Authored by:
John Pieri
Product Line Manager
Thomson Industries Inc.
Wood Dale, Ill.

Edited by Robert Repas
robert.repas@penton.com

Key points:
• Clutches typically transfer torque via in-line or parallel shafts.
• Most single revolution clutches can stop only about 10% of their load capacity.
• The torque capacity of a wrap-spring clutch or brake is a direct function of the diameter of the hub and the tensile strength of the spring.

Resources:
Thomson Industries Inc., www.thomsonlinear.com 

Clutches and brakes are widely used to transfer rotary motion from one shaft to another. Clutches typically transfer torque via in-line or parallel shafts. Brakes transfer torque from a rotating shaft to a motor flange or other object permanently affixed in a stationary position to stop or hold the shaft.

The two most widely used types of clutches and brakes are wrap spring and electromagnetic friction. Unique specifications make one type more suitable for some applications than others. Clutches and brakes come in a variety of sizes, generally ranging in torque and speed from 3 to 5,000 lb-in. and from 150 to over 20,000 rpm. Their specifications are spread over four modes of operation: start, index, slip, and hold. The start and slip modes are rated exclusively for clutches, the hold mode for brakes, and the index mode for both clutches and brakes.

Wrap-spring designs
Wrap-spring clutches and brakes are composed of three basic parts: an input hub, a wrap or coil spring, and an output hub. They transmit torque through the interference between the wrap-spring internal diameter and the outside diameters of the input and output hubs.

Wrap-spring clutches and brakes excel in torque capacity, low power, positive engagement, stopping accuracy, and come with a choice of pneumatic or mechanical actuation. They are the best choice for single-revolution operation and match electromechanical clutches and brakes for rapid-cycling capability. They are limited to unidirectional operation and maximum speeds of approximately 1,750 rpm. Depending on the application and torque requirements, wrap-spring clutches and brakes can be specifically designed for higher-speed applications.

Wrap-spring clutches come in overrunning, start-stop or random positioning, and single revolution types. A fourth type is a clutch-brake combination that uses two wrap-springs to form an overrunning clutch. When the input hub rotates at a higher rpm than the output hub, the spring wraps down to engage and lock the two hubs together. Should the input hub turn slower than the output hub, stop, or even turns in reverse, the spring unwraps to release the output hub and lets the load overrun. Overrunning clutches are also used for one-way indexing and backstopping.

The overrunning clutch may be modified to become a start-stop clutch by adding a control tang to the spring. The tang is used to engage and disengage the load by locking into position with the stop collar. When the tang is disengaged, the load coasts freely from the continuously running input.

To create a single-revolution clutch, a second tang is secured to the output hub. When the control tang engages, the output hub cannot overrun because it is secured to the spring. Most single-revolution clutches can stop only about 10% of their rated load capacity.

Wrap-spring clutch-brake combinations use two control tangs to hold the clutch or brake spring open. When the clutch and brake-control tangs rotate with the input hub, the clutch spring positively engages the input hub and the output shaft. When the stop collar locks the brake-control tang, the brake spring wraps down to engage the output shaft to the stationary brake hub. Simultaneously, the clutch spring unwraps slightly and lets the input hub rotate freely.

The wrap-spring clutch is the better choice when the load needs to come into synch rapidly, say, within a predictable time or travel. A friction device slips under certain conditions but a wrap-spring clutch or brake will not slip after the spring wraps down and locks the input and output hubs together. A wrap-spring device should be considered when slip mode is not appropriate for the application. Wrap-spring devices are quite cost effective due to their simple construction. When used properly, they are maintenance-free and do not require periodic parts replacement or lubrication. They install easily and have a long service life.

The torque capacity of a wrap-spring clutch or brake is a direct function of the hub diameter and tensile strength of the spring. The wrap-spring clutch or brake supplies torque demanded up to the mechanical limitations of the spring. When the drive spring wraps down to grip the hubs, the output hub typically accelerates to the input rpm in 0.003 sec. When the brake spring is activated, it stops the output shaft within 0.0015 sec.

The torque demand on the wrap-spring clutch equals the system frictional torque of the load plus the dynamic torque of acceleration. When approaching the stop position of the cycle, enough energy must be available in the rotating mass of the load to let the output hub unwind the drive spring from the input hub. This means that when there is a large frictional load or a torque demand when the load comes up to the top of a cam, the rotating mass must have sufficient energy to open the drive spring. Without it, the input hub could wear excessively and generate noise.

Dynamic torque of acceleration or deceleration is proportional to rpm multiplied by the load inertia and divided by the acceleration time. This fact indicates that spring clutches and brakes are inertia sensitive — the more inertia, the higher the dynamic torque. The torque demand of the spring clutch is equal to the system frictional torque of the load plus the dynamic torque of acceleration.

Selecting wrap-spring clutches and brakes
Three basic steps are required to select wrap-spring clutch/brakes: determine the clutch/brake function, determine the clutch/brake size, and then verify the design considerations.

First, determine the function that provides the best control for the application: overrunning, start-stop, or single revolution. Select the series that best fits the application requirements from the manufacturer’s selection charts.

