RDT experiments

First topic message reminder :

I am inspired by  Patandjliand and Ali but want to incorporate some ideas of my own. I started with the cams. I have an old lathe and mil but neither cuts shapes well that are not either round or straight. Strait lines on the mill are easy but cam shapes, not so much. So I redesigned the cams to use round shapes except for cut outs that are just to lighten the cam. I don't know if they will yield the kind of draw force curve I am looking for yet, I need to build limbs and make strings, but here is what I have so far:


Instead of one elongated power cam I am using a 2 piece cam consisting of one eccentric round main cam and a smaller offset "button" to give similar geometry to the single cam. This allowed me to machine the cam parts on the lathe,
The main or string cam is actually a perfect circle around the axle except for the cut out notch for the string attachment.
More to come. ..

Yes a winch might be a good idea when I get some stronger limbs. Right now these limbs are only pulling 55 LB. I can actually cock it by hand even with the string pinch.
This is what I came up with for a shuttle for the stringer:


And here it is strung:

I got everything fixed and was able to get off a few shots and also put a few through the crony.
I was getting 170 FPS which is a bit disappointing but about what I expected for 55 LB limbs.
These cams give good mechanical advantage but they are way too heavy for a low weight set of limbs. I think next I will experiment with seeing how much weight I can take off the cams and how much it effects the FPS. But the main thing will be to design and build a new and heaver set of limbs. I am thinking composite with bo-tuff,  and wood core like I used in the past for re-curves but much thicker. I have some designing and figuring out to do. And then I have to order materials. That will take time but while I am waiting I have lots to experiment with.
Ali, good luck with your mini design. Please keep us posted.

Hey  Jaeger; possible more to gain if you looked at your limb mounts.I was in the same boat at 13# When I redone things I ended up a little over 8#. Just flapping my jaws, but it looks like a lot involving the mounts. Bumpers also, I took mine off. I noticed the store bought mostly don't use them. Their claim to fame is supposed to be for speed. I don't need speed. I'll take kinetic energy over speed any day. Besides, a heavy arrow helps make a quite bow. It helps suck all the energy out of it.Stock from the trigger back , close to 3/4 of a pd. I chopped mine off and went with a sliding aluminum butt plate. I guess that's what they call a bull pup.When I sacrificed the butt end I was able to trade for my winch.No matter what you have a great machine started there.

Geezer here: Pardon me for sticking my oar in here.  I don't do anything with pulley-bows, but generally speaking, you won't see bumpers on any sort of medieval bow.  In fact, I think they'll cause excessive wear to your bowstring, maybe the prod as well.  I think you'd be better off extending the fore-end of the bolt-track further forward, to let the string bring itself to a halt, or perhaps look into fitting a bit of a deceleration-ramp ahead of the stopping point. If you do it right, the ramp need not cause the bolt to loft.  Medieval bows that were fitted with horn/sinew prods often had a rise in the 'table' about where the string should rest... I suspect that was to soften the stop at the end of the run... working on the assumption that hard stops on a glued together prod probably eventually ended up making the glued-up prod delaminate.  Anyhow, a longer fore-end with some sort of decleration ramp would probably result in less string wear and a quieter bow... though it would be a bit longer.  Just thinkin.... Geezer.

Good point Geezer and I thought about the same thing, just make the barrel a few inches longer. But I noticed that every RTD I have seen, both commercial and homemade seem to have some kind of stop. Maybe just for noise or to prevent the string from getting caught on the end. But maybe there is a tendency for the string to jump off the cam if there is no stop!  I don't really know. So in this case I just assumed that since everyone seemed to be adding the stops, there is a very good reason for them and did the same. They are soft rubber and should not hurt the string. 
As far as limb mounts, yes that is a key factor in the pull weight. I played around a bit with the angle and width to try to get the "right" amount of pre-stress. I basically want as much pre-stress as I can get without the limbs being over stressed at full draw. I may be able to get a bit more but will not really know when I get to the "too much" level of pre-stress until the limb explodes. I don't know how much these solid glass limbs can stand so I have tried to error on the conservative side.
Sounds like another good experiment to try after I finish playing the "how light can I make the cams" game.

Wow! I am shocked! This experiment was definitely a learning event. . . . .
I machined more material off the cams and then drilled a bunch of holes everywhere I thought I could get away with it and not weaken the cam too much. Before this the cams weighed 240 Grams each. This lightening exercise reduced it to 195 grams each. About a 18% reduction. This is what they look like now:


So this should make the arrow velocity jump way up right? . . .NOT!!! It went from an average of 170 FPS to 171 FPS A whole 1 FPS faster for 18% lighter cams.
I conclude that it is not worth spending a huge amount of time reducing the cam weight.
Next I plan to increase pre-stress to see if I can get the pull weight up without breaking the limbs.

