Since belt conveyors vary so widely in their duty (1 te/hr - 10,000 te/hr) and dimension (10m - 10 km) I would have thought that it would be wholly impractical to specify a stopping time as x number of seconds.

On this basis "as quickly as possible" seems a sensible statement. ■

If you stopped 'a bit quicker' then a bit of damage may be prevented,

or,

If you stopped 'a bit quicker' then you might cause more damage than if you stopped 'a bit slower'.

You just have to use 'due diligence' in your risk analysis.

There will always be some smart ass who can pick holes in what you do, but give them a clean sheet of paper and it will still be clean in a years time. Such is life ■

Usually, this condition refers to downhill conveyors (i.e. regenerative). However, my comment refers to all conveyors.

A typical conveyor is engineered to modern standards (having interpretation in rubber mechanics, control dynamics, equipment mechanical safety factors, etc.) that have built-in allowances such as consevative brake rating, pulley traction, system drag, etc.

Nominal vs. shortest stopping time was petitioned to the Governing Safety Board and was found to be adequate. The petition shows typical design criteria with an engineered stopping time that does not unduly penalize the mechanical component selection, such that, by applying design limit safety factors, can produce a far shorter stopping time. The Safety Board considered these as adequate protection.

A case-in-point, a major mine in Chile, had a stopping time, based on good mechanical design practice, that stop the conveyor in more-or-less 60 seconds, accounting for typical belt stress factors. When all critical design limits/allowances are evaluated, the stopping time was closer to 40 seconds.

To stop the conveyor in 20 seconds, would create a extreme cost penalty in terms of belt strength, brake and structural sizes, and not produce a significant reduction in operator risk. In fact, this may produce the opposite effect as the limits on known technology were stretched, such as the belt construction and splice endurance limit.

To stop the conveyor in 10 seconds would require a design criterion with stress factors and costs between 4 and 6 times the original design and make the project cost untenable. One issue was the belt strength specification. It was already at known manufacturing limits. The authorities did see the senseless side of an extreme criterion. Thus, a case is made to apply good judgment and modern standards together with an update in the technical methods operators, engineers, and safety officials collectively can agree upon.

In summary, applying prudent engineering while taking the components to their extreme design capacities can demonstrate equipment capabilities and their reduced stopping time, that can be fractions of every day stopping time operation, and that are only taxed a few times in the life of the machine. Two standards emerge, nominal and extreme. Each has their place in the engineering repertoire.

I believe each case has unique merits on how to evaluate the extreme.

Sorry for the pedantic excersize in words.

Happy New Year to All ■

Dear Mr.Nordell ,

Thanks a lot for your new year greetings & wish you the same.

Brakes can be applied in two ways. External or Internal.

External : Mechanical brakes, by thrusters or electromagnetic brakes.

Internal : This may be provided by stopping the conveyor by reducing the ramp down time to bare minimum or by giving the motors breaking current.

I feel external brakes may be giving more stresses on belts & other mechanicals than the internal brakes. ( It is like this : When I am running, am I stopped forcefully by someone else or I am stopping myself ? )

When I need to stop the conveyor by "Emergency switch or pull-cord ropes", I expect the conveyor to be stopped immediately.

Requesting your valuable opinion.

Regards, ■

Originally Posted by sganesh

When I need to stop the conveyor by "Emergency switch or pull-cord ropes", I expect the conveyor to be stopped immediately.

Sounds like a recipe for disaster to me

What is going to happen to all the energy if the conveyor stops "immediately" ■

Dear Mr.Designer,

I am not from design. I am from maintenance & the end user. Please assume ( repeat assume ), when I notice that a conveyor is getting damaged when it is under operation, I will stop the conveyor by emergency switch or by pull cord.

Should not I expect that the conveyor to be stopped immediately ? Because the more time we allow, more it will get damaged.

Regards, ■

Originally Posted by sganesh

Should not I expect that the conveyor to be stopped immediately ?

Simple answer - NO

This may be difficult for someone without an engineering training to understand, but think, an immediate stop is like driving your car into a brick wall at 60 mph, result is disaster (and death of the driver). ■

Originally Posted by sganesh

Dear Mr.Nordell ,

...Thanks a lot for your new year greetings & wish you the same.

Brakes can be applied in two ways. External or Internal.

External : Mechanical brakes, by thrusters or electromagnetic brakes.

Internal : This may be provided by stopping the conveyor by reducing the ramp down time to bare minimum or by giving the motors breaking current.

I feel external brakes may be giving more stresses on belts & other mechanicals than the internal brakes. ( It is like this : When I am running, am I stopped forcefully by someone else or I am stopping myself ? )

When I need to stop the conveyor by "Emergency switch or pull-cord ropes", I expect the conveyor to be stopped immediately.

Requesting your valuable opinion.

Regards,

Thanks a lot Mr.Designer.

May I have your comments about internal breaking ?

Regards, ■

I suppose you could consider direct reversing the motor then cutting the power just as the motor starts to reverse. But it's all very dependant on your drive arrangement and conveyor size.

