Patent GB-1351546-A (GB)
Title: Safety devices for centrifugal clutch mechanisms
IPC: F02B75/02
Filed: 1972-02-21
Abstract: 1351546 Machinery safety devices; friction and positive clutches; clutch-brake combinations R A G JOHANSSON 21 Feb 1972 [24 Feb 1971] 7915/72 Headings F2J F2C F2L and F2U In an injury preventing safety device in a chain saw in which the saw cutting chain 7 is driven from a motor (not shown) via a chain driving sprocket 9 of a centrifugally engaged segmented shoe type friction clutch 59, 61 freely mounted on the motor shaft 49 and connectible thereto by a centrifugally disengagable pivotally mounted (or radially slidable, not shown), positive clutch pawl 77 which is carried by a disc 69 fast on the motor shaft 49 and inwardly sprung biased at 85 into driving engagement at 79, 81 with an extension 83 of the friction clutch hub 59 and normally locked in this position against centrifugal disengagment by a radially projecting lock pin 89 pivotally mounted on the disc 69, a safety device is effective in the event of an operator's hand slipping off the handles or by sudden kick-back of the saw in use to electrically, mechanically, hydraulically or pneumatically downwardly move a release arm 11 to engage and unlock the pin 89 to release the pawl 77 and disengage the friction clutch 59, 61 and additionally engage shoulders 111 or 113 of a ring 101 to apply a brake 107, 109 to rapidly stop the friction clutch 59, 61 and hence the saw cutting chain 7 to prevent injury to the operator. The pawl 77 is re-engagable with the clutch hub 59 via spring 85 and relocked in this engaged position by the centrifugally operated, i.e. engaged pin 89 when the motor shaft speed falls below the engaging speed of the centrifugal friction clutch. As shown, an armature 19 of the safety device is energized via a circuit 21-29 to release the pawl 77 by releasing safety levers 31, &c. in the handles (i.e. dead-man's type handles) of the saw to disengage the friction clutch from the motor shaft for starting purposes, &c., and by a pivotally mounted safety bar 35 arranged in front of the front handle 5 whereby if the operator's hand slips off this front handle, i.e. in the proximity of the saw cutting chain 7, it strikes and rocks the bar and energizes the armature 19 through contacts 39 and circuit 41 to operate the safety release to disengage the motor drive to the saw chain which is rapidly stopped by the brake 101, 109. A saw kick-back sensitive transmitter 43 is additionally incorporated on the chain saw guide bar 7 and responds to sudden kick-back action of the saw to operate, via a circuit 45, the clutch release safety device to stop the saw chain as before. To reduce shock loads in the clutch mechanisms during rapid safety stops of the saw chain, and reconnecting the motor drive thereto, the release arm 11 is resiliently mounted at 117 on its pivot 13 and, additionally, a spring ring friction slip coupling 121 (Fig. 5) is arranged between the driving disc 69 and the clutch hub 59 to maintain a slipping drive to the clutch hub 59 during disengagment of the positive clutch pawl 77. In Fig. 10 (not shown) the armature is replaced by a push rod (143) linked to the safety handles, &c. and operating a band brake (135) having a projecting strip (145) serving in the manner of the release arm 11  (Fig. 1) during a safety stop &c. to initially unlock the pin 89 prior to engagement of the band brake.