shrink fitting

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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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http://en.wikipedia.org/wiki/Transformer

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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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shrink fitting

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