You'll need a qualified design engineer or consultant to investigate each application individually, no place for "rules of thumb" here. ■

Engineers must be prudent in spending client's money. You will not get consensus on the procedures. My earlier discussions is a result of designing many conveyor with significant downhill slopes and having normal belt stress and structural stresses, which can be amplified with momentary and infrequent stopping forces.

When these critieria are used, fatigue and fracture analysis is evaluated for structures, pulleys, drive train, and belt assemblies. I believe I quoted a circumstance where our design allowed momentary braking force to SF=4:1 on a very infrequent basis. After further analyzing the details, we found the operators were applying values down to SF=3.5:1 in the extreme. There is not operating systems with successful track records in these regimes. The tricky question is: What is SAFE ????

To argue SAFE limits with peoples lives, you need a high degree of competence that all things are considered. Many engineers are not equipped to accurately define the necessary factors and apply them in a competent fashion. Special computer based tools have to be vetted to give comfort in the results. ■

A bit like how long is a piece of string

There are a number of things you need to consider. The way it was put to me was if someone gets caught in a conveyor, then unless it stops instantly, the person is free minus some part of him/her. Sorry, it is that harsh. If personnel safety is the issue, make sure you design with safety in mind and carry out a PHA before you get too involved in the design. So you really need to look at the practicalities of the system. You need to consider the feed and discharge as well as the stresses involved and the ability to stop quickly.

If the feed system takes say 10 seconds to stop and you stop the conveyor in 5 seconds, you need to be able to accommodate 5 seconds of feed somewhere otherwise it is all hands to the shovels. The same goes for the discharge. This is what has to be taken into account when designing chutes. They are not just to divert material, they also act as surge bins or buffer bins. By the way, I hate the use of the term "buffer bin" as this was the term used to justify one of the biggest disasters in Australian coal terminals. It is now being rectified some 30 years later at a huge cost. But we won't go there...you work it out.

So you need to ensure that you have sufficient return tension or counterweight when trying to brake a conveyor. If you simply apply high braking force, the return side now becomes the tight side. This can pull the counterweight up and I have seen takeup towers as a pile of twisted metal following an emergency stop where this was not considered in the design.

It is all a case of design.

Oh and NEVER use the term ASAP or any of its derivatives. This covers everything from let it run down on its own to stop it instantly. I have had clients tell me something is not urgent and so I put it at the bottom of the pile. Then a couple of days later they wonder where it is!

In this case, work out a stopping time and set it as part of the criteria, then incorporate it in the design. ■

Originally Posted by cancengiz

I have another question, related to this one? Can we apply brakes to non-driven, tail pulley, as well? That could a bit improve stopping time.

Lunch time

You can apply brakes to any pulley as long as you have the correct tension conditions around the pulley, otherwise all you will do is stop the pulley and the belt will keep going (with a bit of noise and smoke ).

Then you have the issue of balancing the braking. This is one reason why you normally have the brake at the drive. ■

Hi all..

Stopping time must ensure that you don't get spillage due to incompatability with what the conveyor is feeding. Simple as that

Otherwise, regarding safety, I remember what Graham Shortt said:

"What is the difference between being killed in 10 seconds, and being killed in 20 seconds?"

Cheers

LSL Tekpro ■

I know of two deaths due to belt conveyors.

One chap in Israel went arround the tail pulley, and a chap here got sucked into the return belt.

Both instantaneous deaths.

Stopping time irrelavant ■

You cannot stop a belt immediately regardless of circumstances just like you cannot stop a moving car immediately. The explanation by Mr Nordell is very sound and is the way most OH&S authorities view the issue. In practical terms, if a person is trapped or in danger and somehow you manage to stop a belt immediately, there is a real possibility that other catastrophic events may occur that puts many other at risk so the interpretation we apply is "we stop the belts as quickly as possible and in such a manner that it will not cause damage to the conveyor system components and through this create a possible hazard for others"

Cheers

Colin Benjamin

Gulf Conveyor Systems P/L

www.conveyorsystemstechnology.com ■

Normal stopping sequences do apply rational thinking regarding spillage, alternative control of excessive material flow of the discharge pulley, and mechanical system stresses and fatigue limits of equipment.

We do not consider a stopping time, in an extreme emergency, to control spillage. Extreme emergencies include failure of major control components that would otherwise regulate spillage. The concept of "what if" scenarios are investigated with major overland and downhill systems. There can be up to seven stopping scenarios with differing degrees of control. The conveyor's braking system must be designed in unconventional ways. The practices of 20-30 years ago is not relevant to today's design guidelines for large and complex systems.

The new designs are more competent with short stopping times and with better design features that limit the potential for component failures. Modern predictive dynamic behavior tools render a more realistic approach to system control.

Spillage is the least of the worries when considering component failures that impede predictive and regulated control of stopping. The most critical aspect is we do not break it, if things go wrong. Safety is paramount to such designs, spillage is not. ■

Well.. spillage not paramount eh?... I would'nt like to be the poor chap who has to operate and clean up in your transfer stations Larry!!!

Cheers

LSL Tekpro ■