Although you can apply a brake to act as a holdback. There is a potential problem that makes it unwise.

During the initial start phase of an inclne belt with load, when the drive is applying full torque to put the conveyor in motion, asume you have a motor trip. The abrupt stop of tractive tension will cause a large elastic shock and displacement wave to act in reverse to belt's normal motion. The brake cannot catch this reverse action quick enough to inhibit reverse motion. A holdback can.

The question is what is acceptable reverse motion? Noting the belt may push the product backward through the chute and skirt system. Large high tension incline belts are especially prone to such problems.

It is a matter of timing of the brake action and costs to get the valving to apply in the reasonable time to prevent damage to belt, skirts, chute and idlers.

A second point is alll the added maintenance which needs to be considered to check and repair: pump seals, filters, motor, valves, instruments....... Not simple not KISS. ■

Hello Kiruba..

I personally have had no problems with holdbacks... probably because they are:

- Properly sized for the duty

- Correctly mounted with the support stirrup giving correct float to the torque arm

- and filled with the right amount of the correct lubricant

As Larry says, and I always say..keep it simple.

Only use a brake for a holdback if the conveyor is reversible.

Regards

LSL Tekpro ■

Kiruba

I would start by doing a complete failure analysis of the holdback rather than trying to reinvent the wheel. The holdback manufacturer would be a good place to send the failed backstop for this analysis.

As Graham has touched on some of the key points of holdback installations.

The one I see most often in the field is improper lubrication. I would start with the holdback's maintenance manual and follow the installation and lubrication procedures exactly as they are written. EP based oil lubricants will destroy the internals of a backstop over a relatively short period of time.

Gary Blenkhorn ■

There is nothing wrong with mech holdbacks if obv. selected and installed right

One question for Allan Tapp > how can U tell (by PM activities) if a holdback is due for replacement > ie prevent unwanted runback.

Do U suggest CM inspections or time based replacments

In Aust. the code recc. 2 holdbacks for critical applications > riases the question over how U determine one has failed etc.

Thanks

James ■

Dear Mr. Kiruba Sankar,

In case of belt conveyors, it is not advisable to use brake as a holdback. The torque from hold back would be very high and its application for conveyor stoppage would result in to abnormally high deceleration forces in the belt and system. One can minimise such adverse affect by complicated electronic controls but the same would be counter productive. The brake is used as a hold back only in case of reversible belt conveyors (boom conveyor of stacker reclaimer machine) as a no option and also such conveyor is short.

The modern practice is to use free-wheel type hold back. This can be of roller type or sprag type etc. these are tried and tested items and if selected properly should give the desired service life. One should also know the expected service life of such holdback and replace it according to the service life, rather than waiting for the failure and creating accident.

These holdbacks are generally in-built in gear box on higher speed shaft. Most of the buyers opt for this because of economy. Same hold back directly on drive pulley shaft is a better solution but is very expensive and I have found very few buyers who are prepared to expend more money for such an arrangement even though it is superior. The superiority comes because it is rotating at a very slow speed and being separet item receives proper attention for inspection and servicing.

The unsatisfactory service at one location is not sufficient to formulate opinion about the item. It could be more due to wrong selection, source of supply, maintenance care, etc.

Regards,

Ishwar G Mulani.

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

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Originally Posted by nordell

Although you can apply a brake to act as a holdback. There is a potential problem that makes it unwise.

During the initial start phase of an inclne belt with load, when the drive is applying full torque to put the conveyor in motion, asume you have a motor trip. The abrupt stop of tractive tension will cause a large elastic shock and displacement wave to act in reverse to belt's normal motion. The brake cannot catch this reverse action quick enough to inhibit reverse motion. A holdback can.

The question is what is acceptable reverse motion? Noting the belt may push the product backward through the chute and skirt system. Large high tension incline belts are especially prone to such problems.

It is a matter of timing of the brake action and costs to get the valving to apply in the reasonable time to prevent damage to belt, skirts, chute and idlers.

A second point is alll the added maintenance which needs to be considered to check and repair: pump seals, filters, motor, valves, instruments....... Not simple not KISS.

Dear sirs

1) Why do we use stringer support at incline portion of belt conveyor

2) Secondly ,we been advised to use ceremic lagging at pulley in place of diamond grooved rubber lagging for lignite handling system . Why specific reason ? ■

Where can I get hold of a brochure about (eg Bibby) Gradual Release ARU's? These were mandatory to release locked in tension before restarts underground.

Guddu: your questions are just not guddunuff!■

Hi Guddu,

I appreciate your question, however; with due respect, I think the Moderator has to call you to order by refering you to another thread or for you to start a new thered because the issue at hand in the thread is Holdbacks.

