The braking torque is applied through the action of compression springs that operate the shoes when the current is interrupted. When the current is switched on the shoes is lifted from the drum, the electro-hydraulic thrustor unit acting against the pressure of the springs. The normal position of the brakes is, therefore, 'ON' (shoes gripping the drum), and the brake will return to this position when the current fails.
The brakes are robustly made with few and simple working parts, The shoes and base of close grained cast iron and the fulcrum pins are of ground mild steel working in long, reamed bearings. The shoes are fitted with easily renewable woven, fibre, linings held by countersunk rivets, They are adjustable for wear and the pressure exerted by them is easily varid Pivoted shoes are standard on all thrustor-operated brakes.
These are standard electro-hydraulic supplied complete with their first filling of oil. This is 'insulating' oil, Grade 8.30 to British Standard 148, which is readily available at home and overseas. The thrustor cannot be injured by overload or under load.
All size can be wounded for 3 phase A.C. supplies up to 550 volts and outputs are given here for 50 and 60 C/S supplies. For lower supply frequencies the outputs will be reduced and enquiries should-be referred to us. All thrustors are continuously rated.
All standard units are totally enclosed. Dust seals are fitted as standard, but in extremely dusty situations, or for mounting in the open, a cowl or cover the thrustor is recommended.
Standard units have tapped ¾ inch BS conduit thread. Supply leads must be flexible to allow free movement, and cables should have 18 inches free length and be not larger than three-core 7/0.029'' to avoid side drag.
Optional Special Features
A level for releasing the brake can be fitted at a small extra cost; this lever is self-resetting.
Hydraulic control, which enables the brake to be applied by the operator by means of a lever or pedal while the thrustor is energized, can be fitted at extra cost. These brakes can be supplied complete with master and brake cylinder, copper pipe and pedal assembly.
Brake drums, or drums combined with rigid or flexible couplings, can be supplied to customers' requirements at extra cost. A stout high duty cast-iron drum will be supplied with the brake, if specified. Drums are machined and keywayed ready for fitting to the motor shaft at cost.
Asbestos linings can be fitted to any of the brakes listed in the table. These linings withstand higher temperatures but result in reduced torques, about 0%, because of their lower friction coefficient. They should only used when drum temperatures of over 2200F have to be tolerated and should be the subject of a special enquiry to us.
Choosing The Correct Brake
When selecting a brake from the rating table, opposite, it is important to consider not only the brake torque but also its duty, i.e. its frequency of application and the duration of each stop.
When a large amount of store energy has to be dissipated or when the stopping times is specified, it will be necessary to caculate the torque from inertias and speeds of the moving parts. For many purposes, however the braking torque can be related to the torque being transmitted. The full load torque of a shaft transmitting a known horsepower is given by:
Torque (lb-ft.) = hp x 5250
rev/min of shaft.
Usually, sufficient will be known about the drive to enable a suitable brake torque to be determined.
The bake must be chosen with consideration of its intended duty to ensure that the linings will not overheat or wear down too quickly. Selection of a brake for torque only would not ensure this. Since the speed of the drum will also affect the rate at which the drum can dissipate the absorbed energy without overheating. The higher the drum speed for a given torque, the higher will be the rate of energy dissipation in the brake linings during each stopping period the rate being expressible as a horsepower. The column headed 'Nominal horsepower based on linings' in the table is intended as a rough guide to the horsepower of the motor for which a brake is suitable when the duty is equivalent to 'normal' crane duty, i.e. not more than 120 operations per hour continuously applied and with a stopping time not exceeding about one second. If the duty of the brake is less than this 'normal' the horsepower given in the table can be increased. If however, the frequency of application is greater or the stopping time is longer, a larger brake may have to be used; asbestos linings can be fitted and a higher drum temperature accepted with a lower torque. Details of brakes with non-standard features may be obtained from us.
"EMC" THRUSTOR OPERATED BRAKES
AC THRUSTOR-OPERATED BRAKES have a wide application; they can be used with such machinery as electrically driven hoists, cranes elevators, ropeways, machine tools, mixers and laundry machinery.