- Consider total system inertia.
One of the most important — and most overlooked — steps in selecting the appropriate brake for a drive system is considering the system's total inertia.
A standard electric motor brake usually can stop the driven load as often as the motor can safely start it. However, in more demanding applications, such as those with high inertial loads, frequent cycling, and where short stopping times are important, total system inertia should be calculated to accurately determine the brake's requirements and suitability.
The total reflected inertia at the brake, combined with shaft speed at the brake, determines the amount of energy that must be absorbed by the brake during stopping. The brake must be able to absorb this energy as well as dissipate the heat associated with doing so.
Applications with high inertial loads or frequent cycling require a brake with higher thermal capacity. If the brake's thermal capacity is surpassed, excess heating occurs, resulting in premature brake wear, loss of torque, or brake failure. Total inertia is also used with shaft speed and dynamic brake torque to determine the time required to stop a given load.
Thus, for a given brake torque rating, an increase in total reflected inertia results in longer stopping times. Applications requiring short stopping times may require a brake with a higher dynamic torque rating.
William Tarr, Baldor Electric Co.
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- Properly dissipate excess heat.
Beyond the basic assumption that more cycles lead to a shorter life for the clutch or brake, the other factor to consider is heat. Every time a clutch or brake engages, heat is generated between the contact faces. Depending on the inertia and speed, temperatures can exceed 400°.
If a clutch or brake is cycled to the extent that it generates more heat than it can dissipate, the heat will cause damage (warping, cracking, melting) and can literally burn up the friction surfaces.
In electromagnetic units, the heat rise can reduce the magnetic flux generated; this can reduce the ability of the clutch or brake to generate torque and cause slippage if the field degrades too much. Too much heat also adversely impacts other components, such as seals and bearings.
Frank Flemming, Ogura Industrial Corp.
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- Beware of the effects of brinnelling.
If a clutch with a ball bearing is engaged for long periods of time (no relative movement between input and output of clutch) and vibration is present in the application, brinnelling can occur to the bearing races. This indentation of the races causes vibration when the bearings rotate, which can cause an objectionable noise and eventually, bearing failure. Brinnelling often goes undetected in initial lab prototyping and may not show up in an application until after significant field hours. The amount of damage to the bearings is a function of the vibration level and can vary from one machine to another. Although alternate bearing designs can solve this, a simple trick to prevent brinnelling is to induce occasional cycling into the clutch to allow the balls to move and lubricate the bearing races.
Frank Flemming, Ogura
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- Think about safety first.
Selecting the wrong type of brake is a concern, especially when worker safety is involved and people are working in and around the operation. We often ask questions about safety when we're consulting with an end-user. Many times, they don't think about the safety implications of a hydraulically or pneumatically applied brake if power is lost. In those cases, it is better to have a spring-applied brake with a hydraulic or pneumatic release. That way, if power is ever lost, the brake will engage. Another issue is over-designing a brake, especially for an in-plant operation. Specifying a hydraulic brake where pneumatic power is readily available and fully capable of doing the work requires the installation of an intensifier to convert pneumatic power to hydraulic, with added expense and reduced efficiency. To avoid over or under-designing a brake application, it's important to carefully calculate the amount of dynamic and static force required.
Keith Hogan, Tolomatic Inc.
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- Consider a motor-brake module.
For high cycle applications, it's important to do a complete and accurate application analysis (taking into account all load inertias, speeds, and potential sources of friction) to maximize cycle rates and maintain repeatability. In spite of their growing popularity, variable frequency drives are limited in controlling motion and providing controlled stops and holding. Even against servomotors, because of the extremely high torque-to-internal-inertia ratio of typical electromagnetic clutch/brake combinations, clutch/brakes can match or exceed a servo's accuracy — particularly on a cost/value basis. There is no substitute for brute force. If you have a modest cycle rate, say 10 cycles/min, your cycling motor application may be enhanced by a high performance, dynamic cycling, permanent magnet motor-brake module. Such a module takes half the heat out of the motor and drive, dissipating it in the brake, and provides long life dynamic stopping and no-power holding.
Joel Hable, Warner Electric
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- Consider a quiet-duty rotor.
