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Actuation of a clutch or brake is the method by which it is energized and de-energized.
The actuation method is used to either engage or disengage the transfer of torque through the clutch. For example, Carlyle Johnson offers ‘spring engaged-electrically released’ as well as ‘electrically engaged’ clutches and brakes. In the case of an electrically actuated spring applied clutch or brake, applying electrical power to the coil disengages the unit, and it will remain disengaged as long as the electric power is applied. In the case of an electrically engaged clutch or brake, applying electrical power to the coil will engage the unit, and it will remain engaged as long as the power is applied. Springs hold the clutch in a disengaged state when electricity is not applied to the clutch coil. This same option can be supplied using any of the available actuation methods. Mechanical clutches and brakes offer an additional advantage in that they can be supplied such that they are locked in either the engaged or disengaged position, thus it is not necessary to sustain the actuation energy to maintain the state of the device. Once engaged they will remain locked in the engaged position until they are manually released by moving a shifter sleeve into the disengaged position, or automatically released - for example in the event of a torque overload. |
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Electrically Engaged : Electrically engaged clutches or brakes require that electricity be supplied to a coil for engagement. While electricity is being supplied, a coil generates a magnetic field and the unit is engaged. The magnetic field is used to pull on an armature, which squeezes a set of friction discs together allowing the clutch to transmit torque. When the electricity is turned off, the discs are separated by wave springs, and the clutch is disengaged. An in-depth description of the actuation of each of our electric clutches and brakes is available in our product bulletins. Advantages of Electrically Engaged Clutches and Brakes:
Electrically Released (Spring Engaged): Electrically released clutches or brakes are engaged when electricity is absent from the coil. When electricity is not supplied (that is when the electrical power is OFF), springs are used to squeeze a set of friction discs together allowing the clutch or brake to transmit or arrest torque. The unit is not disengaged until electricity is supplied. Under that condition, the coil generates a magnetic field which is used to force an armature plate against the springs, overcoming the spring force and allowing the friction discs to separate. The discs are now allowed to spin freely without the ability to transmit or hold torque. An in-depth description of the actuation of our spring applied electric clutches and brakes is available in the product bulletins. Advantages of Spring Applied-Electrically Released Electric Clutches and Brakes:
Air (Pneumatic) / Hydraulic Clutches and Brakes Pneumatic or Hydraulically Engaged: Pneumatic (Air) and Hydraulically engaged clutches or brakes require that air or hydraulic pressure be supplied to an internal piston for engagement. As long as the pressure is supplied, the unit is engaged. When the pressure is removed the clutch or brake is disengaged. Air or hydraulic fluid forces a piston against a pressure plate that squeezes a set of friction discs together allowing the clutch or brake to transmit or hold torque. An in-depth description of the actuation of our pneumatic/hydraulic clutches and brakes is available in our product bulletins. Advantages of Pneumatic or Hydraulically Engaged Clutches and Brakes:
Pneumatic/Hydraulically Released (Spring Engaged): Pneumatic or Hydraulically released clutches or brakes are engaged whenever air or hydraulic pressure is absent. While the air or hydraulic pressure is off, the unit will remain on. Internal springs are used to push on a pressure plate. The pressure plate then squeezes a set of friction discs together allowing the clutch or brake to transmit or hold torque. The unit is disengaged when air or hydraulic pressure is supplied to the piston. The piston forces a pressure plate against the springs thereby removing the spring force from the friction discs. The discs are now allowed to spin freely without the ability to transmit or hold torque. An in-depth description of the actuation of our pneumatic or hydraulic clutches and brakes is available in our product bulletins. Advantages of Spring Applied-Pneumatically/Hydraulically Released Clutches and Brakes:
Mechanical Clutches and Brakes Mechanical clutches fall into two general categories:
Mechanically Engaged and Disengaged: Mechanically engaged and disengaged clutches or brakes are activated when a part of the unit (a ‘shifter sleeve’) is physically moved from the disengaged position to the engaged position (or vice-versa). This is accomplished when a force is placed against the shifter sleeve through a bearing or a set of shifter shoes and a shifter yoke. The shifter sleeve, acting through a lever mechanism, squeezes a set of friction discs together allowing the unit to transmit or arrest torque. Mechanical clutches and brakes generally have a locking mechanism built in such that once the unit is engaged, it will remain engaged even after the shifter is released or the force against the shifter sleeve is removed. A force in the opposite direction must be supplied to disengage the unit, and it will remain disengaged even if the disengagement force is removed. Carlyle Johnson offers several engagement variations in these clutch and brake lines. Some models, for example, do not lock and require that the engagement force must remain ‘on’ in order for the unit to stay engaged. Advantages of Mechanical Clutches and Brakes:
