Supplied B1 Module 07A textbook: PDF pages 163–174.
Hole preparation and drill selection
A bolt hole must meet the drawing's size, location, alignment and finish. Select a drill for the specified final process; a hole to be reamed needs a suitable preliminary size. Drill identification may use metric diameters, fractional inches, numbered sizes or letters, so confirm the actual dimension. Secure the work and guide the drill when required. Deburring should remove sharp edges without excessive chamfering. Oversize, oval or misaligned holes cannot be made acceptable simply by fitting a larger fastener without approved repair data.
What determines the acceptable bolt-hole size? The applicable drawing or approved maintenance data.
Clearance, interference and transition
A clearance fit allows space between the shaft and hole throughout the tolerance range. An interference fit has the shaft larger than the hole at the applicable limits, so assembly needs an approved method. A transition fit may produce clearance or interference depending on the actual dimensions. Choose the fit by function: free movement, accurate location or firm retention. Do not use force as a diagnosis of fit type. Thermal installation must observe temperature limits and protect materials and seals.
Which fit may result in either clearance or interference? A transition fit.
Calculating limit clearances
Maximum clearance equals the largest hole minus the smallest shaft. Minimum clearance equals the smallest hole minus the largest shaft. A negative result means interference for that combination. For a hole of 20.00 to 20.02 mm and shaft of 19.97 to 19.99 mm, minimum clearance is 0.01 mm and maximum is 0.05 mm. Always carry units through calculations and compare the result with the prescribed fit schedule. Nominal sizes alone cannot show worst-case assembly conditions.
How is maximum clearance calculated? Largest hole minus smallest shaft.
Runout, bow and twist
Runout is the indicated variation as a part rotates against a fixed measurement reference. It may include eccentricity and form errors, so it is not automatically the same as shaft bow. Support the part at specified locations and use the required indicator position. Total indicated runout is the full maximum-to-minimum change. Under a specified simple symmetric setup, bow may be derived as half the total indication, but the manual's method governs. Twist is angular distortion and must be checked against the proper reference surfaces.
What is total indicated runout? The difference between maximum and minimum indicator readings.
Wear, taper and ovality
Measure mating surfaces at specified axial and angular positions. Different diameters along the length indicate taper; different diameters around one section indicate ovality. Check journals, bearing seats, bushes and gear contact surfaces for wear, scoring, fretting and corrosion. Record actual results and the applicable limits rather than only writing 'looks good'. Use calibrated equipment with adequate resolution. A part may meet diameter limits at one point but fail because of local wear or an unacceptable surface defect.
What measurement pattern detects taper? Diameters at different positions along the axis.
Assembly and acceptance
Before fitting, clean parts, verify dimensions and inspect for burrs or damage. Use the prescribed lubricant or retain a dry condition where specified. Apply fitting loads through the correct load-bearing surface; a bearing should not be pressed through its rolling elements. Thermal methods alter dimensions temporarily, so assembly clearances after temperature equalisation still matter. Check freedom of movement or retention as required. If fit limits are not met, use an approved repair or replacement decision rather than informal polishing until the parts move.
Why should a bearing not be installed by loading through its rolling elements? Installation load can damage the raceways and rolling elements.
Apply the knowledge
Work through these examples before starting the quiz. Explain why the other choices would not meet the requirement.
A hole is 25.00–25.03 mm and a shaft 24.96–24.98 mm. What is minimum clearance?
0.02 mm — Subtract the largest shaft from the smallest hole: 25.00 − 24.98.
The same hole and shaft have what maximum clearance?
0.07 mm — Subtract the smallest shaft from the largest hole: 25.03 − 24.96.
A dial indicator varies from −0.02 to +0.04 mm. What is total indicated runout?
0.06 mm — The full maximum-to-minimum span is 0.04 − (−0.02).
A journal is within size at the centre but undersize near an end. What is appropriate?
Assess all specified locations against the limits — Local wear can make the journal unacceptable.