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Motion System Design
Catch and release

Catch and release

With the stop-and-go of packaging machinery, a clutch-brake is often the key, providing the primary motion control while the motor runs at constant speed.

Electric clutch-brakes are relatively simple and inexpensive, sparing other drive components, particularly the motor, from the impact and wear associated with abrupt variable motion. Their value can be appreciated even more if you consider that, even today, many motors used in packaging operations are not electronically equipped to handle the frequent start-stop, and without a clutch-brake they’re going to run hot and wear out.

The right fit

Incremental and indexing packaging operations sometimes require very fine resolution, such as in the precision placement of labels. These strict “positioning” applications may be best served by a purely electronic approach, using inverter drives or stepper motors.

However, when the level of precision isn’t as high, electric clutch-brakes provide a fairly simple, cost-effective solution. They are friction-based, not requiring resolvers, encoders, and additional control circuitry, as do the electronic alternatives.

There’s a long list of packaging machines where clutch-brakes are appropriate. It includes: rotary tables, as used in shrink wrapping, canning, and beveragefilling processes; labeling and marking equipment; palletizers; mechanisms that control the flow and placement of packing materials such as plastic peanuts; kick arms that push packages from conveyors to bins, seen in luggage handling and package sorting; feed augers that control product flow into containers; bag-making and box-making machines; closing and sealing devices; equipment for bundling newspapers and magazines; as well as the many ergonomic tools that position, lift, and convey products, containers, and packing material in order to cut down on workers’ physical efforts.

Zap on, zap off

Electric clutch-brakes are most often used with single-speed motors ranging from 0.5 to 10 hp. The motors can then run continuously at their set speed. To index or position a load, the clutch engages and disengages from the motor while the brake brings the load to a stop at its required position. The clutch disengages before braking occurs, so setting the brake does not slow or stop the motor. Keeping the motor constantly running means the motor’s inertia is readily available to help get the load started. This setup avoids the high starting currents that would otherwise be required to bring motor and load to speed, and allows fairly frequent stopand- go cycling.

Start-stop cycles can be actuated by a timer, photo sensor, limit switch, proximity switch, or Hall sensor, among other types of sensors. The cycle begins with the electric clutch powering down to disengage, and the brake powering up to set itself. A switching relay in the control panel sends an output signal to one of the coils to prevent the two actions from overlapping and causing the brake and motor to strain against one another.

The set and release times that can be achieved are affected by numerous factors, including the electrical circuitry and the response time of the photocell, relay, or other type of sensor that controls input power to the coil. Furthermore, the buildup of coil voltage required to pull in the armature will vary according to the physical size of the coil and its winding. Times for power-up and power-down typically range from 50 to 350 msec.

If it’s necessary to bring the armature in more quickly, it may be possible to overexcite the coil by overpowering it briefly – for example, at 230 Vdc for a half-second before dropping to a 90-Vdc holding voltage. Another approach uses a built-in circuit to cause a faster decay of the electromagnetic field and shorten release time.

The performance requirements of a clutch-brake are largely based on speed and required stopping frequency. Generally, anything more rapid than 20 cycles/ min may require the use of an overexcitation circuit. Also, because the frictional coefficient of the clutching and braking elements changes with temperature, there could be differences in braking or clutching times as the component goes from cold startup to full running speed.

In operations such as beverage filling, where spillage is possible, it may be necessary to avoid jerk. A soft start or stop can be achieved by using a coil voltage that’s lower than the actual coil rating. By controlling input to the armature, the action can often be extended to one or two seconds.

Positioning accuracy may depend on a conveyor’s linear speed, the reaction time of the controls, the size and nature of the load itself, and, obviously, the action in the coil and the friction surfaces. Clutch-brakes usually provide repeatability within 4 to 6 in. of true position; they are not highly suitable if the placement needs to be dead-on unless the linear speed is slow. In other words, use them where there is a window of acceptable location rather than a finite stopping requirement.

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Go configure

Equipment designs often include a clutch or clutch-brake from the beginning. Many times in the field, however, machinery that is not so equipped must be modified to meet changing requirements – perhaps the process flow needs to be altered or cycle time increased. In these cases it’s necessary to deal with the constraints, to work with what is available. Usually such situations involve stock single-shaft motors (rather than double-shafted motors that are better able to take on a clutch-brake). This calls for a coupler-type clutch or clutch-brake, mounted between the motor and the load, either as a free-standing unit or as a C-face attached to the motor housing. Besides being placed just beyond the motor (where input speed is often around 1,725 or 1,750 rpm) the clutch -brake can be located at mid-reduction (past the first speed reduction, which is usually around 3:1 or 4:1) but before the gearbox representing the final reduction. The low-speed end is an unfavorable location because the clutch-brake speed should be at least 200 rpm for proper burnishing or run-in. Furthermore, since the highspeed side is also the low-torque side, a smaller clutch-brake can be used without sacrificing reliability.

Clutch-brakes can be obtained as modular, ready-made units. This approach is beneficial from many design standpoints. Enclosed construction is a common feature, protecting against paper dust, drips, and the like, which can impair the performance and shorten the life of the component. Epoxy coating (for the food industry) and washdown capabilities can often be made part of the package.

Standardized as they are, modular clutch-brake designs can be configured with numerous types of bases, output shafting, and other add-ons. This lets a facility stock a single modular clutchbrake unit as a spare for several applications, as it can be quickly customized to fit into a particular section of machinery. To further broaden and simplify their use, some modular clutch-brakes are made to handle both in-line and overhung applications, and can be positioned either vertically or horizontally.

Modular clutch-brakes tend to be “consumable” products that are installed, wired, and left to run. They are seldom repaired or adjusted, and are replaced in their entirety when worn out.

Sizing considerations

Because of the fast, repetitive action to which clutch-brakes are subjected, the industry applies a healthy service factor (typically 2.75) in the selection process. Usually selection is based on the clutch-brake shaft speed along with motor horsepower or frame size. Tables often provide basic ranges for these parameters, but sometimes there is uncertainty about whether a basic model fills the requirements. Extreme factors may warrant selection and analysis that goes beyond manufacturers’ published figures and ranges. The result may be that the designer decides to use a higher service factor to account for harsh conditions, and standardized catalog ratings must be adjusted accordingly. When general tabular ranges for horsepower versus rpm are insufficient, the average dynamic torque is calculated.

Other drive components operating at various speeds may reflect rotary or linear inertia to the clutch-brake, and this inertia may figure into, and somewhat complicate, the selection process.

James Klann is Field Service Manager and Patricia A. Watson is Product/Application Manager with the Stearns Div. of Rexnord Corp., an Invensys Company, Milwaukee.

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Clutch-brake plus closed-loop controller offers economical alternative to servos for precision motion systems

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