A Comparative Study of the Life and Durability of Cast Iron and Forged Steel Clarifier Drives
Nils Young, PE, Director of Engineering
Water treatment clarifiers and thickeners were among the earliest commercial devices to utilize large diameter anti-friction bearings and gears. The successful performance of these bearings and gears while operating continuously in adverse outdoor environments is a tribute to those early designers and manufacturers.
Those early manufacturers were constrained by the manufacturing technology that was available in the early 1900s. At that time, the only economical way to manufacture a large diameter gear or bearing was to make an iron casting. This method of manufacturing clarifier drives was successful, and is still used today by some manufacturers on their light duty clarifier drives. The chart below illustrates the advantages and disadvantages of using these materials in drive construction in comparison with DBS drive units.
|Cast Iron Drive||DBS Fabricated Drive|
|General||Typically used in worm gearbox, cast iron final gear with replaceable steel strip liners and ball bearing||Always used planetary gear, forged alloy steel final gear and four point contact precision bearing|
|Primary Reducer||Helical or worm gear-motor||Helical gear-motor|
|Connection between the Primary and Intermediate Reducers||Chain & sprocket||Direct drive shaft|
|Intermediate Reducers||Worm type reducer||Planetary type reducer|
|Lower Pinion Bearing Support.||Yes||No|
|Final Main Gear||Spur gear ductile iron or cast Iron||Spur gear forged alloy steel 4340 – induction hardened for maximum strength and durability|
|Pinion Gear||Spur Steel Gear||Spur gear induction hardened for maximum strength and durability|
|Main Bearing||Steel bands (strip liners) and balls.||4 point contact precision bearing.|
|Housing||Cast Iron||Fabricated steel ASTM A36|
|Shear Pin Protection||Yes||Yes|
|Torque Gauge Indication||Yes||Yes|
|Alarm & Cut Off Switches||Yes||Yes|
DBS drive units are all direct driven, and contain no chains, sprockets or guards. This makes the drive unit efficient, maintenance free, and safe with no exposed rotating components. DBS drive units use highly efficient (92%) planetary gearboxes. Worm gearboxes used by cast iron drive units are not efficient (70% or less). The planetary gearbox is a heavy duty gearbox designed to produce high torque and have high capacity output shaft Timken taper roller bearings to handle the radial forces between the final gear and pinion. Planetary gearbox spur gears have infinite theoretical life per AGMA standards. Worm gears have a 25,000 hour life per AGMA standards.
Cast iron drive units have bearings placed below the pinion to support the pinion and handle the radial forces between the main gear and pinion. However, these bearings are prone to fail, exposed to contamination due to wear particles and condensation. This is typically a major failure problem with cast iron drives. DBS drive units do not have a lower support bearing. The planetary gearbox heavy duty output shaft bearings eliminate the need for a lower pinion bearing support.
Cast iron drive units have "Strip Liner" bearings which are steel bands inserted into the main housing. This bearing system has a short life because of the low hardness of the steel bands. Balls have a much higher hardness, Rc 55 – Rc 60 riding on steel bands with hardness of Rc 30 – Rc 35 causing accelerated wear on the steel bands. DBS drive units use a precision 4 point contact bearing with nylon spacers, minimizing the pressure applied across the bearing and avoiding direct ball to ball contact. The DBS bearing has a life that is 400 times more than the strip liner bearing design.
The design and engineering of DBS drive units results in very low maintenance and exceptionally long life. Except for the final reducer which includes the main gear and pinion, all reducers are semi-fluid grease filled. The lubricant in the primary reducer is recommended to be replaced after 5 years and the intermediate reducer to be replaced after 10 years. Cast iron drive units require replacing balls and strip liners several times during the life of the drive unit. Replacement of the balls and steel bands are very difficult tasks to perform in the field and many times they are done in a shop. The DBS drive unit does not require replacing the main bearings. The life expectancy of the DBS bearing is in excess of 100 years.
One key difference is that a steel plate is better than cast iron. Cast iron may have inclusions and imperfections, is not ductile and prone to cracking. ASTM A36 steel plate yield properties allow bending without breaking. Cast iron is simply not as good or as strong as steel. Note that modern gearboxes and machine tools made today no longer use cast iron and use fabricated steel. Castings are becoming difficult to obtain in North America due to environmental issues. There is no definitive corrosion resistance advantage between painted cast iron and steel. Furthermore, entire clarifier mechanisms are made from carbon steel, typically ASTM A36.
An advantage of the DBS gear is that it has more strength and durability. Due to the limitation of cast iron material, based on AGMA standards, the main gear has much less capacity than the forged alloy steel gear used in DBS drive units. Most cast iron drive unit capacities are overstated. The surface durability is generally one half of the torque rating stated and completely ignored.
Another advantage of the DBS torque gauge is highly accurate and has a large diameter face (6”) for easy visual reading. The torque gauge indicates real units of torque and is made of stainless steel construction. On the other hand, the he torque gauge system of a cast iron drive unit, which measures the thrust movement of the worm gearbox high speed shaft, is inaccurate, unreliable, and susceptible to temperature changes and requires periodic recalibration. Torque indication is 0-100%, 1,2,3,4, or a color range (green, yellow, and red).
