Motion System Design

Linear friction brakes

Linear brakes are mechanical devices on linear motion systems that hold loads stationary or are used as failsafe devices. Typically, an actuator (a linear motor, fluid-power cylinder, or ballscrew) positions the load, and then the linear brake springs into action to hold it in place. To keep loads stationary, these spring-applied units must provide enough force to resist external forces due to gravity, inertia, and machine operation. Another challenge: The kinetic energy of moving loads. This is converted into friction-surface heat, and if it exceeds the design's maximum allowable heat it can reduce brake performance and life.

This month's tips on linear brakes provided by Bill Granchi, P.E. of Orttech Inc., Solon, Ohio. For more information, call(440)4987458 or email the editor at [email protected].

Q & A

How does the plane of action affect brake performance during operation?

Linear brakes can be installed in any plane of action. However, required holding force and stopping distance are very much affected by inclines and declines. If a brake is horizontally mounted, the stroke's gradient angle α = 0; if not, the weight forces acting in the direction of motion must be defined. The force component when there is no weight compensation is:

FL = m g sin α, in N

where g = Acceleration due to gravity

m = masses (in kg) to be decelerated — moving motor parts, guide carriage, and traverse bench (including additional load.)

If they're properly sized, linear brakes can perform holding operations even on vertically orientated designs. One example: In many material-handling elevators, mass is pneumatically lifted into position and then the air supply is isolated from the cylinder. Seals can have trouble maintaining the isolated air, and often allow pressure to fall. With reduced pressure, the load may slip, sometimes by several inches, and pose a hazard. This is where linear brakes can be quite useful, preventing loads from falling during air-supply removal.

Do they make emergency stops?

Spring-applied linear brakes can also work as failsafe devices stopping loads upon power loss. It works like this: The brake has a continuous supply of electrical, hydraulic, or pneumatic releasing energy. During an energy failure, that supply is cut off and velocity changes:

3V = g sin α t21, in m/sec

where t21 = Manufacturer-specified value expressing reaction delay.

The spring-applied brake then activates, bringing the mass to rest.

If a predetermined maximum allowable stopping distance is available, it can be used to predict motion during an emergency stop. The target overall braking distance Sdesired for an emergency stop should be calculated for velocity Vmax. This figure can also be used to determine the forces to which brakes are subjected, and the heat generated during braking.

How should designers select the release mechanism?

Application environment determines which mode of operation is most suitable. For example, an electromagnetically released unit may be more appropriate than a pneumatic unit in cleanroom-type environments — especially if it's inconvenient to exhaust compressed gas into a filter system. Load capacity in turn depends on the release method of the body, so a linear brake's disengaging system dictates what level of holding force it's capable of producing.

So what's a designer to do when the environment forbids stronger modes of operation? (For example, in our cleanroom application, pneumatically released brakes — which have more holding force than similarly sized electromagnetic versions — might not be possible.) In these situations, some linear brakes come in modular forms to permit multiple brake bodies on one track. Their holding forces are additive.

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