7.6 Fits and Clearences Flashcards
What determines the size/diameter of a drilled hole for bolts?
The type of fastener fitted in that hole and the application for the hole/shaft combination
Different fasteners require different hole sizes, e.g., a solid rivet vs. a threaded screw.
What is a drill bit?
A circular cutting tool used to remove material to produce a certain sized hole in different materials
Drill bits are attached to powered drills to cut through materials by rotation.
What are standard drill sizes based on?
The number system, metric and imperial measurements
Common imperial sizes range from 1/16 - 1 in and metric sizes from 0.5 - 25 mm.
What is the diameter of a drilled hole assumed to be?
As close to the drill size as possible with no tolerance
This means precision is critical in drilling.
What factors affect the accuracy of a hole drilled with a standard two fluted twist drill?
- The accuracy of the drill point
- The size of the drill
- Use of a drill guide bushing
- The material being drilled
- The length of the drill
- Accuracy of the spindle and chuck
- Rigidity of the drill and part being drilled
- Cutting fluid used
Each factor can influence the final size and quality of the drilled hole.
What can cause a drilled hole to be undersized?
Materials like light alloys and plastics may expand when drilled and contract when cooled
This thermal expansion can lead to holes smaller than the drill diameter.
What does the term ‘fit’ refer to in engineering?
The grade of tightness or looseness between mating parts
It determines the classification of fit.
What are the three basic classes of fit?
- Clearance Fits
- Interference Fits
- Transition Fits
Each class defines how tightly or loosely parts fit together.
What characterizes a clearance fit?
There is always some play after assembly; the hole is larger than the male component
This allows for movement and lubrication.
What is an interference fit?
No movement is possible after assembly; the inner part is larger than the outer part
This fit requires the outer component to be slightly larger than the hole.
What are the three classes of interference fit?
- Driving Fit
- Force Fit
- Shrink Fit
Each class describes how tightly parts fit together and the method of assembly.
What is a transition fit?
An intermediate fit between interference and clearance fits, with very small differences in size
It can result in either a clearance or interference fit depending on tolerances.
What are the two systems used for determining fits based on hole and shaft?
- Hole-based
- Shaft-based
These systems depend on which component has a constant size while the other varies.
What is the term used for putting parts together in hand and machine fitting?
Fitting
Fitting means putting parts together, where they either touch or join each other.
What are the two systems of fits based on the basis of the part being dimensioned?
Hole basis system and shaft basis system
The hole basis system dimensions the hole to include the allowance, while the shaft basis system dimensions the shaft.
What does an allowance represent in the context of fits?
The difference in dimension necessary to give a particular ‘class of fit’ between two parts
This is applied by the designer and is crucial for ensuring proper assembly.
Define tolerance in manufacturing.
The total amount of variation in the size of a part
Tolerance is calculated by subtracting the smallest size from the largest size.
What is bilateral tolerance?
The amount of dimension variation can be above, or below the design size
For example, a dimension of 2.5 mm +/- 0.05 mm indicates bilateral tolerance.
What is unilateral tolerance?
The amount of variation in one direction, either above or below the design size
An example is a dimension of 2.5 mm +0.05 mm/0.00 mm.
What are the classifications for holes and shafts in the British Standards System (BS)?
21 hole classifications (A, B, C, etc.) and 21 shaft classifications (a, b, c, etc.)
Each classification is provided with 16 grades of accuracy (1 to 16).
How many classifications does the ISO 286 standard provide for holes and shafts?
28 classifications for holes and shafts
The ISO system is similar to the British Standards but offers more classifications.
What does the American National Standards Institute (ANSI) standard for metric fits follow?
The ISO standard
For Imperial units, it uses symbols like RCx for running and sliding fits.
What are the two grades of holes in the Newall System?
Class A and Class B
Class A has higher accuracy than Class B.
What type of fit requires mechanical pressure for assembly?
Interference fit
Once assembled, no dismantling is likely to be required.
What distinguishes a transition fit?
It can provide either clearance or interference
Slight manual effort is required to assemble the parts.
In a clearance fit, what is true about the shaft and hole dimensions?
The shaft is always smaller than the hole
This allows for movement of the shaft within the hole.
What does the term deviation refer to in fitting?
The difference between a size and the corresponding basic or nominal size
It is crucial for understanding how parts will fit together.
In BS 4500, what type of fit is designated as H7/e8?
Easy Running fit
This fit is suitable for various types of moving parts.
What are the limits for a 20 mm nominal diameter shaft in an H8/f7 fit according to BS 4500?
Hole: 20.000 to 20.033 mm; Shaft: 19.959 to 19.980 mm
These dimensions ensure proper clearance fit.
Fill in the blank: The _______ system is based on standard sized holes and varying shaft sizes.
British Standards BS 4500
This system allows for the use of standard hole manufacturing tools.
What is the purpose of fits and clearances for aircraft and engines?
To ensure interchangeability of components and ease of replacement
Limits are applied on manufacturing errors to facilitate the manufacturing process.
What must be considered when assembling two mating parts in aircraft or engines?
Manufacturing limits or tolerances specified on the design drawing
This ensures that parts can be easily assembled.
