Certain types of conveyors require fail-safe brakes for stopping. These conveyors, often used in the mining and aggregate industries, include downhill conveyors (e.g., discharge and transfer conveyors), as well as overland conveyors with downhill sections. Not having fail-safe brakes on these types of conveyors can result in catastrophic damage if power is lost. For example, overspeed can result in damage to drive system components, clogging of chutes at transfer points, or general safety issues stemming from performing an e-stop. Some unique considerations related to designing a conveyor brake system include:
Stopping Brake vs. Holding Brake
When specifying a conveyor brake, first determine how the brake is going to be used. Is the brake going to be used to stop the conveyor regularly or will it normally be used for holding (perhaps with an occasional an e-stop condition)?
Some conveyors are designed to be completely dependent on the brake to stop the belt in a specific time period, as well as holding the conveyor in place. Other conveyors will be equipped with a variable frequency drive (“VFD”) that is capable of dynamic braking using the motor torque itself. In this case, the brake is only used for holding the load when the motor is stopped or for e-stopping in an emergency or power failure condition.
Ultimately, whether you choose to use your fail-safe brake to stop and hold the conveyor or whether you rely on a VFD to stop the conveyor can be a complicated decision involving considerations such as how the conveyor will be used. For example, if your conveyor application requires frequent changes in speed and a lot of stopping, you will wear the brake shoes/linings fairly quickly and may be better off using a VFD. That said, including a VFD in your design will require more upfront investment.
Whether the conveyor brake will be used as a holding brake or a stopping brake will have implications on where the brake should be located in the drive system – that is, whether the brake should be placed on the input shaft of the gearbox or at the output on the conveyor pulley shaft itself. The torque requirement will always be less on the input shaft than the output shaft. When braking on the output shaft, it’s important to consider the overhauling torque from the motor inertia that must be absorbed by the gear unit.
Having determined the brake service type and location, the next step is specifying the required torque for the brake. There are several different ways to determine the torque requirement for a conveyor brake.
First, the torque requirement can be determined using a CEMA calculation. Most conveyor design companies use a computer program that takes into consideration the retarding torque caused by the drag of the idlers, as well as conveyor profile, to estimate both the braking torque and conveyor stopping time.
Another method simply uses the rated motor horsepower and RPM to allow for calculation of full load motor torque. If no CEMA calculation is available, we use a 1.7 safety factor beyond nominal motor full load torque. In this case, we recommend selecting a brake with maximum torque adjustability in case the braking cycle is too harsh.
Disc vs. Drum
Since required braking torque has been established, the next step is deciding if a disc brake or drum brake should be used and which type of release actuation should be selected.
If the brake is located on the input shaft, the largest drum brake that we recommend using is 400mm diameter. This is constraint stems from the fact that most conveyor motors are rated at 1800 RPM and because the brake lining material on a drum brake can only withstand 40 m/sec of circumferential speed.
Should the brake be located on the output (or tail pulley) shaft, the required torque cannot be more than 10,000 N-m, as this is the maximum torque allowed for the 710mm diameter drum brake. The drum brake, used within the above parameters, is a good choice as long as a variable frequency drive with dynamic braking is utilized or the number of conveyor stops/braking cycles is very low.
If the brake is the only device used to stop the conveyor, a disc brake is a better selection, because the brake lining material can handle circumferential speeds up to 85 m/sec and temperatures up to 900K. Disc brakes can be used on either the input shaft or output shaft quite effectively.
The method of actuation for either disc or drum brakes is normally electrohydraulic thruster, electromagnetic solenoid, or hydraulic. Hydraulic release is normally used when required torque capacity exceeds that which is available from thruster or solenoid types.
Having established brake location, brake service type, required torque, drum or disc brake type, and means of release actuation, you are now ready to select your conveyor brake.
If you’re looking for fail-safe brakes for a conveyor application, we encourage you to contact us. The engineers at Hindon have decades of experience designing industrial disc and drum braking systems for a variety of applications including conveyors. We will work to understand your application and priorities and provide the best industrial brake for the job.