Introduction: The alloys used in the production of new heavy machinery must meet high application properties and, in many cases, require various types of wear resistance.
Low-alloy cast steels belong to a broad range of ferrous alloys used in parts subjected to heavy loads. They have many applications in mechanical engineering, including durable machinery for mining and power engineering (such as mining machinery, stone crushers, loaders, mills, etc.). These cast parts are mainly produced with fine-grained cast steels related to the registered American invention for particulate steel, such as HY100 or grade T1.
At the Foundry Faculty of the Silesian University of Technology, efforts were made to use low-alloy cast steel grade L2HGSNM for the manufacturing of heavy mining machinery parts. This alloy is from a group of wear-resistant cast steels with high mechanical strength (Rm > 1300 MPa, Re > 1100 MPa) and relatively low ductility (A5 = 6%). Given these properties, improving the chemical composition and production technology of this alloy was considered to enhance its ductility properties.
Pins and bushings used in the movable joints of heavy machinery, which are subjected to high and variable loads, are often exposed to dust and a saline environment. In mining machines, the connections and moving interfaces between the working arms and the housing are in such conditions, which exposes them to high corrosion and abrasive wear. At the Foundry Faculty of the Silesian University of Technology, after studying the properties of cast steel groups, attention turned to cast tool steels, especially cast steels containing chromium.
This study describes the results of abrasive wear for low-alloy and chromium-containing cast steels prepared under laboratory and industrial conditions.
Materials and Testing Method
Wear tests were performed on low-alloy cast steels prepared under laboratory conditions (K7, K6, K4) and produced under industrial conditions (KL). Chromium-containing cast steel alloys were prepared under laboratory conditions (T8, T7, T6), and alloy TZ1 was also prepared under industrial conditions.
Source: Foundry Industry Monthly
Gearbox Classification
Gearboxes are classified in the industry into two types based on their input and output speeds: speed increasers and speed reducers. Speed reducer gearboxes, in essence, decrease input speed and increase torque, while speed increaser gearboxes lead to a reduction in torque by increasing input speed.
The purpose of using gearboxes is to reduce or increase speed and torque, and to transfer it from one point to another or change its direction. In theoretical power transmission systems, the power between the driving and driven machines is constant, and force and torque change according to the following relationship. In practice, however, the input power is reduced by the efficiency amount.
In heavy industries, gearboxes are used primarily to increase torque, which is why speed reducer gearboxes account for a larger share of power transmission equipment. Regardless of speed reduction or increase, gearboxes are classified into the following types based on the arrangement of their input and output shafts:
- In-line gearboxes: Gearboxes where the input and output shafts are parallel and in a straight line.
- Parallel shafts: Gearboxes where the input and output shafts are parallel.
- Right angle: Gearboxes where the input shaft is perpendicular to the output.
Of course, there are also special gearboxes where the input and output shafts are neither parallel nor perpendicular and have other angles. When designing power transmission systems, the design engineer follows this procedure:
- They may select two or more gearbox types based on the required torque and speed.
- By examining various factors, they determine which type has the best features for the intended application.
In addition to technical specifications, the following are also taken into consideration:
- Financial cost
- Space constraints
- Ventilation and cooling
- Maintenance and repair costs
Each type of gearbox has its own advantages, some of which are briefly mentioned below.
- Helical Gearboxes with Parallel Shafts:
- High efficiency
- Transmission of very high forces
- Ability to use bearings that withstand high loads
- Optimal, compact, and efficient arrangement
- Suitable for applications such as ball mills, rotary kilns, extruders, conveyors, etc.
- Helical Gearboxes with In-line Shafts:
- High efficiency
- High overhang load capacity
- High operational reliability
- Suitable for mixers, screw conveyors, conveyors, etc.
- Bevel-Helical Gearboxes with Right-Angle Shafts:
- Optimal use of space
- Ability to create high transmission ratios
- Tolerance for heavy torques
- High efficiency
- Suitable for conveyors, elevators, mixers, cooling towers, etc.
- Planetary Gearboxes:
- Optimal use of space
- Tolerance for high torques
- High efficiency
- Suitable for wind turbines, roller mills, crane slewing drives, stackers, etc.
- Worm Gearboxes:
- Optimal use of space
- Very high transmission ratio per stage
- Vibration-free and smooth operation
- Very low cost
- Based on the design of the gear angles, they can prevent reverse motion.
- Due to the very low efficiency of this type of gearbox, they are usually used for light or heavy applications with a short duty cycle.
CARB® Toroidal Roller Bearings
Each type of bearing has characteristics and properties based on its design that make it more or less suitable for an application. For example, deep groove ball bearings can handle moderate radial and axial loads. The friction in these bearings is low, and they can be produced with high precision and in low-noise designs. Therefore, these bearings are widely used in small and medium-sized electric motors.
Spherical and toroidal roller bearings can handle heavy loads and are also self-aligning (they adjust to misalignment between the shaft and the housing). These properties make them suitable for use in heavy industries where heavy loads, shaft deformation, and misalignment are present. In this discussion, we will review the advantages of using toroidal roller bearings.
