Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

Latest topics

» GLOSSARY OF TERMS
by White Oak Yesterday at 2:19 pm

» Reverse draw "compound" build
by FrenckBrambo Yesterday at 6:18 am

» Prod specs
by White Oak Yesterday at 4:23 am

» Crusader crossbow
by c sitas Mon May 22, 2017 8:48 am

» The Arbalist Guild - Around the World
by White Oak Mon May 22, 2017 4:03 am

» Hello from Maine
by White Oak Sun May 21, 2017 11:06 pm

» Need to locate Windlass Crossbow, pre 1600, in Rome, Venice, or Florence
by Vincenti Thu May 18, 2017 7:18 pm

» Leonardo Da Vinci string with suport-rolls
by Oppenheimer Tue May 16, 2017 11:40 am

» Compact repeating compound project
by Phil Abrahams Mon May 08, 2017 2:10 pm

» Trying to reduce the power stroke in my next project
by octavioaugusto.oliveira Thu Apr 27, 2017 1:27 pm

» The Road so far......
by Onager Lovac Tue Apr 25, 2017 4:07 pm

» Its good to be back!!
by Juan galeano Tue Apr 25, 2017 9:31 am

» The crossbow...What is?!
by octavioaugusto.oliveira Sat Apr 22, 2017 2:07 pm

» I need your advices on a new crossbow project
by Daniel Levesque Fri Apr 21, 2017 12:58 pm

» Guidance Designing My First Slinger
by kenh Thu Apr 13, 2017 8:17 pm

» New member
by harry6855 Thu Apr 13, 2017 8:28 am

» Chinese style crossbow trigger 3d modeled with templates.
by Bs1110101 Thu Apr 13, 2017 6:14 am

» Berkhamsted Castle and Glasgow Museum wooden prod crossbow
by hullutiedemies Wed Apr 12, 2017 8:52 am

» Hello from Asia
by Daniel Levesque Wed Apr 12, 2017 6:49 am

» Medieval crossbow modeled in Tinkercad, with variant trigger design.
by Bs1110101 Mon Apr 10, 2017 1:42 pm

» New project"s"
by kenh Sun Apr 09, 2017 2:38 pm

» New member here!
by c sitas Thu Apr 06, 2017 7:49 am

» Fixing a bad delflex
by Daniel Levesque Tue Apr 04, 2017 11:13 am

» H L Bailey Plans
by ora8i Sun Apr 02, 2017 1:49 am

» Top of Crossbow (launch?)
by Geezer Thu Mar 30, 2017 9:41 pm


    Greasing the track

    Share
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Greasing the track

    Post by Basilisk120 on Sat Aug 21, 2010 9:03 pm

    Just thought I would throw this out to the group.

    Does anyone use anything to "lube" the bolt track? Use silicone on the track or a wax on the bolt to reduce friction? Or would such a thing wear off too quickly to actually help. Or worse attract dust and grit.




    Something Something Dark Side
    avatar
    Lightly
    Master Crossbowyer
    Master Crossbowyer

    Posts : 199
    Join date : 2010-01-12
    Age : 57
    Location : Austin, Tx

    Re: Greasing the track

    Post by Lightly on Mon Aug 23, 2010 10:24 am

    Couldn't sleep?!

    This is a good question...

    According to Geezer, the bolts are quicker when we use a micarta or bone groove rather than just the groove in wood, the former being 'more slick'... he has also mentioned that a groove that is well finished with the Tung Oil or maybe lacquer? may also be a bit faster.

    If the grooves are anything like bicycle chains (yeah, not so much...) than anything that attracts dust WILL cause drag (I used to be a bicycle mechanic/instructor of classes re rebuilding all hubs, bottom brackets, headsets before they were all one piece that gets pressed in...) so if one DID use a lube; silicone before something else, likely.

    But, Geezer will know this better.

