Hello Shakee,

Commonly referred belt safety factors are in conjunction with maximum tension occurring during steady state running and momentary during starting or stopping.

In case of “favourable” conditions, these values are 6.7 during steady state operation and 4.8 during momentary condition.

Again the actual safety factor gets reduced at transition zone and also when belt is passing on pulleys, etc. The final actual safety factor will not look so high.

The belt behaviour is different then structural steel and the norm for the safety factors will be different. These safety factors are for generally used design practice and accuracy. The experts for specially designed conveyors take somewhat lesser value because they made the detailed analysis and are sure that the bare minimum value will not deform the belt during continuous use.

Refer the belt manufacturers published data on this subject.

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 ■

Supplementing Dr. Mulani's very good explanation, in fabric belt applications their are additional considerations including impact abuse at loading and splice fatigue as the belt travels through the various transitions. Indeed, in nylon fabric reinforced belts the safety factor, based on steady state running must be at least 12. Nylon has a high permanent stretch and at safety factors less than 12 the permanent stretch may continue indefinitely.

Joe Dos Santos ■

I believe that the 80% load level, by some conservative specifications, refers to the filling of the belt cross-section not the belt tension. Even this matter is ambiguous without a universally standard definition of the correct surcharge angle to use. ■

Shakee,

Thank you for the clarification. I have never previously run across such specification that limits the percentage of rated tension to be utilized. This is very confusing. The whole issue of rated tension for belting is very ambiguous. To their credit the Europeans assign an ST rating to steel cord belts expressed as the breaking strength in kN/m or N/mm which is the same thing. This is now also the case in North America though for many years the working belt rating was considered the norm. With fabric belting, in North America, belts have a working rating assigned to them by the manufacturer. These are not necessarily at a common safety factor against breaking. You just cannot know what you are getting and you will not likely get the same strength at the same rating. The 80% factor that you mention may be someone's reaction to a bad experience but it is ill advised. It is time to publish standards for belt strength and safety factors. All belts should be designated for their breaking strength as with the ST rating of steel cord belts. That way the same designation means the same breaking strength. Recommended (or specified) safety factors should be listed along with the rational for these safety factors. It does make sense that the safety factors against breaking will be different for the different types of reinforcements. As I mentioned in a previous post, nylon warp fabric belting requires a minimum safety factor against break of 13. This is to stabilize the permanent stretch. Polyester is more stable in this regard and the safety factor may be lower. These vary between 6.7 and 10 depending on the manufacturer and this is a problem since there is no basis for that kind of difference.

In the case of high strength steel cord belting the typical starting safety factor tends to be 6.7. With thorough sophisticated analysis including the transient dynamic forces, safety factors are often reduced, with confidence, to levels of 4.5 to 4. Such safety factors, even lower were always common for the cable belt system which utilized bare steel cables as the tension members.

All of the above needs rationalization in the standards. I welcome the comments of other forum members, particularly Mr. Larry Nordell who has a better perspective of this issue than any one else that I know.

Joe Dos Santos ■

Dear Friends,

Has the Belt jointing efficiency has any thing to do with RMBT? In case of fabric belt 80% efficiency is considered for 5 ply construction.

Rgds,

Nalinakshan.

Originally Posted by Joseph A. Dos Santos

Shakee,

Thank you for the clarification. I have never previously run across such specification that limits the percentage of rated tension to be utilized. This is very confusing. The whole issue of rated tension for belting is very ambiguous. To their credit the Europeans assign an ST rating to steel cord belts expressed as the breaking strength in kN/m or N/mm which is the same thing. This is now also the case in North America though for many years the working belt rating was considered the norm. With fabric belting, in North America, belts have a working rating assigned to them by the manufacturer. These are not necessarily at a common safety factor against breaking. You just cannot know what you are getting and you will not likely get the same strength at the same rating. The 80% factor that you mention may be someone's reaction to a bad experience but it is ill advised. It is time to publish standards for belt strength and safety factors. All belts should be designated for their breaking strength as with the ST rating of steel cord belts. That way the same designation means the same breaking strength. Recommended (or specified) safety factors should be listed along with the rational for these safety factors. It does make sense that the safety factors against breaking will be different for the different types of reinforcements. As I mentioned in a previous post, nylon warp fabric belting requires a minimum safety factor against break of 13. This is to stabilize the permanent stretch. Polyester is more stable in this regard and the safety factor may be lower. These vary between 6.7 and 10 depending on the manufacturer and this is a problem since there is no basis for that kind of difference.

In the case of high strength steel cord belting the typical starting safety factor tends to be 6.7. With thorough sophisticated analysis including the transient dynamic forces, safety factors are often reduced, with confidence, to levels of 4.5 to 4. Such safety factors, even lower were always common for the cable belt system which utilized bare steel cables as the tension members.

All of the above needs rationalization in the standards. I welcome the comments of other forum members, particularly Mr. Larry Nordell who has a better perspective of this issue than any one else that I know.

Joe Dos Santos

■

Belt splicing is also an area of ambiguity. Good splicing procedures are written but their flawless execution cannot be guaranteed. At Dos Santos International we routinely hedge on splicing of multi-ply belting by increasing the splice angle. This too is an area of ambiguity and needs rationalization.

Joe Dos Santos ■