1.5 LIMIT SWITCH ASSEMBLY
The standard limit switch assembly comprises 4 -'open' position and 4 -'close' position micro
switches, each bank of switches being simultaneously operated by a lever actuated by a cam on
the pinion shaft of a hunting tooth gear train. An intermediate position bank is fitted when
required providing 2 micro-switches operated simultaneously by a switch lever actuated by a
cam on the pinion shaft of an additional hunting tooth train.
The hunting tooth gear trains are so arranged that each bank is independently adjustable to
provide any limit switch setting from zero to 1,000 actuator turns, i.e. valve fully closed to fully
opened, with the 'intermediate' bank set at any position between the open and close limits, the
accuracy of the settings being 0.1 actuator turns.
Each hunting tooth gear train consists of a pinion shaft with a cam pinned to the non-driving
end, 4 pinions on the shaft within the housing and a drive pinion in mesh with the compound
gear train coupled to the limit switch drive spindle. The pinions within the housing are in mesh
with 4 spur wheels each of which embodies a ball drive to a hunting tooth gear mounted on
the spur wheel boss. The bore of the gear is formed to permit the ball drive to be in any of 10
positions corresponding to the numerals on the gear flange, and the spur wheels are mounted
on a hunting tooth spindle. The spindle is provided with a series of grooves, circlips to limit the
engagement and withdrawal, and a knurled end to facilitate withdrawal. A spring loaded ball
axially locates the spindle in the engaged or disengaged positions.
In the normal driving or set position the hunting tooth spindle is engaged and in this position
the ball drive of the hunting tooth gear and spur wheel is locked in position by the spindle.
Resetting of a switch bank is effected by pulling out the hunting tooth spindle to allow the
spring loaded ball in the housing to register the disengaged position and also to bring the
grooves in the spindle in line with the drive ball of each hunting tooth wheel. With the spindle
in the disengaged position the hunting tooth wheels can be turned to any combination of
digits on the flanges as each driving ball is allowed to move clear of the hunting tooth wheel
bore profile into the spindle groove. Reading from left to right or non-driving end to driving
end the flange numbers are in multiples of hundred, ten, unit and tenth turns respectively. It is
important that the digits of the required combination are in a straight line and in line with the
centres of the pinion shaft and hunting tooth spindle, before pressing home the hunting tooth
spindle.
The engaged position of the spindle is attained without the use of excessive force.
If the spindle does not readily snap home the four digits are not lined up correctly.
It is possible to re-set a bank with the tenth counter flange only in any radial position, but it is
essential that when re-set the correct relationship of this hunting tooth wheel and spur wheel is
maintained in respect of the ball drive. This means that the radial position of the tenth digits
must be the same before and after setting irrespective of the actual numerals. As the numerals
of each flange must be adjacent to alternate tooth spaces of the spur wheels it is possible to
inadvertently position a tenth digit opposite the incorrect tooth space which will prevent
engagement of the hunting tooth spindle. If this occurs the tenth digit wheel should be rotated
one tooth space thereby allowing engagement of the spindle.
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