    Best!
    Lightly
    avatar
    Geezer
    Master Crossbowyer
    Master Crossbowyer

    Posts : 909
    Join date : 2010-01-12
    Age : 69
    Location : Austin, Texas, USA

    greasing the track

    Post by Geezer on Mon Aug 23, 2010 12:33 pm

    Geezer here: As Lightly said, in my experience, it helps immensely if you have a hard, slick track to begin with. Years ago, a fellow crossbow maker replaced the wooden self-groove in his 125 lb (Alchem prod) medieval-style crossbow with a Delrin one, then reassembled everything exactly as before. The unmodified bow was getting 195 fps with his target bolts (11/32nd Cedar shafts, about 15 inches long, with 125 grain heads, as I recall). The new track gave him 215fps with the same bolts. That's a 20 fps improvement, or just over 10% increase in velocity which showed up clearly on the target range. So a slick top will definitely improve things.
    Lubrication should help even more. Beeswax is pretty sticky, and will attract foreign-material. It may help prolong string-life, but probably won't help velocity. Some people use graphite, which works well, but it will leave a dark residue atop the stock, on bowstring and bolts. I recommend silicone, which dries clear and leaves no residue. Try it, you'll like it. Geezer
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Tue Aug 24, 2010 3:32 pm

    Makes sense. I was playing around with some different things I had lying around the house trying to find some way to reduce friction of wood to wood contact. And well to sum it up: A good smooth polyurathane finish is hard to beat. Granted I don't have anything cool like a chronograph to measure speed so it was just a rough 'feel' test.

    i will have to try Silicon, I don't have any at home and to be honest I didn't even think about it till I wrote it down in the first post.



    Something Something Dark Side
    avatar
    basileus
    Crossbow Building Wiki
    Founder & Curator
    Crossbow Building WikiFounder & Curator

    Posts : 86
    Join date : 2010-06-18

    Re: Greasing the track

    Post by basileus on Wed Aug 25, 2010 5:59 am

    This 10% increase in speed interests me Smile. My crossbows have wooden bolt tracks which are treated with linseed oil followed by beeswax. That kind of surface is really sticky, especially until beeswax is dry. However, my bowstrings only barely touch the top of the stock even before being draw. So the only source of friction is the bolt itself.

    Would I still benefit significantly from making the bolt track from metal or ivory?
    avatar
    Lightly
    Master Crossbowyer
    Master Crossbowyer

    Posts : 199
    Join date : 2010-01-12
    Age : 57
    Location : Austin, Tx

    Re: Greasing the track

    Post by Lightly on Wed Aug 25, 2010 9:45 am

    Yes.

    Take a look at this:

    http://www.flickr.com/photos/swifthoundbows/4012783743/in/set-72157622262033595/

    This is a maximilian bow, and the bolt groove inlet is of micarta, and that will be faster than wood. However, Geezer says, that bone or especially Delrin, (which is slick) will be even faster; it was the Delrin inlay that his friend installed that gave that 10% increase...

    This photo shows the inset more clearly, at the tip of the bow.
    http://www.flickr.com/photos/swifthoundbows/3998209017/in/set-72157622262033595/
    avatar
    Regerald
    Master Crossbowyer
    Master Crossbowyer

    Posts : 67
    Join date : 2009-11-30
    Age : 31
    Location : Finland

    Re: Greasing the track

    Post by Regerald on Thu Aug 26, 2010 11:07 am

    I don't believe that bolt itself cause any significant friction.. Only string, which is usually strongly pushed against stock in medieval type of crossbow. My argument - simple physics. If nothing is pushing bolt against it's groove, friction is caused by it's own weight. Friction constant (wood/wood) is about 0.25-0.5, so energy loss can be easily calculated. For a steel crossbow that basileus has, I've got about 0.15 Joules of energy lost by bolt friction. Which is insignificant, I think..
    If something is pushing down a bolt during it's acceleration, problem isn't in a friction.. I think that can be checked correct stiffness of a string or something else..
    Am I correct or is there any other facts that affect a bolt friction?
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Thu Aug 26, 2010 1:04 pm

    Pesky, pesky friction. The bane of simple dynamic equations everywhere.

    The coefficent of friction that you posted Regerald is technically the coefficient of static friction. Coefficents of dynamic friction are generally lower so that gives us a decent upper bound. I was supprised that they were as low as they were.

    For those that are interested a quick friction primer:
    http://en.wikipedia.org/wiki/Friction#cite_note-engHandbook-6
    Friction force = Mu * Fn; Were Mu is the coefficient of friction and Fn is the
    Normal Force
    This also means that contact area is irrelavant with reguards to friction force. Though Total work lost to friction depends on the distance traveled.

    Coefficients of friction are all experimentally determined and, unfortunalty, change with a great number of variables including speed. At high velocies the coefficent of friction decreases.

