Dear Mr. Kayem,

Your query is somewhat special in nature and hence possibly you have not received any response since few days. Therefore, I thought let me respond.

The rolling resistance i.e. resistance to belt motion on rollers is affected by the following particulars:

a) Hardness / softness of the rubber.

b) Cover rubber thickness in contact with rollers.

c) Elasticity / hysteresis losses of the rubber.

The point (a) can be compared with movement of car wheel on soft ground or hard ground. Softer ground like sand results into more resistance to motion.

The point (b) can be compared with sand layer thickness on road. If the sand layer thickness is very small then resistance to wheel is also less because there is no room for car wheel to dig in.

The point (c) concerned with the elasticity and hysteresis losses. If the characteristics of the rubber maintains its elasticity i.e. to regain shape without crossing yield point deformation, the energy put during deformation will be recovered during reformation (recovery of shape). This will result in less resistance. This will be reflected in the arc of contact between the rollers and the belt in forward location and in rear location i.e. if there is no hysteresis losses then both these arcs will be equal. If the rubber characteristics and the level of deformation is such that there are hysteresis losses (which are always there, sometimes less and sometimes more) then this arcs will not be equal which will result into loss of energy and drag resistance.

The phenomenon mentioned for belt cover is also applicable to belt carcass. However, carcass is much harder compared to rubber and its effect is marginal.

So, if you wish to have a very low rolling resistance to belt motion attributed to this phenomenon, you can achieve it by selection of appropriate hardness of rubber, thickness of rubber cover, bigger diameter of roller etc., as could be permitted in a particular application.

The issue has been discussed in my book on belt conveyors i.e. how to account for rolling resistance (instead of how to reduce it).

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Dear Mr. Kayem,

Yes, there is a general lack of general experience on this subject. If you have specific requirements, you may contact the test labs of the major belt manufacturers (Goodyear, Conti, etc.) that have conducted research on the subject, or major university labs (University of Hannover, Germany, etc.).

Although there has been interest in reducing belt / idler resistance in order to lower the overall horsepower requirements to transport material, I have the basic concern that a cover designed to lower idler resistance will also lower the resistance between the belt bottom cover and the drive pulley(s). If this happens, I suspect that the instances of slippage on the drive pulleys will increase and that users will have to resort to ceramic lagging or increased belt tension. Hence the potential savings from reducing idler rolling resitance by reducing belt cover coefficient of resistance can be lost by other costs.

As Mr. Mulani has stated, this is a specialized area which should be discussed with the belt manufacturers. I believe that the subject is only of interest for long overland systems that require high horsepower.

Your initial enquiry suggested that this was a general interest issue for you, or that you were looking for a method to confirm the claims of your suppliers. If the latter is the case, you can check the rolling resistance of one belt over another by putting the belts in question on a test or operating conveyor and recording the motor load (amperage) requirements under varying load conditions.

As far as the basic chemistry is concerned, Mr. Mulani has supplied the information. Generally, softer rubber of each rubber type will give higher resistance values and require more horsepower to move the belt. Also, different rubber types have different resitance values for the same hardness (e.g. natural rubber has a higher friction value than EPDM for the same hardness).

As far as dynamics are concerned, Mr. Mulani has also explained that you can expect higher resitance values with thicker covers.

Regards, ■

Kayem,

PLease visit our website www.conveyor-dynamics.com which gives references on how rubber influences rolling resistance and the power equation.

Conveyor Dynamics, Inc. has invested well over a million USD in the development of how to analyze rubber rolling resistance. Rubber constitutes about 75-85% of all rolling losses for the horizontal conveyor profile. As we have published in many articles, this can be defined as:



1. idler indention resistance,

2. rubber flexural losses in moving between idler stations

3. material (ore) trampling or aggitation losses between idlers

Idlers bearing and seal drag constitute about 11-15%

Some misnomers stated by others should be noted:

1. There is no direct correlation between hardness and rolling resistance - ask the belt mfgs. They have tried such correlations and found little

2. Low rolling resistance compounds don't have a significant influence on drive friction ( not hardness dependent)

3. Some mfgrs. have found that their low rolling resistance compounds produce more cover wear ( less abrasive resistant)

4. Low rolling resistance should be applied to all belts not just overlands - there is little cost penalty and great environmental benefits.

5. Only one belt mfg has the tools to predict power from rubber measurements and CDI provided this technology.

We are the first company to perfect (1989) the method of measuring rubber compound properties and accurately predicting conveyor power from rubber E', E", strain, strain rate, belt speed, temperature, cross-sectional loading, idler trough configuration, idler diameters, % loading, belt construction and cover thickness. We have provided many publications on the concepts and given many field measurements which verify the methods. On two occasions the same conveyor was fitted with identical belts except for the compound of the bottom cover. One was in the USA and one in South Africa. THese are 7 and 6 km belts. Both have greater than 20% drop in power with the better rubber.

CDI can make the measurements from a small sample and predict power draw. Our accuracy is usually within 98% of measured.

We provide this service for a fee. CDI designs belts on all continents including India. Call or write us for any assistance you may require.

Lawrence Nordell

Conveyor Dynamics, Inc. ■