You have a choice as to the roller fixing method you wish to use:
Simple direct shaft rollers,
(see figure 13 and 14)
Ball bearing shaft rollers.
(see figure 15 and 16)
If you chose the simple direct shaft rollers of figure
13 and 14 then you can make the rollers in a variety of ways. One way is
to drill/lath a hole down the length of the nylon roller and push a longer
length of steel rod firmly into the hole. Alternatively you could push a
sized outer sleeve into the roller and fit a smaller threaded rod to have
a more secure fixing method. (see figure 17 ) The roller
choices are numerous. The advantages of making simple test rollers this
way is that you can test which nylon is working the way you would like.
Once you have stable test results you could then opt for the better method
of adding ball bearings into the rollers to ensure a long continuous life
to your machine. (see figure 15 and 16) There are many various, tried
and tested methods with roller materials and their construction.There is
no absolute right or wrong method of construction. Feel free to add your
own tried an tested methods in place of this.
Constructing Rollers With Sealed Bearings.
This is my own preferred method of constructing the
top roller(s). You will need access to a shop lath or machine to do the
lathe work. If you have no access to these machines then you can use a
hand drill machine as the nylon material is very soft and easy to
machine.The outer diameter of the rollers can be any diameter you wish. You should not make the
bottom roller a small diameter, else you will see the high voltage jump
off the roller an short out onto the shaft of the motor. The bigger the
roller diameter the less likely the HV will short out. As a preference I
like to keep my rollers to a diameter of around 40-50mm.(see figure 17
& 18) If you are just starting out you might like to do the same until
you have mastered all the variables. I prefer not to have the top and
bottom roller the same diameter. I believe this helps the voltage
potential's. If the top roller is smaller than the bottom roller diameter
then the top roller will be travelling at a greater rotation.
I believe this greater
rotational speed produces better friction on the belt which produces
greater HV potential on the sphere condenser.Important:
You will need to use two different substance of nylon materials
on the top an bottom rollers. The top roller needs to be made of a
different density of nylon to the bottom roller. If you use the same
material/substance on the top roller and bottom roller, then you will find
problems with low or no HV potential's. If you must use the same nylon
substance on the top an bottom rollers then you will need to sleeve over
the bottom roller with a rubber or nylon sleeve of a different substance. It is the difference of
these substances which allows the VG to develop greater differential
potential's. Only through your own experimentation can you prove which are
the best combination of materials. As a hint, try to avoid black coloured
rubber, plastics or nylons. The black colour could be carbon or iron.
These substances conduct electricity. This conduction property is very
undesirable and can cause untold frustration. --Avoid black materials
in your search for better results. You can use a metal sleeve over a
roller if the centre of the roller is insulated away from any conduction.
I have had moderate success using a hardwood lower roller with a metal
sleeve around the circumference of the wood. You may play with all the
variables to find the best results. I have been informed that teflon can
also produce some good results. You may also like to try skate board
wheels and/or sleeve over them.
My Personal choices for roller materials :
Top roller : Food
grade nylon cutting board is a good material for the top roller. This
nylon is cheap and readily available from supermarkets and hardware
stores. Glue two or more small pieces together to get wider rollers.
Bottom roller : The
best material for the lower roller is nylon66 with a natural rubber
photocopier feed tyre sleeved an glued over the nylon insulator. You
may try different natural rubber compounds if you wish.
The above nylon choices will produce a negative potential on the
vandegraff condenser sphere. Swap the materials top to bottom if you wish
to produce a positive potential on the sphere condenser.
Positive or Negative Potential's
(Checking The Polarity)
If you find that you would like a specific polarity potential from your
vandegraff generator then you will have to experiment with the top and
bottom roller materials. Firstly you will need to know how to check the
voltage polarity potential of the vandegraff unit. Using a multi-meter
from a safe distance, place positive(+) probe of the multimeter directly
on to the static comb or motor mount earth of the vandegraff unit. From a
distance, say 500mm away, point the negative(-) probe in the air towards
the VG sphere condenser. DO NOT touch the negative multi-meter
probe directly on to the metal sphere while the VG machine is running. The
voltage developed on the VG may be in excess of 400kV. Sparks can jump
through the air some 300mm-400mm in distance from the VG sphere condenser.