Next determine the size of the clutch or brake needed. A quick way to select the correct clutch or clutch brake model from a manufacturer’s catalog is to consult an rpm versus shaft-diameter selection chart. Start with the shaft diameter in the bore size column and shaft speed expressed in rpm. Select the correct model from the clutch size model while staying within the shaded “stock shaft-diameter” area. If the shaft diameter is outside of the shaded area for the desired rpm, and no particular requirement for the shaft diameter is specified, consider using a shaft that would fit the chart selection criterion. However, if the required shaft is larger or smaller than the selection chart suggests, check with the manufacturer for a possible custom device.

Finally, verify that the clutch has enough torque to start the load, and that the load and shaft have enough inertia to activate the stop spring of the clutch/brake. Calculate the WR2 inertia of all rotating components, such as shafts, drums, pulleys, and so forth. A reference table is used to determine the inertia of steel shafting based upon its length and diameter. For materials other than steel use the conversion multipliers usually included with the listing. For hollow components, calculate the inertia of a solid component of the same outside diameter then calculate the inertia of the hollow area as if it were a shaft. Subtract the inertia of the hollow area from the solid inertia and multiply the per-inch number by the length of the component.

For an overrunning or random start-stop clutch, calculate torque using:
T = (WR2 × S)/(3,700 × t) – Tf

where T = torque needed to wrap down the spring, in lb-in., WR2 = load inertia determined above in lb-in.2, S = shaft speed at clutch/brake rpm, t = time to disengage (0.003 sec for clutch) in seconds, and Tf= friction torque (torque required to overcome static friction) in lb-in.

For a single revolution clutch and clutch brakes, consult the manufacturer’s detailed product specifications and verify that the selected clutch/brake model exceeds the torque requirements. From the same specification, find the unit inertia (inertia of the rotating component of the selected clutch/brake) and calculate the torque requirement more accurately by adding it to the load inertia:

Tt = [(WR2LOAD + WR2UNIT)S]/(3,700 × t) – Tf

where Tt = total system torque in lb-in., WR2LOAD = load inertia in lb-in.2, WR2UNIT = clutch/brake inertia also in lb-in.2, S = shaft speed in rpm, Tf = friction torque in lb-in., and t = time element from braking, 0.0015 sec.

The second aspect to verify is whether the load has sufficient inertia to fully engage the stop brake spring and disengage the clutch spring to accurately stop the load. The minimum inertia can be found with:

I = [(t) (Tc + To) (3,700)]/SIc

where I = minimum inertia needed to fully engage the stop spring and disengage the clutch spring in lb-in.2, t = time in seconds (obtain this value from the catalog factory chart), Tc = torque specified to activate the selected clutch/brake in lb-in., To = drag torque in lb-in., S = shaft speed in rpm, and Ic = inertia of the clutch/brake output side in lb-in.2

If the result is zero or negative, the overall system has enough inertia for stopping within the specified accuracy. When the result is positive, the springs will not wrap down and release properly. Additional inertia equal to or greater than the calculated minimum inertia should be added to the system. Use the equation below to determine the maximum load inertia that a given clutch/brake model can handle without excessive wear or failure:
WR2 = (T × 3,700 × t)/S

where T = clutch/brake torque in lb-in., t = the time of 0.0015 sec, and S = speed in rpm.

Manufacturers’ data sheets and catalogs provide a detailed list of options to help build a complete product part number. To fully specify the clutch/brake, review the design considerations and make the needed selections. For example, you can choose clockwise or counterclockwise direction of rotation. Then select the stop collar with one to 24 stops. This sets either full or fractional rotation of the shaft. Next, select the bore size. Finally, choose the activation method. Activation typically uses ac or dc solenoids, but it could use a pneumatic option.

Electromagnetic friction clutches and brakes
An electromagnetic friction clutch or brake has an input, typically a motor, connected to an input rotor-shaft assembly bore and a load connected to the armature of the clutch with a pulley or gear. When the coil of the power-on version of the electromagnetic clutch or brake is not energized, a spring within the armature assembly separates the assembly from the input rotor shaft so that it does not rotate. When the coil is energized it attracts the armature plate which engages the rotor assembly and drives the load.

A power-on electromagnetic brake operates using the same principle as the clutch, but with only a single rotating component — the armature assembly. The brake is generally positioned on the load shaft with the armature assembly secured to the shaft while the field assembly mounts to a nonrotating component or bulkhead. The armature assembly rotates freely until the coil is energized. The field assembly becomes an electromagnet when energized and attracts the armature plate braking the load.

A power-off electromagnetic spring-set brake operates on a slightly different principle. The actual braking force is applied by compression springs within the field assembly. In normal power-off mode these springs apply pressure to the fixed armature plate which, in-turn, applies pressure to the rotor. This rotor has the ability to “float” back and forth when power or voltage is supplied. It is coupled to the load shaft by a spline or hex-milled hub. Some rotors are suspended between two diaphragmlike springs to achieve the floating state for zero-backlash applications.

