Conveyor Equipment Manufacturers Association (CEMA) established standards for the manufacturing of conveyor rollers in the 1900s. These standards have been updated over the years, and have come to encompass not only conveyor idlers, but also a host of items commonly used in the material handling industry. This discussion will be confined to the CEMA B series and the CEMA C series belt idlers. Below is an excerpt directly from the CEMA website:
Troughing and Return Idlers (CEMA Standard 502)
- Assurance to users of conveyor idlers interchangeability of complete idler assemblies but does not restrict the manufacturer, who has complete freedom to design all parts of the idler according to its best engineering judgment.
- Uniform dimensional load and capacity information for: Troughing Idlers with equal length rolls; Picking Idlers with unequal length rolls; Return Idlers with single steel roll but typically available with rubber discs; V Return Idlers with a pair of steel rolls but typically available with rubber discs; Live Shaft Idlers with steel or rubber surfaces.
- Nomenclature and selection methods to provide a versatile and realistic means of classifying idlers.
- Includes Idler selection criteria along with associated formulae and supporting tabular data.
- It is very important to note that these standards refer to dimensional, load, and capacities only. The responsibilities of improving product life and durability are clearly left to individual manufacturers. Life, durability and MTBF are all effected by various construction techniques, bearing selection and idler seal design. As a result, all of these components can be made of whatever manufacturers see fit, as long as dimensions, load and capacity specifications meet or exceed CEMA standards for product sizes and series.
Below are published load and capacity standards from the PPI Idler Catalog. This data corresponds to published CEMA standards. The most popular idler sizes used in ready-mix applications are highlighted in yellow.
CEMA B Standards
|Belt Width||Troughing Angle||Return & Flat||Unequal|
|20° (lbs.)||35° (lbs.)||45° (lbs.)||20° (lbs.)||35° (lbs.)|
CEMA C Standards
|Belt Width||Troughing Angle||Return & Flat||Unequal (Picking)||Live Shaft|
Please note that the roll diameter is not relevant to the load rating. 4″ cans or 5″ cans do not affect capacity. Diameter, however, does directly affect roller RPM and theoretical bearing life.
A quick review lends credence to the claim that bigger is better, especially when you see that the average load rating of the C series is more than twice as large as that of the B series. This assumes that idler life expectancy is tied exclusively to load and capacity. While these issues factor into the overall theoretical life calculation, there are plenty of other factors to consider when calculating life expectancy.
Idler supporting shaft assemblies are key limiting factors for CEMA standard load ratings. This is important, as bearings are frequently attributed as the keys to unlocking improved capacity. CEMA established these minimum standards based on supporting shaft fatigue strength and deflection tolerance and not the bearing load rating. So, every CEMA B idler, regardless of manufacturer, uses a 17mm shaft to support the rolls. For CEMA C products, that diameter increases to 20mm.
When looking at products produced by different manufacturers, you should expect variations in published load capacities, life expectancies and durability ratings. These variations are present due to the variation in individual manufacturer design characteristics. Keep in mind that even though manufacturing tolerances, finishing processes and raw material specifications have improved, the capacity will always be determined by the diameter of the supporting shaft.
Other Factors that Affect Idler Life
Most manufacturers (if not all) use the same materials when constructing CEMA B and C series idlers. The following table is provided by Superior Idlers and details the construction of idler cans. These specifications may vary for extreme duty products.
Idler Material Specifications
|CEMA B||17mm||4″, 5″||11, 9 Ga||18″ – 48″|
|CEMA C||20mm||4″, 5″, 6″||11, 9, 7 Ga||18″ – 60″|
As is apparent, can diameter and gauge thickness are exactly the same for both the B and C series. This means that the can construction from series to series is the same, and there are no net benefits associated with roller life and thicker gauge materials.
Most manufacturers use 6200 series deep groove ball bearings when constructing B series idlers, and either 6300 series ball bearings or regreasable tapered roller bearings when constructing C series idlers. In general, manufacturers have standardized to using sealed deep groove balls for the non-greasable idlers, regardless of the series. Even though CEMA standards are based on shaft strength and size, idler life expectancy is tied to bearings, which leads us to a discussion of bearing L10 life.
Conventional methods of projecting a bearing’s lifespan are based on standardized calculations, also known as the catalogue method. These methods are stipulated in the ISO 281 norm. The parameters are bearing load, speed of rotation, load rating and bearing type. The resulting bearing lifespan is L10. Basic life or L10 is defined in ISO and ABMA standards as, “the life that 90% of a sufficiently large group of apparently identical bearings can be expected to reach or exceed.”
Let’s look at a bearing that has a published 40,000-hour L10 life. In practical terms, this means that if we select a sample of 200 bearings (same size, manufacturer and part number) and test them at speed and under moderate loading, they will continue to perform as designed for 40,000 hours, with a failure rate of 10% or less.
For our purposes, most CEMA idler manufacturers publish life expectations for 30,000 hours L10 life at 500 RPM. This applies to both the CEMA B and the CEMA C series. This means that under moderately loaded conditions, relative to their design limits, the CEMA B and the CEMA C idlers have the same expected L10 life. Theoretically, C series may provide some advantages in moderately-loaded applications over the use of B series idlers in redi-mix plant applications, in which the C series idlers would be “lightly” loaded. Because load factors into the L10 calculation, you could presumably expect longer bearing life–and ultimately longer idler life–if all things were equal. Unfortunately, though, other factors have much more impact on life.
