Shear transmission sheer genius

In oil-shear brake and clutch units, friction plates and oil work together to slow and accelerate systems. Consider oil-shear clutching: Upon startup, rotating input discs move toward stationary output discs, and oil between them shears to make the output shaft rotate. Torque is transmitted through the oil films until the speed of the output shaft reaches that of the input. Besides transmitting power, oil films dissipate heat generated by the dynamic torque of engagement, and lubricate to prevent wear. That makes oil-shear components suitable for a variety of tough applications, including constant-slip tensioning and on systems with adjustable speed drives.

The discs that do it

A clutch disc pack or stack is that bank of steel discs (or drive plates) keyed to the input shaft, which alternates with carefully textured frictional discs splined to an output. (In a brake disc stack, the steel plates are keyed to the housing.) During acceleration, torque transfers from input to output shaft through viscous shear in the oil between plates.

Frictional discs include grooves that boost torque ratings. Within the limits of a particular unit, the amount of torque transmitted is directly proportional to the amount of pressure applied to the clutch stack. To get significantly more torque, two options exist: Increase either the diameter — or the quantity — of the discs inside. Keeping units small offers both design and inertial benefits; a disc's inertia increases by the fourth power of diameter, and torque increases proportionally with diameter or number of discs. So, many oil-shear units have several discs, sometimes even dozens. There is no derating for multiple-disc construction, and adjusting the number of discs (and actuating elements) allows many torque ratings from one brake size.

What spurs them to action

Fluid power or spring actuating elements press oil-shear units into motion. In clutching, the actuating system applies pressure to a nonrotating central piston that in turn exerts clamping pressure on the disc stack. In braking, the actuating system forces the piston toward a stationary stack, and the piston clamps the disc stack that way. For units that use external brake actuation, light-duty return springs are often used to move the piston and provide some braking in a loss of power situation; heavier springs can be used to provide “safety” brakes. Loss of electric power or actuating pressure automatically releases oil-shear clutches and engages brakes.

Controlling oil-shear units

With spring-set oil-shear brakes, electrical releasing action requires no plumbing and is economical. Here, the brake stack is released when 115 or 230 Vac power (either 50 or 60 Hz) is supplied to the brake coil. This is helpful on cranes, winches, conveyors, palletizers, and other start-stop devices. However, applications requiring the brake to be released more than half the time, or for long durations, must typically be reviewed and approved by the manufacturer's application engineering department.

Pulse width modulation (PWM) control is used to minimize heat buildup. Control logic is simplified with motor starter auxiliary contactors, and back EMF effects from the motor windings are eliminated. Besides their cushioned engagement (see sidebar on oil's useful behavior) oil-shear units also offer adjustable engagement time; depending on control logic and methods of actuation and cooling, this can be set for any period from 50 msec to several minutes. Static torque ranges vary from 6 to 12 lb-ft up to 450 to 1,250 lb-ft for electrically released brakes. For hydraulic or air-released brakes, torques can reach 200,000 lb ft.

Sometimes (particularly on high-torque brakes) braking must be highly controlled — to gradually slow the decent of a big mining conveyor, for example. Instead of bringing the full braking torque on at one time, here designers install a closed-loop PID controller and adjust the torque based on the speed and load on the conveyor.

Integrated two-speed motor/clutch/brake units offer versatility with high speed, inching speed, and spring-set braking modes. High speed and inching-speed modes are selected by energizing the appropriate electronically controlled coil. The spring-set braking mode is activated when both coils are de-energized. An internal worm gear reducer provides inching speed output. (Here, the reducer shares the oil sump with the rest of the drive package for easier maintenance.)

Inching output rpm can be varied for an application by changing either the internal reducer or the feed motor/belt ratio, if this style is used. With a speed reducer, load inertia referred to the drive is reduced by the square of the reduction ratio. So, the majority of motor output goes to accelerating the drive, and most of the heat generated comes from starting and stopping the cyclic parts of the clutch/brake — useful in high-cycling work.

Sometimes, to run integrated units in high-speed mode, the clutch coil is energized to pull the clutch armature plate assembly away from the primary clutch stack. This releases the disk stack and allows the splined hub and main motor to rotate independently from the inching input assembly.

To stop the main motor, the brake stack is left engaged and the clutch coil is de-energized. This allows the primary clutch springs to push the clutch armature plate assembly to clamp the primary clutch disc stack and stop the drive.

To run integrated motor/brake units in inching mode, the clutch coil is de-energized and the brake coil energized to release the brake, and allow the worm gear to turn freely. Torque is transmitted from worm to worm wheel and through the unreleased primary clutch stack, which transmits torque to the splined hub and through the main motor. When the brake coil is de-energized, the brake springs force the brake armature plate to clamp the secondary brake stack, causing a stop. Because the primary clutch stack is clamped, the splined hub and main motor are also stopped.