Electromagnetic friction clutches and brakes excel in random stop/start, power-on and power-off braking, soft start/stop, bidirectional rotation, and speeds exceeding 1,750 rpm. Motion systems that require a soft start normally use electromagnetic friction clutches because friction can be gradually reduced or increased by varying voltage across the coil.

Selecting electromagnetic friction clutches or brakes
Braking action can be either static (holding) or dynamic (stopping). Static braking holds an already stopped shaft in position, preventing it from turning. Dynamic braking brings a rotating shaft to a standstill.

For static-brake applications, determine the required static torque to hold the load under worst-case conditions, including system drag. Select a brake model from the manufacturer catalog with a static-torque rating greater than the required torque. Verify that the selected brake fits into the available application envelope and mounting configuration.

For dynamic-braking applications with a specific stopping time requirement, first calculate the dynamic torque (TD) necessary to decelerate the load using the inertia-time equation:
TD = (0.104()/t) − D

where I = total system inertia (lb-in.-sec2), ω = shaft speed (rpm), t = time to zero rpm (sec), and D = load drag (lb-in.) To convert dynamic torque to static torque, multiply the result by 1.25.

When the inertia or engagement time of the clutch or brake initially selected represents more than 10% of the load inertia or acceleration time, use the inertia-time equation to solve for acceleration time. Use an inertia value equal to the sum of the load inertia and the clutch or brake inertia. Then verify that the sum of the acceleration and clutch or brake engagement time is still within the required acceleration time for the application.

For dynamic-braking applications that need only stall a load, calculate the appropriate static torque (TS) using the horsepower-rpm equation:
TS = (1.25)(63,000)(PK)/ω

where P = horsepower (hp), K = service factor. and ω = rotational speed (rpm).

Select the brake model from the manufacturer’s catalog with a static-torque rating greater than the required torque. Finally, verify that the selected brake fits into the available application envelope and mounting configuration.

For applications using an electromagnetic friction clutch that needs a specific acceleration time, first calculate the dynamic torque (TD) required to accelerate the load using the inertia-time equation:
TD = 0.104()/t + D

where I = rotational load inertia (lb-in.-sec2), ω = differential slip speed (rpm), t = time to speed (sec), and D = load drag torque reflected to the clutch (lb-in.).

Again, you can convert dynamic torque to static torque by multiplying by 1.25. For applications that only need to accelerate a load, calculate static torque using the horsepower-rpm equation:

TS = (1.25)(63,000)(Pk)/ω

where P = horsepower (hp), k = service factor, and ω = differential slip speed (rpm).

Select a clutch model from the manufacturer’s catalog with a static-torque rating greater than the required torque and verify that the selected clutch fits into the available application envelope and mounting configuration.

One must carefully consider proper energy dissipation when engaging a clutch dynamically. To calculate the total energy dissipated per minute, use:

E = (Ek + Es)N

where Ek = kinetic energy (ft-lb/min), Es = slip energy (ft-lb/min), and N = cycle rate (cpm).

Consider using a larger series clutch if the total energy dissipation is more than that permitted for the chosen clutch.

In some applications, it may be necessary to look at clutch or brake inertia and engagement time in calculating load acceleration. If the inertia or engagement time of the clutch or brake is more than 10% of the load inertia or acceleration time, use the above mentioned inertia-time equation to solve for acceleration time (t), using an inertia equivalent to the sum of the load inertia and the clutch or brake inertia. Then verify that the sum of the acceleration and clutch or brake engagement times is still within the required acceleration time for the application.

Sometimes the brake must mount on an arm to keep a load from falling when power is removed. Simply calculate the torque needed to hold the load the weight of the arm plus the load times the length of the arm (force × distance). Add a service factor (typically, boost the value 50% by multiplying by 1.5), then select the appropriate brake.

Specialized clutches and  brakes
The bidirectional no-back (BDNB) design is for applications requiring automatic position holding and rotary driven capability. The BDNB design can be turned only when torque is applied to the input shaft. The input shaft may be driven in either direction with torque being transmitted directly to the output shaft. When there is no torque on the input, the output shaft locks and cannot rotate in either direction. Any torque applied to the output shaft is transmitted directly to the clutch body and will not be reflected to the input.

The torque feedback device (TFD) is a critical consideration in moving to electronic steering in that operators are used to the tactile response, or “feel,” provided by both direct mechanical and hydraulic steering systems. The TFD works much like a brake by using a magnetic-actuation system to apply force to a friction disc that impinges upon a rotor. The friction disc uses an innovative material whose static-to-dynamic-friction performance is not subject to the slip-stick effect that conventional brakes generate. Standard brakes display a higher friction when the shaft is stationary. The new material generates a consistent frictional force over its life, yet it does not generate any frictional force when current is turned off. The TFD also provides faster response to very small changes in current.

Though this article provides basic selection and application information for wrap-spring and electromagnetic clutches and brakes, do not overlook the extensive technical assistance provided by clutch and brake manufacturers. They are willing to work with you in selecting a clutch or brake for optimal performance and long service life.

© 2011 Penton Media, Inc.