ACHIEVING LOW PROFILE CONVEYOR RELIABILITY
For those concerned with improving production, and reducing maintenance
costs in systems involving Low Profile Belt Conveyors, this paper addresses the
reliability aspect of these units. Data is provided which permits selections
that can result in a trouble-free greatly extended service life, at reduced
Bearings are the most crucial component of the conveyor, operating under the constant preload belt tension, even in the absence of any product on the conveyor.
Bearings should have reserve
capacity to contend with over-tensioning, and be a complete self-contained
sealed unit. Utilizing the pulley shaft as the bearing inner race should be
avoided. If the shaft is not made of bearing quality steel, and properly heat
treated, it can become the weak link in the bearing system. Failure would then
require replacement of not only the bearings, but also a more costly pulley or
shaft. This construction also exposes these multiple bearing elements to
contamination during storage, assembly, and replacement. Driver bearings can be
subject to harsh localized stresses induced by pulley deflection, and
misalignment, if they do not incorporate a self-aligning feature. Fixed needle
type bearings on wider conveyors would be especially susceptible to this
condition. Since needle bearings normally do not have any axial load capacity,
they must incorporate a separate axial bearing system which compounds bearing
wear. Pulley size also has a direct relationship to bearing life. A 1 inch
pulley conveyor utilizing identical bearings and belting conditions as a 2 inch
pulley conveyor, would have an average bearing life only one half that of the 2
inch unit. This is because the 1 inch pulley must rotate at twice the R.P.M. of
the 2 inch pulley to produce the same belt speed. Bearings that are sealed for
life with an adequate supply of lubricant will normally out-perform bearings
that require re-lubrication. This is due in part to neglecting to re-lubricate,
or inaccessibility. Re-lubrication can also cause bearing contamination, since
it is not performed under the Clean-Room conditions of the sealed for life
bearing. Used lubricant expelled from the bearing during re-lubrication can also
be a major cause of belt and product contamination.
Low profile belt conveyors
normally utilize pulleys from 1-2 inches in diameter;
However, the actual overall difference in the conveyor height is
relatively small, about .36 between a 1.31 pulley and a 2 pulley
system. Pulley diameters near the minimum of the
Chart A indicates the suggested minimum pulley diameter versus belt width.
Failure to comply with these minimum diameters when relying on crowning could result in excessive deflection which would neutralize the crowns tracking ability.
Pulley diameter has a major effect on this deflection. A 1.25 steel pulley will deflect 6.5 times that of a 2 pulley under identical loading.
The Chart is based on data from a leading Belt Manufacturer, and assumes belting commonly utilized on low profile conveyors. Belt rating is 8N/mm using a minimum recommended tension of .3% to assure automatic belt tracking.
Belt failure can occur in several different forms, most
of which can be prevented or delayed by proper selection of conveyor features.
All belting is rated at a
specific minimum pulley diameter. However, most conveyor manufacturers do not
list this data. An average value for a high percentage of belting used on low
profile conveyors is 1 minimum diameter. The closer the pulley diameter
approaches this value the lower belt fatigue life becomes. This fatigue failure
will normally occur in the splice since this is the belts weakest point.
Insufficient initial belt
tension can induce belt elongation which may cause slippage and wear. Once this
elongation exceeds the conveyors tension range, a serviceable belt may have to
Pulley crowning (recommended
by all belt manufacturers) is the most utilized method of
achieving an automatic wear-resistant belt guiding system.
A vee profile on the bottom of
the belt creates an additional wear point that does not exist when using crowned
pulleys. In some instances this guide has had minimal surface contact in the
conveyor bed plate. Any force causing the belt to miss-track will result in wear
on the vee guide at this contact point. Since this condition is not apparent, it
may continue until complete failure occurs. This belt restriction is more often
used in conjunction with crowned pulleys to absorb intermittent off-center
loading induced by diverting or side loading / unloading of products.
Use of sharp diamond knurling
on pulleys to off-set the lack of adequate pre-tensioning can inflict several
belting problems. This type knurling can cause belt fiber abrasion which can
build up in the knurl causing miss-tracking and slippage along with increased
abrasion. Should the belt become jammed and stop, this knurling can destroy the
belt in a few minutes. This abrasive action can eventually wear the knurl,
requiring pulley replacement to prevent slippage when operating near rated load.
Because of damage to the belt, and its negative effect on crowning, most belt
manufacturers do not normally recommend knurling.
Conveyor load capacity is often
over-looked in the selection process. A conveyor with a 60 pound load capacity
may not be able to handle 25 pounds if the load is accumulating, inclined, or
running in reverse. Attempting to increase belt tension to off-set this slippage
can result in greatly reduced bearing life. If increasing belt tension does not
achieve the desired results, the conveyor may have to be replaced with a higher
Conveyors that do not offer a
proportional increase in load rating as width increases, may present more
performance, and maintenance problems on wider units.
Reduced load rating per inch on wider units, is normally an indication of
load limitation of pulley diameter, or bearings, or possibly both.
This reduced belt tension on wider units can contribute to slippage, and
increased belt abrasion wear.
Conveyors with a minimum load
capacity of 20# / inch of width throughout their full width range will permit a
much broader range of application. This will also allow effective use of
crowning and have a positive effect on bearing and belt life.
RELIABILITY COMPLIANCE LIST
The purpose of this list is to provide a systematic
selection process which can result in the highest degree of reliability and
performance at the lowest operating costs.
This list can also assist in indicating those conveyor manufacturers that
are concerned with long term reliability and its' impact on consumer cost.
With an increasing number of engineers requiring 3D CAD drawings of
conveyors to reduce their cost of engineering, it should be equally important
for production and maintenance personnel to require a higher degree of
reliability to avoid costly production delays, and unnecessary maintenance
PROFILE CONVEYOR RELIABILITY COMPLIANCE.
Send mail to WebMaster@ConveyorTechLtd.com with
questions or comments about this web site.