7.6 Fits and Clearances Flashcards
What determines the size/diameter of a drilled hole?
The size/diameter of a drilled hole is determined by the type of fastener fitted in that hole.
How does the hole size differ for various fasteners?
A hole drilled for an aluminium solid rivet differs from a hole drilled for a threaded screw or a plain shank bolt of the same diameter.
What determines the hole size for a metal shaft?
The size/diameter of the shaft and the application in which the hole/shaft combination is to be used.
What must be done if a hole is required to be reamed?
A smaller hole size/diameter must be drilled to allow for material removal during the reaming process.
What is a drill bit?
A drill bit is a circular cutting tool used to remove material to produce a certain sized hole in different materials.
How are drill bits used?
Drill bits are attached to a powered (electric or pneumatic) drill, which cuts through the material by rotation.
Where can standard drill sizes be found?
Standard drill sizes can be found on charts that list each size and include equivalent sizes in metric, fractional, letter, and number/letter systems.
What are the common imperial and metric drill sizes?
In imperial measurements, common drill sizes range from 1/16 - 1 in. In metric measurements, they range from 0.5 - 25 mm.
What is the largest and smallest letter size in the letter system?
The largest drill diameter is given the letter ‘Z’ (10.5 mm), and the smallest letter ‘A’ (5.85 mm).
What does the number system for drill sizes start and end with?
The number system starts from 1 (5.79 mm) and reduces to 80 (0.34 mm).
What is the tolerance for the diameter of a drilled hole?
The diameter of a drilled hole has no tolerance; it is assumed to be as close to the drill size as possible.
What factors affect the accuracy of a drilled hole?
Factors include the accuracy of the drill point, size of the drill, use of a drill guide bushing, material being drilled, length of the drill, accuracy of the spindle and chuck, rigidity of the drill and part, and cutting fluid used.
What can cause a drilled hole to be undersized?
Materials like light alloys and plastics have a high coefficient of thermal expansion, causing the hole to be smaller when the material cools.
What does the term ‘fit’ refer to in engineering?
The term ‘fit’ refers to the grade of tightness or looseness between mating parts.
What are the three basic classes of fit?
Clearance Fits, Interference Fits, and Transition Fits.
What characterizes a clearance fit?
There is always some play after assembly, allowing for movement between parts.
What is an interference fit?
An interference fit allows no movement after assembly because the inner part is larger than the part into which it fits.
What are the three classes of interference fit?
Driving Fit, Force Fit, and Shrink Fit.
What is a transition fit?
A transition fit occurs when the mating parts are almost the same size, with very small differences in size.
How can the type of fit be determined?
The type of fit can be determined by selective assembly of pieces on either side of the allowance.
What is meant by ‘hole-based’ and ‘shaft-based’ systems?
‘Hole-based’ means the hole is constant and the shaft varies in size, while ‘shaft-based’ means the shaft is constant and the hole varies in size.
What does the term ‘fitting’ mean in hand and machine fitting?
Fitting means putting parts together, where they either touch or join each other.
How are parts machined to achieve a specific fit?
Parts must be machined, filed, or ground to a desired size.
What has improved the accuracy and uniformity of fitted parts?
The advancement of precision machinery has significantly improved the accuracy and uniformity of fitted parts.
Can you provide examples of fitted parts?
Examples include shafts fitted to a bearing, a piston running in a cylinder, a propeller splined to its shaft, or a bolt fitting into a nut.
What are limits in the manufacturing process?
Limits ensure that if two mating parts are manufactured to the dimensions stated on the engineering drawing, they will assemble without further adjustments.
What are allowances in fitting?
An allowance is the difference in dimension necessary to give a particular ‘class of fit’ between two parts.
What is the hole-based system?
The hole-based system is commonly used, where the item dimensioned to include the allowance has high and low-limits and therefore a tolerance.
How is tolerance defined?
Tolerance is the total amount of variation in the size of a part.
How do you calculate tolerance?
To find the tolerance, subtract the smallest size from the largest size.
What is bilateral tolerance?
Bilateral tolerance allows for variation above or below the design size.
