Allowance (engineering)

Last updated

In engineering and machining, an allowance is a planned deviation between an exact 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 known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An allowance, which is a planned deviation from an ideal, is contrasted with a tolerance, which accounts for expected but unplanned deviations.


Allowance is basically the size difference between components that work together. Allowance between parts that are assembled is very important. For example, the axle of a car has to be supported in a bearing otherwise it will fall to the ground. If there was no gap between the axle and the bearing then there would be a lot of friction and it would be difficult to get the car to move. If there was too much of a gap then the axle would be jumping around in the bearing. It is important to get the allowance between the axle and the bearing correct so that the axle rotates smoothly and easily without juddering.

Examples of engineering and machining allowances

Confounding of the engineering concepts of allowance and tolerance

Often the terms allowance and tolerance are used inaccurately and are improperly interchanged in engineering contexts. This is logical because both words generally can relate to the abstract concept of permission — that is, of a limit on what is acceptable. However, in engineering, separate meanings are enforced, as explained below.

A tolerance is the planned limit of acceptable unintended deviation from a nominal or theoretical dimension. Therefore, a pair of tolerances, upper and lower, defines a range within which an actual dimension may fall while still being acceptable.

In contrast, an allowance is a planned deviation from the nominal or theoretical dimension.


An example of the concept of tolerance is a shaft for a machine is intended to be precisely 10 mm in diameter: 10 mm is the nominal dimension. The engineer designing the machine knows that in reality, the grinding operation that produces the final diameter may introduce a certain small-but-unavoidable amount of random error. Therefore, the engineer specifies a tolerance of ±0.01 mm ("plus-or-minus" 0.01 mm).

As long as the grinding machine operator can produce a shaft with actual diameter somewhere between 9.99 mm and 10.01 mm, the shaft is acceptable. Understanding how much error is predictable in a process and how much is easily avoidable; how much is unavoidable (or whose avoidance is possible but simply too expensive to justify); and how much is truly acceptable involves considerable judgment, intelligence, and experience.

An example of the concept of allowance can be shown in relation to the hole that this shaft must enter. It is evident that the above shaft cannot be certain to freely enter a hole that is also 10 mm with the same tolerance. It might, if the actual shaft diameter is 9.99 mm and the actual hole diameter is 10.01 mm, but it would not if conversely the actual shaft diameter is 10.01 mm and the actual hole diameter is 9.99 mm.

To be sure that there will be enough clearance between the shaft and its hole, taking account of the tolerance, an allowance is intentionally introduced in the dimensions specified. The hole diameter might be specified as 10.03 mm with a manufacturing tolerance of ±0.01 mm ("plus-or-minus" 0.01 mm). This means that the smallest acceptable hole diameter will be 10.02 mm while the largest acceptable shaft diameter will be 10.01 mm, leaving an "allowance" of 0.01 mm. The minimum clearance between the hole and the shaft will then be 0.01 mm. This will occur when both the shaft and the hole are at maximum material condition.

Related Research Articles

Bicycle wheel Wheel designed for a bicycle

A bicycle wheel is a wheel, most commonly a wire wheel, designed for a bicycle. A pair is often called a wheelset, especially in the context of ready built "off the shelf" performance-oriented wheels.

Ball bearing Type of rolling-element bearing that uses balls to maintain the separation between the bearing races.

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.

Engineering tolerance permissible limit or limits of variation in value or dimension

Engineering tolerance is the permissible limit or limits of variation in:

  1. a physical dimension;
  2. a measured value or physical property of a material, manufactured object, system, or service;
  3. other measured values ;
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel ;
  5. in mechanical engineering the space between a bolt and a nut or a hole, etc.

The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

Interchangeable parts are parts (components) that are, for practical purposes, identical. They are made to specifications that ensure that they are so nearly identical that they will fit into any assembly of the same type. One such part can freely replace another, without any custom fitting, such as filing. This interchangeability allows easy assembly of new devices, and easier repair of existing devices, while minimizing both the time and skill required of the person doing the assembly or repair.

A reamer is a type of rotary cutting tool used in metalworking. Precision reamers are designed to enlarge the size of a previously formed hole by a small amount but with a high degree of accuracy to leave smooth sides. There are also non-precision reamers which are used for more basic enlargement of holes or for removing burrs. The process of enlarging the hole is called reaming. There are many different types of reamer and they may be designed for use as a hand tool or in a machine tool, such as a milling machine or drill press.

Grinding machine power tool or machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool; mechanical hardware for grinding

A grinding machine, often shortened to grinder, is one of power tools or machine tools used for grinding, it is a type of machining using an abrasive wheel as the cutting tool. Each grain of abrasive on the wheel's surface cuts a small chip from the workpiece via shear deformation.

Screw thread helical part of screw

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

A go/no-go gauge refers to an inspection tool used to check a workpiece against its allowed tolerances via a go/no-go test. Its name is derived from two tests: the check involves the workpiece having to pass one test (go) and fail the other (no-go).

An interference fit, also known as a press fit or friction fit is a form of fastening between two tight fitting mating parts that produces a joint which is held together by friction after the parts are pushed together.

Dowel solid cylindrical rod used as structural reinforcement

A dowel is a cylindrical rod, usually made from wood, plastic, or metal. In its original manufactured form, a dowel is called a dowel rod. Dowel rods are often cut into short lengths called dowel pins. Dowels are commonly used as structural reinforcements in cabinet making and in numerous other applications, including:

Boring (manufacturing) process of enlarging a hole that has already been drilled

In machining, boring is the process of enlarging a hole that has already been drilled by means of a single-point cutting tool, such as in boring a gun barrel or an engine cylinder. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole. Boring can be viewed as the internal-diameter counterpart to turning, which cuts external diameters.

Trapezoidal thread form Screw thread profiles with trapezoidal outlines

Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews. They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a vise or the leadscrew of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule. NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which also abbreviates as "NPS". The European and international designation equivalent to NPS is DN, in which sizes are measured in millimetres, see ISO 6708. The term NB is also frequently used interchangeably with DN.

Roundness is the measure of how closely the shape of an object approaches that of a mathematically perfect circle. Roundness applies in two dimensions, such as the cross sectional circles along a cylindrical object such as a shaft or a cylindrical roller for a bearing. In geometric dimensioning and tolerancing, control of a cylinder can also include its fidelity to the longitudinal axis, yielding cylindricity. The analogue of roundness in three dimensions is sphericity.

When 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 fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined together. Engineering fits are generally described as a "shaft and hole" pairing, but are not necessarily limited to just round components. ISO is the internationally accepted standard for defining engineering fits, but ANSI is often still used in North America.

The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present.

Production drawings are complete sets of drawings that detail the manufacturing and assembly of products.

Rule based DFM analysis for forging. Forging is the controlled deformation of metal into a specific shape by compressive forces. The forging process goes back to 8000 B.C. and evolved from the manual art of simple blacksmithing. Then as now, a series of compressive hammer blows performs the shaping or forging of the part. Modern forging uses machine driven impact hammers or presses which deform the work-piece by controlled pressure.