    All that said. Did some quick math based on the work done by friction and found at worst a 5.6 ft/sec drop in speed from friction (assuming a constant wood to wood coeffient of friction of .5 and a 1 foot track)
    I could see the string loosing more to friction as it could have a higher normal force due to it pulling down on the track. and a possible higher coefficent of friction so anything that would lower help with that could be more useful.

    So basically all that to say that experimental results are more reliable than quick mathmatical models. expecially with so many variables like surface finishes, fletching drag, string contact force and high speeds. and friction is a complex area of study.



    Something Something Dark Side
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Thu Aug 26, 2010 2:12 pm

    I was trying to figure out how much velocity would be lost due to the slightly lower acceleration due to friction but that was not as simple as string drag would play a decent part of that.

    I could see that adding up to a 15 to 20 ft/sec drop itself.



    Something Something Dark Side
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Thu Aug 26, 2010 3:28 pm

    Wow! The things one learns on this 'site eh. Simply amazing.

    Robin always said that one should be just able to lift the string from the deck with one's little finger when in the unspanned position. Any more effort needed than this means that you'll have too much friction. And friction means heat. So be considerate about what you use for a barrel (plastics) and serving material on the string, as well as the "lube".

    Pavise
    avatar
    Lightly
    Master Crossbowyer
    Master Crossbowyer

    Posts : 199
    Join date : 2010-01-12
    Age : 57
    Location : Austin, Tx

    Re: Greasing the track

    Post by Lightly on Thu Aug 26, 2010 3:37 pm

    Pavise, that is terrific advice that I will keep in mind... I just had to spend some good amount of time fixing a client bow that had too much string drag. I had improperly calculated the angle of the prod. Or rather, the prod socket. Lucky for me, the prod was attached by bow irons, so, I did not have to cut the prod off and re tie it back on two or three times... I corrected the angle, finally...

    Thank you!

    Lightly
    avatar
    Regerald
    Master Crossbowyer
    Master Crossbowyer

    Posts : 67
    Join date : 2009-11-30
    Age : 31
    Location : Finland

    Re: Greasing the track

    Post by Regerald on Thu Aug 26, 2010 3:59 pm

    basilisk120 wrote:
    The coefficent of friction that you posted Regerald is technically the coefficient of static friction. Coefficents of dynamic friction are generally lower so that gives us a decent upper bound. I was supprised that they were as low as they were.
    That doesn't matter. Even if we have a friction constant as large as 2 or 3, energy loss still remains insignificant. And, at last, coefficients for dynamic friction (sliding in this case) are always smaller than for a static friction. I took the largest as "worst case scenario"

    basilisk120 wrote:
    All that said. Did some quick math based on the work done by friction and found at worst a 5.6 ft/sec drop in speed from friction (assuming a constant wood to wood coeffient of friction of .5 and a 1 foot track)
    Can you expose this math? 5.6 ft/sec drop at which initial velocity? Velocity dependence of bolt's kinetic energy isn't linear, so velocity drop strongly depends of an initial velocity, while the energy drop can be assumed (more or less) constant. In my case: work done by a friction W = C*m*g*s, where weight of a bolt m*g is this "normal force" (if you shoot horizontally). By substitution of a values for one fit draw length, 50gramm bolts (0.05kg in CI), friction constant "C" ,say, 0.5, we get
    0,5*0,05(kg)*9,81(m/s^2)*0,3048(m) = 0,074 Joules. At initial velocity 50m/s this energy loss corresponds to velocity loss of ~0.03 m/s or near 0.1 fps Smile
    basilisk120 wrote:
    So basically all that to say that experimental results are more reliable than quick mathmatical models. expecially with so many variables like surface finishes, fletching drag, string contact force and high speeds. and friction is a complex area of study.
    If something isn't possible even in theory - even for any possible values of variables - there is no need for a practical experiment. Otherwise you'll find yourself constructing a perpetual.. out of bolts with a huge friction Smile
    But, a practical think: one can put some pigment on a bolt (like pencil graphite), and check, does it even leave the trace on a groove (does bolt even touch the groove, or is it "lifted" by acceleration)..
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Thu Aug 26, 2010 4:59 pm

    Wow!

    Now I wonder why Robin Allen never talked about all this mathematical theory.

    But he was a practical man and could truly shoot well with any of his crossbows.

    And I'm honoured to be able to pass along what I learned from him; especially to you Lightly.

    Pavise
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Thu Aug 26, 2010 5:01 pm

    Regerald wrote:
    But, a practical think: one can put some pigment on a bolt (like pencil graphite), and check, does it even leave the trace on a groove (does bolt even touch the groove, or is it "lifted" by acceleration)..

