Most seam welding machine requirements are met by the use of continuous drive. However, in cases where heavy work parts are to be welded or more than a single weld pulse is required (for example, where it is necessary to apply preheat and quench and temper times or long weld times) intermittent motion must be used in order to retain electrode force over the weld nugget while the weld is being completed. Continuous motion would result in premature release of electrode force and therefore cause cracking, porosity and unacceptable welds. The work travels between the electrodes the distance required for each succeeding weld increment. The work stops during the quench and temper time where it is not required to move the job. The portable welding head is moved by motor driven wheels and air cylinder mechanism provides the electrode force. Standard seam welding machines are classified by throat depth.