Thomson-Deltran Application Specialist
Wood Dale, Ill.
Machines that move loads to specified positions and hold them within a few thousandths of an inch need highly controllable, precision brakes. Engineers typically spec friction brakes for these applications. But a wrap-spring clutch/brake can also do the job. It combines in a single package a clutch and brake plus an actuator (solenoid or pneumatic cylinder) to engage and disengage the unit.
Wrap-spring clutch/brakes work best for machines that start and stop multiple times during a single revolution. Printing presses, postage and packaging machines, and food and material-processing equipment are some examples. Wrap-spring clutch/brakes operate on the same basic principle as wrap-spring clutches, though the latter is suitable only for applications that don't need high positioning accuracy, or where final stopping position is unimportant, as with conveyor-feed mechanisms.
A wrap-spring clutch in its simplest form consists of an input and output hub that attach, respectively, to a motor and load. A helical-wound spring spans the two hubs. The spring ID is slightly smaller than the OD of the hubs to create an interference fit. Rotating the input hub in the direction of the spring helix forces the spring to wrap down onto the hubs, coupling the motor and load without slippage. Stopping the motor or reversing its direction unwraps the spring and releases the output hub, letting the load freely rotate (overrun). In other words, wrap-spring clutches are unidirectional.
Friction clutches, on the other hand, work for machines that apply bidirectional torque to a load. Motion systems that need a "soft-start" capability typically use friction clutches because friction can gradually raise or lower by varying voltage to the clutch-control coil. Conversely, a wrap-spring clutch is a better choice when loads must rapidly sync-up with the drive motor within a predictable time or rotation angle. Unlike friction clutches that can slip under certain conditions, wrap-spring clutches won't slip when engaged. This property is extremely important in printing presses, for instance. Here, a wrap-spring clutch maintains the paper's linear start-stop position as paper pays out and inertia drops.
The addition of a stop collar and a control tang on the spring lets the output hub start and stop while the input hub spins. Stop collars come with one or more stops, up to 24/rev or 15° between stops. The control tang anchors to the stop collar, which surrounds the spring and hubs. An external mechanism engages a lug on the stop collar OD. Halting the collar unwraps the spring and releases the output hub. The arrangement applies no braking to the output hub.
So-called single-revolution clutches give limited braking by virtue of a spring incorporating two control tangs. The second tang affixed to the output hub prevents the hub from overrunning when the clutch engages. Such clutches have a stop-load rating roughly one-tenth that of the start load, which may be a design limitation for some applications. Positioning accuracy for single-revolution clutches is about ±20°.
Wrap-spring clutch/brakes, by comparison, have significantly higher positioning accuracy and braking capacity than single-revolution clutches. They add a brake spring with control tangs anchored to the output shaft, and a brake hub bolted to a stationary plate. Engaging the stop collar unwinds the drive spring from the input hub, which, in turn, wraps down the brake spring. Braking torque equals the drive-torque rating.
An antioverrun spring keeps the input and output hubs synced. Without it, the input hub could accelerate faster than the output. An antiback spring keeps the output stationary when the brake spring wraps down. The brake and antioverrun springs boost stopping accuracy to ±0.5°, which is not cumulative cycle to cycle.
Wrap-spring clutches and clutch/brakes come in both clockwise and counter-clockwise-rotation models. The units need no maintenance and work at speeds to 1,800 rpm and torque to 5,000 lb-in. Standard wrap-spring clutch/brakes typically last about 8 to 10 million cycles, while extended-life versions can last three to five times that long.
Wrap-spring clutch/brakes by the numbers
The following calculations verify that a clutch has sufficient torque to start a load, and that the load and shaft have adequate inertia to activate the clutch/brake stop spring.
First, calculate inertia of all rotating components including shafts, drums, and pulleys. Then the torque needed to wrap down the spring, or clutch/brake actuation torque, T (lb-in.) is:
T= WR2loadS / 3,700t –Tf
WR2load = load inertia, lb-in.2
S = shaft speed at the clutch/brake, rpm
t = disengage time, sec (0.0015 sec when braking)
Tf = torque needed to overcome static friction, lb-in.
Next, add inertia of the clutch/brake rotating components to load inertia and figure total system torque, T T (lb-in.):
TT = (WR2load + WR2unit ) S/3,700t – Tf
WR2unit = clutch/brake inertia, lb-in.2
Verify that the load has sufficient inertia, I (lb-in.2), to fully engage the stop brake spring and disengage the clutch spring:
I = 3,700t( Tc+ To)/S – IC
Tc = clutch/brake actuation torque, lb-in.
To = drag torque, lb-in.
Ic = inertia of the clutch/brake output side, lb-in.2
A zero or negative result implies the overall system has adequate inertia to stop within a specified accuracy. A positive result indicates that clutch springs won't properly wrap down and release. In this case, add inertia to the system equal to or greater than the calculated minimum inertia.
Next, calculate the maximum load inertia that a given clutch/brake size can handle without excessive wear or failure:
WR2 = 3,700 Tt/S
t = 0.0015 sec
Select an appropriate bore size for the application. The gray area on the chart designates workable rpm/clutch size combinations. For example, at 200 rpm, clutches are available for all listed shaft diameters. Shafts larger or smaller than what the chart recommends may need a custom unit.