Chapter 9 - Bearings Flashcards
Briefly describe friction bearings and anti-friction bearings.
Note: These bearings may also be called contact bearings and rolling element bearings
- friction bearings
- have a sliding contact between a shaft and the bearing
- a special, low-friction material lines a rigid housing
- the lining directly contacts and supports the shaft
- in use, the shaft slides over the liner material, separated by a thin film of lubricant
- the area of contact is relatively large
- pressure on the bearing is low
- also referred to as plain, sleeve, and journal bearings
- anti-friction bearings
- have a rolling contact between the shaft and bearing using balls, needles, or rollers
- the area of contact is very small
- pressure on the rings, balls or rollers is quite high
What are the principle parts of a friction bearing housing?
- base
- base bolt slots
- cap
- cap bolt holes
What are friction bearing housings made from and what factors influence the choice of housings?
- cast iron
- general use with light-medium loads
- cast steel
- general use with light-heavy loads
- fabricated steel
- special applications
- choice of bearing housing depends on:
- load
- rotational (rubbing) speed
- direction of pull
- support design
How are friction bearing dimensions expressed?
- nominal size
- size of the bore of the bearing (ID)
- dimensions are fractional, not decimal
Do all new bearings of the same shaft size have the same base-to-eye measurement?
- bearings of the same size, design and service weight from the same supplier have the same shaft centre dimension
- bearings from different suppliers may have different values for this and other dimensions
What factors influence the choice of bearings?
- load
- rotational (rubbing) speed
- direction of pull
- support design
What are the applications for the following bearings:
- solid bearings?
- used when speed and load are low
- designed to be used in locations where load is applied to the top part of the bearing
- frequently mounted in the inverted position
- must be slid on or off the shaft
- bearings must be put on as an assembled unit
What are the applications for the following bearings:
- split bearings?
- two-bolt bearing
- used for light-medium loads
- four-bolt bearing
- used for medium-heavy loads
What are the applications for the following bearings:
- angle bearings?
- when the load is applied parallel to, or at a slight anlge to, the horizontal
- when the bearing is mounted on a vertical support
What are the applications for the following bearings:
- integrated bearings?
- heavy-duty machines where bearing base is cast as integral part of machine
- bearings are either babbitted or fitted with shells or liners like other journal bearings
How does friction bearing joint design affect its integrity?
- flat joints
- the bolts hold the cap in alignment
- the opposite sections of the joint may be completely level with each other (flat joint) or be at an angle (flat-joint angle)
- gib in the joint
- have a little “step” in them, they keep the cap from moving sideways
- combines the rigidity of a solid block with the advantages of split construction
- suitable for limited side loading
- it need not be loaded on its bottom half
What qualities should a good bearing material have?
- softer than the shaft material, so it deforms slightly under heavy loads
- of low coefficient of friction
- wear-resistant
- good heat conductor and remain relatively stable with heat changes
- readily available
List the metals generally used for bearing liners and state their applications
- babbit
- general use
- brass (copper/zinc alloy)
- higher speeds and heavier loading than babbitt
- bronze (copper/tin alloy)
- higher speeds and heavier loading than babbitt
- aluminum
- common in hydraulic pumps
- sintered bronze (used for self-lubricating chain)
- porous bronze with a built-in oil supply (oilite)
- oil to bronze ratio = 1 : 2 by volume
What characteristics of nylon polyurethanes and phenol laminates affect their applications?
- nylon and polyurethanes
- generally inert to most mild acids and alkalis
- lubricant can be oil or grease, but water is also used
- with low rubbing speeds and temps, some grades of nylon will run with no lubrication
- higher coefficients of expansion than metallic bearing materials
- some synthetic materials expand when saturated with water
- basic nylon = 10X thermal expansion compared to steel, and a fully saturated water expansion rate of 0.0256”/per inch - important when dry fitting a nylon bearing which will run with water lubrication
- phenol laminates
- have layers of cotton or other natural fillers bonded with phenolic resin
- strong
- shock resistant
- compatible with most fluids
What is the hardest and toughest Babbitt made from?
- tin-based
- copper or antimony, with up to 90% tin
- sometimes called “nickel babbitt”
- used in conditions of heavy service and extreme pressures
How does overheating affect Babbitt?
- service life of the material is greatly reduced
- equivalent to overheating an anti-friction bearing to install it on a shaft
- physical shape is not altered, but normal service life is reduced
How is Babbitt held in the bearing casing?
- by casting “keys” which are grooves or slots in the casting
- any bearing liner to be re-babbitted must have the keys cleaned out
Describe the procedure for pouring a Babbitt bearing using a mandrel
- when you pour into a bearing and mandrel assembly make sure that it has been heated to 94C (200F) first to remove any free water or surface moisture. Free water causes a “blowback” or explosion
- use all protective equipment as called for by:
- WCB rules and regulations
- company safety policy
- the shaft must be brought up to level or aligned to other parts of the machine
1 - gather the following equipment:
– fire extinguisher
– oxyacetylene torch
– putty
– pliers
– cake of babbitt
2 - fit the torch with a large tip and melt out the remaining babbitt in the casting
3 - smoke the shaft with a straight acetylene flame
4 - put a putty dam around the shaft and bearing
5 - hold the babbitt with the pair of pliers
6 - melt it into the bearing with the torch
What is the purpose of tinning?
