Section 2.17 Car and Counterweight Safeties
The car of every elevator suspended as required by 2.20.1 shall be provided with one or more car safety devices of one of the types identified in 2.17.5. Safeties shall be attached to the car frame, and at least one safety shall be located within or below the car frame.
All car safeties shall be mounted on a single car frame and shall operate only on one pair of guide rails between which the frame is located.
Where duplex (two) safeties are provided, the lower safety device shall be capable of developing not less than one-half of the force required to stop the entire car with the rated load (see 2.16.8). Duplexed safety devices shall be arranged to function approximately simultaneously.
Type A or Type C safety devices (see 2.17.5) shall not be used in multiple (duplexed).
Type B safeties shall stop the car with its rated load from governor tripping speed within the range of the maximum and minimum stopping distances as determined by the formulas in 8.2.6. Table 2.17.3 and Figure 8.2.6 show the maximum and minimum stopping distances for various governor tripping speeds, when tested in conformance with Sections 8.10 and 8.11.
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Car safety devices (safeties) are identified and classified on the basis of performance characteristics after the safety begins to apply pressure on the guide rails. On this basis, there are three types of safeties.
Counterweight safeties for rated speeds of not over 0.75 m/s (150 ft/min) shall be permitted to be operated as a result of the breaking or slackening of the suspension ropes and shall be permitted to be of the inertia or other approved type without governors.
A switch operated by the safety mechanism is not required on counterweight safeties.
When overspeed occurs, with the hoisting rope intact, such safeties shall be actuated by the governor.
On the parting of the hoisting ropes (free fall), Type A governor-operated safeties shall apply without appreciable delay, and their application shall be independent of the speed action of the governor and of the location of the break in the hoisting ropes (inertia application), and shall be permitted to be accomplished by the use of a governor and governor rigging having a sufficiently high value of inertia to apply the safety on free fall independently of the speed action of the governor (see Section 8.10 for inertia-application test of car safety).
Buffers shall be located in line with and symmetrically between the guide rails.
In the normally retracted position of the safety, the distance between the rail-gripping faces of the safety parts shall be not less than the thickness of the guide rail plus 3.5 mm (0.14 in.), and the clearance on any side between the gripping face and the guide rail shall be not less than 1.5 mm (0.06 in.), as measured on the side of the rail toward which the car frame is pressed with sufficient force to take up all clearances in the guide-shoe assembly. Safety jaws, while in the retracted position, shall be restrained to prevent a reduction of this minimum clearance.
For all Type B safeties, the movement of the governor rope, relative to the car or the counterweight, required to operate the safety mechanism from its fully retracted position to a position where the safety jaws begin to exert pressure against the guide rails, shall not exceed the following values based on rated speed:
(a) for car safeties
(1) 1 m/s (200 ft/min) or less, 1070 mm (42 in.)
(2) 1.01 m/s (201 ft/min) to 1.9 m/s (375 ft/min), 915 mm (36 in.)
(3) over 1.9 m/s (375 ft/min), 756 mm (30 in.)
(b) for counterweight safeties, all speeds, 1070 mm (42 in.)
Drum-operated car and counterweight safeties, requiring continual unwinding of the safety drum rope to fully apply the safety, shall be designed so that not less than three turns of the safety rope will remain on the drum after the overspeed test of the safety has been made with the rated load in the car.