You lightened the cams weight wise, but you also made them more tender. Make sure there are no more dry fires, or the cams may suffer also.Your speed will come up along with the pull weight.More ump, more zip.If you get into the 270 range your doing well.Also ,you might want to look at making a washer type catch on your sled to make sure the string don't jump off. Don't ask how I know this.

my firest homemade rdt crossbow in my firend,s home
 

 

 

Looks like an arsenal Ali!
I wanted to increase the  pre-stress a bit as an experiment so I added some PVC shims made from thin wall PVC pipe I heated and flattened. They are about .072 thick. Two short (.75") rectangles, one under the inside front and one under the outside back of each limb mount. That resulted in about 1 degree additional angle and about 3/4 inch additional spread. The weight change didn't feel too noticeable but the speed did increase from 171 FPS to 181 FPS. Not wonderful but a nice increase for just a few zero cost shims. Tomorrow I plan to increase it about that much more and hope the limbs don't explode! The only way to find the limit is to go past it.

Hey , way to go jaeger, don't forget where you hung your hard hat and googles.Been there and it's not for the faint of heart.Most likely there won't be any discernable warning. This is where a winch would be nice.

jaeger22 wrote:

So this should make the arrow velocity jump way up right? . . .NOT!!! It went from an average of 170 FPS to 171 FPS A whole 1 FPS faster for 18% lighter cams.

How much did you expect?
Quickly spinning the numbers, assuming initial 70% efficiency, gives theoretical maximum speed increase of 3% or 5fps (zero friction , massless string).

Anyway, in this type of system bulk of the inertia concentrates in string and rims of the cams - these parts need to move allmost with arrow speed. Limbs, cables and cam bearings are accelerated so little that their mass is practically irrelevant.

plz see my new video to learn hoe made composite fiberclass limbs 

hullu_tiedemies wrote:How much did you expect?
Quickly spinning the numbers, assuming initial 70% efficiency, gives theoretical maximum speed increase of 3% or 5fps (zero friction , massless string).

Anyway, in this type of system bulk of the inertia concentrates in string and rims of the cams - these parts need to move allmost with arrow speed. Limbs, cables and cam bearings are accelerated so little that their mass is practically irrelevant.

Yes I understood that the returns would be much less than the 18% weight reduction and that the string and outer rim of the cam were the most critical. For one thing I am fighting the V squared thing. However the increase I got was only 0.6%. which was disappointing to say the least. Not worth the effort. I would be interested to see how you came up with the 3% theoretical maximum speed increase. Not disagreeing, just can't see how you came up with it. 
I did continue increasing the pre-stress a bit at a time and got the FPS up to 196 now. But I don't think I can push these limbs too much more. On hold now due to travel for work.  
Ali, interesting video! I have made a number of bow limbs but I always bought the glass lams. This is the first time I have seen someone make their own from raw fiberglass and epoxy. Cool!

jaeger22 wrote: I would be interested to see how you came up with the 3% theoretical maximum speed increase. Not disagreeing, just can't see how you came up with it. 

Just assumed 70% efficiency. A simple educated guess.
So bolt+bow= 70+30 points of inertia.
Reduced to 70 + 24.6
velocity = square root of mass = sqrt( 100/94,6) ~=103

So how efficient is your system in terms of output/input energy?
Judging from the .6% number it might have allready been somewhere around 90% (unless there is some serious friction issue).

Thanks Hullu.
Reduced to 70 + 24.6???
In my simple math, E = 1/2 M * V squared 
So V = the square root of  E/(1/2 M) . But that is ONLY the projectile.
I expect that you have managed to manipulate the equations to give efficiency but the math escapes me. Been too long since I learned or used any of that. But not important. I don't want to turn this into a detailed math discussion. What really matters is your point about input and output energy. Which can be measured. When I get home, I will have to transfer the bow back to my draw force testing rig and plot a draw force curve to find the new input energy with the higher pre-stress. If I remember the arrow weight right, the output energy is 44.59 Ft-LB. (520 grain arrow at 196 FPS). It will be interesting to see and maybe compare the efficiency to a conventional xbow.
Thanks,
John

today. with a red doe 1x40 walther. 

25 meters...






55 meters 

Looking good Ali! That is good shooting. How much does your total Xbow weigh with the sight?
I took mine apart and cut away as much metal as I could to lower the weight. That brought it down from 15 LB (6.8 Kg) to 11.25 LB (5.1 Kg) (with sight) but it is still heavy. But I don't feel too bad because the commercial Scrorpyd bow is 11 Lb and I think that may be without sights without sight.
The good news it that because the weight is centered it is not a problem when shooting but it is a pain to carry around!

the black box size: 25x40 cm .

this cross bow now is 5.1 kg 

today i made new string . 106 cm to 99 cm and 45 to 37 cm. is so beter letof sistem.

see my froum
http://rifles.ir/showthread.php?tid=311&pid=8776#pid8776

Thanks Ali, that is very good as you are using steel limbs. Mine weighs the same with fiberglass limbs and the steel has to weigh more. So the rest is lighter.
Nice site. Just wish I could read it.

I am following your work with great interest, Ali, Patandjali Jaeger and Asken. My well-meant compliments!
I have allready made cams, but they are too small (because of the aluminium plate that was only 10 cm wide), and laminated a limb that seemed too strong. So I have a lot of poblems to overcome.

But I want to put one question here: I am planning a screw on the back end of the limbs, just behind a fixed resting axle on the riser, by which you can adjust the angle, and thus the tension of the limbs. Why did none of you incorporate that?
If you do that you can increase the tension when all works well.