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Thread: Conveyor Belt Strength Rating

  1. Conveyor Belt Strength Rating

    Can anyone explain why the Recommended Working Tension Rating (Vulcanized Splice) quoted by almost all manufacturers is only 10% of the Minimum Full Thickness Tensile Strength of the belt ? What is the basis for this and should we ignore the Tensile Strength and use only the Working Tension Rating in design calculations ? Would appreciate a response.

  2. 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,
    Dave Miller
    ADM Consulting
    10668 Newbury Ave., N.W.,
    Uniontown, Ohio 44685 USA
    Tel: 001 330 265 5881
    FAX: 001 330 494 1704
    E-mail: admconsulting@cs.com

  3. #3
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

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    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.

  4. 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

  5. Dear Gentlemen,

    Thank you for your answers. Indeed, as mentioned by all of you, the method of arriving at the safety factor is really an emperical one. I looked up a textbook and there it was; n ranging from 10 to 15 depending on the type of belt and the ultimate strength.

    The reason for my original question was that a customer of mine has asked me recently to analyse the design of a particular stretch of conveyor in his plant where the joints failed very often, with catastrophic results. Now, I've worked out the maximum running tension to be about 31 N/mm. The belt is an EP400 x 1050mm BW with Grade BS:N17 covers. Using n=10, this meant the belt was running close to 80% of it's rated tension! I thought this figure should have been closer to 50 or 60%. What is CEMA's and ISO's recommendation?

    For a line of only about 210m cc horizontal, the belt had six joints, quite probably from repeated repairs. To make matters worse, I also noticed that, during offload condition, the inclined stretch of the belt was not in contact with most of the middle carrying idlers. Is there anyway of calculating the extra tension caused by this? Also the repairs were made by cemented or cold joints. Can anyone recommend an appropriate safety factor to use?

    Regards.

    Barry Chung
    Aurium Merchant Corp. Sdn Bhd

  6. 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!
    Dave Miller
    ADM Consulting
    10668 Newbury Ave., N.W.,
    Uniontown, Ohio 44685 USA
    Tel: 001 330 265 5881
    FAX: 001 330 494 1704
    E-mail: admconsulting@cs.com

  7. Dear Mr. Miller,

    Thank you for your reply. To answer your first question, this is the condition that exists when the belt is at running speed but with no load as opposed to the belt lift one usually sees when the belt is accelerating soon after startup. The same condition exists even when the belt is at rest. I would guess that if the belt is adequately strong, this condition will not produce any significant elongation as this would be compensated by the movement of the takeup. But would it however induce some additional stresses due to flexure? Also the total length of the belt is about 460m.

    1. I have been trying out some conveyor design software lately and noticed that the programs usually chose a belt with a rated tension of about 50 to 60% of the running tension. Is there any significance to this?

    2. My thoughts on this are described above. But in addition, wouldn't there be additional stresses if the belt were not properly supported when running with load?

    3. I am inclined to agree that my client should move one up on the belt specifications say EP500 but with the same number of plies and total belt thickness. I will try to work out if this would produce any significant extra load on the drive unit.

    Regards.

    Barry Chung
    Aurium Merchant Corp. Sdn Bhd

  8. Dear Mr. Miller,

    My response in Item 1 of my previous message should read '....... chose a belt where the running tension is about 50 to 60% of the rated tension.' Wish to apologize for the error.

    Regards.

    Barry Chung
    Aurium Merchant Corp. Sdn Bhd

  9. 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
    Graham Spriggs

  10. 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,
    Dave Miller
    ADM Consulting
    10668 Newbury Ave., N.W.,
    Uniontown, Ohio 44685 USA
    Tel: 001 330 265 5881
    FAX: 001 330 494 1704
    E-mail: admconsulting@cs.com

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