What is unilateral tolerance?
Unilateral tolerance is the amount of variation in one direction, either above or below the design size.
What systems of fits and clearances have been developed?
Various standard fits have been developed, including the British Standards System, ISO, ANSI, and the Newall System.
What is the British Standards System?
The British Standards System provides 21 hole classifications and 21 shaft classifications, each with 16 grades of accuracy.
What does the ISO 286 standard provide?
The ISO 286 standard provides 28 classifications for holes and shafts with 20 grades of accuracy.
What does the ANSI standard follow?
The ANSI standard for metric fits follows the ISO standard, while for Imperial units, it uses specific symbols for fits.
What is the Newall System?
The Newall System was the first standard in the UK for limits and fits, providing a range of clearance, transition, and interference fits.
What types of fits are defined in the Newall System?
The Newall System defines interference, driving, transition, and clearance fits.
What is the British Standard Specification 4500?
BS 4500 is a system of limits based on standard sized holes, allowing the use of standard hole manufacturing tools.
What is an interference fit?
In an interference fit, the upper and lower limits of the shaft are greater than those of the hole, requiring force for assembly.
What is a transition fit?
In a transition fit, the differences in the upper and lower limits of both items are negligible, requiring only light effort for assembly.
What is a clearance fit?
In a clearance fit, the shaft is always smaller than the hole, allowing easy movement.
What is the difference between bilateral and unilateral tolerance?
Bilateral tolerance allows variation on either side of the nominal dimension, while unilateral tolerance has one tolerance as zero.
What is the International Tolerance Grade (IT)?
IT numbers indicate the same relative level of accuracy for a particular IT number but vary depending on the nominal size.
What is the hole basis system?
The hole basis system relates to basic or nominal hole size, with fundamental deviation ‘H’.
What is the shaft basis system?
The shaft basis system relates to basic or nominal shaft size, with fundamental deviation ‘h’.
What is the H8/f7 fit for a 20 mm diameter?
For a 20 mm diameter, the H8 limits are +0.033 and 0.000 mm, and the f7 limits are 0.020 and 0.041 mm.
What types of fit are defined in the BS 4500 system?
Types of fit include Easy Running, Normal Running, Slide, Location, Push, Light Press, and Heavy Press.
Why is interchangeability of components important in aircraft manufacturing?
It ensures ease of replacement and limits manufacturing errors.
What is specified on the design drawing for aircraft components?
Limits or tolerances for manufacturing, ensuring easy assembly of mating parts.
What is clearance in the context of an engine shaft and bearing?
Clearance is the positive allowance where the inner part is smaller than the outer part, allowing for lubricant use.
How does wear affect the clearance between a shaft and bearing?
Wear reduces the shaft size and increases the bearing size, increasing clearance over time.
What happens when the clearance becomes too large due to wear?
Parts are no longer deemed serviceable, even if within permissible worn size limits.
What must be considered when determining the serviceability of worn parts?
Both the size of the part and its clearance in the other part.
What is the basis for the limits of allowable wear?
The tolerances provided by the manufacturer of the part.
What types of motion can cause wear between parts?
Intentional motion (e.g., shaft rotation) and accidental motion (e.g., chafing of immovable parts).
What does the maintenance documentation include for moving parts?
A schedule of fits and clearances based on the limit system for each mechanism.
How are parts that are not intended to move together inspected?
Through inspection schedules to identify defects.
What does the schedule of fits and clearances contain?
Tables specifying limits on wear and characteristics like ovality, bow, and twist.
What processes are considered during engine assembly for fit?
Interference fit, transition fit, and clearance fit.
What is an example of a heat/shrink fit in turbine assembly?
The turbine shaft is shrunk with dry ice or liquid nitrogen to fit with turbine wheels heated with a heating element.
Where else are heat/shrink fits used?
In turbine wheel to turbine shaft assembly, bearings to shafts and gears, and sleeves to cases.
What is a press (transition) fit in turbine assembly?
The turbine shaft bearing is pressed onto the turbine shaft assembly.
What components commonly use press fit?
Bushings, bearings, and sleeves.
