One of the many attributes of versatile fluid power systems is the ability to control speed and torque delivered to a load from a prime mover. But hydraulic system designers often overlook clutches as an alternative.

Installing a clutch between a system’s prime mover and load provides an efficient means of placing the system on standby. But saving energy by unloading an entire power system is just one important function of clutches. They can also reduce system cost by tapping into a pressurized hydraulic system to engage or disengage many rotating components. They do this with negligible drain on a hydraulic power system because they require high pressure but very low fluid volume for actuation.

For example, a mine machine might use separate motors and controls for it’s the cutter head, gathering arms, conveyor, and a separate drive system for propulsion. By incorporating clutches into each of these drives, any particular function can be engaged or disengaged independently from one main power source.

Available types

In addition to fluid power, clutches also rely on mechanical, electrical, and electromechanical methods of actuation. These other types, however, will not be discussed beyond their general characteristics.

Mechanically actuated clutches are simple, inexpensive, and provide the operator with a good feel of engagement. However, actuation usually depends on manual positioning of levers and linkages, thereby limiting sensitivity of control, response time, and frequency of engagement.

Many mechanical clutches, such as jaw clutches, provide only full engagement or full disengagement. When these clutches engage, instantaneous transmission of full torque occurs. So the drive either experiences a sever shock load when the clutch engages, or the drive must be engaged at or near zero rpm. These disadvantages limit application of mechanical off-on clutches.

Electrical clutches include such designs as magnetic particle and eddy current. These clutches use voltage control to achieve partial engagement of input and output shaft speed so the clutch can control speed and torque. Unlike fluid power clutches, electrical clutches consume power continuously while engaged.

Electromechanical clutches use a solenoid for engagement and springs for disengagement. This allows very rapid cycling rates, but, again, they can consume substantial electrical power. They also generate heat in their coils during periods of engagement. Furthermore, large solenoids must be used to accommodate heavy loads. Solenoid size and allowable voltage in controls thus limits the size of these clutches.

Fluid power clutches

Hydraulically and pneumatically actuated clutches, on the other hand, generally use fluid (oil or air) pressure to engage a load and springs to disengage. Alternative designs may use springs to keep loads engaged and fluid pressure to disengage. Because the pressure fluid is static, very little power is consumed once the clutch is engaged.

Disc types are the most widely applied fluid-actuated clutches. A single plate, air-actuated design is shown in Figure 1. Pressurized air enters the clutch and pushes a driving pate against a driven pate. A high-friction-coefficient material, bonded to the driving plate, transmits torque from driving plate to driven when the clutch is engaged.

The friction disc lining usually is of paper, elastomeric, graphitic, or sintered material, predicated by the application. Consideration is given to friction characteristics, elevated operating temperature, heat dissipation, and other variables. The driven plate usually is of high-carbon steel to resist distortion under hi-temperature conditions.

But a clutch’s load capacity is limited by the area of the friction disc contacting the driven plate. Because the friction lining has a finite coefficient of friction, load capacity can be increased only by increasing the lining’s contact surface area for the engagement force. For this reason, single-dic air clutches have limited capacity because operating pressure usually does not exceed 110 psi, and increasing the lining area means making the clutch larger and more massive.

Increasing capacity

Multiple disc clutches offer one solution. By engaging many friction discs against many driven discs, capacity increases dramatically with only modest increases in clutch size and weight. In this design, Figure 2, fluid pressure acts on a piston that pushes driving and driven discs together. Splines, lgus, or other geometries allow discs to move axially to contact each other and transmit torque. A drive cup surrounds the OD of driven discs and serves as the output member. Release springs between driving discs keep discs separated-and the clutch disengaged-in the absence of pressure acting on the piston.

Moreover, by actuating the clutch with hydraulic fluid instead of air, much higher actuation forces can be generated because higher pressures—often to 500 psi—are available. Combining these two features means that a multiple-disc hydraulic clutch can transmit much higher torque in a smaller envelope than pneumatic clutches can.

The designs shown in Figures 1 and 2 use stationary mounts through which pressure fluid is introduced into the rotating members of the clutch. The rotating cylinder design, Figure 3, does not have a stationary cylinder, and therefore, requires no anti-friction bearings. Eliminating the bearings shortens the axial length of these style clutches, so they are slightly smaller and a little lighter than the stationary cylinder design. This rotating cylinder design is popular in off-road and mining equipment transmissions where high torque must be transmitted within a small envelope.

However, the rotting cylinder design requires pressurizing the clutch through axial holes drilled in its mounting shaft. These holes must be supplied with fluid through a rotary seal at the end of the rotating shaft. If two or three clutches are mounted on the same shaft, three oil supply lines must  be drilled and a triple-passage rotary seal provided.