Design of Cams for Actuating Limit Switches
FLUID POWER - Design Data Sheet 8
(Some of the material on switch cams was adapted from
information published by R.B.Denison, Inc.)
Too often there is a lack of attention given to properly
actuating a limit switch. The designer may be under the mistaken
impression that the size, shape, and contour of the dog or cam
which actuates the switch is unimportant, provided it actuates at
the right time and has good repetitive accuracy. The facts seem to
indicate that the highest percentage of failures in an electrical
control system have in the past been due to limit switch failures,
and these failures have largely been caused by one or more poor
design practices described in this sheet:
Figure 1 - Cam Angle.
Leading Edge of Cam Should be Parallel to Switch Lever
Figure 2 - Cams with
Straight Slope on Leading Edge are Suitable for Moderately High
Figure 3 - For Very High Cam
Speeds, Leading Edge of Cam Should be Contoured.
1. Many failures have been due to the use, for
reasons of economy, of light-duty, non-enclosed, or unprotected
switches on applications where heavy duty industrial limit switches
should have been used.
2. Improper Cam Angle. Figure 1.
The leading edge of the actuating cam should be approximately
parallel to the switch arm. Not more than a 5-degree variation
should be allowed in either direction.
3. Excessive Rotational Movement. Cams should be
designed so they do not over-rotate switch levers. Excess movement
results in unnecessary strain on mechanical parts and springs in
4. Excessive Impact. Sloped or contoured surfaces
in Figures 2 and 3, should be
used on fast-traveling cams. High impact on the switch mechanism is
a major cause of premature switch failure. At slow cam speeds there
is no cause for concern, but at high speed, impact of cam on switch
mechanism may cause physical damage to mechanical parts of the
switch, but more important, it causes the contacts to bounce,
making and breaking the circuit more than once, and during a period
of high inrush current. This reduces contact life in proportion to
the severity of the bounce.
5. Cam Contour. For cam speeds of 50 feet per minute
or less, a fairly abrupt cam approach as in Figure 1 is usually
satisfactory. For higher speeds, the cam approach angle should be
more gradual as in Figure 2. While this straight
slope may give a slight impact to the switch roller, it is
satisfactory for speeds up to 200 feet per minute provided the
approach angle is very small. The switch should be mounted, at an
angle if necessary, so its arm can be adjusted to a position
parallel to the slope within ± 15°.
At cam speeds faster than 200 feet per minute,
the cam surface should be curved as in Figure 3,
to reduce impact on the switch mechanism at the moment of contact,
and to give a smooth acceleration to the switch arm. Length of
contour should be at least 3 inches, longer if possible. As in all
cam designs, the switch should be mounted so its lever can be
adjusted to be as nearly parallel to the cam surface as
At cam speeds over 400 feet per minute, it is
especially important to have as long a contoured cam surface as
space permits at least 6 inches or longer.
Figure 4 - The Trailing Edge
of the Cam should be Sloped to Prevent "Fly-back".
6. Overriding Cams. Figure 4. On
those applications where a cam must travel past a limit switch, the
switch arm should never be allowed to drop abruptly off the cam
surface. This "fly-back" will produce severe strains on mechanical
parts and springs in the switch, and will greatly shorten switch
life by causing multiple pulsing of the contacts.
The trailing edge of an overriding cam should be
sloped or contoured to prevent rapid "fly-back" of the lever. While
not as critical as slope of the leading edge, the angle between
trailing edge and switch lever should be no greater than 30°.
Remember, too, that the cam must pass back over the switch arm on
its return stroke unless a 1-way actuator is used.
7. Excessive Current. Handling an excessive
amount of current will reduce contact life. Inrush current to a
hydraulic valve solenoid coil may be 4 or 5 times its normal
holding current after the solenoid armature has seated. Rather than
handle excessive inrush current, the switch may energize the coil
of an industrial relay or contactor rated to handle the high
solenoid inrush current.
Notes on the Placement of Limit
Figure 5 - Sustained
Figure 5. Sustained Signal.
While in most applications a limit switch is placed to actuate very
close to the end of a cylinder stroke, there are cases where a
switching signal is needed at some intermediate point in the stroke
of a cylinder. If the switching signal then must be maintained
during the remainder of the stroke, an extended cam attached to a
moving member of the machine will keep the switch actuated.
Note: As a safety precaution, the long cam may be
used on any application where there is a possibility of the
cylinder accidentally overriding the switch. A short cam might get
behind the switch lever and damage it on the return.
Figure 6 - Momentary
Figure 6. Momentary Impulse. If a momentary
switching signal must be produced at some point in a cylinder
stroke, a limit switch with 1-way roller actuator may be used. The
"knee-action" of the switch arm causes the switch contacts to
actuate as the cam on the cylinder moves outward. On the return
stroke, the switch lever folds down without actuating the switch
contacts. If the switch is turned to face the other direction, the
switching impulse occurs on the return stroke of the cylinder.
The same 1-way action is produced with a
half-roller actuator which actuates the switch contacts in one
direction of motion, and rotates out of the way of the actuating
cam in the other direction of motion.
Figure 7 - Impulse Limit
Figure 7. Impulse Switch. The
impulse type limit switch provides a means of generating a
switching signal only during the last one-half inch of cylinder
stroke. It must be protected by a positive stop to limit cylinder
travel. When the cylinder reaches the positive stop, the switching
signal is cut off. On the start of the cylinder return stroke, the
switch plunger springs back out without generating another
switching signal. This switch is useful in circuitry where the
cylinder, at the end of its forward stroke, must "stand" on a limit
switch, and the maintained signal thus generated would be difficult
to override later in the cycle.
Figure 8 - Head-on
Figure 8. A plunger-type limit switch may be
used in circuits which require high sensitivity or great accuracy
and repeatability in switch actuation. The switch is mounted for
head-on actuation, but must be backed up with a positive stop to
prevent damage if the cylinder should accidentally try to override
the switch due to a control circuit failure.
Download a PDF of Fluid
Power Design Data Sheet 8 - Design of Cams for Actuating Limit
© 1988 by Womack Machine Supply Co. This company assumes no
liability for errors in data nor in safe and/or satisfactory
operation of equipment designed from this information.