Brakes and clutches

Productivity of a system is characterized by uptime and output. Designers, component makers, and endusers all affect it. In this report, the editors of Motion System Design polled brake and clutch experts for their advice on optimizing productivity with this particular component. Here are the responses, which we believe you'll find most helpful.

What particular design and construction features in clutches and brakes contribute to higher productivity, and why?

Greg/Orttech: Clutches and brakes reduce productivity when they're unable to start and stop frequently, or when they take too long to stop. Many newer designs use oil to cool friction surfaces, dissipating energy that normally increases start/stop frequencies. Newer electromagnetic-clutch or brake designs also optimize current use, thereby reducing the heat generated. Lastly, newer clutch/brake combinations have increased brake torque compared to clutch torque for faster stops at higher speeds.

Ravi/Eaton: Selecting, sizing, designing, and testing friction material for optimal brake or clutch functionality is critical to productivity. Assembly ease improves maintenance, longevity, and performance. Adequate heat dissipation also affects energy handling. Finally, adhering to design and machine tolerances produces smooth functioning.

Greg/Warner Electric: Ceramic friction materials provide three times the friction face life (at cycle rates of 100 per min.) compared to standard materials for the same application. Also, over-excitation in the control circuit can decrease unit engagement time and permit rapid unit cycling.

Jeff/Placid: For best productivity, select the right clutch (or brake) for the job. Magnetic particle and hysteresis clutches and brakes are best for tensioning (winding systems and load simulation applications). A friction-type clutch has less accurate torque control, and shorter life. Critical to performance and life is heat dissipation. A hysteresis brake can dissipate more heat than a comparable magnetic particle brake, with similar rated torque ratings. But, hysteresis brakes and clutches are physically large (and expensive) for their rated torque.

Galen/Dodge of Rockwell: Wear-resistant and tough materials better maintain an effective coefficient of friction between mating components. Additionally, brake manufacturers can extend friction disk life by designing proper shaping characteristics and minimizing the number of moving components in the unit. Utilizing diodes, internal rectifiers, and metal-oxide varistors (MOVs) in the design can improve the unit's electrical forgiveness.

How can designers increase productivity from the clutches and brakes they place in machines? What mistakes do endusers sometimes make?

John/Deltran of Danaher: Many problems can be traced to misapplied end-user installation, which causes clutches and brakes to appear application-sensitive. The application's speed, inertia, friction, radial and axial forces associated with mounting hardware, and plate attachment (to the stationary frame) affect performance and life.

Galen/Dodge of Rockwell: Typically, the more parts in a product, the more likely the machine will fail. Minimizing, standardizing, and selecting high-quality components yield the best results. One example is in dc brakes, where fewer components are found, parts are quiet, and long life spans are achieved.

Greg/Warner Electric: A key step in maximizing unit life and productivity is to choose units based on manufacturers' selection criteria. Similarly, following their guidelines on mounting tolerances ensures proper functioning. If units must be mounted or used in a way that does not meet recommended guidelines, manufacturers should be consulted to avoid problems.

Greg/Orttech: Designers should consult clutch and brake suppliers, rather than choosing products from a catalog based on torque. Oftentimes, these suppliers have unique products not advertised in catalogs and can help designers optimize productivity.

Another design fault is a lack of information. Often, machine designers don't research clutch and brake details, leading to problems. For example, they might estimate the inertias or add too much safety factor to the torque. This makes for a larger clutch or brake and reduced drive productivity. However, a bigger clutch or brake is not better. Installing a bigger clutch or brake increases the system inertia, which increases the starting and stopping times and the amount of heat generated. Increased heat generation reduces the life of various clutch and brake components. Longer cycle times and more downtime lowers the overall productivity of the machine design.

Galen/Dodge of Rockwell: System designers should consider proper brake and clutch selection. For instance, if a brake is too strong, the sudden impact of braking may lead to other system problems, such as gear teeth shear or key wear. A comfortable stop is often best for the entire machine. On the other hand, the brake or clutch can be undersized. An example: A car rolling past a stop sign after the driver's attempt to stop before it.

Jeff/Placid: Magnetic particle brakes and clutches are best used in lower rpm applications. Tip: Place a magnetic-particle clutch after gear reduction, rather than between the drive motor and gear reduction. Sizing a magnetic particle clutch or brake generously improves life. Since inertia of this type unit is small, system performance is not affected. But hysteresis units have significantly higher inertia, so choosing the smallest unit improves performance. Since hysteresis brakes or clutches generated torque across an air gap, life is not affected by running at maximum torque, even continuously.

Ravi/Eaton: OEM designers should provide application details such as hp, inertia, speed, and stop/start times to clutch/brake manufacturers. In addition, machines should be designed by exploring the application at hand in order to consider all alternative technologies and solutions.

What can endusers do to ensure higher productivity from the clutches and brakes on their machines?

