Download Other types of electromagnetic clutches

ELECTROMAGNETIC CLUTCHES & BRAKE
Electromagnetic clutches operate electrically, but transmit torque mechanically. This is why they used to be
referred to as electro-mechanical clutches. Over the years, EM became known as electromagnetic versus
electro mechanical, referring more about their actuation method versus physical operation. Since the clutches
started becoming popular over 60 years ago, the variety of applications and clutch designs has increased
dramatically, but the basic operation remains the same.Single-face clutches make up approximately 90% of all
electromagnetic clutch sales. This article mainly deals with these types of clutches. Alternative clutch designs
are mentioned at the end of this article.
Construction
Electromagnetic clutch
Horseshoe magnet red silver iron
Double Flux clutch
Triple flux clutch
Ogura Industrial Typical 2 pole clutch
Operation of a clutch
Triple flux rotor with banana slots and bridges
A horseshoe magnet (A-1) has a north and south pole. If a piece of carbon steel contacts both poles, a
magnetic circuit is created. In an electromagnetic clutch, the north and south pole is created by a coil shell and
a wound coil. In a clutch, (B1) when power is applied, a magnetic field is created in the coil (A2 blue). This
field (flux) overcomes an air gap between the clutch rotor (A2 yellow) and the armature (A2 red). This
magnetic attraction, pulls the armature in contact with the rotor face. The frictional contact, which is being
controlled by the strength of the magnetic field, is what causes the rotational motion to start. The torque
comes from the magnetic attraction, of the coil and the friction between the steel of the armature and the steel
of the clutch rotor. For many industrial clutches, friction material is used between the poles. The material is
mainly used to help decrease the wear rate, but different types of material can also be used to change the
coefficient of friction (torque for special applications). For example, if the clutch is required to have an
extended time to speed or slip time, a low coefficient friction material can be used and if a clutch is required
to have a slightly higher torque (mostly for low rpm applications), a high coefficient friction material can be
used.
In a clutch, the electromagnetic lines of flux have to pass into the rotor, and in turn, attract and pull the
armature in contact with it to complete clutch engagement. Most industrial clutches use what is called a single
flux, two pole design (A-2). Mobile clutches of other specialty electromagnetic clutches can use a double or
triple flux rotor (A-4). The double or trip flux refers to the number of north/south flux paths (A-6), in the rotor
and armature. These slots (banana slots) (A-7) create an air gap which causes the flux path to take the path of
least resistance when the faces are engaged. This means that, if the armature is designed properly and has
similar banana slots, what occurs is a leaping of the flux path, which goes north south, north south (A-6). By
having more points of contact, the torque can be greatly increased. In theory, if there were 2 sets of poles at
the same diameter, the torque would double in a clutch. Obviously, that is not possible to do, so the points of
contact have to be at a smaller inner diameter. Also, there are magnetic flux losses because of the bridges
between the banana slots. But by using a double flux design, a 30%-50% increase in torque, can be achieved,
and by using a triple flux design, a 40%-90% in torque can be achieved. This is important in applications
where size and weight are critical, such as automotive requirements.
The coil shell is made with carbon steel that has a combination of good strength and good magnetic
properties. Copper (sometimes aluminum) magnet wire, is used to create the coil, which is held in shell either
by a bobbin or by some type of epoxy/adhesive.
To help increase life in applications, friction material is used between the poles on the face of the rotor. This
friction material is flush with the steel on the rotor, since if the friction material was not flush, good magnetic
traction could not occur between the faces. Some people look at electromagnetic clutches and mistakenly
assume that, since the friction material is flush with the steel, that the clutch has already worn down, but this
is not the case. Clutches used in most mobile applications, (automotive, agriculture, construction equipment)
do not use friction material. Their cycle requirements tend to be lower than industrial clutches, and their cost
is more sensitive. Also, many mobile clutches are exposed to outside elements, so by not having friction
material, it eliminates the possibility of swelling (reduced torque), that can happen when friction material
absorbs moisture.
