CONTROL AND TRANSFER SWITCH
TYPE SBM
DESCRIPTION
INTRODUCTION
The Type SBM switches are cam operated
devices having two mechanically and electrically
separate contacts per stage. The switch is totally
enclosed, having no cover. The contact terminals
are brought out to the corners, allowing screw
connections to be made over a large angle. The
switch is so constructed so as to allow the addition
of
1
or
2
extra stages to the switch with a minimum
of effort.
APPLICATION
The Type SBM switches are intended primarily
for the control of electrically operated devices such
as circuit breakers, small motors and magnetic
switches, and for the transfer of meters, instruments
and relays.
OPERATION
The Type SBM switches are rotary cam operated
switches. Rotation of the shaft causes contacts to
open or close, depending upon the shape and setting
of the cams. Each stage consists of two mechanically
and electrically separate contacts.
This is ac
complished by means of two cams and two cam
followers, assembled with moving contacts.
Each cam is constructed so as to have two
operating surfaces. These surfaces operate on the
cam follower. The cam follower has two tips which
are located in offset horizontal planes lining up
with the two cam operating surfaces. Thus, as the
cam is rotated, one surface operates against the
closing cam follower tip, while the opening cam
follower tip is relieved. Both cam follower tips
are always in contact with the cam surfaces. This
allows for a positive closing and opening action not
dependent upon springs.
Each cam follower has a spring loaded moving
•
contact assembled to it. The compression spring
acts to give adequate contact pressure when a con
tact is closed. The moving contact is held to the
cam follower by a pin passing through a hole in the
cam follower and angled slot in the moving contact.
As the contacts close, the moving contact slides
along this slot while compressing the spring thus
causing relative motion or "wipe" between moving
and stationary contacts.
Some applications, particularly of momentary
contact switches, which have a torsion spring
to
return the switch to a central-neutral position,
require a contact action which lags behind the
switch motion (lost motion or slip contacts). Such
contacts use cams with a special loose fit on the
shaft. When the shaft has turned far enough to close
or open these contacts, it can be rotated
45
degrees
in the reverse direction without moving the cams,
but beyond this point, the cam moves with the shaft
and the contacts either open or close as the case
may be.
Momentary contact switches have a torsion
spring that returns the switch to a central or neutral
position when the handle is released after operation
to a side position or positions. This torsion spring
is designed for maximum of
90
degrees operation
to each side of the central position. The torsion
spring may have one end cut off or tied back in such
a manner as to be effective on one side of the
central position only. That is, the switch may have
momentary contact to one side of the central position
and maintaining contacts to the other side.
In
some momentary contact (spring return)
a locking device is provided by which
the
may be held against the action of the
torsion spring by pulling out the handle when the
switch is turned to one of the side positions.
POSITIONING
A detent wheel, mounted on the square shaft and
acted upon by a spring loaded roller arm, gives
positive positioning action to the switch.
REMOVABLE HANDLES
The removable handle option may be obtained
with up to
3
keyways in the escutcheon. The keyway
locations are at the discretion of the customer. The
handle is removable in one position.
Any style
handle can be used for this operation.
ADD-A-STAGE
A shaft extension is provided to enable an
additional stage to be coupled to the existing switch
in the event that more contacts are required when
the switch is in the hands of the customer.
Theae ill$#ructiona do not purport to cover all detaila or variation• in equipment nor to provide for every pouible
contingency to be met in connection with innallation, operation or maintenance. Should further information be deaired
or ahould particular problem• arise which are not covered aufficiently for the purchaser' a purpo1e1, the maHer should
be referred to the General Electric Company.
3
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