With Kindness,

Bonginkosi B. Sokawukile ■

Dear experts,

I have seen the external hold backs fixed at the discharge pulleys. These pulleys are at higher elevation than other pulleys. These may apply very high torque on the hold backs because of additional belt & material load. If these hold backs are installed at possible lowest loaction, hold backs may give better life.

Integral hold backs in the gearboxes are comparatively very smaller & cheaper. Because the torque applied on these hold backs are less compared to the external hold backs mounted on the pulley shafts.

Please give or valuable opinion.

Regards, ■

Hi there Sganesh..

FYI

The run-back torque is the same where-ever, at head or on the ground

The belt mass does not feature in the equation, as what goes up must come down.

An integral holdback is OK for smaller installations only. If you need to change out the gearbox and the belt is full, you will have problems!

You can use an integral holdback as a back-up to the main low-speed holdback though.. often a good idea

Cheers

LSL Tekpro ■

We have installed intergral holdbacks on drives to 1000 kW. The key factor is to have them fitted with an adjustable spring-applied friction clutch. The clutch will not allow an unacceptable overload to be applied by the belt or reversing action of the belt. Thus, the holdback does not need to be designed for 2 or 3 x time the gravity force. The clutch is set to slip beyond 25-40% of the gravity force action rating. If the holdback is sized for 2 x gravity force, there is no likelyhood of an overstress that will defeat the holdback. Periodic inspection of the clutch will reveal whether the clutch is being used beyond its intention. If so, other factors are at play that must be understood.

Vivo el tinto ■

Dear Shri Guddu,

1) Stringers are provided all along the length of conveyor to support idlers, decking plate etc. So, this will be also necessary for the incline portion of the conveyor.

2) The friction coefficient between rubber belt and ceramic lagging is more compared to between diamond groove rubber lagged pulleys and belt during wet & dirty condition. For worst application condition wet & dirty (clay or loam) this coefficient value is 0.35 for ceramic lagging and it is 0.275 for diamond groove rubber lagging, not so remarkable difference. For dry condition this value is 0.4 to 0.45 for these both types of lagging.

So you have option as below :

You can design conveyor taking friction grip between belt and pulley in accordance with the type of pulley lagging. In case of rubber lagging the required belt tension will be somewhat more for no slippage, when it is wet and very dirty. Otherwise it will not make any difference between the two laggings.

You can use rubber lagging but you may require somewhat stronger belt.

With the ceramic lagging the belt tension will be somewhat low if worst friction grip is applicable. However while using the ceramic lagging please see that there is no belt slippage in any situation as it will result in to faster wear of the belt. In case of rubber lagged pulleys, momentary slippage is not so detrimental.

Possibly, Neyveli (who handles lignite extensively) has widespread use of rubber lagged pulleys.

Regards,

Ishwar G Mulani.

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

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: parimul@pn2.vsnl.net.in ■

1. Stringer's are the horizontal channels on which Idlers are mounted and Short Post/Supports are also channels on which the stringers are mounted. These short supports can be on ground, Galleries, etc. based on the physical senario and profile of belt.

2. Based on belt capacity and length the lagging is engineered. for small belts ceramic lagging is incorporated whereas for high capacity long belts Dimond Groove Rubber Lagging is preffered. Also, the Friction coefficient between rubber belt and ceramic lagging is more when compared to diamond groove rubber lagged pulleys. ■

In our system here, this holdback system is provided in Gear Box by the OEM itself.

I think normally all the manufacturers provide this to prevent belt running reverse.

Breakes are not at all success in belt conveyors and are not recommended by experts across the world.

Hold backs are perfect substitute for the Breakes in Conveyors. ■

Dear All,

This thread has had a long life. It deserves a little more updated commentary.

It is possible to monitor the long term health of mechanical holdbacks with annunciation on the potential for failure and loss of capacity.

Conveyor Dynamics, Inc. (CDI) developed some years ago a holdback load sharing device based on our 7100 kW incline conveyor design at Palabora in 1988. Palabora was designed to load share and from field measurements, it did the job. More recently, we have supplied a commercial version for Australia and Brazil. This holdback can be fitted with a tell-tail indicator ( non-electronic) that gives the value of the force on the holdback arm. Thus, an operator can see the value and record the trends or have an electronic sensor (LVDT) type report each stop the value and note the trend if the holdback begins to fail.

The main focus was to provide load sharing on multiple driven pulleys. However, it can also report the force on a single drive pulley with one or more motors.

The device can be fitted with both force and displacement monitors. Thus, if the device is still capable of carrying the load, but, the wind-up displacement is exaggerated beyond an acceptable limit, the operator can be forewarned of a pending problem. ■