Bearing Basics and Types of Bearings
Bearing Basics and Types of Bearings
The term bearing is derived from the verb ‘to bear’, a bearing being a machine element that allows one part to bear (i.e., to support) another. Bearings are highly engineered, precision made components that enable machinery to move at extremely high speeds and carry remarkable loads with ease and efficiency. Generally speaking, bearings are devices that are used to enable rotational or linear movement, while reducing friction and handling stress. Bearings are considered to be the most critical components of machinery.
Bearings are machine elements that constrain relative motion and reduce friction between moving parts to only the desired motion. The design of the bearings may provide for free linear movement of the moving part or for free rotational around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Many bearings also facilitate the desired motion as much as possible, such as by minimizing friction.
Bearings can have many forms, but supports only two types of motions namely linear motion or rotary motion. There are two types of bearings, contact and noncontact.
- Contact bearings – They have mechanical contact between elements, and they include sliding, rolling, and flexural bearings. Mechanical contact means that stiffness normal to the direction of motion can be very high, but wear or fatigue can limit their life. Fig 1 shows different types of sliding and rolling bearings.
- Non contact bearings – They include fluid bearings and magnetic bearings. The lack of mechanical contact means that static friction can be eliminated, although viscous drag occurs when fluids are present; however, life can be virtually infinite if the external power units required to operate them do not fail.
Fig 1 Different types of sliding and rolling bearings
Contact bearings are classified under two main categories namely (i) plain or sliding bearing and (ii) rolling bearing. Sliding bearings are those bearings in which the rotating shaft has a sliding contact with the bearing which is held stationary. Due to large contact area friction between mating parts there is higher requirement of lubrication. In case of rolling bearing due to less contact area rolling friction is much lesser than the sliding friction. These bearings are also known as antifriction bearing.
Sliding contact bearing
Sliding contact bearings do best when the materials in contact are dissimilar. A steel shaft should not run in a steel bore, but it can run in a bronze lined bore (e.g., a bronze bushing). If similar materials must be used, such as often the case in construction equipment where a steel pin runs inside a steel bore, one of the elements should be harder than the other. Due to it wear would be concentrated in only one element, which can then be replaced instead of two. In addition, any abrasive particles tend to get forced into the softer material where they stay and instead of continually abrading the parts, they just wear a groove in one of the parts.
A sliding contact radial bearing element that is used to support a shaft is called a bushing (bush bearing), and sometimes they have integral flanges to also support axial loads. In this the ‘bush’ of soft material like brass or gun metal is provided and the body or main block is made of cast iron. Bush is hollow cylindrical piece which is fitted in housing to accommodate the mating part. When the bush gets worn out it can be easily replaced. When a bearing, such as a bushing, is placed in a modular structural housing, the assembly is called a pillow block. Hinges are another type of special mounting, and most also have sliding contact bearing elements between the two sides.
Sliding contact bearings are commonly used for low modest speed applications. They need boundary lubrication to reduce wear and friction. Polymers, brass, and ceramics are commonly used materials for slide bearing.
Sliding contact bearings have coefficients of friction in the order of 0.05 to 0.1, as compared to rolling element (anti friction) bearings with up to two orders of magnitude lower coefficient of friction. However, there is always the performance trade off. Sliding contact bearings have more friction and are less accurate than rolling element bearings.
The following are the types of sliding bearings
- Journal bearing – in this the bearing pressure is exerted at right angles to the axis of the axis of the shaft. The portion of the shaft lying within the bearing is known as journal. Shaft is generally made of mild steel.
- Foot step or pivot bearing – in this bearing the bearing pressure is exerted parallel to the shaft whose axis is vertical. Note that in this case the end of the shaft rests within the bearing.
- Thrust bearing – It is also known as collar bearing. In this bearing supporting pressure is parallel to the axis of the shaft having end thrust. Thrust bearing are used in bevel mountings, propeller drives, turbines, etc. In this bearing the shaft, unlike foot strep bearing passes through and beyond the bearing.
Sliding contact linear bearing systems are comprised of a moving structure, often referred to as the carriage or slide, and the bearing elements and the surfaces on which they slide are called the rails, ways, or guides. It is more complex to make the inner surface of the carriage match the outer surface of the rails than it is to bore a hole in a structure for a rotary bearing. If a snug fit is required for accurate motion, an adjustable plate (a gib) can be used to take out the clearance.
Sliding contact linear bearings are essentially just sliding contact rotary bearings with a very large radius of curvature. There are three basic configurations for linear sliding contact bearings. They are boxway, dovetail, and twin rail. Boxway bearings support the largest loads in all directions. Dovetail bearings have less load capacity but fewer required precision surfaces and can be easier to adjust. Twin rails are the easiest to build, but the rails can be subject to bending deformations unless more complex supports are used.
Rolling bearings or anti friction bearings are essential for reducing friction which enables most machinery to operate efficiently and indeed even to exist. In fact, the first widespread application of rolling element bearings was to enable the first transportation revolution (the bicycle) to take off. Rolling element bearings also enabled a revolution in manufacturing by making it possible to rapidly design and manufacture low cost high precision machines, such as lathes, mills, and robots which in turn have helped to rapidly increase productivity. Although rolling bearings are ubiquitous, designing with them requires significant care.
Rolling elements bearings use rolling elements (balls or rollers) to reduce friction. In these bearings axial, thrust, and moment loads can be supported depending on the bearings and how they are mounted. These bearings can be non recirculating (limited range of motion, extra low friction) or recirculating (unlimited range of motion).
