Mechanical Clutches Information
Mechanical clutches are the simplest mode of clutch actuation and often the least expensive. Mechanical clutches can be actuated manually or by foot. Hand operation of mechanical clutches involves actuation directly via cams or levers or, in larger equipment, through compound linkages. In most cases, mechanical clutches are practical only when the lever or pedal can be located near the clutch; longer distance can be achieved by complex linkages, but friction losses in the linkages will create higher actuation resistance for the operator. Mechanical clutches often have a locking mechanism that keeps the system connected once the unit is engaged. It will remain engaged until a force in the opposite direction disengages the system. Most mechanical clutches use friction to operate. Friction force between the drive and driven components allows a moving component to synchronize with another that is moving at a different speed creating a solid connection for power transmission. To maximize efficiency, as little slippage as possible between the input and output shafts is desirable.
Important specifications to consider while selecting an electric clutch include but are not limited to:
- Torque Rating: The maximum torque rating for the clutch should equal or exceed the application's requirement.
- Power: The maximum power rating for the clutch.
- Speed: The maximum rotary speed rating. This specification applies only to rotary clutches.
- Shaft Configuration: Clutch may be mounted in-line, parallel, or right angle.
- Drive/Load Connection:
- In-line Shafts: The drive and the load have shafts that attach to a through-bore.
- Through Shaft: The drive shaft attaches to a bore and the load is driven through the outer diameter.
- Shaft: Pulley/Gear/Sprocket:The drive shaft attaches to a bore and the output is a drive component such as a pulley, gear, or sprocket. These clutches are often designed to accept different drive components.
- Flange: The clutch is mounted to the object in motion via a flange.
There are several types of specialty mechanical clutches that are worth noting.
- Slip Clutch/Disengagement Torque Limiter: Slip clutches or torque limiters disengage the drive in the event of jamming or overload. They have a threshold torque and are used for personal safety and equipment protection.
- Overrunning Clutch: Overrunning clutches are used as a safety mechanism. The drive element stops and the load winds down freely.
- Backstop Clutch: Backstop clutches are freewheeling devices that transmit torque in one direction. They prevent drives from backing up and keep driven systems, such as inclined conveyors, from going down.
- Freewheeling Clutch: Freewheeling clutches are unidirectional torque transmitters that spin freely in the direction opposite of the intended drive rotation.
Several mechanical clutch engagement methods are available, including:
- Noncontact: Braking action is achieved through a non-contact technology such as a magnetic field, eddy currents, etc.
- Friction: Friction between contact surfaces transmits power. This is the most common configuration.
- Toothed: Toothed contact surfaces transmit power without slipping or heat generation. Teeth are engaged only when stopped or running at a slow speed (< 20 rpm).
- Wrap Spring: A coiled spring wraps downward onto the rotating element. The device is disengaged when the spring is uncoiled via a control tang at its end.
- Oil Shear: Braking action is engaged via the viscous action of the shearing of transmission fluid.
- Plate/Disc: The torque level is controlled by compression springs that force plates together.
- Ball Detent: Ball detent is a slip mechanism in which, upon overload, balls ride up out of seats to overcome springs or air pressure engagement.
- Roller Detent: Rollers, held in place by springs, wedge between the inner and outer races to engage the clutch.
- Pawl Detent: Pawl detent is a slip clutch mechanism in which, upon overload, the pawl overcomes spring or air pressure engagement and rotates out of its detent.
- Sprag: Sprags are steel wedges that tip in one direction to wedge between inner and outer races. They can be configured with either the inner or outer race as the input or output. Too much torque makes the sprags tip so much that contact is not maintained. Often, sprag clutches can transmit more torque than other designs of slip or overrunning clutches.
Additional capacities can be added to mechanical clutches. These features help customize the unit to meet special or unique requirements. Typical features and options include:
- Adjustable Torque: Adjustable torque is used primarily for slip clutches and torque limiters. Users can adjust the torque at which the clutch disengages or slips.
- Zero Backlash: There is no play or backlash during the engagement of the load and no load disengagement during a direction reversal.
- Washdown Capable: The housing is rated for washdown cleaning.
- Bi-directional: Devices can be set-up to rotate in either direction.
- Automatic Re-engagement: The clutch re-engages the load when the torque drops to an acceptable level.
- Slip Indication: Slip indication can move a pin radially when an overload occurs, or send an electrical signal to the drive motor.
- Feedback: Feedback provides an electrical or electronic signal for monitoring parameters such as position, speed, torque, lockup, or slip status.
Mechanical clutches mostly rely on operator actuation to engage and disengage the clutch using hand or foot operation. This human factor limits actuation forces to about 75 lbs. Because this is a relatively low engagement force, torque transmission is limited to 25,000 lb-ft or about 2,500 hp (an order of magnitude less than the power transmitted by many large machines). Consequently, mechanically operated clutches are relegated to use in light vehicles and small agricultural and industrial equipment like hoists and wood chippers. Aside from low cost, a major “soft” advantage of mechanical clutch actuation is the control the operator has over clutch engagement; physical feedback or touch allows him or her to control the engagement and disengagement speeds of the equipment. This advantage is also the mechanical clutch’s major disadvantage; the need for an operator. Manual engagement limits clamping force and torque. It also limits response and cycling times. Cycle times of more than a few times per minute can affect the clutch elements or fatigue the operator and prematurely wear the clutch engagement elements.