Basic operation
The clutch has four main parts: field, rotor, armature, and hub (output) (B1). When voltage is applied the
stationary magnetic field generates the lines of flux that pass into the rotor. (The rotor is normally connected
to the part that is always moving in the machine.) The flux (magnetic attraction) pulls the armature in contact
with the rotor (the armature is connected to the component that requires the acceleration), as the armature and
the output start to accelerate. Slipping between the rotor face and the armature face continues until the input
and output speed is the same (100% lockup). The actual time for this is quite short, between 1/200th of a
second and 1 second.Disengagement is very simple. Once the field starts to degrade, flux falls rapidly and the
armature separates. One or more springs hold the armature away from the rotor at a predetermined air gap.
Voltage/current - and the magnetic field
Right hand thumb rule:If a piece of copper wire was wound, around the nail and then connected to a battery, it
would create an electro magnet. The magnetic field that is generated in the wire, from the current, is known as
the “right hand thumb rule”. (V-1) The strength of the magnetic field can be changed by changing both wire
size and the amount of wire (turns). EM clutches are similar; they use a copper wire coil (sometimes
aluminum) to create a magnetic field.
The fields of EM clutch can be made to operate at almost any DC voltage, and the torque produced by the
clutch or brake will be the same, as long as the correct operating voltage and current is used with the correct
clutch. If a 90 V clutch, a 48 V clutch and a 24 V clutch, all being powered with their respective voltages and
current, all would produce the same amount of torque. However, if a 90 V clutch had 48 V applied to it, this
would get about half of the correct torque output of that clutch. This is because voltage/current is almost linear
to torque in DC electromagnetic clutches.
A constant power supply is ideal if accurate or maximum torque is requiried from a clutch. If a non regulated
power supply is used, the magnetic flux will degrade, as the resistance of the coil goes up. Basically, the
hotter the coil gets the lower the torque will be, by about an average of 8% for every 20°C. If the temperature
is fairly constant, but there may not be enough service factor in your design for minor temperature fluctuation.
Over-sizing, the clutch would compensate for minor flux. This will allow the use a rectified power supply
which is far less expensive than a constant current supply.
Based on V = I × R, as resistance increases available current falls. An increase in resistance, often results from
rising temperature as the coil heats up, according to: Rf = Ri × [1 + αCu × (Tf - Ti)] Where Rf = final
resistance, Ri = initial resistance, αCu = copper wire’s temperature coefficient of resistance, 0.0039 °C-1, Tf =
final temperature, and Ti = initial temperature.
Torque
Burnishing can affect initial torque of a clutch but there are also factors that affect the torque performance of a
clutch in an application. The main one is voltage/current. In the voltage/current section, it was shown why a
constant current supply is important to get full torque out of a clutch.
When considering torque, is dynamic or static torque more important? For example, if a machine is running at
a relatively low rpm (5 – 50 depending upon size) then dynamic torque is not a consideration since the static
torque rating of the clutch will come closest to where the application is running. However, if a machine is
running at 3,000rpm and the same full torque is required the result will not be the same because of the
difference between static and dynamic torques. Almost all manufacturers put the static rated torque for their
clutches in their catalog. If a specific response time is needed, the dynamic torque rating for a particular clutch
at a given speed is required. In many cases, this can be significantly lower. Sometimes it can be less than ½ of
the static torque rating. Most manufacturers publish torque curves showing the relationship between dynamic
and static torque for a given series of clutch. (T-1)
Over-excitation
Over-excitation is used to achieve a faster response time. It’s when a coil momentarily receives a higher
voltage then its nominal rating. To be effective the over excitation voltage must be significantly, but not to the
point of diminishing returns, higher than the normal coil voltage. Three times the voltage typically gives
around ⅓ faster response. Fifteen times the normal coil voltage will produce a 3 times faster response time.
For example, a clutch coil that was rated for 6 V would need to put in 90 V to achieve the 3 times factor.With
over-excitation the in-rush voltage is momentary. Although it would depend upon the size of the coil the
actual time is usually only a few milliseconds. The theory is, for the coil to generate as much of a magnetic
field as quickly as possible to attract the armature and start the process of acceleration or deceleration. Once
the over excitation is no longer required the power supply to the clutch or brake would return to its normal
operating voltage. This process can be repeated a number of times as long as the high voltage does not stay in
the coil long enough to cause the coil wire to overheat.
