Mr. Vishy,

The best arrangement is to have a proper design fitted to the trolley for the purpose of removal. Many competent design firms have standards that are made for good maintenance of the takeup pulley assembly. The design allows for quick and easy removal and refitting without pulley realignment and belt retracking.

The takeup carriage must first be decoupled from the counterweight mass.

There are two basic ways of designing for change-out of the pulley assembly of a vertical takeup sled. Ultimately, the best method is the most cost effective method.

1. Large transport systems, where downtime penalties are significant use a motorized winch maintenance system of raising the counterweight mass to so there is no belt tension. Fasten the takeup mass to the vertical support structure thereby decoupling it from the trolley. There is no belt tension left pulling on the trolley.

2. Less critical smaller conveyors have a design that allows the counterweight to be lifted and blocked using: a) a chain fall, b) a manual winch with cable and sheave system, using the appropriate mechanical advantage, or c) a portable crane. The trolly is decoupled from the counterweight mass as noted above leaving no belt tension.

With the counterweight mass decoupled from the trolley, a lifting C- sling can be rigged with a mobile crane or monorail and chain fall to support the pulley and disconnect it from its takeup frame. A proper design should have the replacement bearing assemblies have their housings premachined and matched to the dimensions of the existing pilllow block housing so no belt realignment and shimming is required when the new assemblies are reconnected to the trolley.

Selection of the best take-up position should include provisions of how to remove the pulley so that no significant elevation is required.

The specific details can be provided on a consulting bases.

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor dynamics.com

nordell@conveyor-dynamics.com ■

Mr. Vishy:

Without knowing the details of the system in question, it is difficult to provide specific advise. However, you do have some choices in the replacement of the two bend pulleys and take-up pulley that are typical with a gravity take-up system.

The belt (if it is to be left in place) must be clamped in position -- one clamp on either side of the take-up system-- to prevent belt from moving away from the take-up area. The gravity weight must be secured and disconnected from the take-up trolley.

Once this is done, the take-up pulley can be unbolted and lifted from the belt; and the new pulley positioned. The take-up pulley can then be lifted (crane, chainfall, etc.) so that there is slack in the take-up. Once you have sufficient slack, lift the belt off one of the two bend pulleys and support it, so that the bend pulley can rotate freely. This bend pulley can then be unbolted and swung/slid out from under the belt. The replacement pulley can then be positioned and bolted in pace. Lower the belt so that it is supported by the new pulley and repaet the procedure for the other bend pulley.

Depending on the type of splice, the weight involved, the amount of available belt, and other considerations, it may be preferable to open the belt near the take-up, do the pulley changeouts, rethread the belt, and resplice it. System specifics will dictate which method is most viable.

Although the above sounds easy, you are correct in stating that it can be a rigging nightmare. Too often, system designers or the plant itself make part replacement maintenance difficult by not providing sufficient room to work.

If you have any specific questions, please contact me.

Good luck with your project. ■

Gentlemen,

The complexities of handling, at a gravity take-up system, make a great argument for automatic hydraulic take-up systems. Relief of belt tension requires merely moving a lever. Such sytems are compact, modular, economical and reliable.

The writer has engineered various hoisting systems including counterweight lifting systems. Please contact me if you would like consulting help on this matter. ■

Comment on Mr. Dos Santos Statement:

"The complexities of handling, a gravity take-up system, make a great argument for automatic hydraulic take-up systems."

I do not find it logical that because a gravity takeup must be equipped with a device for relieving the mass from the pulley sled that it makes a strong arguement for a hydraulic or powered takeup.

You will find powered takeups (hydraulic or otherwise) replaced by gravity takup systems due to the winch, load cell or pressure transducer, control electronics, etc. failures in service. I know of no gravity systems replaced by powered winch for lack of reliablility.

I have a list powered winch takeup systems converted to gravity takeups due their failure in service. Want to compare lists?

The "mere moving of a lever" is no susbstitute for reliablility.

We too have designed and installed powered winch systems. There is a time and place where they are preferred. Let us not generalize less we seem to be the salesman for gadget sake.

Lawrence Nordell

Conveyor Dynamics, Inc. ■

Larry,

Again, thanks for your comments. I don't have any sales interests in gravity versus hydraulic take-ups. I do both and as a structural engineer I enjoy the extensive stucture, tower and hoisting and reeving associated with gravity take-ups. My argument is one of economics, both upfront and afterwards, as cited in this forum. I don't share your bad experiences with hydraulic take-ups. If I do in the future I may begin to agree with you. ■

Dear Joe,

I feel your passion for the "extensive", for the flow of fluids, seals, pumps, filters, gauges, piping, motors, instruments, controls, guards, spares and all those reelated gadgets.

I do expect capital can be saved and sometimes it can be justified. The big picture and design mantra for many clients is: "less maintenance, is less downtime, is less costly; is more profit".

Oh, my experience is: learning from my employers in not repeating their experiences of bad habits (gadgets), less I sooner or later lose their employment and others like them.

Your friend and collegue

Larry ■

Larry,

I'm sorry that you have had such bad experiences with hydraulic take-ups. For me they have been very reliable and there is very little maintenance. As I said, when my experiences (and those of my customers) become as bad as yours I'll probably agree with you. But for the last 28 years my experiences don't take me to the same conclusion as they do you.

