Principle of Operation :
In general, it is not easy to mechanize manual metal arc welding.
The short length of the electrodes and the difficulties of controlling the arc length have hampered the development and exploitation of suitable systems. One technique which has been successfully used in a number of applications is gravity welding (in figure below).
The electrode is clamped in a holder which can slide up and down a bar or rod held at a predetermined angle to the joint. The joint is almost invariably a T’, and the top of the electrode is positioned in the root of the joint.
Once the arc has been initiated, the end of the electrode melts, the electrode gets shorter, and the holder moves down the bar, keeping the tip of the electrode in contact with the joint. The weld is thus deposited until the electrode has been reduced to about 50 mm, at which point the movement of the holder ceases and the arc is extinguished.
A fresh electrode is placed in the holder, the slider bar is moved along the joint, and the weld is restarted at the point at which the previous electrode stopped.
The successful operation of gravity welding depends on two factors.
Firstly, the electrode must have a flux which melts in the form of a cone at the tip (in figure below). By resting the outer edge of the flux cone on the plate in the joint, the arc length is controlled by the melting of the flux.
Secondly, the angle between the slider bar and the joint must match the melting rate of the electrode if the system is to be stable.
- Electrodes for gravity welding are made especially long so there are fewer electrode changes for a given run out length than in manual welding.
- As the electrode burns at the same rate this is the only speed advantage over manual welding for a single gravity welding unit.
- The simplicity of the equipment, however, allows several gravity welding units to be operated simultaneously by a single operator and there is increased output and thus considerable savings.
- The process is suitable for repetitive work such as the attachment of stiffeners to pencil, a common requirement in the shipbuilding and structural engineering industries.
Inspite of its simplicity, gravity welding is still not widely used.
References : A Textbook of Welding Technology by O. P. Khanna