Dear Goitse Choabi,

For generally used multi-ply synthetic fabric belts, the vulcanized joint percentage efficiency is given by formula 100 x (N - 1) / N, where N is the number of plies. Thus, a 4 ply belt will have 75% efficiency at joint. If the belt is 1000/4, its carcass strength is 1000 N/mm (in portion where there is no joint). The joint breaking strength will be 1000 x 75 / 100 i.e. 750 N/mm.

There are special two ply belts with thick inter-ply rubber, which can have joint strength of 100%. Refer belt manufacturer’s catalogue / data on this subject.

This issue has been also extensively discussed with practical examples in the book authored by me.

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in ■

Breaking strength rating of splice with respect to ply strength times no. of plies has a limited meaning. All belts, in use, are rated by their dynamic splice efficiency. DIN 22101 makes a stab at this rating system. THis is the natural loss in strength associated with cyclic strain loading.

I assume you are aware that a belt's usable strength is set by its safety factor (SF) where:

SF = breaking strength/ operating load( your breaking load )

SF = 6.7:1 steel by DIN22101

SF = 5.5:1 steel in practice by many manufacturers and designers

SF = 4.5:1 steel on some installations

SF = 3.8:1 steel practiced on one installation

SF = 10:1 fabric practiced by most

SF = 8:1 fabric practiced by some and according to the belts fab.

Restated, the operating load shall not exceed 1/8=12.5% of the total ply strength rating.

The dynamic splice strength is governed by the strain magnitude and cyclic loading reference DIN22110 (If my memory is fit). Polymers have a finite strain/cycle life. Rubber is a polymer as are fabric materials. The dynamic strain/cyclic efficiency, of the interply rubber and its adhesion bond strength to the fabric, are also governed by the stress distribution along the ply interface between opposing fabric action. Fabric construction, interply rubber endurance, and other enhancement procedures can make significant difference to the fatigue (dynamic) strength or the load capacity for a set number of cycles.

A low elastic modulus fabric will have a poor dynamic strength compared to a high modulus fabric. The elongation growth, of the fabric, as it is loaded along the splice path, adds to the inherent rubber shear stress. More elongation means more added stress and early failure.

Every time the splice travels over the drive pulley, where T1 drops to T2, the splice undergoes a load strain cycle. The life can be determined by testing which is often done for steel cord, but less common for fabric.

Hanover University used to have a hydraulic fatigue test machine for fabric. Some other universities and manufacturers also have such equipment.

As Mr. Mulani states, the number of plies also aids in setting the splice strength rating. If you draw a sketch, of the ply configuration for the splice pattern and lay arangement, with a hatching to indicate the direction of the shear stress / strain, you will see his comment illustrated. HE also said that there are ways to overcome this deficit. These procedures continue to be studied for low ply count belt construction. By example, the scissor splice overcomes this problem.

I think I have gone on to long on the point. See our website of the Finite Element Analysis of the steelcord pattern. This is similar to the action of the fabric and rubber.

Lawrence Nordell

Conveyor Dynamics, Inc.

web: www.conveyor-dynamics.com ■