Belt slip between drive pulley and belt wrapped around the drive pulley is detected by monitoring. The drive pulley RPM = belt surface speed and a adjacent non-driven pulley, such as a snub pulley or a idler wheel riding on the belt, have attached speed sensors which annunciate the two belt speeds. If the drive does not maintain the same speed as the snub, a speed comparitor will alarm due to the speed differential exceeding a alarm set point.

Slip-stick is the phenomena where the belt undergoes shorting as the belt tension is reduced across the drive pulley surface. The degree of slip is associated with the belts elastic modulus and relationship of T1 and T2. How much of the wrap angle is needed to transmit the tension differential.

Slip-stick can be heard on some belts as a chatter. It has been measured with strobes and other elaborate sensor devices. ■

Hi Shane..

We detect belt slip as described by Larry, but we normally put the other rev counter on the tail pulley instead of the drive snub.

The reason for this is that for many configurations of conveyor if the belt breaks, the tail pulley will stop and the conveyor will trip out.

The system can therefore act as a slip detector as well as a break detector

Cheers

LSL Tekpro ■

Please use caution in applying Graham's suggestion. It can work, but, if you wish to annunciate drive slip, this will not work well when the drive and tail are remote such as with overlands.

The drive will start and tail will not. The lag time depends on the distance. Then the tail velocity can whip to a higher velocity than the drive reference velocity due to the belt's eleastic response. THis can be for up to 15 seconds on a long overland. The tail velocity will mismatch the drive for much of the start cycle. ■

Note...

I suggest everyone uses caution whilst exercising caution with detecting slip as I do.

The only time I have ever had slip detection next to the drive was on a 2 x 400 kW overland. On this one recent occasion the Secondary tripped but the Primary didn't due to a PLC glitch, but fortunately a passing operator, (who had an allergy to the smell of burning rubber) quickly shut the unit down .

The client had insisted on a doppler motion sensor next to the drive.

This fancy thing did nothing for me or the conveyor. If I had used my old faithfull unit on the tail pulley, I could have avoided the operator's nasal discomfort.

LSL Tekpro ■

Dear Mr. Shane,

Mr. Nordell and Mr. Graham have given the detailed information. In general the switch being used for this purpose is known as belt monitor switch or zero speed switch. You can refer to any manufacturer of conveyor switches and you will get the price and literature about such switch. This switch is normally doing more than one function such as:

- Motor starts but belt doesn’t start within finite time. In this situation it will trip the system.

- The motor starts and belt also moves but it has not reached the particular speed within particular time limit. This is a faulty situation and it can automatically sop the conveyor in system.

This switch is coupled to return roller of live shaft or to non-drive pulley such as snub pulley, tail pulley, etc. As I understand mostly it works on simple principle of frequency being generated by rotating item. In a broad sense you can say it is like a mini-generator. This is only to give you an idea about this item. Refer manufacturer’s literature for specific information.

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.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■