Engineering -Fits and tolerances

Fits and Tolerances : Practical
application in fitment of bearings
and couplings
Presented By
Sandeep Gupta

Fits & Tolerances
Why a proper understanding of fits and tolerances are required for a maintenance
engineer ?
• How to decide acceptable lower / upper limit of bore size of a coupling
• How to decide acceptable lower / upper limit of a shaft OD at bearing seat,
coupling hub seat, or oil seal location
• How to decide whether bore dimensions of an old bearing housings are
within allowable limits or not
• How to decide that OD at bearing / coupling seat of a old worn out shaft are
acceptable
• How to calculate the estimated thrust force required to engage / disengage
two matching parts with interference fit

• Exact size is impossible to achieve.
• Establish boundaries within which deviation from perfect
form is allowed YET fulfil its design intent.
• Enable interchangeability of engineering components
during assembly.
Why Tolerances are required ?

Lower limit =  27. 8
 0.2
Upper limit =  28 . 2
Shaft
Limits (extreme sizes of a part)
Tolerance
zone
Tolerance = UL – LL
= 28.2 – 27.8
= 0.4
Or  0.2
Ø28
(Basic)
Zero line

Limits of Size
Unilateral Limits occurs when both maximum limit and minimum limit are
either above or below the basic size. e.g. Ø25 +0.18
+0.10
Basic Size = 25.00 mm
Upper Limit = 25.18 mm
Lower Limit = 25.10 mm
Tolerance = 0.08 mm
e.g. Ø25 -0.10
-0.20
Tolerance = 0.10 mm

Limits of Size
For Unilateral Limits, a case may occur when one of the limits coincides
with the basic size,
e.g. Ø25 +0.20 , Ø25 0
0 -0.10
e.g. Ø25 ±0.04
Tolerance = 0.08 mm
Bilateral Limits occur when the maximum limit is above and the minimum
limit is below the basic size.

Fits (assembly condition between “Hole” & “Shaft”)
Hole – A feature engulfing a component
Shaft – A feature being engulfed by a component

Hole
Shaft
Min C Max C
Clearance Fits
Tolerance zones never meet
Max. C = UL of hole - LL of shaft
Min. C = LL of hole - UL of shaft

Shaft Min I
Max I
Hole
Interference Fits
Tolerance zones never meet but
crosses each other
Max. I = LL of hole - UL of shaft
Min. I = UL of hole - LL of shaft

Shaft
Max I
Max C
Hole
Transition Fits
Tolerance zones always overlap
Max. C = UL of hole - LL of shaft
Max. I = LL of hole - UL of shaft

Terminology Related to Limits and Fits

ISO limits and fits terminology

• It is defined graphically
by the magnitude of the
tolerance and by its
position in relation to the
zero line.
Tolerance Zone
Basic Size
µm
55
20

Fundamental Deviation
is chosen to locate the tolerance zone w.r.t. the zero line
Holes are designated by capital letter:
Letters A to G - oversized holes
Letters M to ZC - undersized holes
Shafts are designated by small letter:
Letters m to zc - oversized shafts
Letters a to g - undersized shafts
H is used for holes and h is used for shafts
whose fundamental deviation is zero

Grades of Tolerances
Grades of Tolerances
• Grade is a measure of the
magnitude of the tolerance.
• The lower the grade the finer the
tolerance.
• There are total of 18 grades
which are allocated the numbers
IT01, IT0, IT1, IT2 ..... IT16.
• Fine grades are referred to by the
first few numbers.
• As the numbers get larger, so the
tolerance zone becomes
progressively wider.
• Selection of grade should depends
on the circumstances.
• As the grades get finer, the cost of
production increases at a sharper
rate.

International tolerance (IT) grade

Basis of Fits - Hole Basis
• In this system, the basic
diameter of the hole is constant
while the shaft size varies
according to the type of fit.
C
T I
Hole Basis Fits
C - Clearance
T - Transition
I - Interference
Hole
Shaft
Tolerance
Legends:
• This system leads to greater
economy of production, as a single
drill or reamer size can be used to
produce a variety of fits by merely
altering the shaft limits.
• The shaft can be accurately
produced to size by turning and
grinding.
• Generally it is usual to recommend
hole-base fits, except where
temperature may have a
detrimental effect on large sizes.

