Overview of clutches and brakes

This is a slip clutch from Mach III Clutch Inc. adapted to diameter restriction. In fact, brakes and clutches from the manufacturer come in torque capacities to 62,000 lb-in. and with mounting configurations for through-shaft, end-of-shaft, flange, NEMA frame, IEC frame, and custom motor-rame mounting.

Power-transmission and motion designs rely on stopping loads predictably, especially to ensure machinery and operator safety. To that end, torque limiters, clutches, and brakes stop, hold or index loads. Migration towards application-specific designs has quickened as several industries are pushing the performance envelope of stock components.

Brakes stop loads (typically rotating loads) and go in applications that need accurate stopping of the load with motors that stop as well. Clutches transfer torque and go in applications where the machinery must engage or disengage a load and motor while letting the motor continuously run. With a clutch, the design usually lets the load coast to a stop. Clutch and brake combinations go where a machine stops and starts a load while the motor continues to rotate. In fact, both clutches and clutch-brake combinations can mount to a motor shaft or mount to a base and engage the drive shaft with a belt drive, chain drive or coupling.

A machine’s motor frame size and horsepower dictate brake and clutch types suitable for a given design. The general steps to pick a given unit are to categorize axis orientation; determine total axis load and kinetic energy to stop it; and calculate 1. Allowable travel before stopping or slowing and time of engagement 2. Maximum load velocity and required clutch or brake force 3. Driving and backdriving torque and 4. Required brake or clutch geometry.

Manufacturers provide quick-selection charts that list clutch and brake sizes for given motor horsepower ranges and shaft speed. Most of these charts are based on the dynamic torque capacity of the clutch or brake and the motor’s torque capacity plus an overload factor of some value. This presumes that the motor is appropriately sized to the application. Tip: Designs with aggressive cycle rates need manufacturer input to address heat-dissipation capacities.

Harmonization of international safety standards is a factor in brake and clutch selection even as controls have come to integrate more safety features. Those include ANSI B11 Series and OSHA rules in the U.S. and EN ISO 13849 as a global standard on safety to sufficiently mitigate risk — with EN ISO 13849 and EN IEC 62061 possibly merging (as IEC/ISO 17305 should it go forward) requiring compliance of machines that go through the E.U. Many OEMs adhere to these standards for competitive advantage.

Sizing to torque, speed, and cycles

Expressed in lb/ft, N/m, or lb/in., static and dynamic torque values express clutch or brake output capability. Applications needing dynamic braking are those in which the brake controls rotating-axis motion by absorbing kinetic-energy changes. Dynamic clutching is that during which a clutch brings a stationary output to the input rpm by assuming the slower axis’ kineticenergy delta.

Static-torque ratings (values that describe clutch and brake behavior when the units aren’t absorbing any kinetic energy) depend largely on torsional load. Reaching static torque operation with a clutch needs a clutch that must engage prior to rotating the input — so the clutch effectively functions as a coupling between in and output. In contrast, reaching static torque operation with a brake just takes holding the output element stationary.

Shown here is a RSCI 20-130 series backstop clutch from Stieber of Altra Industrial Motion.

Static torque equals clutch holding torque when there’s no relative shaft rpm difference between input and output — or (in the case of a brake) when the shaft is stopped. The point at which a system exceeds a unit’s static torque is aptly called the breakaway torque. This is what a machine assembly must reach before relative motion arises between the shafts (in the case of a clutch) or before the shaft starts turning (in the case of brake).

Designers sometimes boost dynamic torque rating (and shorten response time) by selecting larger brakes and clutches — though keep in mind that this also increases torque and shear forces (due to torsion) on mounts to nonrotating machine-frame segments.

Dynamic torque depends on the rpm delta between in and output (for a clutch) and operating and zero rpm for a brake. Dynamic torque is usually about 50 to 80% of static torque. In friction-based designs, this value depends on the contact surfacres’ friction coefficient. Because that changes slightly (with the portion of operation the brake or clutch is delivering) designers usually employ an average coefficient for design calculations.

For axes where the brake must stop vertical loads, engineers must account for how motors can temporarily draw higher current to output more than rated torque. As always, refer to published performance curves to get dynamic torque ratings for operating speed ranges and match brakes and clutches to the output torque.