- process used when a thin skin of babbitt must be firmly attached to a metallic backing
- similar to that used in brazing a brass coating
- requires a heat source such as an oxyacetylene or propane torch
What must be checked when fitting a bearing?
- surface finish and area of contact
- running clearance
- lubrication entry (for setting up the oil wedge)
- groove(s) (for distributing the lubricant)
Describe the tools used to fit a bearing
- scrapers
- curved scrapers (wavy finish)
- straight scrapers (flat finish)
- flat file or machinist’s file
- triangular file
- groove cutter cape chisel
Describe the procedure for fitting a bearing
1 - after the bearing has been babbitted, the corners of the bearing must be relieved to keep the shaft from binding
2 - the top edge of the bearing must be chamfered almost to the corners. This channels the lubricant to the shaft. The amount of chamfer is usually up to the individual, but the chamfer is often greater for grease lubrication than for oil. For heavy loads, the bottom part of the bearing is chamfered also.
3 - another alternative is to cut an oil groove in the centre of the bearing to help maintain an effective oil wedge. This method is used mainly for casual, marginal lubrication
4 - after the chamfers/oil grooves have been cut in the load-bearing surfaces, check the bearing for contact. To do this, lightly coat a mandrel or shaft with mechanic’s bluing, and rotate it in the bearing. Apply the bluing in a light, smooth, even coat. Do NOT use layout ink.
5 - Remove bearing high points by scraping to allow the shaft to make contact with the bottom of the liner. >80% contact is acceptable
Describe the procedure for shimming a bearing to the correct amount of clearance
1 - put between the bearing halves the original shim used for babbitting the cap (or a shim of the same thickness) and an additional thin shim
2 - make sure that the shims do not touch the shaft. Shape them to clear the chamfer cut in the babbitt
3 - tightly bolt together the cap and base and then try to rotate the shaft or mandrel
4 - if the shaft does not rotate:
- for a gib-style = add more shims
- for a flat-style = pull the cap slightly to one side by the cap bolts and align the cap with a few sharp blows with a ball-peen hammer. If this does not work, add more shims until the shaft turns freely
Describe the procedure for checking bearing clearance with a dial indicator, plastigage and lead wire.
- dial indicator
1 - mount a dial indicator
2 - pry or mount jacks at both ends of the shaft to lift the shaft evenly for an accurate reading. If a jack is mounted at only one end of the shaft, the shaft will tilt in the bearing, and give a false reading
3 - note that the dial indicator gives the total clearance in a bearing. It does NOT indicate high and low spots. Check the wear by visual inspection - Plastigage
1 - choose plastic gauge stock with diameter about equal to the clearance
2 - take off the bearing cap and place lengths of the plastigage across the shaft in several places. They should reach from one edge of the base to the other. Number each piece of plastigage on the shaft
3 - have a record sheet handy, with space to record all readings
4 - check that the plastigage is not pinched at the corners of the top cap of the bearing
5 - tighten down the cap, then take the cap off carefully because some plastigage pieces may stick to it
6 - compare the width of the crushed plastigage with the chart on the packet - lead wire
1-5 - use the same procedure as plastigage
6 - measure the thickness of the wire with a micrometer. Start from the same end of each wire and take the wires in sequence
7 - record the thickness of each compressed wire
Describe how preformed bearings are installed and held in place
- held in position by pins or dowels, screws, special bearing designs, compression, or a combination of these
- when shells are first installed, they frequently extend past the face of the bearing by a small amount, which is knows as the “crush allowance” or “crush clearance”
- if the shells are machined for the bearing, the crush allowance should not be changed
- Shells can be installed and used in 2 cases:
1 - where there is no adjustment for wear - that is, when the shells are worn a certain amount, they are replaced (compressor/piston connecting rods)
2 - where adjustment for wear is obtained by shims between the cap and the base. To obtain effective crush or clamping action, the shims must be between the shell halves as well as between the cap and base - one advantage of shell liners is that, in some designs, the worn liner may be replaced by taking weight of the shaft off the liner (by removing the cap) and rolling out the old liner and rolling in a new liner in the same manner
Describe the procedure for installing bushings
- fit an clearance is usually set by the dimensions machined into the housing and bushing
- held in position by a press fit, a press fit and dowels, or by dowels and with clamping action between cap and the base
- small bushings can be reamed to bring the bore to the correct diameter
- large diameter bushings often require honing or scraping
- if the bushing is supplied with an oil hole, you must align the oil hole with the supply line when pressing in the bushing.
