POSITION
AND SAFETY SWITCHES Features
and Usage of electromechanical limit switches
Introduction
For automation, one is often faced with the need to deal with
moving mechanisms, to control the correct sequence of
operations and to detect the maintained or attained position,
in every instance, of various parts in motion or to simply
stop them. All this is very obvious for industrial machinery
where in addition to the previously specified operations, it
is essential to monitor mobile protections and warrant total
safety to machinery as well as personnel who operate them.
Position limit switches can be used in all of these cases,
working together with moving parts of machines and signalling
when well defined positions are reached. In this relation, the
control of the grinding machine saddle, the correct position
of packages on a conveyor belt, the stopping of a platform or
fork lift at the end of travel or, again, the disconnection of
supply when a mobile protection of a press is opened can cross
one's mind.
An
assortment of position limit switches exists in trade and each
can differ per operational or design features; one typically
finds electromechanical, electronic, inductive, magnetic or
optical position switches. However, not all of these are
suitable for every application since some may not correctly
operate in particular circumstances, for instance, in dirty or
dusty ambient, common to the industrial field. In other cases,
like safety applications, there are standards like EN 60204-1,
which tend to mainly take into account the electromechanical
types only since indeed it does assure the disconnection of
supply by means of physically opening an electrical contact.
Within the family of electromechanical position switches,
diverse types can be found as well, basically related to the
evolution of standards over the years, which have laid down
rules for dimensional standardisation. It is many years now
that all manufacturers and users of limit switches prefer
types complying with EN 50047 standards, which dictate the
fundamental dimensions of the device.
The EN 50047
define the characteristics of compact limit switches,
generally used in modern industrial machinery. Before this,
the EN 50041 standards had been considered and, although still
valid, prescribe larger dimensions and thereby formulate
bigger products.
Other even more compact position switches are prevalent in
trade, typically called microswitches, which however cannot
guarantee direct opening operation of the NC contacts.
LOVATO ELECTRIC PRODUCT PACKAGE
The preference of LOVATO Electric has resulted in developing
electromechanical limit switches for the reasons stated above.
These can well suit each situation and can be extensively used
thanks to the three main features: versatility and array of
operating heads available, the part in contact with moving
mechanisms, usually called the head or operator of the limit
switch, IP65 protection of the body against the external
ambient and the direct opening operation principle of the NC
contacts, specifically requested for safety applications.
Concentrating on the EN 50047 typology, the package offered by
LOVATO Electric has been renewed with the introduction the KB
series, with plastic body, and KM, with metal body.
The KB and KM series are electromechanical limit switches of
unique design and operational innovations, implementing new
modularity standards up to now never considered for this type
of equipment and providing major capability of use and
operation flexibility. Many different configurations are
available, making them suitable for position detection or for
safety application, as well.
Limit switch for
position detection
As mentioned, one of the major usages of electromechanical
limit switches is detecting the position of moving mechanisms.
The LOVATO Electric KB and KM series include an extensive
range of operators and electrical contacts, able to best work
together with the mobile parts. Possible movements of a mobile
mechanism are, in fact, wide-ranging so a limit switch must be
capable of detecting linear movements either in vertical or
horizontal direction, offset or along the same axis of the
limit switch body. The movement can also be rotary, like a cam
of a rotating shaft, which then operates the limit switch.
This same movement can be very ample or limited and all this
affects the choice for the most appropriate operator.
An additional
discriminating factor can be the material that the mobile
mechanism is made of, so the choice of the operator must
thereby fall on metal or plastic parts of the operator roller
or lever. A specific but emblematic case is the operator
undergoing continuous fretting, i.e. friction, thus a lever
with ceramic rod would be the best choice since this material
warrants the required endurance in time.
To meet all the application requirements, the new LOVATO
Electric range includes rod plunger, roller plunger, roller
lever with centre or side push operation, adjustable roller
lever, adjustable lever plunger, ceramic or adjustable rod
lever along with wobble sticks. All the operating heads of the
KB and KM series are made of metal, to guarantee the required
sturdiness; it is in actual fact the part that picks up the
external movements and is therefore exposed to continuous
solicitations and wear.
The KB and KM series also offer a wide choice of electrical
contacts, with snap or slow break action, with 1NO+1NC, 2NC,
2NO, 1NO+2NC, 2NO+1NC, 3NC configurations.
In
all cases, the NC contacts operate according to the direct
opening operation principle.
Typical features customers focus on when they find themselves
using this device are the configuration simplicity and wiring
speed. Configuration: the need to install the limit switch in
the most diverse situations; the switch operator may need to
be repositioned so it must be rotated on its axis and/or the
initial lever or rod position may need to be adjusted. Wiring:
the space is reduced or awkward to connect electric cables.
LOVATO Electric responds to these particular needs by
introducing new concepts.
Operators totally interchangeable and "self-contained":
The head can be quickly removed from the body and no internal
component is loose and cannot be lost, obtaining secure and
safe handling of the switch at all times.
A
convenient bayonet insert fixes the operator to the body: The
head can be easily repositioned with no need for particular
tools; traditional screws for operator fixing have been
eliminated providing timesaving installation.
Heads can be repositioned at 45° angles: In the past, only
90° angles were available. As a result, there is major
operation flexibility.
The body cover has a captive fixing screw and is hinged on the
bottom for easy removal.
Contact blocks can also be removed from the body to simplify
cabling and connections.
The new contact blocks are supplied with
terminal screws open and have IP20 protection against
accidental touching of terminals.
Cable entry has been designed to facilitate access and to
provide major ease during installation.
NC contacts have direct opening operation hence the switches
can also be used in safety applications, if correctly
installed.
Limit
switches for safety application Although it has just been
mentioned, the models, usually referred to position switches,
are also suitable for safety applications. At the same time,
specific types have been designed for safety purposes in
general.
The types consist of key-operated, hinged or slotted lever
version. Essentially, they are used for the control of mobile
protections, sliding or hinged doors, which protect the user
and prevent access to specific areas during machinery
operation.
The key-operated type includes a shaped key that causes a cam
to operate electrical contacts when withdrawn from the switch
head. It is a method quite suitable in cases where mobile
protections have either rotary or linear movement like hinged
or sliding doors.
The
slotted and hinged lever types are commonly used for hinged
doors and must be installed next to the rotating hinge to
detect door movement. LOVATO Electric sought to differentiate
these models from the position switches and therefore adopted
a red colour for body cover for easy identification on board
machines.
These models implement the same concepts as the others to ease
installation, that is the removal of the contact blocks for
cabling and the bayonet insert of the head fixing; the only
exception being the key-operated types, which maintain
operator fixing screws. The possibility to have configurations
up to 3NC contacts benefits and provides for additional safety
signals just in one switch.
