Thread: Snub Pulleys in Belt Conveyors

1. Axel Witt Guest

Snub Pulleys in Belt Conveyors

Snub pulleys are mostly installed in belt conveyors in order to achieve a higher angle of wrap on the drive pulley; in this cases the wrap angle on the snub pulley is in the area of 15° to 30°. At other locations in the belt conveyor pulleys with smaller diameters (similar to the diameter of snub pulleys) are installed to lead the belt into another angle; sometimes these pulleys are called deflection pulleys. These pulleys have wrap angles between 3° and 45°. My question is: does anybody have experiences with negative influences of such pulleys installations with regard to the pulley shell (wear or failure) and to the rubber lagging. What is the limitation in surface pressure of the (rubber) lagging? What is the minimum shell thickness of the pulley. I have enough experience in pulleys with wrap angles between 90° and 180° (210°), but I heard that in the area of a few degrees upto 45° different criteria should be considered.
Looking forward getting any help from the theorectical point of view (calculation), as well as any practical experiences.
Best regards,

2. An attempt at a possible cause:
Excessive wear would tend to indicate some differential speed between shell and belt.
Relatively small wrap angle coupled with small shell diameter on a pulley with high rotational friction can result in belt 'skidding' over surface of the pulley resulting in accelerated wear.

Regards,

3. Lyle Brown Guest
Some of the problem, maybe the snub pulleys are in LT areas, where as deflection pulleys may be in HT areas (or maybe not), hence for a given contact area, there would be increased pressure.
Are both types typically on clean or the dirty side (compare apples, with apples, per se) of the belt?

Suspect you are aware of mechnical issues associated with relatively small angles of wrap:

www.engineering.com.au/pdf/Design.pdf (look under shell)

http://www.bulk-online.com/Forum/sho...?threadid=1555 (scroll through, see words from Nordell)

Regards,
Lyle

4. Axel, I thought you read the forum points on pulleys?

Any triaxial stress analysis program (Prestress v3.0; ANSYS, et al) will tell you that the arc angle can cause varying degrees of buckling stress when the shell contact angle is in the 60 degree neighborhood verses 180 wrap. Below 20 degrees it again becomes less critical.

Lyle's second reference leads to some earlier corrections of the hoop stress sensitivity by degrees of wrap. This is a so-so substitute for a proper analysis.

Make-em fat and sassy, the iron ore industry will love you.

Contact stress vs pulley diameter is another matter. Now you get into twist wind-up on wires et al.

Since contact pressure or normal force is proportional to Fn = Tension/Radius, you can do the sums with respect to any reduction in diameter that will not exceed your shell theory of the moment.