This topic has obviously sparked some contraversy. I am very interested to know what others think, especially our customers. I encourage others to please offer your opinion on this matter. ■

For years hydraulic take-ups have been used on underground conveyors where space and headroom are at a premium. Dowty Meco used to have a very good system that employed a three piab switch system of applying a variable hyraulic load to meet the variable conveyor belt tensions (not just a constant tension).

My advice would be to go with a GTU, even if it has to be located at the side of the belt (due to insufficient vertial height on a horizontal belt),whenever possible. When all is said and done, you can't beat gravity and it's free. ■

Dear Mr. Vishy

Please forgive me on the following comments. These late night hours dull diplomacy.

Personally, I blame that "Joe Guy" for steering this intellectual train into the hydraulic patch.

Look at it this way - you get free advice on subjects yet unexpored.

Ra Ra and right on Catkin.

Given there is space available to apply either a GTU or hydro, one point is overlooked in this dialog. Stress distrubution around the belt loop can be a serious dynamic problem. - more so with hydro. Hydraulic takeups do not normally respond as fast as the acceleration of gravity. As a consequence, during starting and stopping, the belt line forces can cause unacceptable conditions of: structural overloading, pulley overloads, belt and splice overloads, drive and brake pulley slip against belt accel/decel action, and higher fatigue on the same. Hydro. logic control requires deadband allowance which further ups the variances in belt line tension vs GTUs. There are ocassions where this is not true as others will profess. We are talking in the general.

How about the "what if" failure scenarios. Aside from loss of downtime, what are the risks?

Selby, UK (10100 kW) 12 km incline, measured the fixed takeup condition, during stopping, and found the tail pulley force exceeded the steady-state force by 4 times. Had to rebuild the tail station. Engineer stated they thought the shock wave would disapate in 12 km. We published British Coal measurements and our analysis at SME in 1989-90.

Chuquicamata, Chile (6000 kW) - main slope belt - using a powered winch, had the tail pulley foundation bolts elongating from a normally loaded shutdown. The tail pulley design loads were about 10 times the estimated steady-state loads. The engineer had made the tail station 10 times stronger to standardized to the takup loads. The FTU was located at the head station. With a 10 times boost, they still missed the magnitude of dynamic forces at the tail, due to the locked FTU.

Alcoa, Pinjara, Australia 9km downhill and others, removed all their powered winch takeups on their overlands some years back after suffering control coomponent failures and other reliablility issues - read note below on US Borax.

Prosper Haniel, Germany (6500 kW) main slope belt destroyed the tail station from a dynamic shock wave during stopping - had a fixed takeup. The conveyor is in a tunnel. HAd anyb

odybeen near the tail station, the flying idlers could have had bad consequences.

US Borax, USA, had a number of control logic runaways with the powered winch. You can not predict the state of logic control failure, using operational amplifiers, for the strain bridge - they can fail open or closed sending a signal to boost or release the takup force. Other transducers can have similar behavior.

Just a few of the many. To be fair, there are cases where gravity TUs can have dynamic problems. Proper engineering can identify them and control themin a more reliable fashion. The GTU "what if list" is much smaller. GTU failures, as Catkin implies, are in the dominion of God - gravity does not fail. Some of the anacilliary devices do. Their list is small and consequences almost insignificant.



The beat goes on.

Lawrence Nordell

Conveyor Dynamics, Inc. ■

Dear Mr. Vishy,

Have I missed something in your question?

The way that I read your question is that you have an existing gravity take-up unit and were wondering how to handle the take-up and bend pulleys in the event that you had to replace them. However, this thread has developed into a discussion between GTU and hydraulic take-ups (at least nobody has weighed in with pneumatic take-ups).

Please advise.

P.S.: Pneumatic take-up systems can also be difficult to work on, if you have to change out their take-up and bend pulleys -- the same space availability problem persists.

P.P.S.: As a word of agreement with both Misters Nordell and Dos Santos, both gravity and pneumatic take-up systems have their place. The best for the application depends on the system -- of which we have no information on yours. Generally, pneumatic systems are easier to work with, if the main criterium is to have the ability to easily remove or apply take-up pressure. Gravity systems normally have the advantage in allowable take-up travel, thereby permitting belt load surges without doing damage to the system.

Regards, ■

Gentlemen, Larry,

I'm happy to have sparked such lively discussion. Indeed the GTU is great and reliable but so are others when they are properly analized, engineered and manufactured. Simple hydraulic systems can easily be overcomplicated by a so called "gadget freak". Properly designed with ample accumulator volume a hydraulic system just sits there and does nothing but hold pressure. Response time can be handled erroniously by ever bigger pumps or wisely, by simply calculating the tension change due to the system's response speed. Indeed some very large and long conveyors use fixed take-ups by winches. These have load cells to set the desired tension at start-up and again at running but remain fixed during running. Of course the pulleys and terminal structures must be sized for all operating conditions not just steady state running at desin rate. Can you imagine trying to advance the largest shiftable conveyors if equipped with gravity take-ups, or relocating conveyor lines underground.

I do concede that in operation you cannot beat the reliability of a GTU. But there are other ways to do it reliably.

Joe Dos Santos ■