Actuation Types for Force Control Clutches and Brakes
Selecting the correct actuation for your Clutch Brake or Posistop Brake
What is actuation type? Engaging a Force Control Clutch or brake requires putting pressure on the friction stack squeezing the discs and plates together. This is done through the use of a centrally located piston (or pressure plate in the case of the electric MagnaShear brakes). Pressure is applied to the piston or pressure plate either through spring force, air or hydraulic pressure. This is actuation. This combination of design, and pressure media is called actuation type. In addition there is logic type for Posidyne clutch brakes which is the design combination of springs and controlled pressure.
Controlling the actuation pressure determines the torque transmitted by the clutch or brake.
Spring set – Springs can be used to apply pressure to the piston exerting pressure on the friction stack. This is typically used to engage the brake stack. In a clutch brake the brake can be engaged only by springs ( A and B logic), or in combination with additional air or hydraulic pressure. Brakes are typically spring set, but may be pressure set, spring release. Spring set brakes are often used to assure the brake engages if control power is lost.
Pressure Set – There are two types commonly used in Force Clutches and brake, air and hydraulic. There are advantages to both and normally relate to the type of application.
Air Set Actuation – Air pressure is used to apply pressure to the piston applying pressure on the friction stack. This will cause the friction stack to apply torque to the output shaft. Using air to control a clutch or brake is common in factories, food processing plants, and other manufacturing processes where air is readily available. Air is efficient, easy to control, inexpensive, and environmentally clean. Force Control clutches and brakes are designed to operate on air which not only provides quick actuation for high cycle application, but offers some cooling benefit. Air is typically supplied from a main compressor system and piped to the application. For control purposes pressure regulators and solenoid operated control valve are needed.
Hydraulic actuation – Hydraulic actuation offers the advantage of being self contained for use outdoors and is often used where air is not available. It is also useable in cold temperatures with no problems with freezing. The transmission fluid used for the oil shear technology is also designed for hydraulic pressure systems so the same fluid is used for both actuation and cooling.
A typical external hydraulic actuation package for the Posidyne brake or clutch brake includes an electric pressure pump, control valve, over pressure relief valve, filter and in some cases an external cooling system. This provides the longest life for a clutch or clutch brake as the fluid is being filtered removing any metallic particles, and the lack of rust or corrosion build up in the piston area reduces piston leaks.
Electric Actuation – Electric actuation is only available in the MagnaShear brake product. These are spring set motor brakes, electrically released. Springs exert pressure to the pressure plate which in turn applies force on the friction stack. The torque is determined by the force of the springs. An electromagnetic coil pulls the pressure plate back to release the brake.
These are ideal for cranes, conveyors, and other areas where a motor brake is needed and air or hydraulic are not a viable option.
Valve Installation and Design Suggestions
The internal piston volumes of Force Control Clutch/Brake and Brake Products are quite low. The control valves should be located as close as possible to the unit, as this directly affects the response time and consistency. Many of the products have manifold mounted valves available, which is the best arrangement because it eliminates the plumbing between the valve and the Force Control unit.
Air Line Sizes and Fittings
The optimum air line size is 3/8” for sizes 01.5 through 10 Posidyne clutch/brakes, and 056 through 280 Posistop brakes. The size 5/8” for Size 11, and 3/4” for size 20 and 30. 20 Posidyne clutch/brake and size 320 Posistop brake should have 1/2” air lines. The fewest number of fittings should be used and all fittings should be maximum flow type. A tee and pressure gauge located near the actuation port is often helpful for troubleshooting.
In High Cycle applications, for the best response and consistency, accumulators should be used – one for the clutch and one for the brake on Posidyne clutch/brakes. This will maintain a constant pressure to the unit. Regulators should be located on the inlet to the accumulators. Accumulators should be sized at 10 times the volume per engagement.
Air Line Connections and Air Supply
Both top and bottom porting is supplied in many of the models. Whenever possible, bottom porting is recommended to purge any contamination from the piston chamber. The air supply should be dry and free of all contamination. The cleaner the air is the longer the control valves and drive unit will last. Lubricated air will make the control valves last longer but... too often oil will fill up the piston chamber with oil and cause sluggish action of the piston.
Accurate pressure gauges should be used in all applications to maintain the proper operation. Glycerin filled gauges are recommended because cycling the dry gauges do not hold up, and often give false readings.
High Speed - High Accuracy Applications
For High Speed and High Accuracy Applications the system should be equipped with a consistent air supply, accumulators and hoses of the proper size, Glycerin gauges, manifold mounted valves, and 24 VDC control voltage.
Solenoid Control Valves
Solenoid control valves are available in several voltages depending on application. 120 volt AC and 24 volt DC are most common. For high cycle applications or those where accurate positioning is required DC voltage is recommended. External pilot is another option for best response, and highest cycle rates. The external pilot applies pressure to move the spool at a fixed pressure rather than the regulated pressure normally used. This is especially applicable to systems where a low pressure may be used to actuate the clutch brake.
The standard valves are water resistant and approved for outdoor use. Special valves are required for hazardous duty applications.
Quick Exhaust Valves
The Quick Exhaust valve is used to allow air to escape directly to atmosphere rather than going back through the line to the solenoid valve. This method is particularly effective for high cycle clutch brake applications and brakes where quick response is needed when the solenoid valve is not mounted directly on the unit.
Many of the positioning problems associated with the clutch/brake can be traced to the control system. PLC controls often include scan time delays depending on the speed of the control and number of lines of code used. High-speed cards may be required. The type of limit switches can also cause position error.
Force Control has developed the CLPC Closed Loop Positioning Control which eliminates scan time problems. The CLPC will correct positioning errors and compensate for cold start to hot run shifts, as well as adjustment for changing speeds, loads and other variables.