John/Deltran of Danaher: Performance and life problems can ensue during and after installation. Most pertain to rigid clutch-plate mounting, accumulating foreign materials on outer surfaces, stopping accuracy, breaking springs, or solenoid actuation problems. For fairly new units, adding one inch of radial and axial movement corrects intermittent clutch performance. Older units may need to be replaced.

Galen/Dodge of Rockwell: Since brakes are wear components, they require long-term maintenance. Many clutch/brake designs have airgaps between friction materials and pressure plates. Over time, these airgaps may need to be reset, due to wear on friction material faces. But some clutch/brake designs address this issue, automatically resetting airgaps as the friction material wears away. Since all brakes and clutches operate on friction and pressure, the presence of oils, greases, or other lubricious substances drastically affects operation. Therefore, it is best to keep these materials from contacting the surfaces. When this isn't possible, units offering washdown features are useful.

Greg/Orttech: Productivity depends on cycle times and uptime. That's why endusers should make sure the machines they invest in include clutches and brakes from a credible source with available spare parts. It doesn't matter if the initial cost of the machine is less if you have to wait weeks to a get a spare part for the clutch or brake. Uptime also improves when endusers specify newer clutch and brake designs. Many times, OEMs are unwilling to redesign a newer clutch or brake into their machine, so endusers end up with older designs. For this reason, endusers should understand what is current and should not assume their machine OEM is giving them the most current technology.

Give one example of how a designer or enduser improved machine productivity by good design and maintenance practices.

Greg/Warner Electric: A steel heat-treat operation in Chicago wanted to control material flow more accurately within an oven. They installed a clutch coupling in the middle of a line shaft, but did not address proper shaft alignment and bearing support. Clutch couplings require that input and output shafts align within certain limits, which was disregarded. In little time, the unit's output hub wore to the point of failure. Adhering to tolerances in the catalog could have prevented this.

Galen/Dodge of Rockwell: One customer contacted us to discuss gearbox failures. After detailing the application and inertia and cycle rates, we determined that the current brake's static-torque rating was too high. The impact of a sudden stop initiated by the brake did not dampen the load through the machine system. Rather, it produced excessive stress on the gear teeth and ultimately, gear failure. After further review, we recommended a brake with less static torque, which could better dampen the load. Because the stop time was not as abrupt, the gearbox was not damaged. This is an excellent example of how proper component selection is vital to improved machine productivity.

John/Deltran of Danaher: Most applications mount horizontally with the clutch supported in the center by two pillow-block bearing supports. Vertical mounting generates additional forces through the weight of the clutch and the mating drive hardware. Endusers requiring vertical mounting tend to mount the input hub at the six o'clock position. They also attach the mating drive hardware to the input hub, which adds weight to the clutch assembly. During operation, this added weight causes premature wearing of the input hub and retaining ring surfaces, leading to clutch failure. The failure is usually a drive spring fatigue break, excessive radial concentricity, vibration, noise, or the unit starting to disassemble.

For vertical mounting applications, the input hub should face the twelve o'clock position. The two pillow-block bearings must support the clutch and ride against the output clutch shaft. Mating drive hardware should have a bearing (sleeve or ball) in the bore, use drive pins to couple to the input hub, and must not be hard mounted to the clutch, but float freely.

Ravi/Eaton: In response to one mining company's request, we developed an eight-foot diameter, air-actuated clutch for grinding mills. The drum clutch transmits 7.3 million lb-in. of torque with 9,500 hp at 200 rpm. Materials science technology and innovative engineering reduce peak-electricity demand.

Ore is pulverized in large grinding mills that demand high-capacity drive systems, as they typically start from a stationary position. Therefore, it's critical to manage peak loads to minimize power costs. Large drum clutches allow grinding mill motors to start in the unloaded condition, keeping current demand within limits agreed upon with local utilities. The result is a substantial savings in power costs. No-load starting also prevents blowing fuses or opening circuit breakers.

The first of three drum-clutch systems ordered was for a gold mine in Nevada. A low-torque synchronous motor using such a clutch reaches peak amperage of 2.5 times a normal working motor (as opposed to the high-torque synchronous motor).

Greg/Orttech: Many machine designs today require a combined clutch and brake. Older designs with separate clutch and brake require a dwell time between when the clutch disengages and the brake engages, reducing the machine design's overall productivity.

Jeff/Placid: Winding rolls of paper makes a lot of paper dust that coats everything in the machine. Machine operators are often tempted to clean their machine by an air gun. But this drives the dust into the bearings of any clutch, brake, motor, gearbox, and shafting, causing early bearing failure. Tip: If dust must be removed, vacuum and wipe with a rag instead.

Also: Many winding systems have variable-speed motors that drive the clutch input shaft. It's important to drive magnetic-particle clutch input shafts just a little bit faster (30 to 50 rpm is ideal) than the output shaft for low heat input and least wear. Excessive slip rpm shortens life with no improvement in performance.