Interference fit
Encyclopedia
An interference fit, also known as a press fit or friction fit, is a fastening between two parts which is achieved by friction
after the parts are pushed together, rather than by any other means of fastening. For metal parts in particular, the friction that holds the parts together is often greatly increased by compression of one part against the other, which relies on the tensile
and compressive
strengths of the materials the parts are made from. Typical examples of interference fits are the press fitting of shafts into bearing
s or bearings into their housings and the attachment of watertight connectors to cable
s. An interference fit also results when pipe fittings are assembled and tightened.
dimension. The word interference refers to the fact that one part slightly interferes with the space that the other is taking up. For example: A shaft may be ground slightly oversize, and the hole in the bearing (through which it is going to pass with an interference fit) may be ground slightly undersized. When the shaft is pressed into the bearing, the two parts interfere with each others occupation of space; the result is that they elastically deform slightly, each being compressed, and the interface between them is one of extremely high friction—so high that even large amounts of torque
cannot turn one of them relative to the other; they are locked together and they turn in unison.
" (planned difference from nominal size) that will result in various strengths of fit such as loose fit, light interference fit, and interference fit. The value of the allowance depends on which material is being used, how big the parts are, and what degree of tightness is desired. Such values have already been worked out in the past for many standard applications, and they are available to engineers in the form of table
s, obviating the need for re-derivation. Thus if a loose fit is desired for a 10 mm (0.393700787401575 in) shaft made of 303 stainless steel
, the engineer can look up the needed allowance in a reference book or computer program, rather than using a formula to calculate it.
Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses are generally hydraulic, although small hand-operated presses (such as arbor press
es) may operate by means of the mechanical advantage supplied by a screw jack
or by a gear reduction driving a rack and pinion
. The amount of force applied in hydraulic presses may be anything from a few pounds for the tiniest parts to hundreds of tons for the largest parts.
Often the edges of shafts and holes are chamfer
ed (beveled). The chamfer forms a guide for the pressing movement, helping (a) to distribute the force evenly around the circumference of the hole, (b) to allow the compression to occur gradually instead of all at once, thus helping the pressing operation to be smoother, to be more easily controlled, and to require less power (less force at any one instant of time), and (c) to assist in aligning the shaft parallel with the hole it is being pressed into.
ed and shrink when cooled. Enveloping parts are heated (e.g., with torches or gas ovens) and assembled into position while hot, then allowed to cool and contract back to their former size, except for the compression that results from each interfering with the other. This is also referred to as shrink-fitting
. Railroad axles, wheels, and tires are typically assembled in this way. Alternatively, the enveloped part may be cooled before assembly such that it slides easily into its mating part. Upon warming, it expands and interferes. Cooling is often preferable as it is less likely than heating to change material properties, e.g. assembling a hardened gear onto a shaft, where heating the gear would alter its hardness.
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
after the parts are pushed together, rather than by any other means of fastening. For metal parts in particular, the friction that holds the parts together is often greatly increased by compression of one part against the other, which relies on the tensile
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
and compressive
Compressive strength
Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...
strengths of the materials the parts are made from. Typical examples of interference fits are the press fitting of shafts into bearing
Bearing (mechanical)
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
s or bearings into their housings and the attachment of watertight connectors to cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
s. An interference fit also results when pipe fittings are assembled and tightened.
Introducing interference between parts
An interference fit is generally achieved by shaping the two mating parts so that one or the other (or both) slightly deviate in size from the nominalReal versus nominal value
In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...
dimension. The word interference refers to the fact that one part slightly interferes with the space that the other is taking up. For example: A shaft may be ground slightly oversize, and the hole in the bearing (through which it is going to pass with an interference fit) may be ground slightly undersized. When the shaft is pressed into the bearing, the two parts interfere with each others occupation of space; the result is that they elastically deform slightly, each being compressed, and the interface between them is one of extremely high friction—so high that even large amounts of torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
cannot turn one of them relative to the other; they are locked together and they turn in unison.
Tightness of fit is controlled by amount of interference ("allowance")
Formulas exist to compute the "allowanceAllowance (engineering)
In engineering and machining, an allowance is a planned deviation between an actual dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some...
