Common mistakes with clutches and brakes
By Joel Hable
Perhaps the first potential common mistake in motion control is not specifying a clutch and/or brake when necessary. Specifically in applications with high cycle rates, for accurate positioning in mot...
By Joel Hable
Perhaps the first potential common mistake in motion control is not specifying a clutch and/or brake when necessary. Specifically in applications with high cycle rates, for accurate positioning in motion control applications, and for large starting loads, motors alone are quite limited.
Whether or not clutches and/or brakes should be specified depends on the application details, but certainly as cycle rates exceed 10 cycles/min. they should be considered. However, even in cycle rates below 10 cycles/min., clutches and or brakes might be necessary in applications with larger loads, in low-friction systems when shorter stopping times or distances are required, and in processes requiring accurate positioning.
Getting more specific with regard to traditional electromagnet clutches and brakes, a common mistake is in sizing these devices in applications operating below 100 rpm. This is most evident at the extreme of low speed and static engagements, where electromagnetic friction devices will not maintain rated torque. Conversely, spring set, electrically released brakes, ideal for zero-speed-engagement holding applications are often not optimum for continuous dynamic stopping (Fig. 1).
Wrap spring clutches and brakes are simple and effective, accurate, non-accumulative error indexing and positioning devices. However they operate in a relatively narrow band of application parameters. Problems related to too much or too little speed and inertia are common (Fig. 2).
Closely related to clutches and brakes are the controls used to power them. While controls are a critical part of the clutch/brake system, they are often not given proper consideration at the time the system is designed. Users will often call their supplier for help at the time the clutch and/or brake is being installed, realizing they need something to control it. Some specific oversights involve ‘permanent magnet’ electrically released brakes, where an adjustable output control is required to effectively disengage the brake. Fixed output controls are often used in these applications.
Certain electromagnet clutch/brake controls require customer-supplied switching. Since clutch brake systems can cycle in the range of many millions per year, electro-mechanical relays with only hundreds of thousands in their cycle life rating can be a maintenance issue.
Problems that occur
Overheating and premature failure of motors are often the result of cycling a motor alone too fast. Low throughput, poor quality and high scrap can often result without the fast and accurate starting and stopping of a clutch and brake. Variable frequency drives alone are also limited in cycle rates and the quickness and accuracy of stopping and holding.
Engaging a traditional electromagnetic device continuously below 100 rpm will not maintain rated torque and can result in inconsistent stopping and compromised load moving and holding. Starting a very large inertia or friction load without a low or no-load starting clutch unnecessarily stresses the motor and puts an extreme load on the electrical service.
Operating a wrap spring clutch with too much speed and inertia will result in breaking a spring. Operating with too little speed and inertia do not allow the clutch and brake springs to fully wrap and unwrap, causing inconsistency in positioning and premature wear of the springs.
A fixed output (clutch) brake control may not fully release the armature of a permanent magnet electrically released brake, causing armature drag, excessive heat, premature wear and failure.
For high cycle-rate applications, it is simply necessary to do a complete and accurate application analysis, being careful to take into account all load inertias, speeds and potential sources of friction, to maximize cycle rates and maintain repeatability.
In spite of the growing popularity of VFDs, they are limited in controlling motion and providing controlled stops and holding. Even against servo motors, because of the extremely high torque to internal inertia ratio of a typical electromagnetic clutch/brake-combination, clutch/brakes can match or exceed a servo’s accuracy, particularly on a cost/value basis (Fig. 3).
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 (Fig. 4). It 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.
Avoiding other specific pitfalls, in low-speed/ no-speed engagement applications, pre-burnishing (pre-running the clutch or brake under certain conditions) helps, but it is not always a permanent solution. Over-sizing is effective if spacing and proper funding is available. This is a common situation on the output side of a speed reducer, when applications require the decoupling of the load so it can be moved independently of the drive.
The most elegant solution is an electromagnetic tooth clutch. The caution here is it can only be engaged at zero speed. For zero-speed-engagement brake applications, a simple spring set brake is a cost-effective solution. But many applications require a combination of low/zero-speed and higher speed (>100 rpm) engagements. For these applications it may be a good idea to get the manufacturer’s technical support involved.
In starting a large load, a fluid coupling or centrifugal clutch is effective in reducing the starting load on the motor and the electrical supply.
Wrap spring clutch and brake applications are another place where accurate system analysis is crucial in achieving the desired performance and maximizing life.
Regarding controls, there are numerous options on the market today, making this another good opportunity to take advantage of the manufacturer’s technical support for assistance with your application. Some specific features of clutch/brake controls that can help avoid problems and improve results are adjustable outputs, internal transistor switching and over-excitation (OEX).
An adjustable output control is required for all permanent-magnet electrically released brakes to get a clean release of the armature.
There are a number of mounting and switching options to accommodate most application requirements (Fig. 5).
To avoid maintenance issues with electromechanical relays, there are controls available with internal transistor switching, giving millions of cycles of maintenance-free service.
When ultimate repeatability is required, and OEX-control shortens the coil current build time, reducing variations in engagement times and variation in position control is necessary.
Conversely, when clutch or brake engagements are too abrupt, disrupting material flow, unnecessarily stressing power transmission components and supporting structures, or causing noise and vibrations unacceptable to personnel, an adjustable control can soften the engagements to acceptable acceleration rates.
Joel Hable is a senior application engineer with Warner Electric. Technical support is available from the company by calling 1-800-825-9050.