- some bushings the oil hole may be drilled after installation and should be scraped to remove high spots caused by the drill
- when bushings are worn, they must be replaced
Describe how thrust is controlled with plain bearings for both low- and high-speed applications
Low Speed Applications
- thrust (axial force) is mainly controlled by the use of other components:
- controlled by a shaft collar fastened on the shaft
- thrust washer backed by a sprocket or any other hub
High Speed Applications
- thrust of the shaft is controlled by the shaft shoulder and a collar that adjusts the amount of end float
- end float is the amount the shaft moves axially in a bearing. Also called end play, axial float, or axial displacement
Describe the basic design of Kingsbury bearings
- a revolving ring with a flat contact surface, usually hardened and ground
- a stationary ring with a number of flat contact surfaces of a low-friction material - either brass, bronze, or a tinned-on, high-capacity babbitt on a brass backing
- these surfaces are separated by lubricant grooves and have chamfers or slopes on the leading edges to set up oil wedges
- due to centrifugal force of the bushing ring throwing oil outward, lubricant must be supplied to the centre of most bearing designs
What styles do Kingsbury bearings come in?
- fixed pad
- a backing which has a convex shape and fits into a mating housing to correct for minor misalignment
- floating or tilting pad
- individual load blocks are pivoted on the fixed ring to adjust for thrust and load
What type of bearing is used to control linear motion ?
- guide bearings
- used for such things as machine tools and gas compressors
- often has a thin layer of bearing material on its surface to reduce friction, but normally operates with lubrication
What is the application for pivoted shoe bearings?
- split journal bearing, used mostly for high peripheral speeds and shaft stabilization
- shoes are machined cylindrically to fit freely in grooves in a retaining ring
- operation is like a flat, tilting pad, Kingsbury bearing
- surfaces of the shoes and rings are corrosion resistant and low-friction
- bearing is dowelled for installation and to position the bearing axially and prevent rotation
- each shoe has a separate oil inlet, which keeps the operating temperature lower and uniform
- plates are bored so discharged oil is regulated without shaft contact
- carefully inspect the babbitted surfaces, pivoting surface and seat, and the diametral clearance between shoe and journal for signs of wear
What are the merits of anti-friction bearings?
- versatility
- low-friction operation
- ability to be packed to avoid frequent lubrication
List the main parts of anti-friction bearings
- two hardened steel rings, cup and cone for tapered bearings
- hardened balls, rollers or needles
- separator
- for specific applications, bearings may have other parts such as a snap ring used to set axial location
List the three styles of rolling elements
1 - ball bearings
2 - roller bearings
- cylindrical
- tapered
- spherical (surfaces of the rollers are curves across their width and run in concave channels)
3 - needle bearings
Describe the operating principle of ball bushings
- can be used as a guide for axial shaft measurement using recirculating ball bushings
- the balls circulate through radial grooves
- each carries only a small amount of the toatl load at any one time
Describe the applications for open bearings, shielded bearings, sealed bearings and sealed and sealed and shielded bearings
- open bearings
- foreign material is kept out of the housing by shaft lip seals or the equivalent
- the lubricant is expected to work through the bearing from one side to the other
- shielded bearings (one or both sides)
- the bearing is exposed and the shield will keep out solids but not fluids
- the lubricant is metered out of the housing into the bearing
- sealed bearings (one or both sides)
- the bearing is exposed and the seal will keep out solids and fluids
- the lubricant is to be kept in the bearing
- sealed and shield bearings (pillow block cartridges)
- conditions are very dirty and abrasive
- when replacing a bearing with a single shield or seal, take care that the new bearing orientation during installation
How are the directions of bearing loads expressed?
- radial load
- maximum or total radial load means that all forces on the bearing are in a radial direction, with little or no sideways thrust load
- example - needle bearings / single-row ball bearings
- thrust load
- a pure thrust (axial) load means there is no radial load.
- all forces run parallel to the shaft axis
- example - ball / needle thrust bearings
- combination thrust and radial load (angular load)
- tapered and roller bearings
- ball bearings with angular contact
- spherical roller, self-aligning bearings
What are the normal speed ranges for ball bearings and roller bearings?
- ball bearings
- high-speed applications
- up to 40,000 RPM or as “rubbing speed”
- roller bearings
- low-speed applications
- up to 20,000 RPM, normal range is 2000-20,000 RPM
How are bearings classified according to their service use?
- as the rated load capacity is increased, the bore stays the same but the diameter of the rolling unit and the width and thickness of the rings are increased
- “service weight” or “service use” is a means of classifying bearings with the same types of rolling element and bore for their load capacity
- service weight is shown in the code number of the bearings
- common service weight groups: 00 series -- ultra light 100 series -- extra light 200 series -- light 300 series -- medium 400 series -- heavy-duty *other grades also available
What measurements are matched to the code markings on a bearing?
- outside diameter (OD)
- bearing width (W)
- bore (ID)
– last 2 digits of bearing number
– up to 20 mm, the bore is designated as
00 = 10 mm
01 = 12 mm
02 = 15 mm
03 = 17 mm
– from 20 mm to 480 mm, to find the bore, multiply the last two figures from the bearing number by five (example - 6204 ball bearing has a bore of 04 x 5 = 20mm)
– above 480 mm (number 6296), the bearing size is directly included in the bearing number - indicated in millimetres (mm)