The use of a variable frequency drive allows controlled acceleration and deceleration, as well as variable speed operation. However, at some speed point, the motor may develop a harmonic vibration, which causes a rattle between the hub and rotor of most brakes. Typically, this does not damage components, but the noise itself can be a problem or cause concern. To eliminate the rattle at critical frequencies and improve lubricity between the spline rotor and hub, consider using a quiet-duty rotor with a plastic sleeve inserted in the rotor's spline bore.
Alex Himmelberg, Lenze-AC Tech
- Know when to use a flexible coupling.
Sometimes a less experienced engineer assumes that a brake (or clutch) can be run at maximum torque and slip rpm simultaneously. However, heat dissipation limitations reduce the allowable simultaneous application of torque and slip rpm to a fraction of the maximum of each individual rating.
Improper mounting is another concern. Whenever a shaft that is fully supported by bearings (at the brake or clutch) is connected to another shaft that's also fully supported by bearings (at the load), a flexible coupling must be used to avoid destructive bearing loads due to unintended misalignment.
Jeff Pedu, Placid Industries Inc.
- Choose the right design for the job.
The wrong type of clutch or brake may have a shorter life or not perform as expected. Each type has subtle characteristics that make one a better choice for a certain application than another. For example, magnetic particle and hysteresis brakes are both suitable for precise torque and tension control, but a hysteresis type works well at all rpm. A magnetic particle type may not slip smoothly at dead-slow slip rpm, and may overheat or fail faster at high-slip rpm. That said, for load simulation, a magnetic particle type is better than a hysteresis type. Torque and rotation direction can be changed at any time without the unusual output torque that's typical of a hysteresis type during these kinds of transitions.
Jeff Pedu, Placid Industries Inc.
- Bigger is not always better.
When selecting power transmission components, specifying one size larger is often advantageous. This is not true of clutches and brakes. If the torque is too high, it can cause banging, severe shock loads, broken components, and other failures. In addition, the internal inertia of the unit may place undue load on the motor or require that the complete drive system be upsized.
Stan Porter, Force Control Industries
- Understand and use correct terminology.
A few guidelines: Know the operating conditions of the machine and provide the clutch and brake manufacturer with drawings of the application, showing component descriptions to communicate how the machine works. Be sure to identify the machine's function as well as type of application:
Cyclic start or stop: Accurate positioning, inching and jogging, indexing. Consider response time, inertia, speed, and thermal capacity.
High inertia start or stop: Controlled acceleration/deceleration or emergency stops of heavy loads in a specific time period. Thermal characteristics and torque are important considerations.
Continuous slip: Used for winding and unwinding of materials on a roll or spool. Heat dissipation and facing wear life are primary concerns.
Occasional starts or stops: Connect/disconnect or holding applications where cycle rates are less than five per minute. Transmitted torque and horsepower are primary considerations.
Also provide details and specifications of the machine's components that interface with the clutch or brake.
Drive components used with clutches and brakes typically include motors, gear reducers, couplings, shafts, bearings, sprockets, pulleys, chain, and belts.Terminology commonly associated with clutch and brake selection:
Torque: Transmitted horsepower (hp) and speed (rpm) must always be considered. The relation to each other is expressed as: Torque (T) = 63,025 (hp)/rpm
Rotational inertia: Commonly called WK2, it measures the unit's resistance to a change in rotational speed. It is expressed in lb-ft2 and is an important consideration in cyclic applications.
Friction facing life: Friction facings are the consumable component in clutches and brakes and have a usable volume of material measured in hp-hours of work or work-energy capacity.
Dividing the work-energy capacity by the energy produced each cycle gives the number of cycles before facing replacement is necessary.
Response time: The time when control power is turned on or off until the clutch or brake reaches a percentage of torque output.
Energy consumption: Typically, clutches and brakes are activated by a coil or by air pressure. Both modes of operation require solenoids; the solenoids' power consumption is typically rated in watts.
Limiting speed: A product's limiting speed is determined from the bearing manufacturer's speed limits and the safe operating speed of the spinning part's material.
Thermal capacity: A unit's thermal capacity is rated as hp or an equivalent value of ft-lb per minute. Continuous thermal dissipation is a measure of the average rate that heat is generated at the interface without causing damage to seals, bearings, and interface components.
Edd Brooks, Nexen Group Inc.
Baldor Electric Co.
Ogura Industrial Corp.
Placid Industries Inc.
Force Control Industries
Nexen Group Inc.