    I unintentionally did that. My first set of bolts were painted red and they left a strip of paint on the top plate. Though when I switched to a new set of arrows with different fletching the new bolts left a paint mark in a different place. scratch i should add my crossbow has a ramp on the front ent with the track in it so the bolt only touching in a small area makes sense as compared to a bow with a full length track


    So here is the math I was using. Maybe a case of me trying to be too clever.
    (Reginald just spelling it out so it obvious and I'll rember how I did it next time I read this topic)
    W= F * x (Work = force * Distance)
    W = .5*m*(v2^2 - v1^2) (Work = delta E or E2-E1)
    F= C *Fn (Force = friction coefficent * Normal Force )
    Fn = m*g (Normal Force = mass * gravity - assuming same assumtion about bolt being horizontal or close enough)
    so
    W=W
    C*m*g*x = .5*m*(v2^2-V1^2) were V1 is the intial velocity so it is 0 and masses cancel
    2*C*g*x=v2^2
    sqrt(2*C*g*x)=v2

    assuming C = .5 g =9.81m/s^s x=.3048m (1 foot) v2 = 1.73 m/s
    hence slowing by 1.73 m/s
    This was making the assumtion that friction was doing work for the entire distance. It was not as the paint marks will attest and only the rear of the bolt will travel the entire track.

    Well geezer put some results from testing that showed that changeing the tarack material resulted in a decent jump in speed. Though you are right that it appears that unless something else is going on then most of that speed jump is from something other than just bolt friction.



    Something Something Dark Side
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Thu Aug 26, 2010 5:05 pm

    Does anyone have plans for a cheap wind tunnel?

    Pavise
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Thu Aug 26, 2010 5:10 pm

    Here are some from NASA: http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html

    Seems like something that should be realativly easy in theory to do. As long at its a sub sonic wind tunnel, making a supersonic wind tunnel would take more work as the compressablity of air makes it tricky.



    Something Something Dark Side
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Fri Aug 27, 2010 10:22 am

    Thank you basilisk. I will probably build such a wind tunnel to test my theory about aerodynamic losses due to poor limb design.

    Now as for frictional losses due to the bolts contact with the barrel, there are some basics to consider. No matter what we do: the bolt will bend before it starts to move! (See any high speed video of this to verify.) It is the resulting slight wobble from side to side and up and down that leaves the varying and incomplete signatures we see from painted or crested bolts etc. The slot width should be made with a certain bolt in mind and again the late Robin Allen covers this well on his website, under Track Dimensions I believe it is. And it is well to note that dedicated target crossbows invariably have very long tracks or barrels. This is in order to stabilize the shaft for as long as is practical and thus improve intended accuracy; not unlike a long gun barrel in effect. Delrin is a good material but UHMW is even better as far as coefficient of friction is concerned, but it requires special techniques and adhesives to fasten it to wooden stocks. None of this is new. Crossbow makers were doing it years ago and had to work with what they had available for the most part. And as dear Robin always used to say, "Try it mate and see if it works. That's how we learn!" God, how I miss my friend.

    Pavise


    Last edited by Pavise on Fri Aug 27, 2010 2:15 pm; edited 1 time in total (Reason for editing : Typo)
    avatar
    Lightly
    Master Crossbowyer
    Master Crossbowyer

    Posts : 199
    Join date : 2010-01-12
    Age : 57
    Location : Austin, Tx

    Re: Greasing the track

    Post by Lightly on Fri Aug 27, 2010 11:25 am

    Pavise, I looked up UHMW material, and it looks interesting...

    You mention special adhesives for that material, do you know which? We use Gorilla Glue for the micarta (in addition to pinning it) for when we use it for a bolt groove inset. And, scratch it up, to help it grab.

    In your opinion, (altho not traditional, of course!) would it be worth trying some of the UHMW stuff for bolt grooves, like we use the micarta? For personal bows...

    We use the micarta on client bows as it somewhat resembles bone, and even, has a tendency to 'yellow' with age, like bone...

    Thanks!