The vandegraff generator only utilises micro amperage of current, the
400kV potential sparks will not harm you to any great degree, but there
will be enough potential to probably destroy your multi-meter. If you
still have no reading of the polarity then slowly move the negative lead
in the air toward the VG sphere. If you start to get a stable reading and
the meter reads as a normal reading, then you have confirmed that you have
a negative potential on the VG sphere. If your multi-meter reading is
showing a negative reading then you have a positive potential on the VG
sphere. Be watchful that you have the flying leads of the multi-meter in
the correct locations of the multi-meter. The reading you get from the
multi-meter may be in milli-volts. You will never be able to read the true
voltage reading from the vandegraff generator unless you own a special EHT
meter or EHT probe.
You can make an EHT probe by connecting 10 or more 10mega-ohm resistors
in series. This will give you a slightly more accurate reading but will
not be truly accurate. A rule-of-thumb for high voltage transmission
through dry air is around 20,000 volts per 25mm(1") air gap.
Example rule-of-thumb: Hold one end of a plastic ruler in the air
while touching the other end directly onto vandegraff sphere. Move your
finger down the ruler until a spark jumps to your finger. If the sparks
are jumping over a 250mm air gap, then you can roughly calculate you have
a high voltage potential of 200kV. This depends on the moisture content in
the air. The drier the day the better the build up of charge on the VG
sphere.
Once you know how to check the polarity potential on the vandegraff;
all you need do is swap the top & bottom rollers over. That is move
the top nylon roller down to the bottom roller, and move the bottom nylon
roller up to the top. You will now have reversed the polarity potential of
your vandegraff. If the VG was first a negative potential then it will be
a positive potential (or visa versa) when you swap the rollers from top to
bottom. I prefer a negative potential vandegraff as there are some small
health benefits from using negative ions, plus other interesting
conditions when you treat plastics and nylons with HV potential's. These
interesting conditions are principles based on how to make diodes and
other semi conductor devices. I will leave this semi conductor topic for
another web page article.... Yes your vandegraff generator can become more
than an interesting toy. It has the potential to be a useful work tool or
valued test bench device.
Alternative
(see figure 17 ) The roller
choices are numerous. The advantages of making simple test rollers this
way is that you can test which nylon is working the way you would like.
Once you have stable test results you could then opt for the better method
of adding ball bearings into the rollers to ensure a long continuous life
to your machine. (see figure 15 and 16) 
There are many various, tried
and tested methods with roller materials and their construction.There is
no absolute right or wrong method of construction. Feel free to add your
own tried an tested methods in place of this.
You should not make the
bottom roller a small diameter, else you will see the high voltage jump
off the roller an short out onto the shaft of the motor. The bigger the
roller diameter the less likely the HV will short out. As a preference I
like to keep my rollers to a diameter of around 40-50mm.(see figure 17
& 18) If you are just starting out you might like to do the same until
you have mastered all the variables. I prefer not to have the top and
bottom roller the same diameter. I believe this helps the voltage
potential's. If the top roller is smaller than the bottom roller diameter
then the top roller will be travelling at a greater rotation. 
condenser.Important:
You will need to use two different substance of nylon materials
on the top an bottom rollers. The top roller needs to be made of a
different density of nylon to the bottom roller. If you use the same
material/substance on the top roller and bottom roller, then you will find
problems with low or no HV potential's. If you must use the same nylon
substance on the top an bottom rollers then you will need to sleeve over
the bottom roller with a rubber or nylon sleeve of a different substance. 
It is the difference of
these substances which allows the VG to develop greater differential
potential's. Only through your own experimentation can you prove which are
the best combination of materials. As a hint, try to avoid black coloured
rubber, plastics or nylons. The black colour could be carbon or iron.
These substances conduct electricity. This conduction property is very
undesirable and can cause untold frustration.
--Avoid black materials
in your search for better results. You can use a metal sleeve over a
roller if the centre of the roller is insulated away from any conduction.
I have had moderate success using a hardwood lower roller with a metal
sleeve around the circumference of the wood. You may play with all the
variables to find the best results. I have been informed that teflon can
also produce some good results. You may also like to try skate board
wheels and/or sleeve over them.