Basis of Fits - Shaft Basis
• A series of drills and reamers is
required for this system,
therefore it tends to be costly.
• It may, however, be necessary to
use it where different fits are
required along a long shaft. For
example, in the case of driving
shafts where a single shaft may
have to accommodate to a
variety of accessories such as
couplings, bearings, collars, etc.,
it is preferable to maintain a
constant diameter for the
permanent member, which is the
shaft, and vary the bore of the
accessories.
Shaft Basis Fits
C T I
Legends:
Hole
Shaft
Tolerance
C - Clearance
T - Transition
I - Interference
•Here the hole size is varied to
produce the required class of fit with a
basic-size shaft.

Selected ISO Fits- Hole Basis
• The ISO system provides many holes and shaft tolerances so as
to cater for a wide range of conditions.
• The following selected hole and shaft tolerances have been
found to be commonly applied:
Selected hole tolerances: H7, H8, H9, H11
Selected shaft tolerances: c11, d10, e9, f7, g6, h6, k6, n6, p6, s6
• It covers fits from loose clearance to heavy interference and are
suitable for most general engineering applications.

Selected ISO Fits- Hole Basis
+ 130 - 110
0 - 240
+ 21 + 35
0 + 22
+ 21 + 15
0 + 2
Clearance Transition Interference
H11 - c11 H7 - k6 H7 - p6
30

Hole Shaft Fit 100 H7 / g 6
Calculate
1. Identify fitting conditions from Fundamental deviation.
2. Convert from F.D to limits of tolerance for hole and shaft.
3. Calculate max. & min. limit of size of hole and shaft.
4. Max./ Min. Clearance or Interference
Let us do an exercise ….

Application of Tolerances to Dimensions
• Tolerances should be specified in the case where a dimension
is critical to the proper functioning or interchangeability of a
component.
• A tolerance can also be supplied to a dimension which can
have an unusually large variation in size.
• General tolerances are generally specified as a note at the
bottom of the drawing.

Specifications of Tolerances on Drawings
(Detailed Parts for Critical Linear Dimensions)
(a) Maximum and Minimum Limits
By specifying directly both limits of size, the
maximum limit being quoted first and the same
number decimal places used in both figures. This
method eliminates calculations on the shop floor.
(b) Limits of Tolerance
By specifying a size with limits of tolerance in both
direction stated. In this case, again, the same
number of decimal places must be used in both
limits. The larger limit is usually quoted first. If one
of the two deviations in nil, this should be
expressed by the figure 0 (zero).
(c) Deviation and Grade
By using the appropriate deviation and grade
symbols H8, g6 and so on, may be quoted with or
without the limits specified.

Specifications of Tolerances on Drawings
on Assembled Parts for Critical Linear Dimensions
By specifying the basic size followed by the deviation and grade
symbols. The symbols for the hole must be placed before that of
the shaft or above it. If it is necessary to specify also the
numerical value of the deviations, they should be written in
brackets.

Rolling Contact Bearings – Recommended fits for shaft
and bearing housings
While deciding upon the type of fit to be used on shafts and bearing housings for
fitment of rolling contact bearings, it is important to consider following
1. Type of loading – constant or rotating direction
2. Size of the bearing
3. Magnitude of the applied load
4. Bearing internal clearance
5. Temperature conditions
6. Design and material of shaft and housing
7. Ease of mounting and dismounting
8. Displacement of non locating bearing

Rolling Contact Bearings – Type of loading
• Type of loading refers to the direction of the load relative to the bearing ring
being considered for fit
There are three different conditions
Stationary load ( constant direction of load)
Rotating Load ( direction of load rotating )
Direction of load indeterminate
• Rotating load : Refers to a baring ring that rotates , while the direction of
applied load is stationary. A rotating load can also refer to bearing ring that is
stationary and the direction of applied load rotates so that all point on raceways
are subjected to load on in the course of one revolution
• Bearing ring subjected to rotating load will creep / turn on its seat if not
mounted with required interference fit.
• Fretting corrosion / scoring of ring seat - if proper interference fit is not
provided

Rolling Contact Bearings – Type of loading
• Stationary load : Refers to a bearing ring that is stationary , while the direction
of applied load is also stationary.
•A stationary load can also refer to a bearing ring that rotates at same speed as
that of applied load so that the load is always directed towards the same position
on the raceway.
• Bearing ring subjected to stationary will normally NOT turn on its seating.
• Hence an interference fit is NOT required
• Direction of load indeterminate refers to cases variable external loads , shock
loads and unbalance loads.
• Where direction of load is indeterminate and particularly where heavy loads are
involved , it is desirable that both rings to have an interference fit.

