If you are expecting the rolled up belt to contain the material and block the system when it accumulates in the curve you are on very exiting ground. In might be workable depending on the idler pitch around the curve. The cost of extra idler sets will probably exceed the holdback cost and you would still have the risk. Then you would have to start the beast and would need more power and a bigger chute to accommodate the surge. So definitely you need a holdback if there is any chance of roll back. A brake alone will not react to the system whereas a holdback is reasonably fail safe.

One ancient rule of thumb is that if the lift power exceeded one third of the total then you needed a hold back. Don't ask me why though: conveyors were shorter then. There is an interesting article on multiple holdbacks lurking within the forums and it makes very worthwhile reading. I thought so anyway.

Then again! Since rolled up conveyors are usually long you need to examine the location of any potential rollback before contemplating the earlier mentioned risk. When you analyse such a beltline you will most probably realise that you need both because runback/overrun can often occur in either direction.

It is nice to see such a can of worms opened so early into the new year. ■

Hold back is required in pipe conveyor in just the same way as conventional conveyor.

You just have to calculate resistance to run back depending on incline, there is a little more resistance to run back in pipe conveyor.

Paul Holt ■

Thank you Paul.

The Oracle on the subject has spoken quite definitely. No more needs to be said. ■

Thank you John your confidence is appreciated. ■

Hello,

Hold back is required if lift resistance is more than 60% of sum of all other resistances calculated for power calculation.

Now ‘other resistances’ are mostly frictional in nature. The friction coefficient value can vary from minimum to maximum. So resistance calculated for power are obviously on the basis of upper value of friction coefficient. In order to have safe size i.e. torque rating of hold back, the sum of ‘other’ resistance is multiplied by 0.6. This 0.6 value is with reference to DIN / ISO value of conveying basic coefficients ratio 0.012 / 0.02.

The rule is same whether belt conveyor or pipe conveyor.

Hold back torque rating would correspond to roll back force (Lift resistance – 0.6 of other resistances) x (Design safety factor). The design safety factor could be say 1.5 or as per choice, less or more.

Ishwar G. Mulani

Author of Book: ‘Engineering Science And Application Design For Belt Conveyors’. Conveyor design basis ISO (thereby book is helpful to design conveyors as per national standards of most of the countries across world). New print Nov., 2012.

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: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■

Accurate calculation was not considered in earlier times: by me anyway. Even today there is an acceptance of rules of thumb.

Rolling resistance in the, assumed, forward direction accounted for the newly built condition and the resistance chosen was assumed to be the same in both directions. After in-service reduced rolling resistance has been taken into consideration there is an increased net accelerative roll back condition. Should not the lower DIN/ISO values be used to advise the holdback supplier when estimating the rating?

Just a thought.

(A most disturbing rule of thumb exists in heavy vehicle applications where articulated vehicles are interconnected by a 2.5" kingpin formalised in the 1930's, at the latest, when the vehicle mass was about 24t. The application persists to the present day where vehicle mass is 42t and slightly more for container transport. Vehicle manufacturers ignore this uprating. Although I am not the greatest fan of superfluous mathematical analyses I get more and more alarmed at complacent reliance on ancient rules of thumb. Accurate analysis methods are readily and cheaply available, especially in Asian shopping malls. However it is difficult, nigh impossible, for authorities like DIN/ISO & SAE to react and implement because of market forces. Do we just carry on until something happens?) ■

[QUOTE=johngateley;87444]Accurate calculation was not considered in earlier times: by me anyway. Even today there is an acceptance of rules of thumb.

Rolling resistance in the, assumed, forward direction accounted for the newly built condition and the resistance chosen was assumed to be the same in both directions. After in-service reduced rolling resistance has been taken into consideration there is an increased net accelerative roll back condition. Should not the lower DIN/ISO values be used to advise the holdback supplier when estimating the rating?

Just a thought.

Yes I agree with all conveyors it is necessary in the first instance to do the best theoretical calculation that you can but then to be conservative with the answer as run back resistance friction can vary with conditions new / old, heat, freeing / icy conditions etc.

Further in some industries in UK such as coal industry (not many now I know?) it was compulsory to use controlled torque release holdbacks for safety. ■

Hi Paul,

I remember the Holset, I think, gradual release holdback.

The salesman used to bring a cutaway model to demonstrate the operation.

Is it still on the market somewhere?

The story was that a miner had started to clear away tail end spillage and the locked in tension got released and dragged him through the tail drum. The coal board subsequently commissioned the gradual release model. That was always the story I heard. When I mention gradual release nowadays nobody knows what I'm talking about so I guessed they had gone out of production. ■

Hi John

Yes the Renolds Holset anti runback was developed following some serious incidents and 2 fatalities around 1981 in UK coal mines. As you know most holdbacks were and are now gearbox integral (sprag clutch) type units but following governing specifications by UK coal and a few others to prevent belt conveyor windup these controlled tension release units where designed and required to be fitted on the shaft of the non drive side of the pulley so to be independent of the gearbox.

Renolds still manufacture these and I believe are used in Australian coal mines. ■

Thanks Paul,

This link is available for the new lads. http://www.renold.com/media/162187/F...els-TR-TLB.pdf

Cheers. ■