    Lightly
    avatar
    Thee Apprentice
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 4
    Join date : 2010-08-23
    Age : 41
    Location : Oregon

    Re: Greasing the track

    Post by Thee Apprentice on Fri Aug 27, 2010 1:17 pm

    Why not go all out for performance and put a PTFE/Teflon track in place? Its white typically and tends to be very soft and easy to machine. Not sure how well you could glue it in because its so slick but you could probably pin it.

    http://www.mcmaster.com/#plastics/=8l7qc6
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Fri Aug 27, 2010 2:40 pm

    Hello Lightly,

    If you Google "gluing UHMW" you will find several answers, but I always use G2 epoxy and flame heat the surface (3 sides) of the plastic insert first. I also provide strategically placed recesses in the plastic where a mechanical bond will be generated as the epoxy sets and adheres to the more accepting wood. Again, I learned this from Robin who himself learned it from the Samurai. Some say that Loctite 401 in combination with their 770 Primer works well for this application too. I see no reason, other than what you have raised, why UHMW would not work for common bolt grooves except that it is vulnerable to accidental impact damage despite its extreme resistance to wear. As well as being easily chiselled and planed, UHMW may be machined and shaped with router bits and end mills but is almost impossible to sand without overheating and spoiling the desired finish.

    Kind regards,

    Pavise
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Fri Aug 27, 2010 2:55 pm

    Pavise wrote:Thank you basilisk. I will probably build such a wind tunnel to test my theory about aerodynamic losses due to poor limb design.

    Now as for frictional losses due to the bolts contact with the barrel, there are some basics to consider. No matter what we do: the bolt will bend before it starts to move! (See any high speed video of this to verify.) It is the resulting slight wobble from side to side and up and down that leaves the varying and incomplete signatures we see from painted or crested bolts etc. The slot width should be made with a certain bolt in mind and again the late Robin Allen covers this well on his website, under Track Dimensions I believe it is. And it is well to note that dedicated target crossbows invariably have very long tracks or barrels. This is in order to stabilize the shaft for as long as is practical and thus improve intended accuracy; not unlike a long gun barrel in effect. Delrin is a good material but UHMW is even better as far as coefficient of friction is concerned, but it requires special techniques and adhesives to fasten it to wooden stocks. None of this is new. Crossbow makers were doing it years ago and had to work with what they had available for the most part. And as dear Robin always used to say, "Try it mate and see if it works. That's how we learn!" God, how I miss my friend.

    Pavise

    You must have read my mind Pavise. I was just wondering about the bolt flexing. I was just looking at the paint marks on my bow and noticed that the tail end of the bolt is what is dragging and leaving paint. It also takes a bit of force and speed to leave that much paint. So it does look like the normal force on the bolt is greater than merely the weigh of the bolt.

    Three Apprentice:
    I noticed that McMaster Carr has sticky back PTFE film. Hmm wonder how that stuff would hold up to repeated use on a crossbow .



    Something Something Dark Side
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Fri Aug 27, 2010 4:38 pm

    I ordered some of the sticky backed PTFE film to try on my crossbow. I figured it can't do anything to the bow that I can't fix sander Besides if all else fails I have a new crossbow on order with New World Arbalist Very Happy Soo can't wait.
    But I'll let everyone here know how it turns out and how it takes to repeated abuse.



    Something Something Dark Side
    avatar
    Pavise
    Dear Friend, You will be Greatly Missed.
    Dear Friend, You will be Greatly Missed.

    Posts : 128
    Join date : 2010-02-07

    Re: Greasing the track

    Post by Pavise on Fri Aug 27, 2010 5:32 pm

    Don't forget to patent and register your findings too. Rolling Eyes

    Pavise
    avatar
    Lightly
    Master Crossbowyer
    Master Crossbowyer

    Posts : 199
    Join date : 2010-01-12
    Age : 57
    Location : Austin, Tx

    Re: Greasing the track

    Post by Lightly on Fri Aug 27, 2010 7:51 pm

    Wait! You DO?! Which one?! Just want to see if *I* have that order!

    (At Basilisk!)

    Best!

    Lightly
    avatar
    Basilisk120
    Moderator
    Moderator

    Posts : 548
    Join date : 2010-03-01
    Age : 38
    Location : Arizona

    Re: Greasing the track

    Post by Basilisk120 on Fri Aug 27, 2010 9:45 pm

    Lightly
    Walnut Maximillian with steel prod, bow irons and micarta track. Its going to be my Christmas present. My wife was threatening that when it arrives she was going to wrap it up and make me wait till Christmas to open it.



    Something Something Dark Side

    Sponsored content

    Re: Greasing the track

    Post by Sponsored content


      Current date/time is Wed May 24, 2017 2:04 am