DBS drive units offer both convenience and ease of maintenance. They use commercial components that are easily available across the country. Cast iron drive units use proprietary components and do not offer other alternatives. The customer has no choice but to go back to the manufacturer for parts resulting in extensive delivery times and steep prices. The DBS shear pin is designed to be replaced very easily. Unlike the cast iron drive units, the shear pin is located above the primary speed reducer and is as simple as removing a cap with two screws and a snap ring. Flexibility is another advantage DBS offers with their products. Many options available with the DBS drive unit are not available with cast iron drive units, such as 4-20mA transmitters for remote indication of torque, reversible with torque overload protection, stainless steel housings, and a hydraulically sealed housing with desiccant breather for moisture removal.
The traditional cast iron drive unit uses an inserted raceway construction bearing. This bearing is made up of four hardened steel bands that are inserted in grooves machined in the cast iron drive housing and gear. This bearing uses a full complement of bearing balls – that is no spacers between the balls to prevent ball-to-ball contact. This type of bearing is very inexpensive to manufacture, however, it has several weaknesses. First, the bearing can handle only compressive and radial loads whose resultant is compressive and whose line of action is within the diameter of the bearing raceway. Second, the bearing has wide tolerances which lead to inconsistent loadings in the bearing and on the gear teeth. Third, due to the simplicity of the bearing design, the life is relatively short, usually 100,000 hours, or approximately 11.5 years, in clarifier service.
Modern drive units utilize the four-point contact ball bearing. This type of bearing is widely used in many heavy duty applications such as excavators and cranes. These types of bearings are manufactured by several manufacturers and are relatively economical. In relation to the inserted raceway construction bearing, the four-point construction bearing has several important advantages. First, the four-point bearing can take compressive, tensile, and radial loads in any combination and the resultant load position has no effect on the performance of the bearing. Second, the tolerances within the four-point bearing are tight, usually approximately 0.003”. Third, the life of the four-point bearing is very high, usually over 10,000,000 hours (approx. 1,000 years) in clarifier service.
These bearings are produced with or without gears cut on the inner or outer raceway forgings. The typical bearing unit is made from roll forged alloy steel heat treated to 250 to 300 BHN. The bearing raceway is machined in the forgings then hardened to approximately 60 Rc and ground to the proper dimensions. The bearings balls are separated with nylon spacers that prevent adjacent balls from rubbing against each other. These bearings typically employ a 60 degree contact angle, which means the bearings are intended for predominant thrust loads. This usually is the case when the bearings are used in cranes, excavators, and large clarifiers.
Large diameter bearings and gears were used in these early drive units for two reasons. The first was that a large diameter gear was necessary to produce the torque required for driving a clarifier or thickener. The second reason was that the simple bearing used to support the cast iron gear had to have sufficient diameter to keep the center of load of the clarifier mechanism within its ball path. If, due to overturning moments, the center of load were to fall outside the ball path the gear would tilt up in the bearing raceway causing high point loads on the gear and precipitating early gear and bearing failure. In many clarifier designs the need for the large diameter bearing was of prime importance as can be seen in some job specifications that specify the ball path diameter, not gear size or actual torque requirements.
Today, due to many advances in manufacturing technology and metallurgy, there are modern bearings and gears that can be used in clarifier and thickener drive units that offer superior life and reliability to these mechanisms. In the next section, the commonly preferred economical gear and bearing units will be compared to the traditional cast iron gear and bearing.
Cast iron gears have the advantage of being inexpensive and easy to manufacture; however, they have several weaknesses. First, the casting is subject to internal weaknesses from inclusions or blow holes that cannot be detected except by X-ray or ultrasonic inspection, which no clarifier manufacturers employ. Second, the mechanical properties of cast iron are generally low which can only be overcome by producing gears with sufficiently large faces or diameters to obtain the strength needed to handle the required torque loads.
Drive units that employ the four-point contact bearing will have the gear cut on the outer or inner race of the bearing. Producing a gear that is integral with the bearing has several advantages. First, the material used is a high quality alloy steel forging (usually AISI 4140) heat treated to 250 to 300 BHN. The use of a forging virtually insures uniform mechanical properties in the gear, and forgings offer superior grain formation for toughness and strength. These superior mechanical properties allow forged alloy steel gears to be smaller than a cast iron gear of similar torque capacity. Second, manufacturing the gear integral with the bearing produces a rigid structure which allows minimal gear movement outside of the rotational plane.
Modern clarifier and thickener drive units that use the four-point gear bearing unit have a number of important advantages over the traditional cast iron drive unit. These advantages add up to longer life of the drive through greater reliability. The four-point contact bearing can handle large overturning moment loads and thrust loads. Unlike inserted raceway bearings, the diameter of the ball path is of no consideration as long as the bearing has the capacity to support the clarifier mechanism. The gear that is part of the four-point contact bearing has superior mechanical properties in relation to cast iron gears, therefore the gears can be smaller in diameter or smaller in face width. The net result is that a clarifier drive unit using a four-point contact gear/bearing will have superior gear mechanical properties, and the bearing which supports the clarifier mechanism will outlast the inserted raceway bearing in the cast iron drive by a factor of 100:1. The more rigid mounting of the gear will also be a strong factor in extending gear life, and the extremely long life of the four-point contact bearing eliminates the need to rebuild the clarifier drive every ten years or so, greatly reducing operating cost.
Over 20 years ago DBS made a decision to manufacture forged steel clarifiers to maintain its quality standards. With low maintenance and a sustainable design, DBS Manufacturing is proud to design, build, and install products customers can rely on. We are dedicated to offering the best services in the field to make your experience a great one. Our O&M Manuals include all information regarding every component used in the drive unit providing the original manufacturer's name and part number, down to the smallest of components such as the bearings and seal. DBS is the only drive unit manufacturer that offers a 10 year warranty on the main gear and bearing. See our services page on the site for more information on our warranty.