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    RDT experiments

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    Post by jaeger22 Mon Jan 19, 2015 4:53 am

    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:
    RDT experiments - Page 3 P1020117_zps760c2916

    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. ..
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    Post by c sitas Wed Feb 25, 2015 4:05 pm

    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.
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    Post by ali.j Fri Feb 27, 2015 4:48 pm

    my firest homemade rdt crossbow in my firend,s home
     
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    RDT experiments - Page 3 27_15142508200937592
     
    RDT experiments - Page 3 27_15142508200949873
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    Post by jaeger22 Fri Feb 27, 2015 7:32 pm

    Looks like an arsenal Ali! Shocked
    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. Wink
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    Post by c sitas Fri Feb 27, 2015 7:55 pm

    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.
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    Post by hullutiedemies Sat Feb 28, 2015 5:33 am

    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 Shocked Shocked Shocked 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.
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    Post by ali.j Sun Mar 01, 2015 1:43 pm

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

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    Post by jaeger22 Tue Mar 03, 2015 5:05 pm

    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. Crying or Very sad 
    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!
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    Post by hullutiedemies Wed Mar 04, 2015 6:59 am

    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).
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    Post by jaeger22 Thu Mar 05, 2015 5:02 am

    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,
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    Post by ali.j Sun Mar 15, 2015 11:30 am

    today. with a red doe 1x40 walther. 

    25 meters...


    RDT experiments - Page 3 15_1514264402190332

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    55 meters 

    RDT experiments - Page 3 15_15142644073653181

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    Post by jaeger22 Mon Mar 16, 2015 7:34 pm

    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! Neutral
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    Post by ali.j Wed Mar 18, 2015 4:26 pm

    the black box size: 25x40 cm .

    this cross bow now is 5.1 kg  Very Happy

    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
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    Post by jaeger22 Wed Mar 18, 2015 5:48 pm

    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. scratch
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    Post by ali.j Mon Mar 23, 2015 2:27 pm

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    Post by ali.j Thu Apr 02, 2015 2:46 am

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    Post by FrenckBrambo Wed Jan 11, 2017 3:48 pm

    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.

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