Material For Swinging Crossbow Limbs

I need recommendations for swinging crossbow limbs. The bow will be about 200lbs draw. I was thinking 1/2" aluminium; is this overdoing it? I'm guessing delrin is to weak?

1/2" aluminum what?  Solid bar stock?  Hollow tube or square?  T-section?  I-beam?

Wood or poltruded fiberglass bar will work just as well, and be cheaper probably.

I was thinking 1/4" (sorry, I wrote 1/2") x 1" solid aluminum bar stock. Would wood be strong enough, but also light enough? If so that would be a preferable option. I'm guessing that the fiberglass would flex to much; beside it is not pleasant to machine.

What length ?

3/2"x1/2" hardwood . Tapered to 15x8mm . Teardrop cross section.
There are old 100#plus Turkish bows with thin knife-like wooden ears.
200lbs does not require anything fancier than wood.

Besides wood has superior rigidity/mass-ratio to metals. They made airplanes out of that stuff for a reason.

The length would be 8.25" for each limb. There are a number of mechanical things in this bow so I do think I will go with metal; would aluminium (aircraft grade; very light yet strong) solid bar 3/16x.75" be strong enough? I realize that I am probably going over board with all this and making it way to strong, but better safe than sorry; I am a woodworker and realize woods failings and since this is many mechanical moving parts I want to stick to something more predictable than wood.
However, if you think that aluminum would be to heavy, I would go with wood

There is a huge variation in strength of aluminum alloys, from 20 ksi to nearly 80, depending on alloy and treatment.  

I like both. In your application it might be better to use alloy if it will simplify the pivot assembly, but I would probably start with wood and see how long it lasts

Just my tuppence here.  If you're a woodworker, stick to what you know -- wood.  Wood will be plenty strong enough and light enough.  Ash, Elm, Hickory, Maple would be just fine.

The aluminum alloy I would be using is 6061 T651; the weight of a piece 3/16*1*12" is .2205lbs.
Red oak is 44lbs/cu. ft; p { margin-bottom: 0.25cm; line-height: 120%; } If you do the math, a piece of red oak 3/4*1*12” would weigh .229166lbs.
Aluminum strength (6061) has a strength of ~40,000psi; red oak, on the other hand, has a strength of 6,700psi.

I think, considering these figures, as the weight is pretty much the same, that I will go with aluminium. 

Would 1/8*1 aluminum be strong enough? I am guessing that is plenty strong enough, and it would be lighter than wood.

P.S.
As I was writing this kenh's message was posted; I will consider what you said; still considering the options; please give input

Benjaminh; chase down a fellow called Jeep. Made a cool in swinger. Rubber powered. Bull pup stock.He and I must have been on the same wave length.

I have tried to contact jeep; do any of you guys no where he is at?
I am guessing, with below picture, that the pulleys on the front of the arms make it so you get twice the draw length, but half the strength. Please correct me if I'm wrong;

Ben; I made this same bow ,only with my own slant. The one with the divers rubber that I had mentioned.

What did you use for arms? Would 1/4" x 1" aluminium be strong enough for a 150lbs bow?

No. I used  1.25 square stock . Jeep was using flat stock and was having troubles.So I went stiffer.  No problems.

How thick did you go with the flat bar? Are you using hollow square stock?
How approximately how heavy of a draw weight do you have? I'd prefer to go with flat stock, and the bow I'm planning would only be 150lbs max.

p { margin-bottom: 0.25cm; line-height: 120%; }
p { margin-bottom: 0.25cm; line-height: 120%; }
I plan to make a inswinger crossbow like this one
Here are my specs:

• Distance from the front nock to the back string hook-up on the arms: 12.75”
  
• Distance from front nock to arm pivot: 8”
  
• Distance from back string hook-up to arm pivot: 4.75
  
• Distance from left arm pivot to right arm pivot: 20.5”
  
• Draw length: 22”
  

I am assuming that, with a 22” draw length, I should be able to get decently high fps, although when leverage is calculated out, with 100lbs springs, I will only be getting a ~65lb draw weight.


Any problems with these measurments?

I miss wrote my last post . I meant to say the material was 1.25 square "TUBE stock ".Wall thickness was about .060. Your 1" might work if you could  some how mount your wheels to pull on the center of the bar. Jeep and I also mounted them on top , had trouble twisting. That is why I went with the tube stock , cause I could easily mount in the center . In my case the stock was cheaper also.

Ivo once said:
"If you checked out Igora's bow powered in-swinger ballista, he suggested using a flat strip as a lever. >>> Igora's Ballista Perv.

This kinda makes sense:

• Less air resistance.
  
• Improved rigidity along the x-axis
  
• and...perhaps Less weight. >>>BTW What are the dimensions of the Firefly ballista's levers? how much do they weigh?
  

I also would like to add that doubling the strips with spacers in between might be a good idea...a sheet/plate frame of sort.


Enlarge this image Click to see fullsize


...the lever in the picture is from a different project, so it doesn't fully apply to our ballista discussion...yet still has a few details that I wanted to mention)

Perhaps the levers will weight a tad bit more, but rigidity will improve along other axis as opposed to just flat plate version. The tip being a wheel instead of a knock is another good thought."


I was thinking of maybe using two 1/8x1" aluminium bars with my pulley/nock in between layers. How does this sound? The reason I would like to avoid the square tube is because it adds air resistance, and it is much more expensive in my area than flat bar stock. Please keep in mind the draw weight I am after would not exceed 150lbs;
Can anyone comment on my list of dimensions?
How has your inswinger been performing chuck?

Yes I like the way mine shoots. Your double setup would work,BUT be heavy. Maybe such a thing one could use lightning holes to help.In effect ,with the two pieces you have created a beam that will resist any unwanted flexing.I'm wondering if two pieces of 1/16 steel stock wouldn't be lighter and just as strong as long as you keep the pulls and pivots in the center. Also the length of it could come into play.The way mine is strung and the way it pulls, I more or less have to use the wheel. I don't believe it turns hardly at all. It just makes less trouble pivoting.I really never paid any attention to what is best . I new what I wanted ,and just made it.It's a test bed. I'm on my fifth stock for it. It's easier for me to make a stock than putts with the trigger hole, and that's what I've been doing, making different triggers. I'm liking the china version now.