Brake failure on Hoist applications
Brake failure on Hoist applications
(OP)
Hi,
I have been requested to install encoder on the hoist drum for our hoisting applications ( VSD )
I understand the fact that close loop control on hoist applications is important, but what about the reliability of the mechanical brake. It is a fact that electrical braking is not considered fail safe for hoist applications.
I believe that the mechanical brake is the least fail safe device and the VSD should be able to take control of the load in case of failure of the mechanical brake.
Other opinions are welcomed.
Bob
I have been requested to install encoder on the hoist drum for our hoisting applications ( VSD )
I understand the fact that close loop control on hoist applications is important, but what about the reliability of the mechanical brake. It is a fact that electrical braking is not considered fail safe for hoist applications.
I believe that the mechanical brake is the least fail safe device and the VSD should be able to take control of the load in case of failure of the mechanical brake.
Other opinions are welcomed.
Bob





RE: Brake failure on Hoist applications
Consider a BEI HS35 encoder. I've found them to be rugged and easy to mount.
http://www.beiied.com/main-product.html
RE: Brake failure on Hoist applications
I also agree on the BEI encoders, rugged and user friendly.
I disagree on the mechanical brake being the least fail safe device. When the power goes off the mechanical brake will hold, the VFD will not. There are ways for the mechanical brake to fail and let a load drop but they are very rare compared to the number of times power is turned off.
Barry1961
RE: Brake failure on Hoist applications
A good closed loop vector VFD should then have a "torque proving" algorithm that will apply an output to the motor and determine that it is at maximum torque prior to releasing the mechanical brake. That way it does not need to overcome the negative torque of the falling load.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: Brake failure on Hoist applications
I can't imagine relying on a VFD braking system where safety was a concern.
RE: Brake failure on Hoist applications
Elevators do not have this feature because people do not walk under the elevator car. You can image what would happen if a hoist block on a crane were to just drop. Elevators have other safety mechanisms such as an overspeed brake for rope/traction traction systems or a special line valve that closes if the hydraulic fluid at the bottom of the cylinder does not have positive pressure.
RE: Brake failure on Hoist applications
This would be an excellent test of how good your vector drive is. I've seen it work elsewhere and it's rewarding to watch an AC drive hold a load completely motionless. I agree with the other thoughts that the mechanical brake is designed for safety but had another thought to add- if your drive work's very well it will be impossible to tell if the brake is ineffective. Normally as the brake wears the operator will seem some slip and call for maintenance. Some mechanical brake test method would probably need to be included in the hoist pre-start checks.
Regard,
PowerfulStuff
RE: Brake failure on Hoist applications
Best Regards
Drivesrock
RE: Brake failure on Hoist applications
I consider that relying on the VSD for the braking of the mechanical brake is not safe. This should and must be performed seperately. Actually a N/C contact on the VSD is configured to action the mechanical brake.
I agree with mc5w on the idea of the clutch.
Bob
RE: Brake failure on Hoist applications
If you only use it as a last safety it is very likely that it is stuck in rust or dirt.
Elevator safety brakes for cable operated systems are not overspeed brakes, they are appiled if tke wires are slack by a very simple and reliable mecanical system, often the wires are attached to the elevatorcar via a bladespring across the top, if the wires break, the spring pulls vedges up between the rails and the car stopping it very effectively.
Of cause each cable is monitored by slackwire contacts.
Greetings Aksel
RE: Brake failure on Hoist applications
Tom Grosch