Friction, Fluid Optimize High-Inertia Systems

High-inertia loads found in mining, bulk conveying, and other demanding industrial processes can put significant strains on machinery systems. Photo: Don Burkett

By Greg Cober, Altra Industrial Motion

Centrifugal clutches and fluid couplings allow maximum torque delivery in high-inertia-load applications with less energy and smaller motors.

High-inertia loads are common in a wide range of mining, bulk conveying, and other industrial processes. Centrifugal clutches and fluid couplings reflect mature technologies that are still used in these types of high-performance systems—with no expensive control mechanisms required. Both technologies meet a basic design need: They provide soft engagement of loads while allowing full or nearly full application of horsepower into drive systems.

For example, when a bulk conveyor moving mineral ore is started from a standstill, several dynamics are at work. As the motor accelerates the load, it is operating in a generally inefficient mode. Thus, current draws during acceleration can be many times higher than normal running current as the motor deals with the load inertia. In some circumstances, motors must be upsized several times over normal running torque levels due to the additional current draw needed for acceleration. Estimates indicate that upsizing should be a factor of six or seven times, depending on acceleration time length.

In one scenario, an application requiring a motor to run at full load of 30 A for running torque could use a 25-hp motor. For a requirement of six times higher torque, that same application will require a 200-hp motor to exceed the 180-A hurdle—at a cost differential of more than 600%.

Mechanically, the startup strain on drive-train components can be significant. The excess starting torque needed to accelerate the load can cause V belts to stretch and stress the shafts/keyway interface in gearboxes and couplings. Over time, those added strains cause premature wear and component failure.

A solution

Centrifugal clutches and fluid couplings can be used to mitigate electrical and mechanical strains during startup of high-inertia-load systems. Fully engaged, either can achieve full or nearly full transmission of motor torque to the drive train. The two technologies provide similar functions, but use different paths.

Centrifugal clutches feature interior “spider” hubs. As the hub rotates, centrifugal force causes the shoes with friction facing to expand outward into the driven drum. As rotational speed increases, so does centrifugal force, resulting in greater torque transmission.

The centrifugal clutch works as its name implies. As the interior hub (sometimes called the spider) of the unit rotates, centrifugal force causes shoes with a friction facing to move outward into the driven drum. The greater the rotational speed, the greater the centrifugal force—and the greater the transmission of torque from the shoes on the input hub to the output drum.

In fluid couplings, the function is similar, but the method is different. The fluid coupling is an enclosed system. The input portion goes into a vaned member called the runner. The output part is similarly vaned and referred to as the impeller.  Between the two pieces is a volume of hydraulic fluid. As the runner begins to rotate, so does the fluid. The fluid transfers that rotation to the impeller and from there to the output shaft of the coupling. Like centrifugal clutches, as speed increases, so does the torque transfer. Unlike centrifugal clutches, however, once the unit has achieved maximum speed, a fluid coupling will have slippage of 3% to 5%. Centrifugal clutches have 100% torque transfer when fully engaged.

Where high-inertia loads are a concern, centrifugal clutches and fluid couplings can allow soft-load engagement with full or nearly full application of horsepower and no expensive controls. Photo: Gary L. Parr

The benefits

The benefit of using centrifugal clutches or fluid couplings in mechanical systems is to reduce current draw during acceleration. A motor can reach a more efficient operating level faster when it is not directly tied to a load. One that acts directly on a load will require a much higher current draw. This capability can reduce overall current draws to the point where, in many cases, a smaller motor can be used, as the excess starting torque (and corresponding current draw) are not needed. In addition to the purchase-cost savings of a smaller motor, substantial power-consumption savings are possible. Moreover, the softer load engagement can positively affect the mechanical components in the system, i.e., strain on keyways, couplings, belts, and gearboxes is reduced, helping to extend component life.

Overload protection is an added benefit with centrifugal clutches and fluid couplings.

• If the load exceeds the capacity of a centrifugal clutch, it can slip. While this will eventually damage the clutch, it can often be easily rebuilt by replacing the shoes.
• In a fluid coupling, the fluid will simply fail to continue to drive. Over an extended period of time, this causes the temperature inside the coupling to rise. An option for smaller coupling sizes is to include a fusible plug that melts under excessive temperatures, allowing the fluid to drain from the unit. This effectively disables the coupling. For larger coupling sizes, a sensor can be used to measure input/output speeds and shut the motor down if a pre-set differential is exceeded.

As noted earlier, fluid couplings typically have a 3% to 5% slip at full engagement, compared with no slip for a centrifugal clutch. Other important differences in the two technologies include the fact that centrifugal clutches can be designed to engage and operate at lower speeds.

Fluid couplings are enclosed systems. The input portion of these components goes into a vaned member called the runner. The similarly vaned output part is referred to as the impeller. A volume of hydraulic fluid is housed between the two pieces. As the runner begins to rotate, it starts rotating the fluid. The fluid transfers that rotation to the impeller and from there to the output shaft of the coupling. As speed increases, so does torque transfer.

Fluid couplings typically require operating speeds of 1,200 rpm or higher, while centrifugal clutches can be designed to operate at speeds as low as 400 rpm. On the other end of the speed range, smaller-sized fluid couplings will work at speeds to 3,500 rpm, less for larger units. Centrifugal clutches can operate at higher speeds, if balanced. A stabilizing steel band may be required if operating speeds exceed certain thresholds.

Temperature is another reason why a centrifugal clutch might work better for an application than a fluid coupling. At ambient temperatures below –10 F, the fluid in a fluid coupling can become excessively viscous. Although special fluids for low-temperature applications are available, centrifugal clutches are often the better choice because they contain no fluid, thus eliminating the temperature concern.

Finally, centrifugal clutches can be used in systems driven by combustion engines, whereas fluid couplings cannot. This is because torque spikes in an engine interrupt the free flow of the hydraulic fluid and reduce the torque-transmitting capability of the system. MT

Greg Cober, product training manager for Altra Industrial Motion, Braintree, MA, has more than 35 years of experience in the power-transmission and motion-control industry. Based in South Beloit, IL, he can be reached at greg.cober@altramotion.com.