Dear George:

Some points to consider between fixed, semi-fixed, powerd and gravity take-ups:

1. travel requirements - loading induced, thermal induced, start-stop induced, splice allowance, permaent stretch allowance, sag limitation control, etc.

2. space - by example underground cavity excavation

3. portability - shiftable vs staitionary conveyors and their structures

4. reliability - manual, electrica; electrical-electronic vs gravity

5. manual, powered and automatic tension control adjustment

6. belts stress limits - fixed can induce 15- 25% more fatigue stress in belt due to start-stop and thermal expansion-contraction

7. What if failure scenarios, their consequences and methods to control

All things considered, gravity is the more dependable (reliable), shock damping and tension controllable takeup(tup) system.

On very small conveyors, as you note, where the tup travel is short, might favor a screw take-up

Certain coveyor configurations can have advantages of using a semi-automatic winch controlled tup to control dynamics vs the alternative of developing dynamic control of shock waves such as flywheels.

By example, your crosses a field (50 -5000ft) and rises to the head pulley (10 to 100ft in 5oft) at 250 tph with 100 pcf, with no thermal expansion, splice allowance, permanent stretch or belt sag conditions, then we see the belt tension travel equal:

1. 50ft lg horiz.; 50ft lg x 10ft lift tup travel = ~1 ft. use fixed

2. 500 ft horiz; 50 ft lg. x 10 ft lift tup travel = ~5 ft use gravity

Fixed screw takeups are not practical when the travel exceeds 3 ft. (1 m) or the force exceeds simple man turning torque wrench force, factoring in the scew pitch mechanical advantage and friction loss to turn each screw.

Note, screw takeups also need to be cautious of belt alignment errors and belt damage when changing the travel.

Electrically powerd and self-locking travel winch takeups are used is applications where space is a premeium, where the gravity counterweight is difficult to implement, or where shock wave control is best applied by the winch type tup.

Another question is: when do you placed the tup at tail (free tup pulley) vs building a 3 pulley takeup at the head?

Others will follow.

Lawrence Nordell

Conveyor Dynamics, Inc.(CDI)

www.conveyor-dynamics.com ■

There are four options for take-up.

1) Screw Take-up

2) Gravity take-up

3) Winch take-up (fixed belt length during operation)

4) Active winch take-up (variable belt length during conveyor running)

Screw take-up is the cheapest but would not be practical for take-up travel more than about 1200mm (48 inch). So, for take-up travel more than this value, the remaining three options are available.

The active winch take-up is used in very special cases. Its operation depends upon various sensors, which need high-class maintenance and upkeep for its reliability. This take-up is very expensive.

The winch take-up of serial # 3 is somewhat expensive compared to gravity take-up and it has more mechanical components. Its use becomes essential when gravity take-up mass is very huge, and slackening of belt would require large hoist etc and consequent downtime in maintenance of pulleys, belt etc. This take-up is also essential for shiftable conveyors.

So, for the applications other than mentioned above, the gravity take-up will be the choice. The take-up choice should be further analysed as mentioned by Mr. Nordell.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

There is also the hydraulic ram take-up, that are used in underground conveyor mining.

LKN ■

Hello George..

We are normally bound by the client's specifications which invariably limit "fixed" or screw take ups to conveyors of only 30m, and thereafter use gravity.

In the real world however, the practicalities are somewhat different.

For example, we should differentiate between flat conveyors and ones with vertical concave curves.

If flat, you can go for a long way with a fixed take-up, but if there is a concave curve, you will need a gravity or active winch to avoid any problems.

Interesting for your case is an overland conveyor that worked on sand at the West coast here. It was 1.5km long 900mm wide fabric belt for 800t/h and had a winch that took out the slack out and then simply locked up.

(It was originally designed to operate in conjunction with a load cell, but that never worked properly and so it was by-passed)

Despite therefore being a "fixed" take up on a long stretchy fabric belt, it actually worked fine for many many years, and made me wonder about the need for gravity take-ups. other than to ensure the lowest belt class can be used.

This is especially the case when I think of all the shiftable and underground conveyors with steelcord belts I have built with simple "fixed" winches.

Regards from

LSL Tekpro ■

Hi George

There is no "MAGICAL" number but in the salt industry I find that any conveyor over 150' long is my limit for manual take-ups.

The belting used plays a big factor in the amount of stretch. Therefore as other suggest it is only by formal calculation that the true stretch be confirmed and then the method of take-up be dertermined.

I have been checking into a product called Telescoper by Bryant (see link below). They are being used in the mines around Sudbury with much success.

http://www.bryantpro.com/BryantTelescoper1.htm

Best regards,

Gary Blenkhorn

PS - Did you ever solve your screen tube water contamination problem? ■

Mr. Baker,

Ironically fixed type, screw take-ups, are used in the short conveyors where, as a consequence, belt tensions are most difficult to predict and control. It is much easier to predict and control tensions at longer conveyors that have fixed type take-ups. Shiftable conveyors make use of such, having fixed winch type take-ups.

The problem was very frustrating to me in my early career. I could do a constant strain analysis and determine the correct tensions under any loading condition. Then came impimentation. We set tension and measure it on a stopped empty belt in order to get another set of tensions (as predicted mathematically) when running loaded. Indirect tension measuring using belt sag between returns (according to Tens=(Wb x LxL)/(8 x Sag)) was unreliable as the belt is typically not flat accross the width and it is impossible to determine sag accurately. It was this frustration that led to my developing the DSI Semi-Automatic take-up. This system uses screw take-ups that are mounted on a common slide frame. The slide frame is inturn pulled (by the tensioned belt) towards fixed compression springs of known constant. thus the belt can be tension very accurately and it is easy to see when tension is lost and the take-up needs re-adjusting. These Semi-Auto take ups have been used very successfully at many of our short DSI sandwich belt conveyors. Tension control is very important in such applications.

Joseph A. Dos Santos ■