Shaft Attachments and Fits Flashcards
In any mechanical arrangement the static and dynamic positions of the shafts are established and maintained by the:
Bearings
Any geometric or dimensional inaccuracy in the shaft may directly affect the performance of: (2)
Bearings and the machine
In the simple case of a shaft rotating under a radial load and being supported by two bearings, specific factors related to _____, _____ and _____ can affect the runout of the shaft
Shaft, Bearing components, and Housing
some of the conditions that influence shaft position are: (9)
- Straightness
- Roundness
- Size or Fit with bearing
- Bearing seat concentricity
- Bearing seat squareness
- Radius fillets at shoulders of shaft
- Deflection of shaft
- Means of retaining the bearing in position
- Balancing
Almost all conditions that influence shaft position relate directly or indirectly to:
Bearing installation
the primary requirement for bearing shafting is that it be:
The proper size
dimensions must be accurate for both shaft _____ and _____ locations along the shaft.
shaft diameters and axial locations
Oversize shaft diameters invite: (2)
Overheating or Preloading
Undersize shaft diameters may contribute to: (3)
- Fretting of the shaft
- loosening of the bearing
- excessive internal clearance
Another basic requirement is that a shaft has the correct surface finish. This finish is given in: (3)
- Micrometres (µm) AA
- Microinches (µin)
- Root mean square (rms)
Note: AA refers to the arithmetical average height of surface roughness
Root mean square is considered the better method of determining surface finish as it emphasizes:
Extreme surface deviations
Shaft surface requirements depend on: (2)
Bearing materials and types
most anti-friction bearings require a surface finish of up to ___ µm AA
0.30 µm AA (12 µin)
Note: Too rough a surface finish may result in the loss of interference fit, excessive shaft wear, and fretting of the bearing seat
Babbit and bronze sleeve bearings require a finish of up to ___ µm AA
0.81 µm AA (32 µin)
The shafting should be straight and free of nicks, gouges, scratches or burrs. Imperfections can interfere with _____ _____ and cause _____ _____.
Bearing Lubrication and cause Localized Scoring
Retention of bearings on a shaft is influenced by these three things: (3)
- Fit
- Hardness/finish of material
- deflection of the shaft
most anti-friction bearings are mounted on a shaft with a slight interference fit. Too loose a fit may cause the inner ring of the bearing to:
Creep on the shaft
Non-ferrous shaft materials require special attention. Why is this?
The thermal coefficient of expansion in ferrous vs non ferrous metals means the fit of a bearing on a shaft varies with temperature.
A hub is a machine element fitted to components such as gears, sheaves, or sprockets for the purpose of:
enabling a component to be attached to a shaft
Typically hubs are assembled to shafts prepared with a:
Keyseat
What are the steps to install a hub? (4)
- Install the key into the shaft keyseat
- Lubricate the shaft with with an oil or anti-seize
- align the hub keyseat with the key
- slide the hub into position
the method used to position a hub is dictated by: (3)
- fit
- type of equipment
- style of hub
couplings are used to connect two shafts together at their ends for the purpose of:
Transmitting power
Name the two general types of couplings: (2)
Rigid and Flexible
Rigid couplings are designed to connect shafts together _____, making a single unit.
Rigidly
When installing a coupling, ensure the shaft is straight, free of rust, paint, dirt, burrs or imperfections. This is because they could alter:
Coupling position
For general purposes, a coupling should have a ________ fit to the shaft.
locational interference fit
The class of fit for a coupling is dictated by: (3)
- Type of coupling
- diameter of shaft
- torque
flexible and rigid couplings have similar shaft requirements, however flexible couplings may use:
Other mounting methods
What are three other mounting methods that flexible couplings may use? (3)
- plain taper bushings
- flanged taper bushings
- tapered shaft and locknut
Note: In most cases a key is included in the connection
Many machine components are attached to shafts for the purpose of transmitting torque. The means used for axial location depend heavily on whether or not:
axial thrust is transmitted
v-belt sheaves, chain sprockets and spur gears produce ____ thrust loads.
small thrust loads
Note: many types of gears can produce varying amounts of thrust loads
Gears can be keyed to a shaft, and then locked in place by any of the following methods: (9)
- Interference fit
- set screws
- plain tapered bushing
- flanged tapered bushing
- locking collar and set screw
- pinning
- retaining rings
- tapered shaft with key and locknut or screw
- molding polymer
in applications where a gear is required to slide on the shaft, such as in a gearbox, what methods could accomplish this? (3)
- fixed feather key
- sliding feather key
- spline
compression fittings are a method of:
attaching machine components to standard size shafting
Note: quick to install and dismantle
in the simplest form, a hub on a sprocket can be slotted axially, and a clamp (bolt) drilled:
through the hub
Note: when the clamp is drawn tight, it forces the split hub into contact with the shaft. The pressure on the shaft allows the transmission of torque
To maintain a sprockets relative position on a shaft, what is inserted?