What is a clearance fit in turbine blades?
Turbine blades are inserted into turbine wheels by hand and slide into the dedicated slot.
Where else are clearance fits used?
In turbine/compressor blades to discs, case split line bolts, and piston to cylinder.
What is essential when inspecting shafts and tubes?
The ends must be square with each other, and the centreline must be straight.
What does it mean if a shaft is termed ‘bowed’?
It means the centreline of the shaft is not straight.
What risks are associated with a bowed shaft rotating at high speeds?
There is a risk of vibration, leading to mechanical failures, loosening of fasteners, and fatigue.
What is the bow limit for a drive shaft rotating at approximately 1500 RPM?
The limit is 0.25 mm (0.01 in.) bow over the entire length of the shaft.
What causes twist in a shaft?
Twist results from applying a torsional (twisting) force on a round or square-sectioned shaft.
What happens if the twisting force is above the elastic limit?
The shaft remains twisted even after the force is removed.
How does a shaft twist in relation to torque?
A shaft twists in proportion to the torque applied.
Why does a shaft need periodic inspection?
To check for any permanent twisting or deformation due to continuous (cyclic) loading.
How can a square-sectioned shaft be checked for twist?
It can be checked on a surface table using a dial test indicator mounted on a surface gauge.
What is wear?
Wear is the removal of material and deformation due to mechanical action between moving surfaces.
How can wear be detected?
By comparing the dimensions of a new component to a used component.
What do aircraft manufacturers provide to assist engineers with wear measurement?
Allowable wear charts.
What is the ‘Dimension New’ in wear tables?
It is the size of the part when it is new, showing the relevant tolerances.
What is the ‘Permissible Worn Dimension’?
It is the size to which a part can wear before it must be rejected as unserviceable.
What is ‘Clearance New’?
It is the desired clearance in limit form, with interference fits quoted as negative clearances.
What does ‘Permissible Worn Clearance’ refer to?
It relates to the maximum allowable clearance when reassembling the component.
How can bow in a structural member be measured?
Bow can be measured using a straight edge and feeler gauges. Place the straight edge along the length of the shaft and insert feeler gauges at the point of maximum clearance.
What is the maximum bow allowed over a length of a shaft?
The maximum bow allowed is generally 1 in 600, unless otherwise stated in the repair manual or manufacturers’ tolerances.
What instruments are used for dimensional checks during aircraft servicing?
Precision measuring instruments such as micrometers, vernier calipers, and dial indicators are used.
How is bow calculated?
Bow is calculated by dividing the clearance measured by the length of the member.
Example: If the length of the shaft is 2.4 m and the clearance measured is 4 mm, then Bow = 4 mm ÷ 2.4 m = 1 ÷ 600.
What is the purpose of checking for twist in shafts?
Twist is checked to identify signs of torsional loads that may have affected the shaft.
How can twist be measured in a shaft?
Twist can be measured by mounting the shaft in V-blocks and using strain gauges to measure the amount of twist.
What can cause permanent twist in a shaft?
Permanent twist can occur from over-speeding, over-torquing, or sudden stoppage of the shaft.
What are witness marks on a shaft?
Witness marks are indicators provided by the manufacturer to show where the drive and driven flanges meet the shaft. Shifting at these points indicates permanent twist.
What is ovality and how does it occur?
Ovality occurs as a result of surface wear through friction or linear movement and applies to both holes and shafts.
How can holes be checked for ovality?
Holes can be checked using go/no-go gauges, internal micrometers, or calipers.
What is the method to check the roundness of a shaft?
Roundness can be checked by placing the shaft in a V-block and rotating it under a scribing block or dial indicator.
Why is it important to test for ovality before bow?
Testing for ovality before bow is important because measuring bow first can yield inconclusive results.
What is a Dial Test Indicator (DTI)?
A DTI is a precision measuring instrument used to determine the amount of movement between precision machined components and to check roundness and trueness.
How is flatness checked using a DTI?
To check flatness, the DTI is assembled onto a stand, positioned on the brake disc, preloaded, set to zero, and then the disc is rotated to indicate deviation.
