Dear Barry Chung:

I do not know if I can give you a definative answer on this, except that it is convention to have such a high safety margin. You should be aware that not all manufacturers us the same safety margin.

The reasons for the difference between operating tension rating (i.e. the posted rating) and the ultimate tensile strength of the carcass is to allow for start-up, impact, and other extraneous short term shock tensile loads. Because these cannot be calculated beforehand, an arbritary safety margin was established.

Another factor to consider is the use of mechanical fasteners as a splicing method. The strength of these splices will allow the user the full tensile rating, plus a little bit more. Therefore, plants do not have to recalculate and downgrade belt tension ratings when using this splice technique. Cold cure and vulcanized splices have a much higher strength rating (when done properly).

To answer your question -- yes you should ignore the ultimate tensile strength and use the ratings for your design work -- the peace of mind is worth it.

Regards, ■

Barry,

May I suggest you review the website information below. The third image gives the DIN 22101 splice fatigue strength interpretation.

The working tension is defined by this standard and most mfgs. to be 14.9% or (1/6.7) of the breaking strength. The breaking strength must reflect the following:

1. running tension 1.0 plus

2. starting allowance @ 40 % above running or 1.4 of running plus

3. degradation and elongation = running = 1.0+1.4= 2.4 , plus

4. splice losses 64% of capacity due to cyclic fatigue loading= .36 avail.

5. Safety Factor = Necessary / avail. = 2.4 /.36 = 6.7:1

http://www.conveyor-dynamics.com/about/splice.htm

Lawrence Nordell

Conveyor Dynamics, Inc. ■

Hello Barry,

Mr. Dave Miller & Mr Larry Nordell have already given the answer to the question with very useful information. My following views will help in general understanding of safety factors.

Safety factor numerical value for design of any engineering item (mechanical, electrical & civil), depends upon the degree of exactness / depth of calculation to decide the magnitude of forces or engineering quantity. The safety factor numerical value used in design, decrease as the exactness of design increases.

Generally used / understood belt allowable tension of approximately 10% of breaking strength implies ratio of ten for breaking strength divided by steady state running tension. This does not reflect true safety factor, however it is acceptable for commencing the design. In design for actual application; this is further checked for additional tensions / forces / strains, due to belt bending at pulley, concave curvature, convex curvature, flat to trough transition, material impact, belt constituent material degradation during course of service life, and loss of strength at joint, etc. When all these are counted / added, the available / used safety factor could be as low as 3.0 for momentary (short time) condition and 3.5 for continuous running, even though said ratio is around 10.

Please also note that safety factor w.r.t yield strength only has meaningful utility. Forces beyond yield strength deform / damages item, and the item becomes non-operational quickly. Therefore, safety factor numerical value w.r.t yield strength will be quite low (may be half) compared to value against breaking strength.

The method of using safety factor has evolved during long time (could be in centuries), therefore, sometimes although the practice is not scientific but the same is followed. For example while designing the shaft by particular method, the material breaking strength is say 400 N/mm2 and one is using allowable strength as 80N/mm2. Now, the material yield strength is 200 N/mm2, and hence gross meaningful safety factor just becomes 2.5. Again, if the starting / sudden application of force etc. is counted in this kind of method of design, the actual available safety factor will be quite low, although it appears 400/80 = 5, at a glance.

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 5882916 ■

Dear Mr. Chung:

Please clarify your comment about the belt not making contact during the offload. Do you mean that the belt is lifting when empty, or that the belt lifts in this area when load (i.e. offloading material)? An empty belt lifting off the start of the incline section of a stacker is a fairly common occurrence ? usually the result of an underweight belt or over-tensioning.

Also, you have indicated that there are multiple cold cure splices over a 210m belt length. How long is the belt? If the total belt length is (for sake of argument) 1000m, or over, the condition you have described suggests that the problem might be with one roll of the original belt ? or with a replacement belt section that is not of sufficient strength. With this many splices over a relatively short belt length, I must conclude that either someone is using up short belt lengths, or the original belt has had multiple failures ? not just the joints. If all of the joint failures are in this one area, the ?problem? could be splice material incompatibility; poor splice technique; a bad roll of belt; or (as you are suggesting) a borderline belt specification. That is, I would expect a random splice failure rate around the whole length of the belt, if everything else is equal.

With regard to your three questions:

[1]A belt should be able to run at 100% of its rated tension ? that is the reason for the safety factor.

[2]The belt lifting does not increase the tension on the belt. Excessive tension from other sources, however, can cause a belt to lift of idlers is certain areas, such as inclines and return idlers.

