Roller Nut Material

First topic message reminder :

I was wondering about what materials work for roller nuts. I'll take a stab at recording what I have learned. Please comment with more details or point out were I am wrong.

Delrin: Relatively easy to get and work with, light weight, low friction, stable, looks like bone, works well for lighter (hand pull bows) crossbow.

Horn: light weight, period correct, looks good, strong, but more difficult to get (can't order horn from McMaster Carr )

Wood: Light, really easy to get, easy to work with, needs to be be treated to keep it from swelling from absorbed moisture, not strong enough for heavier crossbows, mostly replaced by Delrin.

Aluminum: Common, easy to work with, light weight, may have galling issues with the trigger especially in higher draw weight bows.

Brass: Shiny, period correct, heavy can cause misfire issues due to slow rotation, may be an decent choice for heavy draw weight bows.

Steel: Similar issues to brass, heavy but strong, could a good choice for really heavy crossbows.

So am I missing anything?

How about Corian countertop material, I have some chunks that are about 1-1/2 inches square and are the color of grey stone, if you polish it up real nice it will be as smooth as a babies ass, Lol

Any thoughts ???

Just looked up the specs on Corian its a bit weaker than Delrin but what might be more of an issue is that it is brittle. Also it seems to be rather intolerant of dull blades. So it might be better to use it as an Ivory or bone substitute.

"Intolerant of dull blades" ??? Isn't everything intolerant of dull blades. LOL !!! just kidding, I've dropped pieces on the cement floor many times with not even a chip, I think I'll give it a try since I have it readily available. Thanks for the info anyway though.

I worked in corian countertop shop for awhile. I think it will work fine. Work with the same tools that you use for wood. I can tell you that we used special corian glue when laminating together, although that was mainly for color reasons and think use can use epoxy.

Todd

Hey Guys,

I'd like to bring this discussion back to life.  I have some ideas I'd like to throw out there for your opinions.

I asked on the Ulrich Bow thread about Roller Nut materials and then spent some time looking over the primary threads such as this one.

I was thinking along the lines of a metal roller nut, probably aluminum.  Delrin and ABS are interesting as well.  Anyway, I spent some time looking around the OnlineMetals.com web-site at potential materials.  In addition to metal, they also have several kinds of plastic available.

I looked at titanium - $50 for a 1.5" x 1.5" rod.  I also looked at bronze, brass, and aluminum.  Then just for grins,  I also looked at some of the plastic products they sell.

This has generated several additional questions, so here goes:

1.  Since weight/mass is a consideration to using metal, does it make sense to drill the roller nut to lighten it?

2.  Acetel (Delrin) looks viable, but of course compared to metal not nearly as strong.  I then looked at some plastics that come is sheet form.  Would it be workable to take something like acrylic and bond several pieces together and then machine it into a roller nut with the axis running perpendicular to the plane of the sheets??

3.  How about doing the same thing with GPO-3, which is fiberglass reinforced thermoset polyester??  The properties of this stuff look very interesting - it's available in sheets up to 1/2" thick and a 12" x 12" x 1/2" sheet is like $14.  I figure you could cut it into 2" strips (you get 6 to a sheet), laminate 3 each so you get 2 - 1.5" x 2" x 12" strips which should yield 12 - 2" x 2" x 1.5" blocks that could be easily machined into roller nuts.

So what do you guys think??  Does this sound viable?  Did I miss anything in my thought processes??

I think elemental to this question is how much weight are you trying to hold?  Maybe someone here came speak to the question of when one needs a steel nut?  I know Todd here makes some of his nuts from wood with great success. 

I currently have an ABS nut holding a 250 lbs prod without issues so far.

Another consideration could be is aesthetics. The ABS comes in a bone colour, whereas the the Delrin seems to only come in snow white and black (I think).  This could be a consideration if you are trying to look medieval.  If the 15c is a priority try to get you hands on a large antler pedical! If you are not trying to be historical there are modern trigger designs of greater efficiency to think about.

Your composite nut ideas are interesting.  There are knife makers laminating layers of denim and other cloth with resin and using this to make handles for their knives.  https://www.youtube.com/watch?v=73BEonHiMM4

They get some interesting effects with different fabrics. It may be strong enough for use as a nut, however it is certainly simpler to saw off a piece of plastic rod  .

The GPO-3 material I mentioned earlier, is substantially stronger than Delrin though not up to snuff in comparison to metals.  If you're getting good results with 250 lb draw weights using Delrin, I'm thinking that this GPO-3 material will be very workable.

