Brake failure on Hoist applications

[maypot]

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

 

[sreid]

For closed loop vector control I would think that one would want to keep the VFD alive even if the brake was set. Then if drum position movement is sensed due to brake failure the VFD could hold the load.

Consider a BEI HS35 encoder. I've found them to be rugged and easy to mount.

http://www.beiied.com/main-product.html

 

[Barry1961]

Having the VFD as a back up for the mechanical brake sounds great to me.

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

 

[jraef]

The general approach to this is that the VFD takes over the day-to-day operation of the hoist, but the mechanical brake is still used to hold the load at standstill for safety reasons, especially power loss as Barry1961 mentioned. Think of the mechanical brake as being a "parking brake" now. Using the VFD under normal circumstances significantly reduces wear and tear on the mechanical brake, and thereby reduces the incidence of failure as well.

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

 

[dpc]

Elevators have been using mechanical brakes to protect against free fall for a long, long time. They are required to be functionally tested periodically, but in general, seem to be quite reliable.

I can't imagine relying on a VFD braking system where safety was a concern.

 

[mc5w]

Hoist are also supposed to have a clutch that requires that the motor be turning in the downward direction in order for the drum to go down. This clutch is silimilar in principle to a seatbelt clutch. This clutch is also the backup for the brake. This clutch could allow the load to slide down very slowly if the brake fails but will not allow the load to just drop.

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.

 

[PowerfulStuff]

maypot,
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

 

[Drivesrock]

Dear Maypot, the mechanical brake is the final safety device in the event of drive trip and mains failure. Encoders are required on VSDs when good torque control is required at very low motor speeds - typically <5% nominal speed. (As well as for other reasons/applications). When positioning loads with a hoist it can be expected to have low speed positioning. Remember that the drive should control the brake - releasing it only after torque control has been established by the drive with zero speed reference - which can be a second or so after the "run" command, depending upon the drive. The brake open signal will then release the speed reference (so that the drive isn't trying to accelerate the load with the brakes still on/releasing). After finishing the movement at (or close to) zero speed the drive will re-apply the brake and the brake closed signal will remove the run command. This is to ensure there isn't a moment when the drive is nolonger controlling the motor and the brake has properly closed.

Best Regards

Drivesrock

 

[maypot]

Thanks all for your inputs.
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

 

[akselj]

I would suggest that you activate the mecanical brake at each stop, when the hoist is nearly stopped by the electrical brake.

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

 

[TCG]

Elevator brakes for cable machines are applied as car stops, or in case of an overspeed, the circuit to motor is opened, and mechanical brake are applied. Only failure I have seen in nearly forty years playing with elevators is when a person oiled the brake shoes.

Tom Grosch