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 costs.
Modern Low Profile conveyors used in thousands of automation applications, are an out-growth of small compact conveyors used to remove scrap from stamping dies. As the transition into automation occurred, more demanding applications of speed, load, accumulating, inclines, reversing, and increasing duty cycles emerged. A portion of those involved in this out-growth failed to recognize the inability of some units to perform reliably under these more stringent demands. The requirements of these early scrap conveyors were quite different from that of the automation applications. The primary consideration for the scrap conveyor was height, and width. These units had to fit between the 2” high parallels used on many dies. This dictated a conveyor of about 1.5” height, requiring 1” pulley diameters and small bearings. The negative effect of these small diameter pulleys on belt fatigue was not a major problem since they were operating at low speed, and normally failed due to cutting and abrasion rather than fatigue. Bearing requirements were less due to light load, low speed, and the fact these units were initially limited to 12” widths. Reliability was not as critical as that of automated systems, since the user could easily revert to his previous method of manually removing the scrap, in the event of failure. The basic Low Profile conveyor covered in this paper is defined as a unit with two pulleys of approximately equal size ranging from 1-2 inch diameter and utilizing a high performance woven polyester belt. Maximum width of these units is considered to be 24”.
1) BEARINGS
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.
2) PULLEY DIAMETER
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 “ Low Profile “ Range ” will reduce bearing life, as well as restrict use of heavier more durable belting selections.
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.
3) BELT FATIGUE
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 be discarded.
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.
4) LOAD PERFORMANCE
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 performance unit.
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.
5) 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 costs.
LOW PROFILE CONVEYOR RELIABILITY COMPLIANCE
| Feature |
Benefit |
| Self-Aligning Driver Bearings with Reserve Capacity |
Eliminates bearing damage caused by bearing misalignment and pulley deflection. |
| Sealed For Life Bearings |
Eliminates periodic lubrication and product contamination from lubricant expelled from the bearing. |
| Complete Self-Contained Bearing Element |
Eliminates the need to replace the drive pulley or tail shaft, and prevents bearing contamination. |
| Belt Tension Calibrator |
Allows rapid equalized tension settings, improving belt life and tracking. |
| Smooth Surface Pulleys |
Eliminates belt abrasion and mis-tracking, reducing down time and costs. |
| Minimum Pulley Dia. Of 1.86” |
Offers greater load capacity and improves belt and bearing life. |
| Crowned Pulleys |
Provides automatic wear-free tracking, avoiding expensive vee-guided belts. |
| Belt Load Capacity of 20#/in of Belt Width |
Reduces belt slippage, improves bearing and belt life, and permits increased usage for various applications. |
| Cantilever Stands |
Eliminates 95% of costs related to belt changes and allows open tiered mounting. |
| 2 Year Warranty with Unlimited Hours |
Reduces downtime and maintenance costs. |