Introduction to clutches
Machines, whether they are vehicles or some kind of equipment, have two essential systems—a power source (known as the “driver” side) and operational components (known as the “driven” side). Both systems are needed to accomplish work. A car without a transmission is just an expensive container for an engine. Similarly, a woodchipper’s rotating drum and knives cannot chip branches if they are not connected to a power source.
No matter the kind of machinery, for it to operate properly there must be a way to connect the two components and to separate them when necessary. This is why clutches are vital to so many pieces of equipment.
Clutches operate either by friction or by using interlocking parts. Friction clutches use discs with high friction coefficients. When the disc on the engine side and disc on the operating side are pressed together, the discs grab each other, transferring power from the engine to the equipment. These discs are made from combinations of metal powders, resins, and woven fiberglass, and the strength and durability of these materials can be matched to the needs of the equipment.
When stronger connections are required, clutches are built with various types of interlocking parts that create what is known as “positive engagement.” These clutches have two pieces, one coming from the driver side and one from the driven side. When the pieces come together, they fit in such a way that a strong and durable connection is made between the two systems.
What does a PTO clutch do?
When most people think of clutches, they probably think of cars with manual transmissions. But clutches are also found in most industrial equipment and are designed to perform many kinds of work. A power take-off (PTO) clutch is used to engage and disengage power from an engine to auxiliary equipment as diverse as hydraulic pumps, woodchipper drums, and farm machinery. Understanding how these clutches work and some of their design features will help you to make informed decisions and provide proper maintenance.
PTO Construction
PTO clutches consist of an external housing and internal parts. Typically, one side of the external housing mounts to the engine, and the other side provides support for the clutch’s output shaft bearing. The parts contained inside the housing are what connect the driver and driven parts of the machinery.
When connection is made with the engine, the power is transferred to the output end of the clutch. Depending on clutch design, that power is sent straight out of the clutch, or the power is transferred laterally (sideloaded) by means of a pulley and belt. Inline clutches are sometimes used with output shafts (like those that drive irrigation pumps and ag equipment), while at other times they are directly connected to things such as hydraulic pumps. Sideloaded clutches are used on equipment like wood chippers.
Spring-applied clutches vs. energy-applied clutches
Clutches fall into two categories. Some equipment is designed to have a constant connection between the driver and driven parts that is only interrupted briefly. This equipment uses clutches that are often called “spring applied” clutches, since springs hold the driver and driven sides in contact unless some other force pulls them apart. An example of this is a car with a manual transmission. The friction plates are always touching and designed to power the car’s transmission, but when the clutch pedal is pressed, the connection is temporarily broken, allowing different gears to be chosen. As soon as the pedal is released, the friction plates reengage, and power is restored.
The other style (often called “energy applied”) keeps the power source and moving parts disengaged until the clutch is activated. Here, energy needs to be applied to bring the parts together. An example of this is a woodchipper, where the engine is connected to a rotating drum by means of a pulley and belt. In this situation, the engine may be started but there is no connection with the pulley that turns the belts until the clutch lever is moved, bringing the friction plates into contact with each other.
Activation
The simplest and oldest way to activate a clutch is through a manual lever. In an industrial setting, this is often accomplished by moving a handle. (In a car, this is done with the driver’s foot.) Levers can also be moved with electric actuators like those used in dump truck bodies; these actuators are activated remotely from the cab.
As an alternative to levers, some equipment uses hydraulic or pneumatic cylinders to engage or disengage a clutch. When the cylinder’s pistons move in or out, the clutch discs are brought together or are separated.
Specific applications and considerations
PTO clutches must be matched to their application since they are built for various stress and output demands. Clutches that engage high inertia loads (like those of a woodchipper), are made differently than those that experience high vibrations (such as those used for reciprocating compressors).
Clutches have traditionally been built with pilot bearings on the engine side; these bearings support the inner end of the clutch shaft. When the clutch is installed correctly, the engine flywheel has a bore of sufficient depth to accommodate the clutch’s pilot bearing and the inner end of the clutch shaft. However, if the clutch is assembled with insufficient depth, or the thrust pressure from installing the bearing is not relieved, engine damage will result. Pilotless clutches eliminate this risk by using bearings along the length of the shaft instead of a pilot bearing.
Rotary motion tends to cause what is known as torsional vibration, or side-to-side movement. These small but constant vibrations can lead to early clutch wear. For this reason, many clutches have some form of torsional dampening system to offset these vibrations and prolong clutch life.
Some clutches contain an integrated speed-reducing gearbox. This allows power to be sent directly from the engine to an output shaft that turns at the correct speed for running farm implements—typically 540 or 1000 RPM.
Clutch maintenance
Clutch maintenance is usually limited to lubrication and adjustment. The need and frequency of this maintenance will depend on clutch design.
Traditional lever-operated, over-center clutches need to be adjusted regularly as their discs wear. If this is not done, these clutches will start to slip. Spring-loaded, “automotive-style” clutches are self-adjusting, meaning that the springs help to compensate for disc wear and require minimal maintenance.
Conclusion
Though clutches are fundamentally a way to engage and disengage power, they are designed with specific needs in mind. Understanding the demands of your equipment will help you to choose and maintain your clutches for the longest possible time.