Mechanically Engaged / Overload Disengaged: Mechanically engaged and overload disengaged clutches can be engaged and disengaged manually in the same manner described above. An additional disengagement mode is also present. This additional disengagement mode occurs when the clutch is subjected to an overload, for example when the clutch is subjected to a torque that is greater than that of the static torque capability of the clutch, and slippage occurs. The clutch will then fully disengage until it is re-engaged by a force on the shifter sleeve. Generally, these types of devices have an adjustment capability so that the overload torque level can be accurately set for each application, and changed if necessary. In addition we offer several other variations in this clutch line including single or multiple position engagement for timing or registration of input to output. Advantages of Overload Release Clutches:
Torque Limiters and Overload Release Clutches “A torque limiter is a frictional device whose primary function is to transmit continuous power until a preset limit is achieved. Upon achieving the preset torque limit the frictional device will slip, thereby transmitting only partial power.” Carlyle Johnson torque limiters are basically clutches that are always engaged. A torque limit is preset (and usually adjustable over some range). When the preset torque setting is reached, the device slips, allowing only the transmittal of torque up to the ‘slip torque’ value. The torque limiter is used to couple two separate bodies of rotation. The two separate bodies may consist of shafts, gears, sprockets, a prime mover or motor, or any combination of these. The components are usually pumps, fans, Power Take-offs, compressors, motors, gearboxes and generators, but it is usually the shaft of the component itself that is used to transfer power whether a driven or driving element. Whenever the torque being transmitted is below the ‘slip torque’ value preset in the torque limiter, the full torque will be transmitted. The preset torque level must be exceeded in order for the torque limiter to slip. When the torque is reduced below that preset level, the device resumes transmitting full torque. The torque transmitted during the slip period is a function of the speed, duration of slip, the frictional materials selected and the type of lubrication - if lubrication is utilized. Some torque limiters are designed to tolerate being run in a continuous slip condition – that is the torque from the driving element may consistently exceed the ‘slip torque’ value, and only the preset level of torque is transmitted. Other torque limiters are not designed to run in continuous slip environments, and when the torque limit is approached – for example in the case of an overload – the output of the driving element must be reduced after a momentary slip condition – or it must then be decoupled from the driven device until the overload is corrected. Consult the factory for additional information on your specific application. Typical Torque Limiter applications are as follows: 1. Torque Limiter and Cup (TL1)
2. Shaft to Shaft Coupling: (TL2)
3. Shaft to sprocket or gear. (TL3)
Overload Release Clutches are clutches that have the additional feature of self-disengagement when a preset torque is reached. Upon reaching the preset torque value the clutch is fully disengaged by an internal cam mechanism and no power or torque will be transmitted. The disengagement point is usually adjustable over some range for each size clutch. Once disengaged, the clutch must then be reengaged manually before power can be transmitted. This feature also allows the clutch to establish rotational positioning (indexing) between the two rotating bodies. When used in indexing, the reengagement point of rotation can be defined in degrees, with 1, 2, 4 or more reengagement points when the clutch is reset. A clutch which can be engaged every 90° will have four reengagement points for each complete rotation of the driving element. This assures that the driving elements are always synchronized with the driven devices when timing is critical. Unlike a torque limiter, an overload release clutch will not tolerate even momentary overload. This is especially useful in applications where safety considerations and protection of sensitive downstream equipment must take precedence over continuous operation. Considerations in Selecting Torque Limiters vs. Overload Release Clutches: The nature of the application and the types of driving and driven bodies will determine the proper device selection. Consider, for example, a conveyor being driven by an electric motor. When a jam or backup occurs which prevents the conveyor from moving, use of a torque limiter may present a safety hazard to operating personnel. In this case, if a jam is manually cleared, the torque required to move the conveyor will be immediately reduced, and the motor, which is not disengaged from the conveyor even in a slip condition, will instantly begin moving the conveyor, even while personnel are still clearing the jam, potentially causing injuries. When a torque limiter is used in a power transmission application, the design engineer must recognize that there will always be ‘stored energy’ in the system when the driving element is energized, even if the driven device is not moving. Careful consideration to safety factors must be given in this type of environment. An overload release clutch, in contrast, will positively disengage the conveyor in our example, and will not restart it again until the jam is cleared, and until operating personnel clear the area, positively reset the clutch, and allow the conveyor to resume operation. Torque limiters are useful in applications where the driven device cannot absorb the full output torque of the driving element. An electric motor driving a bottle capping machine is an example of this kind of application. Sufficient torque must be transmitted to twist the cap into position, but when the cap is tightly fit to the bottle, additional torque would damage the product, the capper, or both. In this case, the torque limiter assures a tight fit of the cap, but not so much torque that any damage occurs, assuring a high level of efficiency with minimum down time in the capping operation. |
