailor the physical properties of their
bow limbs. For example, a designer can move the neutral axis on a limb so
it is stiffer on one side. Today’s engineer is able to optimize the tensile and compressive strengths – along
with sheer strength – and put them exactly where they are needed during
very deep deflection to maximize performance.

A typical laminate construction generally starts with a composite billet
that is machined to a specific shape. Thin layers of composite material are
then laminated to the bar stock, giving this type of bow limb its name. The
added layers can contain high-performance glass or carbon materials, sometimes
arranged in a weft or cross direction. Reinforcements can also be embedded
as a scrim or fabric layer.

This blending of materials and properties takes the design and engineering
of bow limbs to the next level, by not only allowing high and lasting performance,
but also the complex shapes that make modern bows so smooth and accurate.

Stress is applied to a bow limb when the string is drawn.
One side of the composite is actually stretched while the other is compressed.
The fibers resist these two forces and the matrix spreads the load throughout
the limb. When the stress is released, the “elastic” composite quickly “springs” back
to its original shape. This action converts the elastic energy into kinetic
energy and sends the arrow on its way.

Today, engineers at Gordon Composites use computers and software to help
them plot the stress-strain curve for a given material or composite. The programs
actually color code the internal forces, enabling the designer to see how
much and where stress is occurring.

“’If we can see it and understand it, we can develop ways to
make the limbs even better,” says Steve Johnson, General Manager and
Vice President.


The following excerpt is reprinted with permission from Inside
Archery magazine. The article appeared in the September 2007 issue.

Like it or not, we are equipment oriented, and spurts in archery participation
and industry growth have typically been linked to product innovation. Beginning
in the 1950s, it was the development of reliable recurve bows that sparked
expansion. In the 1960s, arrow shafts improved dramatically, and in the 1970s
it was the amazing advent of the compound bow and the treestand that ballooned
archery involvement.
While more recent innovations may not have been quite as monumental, they
have been more frequent. Hunting releases, 3D targets, fiber-optic sights,
expandable broadheads, carbon arrows, drop-away and total-containment arrow
rests, laser rangefinders, scent-containment clothing, portable ground blinds,
scouting cameras and much more have all made archery participation more attractive
and enjoyable.
Through it all, thought, how performance has remained a central theme. As
bows became more dependable, more enjoyable to shoot and more accurate, the
sport and the industry prospered.
Evolving bow performance has hinged on a variety of factors, including refinements
in risers, eccentric systems, bowstrings and cables. But in no area have the
improvements been more enabling of archery progress than in the evolution
of bow limbs. After all, it is the bow’s limbs that allow everything
else to happen. If the limbs aren’t up to the task, no new high-performance
cam, riser or cable system will carry the day.
From the very beginning of the modern archery industry in the late 1940s,
limb development has been central to bow improvement. And surprisingly, a
single longstanding company has been at the forefront of bow-limb innovation
over the past five decades. That company, now called Gordon Composites, has
been and continues to be one of the most influential companies in the industry.
To read the complete text, download the article by clicking here.