Clutch is basically a mechanism which is being used for transmitting rotation, which can be engaged and disengaged. Clutches are being used particularly in those devices that have two rotating shafts. In these devices, one shaft is typically driven by a motor or pulley, and the second drives the another device. The clutch connects the two shafts so that they can either be locked together and spin at the same speed, or be decoupled and spin at different speeds.
The clutch disc(center) spins with the fly wheel (left). To disengage, the lever is pulled, causing a white pressure plate to disengage the green clutch disc from turning the drive shaft, which turns within the thrust bearing ring of the lever. All the 3 rings will never connect, with any gaps.
Requirement of a clutch:
- Torque transmission: The clutch should be able to transmit maximum torque of
- Gradual engagement: The clutch should engage gradually to avoid sudden jerks.
- Heat dissipation: The clutch should be able to dissipate large amount of heat
which is generated during the clutch operation due to friction.
- Dynamic balancing: The clutch should be dynamically balanced. This is
particularly required in the case of high speed engine clutches.
- Vibrating damping: The clutch should have suitable mechanism to damp
vibrations and to eliminate noise produced during the power transmission.
- Size:- The clutch should be as small as possible in size so that it will occupy
- Free pedal play: The clutch should have free pedal play in order to reduce
effective clamping load on the carbon thrust bearing and wear on it.
- Easy in operation: The clutch should be easy to operate requiring as little
exertion as possible on the part of the driver.
- Lightness: The driven member of the clutch should be made as light as possible
so that it will not continue to rotate for any length of time after the clutch has
Types of clutches:
Different types of clutches are as follows:
1. Friction clutch:
(a) Single plate clutch
(b) Multi plate clutches:
- Wet clutch.
- Dry clutch.
(c) Cone clutch:
- External clutch.
- Internal clutch.
2. Centrifugal clutch.
3. Semi-centrifugal clutch
4. Diaphragm clutch:
(a) Tapered finger type
(b) Crown spring type
5. Positive clutch:
(a) Dog and Spline clutch
6. Hydraulic clutch
7. Electromagnetic clutch
8. Vacuum clutch
9. Over running clutch or free-wheel unit
Main Parts of the clutch:
The main parts of the clutch is divided into three parts :
- Driving members:
It consist of a fly wheel mounted on the engine crankshaft.
Flywheel couple with clutch
- Driven members:
The driven members are consist of the disc or plate, called the clutch plate.
Pressure plate and clutch plate
- Operating members:
Operating members are consist of a foot pedal, linkage, release or throw out bearing, release levers and springs.
Pedal, Bearing and spring
Electromagnetic clutches operate electrically, but transmit torque mechanically. That is why they are called electromechanical clutches. They are most suitable for remote operation since no mechanical linkages are required to control their engagement, providing fast and smoothing operation.
However, because the activation energy dissipates as heat in the electromagnetic actuator when the clutch is engaged, there is a risk of overheating. Consequently, the maximum operating temperature of the clutch is limited by the temperature rating of the insulation of the electromagnet.
- In this type of clutch, the flywheel consists of winding from the battery or dynamo.
- When the current passes through the winding. it produced an electromagnetic field which attracts the pressure plate.
- Thereby engaging the clutch.
- When the supply is cut-off the clutch is disengaged.
- The gear lever consists of a clutch release switch.
- When the driver holds the gear lever to change the gear, the switch is operated
cutting off the current to the winding which causes the clutch disengaged.
- At low speed when the dynamo output is low, the clutch is not firmly engaged.
- Therefore, three springs are also provided on the pressure plate which helps the
clutch engaged firmly at low speed also.
A horseshoe magnet 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 the south pole is create by a coil shell and a wound coil. In a clutch when power is applied, a magnetic field is created in the coil. This field overcomes an air gap between the clutch rotor and the armature. This magnetic attraction, pulls the armature on contact with the rotor face. The frictional contact, which is being control 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 fiction 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 to decrease the wear rate, but different types of material can also be used to change the coefficient of friction. For example, if the clutch is required to have an extended time to speed or slip time, a low co efficient of friction material can be used and if a clutch is required to have a slightly higher torque, 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. Mobile clutches of other specially electromagnetic clutches can use a double or triple flux motor. The double or triple flux refer to the number of north- south flux paths, in the rotor and the armature.
