14 - Braking Systems PDF

14. Braking Systems Hoist Brakes BS EN 14492-2 states the requirements for brakes that are used for hoisting and lowering movements. Hoists shall be designed in such a way that movements can be decelerated, the load can be held, and that unintended movements are avoided. In addition the rotating...

14. Braking Systems Hoist Brakes BS EN 14492-2 states the requirements for brakes that are used for hoisting and lowering movements. Hoists shall be designed in such a way that movements can be decelerated, the load can be held, and that unintended movements are avoided. In addition the rotating masses, the triggering limit of the rated capacity limiter and the maximum speed, e.g. in the event of a phase failure, shall be taken into account. Brakes shall engage automatically in the following cases when:  The control device returns to its neutral position; 135  The emergency stop function is activated;  The external power supply to the brake is interrupted;  The power supply of the corresponding drive (= motor) is interrupted or switched off In addition, in the case of 3 phase motors, brakes must engage when two phases of the power supply to the drive (motor) are interrupted. Failure of Power Supply Electric hoists shall incorporate features so that:  The load cannot lower in an uncontrolled manner if a phase should fail  The load cannot drop if a phase should fail ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3 When one phase of a three-phase system is lost, a phase loss occurs. This is also called 'single phasing'. Typically, a phase loss is caused by a blown fuse, thermal overload, broken wire, worn contact or mechanical failure. A phase loss that goes undetected can rapidly result in unsafe conditions, equipment failures, and costly downtime. Phase loss protection devices are relatively inexpensive and simple to install. They provide protection by disconnecting the equipment from the circuit when phase loss is detected. Phase or voltage monitors are the most common solution. Notes: Conical Rotor Motor 136 The sliding rotor principle uses an electric motor specially designed with a conical rotor and stator windings. When power is applied to the motor windings the magnetic field is angular to the centre line of the rotor shaft, operating in effect, two components of force at right angles to each other. The radial component rotates the rotor whilst the horizontal component, pulls the rotor into the windings. ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3  Notes: 137 Adjustment Brake adjustment must always be carried out in accordance with maker's instructions since incorrect settings will upset motor performance. When the brake lining wears the air gap between the rotor and stator increases and, if not adjusted, will produce erratic motor operation because the magnetic forces will not be powerful enough to move the rotor forward and so will not release the brake. To check if brake adjustment is necessary the rotor should be moved forward manually either by pushing directly onto the shaft or by way of a lever between the brake end cap and the brake wheel. When the rotor is forward as far as it will go, measure the distance between the end of the rotor shaft and the front of the brake end cap. Release the rotor and take a further measurement. ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3 The difference between the two measurements is the air gap (stroke length) and is usually be between 0.5 and 1.5mm and must never be allowed to exceed 3mm, however LEEA recommends that the appropriate manufacturer's instructions are always followed. If brake adjustment is required, remove the four screws that secure the brake end cap. The brake end cap (which is threaded) can now be turned, and each 90° turn will reduce the air gap (stroke length) by approximately 0.5mm. The securing screws should now be replaced and the air gap checked once more to ensure correct setting. During the first week of operation or after changing the brake lining, the brake should be checked daily, since the lining may wear unequally until bedded in. Ideally it could be argued that the hoist brake should be positioned as near to the hoist drum as possible. For reasons of design accessibility, cooling etc., it is usually on the end of the rotor shaft. The examiner should take the utmost care to satisfy himself that all components are in good working order. Should for example a key shear or the coupling fail, then the load would fall. This drawing shows a cross section of a conical hoist brake. Adjustment is made by removing shims (3) as the lining wears. As shims are removed, the brake end cap 138 and brake lining (1 – shown in red) moves further toward the brake rotor at the end of the rotor shaft and closes the gap. As the brake linings wear down, the path of the rotor displacement increases to a maximum of 3.5mm, after which the braking would become ineffective. It is essential that the movement of the rotor is regularly checked and maintained within the manufacturers limits (approximately 1-1.5mm). Notes: ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3 Parallel Rotor Principle The brake shown is in the power OFF / brake ON position. The torque springs force the armature plate to the brake rotor and linings preventing the motor drive shaft from moving. The brake shown is in the power ON / brake OFF position with the coil energised and pulling the armature plate against it. This allows the brake rotor to turn as the motor is powered. The air gap is clearly shown between the 139 brake rotor and the coil. The air gap should be checked to ensure that it meets the requirements of the manufacturer to ensure effective use. Brakes (DC Electromagnet) ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3   The hoist brake is a single disc brake, electro‑magnetic spring applied, DC coil release  The coil configuration is of the stator rotor type, direct current is energised to ensure positive action  The brake is directly fixed to the main gear case or motor frame and operates on the primary drive shaft  Torque is pre-set on factory assembly and should not require further adjustment during its working life  The brake is readily accessible for periodic safety checks  For additional safety, it is switched independently of the motor supply  The fail-safe operation maintains the load in the event of an interruption to the power supply  A hand release mechanism is fitted to enable the load to be lowered in the event of power failure  The hand release operating handle is detachable and is stored in the brake cover 140 Notes: ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3 Example of Manufacturer’s Air Gap Settings 141 Travel Brakes Travel brakes may be either the disc or the drum type but in both cases their characteristics need to be a lot different to a hoist brake Braking characteristics have to be finely tuned in order to avoid excessive braking under no load conditions and providing reasonable braking when travelling with a maximum load Disc Travel Brake This is a cross section of a travel brake fitted to a conical rotor motor.  A good example of soft travel braking  The brake has been greatly reduced on disc and the equivalent of a flywheel has been fitted to the motor shaft  This flywheel would have the effect of allowing much smoother acceleration as well as decelerations ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3  Notes: Thrustor Brake  Heavy duty applications  Usually electro-hydraulic in operation  Centrifugal pump and impeller spinning in oil and developing a pressure head  Pressure exerted on a piston directly coupled to the load to be lifted (brake arm)  Centrifugal pump driven by AC motor – pressure developed depends on speed of motor  Class B insulation, 400v 3Ph 50 Hz motor 142 Main Parts of the Thrustor Brake Base/Arms Rigid wielded construction. Shoes Self-aligning, easily removable high grade cast iron filled with best quality linings fitted with stops. Rods/Grid Rods The tie rod transmits the spring force on shoes by simple lever system. Springs Compression springs are vertically mounted through the grid rods and are held securely between guide plates. One or more springs are used depending on the brake size and thrustor capacity so as to obtain the required braking torque. Operation The braking pressure to the shoes is transmitted from the springs and means of extremely rigid and simple lever/tie rod mechanism. Braking is smooth and positive. Release of the brake shoes is by introduction of a 3 phase mains voltage supply to the thrustor which overcomes the spring force and the shoes are moved clear of the drum by the lever/arm linkage system. ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3 Checking the Thrustor ALWAYS REFER TO OEM INSTRUCTIONS! Correct operating limits and settings must be gained from the manufacturers data sheets or the crane maintenance manuals. Notes: 143 ©LEEA Academy Overhead Travelling Cranes – Step Notes – Apr 2017 – v1.3