" (planned difference from nominal size) that will result in various strengths of fit such as loose fit, light interference fit, and interference fit. The value of the allowance depends on which material is being used, how big the parts are, and what degree of tightness is desired. Such values have already been worked out in the past for many standard applications, and they are available to engineers in the form of table
Table (information)
A table is a means of arranging data in rows and columns.Production % of goalNorth 4087102%South 4093110% The use of tables is pervasive throughout all communication, research and data analysis. Tables appear in print media, handwritten notes, computer software, architectural...
s, obviating the need for re-derivation. Thus if a loose fit is desired for a 10 mm (0.393700787401575 in) shaft made of 303 stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, the engineer can look up the needed allowance in a reference book or computer program, rather than using a formula to calculate it.
Assembling an oversize shaft into an undersized hole
There are two basic methods for assembly, sometimes used in combination: force, and thermal expansion or contraction.Force
There are at least three different terms used to describe an interference fit created via force: press fit, friction fit, and hydraulic dilation.Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses are generally hydraulic, although small hand-operated presses (such as arbor press
Arbor press
An arbor press is a small hand operated press. It is typically used to perform smaller jobs, such as staking, riveting, installing and removing bearings and other press fit work. Punches, inserters, or other tools/dies may be added to the end of the ram depending on the desired task. Arbor presses...
es) may operate by means of the mechanical advantage supplied by a screw jack
Jackscrew
A jackscrew is a type of jack which is operated by turning a leadscrew. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.-Advantages:...
or by a gear reduction driving a rack and pinion
Rack and pinion
A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...
. The amount of force applied in hydraulic presses may be anything from a few pounds for the tiniest parts to hundreds of tons for the largest parts.
Often the edges of shafts and holes are chamfer
Chamfer
A chamfer is a beveled edge connecting two surfaces. If the surfaces are at right angles, the chamfer will typically be symmetrical at 45 degrees. A fillet is the rounding off of an interior corner. A rounding of an exterior corner is called a "round" or a "radius"."Chamfer" is a term commonly...
ed (beveled). The chamfer forms a guide for the pressing movement, helping (a) to distribute the force evenly around the circumference of the hole, (b) to allow the compression to occur gradually instead of all at once, thus helping the pressing operation to be smoother, to be more easily controlled, and to require less power (less force at any one instant of time), and (c) to assist in aligning the shaft parallel with the hole it is being pressed into.
Thermal expansion or contraction
Most materials expand when heatHeat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
ed and shrink when cooled. Enveloping parts are heated (e.g., with torches or gas ovens) and assembled into position while hot, then allowed to cool and contract back to their former size, except for the compression that results from each interfering with the other. This is also referred to as shrink-fitting
Shrink-fitting
Shrink-fitting is a technique in which pieces of a structure are heated or cooled, employing the phenomenon of thermal expansion, to make a joint. For example, the thermal expansion of a piece of a metallic drainpipe allows a builder to fit the cooler piece to it...
. Railroad axles, wheels, and tires are typically assembled in this way. Alternatively, the enveloped part may be cooled before assembly such that it slides easily into its mating part. Upon warming, it expands and interferes. Cooling is often preferable as it is less likely than heating to change material properties, e.g. assembling a hardened gear onto a shaft, where heating the gear would alter its hardness.
See also
- Statistical interferenceStatistical interferenceWhen two probability distributions overlap, statistical interference exists. Knowledge of the distributions can be used to determine the likelihood that one parameter exceeds another, and by how much....
- Engineering tolerance
- Engineering fitEngineering fitFit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit...
- Coiled spring pins
- Shrink-fittingShrink-fittingShrink-fitting is a technique in which pieces of a structure are heated or cooled, employing the phenomenon of thermal expansion, to make a joint. For example, the thermal expansion of a piece of a metallic drainpipe allows a builder to fit the cooler piece to it...
External links
- Diagram of an interference fit
- Interference fitting - formulae for calculating clearance reductions when using interference fits for bearings on shafts and in housings