Anti friction bearings use rolling elements (balls or rollers) to reduce friction. The rolling elements are constrained between an inner race (ring) and an outer race (ring). Generally, a separator (cage) keeps the rolling elements spaced apart so they do not rub against each other which would result in skidding.
Full complement bearings are fully loaded with rolling elements and have no cage, which enables them to carry greater loads at low speeds. There are a vast array of different sizes and types of rolling element bearings to meet virtually any need. From subminiature bearings for instruments, to giant bearings for the bases of huge cranes, the bearings are being made for these applications. The challenge is to define the need and then to select the best type.
There are many distinct rolling bearing types, each with particular characteristics which are suited to specific applications. These are given below.
- Ball bearings – Ball bearing use balls that roll on conformal raceways on the inner and outer rings (races) outer and inner surfaces respectively. By having the raceway closely conform to the ball, rather than rolling on a pure cylinder, orders of magnitude greater load capacities are obtained. So common has this basic design become, that the term ball bearinghas come to mean not just a spherical metal ball used in bearings, but a bearing itself that uses balls. There are many different types of ball bearings, but in general, when the term ball bearingis used, it is usually referred to a deep-groove radial bearing, which is sometimes called a Conrad bearingafter Robert Conrad who invented its means of manufacture. Ball bearings are extremely common because they can handle both radial and thrust loads. In these bearings rolling function is provided by a ball. They are low friction high speed bearings meant for light to medium loading. Ball bearings generally are used for lower cost, lower load, or higher precision applications. They are normally being used in light and general machine applications. They are commonly found in fans, roller blades, wheel bearings, and under hood applications on cars etc.
- Ball thrust bearings – Ball thrust bearings are designed to handle exclusively thrust loads in low speed low weight applications. Bar stools, for example, make use of ball thrust bearings to support the seat.
- Roller bearings – Roller bearings are most often used when maximum load capacity is required in a minimum of space. They are cylindrical and needle roller bearings where the rolling function is provided by a cylinder of some kind. Roller bearings are low friction and are designed to carry medium to heavy radial loads. In this type of bearing, the primary roller is a cylinder, which means the load is distributed over a larger area, enabling the bearing to handle larger amounts of weight. This structure, however, means the bearing can handle primarily radial loads, but is not suited to thrust loads. For applications where space is an issue, a needle bearing is usually used. Needle bearings work with small diameter cylinders, so they are easier to fit in smaller applications. These bearings are commonly found in general machine applications including gearboxes and transmissions, machine tool and construction equipment.
- Roller thrust bearings – These bearings much like ball thrust bearings, handle thrust loads. The difference, however, lies in the amount of weight the bearing can handle. Roller thrust bearings can support significantly larger amounts of thrust load, and are therefore found in car transmissions, where they are used to support helical gears. Gear support in general is a common application for roller thrust bearings.
- Tapered roller bearings – It is a tapered version of a roller bearing and is used for combined axial and radial loads. This type of bearing is designed to handle large radial and thrust loads. Because of their load versatility, they are found in car hubs due to the extreme amount of both radial and thrust loads that car wheels are expected to carry. They are commonly found in heavy industrial, truck and wheel applications with combined radial and axial loads. Some examples are manual transmissions, gearboxes, power generation and other process equipment.
- Spherical roller bearings – These are roller bearings which have a barrel shaped roller. These bearings have medium friction, medium to heavy loads and misalignment capabilities. They are generally used for very high load applications with misaligned shafts to housings.
A flexural bearing is a bearing which allows motion by bending a load element. A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame. Another example is a rope swing, where the rope is tied to a tree branch.
Flexural bearings have the advantage over most other bearings that they are simple and thus inexpensive. They are also often compact, light weight, have very low friction, and are easier to repair without specialized equipment. Flexural bearings have the disadvantages that the range of motion is limited, and often very limited for bearings that support high loads.
A flexural bearing relies on the bearing element being made of a material which can be repeatedly flexed without disintegrating. However, most materials fall apart if flexed a lot.
Non contact bearing
Non contact bearings are either fluid bearing or magnetic bearing.
Fluid bearings support their loads solely on a thin layer of liquid or gas. They are usually of two types namely (i) hydrostatic bearings where load is supported by high pressure fluid and (ii) hydrodynamic bearings where load is supported by a lubricant film.
Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and the pressurization is done by a pump.
Hydrodynamic bearings rely on the high speed of the journal (the part of the shaft resting on the fluid) to pressurize the fluid in a wedge between the faces. Hydrodynamic bearings support a rotating shaft and transmit its axial load to a machine foundation by floating it on a self renewing film of oil.
Fluid bearings are frequently used in high load, high speed or high precision applications where ordinary ball bearings would have short life or cause high noise and vibration. They are also used increasingly to reduce cost.
Fluid bearings use a thin layer of liquid or gas fluid between the bearing faces, typically sealed around or under the rotating shaft.
A magnetic bearing is a bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear. Magnetic bearings support the highest speeds of all kinds of bearing and have no maximum relative speed.
Passive magnetic bearings use permanent magnets and, therefore, do not require any input power but are difficult to design. Techniques using diamagnetic materials are relatively undeveloped and strongly depend on material characteristics. As a result, most magnetic bearings are active magnetic bearings, using electromagnets which require continuous power input and an active control system to keep the load stable. In a combined design, permanent magnets are often used to carry the static load and the active magnetic bearing is used when the levitated object deviates from its optimum position. Magnetic bearings typically require a back up bearing in the case of power or control system failure.
Magnetic bearings are used in several industrial applications such as electrical power generation, petroleum refinement, machine tool operation and natural gas handling.