a key
maintaining concentricity of a component can be difficult with this type of connection:
Compression fitting
the difficulties encountered with simple clamp compression fittings can be overcome by using a:
Plain tapered bushing
Note: the bushing has a small taper on its outer surface and the hub has a matching taper in its bore
when a plain tapered bushing is pulled into a mating hub with a prescribed number of screws, it:
compresses tightly to the shaft.
a ____ is used as a positive means of transmitting torque with a plain tapered bushing
key
flanged, tapered bushings are a quick means of installing sprockets and sheaves to shafts. _____ is maintained with this.
Concentricity of the sheave
like with a plain tapered bushing, the hub is machined with a taper in its bore to match the bushings taper. The components can be lightly assembled and then installed, secured it place by:
tightening the cap screws that compress the bushing in the hub tight to the shaft.
a flanged tapered bushing is easily removed from its hub using:
the jackscrews in the holes tapped into the face of the sprocket
A millwright often needs to make decisions regarding the fit of machine components. When doing so, sketches must be fully dimensioned and give the:
appropriate tolerances
A tolerance is:
the total permissible deviation of a dimension on a part
it is the difference between the max and min limits on size
when applying tolerances to dimensions of mating parts, what two systems may be used? (2)
- basic hole system
- basic shaft system
in this system of applying tolerances, the hole is the basic size, and the allowance is on the shaft. For example, when a replacement shaft is to be fitted with a standard anti friction bearing:
Basic hole system
in this system of applying tolerances, the design size of the shaft is the basic size and allowance is left in the bore of the mating part. For example, when replacing a worn out sprocket on an existing shaft:
Basic shaft system
Tolerance may be expressed as: (2)
- Unilateral
- Bilateral
Tolerance being applied in one direction only is known as:
ex: 4.500” +0.000 -0.002
Unilateral tolerance
tolerance being applied in both directions is known as:
ex: 4.500” +0.002 -0.002
Bilateral Tolerance
The terms allowance and fit are used interchangeably. Allowance is the:
prescribed difference between maximum material limits of mating parts (maximum interference)
The terms allowance and fit are used interchangeably. Fit is:
determined by allowance. Allowance determines the fit between two mating parts
positive allowance refers to:
minimum clearance between mating parts (clearance fit)
negative allowance refers to:
maximum interference (interference fit)
the degree of fit is a result of specifying a ____ to each mating part
tolerance
two basic types of fit are: (2)
Clearance and interference fit
This fit allows the mating parts to maintain some clearance between them at all times:
Clearance fit
This fit maintains some resistance between mating parts at all times:
Interference fit
This class of fit provides clearance between mating parts for lubrication. It applies mainly to rotating parts such as a shaft in a friction bearing. fit must not be so close that it cannot turn, but not so loose that it floats about:
Running fit
this class of fit applies to parts which slide on each other, such as a dovetail slide.
Sliding clearance fit
when using a dovetail slide with a gib, the gib:
is normally used to adjust clearance between mating parts.
sliding clearance fits can be subdivided into these types of fit: (7)
- Close sliding
- sliding
- precision
- close running
- medium running
- free running
- loose running
These fits are intended for normally stationary parts which can be freely assembled or disassembled:
locational clearance fit
Note: various classes from snug to medium clearance fits satisfy the needs of various applications
when the tolerance on the mating parts partially overlaps so that either a clearance or interference may result, the fit is called:
transition fit
Note: These fits are used to locate mating parts, such as a dowel pin and mating hole aligning machine parts.
when accurate location and rigid alignment are of the utmost importance, what fit would you use?
locational interference fits
Ex: anti-friction bearing pressed into a housing
Note: Because of their tightness, it’s not recommended to use this fit to transfer frictional loads.
when the tolerance of mating parts fully overlaps, causing an interference fit, the fit is classed as:
force or shrink fits
Note: This fit requires parts to be forced or shrunk together to ensure they act as one unit. Pressure deforms mating parts as they are assembled.
If interference is excessive, the elastic limit of the material will be exceeded and assembled parts distorted. In extreme cases, the ultimate strength of the metal will be exceeded and:
the outer part will burst
ensure that mating parts are within the _____ _____ for the designated fit
prescribed tolerances
