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Shrink fitting
Induction shrink fitting refers to the use of induction heater technology to pre-heat metal
components between 150 °C (302 °F) and 300 °C (572 °F) thereby causing them to expand
and allow for the insertion or removal of another component.[1]
Typically the lowertemperature range is used on metals such as aluminium and higher temperatures are used on
metals such as low/medium carbon steels. The process avoids the changing of mechanical
properties whilst allowing components to be worked. Metals typically expand in response to
heating and contract on cooling; this dimensional response to temperature change is
expressed as a coefficient of thermal expansion.[2]
Process
Induction heating is a non contact heating process which uses the principle of
electromagnetism induction to produce heat in a work-piece. In this case thermal expansion is
used in a mechanical application to fit parts over one another, e.g. a bushing can be fitted
over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then
heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the
shaft, thus achieving a 'shrink fit'. By placing a conductive material into a strong alternating
magnetic field, electrical current can be made to flow in the metal thereby creating heat due
to the I2R losses in the material. The current generated flows predominantly in the surface
layer. The depth of this layer being dictated by the frequency of the alternating field and the
permeability of the material. [3] Induction heaters for shrink fitting fall into two broad
categories:
Mains frequency units using magnetic cores (iron) Solid state (electronics) MF and RF heaters
Mains frequency units using iron cores
Often referred to as a bearing heater, the mains frequency unit employs standard transformer
principles for its operation. An internal winding is wound around a laminated core similar to
a standard mains transformer. The core is then passed through the work-piece and when the
primary coil is energised, a magnetic flux is created around the core. The work-piece acts as a
short circuit secondary of the transformer created, and due to the laws of induction, a current
flows in the work-piece and heat is generated. The core is normally hinged or clamped in
some way to allow loading or unloading, which is usually a manual operation. To covervariations in part diameter, the majority of units will have spare cores available which help to
optimise performance. Once the part is heated to the correct temperature, assembly can take
place either by hand or in the relevant jig or machine press.[4]
Power consumption
Bearing heaters typically range from 1 kVA to 25 kVA and are used to heat parts from 1 to
650 kg (2.2 to 1,400 lb), dependent upon the application. The power required is a function of
the weight, target temperature and cycle time to aid selection many manufacturers publish
graphs and charts.
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Industries and applications
Railway - gearboxes, wheels, transmissions Machine tools - lathe gearboxes, mills Steel works - roll bearings, roll neck rings
Power generation - various generator components
Due to the need to insert a core and also that to be effective, the core has to be in relatively
close proximity to the bore of the part to be heated, there are many application in which the
above bearing heater type approach is not feasible.
Solid state MF and RF heaters
In those cases where operational complexities negate the use of a cored mains frequency
approach, the standard RF or MF induction heater can be used. This type of unit uses turns of
copper tube wound into a electromagnetic coil.[5] There are no cores required, the coil needs
to simply surround or be inserted into the part to be heated this makes automating the processstraightforward. A further advantage is the ability to not only shrink fit parts but also remove
them.
The RF and MF heaters used for induction shrink fitting vary in power from a few kilowatts
to many megawatts and depending on the component geometry/diameter/cross section can
vary in frequency from 1 kHz to 200 kHz, although the majority of applications use the range
between 1 kHz and 100 kHz.[5]
In general terms, it is best to use the lowest practical frequency and a low power density
when undertaking shrink fitting as this will generally provide more evenly distributed heat.
The exception to this rule is when using heat to remove parts from shafts. In these cases it isoften best to shock the component with a rapid heat, this also has the advantage of shortening
the time cycle and preventing heat buildup in the shaft which can lead to problems with both
parts expanding.
In order to select the correct power it is necessary to first calculate the thermal energy
required to raise the material to the required temperature in the time allotted. This can be
done using the heat content of the material which is normal expressed in kW hours per tonne,
the weight of metal to be processed and the time cycle.[6] Once this has been established other
factors such as radiated losses from the component, coil losses and other system losses need
to be factored in. Traditionally this process involved lengthy and complex calculations in
conjunction with a mixture of practical experience and empirical formula. Modern techniques
use finite element analysis and other computer-aided manufacturing techniques, however as
with all such methods a thorough working knowledge of the induction heating process is still
required. When deciding on the correct approach it is often necessary to consider the overall
size and thermal conductivity of the work-piece and its expansion characteristics in order to
ensure that enough soak time is allowed to create an even heat throughout the component.
Output frequency
As shrink fitting requires a uniform heating of the component to be expanded, it is best to try
to use the lowest practical frequency when approaching heating for shrink fitting. Again theexception to this rule can be when removing parts from shafts.