Case 1 : Inner Ring – Rotating ; Outer Ring –Stationary; Load direction
- Constant
Examples : All centrifugal fan and pump shafts bearings ( with small rotor unbalance only),
Belt drives, Belt and chain conveyor shaft, Bucket elevator shafts helical gear box shafts, Ball
mill pinion shaft

Case 2 : Inner Ring – Rotating ; Outer Ring- Stationary; Load
Direction –Rotating
Examples : Vibrating screens, Vibro feeders, Gear box output shaft for
reciprocating feeder, shafts with eccentric weights

Case 3 : Inner Ring – Stationary ; Outer Ring - Rotating; Load
direction – Constant
Examples : Conveyor idlers, VRM Grinding Roller shaft bearing, Car wheel
hub bearings

Case 4 : Inner Ring – Stationary ; Outer Ring - Rotating; Load
direction – Rotating
Examples: Gyratory Crusher, Merry Go Round Drives

Case 5 : Inner Ring – Stationary / Rotating; Outer Ring –
Stationary / Rotating; Load direction – Indeterminate
Inner Ring Fit – Interference
Outer ring fit - Interference
All machines with unpredictable unbalance load like hammer crusher, centrifugal
fans / pumps handling corrosive / erosive fluids

Rolling Contact Bearings – Selection of Fit - Magnitude of Load
• The interference fit of a bearing ring on its seat will be loosened with
increasing load since the ring can flex under load.
•Therefore the amount of interference fit should be related to the magnitude of
applied load.
•The heavier the load, the greater the interference fit that is required.

Rolling Contact Bearings – Selection of Fit – Bearing Internal Clearance
• When the bearing is mounted on its location , the inner ring tend to expand
whereas outer ring tend to compress if interference fits are used for both the
rings.
•Hence the bearing internal clearance decreases after mounting.
•Although all bearing types can run with some preload (negative clearance),
however it is recommended to have small positive operating clearance especially
for cylindrical roller bearing, needle roller, spherical roller and CARB toroidal
bearings.
•With negative clearance i.e. preload there shall be increase in friction and
friction heat , though a small preload may give better bearing life.
•However a small preload is required for applications where stiffness of the
system is required or minimum vibrations are allowed such as in lathe machine
spindle bearings

The required initial internal clearance of an un-mounted bearing can
be estimated using
r = rop + Δrfit + Δrtemp
where
r=required internal clearance for the un-mounted bearing [mm]
rop=desired operating clearance [mm]
Δrfit =clearance reduction caused by the fit [mm]
Δrtemp=clearance reduction caused by temperature difference [mm]

Clearance reduction caused by an interference fit
The reduction equals the effective interference fit multiplied by a reduction
factor using
Δrfit = Δ1 f1 + Δ2 f2
where
Δrfit =clearance reduction caused by the fit [mm]
f1 =reduction factor for the inner ring
f2=reduction factor for the outer ring
Δ1=effective interference between the inner ring and shaft [mm]
Δ2=effective interference between the outer ring and housing [mm]
The reduction factors are a function of the ratio of the bearing bore diameter d to
the outside diameter D. It is valid for a solid steel shaft and a cast iron or steel
housing

d = Bore of the bearing
D = Outside diameter of the bearing

Bearings with an internal clearance other than Normal are identified by the suffixes
C1 to C5.

Rolling Contact Bearings – Selection of Fit – Temperature Conditions
• In many of applications the bearing outer ring has a lower temperature in
operation than the inner ring. The centrifugal fans / pumps handling the hot
gases / liquids are such examples
•In such cases a higher interference fit is required on inner ring as inner ring
tends to expand due to temperature when in operation.
•This causes reduction in operating bearing clearance and hence require use of
bearings with initial higher clearance

Rolling Contact Bearings – Selection of Fit – Temperature Conditions
Clearance reduction caused by a temperature difference between the bearing
rings
The internal clearance reduction can be estimated using
Δrtemp = α dm ΔT
where
Δrtemp =clearance reduction caused by temperature difference [mm]
dm =bearing mean diameter [mm] = 0,5 (d + D)
α=thermal coefficient of expansion [°C–1] = 12 x 10–6 for steel
ΔT =temperature difference between the shaft and housing [°C]

Rolling Contact Bearings – Selection of Fit – Displacement of non
locating bearing
• If a non separable bearing is used as the locating bearing , it is required that one of the
bearing ring ( normally outer ring) is free to move axially on its seat .
•For this , a clearance fit is used for the ring which has stationary load.
•For cylindrical roller bearings , needle roller bearings and CARB toroidal bearing , both
rings can be mounted with interference as these bearings axial displacement can take
place internally.

