referring generally to the drawings and , in particular , to fig1 thereof , one example of an overall helicopter hoist assembly with which the present invention may be integrated is indicated as including a hoist boom 1 , a traction sheave assembly indicated generally at 2 and which will be further described in detail hereinafter as the new and improved combination of the present invention , a hoist cable 3 having an upper end 3a connected to a winch , indicated schematically at 4 , and a lower hoist cable end , indicated at 3b , to which may be attached the rescuee to be hoisted . for this purpose , a standard hook bumper and hook with keeper may be attached in the usual manner to the lower end 3b of the hoist cable 3 as shown . a boom head 5 is used to attach traction sheave assembly 2 to the boom 1 . the actuator for rotating the boom 1 between its stowed and operative , extended positions is indicated generally at 6 , and a vertical column which may mount the boom 1 to the helicopter is shown at 7 . the reference numeral 8 depicts an electrical connector for the cable used to supply power to the traction sheave motor and to provide a circuit to the alarm switch , which elements will be illustrated and described later . with specific reference to fig2 the new and improved traction sheave assembly 2 of the present invention is clearly illustrated as comprising a traction sheave drive motor 9 incorporating a reduction gear 10 both of which are mounted on and supported by housing 12 . the motor - attached housing 12 may , as specifically taught by the present invention , be bearing mounted to a traction sheave housing 13 , as by means of the bearing 14 , to thereby enable relative rotation to take place therebetween , about the axis of shaft 9a , when a malfunction occurs , as will be further explained hereinafter . said housing 13 encloses a traction sheave 15 that is fixed on shaft 9a and over which is fed the hoist cable 3 , the upper and lower cable portions of which being indicated respectively at 3a and 3b , as previously noted . a guide roller 16 ( note also fig3 ), may be utilized to limit the outward swing of the cable relative to sheave 15 . motor 9 , which is energized and runs continuously during operation of the hoist , continuously applies torque to shaft 9a and sheave 15 through a slipping clutch 11 . the clutch may be of any suitable design , such as a friction disc between spring loaded pressure plates . the direction of the torque is clockwise ( as seen in fig3 .). the motor 9 and its mounting 12 , which are free to rotate about the axis 19 of shaft 9a as already noted , are restrained against a resulting counterclockwise rotation in a manner to be explained later . the purpose of the continuously applied clockwise torque is to maintain tautness at all times , and particularly in the absence of a load , in that part of the steel cable between the winch 4 and sheave 15 . any slackness at the winch may cause miswinds on the drum of the winch , jamming of the winch , and possible breakage of the cable . maintaining the necessary cable tautness requires that the sheave have a firm , nonslipping , friction grip on the cable at all times . to insure this , the groove of the sheave may be coated with a friction material and pressure rollers 30 ( fig3 ) may be used to force the cable against the sheave . although torque is constantly applied to sheave 15 , the sheave should turn only when cable is being paid out or taken up by the winch , the direction of rotation being with the applied torque when cable is paid out and against the applied torque when cable is taken up . as is seen more clearly in fig3 and in accordance with the further novel teachings of the present invention , the rotatable , motor - attached housing 12 is uniquely equipped with a rocker arm 17 , which arm is rigidly affixed to , and extends outwardly in transverse relation from , the outer circumference of the said housing 12 . during operation of the present hoist assembly , a torque counterbalancing spring 18 normally retains the rocker arm 17 in a neutral position about midway of stops 23 and 24 , as seen in fig3 so long as normal traction is being applied to the cable by the sheave 15 and drive motor 9 . if , however , as a result of a malfunction , such as loss of power at motor 9 or loss of grip on the cable by sheave 15 , the traction being so applied to the cable should drop to zero or be reduced to an undesirably low value , the torque counterbalancing spring 18 , which normally exerts a force on rocker arm 17 that balances the traction being applied to the cable by the sheave 15 when no malfunction is occurring , would then become automatically operative , against the now reduced counterclockwise force on arm 17 , to move the arm 17 and the attached housing 12 about the pivot axis 19 in a clockwise direction , as viewed in fig3 to an engaged position with switch 20 . explaining more pecisely the above operation , under normal stable conditions with adequate traction being applied to the cable by sheave 15 and arm 17 stationary at a point intermediate stops 23 and 24 , the following relationship holds : ## equ1 ## where f = force exerted against spring 18 by arm 17 r = effective radius of sheave 15 r &# 39 ;= distance from sheave axis 19 to the point of attachment of spring 18 to arm 17 . since both r and r &# 39 ; are constants , equation ( 1 ) may be written as under these conditions , the force f is exactly balanced by the opposite force exerted by spring 18 . as shown by equation ( 2 ), anything that reduces t results in a proportionate reduction in f and a movement of arm 17 upward ( clockwise ) under the now superior force of spring 18 until the spring force has been reduced to equality with the lower value of f , at which point the system is again in balance . further reductions in t cause arm 17 to move further upward until , when t has fallen below a design minimum , switch 20 is actuated . a reduction in t may occur as the result of loss of power at motor 9 or slippage of sheave 15 on the cable . loss of power of course reduces t and f to zero causing arm 17 to be immediately rotated upward by spring 18 closing switch 20 . if the degree of slippage at sheave 15 is sufficient to reduce t below the above mentioned design minimum , arm 17 is likewise rotated upward by the spring to close switch 20 . closure of switch 20 energizes , through the circuit 22 , an alarm light 21 located in the helicopter at a position readily visible to the hoist operator to enable the latter to take any appropriate corrective action . both electrical circuit 22 and alarm light 21 are shown in schematic form only , since the specific details thereof are unimportant to the novel traction sheave assembly of the present invention . of course , by specifically bearing mounting the motor - attached housing 12 to the sheave housing 13 to thereby enable relative rotational movement therebetween in the event of a malfunction , the previously - described rotation of the rocker arm 17 to engage and close the malfunction - operative switch 20 is thus specifically assured by the unique traction sheave assembly of the instant invention . in this connection , the pair of spaced - apart relatively elongated , rod or bar members , at 23 and 24 , are used to limit the clockwise and / or counterclockwise rotation of the yoke arm 17 , as is seen more particularly in fig3 . thus , a new and improved helicopter rescue hoist - traction sheave assembly has been developed whereby the traction sheave drive motor may be attached to a housing that is rotatively adjusted relative to the traction pulley and is placed in a unique and yet relatively simplified combined torque compensating and malfunction - warning light system comprised merely of a yoke arm fixed to the motor - attached housing a malfunction - operative switch located near the yoke arm , and a torque compensating spring attached to , and moving the yoke arm between a neutral position when normal torque is being applied to the sheave and an engaged position with the switch to operate the warning light when the traction being applied to the cable by the sheave is significantly reduced by a malfunction in the traction sheave assembly . of course , in place of the warning light , or as a supplement thereto , a bell or other clearly audible signal might be used , as desired .