What is clearance in the context of engine shafts and bearings?
A positive allowance where the inner part is smaller than the outer part
This allows for lubrication and extends the life of moving parts.
What happens to the clearance between a shaft and bearing over time?
Wear reduces the size of the shaft and increases the size of the bearing
Eventually, this can lead to the parts becoming unserviceable.
What defines the serviceability of worn parts?
Both the size of the part and its clearance must be within permissible limits
These limits are provided by the manufacturer.
What are the two types of wear that can occur between moving parts?
Intentional wear (e.g., shaft in a bearing) and accidental wear (e.g., immovable parts rubbing together)
Motion always results in some kind of wear.
What is included in the maintenance documentation for parts designed to move together?
A schedule of fits and clearances based on the limit system issued for each mechanism
This helps to maintain serviceability.
What is the purpose of inspection schedules for parts that do not move together?
To identify defects and prevent recurrence of those defects
Inspection schedules are critical for non-moving components.
What characteristics are specified in the schedule of fits and clearances?
Limits on wear, ovality of a hole/shaft, bow of a shaft, twist of a shaft
These characteristics are essential for proper assembly and function.
What is a heat/shrink fit?
An interference fit where one component is shrunk and the other is heated for assembly
Commonly used in turbine shaft and wheel assemblies.
Give examples of where heat/shrink fits are used.
- Turbine wheel to turbine shaft assembly
- Bearings to shafts and gears
- Sleeves to cases
What is a press (transition) fit?
A fit where components are pressed together, typically requiring predetermined torque
This is used in turbine shaft assemblies.
Provide examples of components that use press fits.
- Bushings
- Bearings
- Sleeves
What is a clearance fit?
A fit where components can slide into place without interference
For example, turbine blades are clearance type fits.
List examples of where clearance fits are used.
- Turbine/Compressor blades to discs
- Case split line bolts
- Piston to cylinder
What is meant by limits for bow, twist, and wear?
Limits for bow, twist, and wear refer to the allowable tolerances for deformation in shafts and tubes due to mechanical forces and wear over time.
What happens if the center line of a shaft is not straight?
‘Bowed’ condition leading to vibration, mechanical failures, loosening of fasteners, and fatigue.
What is the limit of bow for a drive shaft rotating at approximately 1500 RPM?
0.25 mm (0.01 in.) bow over the entire length of the shaft.
What is twist in the context of shafts?
Twist results from applying a torsional (twisting) force on a round or square-sectioned shaft.
What occurs if the twisting force applied to a shaft exceeds the elastic limit?
The shaft remains twisted even after the force is removed.
How is a square-sectioned shaft checked for twist?
Using a dial test indicator mounted on a surface gauge on a surface table.
What is wear?
The removal of material and deformation as a result of mechanical action between moving surfaces.
How can wear be detected and measured?
By comparing the dimensions of a new component to a used component.
What are the four dimensions typically covered in wear tables?
- Dimension New
- Permissible Worn Dimension
- Clearance New
- Permissible Worn Clearance
What does ‘Dimension New’ represent?
The size of the part when it is new, showing the relevant tolerances.
What is meant by ‘Permissible Worn Dimension’?
The size to which a part can wear before it must be rejected as unserviceable.
What is ‘Clearance New’?
The desired clearance in limit form, with interference fits quoted as negative clearances.
What is the ‘Permissible Worn Clearance’?
The maximum allowable clearance when reassembling the component.
What is the maximum allowable bow over a length of a shaft?
1 in 600
This is unless otherwise stated in the repair manual or manufacturers’ tolerances.
What instruments can be used for dimensional checks during maintenance?
- Micrometer
- Vernier caliper
- Dial indicator
How is the amount of bow calculated?
Bow = Clearance measured ÷ length of member
What is the first step in measuring bow in a shaft?
Place the straight edge along the entire length of the shaft, parallel to its axis
True or False: Visible marks at each end of a shaft indicate it has not been subjected to high torsional load.
False
How can twist in a solid or tubular shaft be checked?
Mounting them in V-blocks and locating the marks in the horizontal position
What type of gauge can measure the amount of twist a shaft has been subjected to?
Strain gauges
What can cause permanent twist in a shaft?
- Over-speeding the shaft
- Over torquing the shaft
- Sudden stoppage
What are ‘witness’ marks in relation to shaft twist?
Marks provided by the manufacturer where the drive and driven flanges meet the shaft
What is ovality?
A condition that occurs as a result of surface wear through friction or linear movement
What tools can be used to check holes for ovality?
- Go/no-go gauges
- Internal micrometres
- Callipers
How can the roundness of a shaft be tested?
By placing the shaft in a V-block and rotating it under a scribing block or a dial indicator
What is the purpose of a Dial Test Indicator (DTI)?
To determine the amount of movement between precision machined components/parts
What must be done before using a DTI on a brake disc?
Apply a preload by pressing it against the disc
Fill in the blank: Ovality applies equally to _______ and shafts.
holes
What should be referred to in order to determine the serviceability of a component when using a DTI?
Manufacturers’ tolerances
Before testing a shaft for bow, what should be tested first?
Ovality
What happens if the measurement of bow is performed before ovality?
The results can be inconclusive
