Patent GB-738909-A (GB)
Title: Improvements in or relating to electromagnetic clutches
IPC: F16D27/08
Filed: 1953-07-20
Abstract: 738,909. Friction clutches. SOC. ANON. FRANCAISE DU FERODO. July 20, 1953 [July 23, 1952 ; Nov. 18, 1952 ; March 5, 1953], No. 20129/53. Class 80 (2). An axially movable armature 27 which, through an attached presser plate 49, clamps a driven disc 52 of an electromagnetic clutch against a back-plate 47 secured to a driving flywheel-yoke 21, is anchored to the said yoke by tangential spring-steel tension strips 28, laminated or single, each attached at its opposite ends to the yoke 21 and armature 27 by pivot-screws 31, 29 at least one of which is magnetically insulated from its support. Also provided are induced circulatory air cooling; segmental friction facings ; permanent friction drag to take up circumferential play in the transmission of a motor-vehicle driven through the clutch ; and compensation for reduction in air-gap with facing wear by (a) yielding means 44, Fig. 4, for opposing the consequent fluxpressure increase, (b) shim adjustment 109, Fig. 7, (c) insertion at will ot electrical resistance in the winding circuit (not shown), and (d) a magnetic shunt 133, Fig. 17. The magnetic circuit is arranged to operate at 50 to 100 per cent saturation, and preferably 60 to 80 per cent thereof, corresponding to 9,000 to 12,000 gauss for high permeability steel. In Figs. 1-6 the driven disc 52, with facings 53, is spring- mounted at 55 on a hub 56 splined on an output shaft 11, which has a permanent friction drag on an input shaft 10 by a coned washer 18 loaded by a spring 17 which reacts through a collar 16 on a flange of a self-lubricating pilotbush 13. The driven disc 52 is clamped between plates 47, 49 of frictional cast-iron attached by screws respectively to a driving magnet-yoke 21 and armature 27, both of high-permeability iron. The yoke 21 has a magnet winding 24 and radially spaced poles A, B spaced by an axial air gap from the armature 27, which latter is guided for rotation with the yoke 21 by clearance-pins 35, the flanged heads of which define under spring pressure 39 a maximum air-gap. Minimum air-gap is defined by the friction facings on the driven disc 52, and the axial engaging pressure of the magnet winding 24 is maintained constant as the gap reduces due to wear by coned spring washers 44, Fig. 4, under headed pins 41 on the yoke 21, which pins, with new friction facings, contact the armature 27 under light spring pressure 45, with the coned washers 44 just ineffective, whilst, as the facings wear, the coned washers 44 provide an increasingly effective opposing force which compensates for the increased magnetic pressure due to reduction in air-gap. The tangential laminated spring-steel tension-strips 28 which drivingly connect the armature 27 and magnet-yoke 21, are attached by brass pivot-screws and washers 29, 31, Fig. 3, to the armature and yoke, in the latter case with or without an interposed brass ring 33. One end of the magnet winding 24 is earthed either directly to the yoke, as shown at 62, or through a brush, whilst the other end is connected to a current source 78 through a brush 75, insulated slip-ring 70 and radial lead 69, Fig. 6, carried by the armature 27, conducting axial spring 66, and lead 63 carried by the yoke. The winding has a time-constant of 0.2 to 1.2 second. The frictionally active part of the driven disc 52 is divided by slote 54, Fig. 5, into sectors, each provided with facings 53 over its whole area as shown, or only a part thereof (Figs. 14 and 15, not shown), the radial sides of the facings in the latter case being inclined or curved to promote cooling air flow over the bare metal parts of the sectors. Fan action of the sectors induces cooling air flow in the direction of the arrows f and radially between the sectors. The facings, a preferred composition of which is stated below, are attached by riveting, bonding or sintering, and the metal sectors carrying them may be integral with the main disc 52, or attached thereto by bolts or rivets. In Fig. 7, the driven disc 52 is clamped against a back-plate 47, bolted and dowelled to the driving magnet-yoke 21, by a presser-plate 49 attached by screws 108 to the outwardlytapering armature 27 arranged between the back-plate 47 and yoke 21 to leave a heatinsulating gap 110. Shims 109 on the screws 108 provide adjustment of the air-gap between the magnet-poles A, B and armature 27. Permanent friction drag is provided by tangential string strips 104, each secured at one end to the driven disc 52 and having at its free end a friction pad 106, which biases axially against the back-plate 47. In Figs. 19 and 20 (not shown) the periphery of -the armature 27 has spaced radial projections having only a small radial clearance from the surrounding backplate 47, to maintain concentricity in the event of fracture of the driving spring-strips such as 28, Fig. 2, but not appearing in Fig. 7. In Fig. 17 the armature 27 has an axially projecting flange 133 which, as the air-gap is reduced by wear of the friction facings, overlaps the magnetyoke 21 to an increasing extent, thus increasingly shunting the magnetic flux to prevent increase in engaging pressure. The percentage constituents of a sintered friction material for the facings 53 is : copper 63-80 ; lead 10-20 ; tin 5-9 ; graphite 6-1-2, powdered to pass 250 meshes per inch. The disc 52 is first electro-plated with copper to 0.02 to 0.03 mm. thickness, the material being then applied thereto and shaped by a preliminary operation at 1,000 to 2,000 kg./sq. cm. pressure, and then heated at 600-650‹ C. for 10 minutes in a reducing atmosphere, followed by a pressure of 2,000 kg./sq. cm. at 800‹ C. for 20 minutes, and a final compression at 2,000 kg./sq. cm.