Scienceshow/Equipment/Vandegraaff Cleaning

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Vandegraaffs are notoriously tricky devices to use, which has led to some teachers abandoning them in the classroom. However, if you keep your machine in good condition it should not give you any problems. This is a guide on how to repair a non-functioning vandegraaff machine. First, the operational mechanism will be described, then the various steps for repairing it will be explained.

This guide was written with the Frederiksen Vandergraaff in mind, but the advice given here applies equally well to other machines.

Operational mechanism[edit | edit source]

The vandegraaff works by separation charges via the triboelectic effect. Charge is sprayed from a metal mesh underneath the bottom roller to the belt. It is then discharged onto a second mesh on the top roller, which is inside the collector sphere. The charge is then stored on the collector sphere.

Troubleshooting[edit | edit source]

There are two basic things which may fail in this mechanism: either the charges are not being transported on the belt, or they are lost as fast as they are accumulated on the collector. To distinguish between these two cases, connect a wire to ground and hold it next to the belt. If you hear a hissing sound of charges going from the belt to the wire, the belt is charged. Do not touch the belt with the wire as this may damage the belt.

If there is no charge on the belt, there are again two possibilities. Either the belt is losing its charge because it is conductive, or it is not being charged at all. In the first case, the belt needs to be cleaned. In the second case, the rollers need to be cleaned, and the mesh needs to be repaired.

If there is charge on the belt, it is being lost in the collector. In this case, the collector and supporting posts needs to be cleaned.

Repairing the mesh[edit | edit source]

The mesh is non-functioning if it is either touching the roller or if there are small pieces of wire sticking out above it. In either case, remove the rollers and belt, and adjust the mesh. It is very important that the mesh is a uniform distance away from the belt. Use nail clippers or precision pliers to cut the mesh to size.

Cleaning the rollers[edit | edit source]

Clean the rollers by removing the rollers and belt from the machine. Remove any oil or grease which is sitting on the outside of the rollers with a leather cloth soaked in petrol. Be careful when replacing the rollers, as the ball bearings of the axle on which the roller rests are also oiled.

Cleaning the belt[edit | edit source]

The most important thing about cleaning the belt is that any trace of oil and grease is removed. This can be done by cleaning the belt in turpentine or pure petrol. Use a leather rag to clean the belt, and dry it with a blow dryer afterwards. Be sure to put to blow dryer on a non-heating setting to avoid igniting the chemicals.

After the belt is cleaned in this way, powder it lightly with talcum powder, then apply the blow drier again to prevent any excess talcum powder from remaining.

Cleaning the collector[edit | edit source]

The collector must be absolutely free from anything which may act as a point for corona discharges - i.e. any hairs, pieces of dust, etcetera. It is important to remove these from the inside of the sphere as well! Clean the collector with a leather rag soaked in petrol.

Cleaning the supporting posts[edit | edit source]

The support posts are made of plastic, which means that the cannot be cleaned with alcohol. Do not use window cleaner on the support posts. Instead, clean them with a towel soaked in petrol or turpentine.

Operational tricks[edit | edit source]

If after this troubleshooting the VDG still doesn't work, there are a few operational tricks. First of all, try blowing hot air into the machine (especially at the mesh). If this area operates at a higher temperature it will release more charges.

Second of all, reduce the humidity in the room. This may be done by pouring some liquid nitrogen on the floor, which will cause any water in the air to condense onto nearby surfaces.