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.

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. This article will compare the most common and economical of these gear/bearing units with the traditional cast iron gear and bearing.

BEARING COMPARISON
The traditional cast iron drive unit uses an inserted raceway construction bearing. This bearing (fig. 1) 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 compliment 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 (fig. 2) 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 (approx. 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.Ê Second, the tolerances within the fourpoint bearing are tight, usually approximately 0.003 inch. Third, the life of the four-point bearing is very high, usually over 10 million hours (approx. 1,000 years) in clarifier service.

Figure 3 shows a typical cross section of a four-point contact ball bearing. 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 theÊ forgings then hardened to approximately 60 Rc and ground to theÊ 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 predominately thrust loads. This usually is the case when the bearings are used in cranes, excavators, and large clarifiers.

GEAR COMPARISON
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 AIS 4140) heat treated to 250 to 300 BHN. The use of a forging virtually ensures 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.

CONCLUSION
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 fourpoint contact bearing eliminates the need to rebuild the clarifier drive every 10 years or so, greatly reducing operating cost.

FIGURE 1 - CAST IRON DRIVE WITH INSERTED RACEWAY BEARING

FIGURE 2 - CAST IRON DRIVE GEAR CAN LIFT OUT OF ITS RACEWAY CAUSING PREMATURE BEARING AND GEAR FAILURE IF SUBJECT TO AN OVERTURNING MOMENT.

FIGURE 3 - FOUR-POINT CONTACT BEARING IS DESIGNED TO HANDLE HEAVY LOADS.

AGMA COMPARISON
Assume two gears are identical in all respects, except one is made of AGMA Class 40 cast iron (the highest grade) and the other is made of alloy steel heat treated to 300 BHN. By comparing the AGMA formulas for surface durability - and strength - the superiority of the forged gear can be demonstrated.
1. AGMA SURFACE DURABILITY - 2001-C95

All factors will be the same except Cp and Sac.
CAST IRON: Cp = 2100 Sac = 85,000
FORGED STEEL: Cp = 2300 Sac = 135,000
A comparison of the surface durability of each gear shows that the forged gear will have 210 percent higher surface durability than an identical cast iron gear.

2. AGMA STRENGTH - 2001-B88

All factors will be the same except Sat.
CAST IRON: Sat= 13,000 FORGED STEEL: Sat = 47,000
A comparison of the strength of each gear shows that the forged gear will have 360 percent greater strength than an identical cast iron gear.