Clutch wear
It is very rare that a coil would just stop working in an electromagnetic clutch. Typically, if a coil fails it is
usually due to heat which has caused the insulation of the coil wire to break down. The heat can be caused by
high ambient temperature, high cycle rates, slipping or applying too high of a voltage. Bushings can be used
in some clutches that have low speed, low side loads or low operating hours. At higher loads and speeds,
bearing mounted field/rotors and hubs are a better option. Like the coils, unless bearings are stressed beyond
their physical limitations or become contaminated, they tend to have a long life and they are usually the
second item to wear out.The main wear in electromagnetic clutches occurs on the faces of the mating surfaces.
Every time a clutch is engaged during rotation a certain amount of energy is transferred as heat. The transfer
which occurs during rotation wears both the armature and the opposing contact surface. Based upon the size
of the clutch or brake, the speed and the inertia, wear rates will differ. For example a machine that was
running at 500 rpm with a clutch and is now sped up to 1000 rpm would have its wear rate significantly
increased because the amount of energy required to start the same amount of inertia is a lot higher at the
higher speed. With a fixed armature design a clutch will eventually simply cease to engage. This is because
the air gap will eventually become too large for the magnetic field to overcome. Zero gap or auto wear
armatures can wear to the point of less than one half of its original thickness, which will eventually cause
missed engagements.Designers can estimate life from the energy transferred each time the brake or clutch
engages. Ee = [m × v2 × τd] / [182 × (τd + τl)] Where Ee = energy per engagement, m = inertia, v = speed, τd
= dynamic torque, and τl = load torque. Knowing the energy per engagement lets the designer calculate the
number of engagement cycles the clutch or brake will last: L = V / (Ee × w) Where L = unit life in number of
cycles, V = total engagement area, and w = wear rate.
Other types of electromagnetic clutches
Multiple Disk Clutches
Introduction - Multiple Disk clutches are used to deliver extremely high torque in a relatively small space.
These clutches can be used dry or wet (oil bath). Running the clutches in an oil bath also greatly increases the
heat dissipation capability, which makes them ideally suited for multiple speed gear boxes and machine tool
applications.
How it works - Multiple disk clutches operate via an electrical actuation but transmit torque mechanically.
When voltage /current is applied to the clutch coil, the coil becomes an electromagnet and produces magnetic
lines of flux. These lines of flux are transferred through the small air gap between the field and the rotor. The
rotor portion of the clutch becomes magnetized and sets up a magnetic loop, which attracts both the armature
and friction disks. The attraction of the armature compresses (squeezes) the friction disks, transferring the
torque from the in inner driver to the out disks. The output disks are connected to a gear, coupling, or pulley
via drive cup. The clutch slips until the input and output RPMs are matched. This happens relatively quickly
typically (.2 - 2 sec).
When the current/voltages are removed from the clutch, the armature is free to turn with the shaft. Springs
hold the friction disk away from each other, so there is no contact when the clutch is not engaged, creating a
minimal amount of drag.
Electromagnetic tooth clutches
Introduction - Of all the electromagnetic clutches, the tooth clutches provide the greatest amount of torque in
the smallest overall size. Because torque is transmitted without any slippage, clutches are ideal for multi stage
machines where timing is critical such as multi stage printing presses. Sometimes, exact timing needs to be
kept, so tooth clutches can be made with a single position option which means that they will only engage at a
specific degree mark. They can be used in dry or wet (oil bath) applications, so they are very well suited for
gear box type drives.They should not be used in high speed applications or applications that have engagement
speeds over 50 rpm otherwise damage to the clutch teeth would occur when trying to engage the clutch.
How it Works – Electromagnetic Tooth clutches operate via an electric actuation but transmit torque
mechanically. When voltage/current is applied to the clutch coil, the coil becomes an electromagnet and
produces magnetic lines of flux. This flux is then transferred through the small gap between the field and the
rotor. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop, which attracts the
armature teeth to the rotor teeth. In most instances, the rotor is consistently rotating with the input (driver). As
soon as the clutch armature and rotor are engaged, lock up is 100%.
