This is an installation tolerance. The idler sets should be evaluated for shortened life based on some idlers are installed above the general-nominal plane. At some tolerance the elevated roller will carry significantly more load and therefore decreased life.

If the allowable tolerance is significant, premature and repeated failures will occur. The question then comes: What is a prudent installation tolerance that will not cause excessive roll life failures.

CDI normally specifies about 1 mm error in initial position. Very hard to do. SPecial procedures are required to achieve the higher accuracy. The high accuracy guarantees superb tracking from the initiation of commissioning. Curragh North, a +20 km overland had no tracking issues from first movement, even with horizontal and vertical curves at many positions along the conveyor.

Often we evaluate general tolerances of about 2 mm +/-. Sometimes we see +/- 5 mm errors on installation. Poor client who suffers with this large installation flaw. ■

A further point:

The equation you cite is not correct. The "IML" value comes from the load lifted off the idler (both sides), based on a simplified version is the true sag equation. This can be observed by understanding your diagram in a different way.

Try the web: http://teachers.sduhsd.k12.ca.us/abr...s/catenary.htm

Observe when the roller lifts the belt at the elevated station, it causes the belt to sag seeking support from the adjacent rollers. The sag is the same geometry as that we see with displacement between rollers. It is the old catenary close-line formula reduced to a parabolic equivalent. You get a bonus, when the elevated roll lifts up the belt, it decreases the vertical force on the adjacent two rolls.

Also observe, with sufficient lift and tension, you may not have any support on the adjacent rolls. ■

Dear Samy,

In answer to your email request:

IML: "Idler Misalignment Load" is the additional vertical force placed on the subject misaligned idler when it is elevated above the common horizontal plane of adjacent idlers.

IML can occur on horizontally installed conveyors. This is an installation tolerance that protects against unanticipated premature roll failures. However, it can be argued, the adjacent rolls are relieved of added pressure on the elevated roll, leaving a net neutral overall loss. This is not a good arguement. It can change the way the owner perceives the maintenance program. ■