Are you using metal inserts at all?

Jeff
Bend,Or

Hi Jeff,

I am using ABS for the 250, and its fine.  I would also be happy with Delrin (but untested).  

Regarding metal nut inserts.  Yes, they are necessary, you need comparable steal touching steal, what I did was to screw a 1/4" stainless steel bolt down through the ABS nut. (spend extra time aligning the tickler and the nut bearing surface and ensure the tickler doesn't  push the nut against the string during release or release will be very hard). Its to do with the pivot point of the tickler. 

I ground a flat vertical surface  in the bottom tip of this bolt to serve as the bearing surface for the tickler.  I cut the head off the bolt so it would recess into the nut.  I applied a bit of lock-tight glue to keep it in place (probably unnecessary)  I cut in a slot in the top of the bolt to act as a screwdriver slot to drive it in...

Ernest (Hotspur)

I've made about a half dozen roller nuts from Delrin for weapons from 100 to 250 pounds, so far with zero failures. I've made a couple of the bone-colored ABS nuts Hotspur has used, but they scared me a bit because the ABS seems to flex more than Delrin. It does look very nice, though, I may try it for an upcoming 110 pound project. I love Delrin. I have a hard-hitting pistol where the entire release mechanism is made of the stuff. I'm currently designing a nut-less, modern release mech to be made entirely of delrin, along with the bolt track and significant other parts of the bow. I'm nuts about Delrin, I tell ya.
I would also like to know the general qualities of the weapon you'd like to build before diving too deep into an academic discussion of exotic roller nut materials. After all, if you want an authentic, historical replica, get ye an olde bit of antlere. If you're going high tech, there are, as Hotspur has pointed out, more efficient release mechanisms than the roller nut.
Gnome

Actually, as far as bow design goes, I'm thinking I'd like to design something like the H bow designs that Nerd Flintstone has come up with.  I like the compound action w/o pulleys and eccentrics.  Probably do something like a bull-pup design.

As for lock mechanisms, I'm not opposed to more modern designs, but I guess I'm intrigued somewhat by the simplicity of the roller nut mechanism.  I am looking for a more conventional type of trigger assembly so maybe the roller nut type of mechanism won't be appropriate.

I'm thinking in the 150 to 250 lb draw weight range.

I like to play around with unconventional uses of various materials.  I've become pretty good at adapting different items and materials for use in my other hobbies (amateur radio and high power rocketry).

I have to admit, I've been amazed at the level of power in some of these devices even something designed and built 500 or 600 years ago and the level of sophistication is eye opening as well!

Thanks for all the feed-back and answers to my questions.  I will strive to make a bow that is worthy of the quality displayed on this forum.

Jeff
Bend, Oregon

I'm a big fan of "unconventional uses of various materials" too- especially when it comes to repurposing esoteric items that would otherwise end up in the landfill. I work in the printing industry and that's how I got interested in Delrin, UHMW and other engineering grade polymers- salvaging gears and other bits of laminators and other machines that the technicians were tossing in the dumpster. I'm eager to see what you come up with, Jeff.
Gnome

Gnome wrote:I work in the printing industry

 Gnome, do you have access to Quoins through your work?  I think they'd be a pretty good way to provide a locking wedge to secure a prod to a take-down cross bow.

Quoins for letterpresses?  Gee, that takes me back thirty years.  I spent 8 years running Miehle and Chandler letterpresses for a printshop in Austin Texas.  Yeah, I think quoins would work well, but the ones we used were a bit bulky.... It might take some clever engineering.  Geezer

Quoins? I had to Google that term. So that's what you call those things! I've never worked a letterpress, but those things were hiding all over the place in the first shop I worked in, twenty-five years ago or so- tool boxes, drawers, filing cabinets, etc. Wish I'd grabbed a few!
Gnome

A machinist friend of mine made me aware of them... They make a very good low-profile clamp if your are fly-cutting a surface.  Sort of an expanding wedge action in a compact unit.

It may be not what are you looking for exactly but for example have you ever considered nylon/ertalon?

ertalon is that ugly looking green plastic but is also found in white colour

it has a tensile strenght of 90 Mpa or N  and weights 1g/cm3

for example a rolling nut with the 2 ''dents'' (dont know the exact term yet) that keep the string with a base area of 10mm each it can hold up to 1 tonne 

10*10*2*90=18000N    18000*0.577 (correction on shear stress) = 10000N

Meaning that it can hold 500kgs easily