When current/voltage is removed from the clutch field, the armature is free to turn with the shaft. Springs hold
the armature away from the rotor surface when power is released, creating a small air gap and providing
complete disengagement from input to output.
Electromagnetic particle clutches
Introduction – Magnetic particle clutches are unique in their design, from other electro-mechanical clutches
because of the wide operating torque range available. Like a standard, single face clutch, torque to voltage is
almost linear. However, in a magnetic particle clutch torque can be controlled very accurately. This makes
these units ideally suited for tension control applications, such as wire winding, foil, film, and tape tension
control. Because of their fast response, they can also be used in high cycle application, such as card readers,
sorting machines, and labeling equipment.
How it Works – Magnetic particles (very similar to iron filings) are located in the powder cavity. Without any
voltage/current they sit in the cavity. However, when voltage/current is applied to the coil, the magnetic flux
that is created tries to bind the particles together, almost like a magnetic particle slush. As the voltage/current
is increased, the magnetic field builds, strengthening the binding of the particles. The clutch rotor passes
through the bound particles, causing drag between the input and the output during rotation. Depending upon
the output torque requirement, the output and input may lock at 100% transfer.When voltage/current is
removed from the clutch, the input is free to turn with the shaft. Since the magnetic particle is in the cavity, all
magnetic particle units have some type of minimum drag associated with them.
Hysteresis-powered clutch
Introduction – Electrical hysteresis units have an extremely high torque range. Since these units can be
controlled remotely, they are ideal for testing application where varying torque is required. Since drag torque
is minimal, these units offer the widest available torque range of any electromagnetic product. Most
applications involving powered hysteresis units are in test stand requirements. Since all torque is transmitted
magnetically, there is no contact, so no wear occurs to any of the torque transfer components providing for
extremely long life.
How it works – When the current / voltage is applied to the field, it creates magnetic flux. This passes into the
rotor portion of the field. The hysteresis disk physically passes through the rotor, without touching it. These
disks have the ability to become magnetized depending upon the strength of the flux (this dissipates as flux is
removed). This means, as the rotor rotates, magnetic drag between the rotor and the hysteresis disk take place
causing rotation. In a sense, the hysteresis disk is pulled after the rotor. Depending upon the output torque
required, this pull eventually can match the input speed, giving a 100% lockup.
When current / voltage is removed from the clutch, the armature is free to turn and no relative force is
transmitted between either member. Therefore, the only torque seen between the input and the output is
bearing drag.
Electromagnetic brakes operate electrically, but transmit torque mechanically. This is why they used to be
referred to as electro-mechanical brakes. Over the years, EM brakes became known as electromagnetic,
referring to their actuation method. Since the brakes started becoming popular over sixty years ago, the
variety of applications and brake designs has increased dramatically, but the basic operation remains the same.
Single face electromagnetic brakes make up approximately 80% of all of the power applied brake
applications. This article mainly concentrates on these brakes. Alternative designs are shown at the end of this
article.
Construction
Horseshoe magnet red silver iron
A horseshoe magnet (A-1) has a north and south pole. If a piece of Iron contacts both poles, a magnetic circuit
is created. In an electromagnetic brake, the north and south pole is created by a coil shell and a wound coil. In
a brake, the armature is being pulled against the brake field. (A-3) The frictional contact, which is being
controlled by the strength of the magnetic field, is what causes the rotational motion to stop. All of the torque
comes from the magnetic attraction and coefficient of friction between the steel of the armature and the steel
of the brake field. For many industrial brakes, friction material is used between the poles. The material is
mainly used to help decrease the wear rate. But different types of material can also be used to change the
coefficient of friction (torque) for special applications. For example, if the brake was required to have an
extended time to stop or slip time, a low coefficient material can be used. Conversely, if the brake was
required to have a slightly higher torque (mostly for low RPM applications), a high coefficient friction
material could be used.In a brake, the electromagnetic lines of flux have to attract and pull the armature in
contact with it to complete brake engagement. Most industrial applications use what is called a single-flux
two-pole brake. The coil shell is made with carbon steel that has a combination of good strength and good
magnetic properties. Copper (sometimes aluminum) magnet wire, is used to create the coil, which is held in
shell either by a bobbin or by some type of epoxy/adhesive.To help increase life in applications, friction
material is used between the poles. This friction material is flush with the steel on the coil shell, since if the
friction material was not flush, good magnetic traction could not occur between the faces. Some people look
at electromagnetic brakes and mistakenly assume that, since the friction material is flush with the steel, that
the brake has already worn down, but this is not the case.