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Industries and applications
There are a huge number of industries and applications which benefit from induction shrinkfitting or removal using solid state RF and MF heaters. In practice, the methodology
employed can vary from a simple manual approach where an operator assembles or
disassembles the parts to fully automatic pneumatic and hydraulic press arrangements.[7]
Automotive starter rings onto flywheels Timing gears to crankshafts Motor stators into motor bodies Motor shafts into stators Removal and re-fitting of a gas turbine impeller Removal and re-fitting of hollow bolts in electrical generators Assembly of high precision roller bearings Shrinkfitting of 2-stroke crankshafts for ship engines
Advantages & disadvantages
Advantages:
Process controllability - Unlike a traditional electric or gas furnace the inductionsystem requires no pre-heat cycle or controlled shutdown. The heat is available ondemand. In addition to the benefits of rapid availability in the event of a downstreaminterruption to production, the power can be switched off thus saving energy.
Energy efficiency - Due to the heat being generated within the component energytransfer is extremely efficient. The induction heater heats only the part not theatmosphere around it.
Process consistency - The induction heating process produces extremely uniformconsistent heat this often allows less heat to be used for a given process.
No naked flame - This allows induction heating to be used in a wide variety ofapplications in volatile environments in particular in petrochemical applications.
The main disadvantage of this process is that, in general, it is limited to components which
have a cylindrical shape
Tolerancing
Interchangeability of manufactured parts is a critical element of present day production. The
production of closely mating parts, although theoretically possible, is economically
unfeasible. For this reason, the engineer, designer or drafter specifies an allowable deviation
(tolerance) between decimal limits.
The definition of a Tolerance, per ASME Y14.5.5M-1994, is the total amount a specific
dimension is permitted to vary. For instance, a dimension shown as 1.498” to 1.502” means
that it may be 1.498” or 1.502” or anywhere between these dimensions. Since greater
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accuracy costs money, you would not callout the tightest possible tolerance, but instead
would callout as generous a tolerance as possible.
Definition of Terms
Example 1
Maximum Material Condition (MMC) – Is the condition where a feature of a finished part
contains the maximum amount of material. That is, the largest shaft or smallest hole. See
Example 1.
Least Material Condition (LMC) - Is the condition where a feature of a finished part
contains the least amount of material. That is, the smallest shaft or the largest hole. See
Example 1.
Nominal Size – Approximate size used for the purpose of identification such as stock
material.
Basic Size – Is the theoretical exact size from which limits of size are determined by the
application of allowances and tolerances.
Tolerance – The total amount by which a given dimension may vary or the difference
between the limits.
Limits – The extreme maximum and minimum sizes specified by a toleranced dimension.
LMCMMC
LMCMMC
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Allowance – An allowance is the intentional difference between the maximum material limits
(minimum clearance or maximum interference) of mating parts.
Refer to Example 1 above: MMC of the hole – MMC of the shaft = Allowance.
MMC Hole = 1.250
- MMC Shaft = 1.248
Allowance = .002
Fits
Clearance fit – A clearance fit results in limits of size that assure clearance between
assembled mating parts.
Refer to Example 1 above: LMC of the hole – LMC of the shaft = Clearance.
LMC Hole = 1.251
- LMC Shaft = 1.247
Clearance = .004
Interference fit (also referred to as Force fit or Shrink fit) – interference fit has limits of size
that always result in interference between mating parts. For example, a hole and shaft, the
shaft will always be larger than the hole, to give an interference of metal that will result in
either a force or press fit. The effect would be an almost permanent assembly for two
assembled parts.
Example 2
Least amount of Interference is:
LMC Shaft = 1.2513
- LMC Hole = 1.2506
Min Interference = .0007
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Greatest amount of Interference:
MMC Shaft = 1.2519
- MMC Hole = 1.2500
Max Interference = .0019
Transition fit – A transition fit might be either a clearance or interference fit. That is, a shaft
may be either larger or smaller than the hole in a mating part.
Example 3
LMC Hole = 1.2506
- LMC Shaft = 1.2503
Positive Clearance = .0003
MMC Shaft = 1.2509
- MMC Hole = 1.2500
Negative Allowance (Interference) = .0003
Basic Hole System – The basic hole system is used to apply tolerances to holes and shafts
assemblies. The minimum hole is assigned the basic diameter (basic size) from which the
tolerance and allowance are applied. This system is widely used in industry due to standard
reamers being used to produce holes, and standard size plugs used to check hole sizesaccurately.