Most manufacturers provide some type of lip seal, centrifugal flinger seal, end disks or a combination of the three as part of the roller assembly. These are features that can potentially separate one manufacturer from another and have the largest potential ability to extend roller life. This is not a CEMA standard, so each manufacturer is free to protect their assembly in whatever fashion they prefer. Seal design and effectiveness are both key to the success and longevity of the bearing, and in turn the longevity of the roller. Comparisons between manufacturers show a variety of configurations, quality of sealing and some very application specific configurations. Each has been designed to protect the bearings and help them reach their L10 life expectations.
To lubricate or not: it’s always a tough question. The most frequently selected CEMA B series idlers are of the no-lube variety. In these products, the ball bearings are provided with a standard 1/3 fill of grease and require no maintenance. Lubed versions are pre-greased as well, but have ports and connections that allow for additional lubrication throughout their life. Issues like lubrication intervals, volume required and type of lubricant can all contribute to the success or failure of the idler to reach its expected life, regardless of series.
Coatings, Seals & Custom Options
These options are manufacturer-specific and can be made from a variety of materials including polymer sleeves, urethane coatings and special paints and epoxies. All can contribute to longer life, but with an additional cost.
Based on discussions with several leading idler manufacturers, can size is the one factor that can directly affect roller life.* In the case of idlers, size does matter, and there is a rule of thumb: 4” cans for 400 FPM, 5” for 500 FPM, and so on. Most ready-mix plants have measured belt speeds that fall in the 300-400FPM range, which typically indicates that 4” rollers should be used. The difference is very common sense – using 5” cans where previously 4” cans were used, will reduce the number of RPMs that each can turns, thereby extending the bearing life. Generally, if the belt size, load and speeds call for a CEMA B 4”size, or the OEM original specification was a 4” B, a 5” roller would provide longer life.
A commitment to replace 4” with 5” is not without pain, however. Extra caution must be used to not mix 4”and 5” rollers on the same conveyor. Mixed installations produce hills and valleys on the belt, overloading the rollers on either side of the peak, and can cause significant belt tracking issues. Just like the issues with mixing idler brands on the same conveyor belt, a changeover should be done across the entire conveyor.
*The CEMA tables are generally selected on the basis of a bearing L10 life of 30,000 or 60,000 hours (dependent on the series) with the idler roll rotating at 500 RPM.
Coating Causes of Premature Failure
CEMA lists a variety of causes for premature roller failure, but the most common are seal effectiveness (contamination of the bearing and shaft), corrosion, abrasion and end design failure. Industry statistics from the roller and bearing manufacturers indicate as much as 43% of premature failures can be tied to bearing contamination issues. These range from external contaminants like fines, dust, dirt and moisture, as well as internal issues like corrosion and rust from idle or aging products in use.
Many plant operators are convinced that the only way to prevent premature roller failure is to use greasable units and instate a regular greasing and purging program. The dilemma is that this type of program leads to shortcuts and improper lubrication, which can result in shortened roller life. Proper lubrication includes cleaning the fittings before attaching the grease gun, slowly purging the lubricant until clean grease emerges and (most importantly) wiping excess grease from the rolls. If done properly, this will extend roller life.
The reality is less positive – reach, personal safety concerns, limited access and limited time all will hinder this process, and are the main reasons that many operations have elected to use the non-relubricable versions.
Regardless of the selection of roller type or size, preventative maintenance is the single most important part of extending roller life. Regularly and thoroughly inspect your rollers. Premature failure due to a locked roll can lead to a much more expensive premature and catastrophic belt failure. These inspections can lead to the identification of issues like contamination, pinched debris, material build up and misalignment issues long before they create unexpected downtime. Replacing a single roller is much easier and less expensive than replacing an entire belt. Keep in mind that there are now several tools to assist the smaller operation in roller and idler replacement, including belt lifting tools, and tensioning equipment.
Challenges to Standardization
Any standardization plan has its challenges, and an idler program is no different. Keeping in mind that CEMA sets some specific standards on B and C style components and assemblies, please keep in mind that there can be marked differences from B frames, to C frames and rollers. Differences between manufacturers (even on the same idler size and series) are also present.
The most significant and problematic change is the height of the center roller. Changing from a CEMA B to a CEMA C will affect the belt riding height, even if the same can is used. For example, the measurement from the base or the conveyor frame to the top of the center roll in a 4” diameter CEMA B is 7”, a 5” diameter B roll is 7-1/2”; the same measurement in a 5” diameter CEMA C is 8-3/4”. Individual idler replacement on a conveyor will lead to variations in the center height creating hills and valleys in the belt run, placing undue load related stress on the higher rolls as well as the potential for disturbing material on a loaded belt in an incline situation. It is therefore recommended that any standardization be done for the entire run.
There are a great many challenges associated with the selection of the appropriate idlers for use on any specific conveyor system. The CEMA Standards provide guidelines relative to loads, speeds, and belt types. There are rules of thumb when it comes to determining the roller diameter relative to belt speed. Personal experience with specific brands, price and longevity also factor into a decision to standardize. Most importantly, the product needs to “fit” the specific needs of a particular requirement. This boils down to not so much as a choice between one series and another, but more of a choice between features, costs and relative benefits from one manufacturer to another.
Estimated Reading Time: 10 minutes
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