Basic OperationThere are three parts to an electromagnetic brake: field, armature, and hub (which is the
input on a brake) (B-2). Usually the magnetic field is bolted to the machine frame (or uses a torque arm that
can handle the torque of the brake). So when the armature is attracted to the field the stopping torque is
transferred into the field housing and into the machine frame decelerating the load. This can happen very fast
(.1-3sec).Disengagement is very simple. Once the field starts to degrade flux falls rapidly and the armature
separates. A spring(s) hold the armature away from its corresponding contact surface at a predetermined air
gap.[4]
Electromagentic brake
Torque
Burnishing can affect initial torque of a brake but there are also factors that affect the torque performance of a
brake in an application. The main one is voltage/current. In the voltage/current section we showed why a
constant current supply is important to get full torque out of the brake.When considering torque, the question
of using dynamic or static torque for the application is key? For example, if running a machine at relatively
low rpm (5 – 50 depending upon size) there is minimal concern with dynamic torque since the static torque
rating of the brake will come closest to where it is running. However, when running a machine at 3,000rpm
and applying the brake at its catalog torque, at that rpm, is misleading. Almost all manufacturers put the static
rated torque for their brakes in their catalog. So, when trying to determine a specific response rate for a
particular brake, the dynamic torque rating is needed. In many cases this can be significantly lower. It can be
less than half of the static torque rating. Most manufacturers publish torque curves showing the relationship
between dynamic and static torque for a given series of brake.
Over Excitation
Electromagnetic-Power-Off-Brake
Over-excitation is used to achieve a faster response time. It is when a coil momentarily receives a higher
voltage than its nominal rating. To be effective, the over-excitation voltage must be significantly, but not to
the point of diminishing returns, higher than the normal coil voltage. Three times the voltage typically gives
around 1/3 faster response. Fifteen times the normal coil voltage will produce a 3 times faster response
time.With over-excitation, the in-rush voltage is momentary. Although it would depend upon the size of the
coil, the actual time is usually only a few milliseconds. The theory is, for the coil to generate as much of a
magnetic field as quickly as possible to attract the armature and start the process of deceleration. Once the
over-excitation is no longer required, the power supply to the brake would return to its normal operating
voltage. This process can be repeated a number of times as long as the high voltage does not stay in the coil
long enough to cause the coil wire to overheat.
Wear
It is very rare that a coil would just stop working in an electromagnetic brake. Typically if a coil fails it is
usually due to heat which has caused the insulation of the coil wire to break down. That heat can be caused by
high ambient temperature, high cycle rates, slipping or applying too high of a voltage. Most brakes are
flanged mounted and have bearings but some brakes are bearing mounted and like the coils, unless bearings
are stressed beyond their physical limitations or become contaminated, they tend to have a long life and they
are usually the second item to wear out.The main wear in electromagnetic brakes occurs on the faces of the
mating surfaces. Every time a brake is engaged during rotation a certain amount of energy is transferred as
heat. The transfer, which occurs during rotation, wears both the armature and the opposing contact surface.
Based upon the size of the brake, the speed and the inertia, wear rates will differ. With a fixed armature design
a brake will eventually simply cease to engage. This is because the air gap will eventually become too large
for the magnetic field to overcome. Zero gap or auto wear armatures can wear to the point of less than one
half of its original thickness, which will eventually cause missed engagements.
Other Types of Electromagnetic Brakes
Electromagnetic Power Off Brake
Electormagnetic Power Off Brake Spring Set
Introduction - Power off brakes stop or hold a load when electrical power is either accidentally lost or
intentionally disconnected. In the past, some companies have referred to these as "fail safe" brakes. These
brakes are typically used on or near an electric motor. Typical applications include robotics, holding brakes
for Z axis ball screws and servo motor brakes. Brakes are available in multiple voltages and can have either
standard backlash or zero backlash hubs. Multiple disks can also be used to increase brake torque, without
increasing brake diameter. There are 2 main types of holding brakes. The first is spring applied brakes. The
second is permanent magnet brakes.
