Dear Mr. Mike,

I understand that you are referring to rollers used in belt conveyors.

In case of belt conveyors, we have belt width ranging from 400 mm to 2400 mm. So, there will be very wide variation of the load acting on the rollers. The load acting on the roller with 400mm belt will be negligible compared to the load action on 2400mm rollers. So, the roll diameter and thickness has to be greater for wider belts.

The rollers will never be 100% aligned, that means they will never be exactly at right angles to the belt motion. This is due to reason that there is installation tolerance and one cannot invest extraordinary efforts to install the rollers, like machine tool alignment. As the rollers are slightly mis-aligned, there will be always some sliding action of belt on the rollers. Now, the belt is contaminated with the dust of the material. There is wide ranging of materials, some are non-abrasive and some are very abrasive. So, thicker wall is necessary when handling very abrasive material (thicker wall gives adequate wear allowance in life).

Then there is wide variation in the belt speed being used ranging from 1 mps to 5 mps. Roller surface wear is proportionate to the rpm i.e. belt velocity. So, to get the same calendar life, one will need thicker wall.

In view of above reasons, there will always be some difference in the wall thickness being used for various applications.

Regards,

Ishwar G Mulani.

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

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Dear TMN..

I normally use the 3.8mm roll thickness, (but go for the next thickness up for the wing rolls on horizontally curved conveyors).

Please remember though, if the wheels of your car are out of alignment, you don't order tyres with extra tread.... you get the wheels aligned!

Same goes for idlers. That is why we always align our idlers very accurately using a home built optical aligner on which is mounted a telescopic sight from a hunting rifle.

In this way, the rolls last very well indeed. and we don't have to train the belt either.

Regards

LSL Tekpro (here in sunny South Africa) ■

Hello TMN,

As discussed by others the wear rate on the idler shell is dependant on many factors.

In general, if the belt is cleaned, if there is no excessive use of the idlers as a belt tracking mechanism and if the roll's inherent rolling friction (seal drag) is within acceptable limits, the standard (in RSA) 3,8mm for 127mm dia tubing and 4mm for 152mm dia tubing would result in reasonable life. What is reasonable life? Again dependant on many factors, but say generally of the order of 3 - 5 years (although I know of many instances where rolls have been running for longer than ten years with only minimal (less than 2mm) wear on the shell.

The above figures would cover about 90% of cases with rolls of 25 and 30mm shaft diameters.

Return rolls, being generally in contact with the dirty side of the belt, are often subject to increased shell wear.

Remember also that as a general rule of thumb wear rate increases as a function of speed cubed - hence for higher speed belts you may consider using thicker shells. (Rolls with 4mm shells have proven themselves on belt speeds up to at least 6m/s).

The most important factors affecting shell wear are summarised as:

- Cleanliness of belt

- Alignment of idlers w.r.t. the belt

- Seal drag (frictional resistance)

Regards,

Adi Frittella

Afripp Projects ■

Dear Mr.Mike,

In most bulk handling system we use minimum wall thickness of roller is 4.5mm. However, if the capacity is too high, then we check with load only.

Regards.

A.Banerjee ■

Another point on wall thickness:

Hard rock mining with significant tonnage is normally specified with 6 mm walls to sustain impact of primary crusher rock and for base rock in dams.

There was a paper on Tarbella dam in Pakistan, designed by PWH which illustrated the kind of roll damage a rock can produce.

As the belt and roll width increases both the end disk and shell require larger metal thickness to carry the stress.

Today conveyors are becoming more common with wider idler spacing and larger shafts and bearings to 6309 and 6310. At 6308 it usually becomes prudent to use a 6mm wall. ■