Rolling Contact Bearings – Recommended fits

Rolling Contact Bearings – Recommended fits – Tolerance for
Solid steel shafts

Solid steel shafts
Notes
1) For normal to heavily loaded ball bearings (P>0.05C), radial clearance greater than
Normal is often needed when the shaft tolerances in the table above are used.
2) Tolerance in brackets applies to stainless steel bearings
3) For stainless steel bearings within the diameter range 17 to 30 mm, tolerance j5
applies
4) Bearings with radial internal clearance greater than Normal are recommended
5) Bearings with radial internal clearance greater than Normal are required for d> 150
mm

Solid steel shafts – For thrust bearings

Rolling Contact Bearings – Recommended fits – Housing fit
tolerances for cast iron and steel housings
1) For ball bearings with D <= 100 mm, tolerance grade IT6 is preferable.
2) For applications such as electric motors and pumps, an H6 should be used to reduce
the amount of looseness in the housing, while still allowing the bearing to float.

Rolling Contact Bearings – Recommended fits – Housing fit
tolerances for cast iron and steel housings

Interference fit – Calculation of required engagement /
disengagement force of coupling hubs / gears

Interference fit – Calculation of required engagement /
disengagement force for bearings

Selection Of Shaft Coupling Fits
Types of fits used between coupling and shaft
1. Clearance fit
2. Transition Fit
3. Interference fit

Selection Of Shaft Coupling Fits
Selection of a clearance or interference fit is influenced by several factors such as
Ease of assembly
System sensitivity to unbalance
Available equipment
Forces generated during operation of coupling
Clearance fits :
• Ease of assembly and disassembly
• Hubs secured on shaft by tightening of set screws. Hence clearance fits work well as
long as the setscrew can generate enough friction force to overcome the forces
generated by coupling. (eg. Due to deformation of disc, elastomeric element and axial
thrust in gear couplings)
• Give rise to potential unbalance due to clearances (as a set screw on a clearance fit hub
is tightened, the hub is pushed to one side. Shift in hub position sets an unbalance.)
Hence not suitable for high speed application more than 3000 rpm.
• Due to clearance fits, hub may rock on shaft and result in fretting of shaft hence not
used on shafts size more than 2” and high power machines more than 30 kW.
• Preferred for vertical VS4 and VS5 type pumps where axial positioning and securing of
coupling hub(s) is required

Selection of Shaft Coupling fits
Transition & Interference fits :
• Preferred where positive axial location of the hubs on shafts is required.
• Preserve the inherent balance quality of the coupling. Hence suitable for
high speed application more than 3000 rpm.
• No rocking of hub on shaft and hence used high power machines of
normally more than 30 kW.
• Interference fits are not suitable for hubs made of brittle materials.
Interference fit stresses can cause the hubs to crack.
• Coupling hubs with interference fits are generally mounted to the shaft by
heating the hubs.
• The best methods for heating the hubs are with an oven, hot plate,
induction heater or a hot oil bath.

Shaft –Coupling Hub Preferred Tolerances And Resulting fits

Shaft -Coupling Hub Preferred Tolerances And Resulting Fits

IEC Electric Motor Shaft Size Tolerances
Mechanical Tolerances (ISO tolerance to DIN748, DIN 7160, DIN 42948, DIN EN 50347)
Shaft End Diameter dimension “D” Up to 28 mm j6
38 mm to 50 mm k6
More than 50 mm m6
Register Diameter, dimension “N” Up to 230 mm j6
More than 230 mm h6

Shaft End -DIN 748/1 and Flender W 0470 Standard

Key - Keyway Fit
• The fitting of key is important to ensure the proper capacity
of the interface.
• AGMA 9002-B04 governs the keys and keyways fits
• As a general rule
– The key should fit tightly in the shaft keyways
– The key should have sliding fit ( but not be too loose) in the hub
keyway
– The key should have clearance with top of the keyway
– The key should have chamfered corners

Key - Keyway Fit
Two classes of fits are used
Normal Js9
Close P9

Common Defects in Coupling Bores / Keyways
• Bore diameter is not correct
 Too large – excessive clearance – rocking of hub on shaft, unbalance
 Too small – excessive interference – problem in hub installation, hub crack due
to stresses generated due to interference
• Bore is eccentric
Eccentric but parallel
to the hub axis. Gives
rise to unbalance

Common Defects in Coupling Bores / Keyway
• Skewed Hub Bore
The bore is at an angle
to the OD of the hub. In
this case misalignment
capacity of the coupling
is reduced
• Key way offset
Centreline of bore and
keyway are offset

Common Defects in Coupling Bores / Keyway
• Keyway Parallelism
The Keyway depth is
not uniform
• Key way Lead
Centreline of bore and
keyway are at an angle

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