[3]Your third question suggests that your client is willing to replace the entire belt. If so, you should ensure that the system is properly aligned; that all idlers are functioning properly; that the loading zone is not creating a problem; and that the take-up is not excessive. Once this is done, recalculate the system numbers to determine the minimum belt tension rating requirements and confirm with the belt manufacturer of your choice. For your client?s comfort level, you can increase the belt rating by one level without doing any harm, except for the added cost.

Although it is true that the joints are the weakest part of the belt, it is unwise to assume that either the splices or the belt specification is at fault with a problem such as you described. I would start by examining the failed joints first, to determine if the craftsmanship and materials are satisfactory. If that proves to be good, then look at the system, including the belt, for causes and react accordingly.

Good Luck! ■

Dear Barry Chung

Don't forget to check the number of plies you have in the belt as well as the type of splice.

For example, if you have 3 plies, and your splicer makes a normal join where the plies butt together at the ends, then the number of overlapping bonded pairs is only two.

The safety factor is therfore already 2/3 of the original 10.

This gives sf=6.7 for starters. The remainder of the de-rating and erosion of the safety factor is as given by Larry Nordell. As you can see there is really nothing left even starting at sf=10.

I have seen belts break where the designer used sf=7 and thought that was enough.

Regards ■

Dear Barry Chang:

Once you have calculated the operating tension, the total expected tension to move the loaded belt, you divide by the belt with to obtain the minimum belt rating needed. You do not have to use another safety factor ? the belt manufacturers have done this for you.

Other concerns that you should take into account, however, include load support (to ensure the belt can bridge the idler junction gaps), impact resistance (to ensure the belt carcass can withstand the load point impact), troughibility (to ensure the empty belt will make contact with the troughing idlers, pulley flexing (to ensure the carcass can bend around the pulleys without separating), and cover rubber suitability (to ensure that the cover rubber will provide the protection needed against the bulk material and elements).

With regard to the belt not running on the idlers in the incline area when unloaded, the problem is one of training instability not of tension. When the belt is lifted in this manner it can easily be pushed aside by wind or other factors.

Regards, ■

Dear Barry Chung,

Be aware that the safety factor of 10 or 6,7 is a convention but it is not absolute.

As example, the lift cable for under ground mines usually have a safety factor of 5 in Europe and in the USA but only 3 in South Africa, because there is no cable available with a factor of 5 which is long enough to go 3000 m deep. The people in South Africa say it is enough. In this case the designers have to calculate more precisely the requirements and all needed devices.

In the case of a simple conveyor, the designers mostly don’t calculate all the components but over dimension them. On the other hand, the splices of the belts made on field have mostly less strength as the tested strength in the very clean laboratory or in the manufacturer’s plant because of environmental difficulties (dust, sun heat, humidity, bad skilled people, wrong material and presses etc.). That why a safety factor of 10 is better.

But if you have only 31 N/mm operating tension (7,75 % of nominal strength of the belt) your problem is another. I suspect that cold splicing is very difficult in Malaysia because the glue will begin to polymerize with the sun heat before the splice is finished. You don’t have to increase the safety factor but to find a easy solution to make the splice.

I will suggest to try the Super-Screw fasteners. It is a new fastener made with a multi-ply synthetic and high elastic textile carcass, a hardened rubber as cover material and metal pieces under the cover. This fastener will be installed on bias by placing the belt carcass between the top cover and the bottom cover of the fastener after removing the cover of the belt, and by fixing them with specific self taping screws and a power screw driver. Due to the rubber cover it is also wear resistant. It is dust proof and compatible with scrapers.

You will find further information under : http://www.minet-france.com

Regards

Edgar Jakob

Attachments

installation with power screw driver (JPG)

■

Barry:

Beware of the reference to Safety Factor (SF). This term, as you are beginning to understand, does not refer to the avaliable working tension divided by the breaking strength.

Mr. Mulani warned of the use of yield verses tensile failure. Mr. Spriggs warned of one of the isssues for fabric splices losing an overlap. Such issues are to be pondered for all fabric and steel cord splices.

Once "all factors" are considered, the available SF is much lower. What are the all factors? I refered you to our website:

www.conveyor-dynamics.com

Din 22101 has further divided the SF term into many terms:

1. running tension

2. starting and stopping dynamic momentary tension additions

3. elongation & degradation tension additions - with age/vertical and horizontal curves/mfg build errors/field build errors/pulley bend stresses/off center running/load station impact/idler trough tranverse flexual support/idler trough transitions/material buildup on pulleys/load sharing anomolies among drives/......

