Patent GB-618830-A (GB)
Title: Improvements in machines for joining sheet materials by the heating effect of electric stress alternating at high frequency
IPC: H05B6/50
Filed: 1946-11-13
Abstract: 618,830. Seaming non-metallic sheet material. BRITISH INSULATED CALLENDER'S CABLES, Ltd., and JONES, P. Nov. 13, 1946, No. 33726. [Class 42(i)] [Also in Groups XXXV, XXXVIII and XL(b)] In a machine for seam-welding superimposed layers of thermoplastic material by passing them between pressure rollers while being subjected to the heating effect of a high-frequency electric field, the speed at which the material is driven through the machine is automatically adjusted by means responsive to variation in the gap between the rollers, due to alteration in the softness of the material, or to the displacement of one of them relative to a monitoring roller engaging the cold material adjacent the pressure rollers. The adjustment means may be mechanical, optical or electrical. In the arrangement shown in Fig. 1, the machine comprises a worktable 1, beneath a gap in which the lower roller 3 is mounted, a standard 13, overhanging arm 15, and a head 14 in which the upper roller 2 is mounted in a stirrup 9 on the end of a rod 10, which can slide vertically in a bracket 11 and is urged downwardly by a spring 12. Both upper and lower rollers are driven at the same speed and in opposite directions from a constant-speed electric motor 25 through shafts and bevel gearing, the transmission including a slipping clutch 27. The lower roller 3 is made of insulating material, and its periphery is provided with electrodes 30 and 31, which are connected by contacts 32 and 33 to an oscillator 28; the potential difference applied to the electrodes and thus the rate of heating of the work, is controlled by a regulator switch 29. A cam 23 engaging a projection 22 on the rod 10, and controlled by a lever 24, enables the roller 2 to be lifted clear of the work. The end 35 of a lever 34 normally engages the top of a slot 36 in the rod 10. Lever 34 is pivoted on a pivot 37 engaging a slot 44 therein, and movable by an external control 45 to enable the lever ratio to be varied. A rod 39 is pivoted to the end of the longer arm of the lever 34, and carries a fork 40 engaging a collar 41 on the shaft 42 controlling the slipping clutch 27. The machine is so adjusted that, with the sheet material between the rollers 2 and 3 and the oscillator switched off, the clutch 27 is disengaged; on switching on the oscillator, the sheets soften and the rollers approach one another, causing the clutch 27 to be engaged and the material to be fed automatically through the machine. Alternatively, as in Fig. 2, the rod 39 may operate a contact arm 51 moving over contacts 50 of a tapped resistance 49 connected, through slip-rings and brushes 47, in the circuit of a solenoid 46 controlling the clutch 27. The control of the drive may be effected optically by the means shown in Fig. 3, wherein a mirror 53 pivoted at 54 reflects the light from a source 55 to a photo-electric cell 56. A lever 57 is pivoted at 58 on an adjustable support 63, and a projection 59 on the rod 10 contacts its shorter arm, while a projection on the longer arm contacts a lever 60 attached to the mirror 53, to rock the latter. The shape of the light beam and the aperture of the light-sensitive relay are adapted to provide progressive variation in the amount of light reaching the latter; or the intensity of the light may be varied by a graduated filter or other means of intercepting or moving the beam. The photo-electric cell may be of the electron-multiplier type, controlling a thyratron acting as a grid-controlled rectifier included in a circuit providing power for a variable-speed electric motor. Or, the movement of the rod 10 may be used to strain a member 65, Fig. 4, pivoted at 67 and anchored to an adjustable member 66. A wire strain-gauge 69 is secured to the member 65 and forms part of a resistance bridge which is supplied with alternating current. Variations in the strain in the wire cause alterations in its resistance and unbalance the bridge, the resulting alternating voltage beiny applied to the input side of a valve amplifier; the output is rectified and applied to the grid of a thyratron forming a grid-controlled rectifier in a circuit providing power for a variable-speed electric motor driving the machine. In order to adjust automatically for variation in the thickness of the sheets, as when crossing seams, the variation in the speed of the machine may be controlled by differential movement between the upper roller 2 and a monitoring roller 96, Fig. 7, mounted on a rod 97 and urged downwardly by a spring 98. A sleeve 100 surrounding the rod 97 is adjustable axially by a screw 101, and on it is mounted a strain-gauge device which is operated by contact with a projection 59 on the rod 10. A projection 102 raises the rod 97 when the roller 2 is raised by the cam 23.