How It Works
Spring Type - When no electricity is applied to the brake, a spring pushes against a pressure plate, squeezing
the friction disk between the inner pressure plate and the outer cover plate. This frictional clamping force is
transferred to the hub, which is mounted to a shaft.
Permanent Magnet Type – A permanent magnet holding brake looks very similar to a standard power
applied electromagnetic brake. Instead of squeezing a friction disk, via springs, it uses permanent magnets to
attract a single face armature. When the brake is engaged, the permanent magnets create magnetic lines of
flux, which can turn attract the armature to the brake housing. To disengage the brake, power is applied to the
coil which sets up an alternate magnetic field that cancels out the magnetic flux of the permanent magnets.
Both power off brakes are considered to be engaged when no power is applied to them. They are typically
required to hold or to stop alone in the event of a loss of power or when power is not available in a machine
circuit. Permanent magnet brakes have a very high torque for their size, but also require a constant current
control to offset the permanent magnetic field. Spring applied brakes do not require a constant current control,
they can use a simple rectifier, but are larger in diameter or would need stacked friction disks to increase the
torque.
Electromagnetic Particle Brake
Magnetic Particle Brake
Introduction - Magnetic particle brakes are unique in their design from other electro-mechanical brakes
because of the wide operating torque range available. Like an electro-mechanical brake, torque to voltage is
almost linear; however, in a magnetic particle brake, torque can be controlled very accurately (within the
operating RPM range of the unit). This makes these units ideally suited for tension control applications, such
as wire winding, foil, film, and tape tension control. Because of their fast response, they can also be used in
high cycle applications, such as magnetic card readers, sorting machines and labeling equipment.
How It Works - Magnetic particles (very similar to iron filings) are located in the powder cavity. When
electricity is applied to the coil, the resulting magnetic flux tries to bind the particles together, almost like a
magnetic particle slush. As the electric current is increased, the binding of the particles becomes stronger. The
brake rotor passes through these bound particles. The output of the housing is rigidly attached to some portion
of the machine. As the particles start to bind together, a resistant force is created on the rotor, slowing, and
eventually stopping the output shaft.
When electricity is removed from the brake, the input is free to turn with the shaft. Since magnetic particle
powder is in the cavity, all magnetic particle units have some type of minimum drag associated with them.
Electromagnetic Hysteresis Power Brake
Electomagnetic Hysteresis Power Brake
Introduction - Electrical hysteresis units have an extremely wide torque range. Since these units can be
controlled remotely, they are ideal for test stand applications where varying torque is required. Since drag
torque is minimal, these units offer the widest available torque range of any of the hysteresis products. Most
applications involving powered hysteresis units are in test stand requirements.
How It Works - When electricity is applied to the field, it creates an internal magnetic flux. That flux is then
transferred into a hysteresis disk passing through the field. The hysteresis disk is attached to the brake shaft. A
magnetic drag on the hysteresis disk allows for a constant drag, or eventual stoppage of the output shaft.
When electricity is removed from the brake, the hysteresis disk is free to turn, and no relative force is
transmitted between either member. Therefore, the only torque seen between the input and the output is
bearing drag.
Multiple Disk Brakes
Electromagnetic Multiple Disk Brake
Introduction - Multiple disk brakes are used to deliver extremely high torque within a small space. These
brakes can be used either wet or dry, which makes them ideal to run in multi speed gear box applications,
machine tool applications, or in off road equipment.
How It Works - Electro-mechanical disk brakes operate via electrical actuation, but transmit torque
mechanically. When electricity is applied to the coil of an electromagnet, the magnetic flux attracts the
armature to the face of the brake. As it does so, it squeezes the inner and outer friction disks together. The hub
is normally mounted on the shaft that is rotating. The brake housing is mounted solidly to the machine frame.
As the disks are squeezed, torque is transmitted from the hub into the machine frame, stopping and holding
the shaft.
When electricity is removed from the brake, the armature is free to turn with the shaft. Springs keep the
friction disk and armature away from each other. There is no contact between breaking surfaces and minimal
drag.