1. Steel Cord Belt Nomenclature

The DIN 22131 standard specifies that the tensile strength of - say - a ST 2000 belt should actually be 2000 x 1.1 = 2200 KN/m, i.e., this belt should break at a load not less than ST 2200.

Since most other specifications do not stipulate this 10% "buffer", which is more of a belt selection consideration than representative of the belt parameters itself, should the nomenclature of such a belt be ST 2200 OR ST 2000 ???

If a belt that breaks at a load of 630 KN/m is called ST 630 or 630/4 why would a belt that breaks at 2200 KN/m be called ST 2000 simply because a certain specification stipulates a 10% buffer strength rating ?

Can someone explain this please ?

Thanks

2. Kayem,

This not the only confusion about some of the DIN specifications. Look at DIN 22129 vs 22131. These are separate controls for underground and above ground steel cord splices. Like two different entities forming dissimilar criteria for the same problem.

They disagree on the pitch distance between cables for the same strength. They disagree about the splice pattern. One standard says use a 123 step while the other says 132.

Author's perogative??

LK Nordell
Conveyor Dynamics, Inc.

3. Paul Haywood Guest
The issue you mention is a misinterpretation of the text of the norm. The belt is classed as a 2000 N/mm belt, but since the belt is made up of cables and in between the cable there is rubber (the cable pitch is larger than the cable diametre) it is required to have cables with strength in excess of the nom. belt strength to compensate for the loss in the spaces in between cables. This is approx. 10%.

Frits Muller
Muller Beltex BV
www.mullerbeltex.com

4. Dear Fritz,

The belt steel cord strength rating is calculated as follows:

Defining strength class (ST) of steel cord belt is =

ST-xxxx N/mm where "xxxx" is the rating ; where,

XXXX N/mm = no. of cables in belt x single cable rated break strength (N) divided by the belt width (mm).

There is no accounting for the rubber.

The rubber gap between the cables in the belts is =

(belt width - edge rubber to cable x 2 edges - cable diameter x no. cables) / ( no. cables -1).

In my opinion the 10% addition is the norm drafters whim. Truth be know, most belt manufacturers already put their own fudge factors on the cable breaking strength by adding allowances such as:

a) First increase is the minimum cable test strength is used and it is typically 5% below the nominal of 6-10 cable breaks in the lab. The test specimen can also reduce the apparent strength as well as how the cable is grabbed. The cable specimen must be correctly socketed in epoxy while the wire strands are able to load share. This is not typically done.

b) Second increase is the belt manufacturer rounds up on the cable number from the "XXXX" calculation and Safety Factor criteria.

c) Third increase is when the cable is encapsulated and fully penetrated by rubber you gain another 3-4% strength which is not included in the "XXXX" strength formula.

d) Fourth increase is when the manufacturer derates the cable break strength to pass the dynamic fatigue test without breaking the cable from bending around the pulleys.

L. Nordell

5. Over a week and still no conclusion ??? Well, Mr. Nordell's response does seem to indicate that the belt should very well be termed as ST 2200.

Is that a 'yay' or a 'nay', forum ?

6. Kayem,

No multiplier is used, unlike DIN 22131.

The formula is: (working strength N/mm x SF) = ST-XXXX N/mm.
If working strength is 500 N/mm; SF= 6.0:1 then ST-3000 N/mm.

Lawrence Nordell

7. Dear Mr. Nordell,

The tensile strength of a belt is an absolute value that can be established through tests. The working tension, on the other hand, is not.

How then, is the belt tensile strength arrived at from the working tension ? Should it not be the other way round ? Or have I not been able to follow your inputs ?

Regards

Kayem

8. Kayem,

You have the idea but backwards.

The responsible engineer makes the calculation of the working tension based on gravity and rolling resistance factors ( idlers, belt rheology or CEMA/DIN/ISO) and parasitic losses. This then becomes absolute. The absolute belt strength tension rating is then derived by muliplying the above absolute working tension by a selected belt strenght safety factor ( 5:1, 5.5:1, 6.0:1, 6.7:1, 7:1, 8:1, 10:1....yours??).

The calculated working tension is an absolute ( or not so absolute) value derived by responsible or not so informed engineers. Although, it varies among engineers and published standards, which use different techniques to determine its true value, there is only one true value. This range of absolute tension can vary from the true working tension by 20-50% as I have published. You can find many publications on this subject in BSH. I assume you have visited our website where we reference many papers on the procedure of absolute working tension analysis which we prefer to call the true operating tension.

Lawrence Nordell
Conveyor Dynamics, Inc.

9. Dear Mr. Nordell,

Here, since we are talking about "christening" a belt, IRRESPECTIVE of what operating conditions it is installed on (and therefore irrespective of the system tension demand and by rote, of the factor of safety thus derived) am I incorrect in stating that the belt tensile strength is the only absolute here ?

In fact, given the usual system parameters, each belt selection software would probably generate different T1 values since the underlying assumptions vary from software to software (and person to person) as well. The system tension, therefore, is really not as sacrosanct a figure as the belt tensile strength.

The factor of safety, which - mathematically - is a derived figure, is probably more a fruit of experience than anything else. In the early 90's, the F.O.S. for steel cord belts was nothing less than 7 : 1 in most Indian installations. Somewhere, over the next decade, the figure 6.7 : 1 was mooted in the designer's circles. Today, we even hear of figures lower than 5.5 : 1 !

Mr. Nordell, as I understand it : when we select a belt for a given installation, the working tension is the starting point - even while we make so many assumptions (viz. friction factors, K factor, factors governing the power transmission - type of coupling, type of starting etc. - and even the most radical assumption that all these factors will remain undisturbed through the seasons and as the system ages).

It is partly (repeat : partly) because of these intangibles - largely catered to through empirical formulae and reference tables - that the need for a factor of safety emerges and therefrom : a minimum tensile strength requirement for the belt.

I would love to be corrected, but the belt tensile strength seems to be really the only absolute here.

Regards

Kayem

10. Gabriel Lodewijks Guest
Dear Kayem,

One important issue to take into account. If people around the world use a standard (DIN,ISO,CEMA) to design a belt conveyor system and that standard prescribes that a belt, that is denoted as an ST 2000, should in fact be able to take 2200 N/mm then that belt is clasified as an ST 2000. You are obviously free to do whatever you want to do, but if you deviate from design standards then you are on your own. There are a number of companies around the world that have the capability to do so, but you take a risk.

Kind regards,

Prof.dr.ir. Gabriel Lodewijks