Computed Clearance Fit using Basic Hole System
.500 = hole basic size .500 basic hole
.002 = Allowance (decided) - .002 allowance
.498 Maximum shaft
Step 1 Step 2
If tolerance of part is = .003 then:
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.498 maximum shaft .500 basic hole
-. 003 tolerance +.003 tolerance
.495 minimum shaft .503 maximum hole
Step 3 Step 4
Calculate clearances:
.500 smallest hole (MMC) .503 largest hole (LMC)
-. 498 largest shaft (MMC) -.495 smallest shaft (LMC)
.002 minimum clearance .008 maximum clearance
Step 5 Step 6
Drawing annotation of tolerance
Example 3
Basic Shaft System – The basic shaft system can be used for shafts that are produced in
standard sizes. When applying this system, the largest shaft is assigned the basic size
diameter from which the allowance for the mating part is assigned. Then, tolerances are
applied on both sides and away from the assigned allowance. One situation for using the
basic shaft system is when a purchased motor, with an attached shaft, from which a mating
hole must be calculated.
Computed Interference fit using Basic Shaft System
.500 = shaft basic size .500 basic shaft
.002 = Allowance (decided) - .002 allowance
.498 Maximum hole
Step 1 Step 2
If tolerance of part is = .003 then:
.498 maximum hole .500 basic shaft
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-. 003 tolerance +.003 tolerance
.495 minimum hole .503 maximum shaft
Step 3 Step 4
Calculate clearances:
.498 largest hole (LMC) .495 smallest hole (MMC)
-. 500 smallest shaft (LMC) -.503 largest shaft (MMC)
- .002 minimum interference -.008 maximum interference
Step 5 Step 6
Drawing annotation of tolerance
Example 4
Preferred precision fits – The American National Standards Institute publishes the
“Preferred Limits and Fits for Cylindrical Parts” (ANSI B4.1-1967) to define terms and
recommending standard allowances, tolerances, and fits for mating parts. The chart data is
provided in thousandths (.001) of an inch. For example: -1.2 and -2.2 (See Example 5) for
calculation purposes would be -.0012 and -.0022.
Running and Sliding fits (RC1-RC9)
Loosest of the class fits, used when a shaft is must move freely inside a hole or bearing, and
the positioning of the shaft is not critical. This fit would always allow a clearance between
shaft and hole.
Clearance locational fits (LC1-LC11)
Tighter than RC fits, but the shaft and hole may be the same size. LC fits allow the shaft to
be located more accurately than the RC fits but may still be loose. With this fit, a shaft would
move less freely inside a hole.
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Transition locational fits (LT1-LT6)
These fits are a compromise between LC and LN (interference/force) fits. These fits would
allow either a small amount of clearance or interference.
Interference locational fits (LN1-LN3)
Used where accuracy of location is the prime importance such as alignment of dowel pins
and other devices where location relative to another part is of prime importance.
Force and shrinks fits (FN1-FN5)
With this fit, the shaft is always considered larger than the hole. These fits are used to
transmit torque such as a motor shaft to a bearing.
Limits Calculations Using ANSI B4.1 Standard Tables
Class RC6 Clearance Fit
Partial Table from ANSI B4.1
Example 5
A nominal hole size of .8750 Diameter and a RC6 Class Fit has been selected.
Hole nominal size range = .71 – 1.19
Minimum clearance = .0016
Maximum clearance = .0048
Tolerance of hole = +.0020, -.0000
Tolerance of shaft = -.0016, -.0028
Calculations:
NominalSize Range,
Inches
Over To
0.40 - 0.71
0.71 - 1.19
Class RC6
C l e
a r a n c e Standard
Tolerance
LimitsHoleH8
Shafte7
1.23.8
1.64.8
0
+2.00
+1.6
-1.6-2.8
-2.2-1.2
Class RC7
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Hole: Basic size .8750 .8750
Tolerance +.0020 -.0000
Maximum hole .8770 Minimum hole .8750
Shaft: Basic size .8750 .8750
Tolerance -.0016 -.0028
Maximum shaft .8734 Minimum shaft .8722
Limits of size for Hole and Shaft
Example 5
Limit Calculations when one Design Feature Exists
When calculating the limit tolerances for features that mate with purchased parts, the
purchased part size must be known. This may be obtained be requesting a drawing from a
vendor or, a caliper or micrometer can be used to obtain an accurate size.
Example:
A shaft diameter of .2500 is to be pressed into a part using a FN4 interference (force) fit.
Limits of size for the shaft diameter are .2500 and .2495.
The table shows a minimum acceptable interference of .0006 and maximum interference of
.0016.
Calculations:
Maximum shaft: . 2500
Maximum interference: -. 0016
Minimum hole: . 2484
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Minimum shaft: . 2495
Minimum interference: -. 0006
Maximum hole: .2489
References:
Dimensioning and Tolerancing, ASME Y 14.5M-1994, The American Society of Mechanical
Engineers.
Technical Drawing Tenth Edition, Frederick E. Giesecke, Prentice Hall, Upper Saddle River,
NJ 07458.
Geometric Dimensioning and Tolerancing, 2003, David A. Matson, Goodheart-Wilcox Co.
Inc., Tinley Park, Illinois.
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