Hello Henk,

Interesting.

A generally acceptable Norm. (dont know if Alex confirms this) is 1m per m/s belt speed between drive pulleys in dual drive situations.

Many apply this also to distance bwtween snubs and snubs/drives.

My enquiries to belt suppliers in this regard have not confirmed this 'rule of thumb'.

The most critical, I would assume, is the distance between drive and snub where typically the distance is very close in order to achieve wrap angles above 190 degrees in confined spaces.

Replies from the Forum will be interesting.

Regards,

Adi ■

I have not commited to memory Alex's publication and therefore will not reference his work.

There are three main issues the CDI evaluates:

1. Proximity between drive pulleys to control telegraphing of geometric diameter variances that can lead to drive load sharing instability

2. Proximity between pulleys that will cause unacceptable strain in belt splices and stress amplification on pulley and its structure

3. Relaxation time of rubber strain dissipation in splice from bending around multiple pulleys

ITEM 1: The first becomes of special interest when using variable frequency drives. The internal gate timing of GTO's, IGBT's, et al can be too fast and set up unacceptable feedback between dual driven pulleys when the belt is stretched or relaxed from the irregular geometry of the two drive pulleys or drive and snub. Often is a big problem when material builds up on snubs in dirty side contact with belt

ITEM 2: When pulleys are in close proximity, high strain variances can occur either from pulley or lagging geometry tolerance irregularities or from material buildup on pulley surfaces. This is most significant in high tension regions. Strain is amplified by decreasing the proximity.

ITEM 3: Rubber has a viscoelasticity component which is time dependent. When analyzing the axial and bending stress behavior of splices, we do not want the multiple bends to have an additive bending stress component. To minimize this conditions, there should be a minimum time between bends to allow the rubber to relax within the splice. Rubbers do have varying strain-rate dependent properities. In general, the rubber will relax to about 90% of its original strain, in about .5 seconds. Therefore, the pulleys should be positioned better than 0.5 second flight time between pulleys, before repeating the bending action, to minimize the bending stress additive.

By example, a 5 m/s belt speed should have the pulleys greater than 2.5 meters apart. This assumes high tension. Lower tension regions can have this condition reduced. ■

Some manufacturers use a rule-of-thumb of 1 second between pulley bends. This does not address: proximity, geometry irregularity potentials, belt speed, and rubber properties. ■

Hello Henk..

I first came across Adi's rule of thumb in the 80's.

It came from Trevor Page, (then chief eng of Rand Mines) and was seemingly not the result of a belting supplier's wet thumb.

Heavens knows where Trevor got it from.

Anyway LT drive snubs are not so much the worry.

It is the often found HT bend pulley adjacent to a primary drive that give most of the hastles, with the belt being at maximum tension and maximum vulnerability.

This is especially the case where there is build up of material on the HT bend as it is invariably on the dirty side of the belt. (refer Larry's comments above)

Keep this pulley as far away as possible from the primary drive. The wrap on the primary is nowhere near as important as on the secondary, so this can easily be achieved.

Cheers

LSL Tekpro ■

Graham,

FYI-

The one second rule. Trevor got it from me in about 1987. I got it from Dr. Rainer Alles at ContiTech. ContiTech got it from Rhinebraun and the many German installations which they noted having problems with too close pulley proximity and their knowledge of rubber relaxation timing properties.

CDI subsequentally investigated the same analytically once we delved into rubber mechanics using the then newly purchase Rheometrics Dynamic Mechanical Analyzer (DMA) machine in 1990. We have since confirmed the conditions on, as you say, mostly high tension bends.

However, dirty side contact and subsequent buildup of bend pulleys, in close proximity to adjacent pulleys, especially drive pulleys, seriously exacerbate the degradation of splices of both steel and fabric belts. Drives power will oscillate with the quick acceleration and deceleration response to the change in surface speed of the dirt build up of a nearby pulley. Drive power amplifies the differential forces between pulleys.

Sufficient distance (timing) allows the speed variation to be reduced, as less Dv/dt belt band stretch, between pulleys. ■