Patent GB-510806-A (GB)
Title: Improvements in electromagnetically operated tools of the percussion type
IPC: B25D13/00
Filed: 1937-12-03
Abstract: 510,806. Electric percussion tools. POLLAK, J. E. (Greiner, M.). Dec. 3, 1937, No. 33520. [Class 124] [Also in Groups XXXV and XXXVII] In an electromagnetically operated percussion tool a tubular casing 15, pole pieces 12, 13, and a laminated or other annular member 14 form two closed circuits for striking and return coils 25, 26, the latter surrounding an insulating or insulated tube 16 in which an armature 11 reciprocates. The armature 11 is secured to, or formed integrally with, a rod 17, their upward movement being limited by a cap 19 surrounded by a buffer spring 20 inside a housing 21, the spring 20 accelerating their downward movement by the coil 25 so that the period of energising the coil 25 is less than that of the coil 26. The number of turns in, and/or the cross-section of, the conductors of the two coils differ so that the electrical resistance of the circuit of the striking coil 25 is from about one half to about one sixth of that of the return coil 26, so as to reduce losses in the tool. The electric time-constant of each coil divided by its period of energisation is at least .5, and preferably from .7 to 1.0. The housing 21 is carried by bolts 23 serving to secure together the casing 15 and a casting 44 forming a handle; the casting 44 carries a clamp 47 for a flexible cable 46 and a finger piece 45 of a switch comprising contacts 42<1> on leaf springs 42. An interrupter is formed by fixed contacts 28' (not shown) and 29<1> and by contacts 28, 29 carried by springs 31, 32 connected to terminals 38. The springs 31, 32 carry cams 33 operated by a sliding insulated member 34 on a sleeve 35 sliding on the rod 17 and adapted to open either pair of contacts, or allow both to close when in its mid-position. The sleeve 35 moves between a shoulder 59 on rod 17 and a nut 18' on the end of the rod. Modified form of interrupters operate on the principle of a toggle, the contacts corresponding to contacts 28<1>, 29<1> being mounted on a common leaf spring which is bent upwards or downwards to open and close the circuits alternately by direct contact with the reciprocating rod 17. Current from a generator or battery 41 passes through a positive conductor 60, switch 42, 42<1>, coils 25, 26, leads 62, 63, terminals 38, contact springs 31, 32, and thus to contacts 28, 29. It then passes alternately through contacts 28<1>, 29', a bracket 30, tube 15 and casting 44, and negative conductor 61 back to the generator. Condensers 39, 40 are provided to prevent sparking at the contacts. These condensers may be electrolytic, the condenser 40 for coil 25 being substantially larger than the condenser 39 for coil 26. The reluctance of the magnetic circuits may be decreased by forming the upper face, or both the upper and lower faces, of the armature 11 and the opposed pole pieces 12, 13 of conical or stepped cylindrical form, as shown at 64, 65. In order to reduce eddy currents, the armature 11, and plates 12, 13, and the annular member 14 may be formed with radial slots, and these slots may be wide enough to provide ventilation of the interior of the tool by fresh air drawn in through holes 15<1> in the casing 15.