Limits Fits and Tolerances Questions and Answers

Answer: C

The heavy fit is mainly used for permanent assemblies. In a heavy force fit the parts are joined with a large interference, so they are not meant to be taken apart. A press fit or a push fit may be taken apart with force, and an easy running fit leaves a clearance, so the keyed answer is the heavy fit.

Answer: B

The cylinder liner in a block is an example of a heavy fit, with a high interference between the liner and the cylinder block. The gearbox spindle in a bearing, the exhaust valve in its guide, and the commutator shell on a shaft are examples of other classes of fit, as the key assigns them.

Answer: B

The fit is a transition fit.

The shaft ranges from 24.990 to 25.010 mm and the hole from 25.000 to 25.030 mm.

• Largest shaft in smallest hole: 25.010 - 25.000 = 0.010 mm of interference.
• Smallest shaft in largest hole: 25.030 - 24.990 = 0.040 mm of clearance.

Because a given pair may end up with either clearance or interference, the fit is transition.

A clearance fit needs the smallest hole to be larger than the largest shaft, and an interference fit needs the largest hole to be smaller than the smallest shaft; neither holds here. 'Linear' is not a class of fit.

Answer: A

The match is P-3, Q-1, R-4.

The limits are: shaft 24.93 mm to 24.96 mm; hole 25.00 mm to 25.02 mm. The shaft is always smaller, so this is a clearance fit.

\[ \begin{aligned} \text{Allowance} &= 25.00 - 24.96 = 0.04\ \text{mm} \\ \text{Max clearance} &= 25.02 - 24.93 = 0.09\ \text{mm} \end{aligned} \]

Allowance is the minimum clearance, the tightest condition: smallest hole minus largest shaft, 0.04 mm (3). Maximum clearance is largest hole minus smallest shaft, 0.09 mm (1). The maximum material limit for the hole is the size at which the hole holds the most metal, its smallest size, 25.0 mm (4).

24.96 mm is the maximum material limit of the shaft, not of the hole, which rules out every option pairing R with 2; the options pairing P with 1 swap allowance and maximum clearance.

Answer: C

The maximum interference is 20 microns.

Interference is greatest when the largest shaft meets the smallest hole:

\[ \begin{aligned} d_{shaft,max} &= 25.040\ \text{mm} \\ d_{hole,min} &= 25.020\ \text{mm} \\ I_{max} &= 25.040-25.020 = 0.020\ \text{mm} \end{aligned} \]

0.020 mm is 20 microns. 40 is the shaft's upper deviation on its own, ignoring the hole. Pairing other limits gives the remaining figures, for example the largest shaft against the largest hole leaves 10 microns, but only largest shaft against smallest hole gives the maximum interference.

Answer: C

The assembly is a transition fit.

The hole ranges from 30.000 to 30.030 mm and the shaft from 29.980 to 30.020 mm.

\[ \begin{aligned} \text{max clearance} &= 30.030 - 29.980 = 0.050\ \text{mm} \\ \text{max interference} &= 30.020 - 30.000 = 0.020\ \text{mm} \end{aligned} \]

Because the tolerance zones overlap, a given pair may have clearance or interference, which is the definition of a transition fit. A clearance (loose) fit would need the largest shaft below the smallest hole, and an interference fit the smallest shaft above the largest hole.

Answer: A

30H7/g6 is a clearance fit.

In the ISO system, H is the hole basis: the hole's lower limit is exactly the basic size of 30 mm. The shaft letter g lies below the zero line, so the largest shaft is still smaller than the smallest hole. There is always a gap, which makes it a close sliding or running clearance fit.

Shaft letters j, k, m and n give transition fits, and p, r, s and beyond give interference fits. 'Tolerance fit' is not a class of fit at all: every fit has tolerances.

Answer: B

A NO GO plug gauge checks the upper limit of the hole (the paper prints 'plus' for plug).

A plug gauge has a GO end made to the hole's lower limit and a NO GO end made to its upper limit. A correct hole accepts the GO end and rejects the NO GO end. If the NO GO end enters, the hole is larger than its upper limit and is rejected.

The lower limit is checked by the GO end. Tolerance and deviation are differences between limits and are not checked by one end of a gauge; it is the pair of ends together that confirms the hole is within tolerance.

Answer: D

Hole diameters are checked with a plug gauge.

A plug gauge is a hardened, precisely ground cylinder, usually double-ended: the GO end must enter the hole and the NOT GO end must not. Together they confirm that the hole lies within its tolerance, quickly and without measuring the exact size.

A ring gauge is the opposite: a hardened ring for checking the outside diameter of shafts. A wire gauge checks the diameter of wires and the thickness of sheet. A feeler gauge is a set of thin blades for measuring small gaps, such as tappet clearance.

Answer: A

The three classes of fit are clearance, transition and interference.

The BIS (and ISO) system classifies a fit by how the tolerance zones of hole and shaft overlap. In a clearance fit there is always a gap; in an interference fit the shaft is always larger than the hole; in a transition fit, depending on the actual sizes, either may occur.

Terms such as sliding fit, push fit, tight fit and medium fit are older workshop names for particular fits within these classes, not the classes themselves. Expansion fit is an assembly method for interference fits, and round and flat fits are not fit classes at all.