Design of a Roller Bearing - Coursework Example

Design of a Roller Bearing A bearing is a machine element that confines relative motion and reduces resistance involving moving parts to only the preferred motion. Often the designs of the bearing supply for open linear movement of moving parts or for frees revolution around a fixed axis or even sometimes prevent motion by having control on a given vector of normal forces that are exerted on the moving parts of the machines (Allan, p11). Almost all bearings do facilitate the preferred motions as much as possible by minimizing frictions between the surfaces in contact with each other during motion. Bearings are usually classified in terms of operations it assists, or the direction of loads or forces applied to the different parts of the machines. There are two types of bearings, which are contact and non-contact. The contact types, as the name suggests, have the mechanical contact with the elements that allow any component to move. Examples of such bearings include the sliding, rolling and flexural bearings. The use of the word mechanical means that inflexibility standard to the direction of movement can be very high. However, tear and wear can limit their service period (Brändlein, p20). On the other hand, non-contact bearings involve superficially pressurized and hydrodynamic fluid-film that include use of liquid, air and magnetized bearings. In this regards, it is evident that mechanical contact is not involved and therefore the static friction can be eliminated despite the fact that viscous drags do often occur in presence of fluids (Allan, p60). This can be very durable and infinite only if the external power that is responsible for the operation does not by any means breakdown. It is worth noting that each type of bearing has its own area of application and therefore the engineers must be familiar with the different types of bearings, and their relevance and limitation (DellaCorte, Christopher & Walter, p30-35). Roller Designing The radius of majority of the bearing is usually made up of a radius curvature across the pathway of an inner ring. This radius is usually 51-52 of the ball diameter while the radius curvature, transversely the pathway of the outer ring, is usually held to 53-54% of the ball diameter (DellaCorte, Christopher & Walter, p138). This is usually standard requirement of the bearing designing. This is usually so due to the fact that as the pathway radius curvature is near 50% of the ball diameter, the stress between the ball and pathway decreases. However, it shifts the contacts higher up the pathway wall generating more friction as the balls rotate round the bearing (DellaCorte, Christopher & Walter, p125). Therefore, to reduce the level of stress, the pathway should be slightly increased above 50% for both inner and outer rings. It is worth noting that for the outer ring, it is higher due to the fact that during the revolutions plain, the outer ring places a concave surface to the balls lowering contact stress in relation to the inner rings which in rotational plane, places a convex surface to the balls thus increasing stress. The shoulder is the raised surface on each side of the pathway (Allan, p81). The shoulder height is the difference between the deepest part of the pathway to the outer diameter of the inner ring and to the inner diameter of the outer ring. This is usually held to be 22-30% of the ball diameter for inner and outer rings, and 18-22% for the outer rings (Brändlein, p63). This is so due to the fact that the two are not same for inner and outer rings. Design height of the shoulder is a balance between being highly sufficient to hold up proportionately high-force loads and low enough to be capable to pull together adequately sized dividing walls. Another part that should be considered while designing the bearing is the wall thickness. This should be held at minimum from the bottom of the pathway of both inner and outer rings (Brändlein, p85). This should be thick enough to hold up pressure and stress compressed by the balls on the inside and hoop pressure from press fit assembly on the outside. Types of Bearings Different types of bearings exist and each one of them is made for a different purpose. Examples of such bearings include ball bearings, roller bearings, ball-thrust bearings, and tampered-roller thrust bearing among others. Ball Bearings: These are one of the most common types of bearing. They are found in almost everything from inline skates to hard drives. This type can handle both radial and thrust loads. Often this is placed where the load is not too large as this is transmitted from the outer diameter and from the ball to the inner diameter (Allan, p190). The balls are usually spherical in shape and therefore present contact to the inner and outer diameter at a very small point hence rotating very smoothly. The life expectancy of this type of bearing can go higher due to its ability to endure high stress. However, it can deform and smash incase of overloading. Roller Bearings: Often these types of bearings are usually installed in conveyer belts rollers in which they are suppose hold very heavy loads (DellaCorte, Christopher & Walter, p16). These bearings contain cylindrical rollers and therefore the inner and outer diameter is not a point but a line. As a result, this provides a wide surface area where the load can be spread. This is one of the reasons why these bearing are used in conveyer belts to hold very heavy loads. However, this type of bearing is not designed to handle much force loading (Allan, p95). Ball Thrust Bearings: These are normally used for low speed appliances. These types of bearings do not handle too much radial loads. Good examples of machines that use such bearing are the barstool and Lazy Susan turntables. Roller-Thrust Bearings: These types of bearings do support large forces of the loads. Such bearing are often installed in the gear sets such car transmissions between gears, and between the housing and the revolving shafts (DellaCorte, Christopher & Walter, p110).The angled teeth on the helical gears are the major causes of force that the bearing must withstand. Tapered-Roller Bearing: Often these types of bearings do withstand large radial and thrust loads. These are usually installed mostly on the car hubs, and are usually mounted in pairs facing opposite directions. The reason behind the opposite direction placement is to enable them withstand the high thrust in both directions (Brändlein, p156). L10 Life This represents the life given to a bearing outside the United State. This is the life at which 10% of the bearings in their application can be projected to have been unsuccessful due to conventional fatigue failure and not by any other mode of failure like lubrication starvation, wrong mounting among others by the engineers or, alternatively, the life at which 90% will still be in service (Allan, p106). The L10 life of the bearing is the hypothetical life and may not stand for service life of the bearing. Bearings are also rated by means of C0 (static loading) value. In this the basic load rating is used as a reference, and not a real load value. Mounting Bearing mounting highly depends on the system quality and performance, proper selection of the bearing and how the bearing supports the moving part. The bearing can be mounted hot or cold depending on the kind of bearing and necessary fit. When the hot bearing is started on the axel, it must be put as fast as possible to its position to avoid seizing. The axel should be arranged and measured to help avoid detrimental effects. If the bearing should start to bind, it must be removed quickly and reheated. For a tight fit of the outer ring, the housing should be brought up to mounting temperature (DellaCorte, Christopher & Walter, p93). This may be inconvenient for large bearing housings. Next method used in the fitting of roller bearings is cold mounting. Ball and roller bearings have great differences in type, design, and size and therefore no common applicable mounting procedure. For non-separable bearings, the mounting and dismounting forces must be put directly to the ring which is being installed. When a bearing is fitted on a shaft, the pressure must be applied to the face of the inner ring (DellaCorte, Christopher & Walter, p311) Lubrication Highly refined minerals oils are the best lubricants for the ball bearings and other types of bearings. With the emergence of the technology, other forms of lubrication have been developed such as synthetics. However, they are not good enough as compared to mineral oil due to the fact they do not form the elastohydrodynamic (EHD) films very well. EHD is a term used to refer to refer to what happens to oil between the ball and the bearing when the bearing is rotating (Allan, p88). From research, it is evident that oil forms films and more so becomes thick such that it completely seals the ball from the pathway. Methods used to deliver the oil to bearings include jet, bath, mist and wick feed. Of all the four, the best is oil jet combined with re-circulating system. This directs hassled pour out of oil into bearing load region. Oil is then pushed back to the sump where it is filtered, cooled and pressurized again into the bearing region and the process continues (Brändlein, p189). This is usually used in variety of loads and speed. In some cases like gear box, the housing is filled with oil such that it touches all the bearings. In some cases there is use of mist systems. This uses pressurized air to atomize oil. This is used in high speed appliances. Wick system on the other hand is whereby oil is absorbed into a material so as to enable to store the oil which it later delivers to the bearing, little by little in a controlled manner. Next method of lubricating the bearing is by the use of grease. Right quantity of grease is applied evenly all over the bearing where it is contained by seals. It is worth noting that grease that is too soft is not good since it will cause a lot of mixing while that which is hard will not lubricate. The grease needs to be consistent (Allan, p137). The best grease include mineral oil grease for general operations from -30F TO +300F, Ester based grease for operation from -100F TO +350F, and Silicon based greases for operation from -100F to +350F which is inadequate in carrying good load capacity. In designing the bearing mounting, the initial step is to decide which type and size of bearing should be used. The choice is usually based on a certain desired life for the bearing (DellaCorte, Christopher & Walter, p199-203).The subsequent step is to design the weight with allowance for the prevailing service conditions. Despite how much care is taken during installation, the bearing will be subjected to one cause of failure such as fatigue of the bearing material. Major failures are caused by defective bearing seats on shafts and in housings, misalignment, faulty mounting practice, incorrect shaft and housing fits, inadequate lubrication, ineffective sealing Vibration while the bearing is not rotating and passage of electric current through the bearing (Brändlein, 206). After cleaning, inspecting, and reassembling a bearing that is to be reused, it must be oiled or re-greased right away, then protected from pollution by airborne dust or dirt, which is always present. Work Cited Allan, R K. Rolling Bearings: A Comprehensive Treatise Covering History, Theory, Design, and the Practical Application and Use of Ball and Roller Bearings. Adapted to Meet the Needs of Designers, Draughtsmen, Plant and Maintenance Engineers, Students, and Others Interested in This Important Branch of Engineering. London: Sir I. Pitman & Sons, 1945. Print. Brändlein, J. Ball and Roller Bearings: Theory, Design, and Application. Chichester: Wiley, 1999. Internet resource. DellaCorte, Christopher, and Walter A. Wozniak. Design and Manufacturing Considerations for Shockproof and Corrosion-Immune Super elastic Nickel-Titanium Bearings for a Space Station Application. , 2012. Internet resource. Read More