(2013-01-04) Characteristics of a rubber compound.
Strict relationships between bands made from the same rubber recipe.
Manufacturers may not always use the same type of rubber for all their bands.
Their nominal formulas may evolve over time and various coloring additives
can substantially influence the physical properties of rubber.
There may also be some slight variations from one batch to the other.
However, long bands made from strips cut from same wide band spool
have consistent physical properties which can be predicted from their geometry alone.
(The flat bands supplied by Thera-Band are 128 mm wide.)
(2012-07-21) Straight Cuts vs. Tapered Bands
Applied forces are inversely proportional to their virtual displacements.
Tapered bands are easily fabricated from wide flat rubber bands, using a cutting mat and
rotary cutter.
The only widely available tapered tubular rubber bands
are the red bands (RRT model) from Trumark.
(2013-01-03) Calibrated Straight Rubber Bands
Characteristics of commercial color-coded rubber bands (tubular or flat).
The Thera-Band company sells high-quality rubbertubing
manufactured for them by Hygenic
in the 7 color-coded characteristics tabulated below.
They also provide, 8 different types of
flat
rubber bands. Those used to be 138 mm wide; they are now 128 mm wide,
with more efficient rubber
(reduced width and thickness makes their new bands lighter but a given length of any of their color
grades retains virtually the same elastic propertiesthe same dynamical properties).
The thickest and stiffest grade for flat bands is dubbed gold
(that strength isn't available for tubular bands).
The improved formula is also used for the newer tubular rubber bands, whose outer diameter
has been reduced to produce the same pull as before (the inner diameters remain the same).
All those bands are primarily intended for the Thera-Band progressive
exercise systems but are also extensively used by slingshot enthusiasts
(who commonly call the tubular bands thera-tube for short).
Straight Tubular Rubber Bands
Brand
Model
Stretch
Draw
Linear density
Inner Æ
Date
Thera Tube
Silver
167 N
0.535 g/cm
0.2 "
2012
Thera Tube
Black
73.5 N
0.43 g/cm
0.2 "
2012
Thera Tube
Blue
0.2 "
Thera Tube
Green
0.2 "
Thera Tube
Red
0.2 "
Thera Tube
Yellow
0.2 "
Thera Tube
Tan
0.062 "
The small inside diameter of tan tubing makes it unsuitable for traditional slingshot rods.
All of the others are perfectly suited for standard ¼" round-rod frames, except
Thera-tube yellow which doesn't provide sufficient grip around a ¼"
(either secure it with a constricting knot, or use 5/16" rods).
Straight Flat Rubber Bands
Brand
Model
Stretch
Draw
Linear density
Date
Thera Band
Gold
Thera Band
Silver
Thera Band
Black
Thera Band
Blue
Thera Band
Green
Thera Band
Red
Thera Band
Yellow
Thera Band
Tan
Isothermic Calibration : Draw (force in newtons) vs. stretch (strain in %).
Strain
Tan
Yellow
Red
Green
Blue
Black
Silver
Gold
0 %
0
0
0
0
0
0
0
0
25 %
4.9
6.7
8.9
12.5
16.0
22.2
35.1
50 %
8.0
11.8
14.0
20.2
27.2
38.0
75 %
100 %
125 %
150 %
175 %
200 %
225 %
250 %
Office Rubber :
Size #107 rubber bands (office supply) are 7" long and 0.625" wide (5/8).
Here are the product numbers for a one-pound box at the Alliance Rubber Company :
27075: 40 per pound.
Alliance Advantage grade (max. 700%).
25075: 50 per pound.
Alliance Sterling grade (max. 750%).
(2013-01-10) The Right Stuff
Rubber, polymer, resin, protectant...
303 protectant.
(2013-01-09) Tools of the Trade
Cutting rubber, punching leather, carving wood...
A rotary cutter is designed to cut fabric on a cutting mat.
It's what's needed to cut strips from a wide roll of rubber.
Hole punch. Rivet.
(2013-01-12) Slingshot Ammo and Calibrated Projectiles
Steel ball bearings (BB) glass marbles, nylon balls, cast lead or bismuth.
Odd-shaped natural rocks remain the mainstay of slingshot ammunition
for casual plinking. For competitive shooting,
however, only spherical projectiles are used.
The density of polypropylene (PP) is 0.946 g/cc. Nylon 6-6 is 1.15 g/cc.
Teflon (PTFE) is 2.20 g/cc.
The density of corundum (ruby or sapphire) is 4.02 g/cc.
Synthetic ruby, sapphire or tungsten carbide are inadequate for casual plinking.
They are reserved for specialized experiments.
(2013-01-23) Ballistic Gel & Ballistic Pendulum
Calibrating ordinary gelatin (foodstuff) for ballistic purposes.
The old NATO formula for "20% gelatin" (3.2 oz/lb) is no longer used.
Professionals use 10% gelatin (1.6 oz/lb) and some amateurs are satisfied
with a less expensive mix (1 oz/lb).
The preparation protocol is critical to achieve consistent results.
In particular, temperature should be maintained below 40°C.