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Actuation Summary In general each method of actuation offers the designer some advantages and each method has disadvantages. It is important to realize what these advantages are when selecting a clutch or brake. This is a general guide and there are always exceptions to the general guidelines but most exceptions will involve additional cost and engineering effort to obtain the same results. Carlyle Johnson has the expertise to resolve the most difficult control problems. For example, circuits are available for reducing engagement times in electric clutches (over excitation), controlling torque through pulse width modulation, reducing coil wattage through step down/up excitation and increasing engagement for soft start and timing applications. Consult the factory for further information. Energy Engaged Clutches and Brakes Electrically engaged clutches require electricity to operate. They can typically operate at higher speeds and have longer periods of usage between maintenance cycles. This is a direct result of longer bearing life. The bearing life of an electric clutch is much longer than that of an equivalent size pneumatic or hydraulic clutch operating at the same speed, as a result of reduced forces placed on the bearings. Reduced bearing forces are possible because of the magnetic force design of our electric clutches. The magnetic force required to squeeze the friction discs together in a Carlyle Johnson Electric Clutch is self-contained. Only a very small portion this force is transmitted to the bearings. Electric clutches will also operate warmer than the equivalent pneumatic/hydraulic clutch because of the heat generated by the coil. The coil wattage ranges from as little as 5 watts to as much as 150 watts, therefore the clutch must be operated with sufficient ventilation to ensure that overheating does not occur. Pneumatic/Hydraulic clutches require that air or hydraulic pressure be applied to the clutch in for operation. The customer normally utilizes a solenoid to turn the pressure on and off. These solenoids typically have much smaller coils than the coil in an electric clutch. Since the smaller coil has less inductance, the clutch will have a much faster response time than an electric clutch. Faster response time also offers more consistent cycling as long as supply pressure is closely regulated.
Mechanically Engaged vs. Energy Engaged Clutches and Brakes Mechanical clutches and brakes are ideal in applications where manual ‘on and off’ engagement is required, or where manual ‘on’ and torque sensing is used to turn the clutch ‘off’. Mechanical clutches and brakes require an external force to shift the device from the engaged-to-disengaged, or disengaged-to-engaged position. Mechanically engaged units utilize an internal lever mechanism to produce the force that squeezes the friction discs together and thereby transmits torque. Mechanical devices require occasional adjustment to engage or disengage properly, due to friction disc wear. The mechanical clutch or brake has no internal bearings and has a positive engagement position. It will remain in a particular state with no risk of turning on or off due to power or hydraulic/air failure
Spring Applied vs. Energy Applied Clutches and Brakes Energy applied clutches and brakes transmit torque – that is they are engaged – when power is applied to the device. They are disengaged – unable to transmit or arrest torque – when the power is removed. Energy-applied clutches and brakes are self-adjusting for wear. Spring-applied clutches and brakes work in the opposite fashion. The application of power to a spring-applied unit will disengage it – overcoming the engagement spring force and preventing the clutch or brake from transmitting or holding torque. The removal of power will allow the internal springs to engage the device, allowing torque to be transmitted. Spring-applied clutches and brakes require periodic adjustment to compensate for wear. When they are assembled after maintenance is performed, careful attention must be paid to instructions to be sure the device will properly engage or disengage. When selecting a spring-applied or energy-applied device, design engineers must consider the operating conditions of the device being powered. Spring-applied clutches and energy-applied brakes potentially allow driven mechanisms downstream to retain ‘stored energy’ when the actuation power fails. That is, they are not safely disengaged or held unless energy (electricity, hydraulic, or pneumatic power) is available for actuation, to disengage or brake the device. Energy-applied clutches and spring-applied brakes will remove power or stop a rotating device when actuation power fails. In applications where personnel are likely to be exposed to injury during jams, power failures, and the like, safety concerns dictate the use of clutches and brakes which do not allow stored energy to remain when equipment is not operating. |
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