Addressing heat buildup

Heat buildup, the mortal enemy of electric motors, can also destroy conventional motor brakes. Heat is the natural product of the braking process, but frequent starts and stops of the electric motor create intense heat in conventional brake devices. Often, heat buildup damages the brake as well as the electric motor — and frequently causes whole-system failure. This can lead to increased downtime and maintenance costs.

Well-sized conventional dry-friction brakes that can withstand industrial motor heat still sacrifice friction material with each stop — so they deteriorate with repeated use. Though the motor may be spared, the brakes still require routine maintenance in the form of coil and friction material replacement.

In oil-shear units, the heat of engagement is generated in — and efficiently circulated away by — the oil film. Typically, oil washes radially across the disc faces, and then collects in a sump where heat is dissipated through its housing. An internal water-to-oil heat exchanger further increases thermal capacity here. Oil-shear maintenance is minimal; an occasional leveling of oil is sufficient.

Installation

Converting existing systems to oil-shear braking is easy because the self-contained units have their own housings, bearings, and input and output shafts. In fact, some oil-shear units mount right to motors with universal mounting flanges and clamping split-quill designs. These stop all play and movement between the brake hub and motor shaft caused by high-torque and rapid-cycling applications.

Some oil-shear models can be furnished in multiple ways: to fit NEMA, U-frame, T-frame, or low-inertia IEC frame motors, to mount on a machine frame or other special mounting configuration, or as a complete motor-and-brake assembly.

Selecting the latter is particularly easy. First, determine what motor hp and rpm an application requires. Then determine the brake torque requirement of the application. (Consult published procedures for this step.) Next, select a correctly sized brake based on torque just calculated, and determine the correct motor specification. Finally, determine the motor/brake unit's mounting position. Contact a local distributor or the manufacturer if thedesign or application has special requirements; custom units are an option in such cases.

Where they're used

Oil-shear brakes are often used for applications in dirty and wet enviorments where position is critical and productivity is a magor concern. Other applications include situations in which the motor reverses each cycle. (Note that clutch/brake combination units are not as practical here, because a brake is needed to stop the motor before reversing.) Completely enclosed housings prevent contamination in these places.

Oil-shear brakes are also used on vertical or over hauling applications for their ability to dissipate significant heat. They can be sized to the correct torque, independent of the motor frame size or horsepower.

For more information, call (513) 868-0900, e-mail [email protected], or visit www.forcecontrol.com.

Look up Cranes … they run many facilities.

How many times have you driven down the street and marveled at a huge construction crane on a building site? In contrast, how many times have you walked through a manufacturing facility and failed to notice overhead cranes?

Useful as their more visible cousins, overhead cranes are lifeblood of many manufacturing facilities. Without these two to 50-ton cranes, production would stand still. But cranes are often the forgotten system when it comes to routine maintenance, and especially when it comes to upgrades.

Cranes are fairly simple systems - a drive system consisting of a motor, gearbox, brake, drum, wire, and of course, the hook — and a structural system consisting of rails, a bridge, and a trolley. As long as it's not overloaded or rusted, a crane structure should last 40 years or more. The drive systems are typically robust as well. Some cranes have as many as five drive systems — two bridge drives, two trolley drives and one hoist drive. The motors are very reliable, in totally enclosed and fan-cooled versions. Gearboxes typically used on cranes are designed with a good safety margin and often use high-grade synthetic oil that never needs changing.

The weak link tends to be the brake: Typical brakes have brake pads that must be continually adjusted and changed every one to two years. If they are not changed, the crane rolls five to 10 feet before stopping. The operator, in turn, learns to reverse the crane to stop it. This “reverse plug” damages the gearbox.

Case in point: AllState Steel Co. upgraded cranes at their steel fabrication facility in Jacksonville, Fla. Because the crane structure was in good condition, they only replaced drive motors and brakes. The oil shear brakes here eliminate brake maintenance and problems, and directly mount to the new motors. They are sealed from the environment and never need adjustment; AllState operators are satisfied with how well the cranes now work.

Sidebar: Oil's useful behavior

In oil-shear drives, friction surfaces operate in a constantly replenished oil film. Oil molecules tend to cling together and to friction surfaces. So, as moving and stationary elements are brought together, a thin but positive oil film is maintained between them, controlled by clamping pressure and designed grooves in the friction discs. Torque is transmitted from one element to the other through viscous shear of the oil film. As long as relative motion occurs between the elements, the oil protects them from wear.

Dynamic coefficients of friction can vary slightly under near-constant speed conditions, and considerably more with varying speed and temperature. However, the coefficient for wet surfaces tends to be more constant than that for dry ones. So a minor limitation in fast, constant-slip applications is that even when an oil-shear clutch or brake is disengaged, its oil film continues to transmit some torque. Drag is more pronounced with higher viscosity and relative slip speed, and smaller running clearance.

On the plus side, oil acts as a cushion; it absorbs the shock of engagement as the film is squeezed between discs and plates to reduce drive-train stress. So, oil-shear drives can produce many torques over a wide speed range — even developing high torque without chatter at speeds below 1 rpm, where other absorbers can't function.