Building Design and Construction Handbook - Merritt - Ventech!

VERTICAL CIRCULATION 16.31
car-speed governor. The safety should be actuated when the car travels at more
than 15% above its rated speed.
Car safeties are generally mounted on the safety plank, or bottom member of
the car frame. When tripped, springs on the safeties push shoes against the guide
rails hard enough to make the car slide to a stop. (When a hoistway is located
above an accessible space, safeties, such as those used for cars, should also be
provided on the counterweight frame.) The safeties are typically released by upward
motion of the car.
The governor may be conveniently located in the machine space, where the
device will not be struck by the car or the counterweight if either should overtravel.
The governor may measure car speed from the rotation of a sheave around which
is wound a wire rope connected to the car and held under tension. When the car
goes too fast, the governor trips jaws that grip a wire rope connected through
linkages to a safety and release a spring to actuate the safety. Also, electrical
switches on the governor and the safety are opened to remove power from the
driving machine and apply a friction brake to the drive shaft.
16.9.6 Counterweights
Power requirements of the driving machine for moving the car are reduced by
hanging a counterweight on the hoisting ropes. Use of a counterweight also is
advantageous for maintaining traction between the hoisting ropes and the driving
sheave. The weight of the counterweight usually is made equal to the weight of
the unloaded car and the ropes plus about 40% of the rated load capacity of the
car (Fig. 16.14).
A counterweight usually is made up of cut steel plates set in a steel frame.
Moving up as the car moves down and down when the car moves up, the counterweight
is kept in a fixed vertical path by upper and lower guide rollers that are
attached to its frame and roll along a pair of guide rails.
FIGURE 16.14 Types of roping for electric traction elevators. Rope tension and loads imposed
on sheaves and supports depend on the type of roping, car weight W, and car capacity C.