Page 319 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 17 Electromechanical Mechanisms 281

Motor Piston
Cylinder
Rod

Two Stage Mount
Vee Belt drive
Controller
Pneumatic (A)
Traction Spool

Cylinder
Counter Weight

Extend Retract Check
Speed Speed Valves
Bypass
Cable Hydraulic (B) Loops
Figure 17-5 Dash Pot Shock Absorbers

Floor the piston and rod assembly to move unimpeded. At higher
Control Cable
Limits speeds the gap restricts the flow and, in turn, places a load on
the motion of the piston and rod.
Hydraulic dash pots operate in much the same manner as
their pneumatic counterparts, except the flow is controlled
through a bypass loop, as shown in the lower illustration (B).
The bypass loop can be set up with a pair of needle valves and
Passenger Car check valves, which allows the damping characteristics of
both the extend and the retract to be tuned independently.
Figure 17-6 shows a pneumatic dash pot used to dampen
Figure 17-4 Basic Traction Elevator the motion of a pendulum accelerometer. The dash pot will
limit sudden impulse loads, while allowing long duration
loads to be monitored.
Figure 17-7 shows a hydraulic dash pot used to limit the
speed at which a solenoid-activated knife switch throws.
someone who has never been in an elevator before. The floor
locations are detected with ordinary limit switches or opto
sets. The controller is generally mounted adjacent to the lift-
ing machinery and a control cable is routed to the car.

Dash Pots
Dash Pot
Dash pots are mechanical shock absorbers that are intended to
Accelerometer
smooth out the actions of a sensor or drive. Figure 17-5 shows
two common dash pots—a hydraulic unit and a pneumatic
unit.
Pneumatic units are generally used for low-load applica-
tions, such as damping the motion of a turn table tone arm or
filtering out high-frequency signals on a vibration sensor.
These units typically consist of a small cylinder with a loosely
fitting piston and rod, as shown in the upper illustration (A). Dead Weight
Air is allowed to leak between the gap formed around the out-
side of the piston and the inside diameter of the cylinder. At
low speeds the flow rate through this gap is sufficient to allow Figure 17-6 Accelerometer Equipped with a Dash Pot