Basic structure of Electromagnetic Clutch
This means that, if the armature is designed properly and has similar banana slots, what occurs is the leaping of the flux path. Which goes north south, south north. By having more points of contact, the torque can be greatly increased. In theory if there were two sets of poles at the same diameter, the torque would double in the 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 slot. 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% 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 the carbon steel that has a combination of good strength and good magnetic properties. Copper magnet wire, is used to create the coil, which is held in shell either by a bobbin or by some type of epoxy.
To 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 do not use the 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, that can happen when friction material absorbs moisture.
Need of using Electromagnetic Clutch:
- Function of clutch is to engage or disengage the engine from the transmission system. Hence it is inserted between the flywheel as well as gear box. It is consists of main important parts like clutch plate, pressure plate, friction disc, operating lever etc. In clutch engage as well as disengage are very important, because due to which clutch is used. But when clutch is applied at that time, some clearance is there in the clutch pedal called clutch pedal play and due to which, proper disengage of clutch is not achieve and clutch will slip or dragged.
- There is a need to use some system incorporated in clutch system, to prevent above situation. Hence, if we shift gear and at that time clutch will disengage hence, it is very simple for driver and force require to engage as well as disengage the clutch is also neglected, hence a new type of clutch used in automobile vehicles called “Renault Car” called electromagnetic clutch.
- In this clutch system, when gear shift lever is applied at that time, due to MMF clutch will disengage and when release lever, clutch will engage.
Basic operation of electromagnetic clutch:
The clutch has four main parts:
- Field Coil
When we apply the voltage, the stationary magnetic field generates the lines of flux that pass into the door. The flux pulls the armature in contact with the rotor, as the armature and the output start to accelerate. Slipping between the rotor face and the armature face continues until the input and the output speed is the same. 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.
- If a piece of copper wire was wound, around the nail and then connected to a battery, it would create an electromagnet. The magnetic field that is generated in the wire, from the current, is known as the “right hand thumb rule”. (FIGURE-21) 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 required 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.
There are actually two engagement times to consider in an electromagnetic clutch. The first one is the time that it takes for a coil to develop a magnetic field, strong enough to pull in an armature. Within this, there are two factors to consider. The first one is the amount of ampere turns in a coil, which will determine the strength of a magnetic field. The second one is air gap, which is the space between the armature and the rotor. Magnetic lines of flux diminish quickly in the air.. Air gap is an important consideration especially with a fixed armature design because as the unit wears over many cycles of engagement the armature and the rotor will create a larger air gap which will change the engagement time of the clutch. In high cycle applications, where registration is important, even the difference of 10 to 15 milliseconds can make a difference, in registration of a machine. Even in a normal cycle application, this is important because a new machine that has accurate timing can eventually see a “drift” in its accuracy as the machine gets older.
The second factor in figuring out response time of a clutch is actually much more important than the magnet wire or the air gap. It involves calculating the amount of inertia that the clutch needs to accelerate. This is referred to as “time to speed”. In reality, this is what the end-user is most concerned with. Once it is known how much inertia is present for the clutch to start then the torque can be calculated and the appropriate size of clutch can be chosen.
Most CAD systems can automatically calculate component inertia, but the key to sizing a clutch is calculating how much inertial is reflected back to the clutch or brake. To do this, engineers use the formula: T = (wk2 × ΔN) / (308 × t) Where T = required torque in lb-ft, WK2 = total inertia in lb-ft2, ΔN = change in the rotational speed in rpm, and t = time during which the acceleration or deceleration must take place.
There are also online sites that can help confirm how much torque is required to accelerate a given amount of inertia over a specific time.
Elements of the Electromagnetic clutch:
Electromagnetic clutches share basic structural components: A coil in a shell, also refereed to as a field: a hub: and an armature. A clutch also has a rotor, which connects to the moving part of the machine, such as drive shaft.
The coil shell is usually carbon steel, which combines strength with magnetic properties.
Copper wire forms the coil, although sometimes aluminum is used. A bobbin or epoxy adhesive holds the coil in the shell.
Activating the unit’s electric circuit energizes the coil. The current running through the coil generates a magnetic field. When magnetic flux overcomes the air gap between the armature and field, magnetic attraction pulls the armature – which connects to the hub -into contact with the rotor.