Some machines turn on and stay on. Others do their work by successively starting and stopping. Anything beyond 60 cycles/ min is generally considered a high cycle rate, and is capable of exacting a heavy toll on individual components and overall system performance. Some functions have strict speed and accuracy requirements, single-revolution applications for instance, and these can be among the trickiest. But the right combination of drive components – motors, couplings, clutchbrakes, and reducers – can make light work of most any high-cycle application.

System stress

Rapid-cycling applications accumulate a remarkable number of cycles over a relatively short time. Consider that in a month, a 24-hour-a-day 100 cycles/min application will have tallied 2,880,000 cycles. At this rate, standard dry friction clutch-brakes may wear down in as little as five months – less if the rest of the system is not in good order.

Heat buildup brought on by high-cycle rates is another source of distress. Heat results from friction surfaces engaging, and with some forms of clutch-brakes it is also a byproduct of the actuation method. In any case, when accelerating the load, a portion of the horsepower (often half) is converted to heat rather than used for work; thermal horsepower is expended while the clutch goes from partial to full engagement. Upon decelerating, all the power of the moving load is turned to heat; the thermal horsepower required in braking the load.

So as a load is indexed, the clutch brake generates heat. With high indexing rates, heat can build up rapidly. It is particularly so with dry friction units which sometimes necessitate forced-convection cooling. Apart from the fact that nobody wants a red-hot clutch-brake in the middle of their drive train, excessive temperatures can weaken friction materials and accelerate wear.

Beyond concerns of clutching and braking, rapid-cycle operations are notorious for shock loads that continually buffet gears, couplings, and the like. High service factors are in order; all components must be prepared to withstand shock. Also, during torque reversals due to constant starts and stops, having low backlash reduces speed buildup prior to re-engaging, so there’s less impact energy. Torsionally rigid components work best down the line. While rubber-insert couplings, for example, are commonly touted for their damping abilities, with torque reversals they can actually add to the shock, winding up to store energy and springing back to create an impact.

Shaft connections are crucial in a shock-load situation. Quill connections used in clutch-brakes and gearboxes take a beating in high-cycle applications. Standard quill connections have a loose shaft fit, for assembly purposes, and with each indexing move the key is hit with hammer-like force, so it distorts or wears out quickly. Furthermore, it can cause galling, making disassembly a problem. Similarly, slip-fit keyed connections used on coupling and pulley attachments are prone to pounding with each torque reversal, and will fail quickly with the high number of cycles involved. It’s expedient, then, to use tight-fit bores and compression-type connections.

Clutch-brake overview

In many machines, some form of clutch-brake is responsible for disengaging and re-engaging the load, thereby providing incremental motion. There are certain mechanical-lockup clutches that work against a positive stop and do not slip. Wrap spring and so-called “single revolution” clutch-brakes are examples. However, they are generally used only in low-inertia applications where near-instant stops are acceptable. Friction- based units are used in the majority of high-cycle drives.

Friction clutches and brakes can be broken down into several categories. Dry friction devices are based around a steel plate contacting a friction disc, which is a second steel plate bonded with friction material. It is common for these to be cooled by the surrounding air, but such versions have limited heat dissipation capabilities. While many use an open design that allows airflow to the contact areas, the easy access also applies to dust, dirt, and unwanted fluids.

Wet disc or oil shear clutches consist of multiple steel plates mated to friction discs in a stacked configuration. Fluid flows over the contact surfaces and removes the heat, transferring it by convection into the housing, which in turn conducts it to the surface to be dissipated to the surroundings by convection and radiation. Sometimes fluid is circulated through a heat exchanger.

An oil shear clutch-brake provides high torque and low inertia by using multiple disc stacks instead of a single large disc. Oil shear units are prone to run fairly cool

Wet clutches can operate on the oil shear principle or as a standard friction clutch. As an oil shear or hydroviscous clutch engages, fluid shear transmits motion until input and output speeds are nearly equal and mating discs make full contact. Because rubbing between metal and friction material is limited, wear is minimized. The other versions are simply wet disc clutches where the fluid is used as a lubricant and coolant and has little hydroviscous effect.

A splined hub in an oil shear clutch-brake fixes a stack of discs to the output shaft. Slots in the hub, and centrifugal pumping action from a vane, promote oil circulation across the plates during rotation. The expedited heat dispersion increases lubricant and component life.

Electric, pneumatic, and hydraulic are the usual actuation methods for frictionbased clutches and brakes. Of the two fluidic types, pneumatic actuation is far more common in industrial applications. Besides pressure-based fluid actuation, there is also vacuum-based.

Electric units have a magnetic coil that pulls friction surfaces together. These are seen in all sorts of low-tomedium torque applications, including high-cycle applications, and are convenient in many ways. However, there are certain difficulties under high-cycle rates. The magnetic coil generates its own heat. With rapid-fire actuation, the accumulation of heat from the coil can contribute noticeably to that from friction. Furthermore, the electromagnets take considerable time to build and decay, and can limit cycle rates and response time. The larger the unit, the more time required. Common sizes take from 100 to 200 msec for buildup or saturation, and 50 to 100 msec to decay.

A compression-type quill shaft connection does well with high-cycle applications, reducing and withstanding impact from frequent torque reversals. In-shear devices like keys and pins can take a pounding from shock loads, particularly if there is a loose fit. This arrangement, however, bears down all around the shaft to form a tight no-slip connection that transmits torque through static friction.

Air clutches and brakes use compressed air to push a piston and force the friction surfaces together. Vacuum systems draw in the actuator to bring the surfaces together. Vacuum actuation is limited, however, in the amount of force and subsequent maximum torque it can develop. Neither fluidpressure actuation nor vacuum actuation produces heat.

For numerous reasons, air actuated oil shear clutch-brakes work very well in extreme high-cycle machinery. They generate the least amount of heat from friction and actuation, and the multiple disc arrangement provides high torque with low inertia by adding discs rather than increasing disc diameter. Additional cooling measures such as a fan or coolant circulating through a heat exchanger increase thermal capacity considerably.

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