4.splice residual capacity after dynamic cycling between steady-state low to high tensions for the given splice design

Applying all of these factors yield a true SF closer to 1.00, as noted by Mr. Spriggs, if all of the factors have been implemented.

Your reference to trying outsome software that produces a rating:

" running tension is about 50 to 60% of the rated tension". This statement appears to be in error. I do not believe any reasonable software/design engineer would suggest a critieria where the belt running tension is only half the rated tension selection. Please reconsider your understanding of this statement.

The forum has given you sufficient information to make a prudent selection.

Lawrence Nordell

Conveyor Dynamics, Inc. ■

The value of safety factor, whish is defined as ratio in between critical load and actual load, depends on following factors:

- Precision of your mathematical model used to depict the problem.

- Reliability of obtained data on load.

- Reliability of data on material used.

- Special safety considerations regarding application in question (an elevator or a bridge, and so on)

Each factor is represented with a coefficient, and the Safety Factor is found by multiplying those coefficients. However, for very many "standard' application

Safety Factor values are already available.

Best regards.

Mike Nemet ■

Dear Barry,

You are getting the nack of it. Yes, the true SF ~ 1.0:1, in fabric or steel, if all conditions are met. The real problem is far more complex, if you want to know the true margin of safety as taken from standards, mfgrs. data, catalog data....... You have to do a lot more, such as review and advance your knowledge of the following:

1. Running Tension Calculation - as Mr. Nemet points out, how accurate is the tension calculation which this is all to be based? What is your choice of analysis: CEMA, ISO, DIN, Goodyear, Bridgestone, ContiTech, Phoenix, Dunlop......, CDI?

For most conditions, these methods, save CDI, yield a 10-25% margin of error toward conservatisim. Fabric consumes more power than steel cord and is the basis of these methods.

As belts age, even after a few months of use, the power consumption will fall ~5%, not counting gravity forces. What about surges? What about motor nameplate picked 10-15% above this tension figure calculated?

Designers calculate the power consumption based on peak conditions which are about 20-25% above the nominal capacity. Make-up capacity is used about 3 in 7 days of a PM week.

What about rubber rheology or rolling efficiency? Todays best compounds will lower the rolling resistance about 20-30%.

Based on the above, I estimate you will get roughly a 20-50% margin of safety over normal running.

2. Dynamic Allowance - DIN 22101 states the motor starting/stopping dynamics must allow for 40% over running tension. As per an earlier publication, I showed, when the true starting value is compared on a fatigue cycle basis, the the multiple is closer to 15% over running tension even though the conveyor is started 10 times a day, with the splice at the same high tension position, witha 60 sec start ramp and 40% overstress.

Thus, we get closer to an added 25% margin of safety over running tension.

3. Degradation and Elongation Allowance - is the "kitchen sink of the unknowns". Do all conditions simutaneously occur? Usually not. Age, accounting for 15% of this condition do not kick in for a while. Vertical curve and belt alignment allowance may not be relevant amd account for another 15%.

Field splice construction is the bad apple. Here, a lot can go wrong and is terribly misunderstood. Few splice contractors know how the splice works, know the meaning of hygiene, how tackifiers and cleaners degrade the splice strength, know how tensile member placement affects dynamic capacity, and so on.

This single point can be the loss of all the above gains. If you want to insure belt splice success invest in knowing and controlling this point.

4. Splice Dynamic Strength - this loss is normally taken as 64% of the breaking strength for steel cord ( SF=6.7:1) and ~90% for fabric (SF=10:1). However, this is highly dependent on the splice design and core rubber compound dynamic efficiency.

A lot of research, by a few belt manufacturers, have advanced our understanding of splice dynamic efficiency. Proper design can yield a > 50% increased dynamic strength and hundreds % in cycles to failure.

The true, in service value, can vary with extremes depending on the belt cycle times to peak stress such as short in-plant conveyors in the range of <100 meters, to longer > 1000 meters, or overlands > 2000 meters. A lot more can be said here, but will become boring.

My point in noting the above 4 points, is that it is not a simple process to pin point the value for the "margin of safety" above the stated standards of SF. These standards have little to do with the margin of safety in proclaiming SF. Scary??

Thanks for the interest on a distressing, poorly researched, poorly published, and poorly understood subject.

Lawrence Nordell

President

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com

email: nordell@conveyor-dynamics.com ■