Sources:
- Why-SKF—CARB-toroidal-roller-bearings—06550_2-EN
- The SKF total shaft solution for industrial—6185EN
- SKF rolling bearings catalogue_tcm_12-121486
- SKF_bearing_ maintenance-handbook
- WWW.SKF.COM
CARB® Toroidal Roller Bearing
A CARB® toroidal roller bearing is a radial roller bearing, which is an acronym for Compact Aligning Roller Bearing, meaning a compact self-aligning roller bearing. It was first introduced by the company SKF in 1995.
This bearing features a single row of long, slightly arched, symmetrical rollers that combine the self-aligning capability of spherical bearings with the axial displacement capability of cylindrical bearings, making it highly versatile. It can also have a small cross-section similar to needle bearings.
Open Bearings (Without Seals)
CARB® bearings are produced in three designs: with cages, without cages, and sealed. The load-bearing capacity of the cageless design is significantly greater than that of the caged bearings. In all three designs, the inner ring is available in both cylindrical and tapered forms, the latter being indicated by the suffix K.
Sealed Bearings
These bearings have seals on both sides and are filled with long-life grease for high temperatures. They require no maintenance and can operate in a temperature range from -40°C to +150°C. More than 70% to 100% of the internal space of these bearings is filled with grease.
Internal Clearance
CARB® bearings are produced with normal clearance as a standard, but most are available with higher clearances, such as C3 and sometimes C4 or C5. An important point is that the internal clearance in these bearings decreases due to the axial displacement of one ring relative to the other. Since these bearings are often used with spherical bearings, their clearance in the same class is greater than that of spherical bearings. As a result of displacement by 6% to 8% of the bearing’s width, the clearance of CARB bearings is reduced and becomes equal to that of spherical bearings. The greater the displacement from the center of the rollers, the more the clearance decreases.
Misalignment
The maximum amount of misalignment without a negative impact during operation is 0.5 degrees. If this amount is exceeded, it will increase friction and reduce the bearing’s lifespan. For CARB® bearings with a machined brass cage (suffix MB), misalignment should never exceed 0.5 degrees.
Misalignment also causes axial displacement of the rollers, causing them to move closer to the side of one of the bearing rings, which may ultimately reduce the allowable axial displacement.
Axial Displacement
Axial displacement can be caused by thermal expansion or a deviation in the bearing’s position. If displacement occurs, it leads to a reduction in clearance, and when this clearance is not sufficient, the rollers may protrude from the side of the ring or come into contact with the locking ring or seal.
Cages
CARB® bearings with a cage have four types of cages, depending on the application:
- Polyamide 4.6 window-type cage, specified by the suffix TN9 in the part number, can operate up to +120°C.
- Stamped steel window-type cage, roller-centered, without a suffix in the part number.
- Machined brass window-type cage, roller-centered, with the suffix M in the part number.
- Machined two-piece brass cage, inner ring-centered, with the suffix MB in the part number.
Bearing Part Number
All standard bearings have a main part number that consists of 3, 4, or 5 digits, or a combination of letters and digits. In a CARB bearing, the part number is as follows:
XXXXC
The letter C indicates a CARB bearing. After the letter C, the next two digits indicate the ISO dimension series, which is a code that can be obtained accurately by referring to bearing tables. The first digit of these two specifies the width series and the second specifies the diameter series.
The third and fourth digits indicate the inner diameter, which is obtained by multiplying by 5 to get the inner diameter in millimeters.
Example: C 123 where 22 is the dimensional series and the inner diameter is mm.
Suffixes in the CARB® Bearing Part Number:
- C2: Internal radial clearance less than normal
- C3: Internal radial clearance greater than normal
- C4: Internal radial clearance greater than C3
- C5: Internal radial clearance greater than C4
- CS5: Seal made of hydrogenated nitrile butadiene rubber (HNBR) reinforced with a steel sheet on one side of the bearing.
- 2CS5: CS5 type seal on both sides of the bearing, filled with 70% to 100% heat-resistant grease.
- HA3: Case-hardened inner ring
- K: Tapered inner ring, taper 1:12
- K30: Tapered inner ring, taper 1:30
- M: Machined brass window-type cage, roller-centered
- MB: Machined two-piece brass cage, inner ring-centered
- TN9: Polyamide 4.6 window-type cage reinforced with glass fibers, roller-centered
- V: Cageless roller set
- VE204: Modified bearing for greater axial displacement
- VG114: Case-hardened steel cage, roller-centered
Example: C 2310KV/C3
The bearing type is CARB because it starts with the letter C. Its dimensional series is 23, from which the outer diameter and width can be obtained by referring to bearing tables. Its inner diameter is 50 mm. The letter K indicates a tapered inner ring with a 1:12 ratio. The letter V indicates that it is cageless, meaning it only has rollers and can handle a very high load. C3 indicates the clearance class.
New SKF Solution in Floating Arrangements Using CARB® Bearings
The arrangement of bearings in a rotating machine component, such as a shaft, generally requires two bearings to hold and axially and radially fix the rotating components relative to the machine’s fixed housing. Depending on the application, load, required movement precision, and costs, the bearing arrangement may be as follows:
- Fixed and floating bearing arrangement
- Adjustable bearing arrangement
- Floating bearing arrangement
Source: Maintenance in Industry Monthly